Structure-informed two-electron electrocatalysis for selective redox synthesis

Oxygen-related two-electron electrocatalysis

Oxygen-related two-electron electrocatalysis constitutes the most archetypal and chemically instructive class within this field because it places selectivity control directly on the question of whether an O-O unit is preserved, generated, or cleaved during interfacial redox transformation^[43]. Its core importance arises from the fact that both the cathodic reduction of O₂ and the anodic oxidation of H₂O can, in principle, terminate at H₂O₂ as a discrete two-electron product, yet the mechanistic logic by which this occurs is fundamentally different on the reduction and oxidation sides^[44-46]. In the two-electron oxygen reduction reaction, the essential chemical requirement is retention of the O-O bond throughout activation and proton-coupled electron transfer, typically described in acidic medium by O₂ + 2H⁺ + 2e^- → H₂O₂ and in alkaline medium by O₂ + H₂O + 2e^- → HO₂^- + OH^-[47,48]. The defining selectivity problem is therefore not oxygen activation itself, but the ability of the catalytic interface to activate O₂ sufficiently to form an adsorbed hydroperoxo-type intermediate while avoiding subsequent O-O scission that would divert the pathway toward the four-electron reduction to H₂O^[49-51]. This makes the oxygen reduction case a highly sensitive expression of intermediate-state control, since the same interfacial reactivity that enables O₂ adsorption and reduction can also facilitate bond cleavage if the surface binds oxygen-containing fragments too strongly. By contrast, in the two-electron water oxidation reaction, described in acidic conditions as 2H₂O → H₂O₂ + 2H⁺ + 2e^- or in alkaline conditions as 2OH^- → H₂O₂ + 2e^-, the chemical challenge is not bond preservation but selective O-O bond formation at the peroxide oxidation level without further oxidation to O₂. The catalytic interface must therefore create an oxygen-coupling route that terminates at peroxide rather than continuing into the thermodynamically and kinetically dominant four-electron oxygen evolution manifold^[52-54]. This distinction is chemically profound. In oxygen reduction, two-electron selectivity is achieved by suppressing overreduction after initial oxygen activation^[55,56]. In water oxidation, it is achieved by suppressing overoxidation after initial oxygen coupling^[57]. The oxygen-related family thus defines a complete redox pair in which the same product, H₂O₂, can be approached either from molecular oxygen or from water, while the central selectivity principle remains the same: the catalytic process must stabilize the peroxide redox level as a chemically persistent endpoint. For this reason, oxygen-related two-electron electrocatalysis has become a benchmark system for understanding how proton-coupled electron transfer, O-O bond chemistry, and product desorption are coordinated under nonequilibrium electrochemical conditions. It also provides a uniquely revealing framework for distinguishing two-electron and four-electron pathways in chemical terms^[58,59]. The two-electron route privileges peroxide formation as a redox-defined molecular outcome, whereas the four-electron route drives the system toward the more thermodynamically relaxed endpoints of H₂O production or O₂ evolution. The competition between these pathways is therefore not simply a difference in electron number. It is a competition between two fundamentally different reaction logics, one centered on selective access to an oxygenated molecular oxidant and the other on complete redox exhaustion of the oxygen substrate^[60,61]. This is precisely why oxygen-related two-electron electrocatalysis occupies such a central place in the broader framework of selective electrosynthesis: it captures, in its most explicit form, the chemical problem of how an electrocatalytic interface can recognize a partially transformed oxygen state as the desired product rather than as a transient species to be consumed further.

Carbon dioxide-related two-electron electrocatalysis

Carbon dioxide-related two-electron electrocatalysis represents a chemically distinctive branch of CO₂ conversion because it seeks to arrest carbon at an early reduction level where the carbon atom remains bound to oxygen, yet has already undergone a discrete change in electron density and protonation state sufficient to generate a synthetically useful product. In its most representative form, this chemistry is expressed by the two-electron reduction of CO₂ to formic acid or formate, written as CO₂ + 2H⁺ + 2e^- → HCOOH in acidic media or CO₂ + H₂O + 2e^- → HCOO^- + OH^- in alkaline conditions^[62,63]. The significance of this pathway lies in the fact that it preserves the monocarbon framework while selectively converting a highly oxidized, thermodynamically stable molecule into an energy-bearing liquid product without requiring the deeper deoxygenation steps associated with CO, hydrocarbons, or alcohols. The underlying chemical problem is therefore not merely reduction of CO₂, but selective control over the first productive branch of carbon-centered proton-coupled electron transfer. Because linear CO₂ possesses a closed-shell electronic structure and a high degree of C=O bond stabilization, its activation at an electrode interface necessarily involves interfacial polarization, bending of the molecular geometry, and redistribution of charge into antibonding orbitals. Once this activation occurs, the reaction network can diverge into chemically different pathways depending on how the carbon center and oxygen atoms are differentially stabilized^[64-66]. A two-electron route toward HCOOH or HCOO^- requires the catalytic interface to favor a formate-type reduction logic in which hydrogen addition and electron transfer are directed toward generating a partially reduced carbon-oxygen framework rather than promoting a carboxyl-type intermediate that evolves toward CO formation. In this sense, carbon dioxide-related two-electron electrocatalysis is governed by a fundamental issue of chemoselective oxygen retention. The catalyst must enable reduction of carbon without driving excessive C-O bond weakening, and it must permit release of the oxygen-containing product before further reduction or competing hydrogen evolution consumes the available interfacial reducing equivalents. This places the two-electron CO₂ pathway in a chemically delicate position. If activation is too weak, the substrate remains kinetically inert because the first electron-transfer event to CO₂ is intrinsically demanding. If activation is too strong or too reducing, the system may be diverted toward CO-producing or more deeply reduced pathways, thereby losing control over the oxidation-state-defined endpoint^[67-69]. Carbon dioxide-related two-electron electrocatalysis is therefore best understood as a selective partial reduction problem in which the central objective is not maximal electron delivery to carbon, but controlled accumulation of two reducing equivalents in a manner that preserves oxygen functionality and defines product identity at the earliest chemically useful stage of CO₂ conversion. Precisely for this reason, this reaction class occupies a special place in the broader landscape of two-electron electrocatalysis. It demonstrates that selective redox synthesis is not always a matter of breaking stronger bonds or reaching more reduced products, but can instead depend on recognizing partial reduction itself as the desired chemical outcome and on constructing an interfacial reaction environment that stabilizes that outcome against both underactivation and overreduction^[70,71].

Nitrogen-oxoanion two-electron electrocatalysis

Nitrogen-oxoanion two-electron electrocatalysis introduces a further level of chemical complexity because the target transformation is no longer defined primarily by O-O bond retention or partial carbon reduction, but by the selective deoxygenation of an already oxidized nitrogen center without allowing the system to descend into extensive hydrogenation or complete reduction. Its most representative expression is the two-electron reduction of nitrate to nitrite, which can be written in acidic medium as NO₃^- + 2H⁺ + 2e^- → NO₂^- + H₂O and in alkaline conditions as NO₃^- + H₂O + 2e^- → NO₂^- + 2OH^-[72].

This reaction class is chemically significant because it occupies the first decisive reduction threshold in the broader electrochemical nitrogen redox network. At this stage, the oxidation state of nitrogen is lowered while the nitrogen-oxo framework remains intact, so the reaction does not yet enter the more deeply reduced manifolds associated with NH₃, N₂, hydroxylamine, or other downstream nitrogen species. The essential feature of this chemistry is therefore selective oxygen-atom removal under conditions where the substrate remains highly susceptible to further proton-coupled electron transfer^[73-75]. Unlike CO₂ reduction, where the central challenge is selective protonation of an activated carbon-oxygen framework, or oxygen electrocatalysis, where the principal issue is preservation or formation of an O-O unit; nitrogen-oxoanion chemistry is governed by the controlled editing of N-O bonding within a multistate reduction ladder. This ladder is intrinsically dense because nitrate, nitrite, NO, N₂O, NH₂OH, NH₃, and N₂ are all chemically accessible nodes connected by interfacial proton-electron transfer, oxygen abstraction, and nitrogen-centered bond reorganization. As a consequence, the two-electron pathway to nitrite is rarely an isolated event. It is the first chemically defined branch point at which the catalytic interface either releases a partially reduced oxyanion or continues to drive the system into deeper nitrogen reduction chemistry. Nitrogen-oxoanion two-electron electrocatalysis is thus best understood as a problem of selective decoupling between initial activation and subsequent overreduction. The catalyst must promote sufficient interaction with nitrate to weaken one N-O bond and allow oxygen removal, yet it must not stabilize the resulting reduced nitrogen species so strongly that additional protonation, deoxygenation, or hydrogenation becomes unavoidable. This requirement gives the two-electron nitrate-to-nitrite route a conceptually important status within selective electrosynthesis^[76]. It reveals how control over redox depth can be exerted not by maximizing reduction power, but by defining a chemically precise stopping point within a cascade of energetically accessible nitrogen transformations. It also distinguishes nitrogen-oxoanion electrocatalysis from many other inorganic redox processes, because the two-electron product is not simply a useful chemical target but also a mechanistically critical discriminator of pathway logic. Once nitrite is released as the dominant product, the catalytic system demonstrates an ability to recognize partial deoxygenation as a terminal outcome. Once nitrite remains adsorbed or is further activated in situ, the chemistry rapidly transitions from selective two-electron conversion to multielectron nitrogen reduction. In this sense, nitrogen-oxoanion two-electron electrocatalysis offers an especially revealing framework for understanding how electrode interfaces govern the balance between controlled oxyanion editing and uncontrolled descent into more reduced nitrogen states, thereby extending the broader concept of two-electron electrocatalysis into a chemically richer domain of valence-state management, oxygen transfer selectivity, and nitrogen-centered reaction-network control^[77,78].

Halide two-electron electrocatalysis

Halide two-electron electrocatalysis defines an anodic class of selective redox chemistry in which the central task is the controlled oxidation of halide anions to molecular halogens through a discrete two-electron process, most commonly represented by 2Cl^- → Cl₂ + 2e^-, 2Br^- → Br₂ + 2e^-, or 2I^- → I₂ + 2e^-. In chemical terms, this class differs fundamentally from the preceding oxygen-, carbon-, and nitrogen-centered systems because the target transformation is not a partial reduction of a highly oxidized substrate, but a selective oxidation in which an anionic main-group species is converted into a covalently bonded neutral molecule. Its conceptual significance within two-electron electrocatalysis arises from the fact that electron count directly determines whether the anodic interface performs productive halogen evolution or diverts oxidative equivalents into oxygen-centered pathways arising from H₂O or OH^- activation^[79]. The key issue is therefore not simply halide oxidation itself, since halides are in many cases readily oxidizable, but the ability of the electrochemical interface to discriminate between halogen-forming redox chemistry and competing oxygen evolution under strongly anodic conditions. This makes halide electrocatalysis an especially revealing case of selective charge allocation at positive potentials. The catalyst must promote interfacial electron withdrawal from X^- to generate a surface-associated halide-derived oxidized state, yet it must also channel that oxidizing power toward X-X bond formation and halogen release rather than toward the buildup of oxygenated surface species that ultimately lead to O₂ evolution. In this sense, halide two-electron electrocatalysis is governed by a distinctive coupling between anion adsorption, outer- or inner-sphere oxidation character, and bimolecular or surface-mediated bond formation between halogen equivalents. Its mechanistic identity therefore depends on whether the anodic interface recognizes halide oxidation as a self-contained molecular synthesis pathway or merely treats halide as a spectator in a broader oxygen evolution environment. This distinction is chemically consequential because halogen evolution represents a selective oxidation-state transition with clear product definition, whereas oxygen evolution corresponds to redirection of current into a fundamentally different redox manifold^[80]. Halide two-electron electrocatalysis also occupies an important position in the general theory of competing pathways because it illustrates that two-electron selectivity is not restricted to partial reduction chemistry. It can also emerge in oxidation processes where the desired outcome requires arrest at a lower oxidative depth than the surrounding anodic environment would otherwise favor. The reaction thus exemplifies a general principle of two-electron electrocatalysis: the relevant selectivity problem is determined not by the absolute ease of electron transfer, but by whether the interface can stabilize the chemically intended redox channel long enough for product-defining bond formation and desorption to occur^[81,82]. In halide systems, that intended channel is molecular halogen generation. Once the interface instead accumulates oxygenated intermediates from solvent or hydroxide oxidation, the chemistry shifts away from selective halide conversion and into the multielectron oxygen evolution regime^[83]. Halide two-electron electrocatalysis therefore provides a chemically clean and conceptually important model for understanding how anodic interfaces partition oxidizing equivalents between substrate-centered molecular formation and solvent-derived overoxidation, thereby extending the broader framework of two-electron electrocatalysis to include oxidation-state precision on the oxidative side of inorganic electrosynthesis.

Electronic structure of active sites

The electronic structure of active sites constitutes the most fundamental determinant of two-electron selectivity because it defines how the catalytic center exchanges charge with the reacting substrate, how strongly key intermediates are stabilized, and whether the interfacial redox process terminates at a partially transformed state or continues into deeper reaction manifolds. In chemical terms, two-electron electrocatalysis is governed by the need to place the active site within a narrow electronic window. The site must be sufficiently electron-rich or electronically polarizable to activate a relatively inert molecule such as O₂, CO₂, or NO₃^-, yet it must not stabilize the resulting adsorbate-derived states so strongly that subsequent bond cleavage, excessive protonation, or further electron accumulation becomes unavoidable. This requirement makes two-electron selectivity highly sensitive to the frontier-orbital interaction between the catalyst and the adsorbate^[89-91]. The energy, symmetry, and occupancy of valence orbitals at the active site determine the extent of σ donation, π back-donation, and charge redistribution during adsorption, which in turn dictates whether an intermediate is bound in a configuration compatible with release at the two-electron level or with progression toward multielectron conversion. For transition-metal-centered sites, this behavior is closely associated with the position of the d-band manifold, the degree of orbital degeneracy or splitting under the local ligand field, and the oxidation-state flexibility of the metal center during proton-coupled electron transfer^[92,93]. A more electronically elevated and weakly filled d manifold can facilitate initial substrate activation by enhancing overlap with antibonding orbitals of the adsorbate, whereas a more stabilized and lower-lying manifold may weaken excessive covalent interaction and favor desorption of a partially reduced or oxidized product. The decisive issue is therefore not strong binding in a generic sense, but electronically discriminating binding that stabilizes the desired precursor state without overstabilizing the states that lead to further transformation. An optimized electronic structure should balance substrate activation and intermediate release rather than simply maximize adsorption strength. Appropriate orbital interactions facilitate substrate activation, whereas excessively strong interactions can hinder desorption and promote deeper redox pathways. Conversely, weak interactions may suppress substrate activation. Thus, electronic-structure regulation should establish an appropriate adsorption-desorption balance to promote the desired two-electron transformation while preventing further conversion^[94]. This principle becomes especially important in reactions where two-electron and multielectron pathways diverge only after the first adsorbed intermediate has formed. Under such conditions, small changes in charge density, spin state, covalency, or metal-ligand electronic coupling can shift the relative energy ordering of surface intermediates and thereby redirect the reaction network. Electronic structure also determines the redox noninnocence of the active center itself. In some catalysts, charge transfer during catalysis is borne predominantly by the metal site, whereas in others substantial electron density is delocalized onto ligands, lattice anions, or electronically coupled neighboring atoms^[95,96]. Such differences are chemically consequential because they alter whether the active site behaves as a localized redox center, a covalent adsorption platform, or a more distributed electronic ensemble capable of buffering charge without excessively deepening the reduction or oxidation state of the bound species. From this perspective, two-electron selectivity can be viewed as a problem of electronic-state matching between catalyst and reaction intermediate. When this matching is appropriate, the active site lowers the barrier for the first chemically productive step while preserving a thermodynamic and kinetic bias toward product release at the two-electron stage. When the matching becomes too strong, the interface begins to favor bond scission, overprotonation, or continued electron transfer, thereby collapsing selectivity into deeper pathways^[97-99]. Electronic structure is thus not merely one descriptor among many. It is the primary chemical variable that governs how the active site interprets the redox identity of the substrate and decides whether a two-electron state is treated as a target product or as a transient species en route to further conversion.

Coordination environment and atomic configuration

Beyond the intrinsic electronic character of the active center, two-electron selectivity is critically shaped by the coordination environment and atomic configuration that define how that center is spatially and chemically expressed at the catalytic interface. This level of control is distinct because it concerns not simply how many electrons are available for substrate interaction, but how the reactive atom is embedded, constrained, and exposed to the approaching molecule. The coordination number, the identity of neighboring atoms, the symmetry of the local ligand field, and the nuclearity of the active ensemble together determine whether the catalytic site presents a highly localized adsorption motif, a cooperative multiatom platform, or a geometrically frustrated environment in which partial transformation is favored over complete redox conversion^[100]. In two-electron electrocatalysis, this distinction is especially important because the difference between selective termination and overconversion often emerges from subtle changes in binding geometry rather than from large differences in overall adsorption strength. A coordinatively unsaturated site may facilitate initial substrate capture by increasing orbital accessibility and local electrophilicity, yet the same site can also promote excessive activation if it allows multidentate binding or bond dissociation geometries that open deeper pathways. Conversely, a more saturated or symmetry-constrained coordination environment may attenuate full bond cleavage while still permitting the end-on or weakly asymmetric adsorption modes that are compatible with two-electron product formation and release^[101,102]. Atomic configuration further refines this behavior by determining whether catalysis occurs at an isolated atom, a pair of adjacent heterometallic or homometallic centers, a cluster-like ensemble, or an extended surface motif. These configurations are not electronically equivalent, but their significance in the present context is fundamentally geometric and chemical. An isolated site often limits the configurational space available for adsorbate reorganization, thereby suppressing pathways that require bridging adsorption, multi-center bond scission, or sequential deep reduction at neighboring atoms. By contrast, binuclear or ensemble sites can either enhance two-electron selectivity through controlled cooperativity or undermine it by enabling excessive intermediate stabilization and further transformation. The decisive question is whether the local atomic arrangement imposes a reaction geometry that favors partial redox editing while disfavoring the reorganizations required for multielectron descent. This is why coordination environment and atomic configuration are best regarded as structural filters of selectivity. They determine the orientation of substrate approach, the multiplicity of accessible adsorption states, the degree of bond polarization induced upon binding, and the probability that the first product-like intermediate will remain terminal or become a precursor to deeper chemistry. Their role is also chemically significant because they mediate the balance between rigidity and adaptability at the active site. An overly rigid coordination framework may prevent the substrate from reaching the transition-state geometry needed for productive two-electron conversion, whereas an overly flexible environment may accommodate multiple downstream configurations and thereby erode selectivity. The most effective coordination motifs are therefore those that combine sufficient structural definition to restrict undesirable pathways with sufficient local lability to permit the specific geometric distortion associated with the desired two-electron event. In this sense, two-electron electrocatalysis depends not only on what the active site is electronically, but also on how it is built atom by atom. Coordination environment and atomic configuration determine whether catalytic reactivity is localized, cooperative, constrained, or overextended, and they thereby translate structural chemistry into a concrete decision about whether a substrate will be released after a two-electron transformation or driven onward into a deeper reaction manifold^[103].

Surface structure, defects, and crystal phases

Two-electron selectivity is further governed by the extended structural state of the catalyst surface, because the same chemically defined active site can exhibit markedly different reaction behavior once it is embedded within different surface terminations, defect populations, and crystallographic phases. This level of control operates beyond local coordination in a narrower sense and concerns the broader structural context that determines how adsorption motifs are distributed, how surface atoms relax under bias, and how reactive intermediates migrate, reorganize, or couple during turnover^[104]. Surface structure is central in this regard because crystallographic orientation and atomic termination define the spatial arrangement, density, and connectivity of adsorption sites available to the substrate^[105-107]. Recent studies on electrochemical CO₂ reduction further demonstrate that phase and facet engineering can regulate catalytic selectivity by modifying surface atomic arrangements, coordination environments, defect structures, and interfacial strain, thereby altering the adsorption behavior and energetic barriers of key intermediates^[108]. A close-packed terrace, a stepped edge, and a kink-rich surface may expose nominally similar elemental compositions, yet they impose fundamentally different adsorption geometries and bond polarization environments. For two-electron electrocatalysis, this distinction is crucial because selective partial conversion often requires a surface that permits the first productive adsorption event but disfavors the ensemble geometries needed for deeper bond cleavage, excessive hydrogenation, or prolonged intermediate retention. Defects introduce a further layer of chemical complexity. Vacancies, antisite disorder, lattice distortion, dislocations, and undercoordinated surface motifs can alter local charge density, redistribute frontier states, and create chemically nonuniform adsorption microdomains in which intermediate-state energetics deviate substantially from those of the ideal lattice^[109-111]. This defect-mediated electronic regulation is also evident in graphene-derived quantum dots, where edge and structural defects introduce surface states and modulate electron density and heterogeneous electron-transfer behavior, demonstrating that defect-induced electronic perturbations can directly influence electrochemical reactivity^[112]. Such perturbations may be beneficial when they lower the barrier for substrate activation or tune the adsorption state into the narrow window required for two-electron termination, but they may also undermine selectivity if they generate highly reactive sites that capture intermediates too strongly and facilitate continued transformation. Defect and phase engineering should therefore be treated as structure-dependent strategies rather than universally beneficial approaches. Appropriate defects and stable phases can promote the desired two-electron pathway, whereas excessive defects or unstable phases may overstabilize intermediates and promote deeper multielectron conversion. Effective regulation therefore requires balancing reaction activation with intermediate release. The role of defects is therefore not reducible to a general enhancement of activity. Their true significance lies in whether they selectively stabilize the product-forming branch without opening additional pathways that consume the partially transformed species^[113]. Crystal phase exerts a similarly profound influence because phase identity determines not only long-range atomic ordering but also metal-anion covalency, bond ionicity, lattice flexibility, and redox adaptability under electrochemical conditions^[114-116]. Even when composition remains constant, distinct polymorphs can present different surface energies, different capacities for structural relaxation, and different propensities for forming metastable adsorbate-bound states during catalysis. These differences matter acutely in two-electron systems, where a modest shift in the surface free-energy landscape can determine whether a reaction terminates at peroxide, formate, nitrite, or halogen, or instead proceeds toward deeper reduction or oxidation^[117]. Surface structure, defects, and crystal phase also share a common mechanistic consequence in that they regulate how far the catalyst departs from its idealized static configuration during operation. Two-electron electrocatalysis often depends on maintaining a surface state that is reactive but not overadaptive. If the surface is too inert, the first electron-transfer event remains difficult and selectivity is lost through underactivation. If the surface is too labile, restructuring under bias may generate highly active motifs that favor multielectron chemistry or transform the original catalyst into a different reactive phase altogether^[118]. The relevant structural question is therefore not simply which surface or phase is intrinsically most active, but which one preserves an adsorption and transformation landscape compatible with controlled arrest at the two-electron level. In this sense, surface structure, defect chemistry, and crystal phase function collectively as mesoscale regulators of selectivity^[119]. They translate bulk structural chemistry into the real interfacial topology encountered by reacting molecules and determine whether the catalyst surface behaves as a platform for selective partial redox conversion or as a continuously evolving environment that drives the system beyond the intended two-electron endpoint^[120,121].

Interfacial microenvironment effects

Even when the active-site electronic structure and the catalyst lattice are held constant, two-electron selectivity can still change profoundly because the electrochemical reaction is expressed not in vacuum but within a dynamically organized interfacial microenvironment. This microenvironment constitutes the final level at which selectivity is determined, since it governs how reactants, intermediates, protons, solvent molecules, counterions, and electric fields are arranged at the moment of electron transfer. Its importance in two-electron electrocatalysis is especially pronounced because partial redox conversion is intrinsically sensitive to whether a newly formed intermediate remains solvated and releasable or is immediately driven into further transformation. In chemical terms, the interfacial microenvironment controls this balance through four tightly coupled effects. The first is local proton activity, which determines not only proton availability for proton-coupled electron transfer but also the protonation state and chemical identity of the product itself^[122]. A nominally identical electron-transfer sequence can therefore terminate in chemically different forms depending on whether the interface favors H₂O₂ or HO₂^-, HCOOH or HCOO^-, and whether partially reduced nitrogen-oxo species are rapidly protonated or remain in less reactive oxyanion forms^[123]. The second is solvation structure and hydrogen-bond organization, because the orientation and mobility of interfacial water molecules influence charge stabilization, transition-state polarity, and the desorption tendency of polar oxygenated or oxyanion intermediates. A hydrogen-bond network that stabilizes a released two-electron product in the outer coordination environment can favor selective termination, whereas a more weakly accommodating solvent structure may leave the product vulnerable to readsorption or continued surface reaction. The third is the ionic environment and double-layer organization, including the identity, concentration, and spatial distribution of cations and anions near the electrode^[124-126]. These species reshape the local electric field, alter electrostatic screening, and selectively stabilize charged or polarizable intermediates through noncovalent interactions. Such effects are chemically consequential because two-electron and multielectron pathways often diverge only after a charged intermediate has formed, so even modest changes in interfacial field strength or ion pairing can shift the relative accessibility of partial versus deep redox conversion. These interfacial factors are closely coupled through the electric double layer, which governs the local distribution of ions, solvent molecules, and electric fields. Changes in proton activity, electrolyte composition, and solvation can reshape the electric double layer, thereby affecting charge transfer, intermediate stabilization, and proton-coupled electron transfer. Thus, interfacial regulation should be viewed as coordinated control of the electric double layer, solvation, and transport processes, which collectively determine two-electron selectivity^[127]. The fourth is mass-transport-defined concentration polarization, which determines how rapidly substrate is replenished, how efficiently product is removed, and whether local accumulation of reactants or products perturbs the reaction network away from its intended two-electron endpoint. This factor is not merely hydrodynamic in a physical sense. It has direct chemical implications because local buildup of H₂O₂, HO₂^-, halogen species, or partially reduced nitrogen intermediates can change secondary reaction probability, alter interfacial acidity, or induce competitive pathways that are absent under dilute boundary conditions^[128]. Interfacial microenvironment effects are therefore best understood as the operational chemical context in which intrinsic catalyst structure is translated into actual selectivity^[129-131]. They determine whether the catalyst experiences the substrate as a neutral molecule, an ion-paired complex, a strongly polarized adsorbate precursor, or a species embedded within a proton-rich or proton-deficient solvent cage. They also define whether the two-electron product leaves the interface as a chemically stabilized molecular entity or remains trapped within a reactive interfacial zone that promotes further oxidation or reduction. In this sense, two-electron electrocatalysis cannot be explained solely by the structure of the solid catalyst. It must also be understood through the structure of the liquid-side interface, where proton distribution, solvation fields, ionic organization, and local concentration gradients collectively decide whether a partially transformed state survives as the desired product or is consumed as a transient intermediate on the way to deeper redox chemistry.

Metals and alloys

Metals and alloys constitute one of the most foundational catalyst classes in two-electron electrocatalysis because they provide electronically coherent surfaces on which adsorption, charge transfer, and bond activation can be regulated with comparatively high chemical precision. Their importance arises first from the fact that metallic bonding creates delocalized electronic states capable of mediating rapid interfacial electron exchange, while at the same time permitting continuous variation in adsorbate binding strength through changes in elemental identity, surface composition, and alloy structure^[139]. This combination is particularly relevant to two-electron chemistry, where the central requirement is not maximal substrate activation but selective stabilization of an intermediate state that is reactive enough to form and yet labile enough to desorb before deeper conversion occurs. In pure metals, this balance is often governed by the intrinsic position and filling of valence states, especially for transition-metal surfaces where adsorption of oxygenated, carbon-containing, or nitrogen-oxo intermediates depends sensitively on metal-adsorbate orbital overlap^[140-142]. A metal surface that binds reaction intermediates too weakly may fail to induce the first chemically productive step, whereas one that binds too strongly can prolong surface residence, facilitate bond scission, or promote additional proton-coupled electron transfer beyond the intended two-electron endpoint. The chemical role of metals in this context is therefore best understood as providing a continuous adsorption-energy platform from which selective partial conversion can be achieved only within a narrow reactivity window. Noble metals are often prominent in this regard because their relatively moderate affinity for oxygenated intermediates can favor molecular release over overreduction, while post-transition metals may promote oxygen-retaining carbon reduction pathways by stabilizing particular adsorption geometries associated with partial reduction rather than deep deoxygenation. The mechanistic significance of metallic catalysts thus lies in their ability to translate band-structure-defined adsorption into oxidation-state-specific product formation, making them especially suitable for reactions in which the distinction between two-electron and multielectron pathways is controlled by subtle changes in surface bonding rather than by wholesale changes in catalytic composition.

Alloying extends this chemistry into a much broader regime of selective control by introducing compositional heterogeneity, altered surface atom distributions, and modified electronic coupling between constituent elements. Its value in two-electron electrocatalysis does not lie simply in averaging the properties of two metals, but in generating new catalytic states through ligand, strain, and ensemble effects that are inaccessible to monometallic surfaces. Through ligand effects, one component can perturb the electron density and orbital energy of another, thereby tuning the binding strength and transformation pathway of key intermediates without necessarily changing the overall metallic character of the surface^[143]. Through strain effects, lattice mismatch can alter bond lengths and orbital overlap, shifting the energetic balance between intermediate formation and further conversion^[144]. Through ensemble effects, alloying can dilute contiguous active atoms, isolate reactive motifs, or create heteroatomic neighborhoods that favor one adsorption geometry over another. These consequences are chemically decisive for two-electron pathways because deeper redox chemistry often requires multi-atom cooperation, stronger adsorption ensembles, or more extensive intermediate stabilization than selective partial conversion. Alloy surfaces can therefore suppress overreaction either by electronically weakening undesired states or by geometrically interrupting the site architectures needed for continued transformation. At the same time, they may preserve sufficient reactivity to promote the first substrate-activation event, which is essential for avoiding the opposite failure mode of underactivation^[144,145]. This makes alloys particularly powerful as catalysts for two-electron electrocatalysis, since they allow the reaction coordinate to be reshaped at both the electronic and atomic scales without abandoning the efficient interfacial charge-transfer properties of metallic systems. From a broader materials-chemistry perspective, metals and alloys thus serve as a canonical platform for understanding selective electrocatalysis in its most controlled form. They show how continuous tuning of surface electronic structure, atomic distribution, and cooperative adsorption behavior can convert a highly reactive metallic interface from a driver of deep redox conversion into a chemically disciplined surface that recognizes the two-electron state as the preferred catalytic endpoint^[146].

In non-noble and low-noble alloy systems, alloying first manifests as a strategy for regulating the electronic structure of active metal sites and the adsorption mode of reaction intermediates. Liu et al.^[147] first pointed out that Bi in BiNi alloys can regulate Ni sites through a dual-isolation effect involving both physical and electronic isolation. At the electronic-structure level, the three-dimensional charge-density distribution of BiNi alloys shows that Bi incorporation reshapes the local electronic environment around Ni and provides a basis for tuning *OOH adsorption [Figure 6A]. Structurally, the High-resolution transmission electron microscopy (HR-TEM) image of BiNi/C, together with the inset diffractogram, further confirms the formation of the crystalline BiNi alloy phase [Figure 6B]. On this basis, the OOH-adsorbed configurations and free-energy diagrams of Bi, Ni, BiNi-Bi, and BiNi-Ni sites at 0.7 V vs. Reversible hydrogen electrode (RHE) suggest that alloyed BiNi sites can optimize *OOH adsorption and conversion [Figure 6C]. At the kinetic level, the Tafel slopes of Ni/C, Bi/C, and BiNi/C show that BiNi/C displays faster ORR kinetics than the corresponding monometallic counterparts [Figure 6D]^[147]. In parallel, Du et al.^[148] focused on a PdCu bimetallic system and showed that Cu incorporation can promote the formation of Pd-O species, thereby enhancing the 2e^- ORR activity of Pd sites. To clarify the adsorption configuration, the computational model of OOH adsorption on an oxygen-containing PdCu surface describes the interaction between the oxygen intermediate and the alloy surface through Pd, Cu, and O atoms [Figure 6E]. Then, the HR-TEM image of PdCu nanowires, with the inset selected-area electron diffraction (SAED) pattern, confirms their crystalline structure [Figure 6F]. From the thermodynamic perspective, the calculated free-energy diagrams for the 2e^- ORR over bare PdCu and oxygen-containing PdCu surfaces indicate that surface oxygen species optimize the energetics related to H₂O₂ formation [Figure 6G]. Finally, the correlation between the H₂O₂ selectivity of PdCu NWs/C and the normalized Raman intensity of Pd-O suggests that reaction-induced Pd-O species are closely associated with improved H₂O₂ selectivity [Figure 6H]^[148]. Taken together, these two studies demonstrate that performance enhancement in non-noble or low-noble alloy catalysts does not arise from composition change alone, but from the synergistic optimization of *OOH adsorption and the 2e^- ORR pathway through electronic regulation, active-site isolation, and stabilization of surface oxygen species.

Figure 6. (A) Three-dimensional charge-density diagrams of the BiNi alloys. (B) HR-TEM image of BiNi/C, with the corresponding diffractogram shown in the inset. (C) OOH adsorption configurations on Bi, Ni, BiNi-Bi, and BiNi-Ni sites, and the corresponding free-energy diagrams at 0.7 V vs. RHE. (D) Tafel slopes of Ni/C, Bi/C, and BiNi/C samples. (A-D) is reprinted with permission from Ref.^[147]. Copyright © 2025 American Chemical Society. (E) Computational model of the OOH adsorption configuration on an oxygen-containing PdCu surface. Silver, blue, and red spheres represent Pd, Cu, and O atoms, respectively. (F) HR-TEM image of PdCu nanowires, with the SAED pattern shown in the inset. (G) Calculated free-energy diagrams for the 2e^- ORR over bare PdCu and oxygen-containing PdCu. (H) H₂O₂ selectivity of the PdCu NWs/C ORR catalyst and normalized Raman intensities of Pd-O. (E-H) is reprinted with permission from Ref.^[148]. Copyright © 2023 American Chemical Society. (I) Charge-density difference of Au-Pd₂Hg₅ (001) induced by OOH adsorption. Green and blue regions represent electron accumulation and depletion, respectively; the isosurface value is 0.004 e Å^-3. (J) HR-TEM image of the heterometal-doped M-Pd₂Hg₅ aerogel. (K) Calculated reaction-coordinate diagrams for Pd₂Hg₅ and Au-Pd₂Hg₅ aerogels, with insets showing the corresponding atomic configurations. This model represents the (100) plane of the Pd₂Hg₅ structure. (L) LSV curves of Pd₂Hg₅ and Au-Pd₂Hg₅ aerogels in O₂-saturated 0.1 M HClO₄ at a rotation speed of 1,600 rpm and a scan rate of 20 mV s^-1, together with the detected H₂O₂ currents on the Pt ring electrode at a fixed potential of 1.2 V vs. RHE. (I-L) is reprinted with permission from Ref.^[149]. Copyright © 2023 Wiley-VCH GmbH. (M) Structural illustration of the obtained Pd@Pt_0.05Au_0.95 core-shell nanocube covered by facets and the corresponding Pt-in-Au square atomic arrangement. (N) HAADF-STEM image of Pd@Pt_0.05Au_0.95-6L core-shell nanocubes. (O) ΔG*OOH values for Pd@Pt-in-Au (100) surfaces with Pt aggregates of different sizes embedded in the Au matrix. In Ptnx, n and x denote the number of Pt atoms and the *OOH adsorption arrangement, respectively, where e and s refer to end-on and side-on adsorption. (P) Electrocatalytic ORR performance of Pd@M-in-Au core-shell nanocrystals measured by RRDE in O₂-saturated 0.1 M HClO₄, where M = Pt, Pd, and Rh. The upper panel shows ring-current polarization curves, and the lower panel shows disk-current-density polarization curves. (M-P) is reprinted with permission from Ref.^[150]. Copyright © 2024 Wiley-VCH GmbH.

In noble-metal alloy systems, Zhang et al.^[149] further demonstrated that Pd-Hg-based pure metallic aerogels can construct single-atom-like Pd sites by suppressing Pd-Pd coordination, while Au doping further tunes the electronic structure of Pd sites without substantially changing their geometric environment. Specifically, the OOH-adsorption-induced charge-density difference of Au-Pd₂Hg₅ (001) shows electron accumulation and depletion around the adsorption region, indicating that Au doping modifies the electronic interaction between Pd sites and oxygenated intermediates [Figure 6I]. Meanwhile, the HR-TEM image of M-Pd₂Hg₅ aerogels confirms that the crystalline alloy aerogel framework is retained after heterometal doping [Figure 6J]. In terms of reaction energetics, the calculated reaction-coordinate diagrams of Pd₂Hg₅ and Au-Pd₂Hg₅ for the 2e^- ORR show that Au-Pd₂Hg₅ follows a pathway closer to ideal H₂O₂ formation [Figure 6K]. Consistently, the linear sweep voltammetry (LSV) curves and H₂O₂ currents detected on the Pt ring electrode demonstrate the enhanced H₂O₂-generation response of Au-Pd₂Hg₅ relative to Pd₂Hg₅ [Figure 6L]^[149]. Extending this site-isolation concept, Lin et al.^[150] developed Pd@M-in-Au core-shell nanocrystals with dilute binary alloy surfaces, using an inert Au matrix to isolate Pt, Pd, or Rh active atoms and thereby regulate OOH adsorption configurations and ORR selectivity. First, the prepared Pd@Pt_0.05Au_0.95 core-shell nanocube and its {100}-related Pt-in-Au alloy surface present the structural basis for isolating Pt atoms within the Au matrix [Figure 6M]. Subsequently, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Pd@Pt_0.05Au_0.95-6L core-shell nanocubes verifies their ordered core-shell structure [Figure 6N]. To reveal the role of Pt aggregation, the ΔGOOH values of Pt aggregates with different sizes embedded in the Au matrix show that the Pt aggregation state and *OOH adsorption configuration strongly affect *OOH binding [Figure 6O]. Finally, rotating ring-disk electrode (RRDE) measurements of Pd@M-in-Au core-shell nanocrystals in O₂-saturated 0.1 M HClO₄ demonstrate that Pd@Pt-in-Au nanocubes exhibit a more favorable H₂O₂-related current response, with the upper ring current reflecting H₂O₂ generation and the lower disk current density representing overall ORR activity [Figure 6P]^[150].

Metal oxides and derived materials

Metal oxides and oxide-derived materials occupy a chemically distinctive position in two-electron electrocatalysis because their catalytic behavior is governed not only by metal-centered adsorption chemistry, but also by the participation of oxygen-containing lattices that can redistribute charge, reorganize local bonding, and stabilize polar intermediates in ways that are fundamentally different from purely metallic surfaces^[119,151]. This distinction is especially important for two-electron pathways, where selective termination often depends on the ability of the interface to support partial redox transformation without triggering deeper bond cleavage or further proton-coupled electron transfer. In oxides, such control arises from the coupled influence of metal oxidation state, metal-oxygen covalency, lattice ionicity, and surface hydroxylation. A metal center embedded in an oxide lattice is not exposed as an electronically isolated atom. It exists within a chemically constrained M-O framework that shapes the distribution of charge density between cationic and anionic sublattices and therefore alters how reactants are polarized upon adsorption. This framework frequently favors more directional and chemically differentiated interactions with oxygenated or oxyanion intermediates, which can be advantageous for two-electron processes because it allows selective stabilization of a product-forming intermediate without necessarily promoting the strong multidentate adsorption modes associated with deeper conversion. At the same time, the oxide lattice can influence catalysis through its capacity for local redox adjustment. Changes in cation valence, transient accumulation of oxygen vacancies, and partial rehybridization of surface metal-oxygen bonds under applied potential can all reshape the energetic landscape of intermediate formation and release^[152-154]. The essential advantage of metal oxides in this context is therefore not simply that they are polar materials, but that they provide a chemically adaptive yet structurally defined environment in which redox chemistry can be modulated through both cation-centered and lattice-mediated effects. Such behavior is particularly relevant when two-electron selectivity depends on preserving oxygen functionality, stabilizing partially reduced carbon-oxygen species, or preventing overreduction of nitrogen-oxo intermediates^[155,156]. In these cases, the oxide surface often acts less as an inert support for electron transfer and more as a chemically active matrix that controls how far a substrate can descend along the redox coordinate before product release becomes favored.

This framework becomes even more interesting in oxide-derived materials, especially oxyhydroxide-type surfaces generated through hydration, electrochemical activation, or near-surface reconstruction, because the introduction of hydroxyl groups transforms the catalyst from a predominantly lattice-controlled oxide into an interface where metal-oxygen-hydrogen connectivity becomes an intrinsic part of the catalytic state. In chemical terms, this means that the active surface is no longer defined solely by M-O linkages, but by dynamic M-OH and M-O(H)-M motifs capable of mediating proton transfer, hydrogen-bond-assisted adsorption, and local acid-base buffering at the reaction interface. Such features are highly consequential for two-electron electrocatalysis, since selective partial conversion often requires careful synchronization between electron transfer and proton delivery^[157,158]. Oxyhydroxide-derived surfaces can lower the barrier for the first productive proton-coupled electron transfer event by providing a more hydrated and proton-responsive environment, while at the same time altering the desorption behavior of polar intermediates through hydrogen-bond interactions and surface hydroxyl exchange. Their catalytic role is therefore not reducible to that of a simple modified oxide^[159-161]. Rather, they introduce a different chemical regime in which the lattice and the interfacial proton network become more strongly coupled. This coupling can favor two-electron selectivity when it supports controlled substrate activation and selective stabilization of partially transformed states, yet it can also undermine selectivity if excessive proton responsiveness promotes continued hydrogenation or deeper oxidation. The decisive issue is whether the oxyhydroxide-like surface maintains a balanced reactivity in which hydroxyl-assisted adsorption and proton transfer remain sufficiently discriminating to terminate catalysis at the desired two-electron level. From a broader structural-chemical perspective, metal oxides and oxide-derived materials therefore exemplify a catalyst class in which selective electrocatalysis emerges from the interplay of cation electronic structure, lattice oxygen chemistry, and surface hydroxyl reactivity^[162]. Their importance in two-electron systems lies precisely in this multidimensional control, because they demonstrate how a chemically complex inorganic framework can encode selectivity not through a single descriptor, but through the integrated behavior of redox-active metal centers, oxygen-containing lattices, and proton-responsive surface structures^[163].

Metal oxides and oxide-derived materials are important platforms for two-electron electrocatalysis because their metal-oxygen coordination, surface facets, oxygen defects, and heterointerfaces can regulate oxygen-intermediate adsorption and reaction selectivity. In Fe-based oxide systems, Yin et al.^[164] reported crystalline nitrogen-doped-carbon-anchored Fe₃O₄ nanoparticles for neutral H₂O₂ electrosynthesis. The charge density difference for O₂ adsorbed on Fe₃O₄(220)-nitrogen-doped carbon with vacancy (CNV) first presents the interfacial electronic redistribution between Fe₃O₄ and nitrogen-defective carbon, providing a theoretical basis for understanding oxygen activation at the oxide-carbon interface [Figure 7A]. The high-resolution transmission electron microscopy (HRTEM) image of Fe₃O₄@TNC then shows the crystalline oxide structure and its contact with the carbon matrix, supporting the structural basis of the proposed interfacial active sites [Figure 7B]. After the structural and electronic features are established, the free-energy profiles at U = 0.7 V reveal the thermodynamic preference of the Fe₃O₄(220)-CNV interface for the two-electron reduction of O₂ to H₂O₂ [Figure 7C]. The capacitance current densities measured at 1.0 V give a higher double-layer capacitance for Fe₃O₄TNC (11.1 mF cm^-2) than for Fe₃O₄@TNC-AE (6.7 mF cm^-2) and TNC (4.6 mF cm^-2), supporting its larger electrochemically accessible surface area [Figure 7D]^[164]. From charge redistribution, crystalline structure, capacitance response, and reaction free energy, this work illustrates how Fe₃O₄@TNC regulates neutral two-electron oxygen reduction reaction through oxide-carbon interfacial coupling. In SnO₂-based systems, Hu et al.^[165] designed Zn/SnO₂ for paired two-electron electrosynthesis. The optimized adsorption configuration of the *OCHO intermediate on the Zn-Sn(101) alloy surface first describes the interaction between the key CO₂-reduction intermediate and the Zn-Sn site [Figure 7E]. TEM images of Zn/SnO₂ at different magnifications, together with the corresponding SAED pattern, then present the nanoscale morphology and crystalline features of the catalyst [Figure 7F]. The calculated reaction energy profiles for water oxidation on the Zn site of Zn/SnO₂ further describe the energetic pathway associated with H₂O₂ formation [Figure 7G]. Finally, the Faradaic efficiency of formate (FE_HCOO^-) over bare SnO₂ and Zn/SnO₂ compares their formate-generation behavior, demonstrating the influence of Zn incorporation on the CO₂ reduction process [Figure 7H]^[165].

Figure 7. (A) Charge density difference for O₂ adsorbed on Fe₃O₄(220)-CNV. (B) HRTEM image of Fe₃O₄@TNC. (C) Free energy diagram at U = 0.7 V for the reduction of O₂ to H₂O₂. (D) Capacitance current densities measured at 1.0 V using 40 mF cm^-2 as the specific capacitance standard. (A-D) is reprinted with permission from Ref.^[164]. Copyright © 2025 The Royal Society of Chemistry. (E) Optimized adsorption configuration of the *OCHO intermediate on the Zn-Sn (101) alloy surface. (F) TEM images of Zn/SnO₂ at different magnifications; the inset is the corresponding SAED pattern. (G) Calculated reaction energy profiles for water oxidation on the Zn site of Zn/SnO₂. (H) Faradaic efficiency of formate (FE_HCOO-) over bare SnO₂ and Zn/SnO₂. (E-H) is reprinted with permission from Ref.^[165]. Copyright © 2023 Wiley-VCH GmbH. (I) Adsorption model of O₂ molecules on ZnO@ZnO₂; red and gray spheres represent O and Zn atoms, respectively. (J) HRTEM image of ZnO treated with 0.4 mol L^-1 H₂O₂. (K) LSV curves of ZnO, ZnO@ZnO₂, and ZnO₂ measured in 0.1 M K₂SO₄, with ring currents in the upper part and disk current densities in the lower part. (L) Calculated electron transfer number and H₂O₂ selectivity in 0.1 M K₂SO₄; the bottom panel shows electron transfer number and the upper panel shows H₂O₂ selectivity. (I-L) is reprinted with permission from Ref.^[166]. Copyright © 2023 The Royal Society of Chemistry. (M) Image of the final obtained In₂O_3-x catalyst. (N) AC-TEM images of In₂O_3-x. (O) LSV curves of RRDE measurement at 5 mV s^-1 and 1,600 rpm in O₂-saturated 0.1 M KOH. (P) Calculated H₂O₂ selectivity of In₂O_3-x. (M-P) is reprinted with permission from Ref.^[167]. Copyright © 2024 The Royal Society of Chemistry.

The role of oxide-derived interfaces is further reflected in ZnO@ZnO₂ and layered Bi-based oxyhalides. Zhou et al.^[166] identified ZnO@ZnO₂ as an oxide-derived interface for neutral two-electron oxygen reduction reaction. The adsorption model of O₂ molecules on ZnO@ZnO₂ first shows the interaction between O₂ and the ZnO/ZnO₂ surface, where red and gray spheres represent O and Zn atoms, respectively [Figure 7I]. HRTEM images of ZnO treated with 0.4 mol L^-1 H₂O₂ then present the structural features of the peroxide-treated ZnO surface, providing microscopic evidence for the oxide/peroxide-related interface [Figure 7J]. On this structural basis, the LSV curves measured in 0.1 M K₂SO₄ show the electrochemical oxygen-reduction responses of ZnO, ZnO@ZnO₂, and ZnO₂, with the ring currents displayed in the upper panel and the disk current densities displayed in the lower panel [Figure 7K]. The corresponding calculated electron transfer number and H₂O₂ selectivity in 0.1 M K₂SO₄ further evaluate the two-electron reaction pathway, with the bottom panel showing electron transfer number and the upper panel showing H₂O₂ selectivity [Figure 7L]^[166]. This set of results forms a continuous logic from O₂ adsorption, interface formation, electrochemical response, and pathway evaluation. In defect-engineered oxide systems, Deng et al.^[167] reported oxygen-vacancy-rich In₂O_3-x for electrocatalytic H₂O₂ production. The image of the final obtained In₂O_3-x catalyst first presents the oxide material produced after the synthetic process, providing a direct visual representation of the prepared catalyst [Figure 7M]. The AC-TEM images of In₂O_3-x then reveal its crystalline lattice structure and provide microscopic evidence for the formed oxide catalyst [Figure 7N]. Subsequently, the RRDE LSV curves measured at 5 mV s^-1 and 1,600 rpm in O₂-saturated 0.1 M KOH record the oxygen-reduction response of In₂O_3-x and reference samples [Figure 7O]. Finally, the calculated H₂O₂ selectivity further demonstrates that In₂O_3-x favors the two-electron ORR pathway for H₂O₂ production [Figure 7P]^[167].

Metal hydroxides

Metal hydroxides represent a chemically distinctive catalyst class in two-electron electrocatalysis because their reactive surfaces are intrinsically built from hydrated metal-oxygen coordination units in which hydroxyl ligands are not merely spectators but constitutive elements of the catalytic state. This gives metal hydroxides a mode of reactivity that differs in an essential way from both metallic surfaces and more lattice-rigid oxides. The catalytic interface is defined by M-OH connectivity, variable hydrogen-bond organization, and a high degree of proton responsiveness, so the surface can regulate substrate activation through coupled electron-transfer and acid-base processes rather than through adsorption energetics alone. Such characteristics are especially relevant to two-electron pathways, where selective termination frequently depends on whether the first partially transformed intermediate can be stabilized and released without being driven into further protonation, deoxygenation, or oxidation. In hydroxide frameworks, this control arises from several interrelated chemical features^[168]. First, the hydroxylated coordination sphere can create a strongly polarized and hydrated local environment that facilitates interaction with polar substrates and oxygen-containing intermediates while preserving a degree of reversibility in surface binding. Second, proton mobility within hydrogen-bonded surface hydroxyl networks can lower the barrier for elementary proton-coupled electron transfer steps, thereby making partially transformed redox states kinetically accessible under milder driving force. Third, the metal center in a hydroxide is often electronically modulated by the σ-donating and hydrogen-bond-active character of surrounding OH ligands, which affects the balance between substrate activation and product release in a manner not captured by dry-lattice descriptors alone^[169,170]. This is chemically important because two-electron selectivity is often lost when the interface either underactivates the substrate or overstabilizes the intermediate state. Metal hydroxides can occupy an intermediate regime in which the surface is sufficiently reactive to initiate conversion yet sufficiently hydrated and proton-buffered to favor controlled arrest at the desired redox level. Their layered or partially disordered structures may further contribute by enabling local reorganization of hydrogen-bond geometry and metal coordination during turnover without requiring the extensive lattice rearrangements that often accompany deeper redox chemistry. The decisive advantage of metal hydroxides, therefore, lies in their ability to couple structural hydration, proton transfer, and coordination flexibility into a single catalytic environment. This makes them especially valuable as platforms for two-electron electrocatalysis, because they illustrate how selective inorganic redox conversion can be regulated not only by the electronic identity of the metal center, but also by the chemical behavior of a hydroxyl-rich surface that mediates when electrons and protons are delivered, how long intermediates remain interfacially trapped, and whether the catalytic sequence terminates at a discrete two-electron product rather than progressing into a more extensive reaction manifold.

Metal hydroxides and hydroxide-derived materials are important platforms for two-electron electrocatalysis because their hydrated metal-oxygen coordination can regulate water activation, proton transfer, and oxygen-intermediate adsorption. In Ni-based hydroxide systems, Nie et al.^[171] designed Ni(OH)₂/carbon nanotube (CNT) for H₂O₂ electrosynthesis in chloride-containing electrolytes. The geometries of Ni(OH)₂ first present the atomic arrangement of the hydroxide structure, providing a structural basis for understanding the Ni-O-H coordination environment [Figure 8A]. The TEM image of Ni(OH)₂/CNT then shows the morphology of the hydroxide/carbon composite; under high resolution, the lattice fringes with a planar spacing of 0.236 nm can be assigned to the (101) facet of Ni(OH)₂ [Figure 8B]. Based on this structure, the wavelet transform (WT) analysis of the Ni K³-weighted extended X-ray absorption fine structure (EXAFS) signals further records the local Ni coordination feature in Ni(OH)₂/CNT [Figure 8C]. The LSV polarization curves of Ni(OH)₂/CNT in NaCl solutions with different concentrations and the Faradaic efficiency in 1 M NaCl then evaluate its electrocatalytic response and peroxide-generation behavior under saline conditions [Figure 8D]^[171]. This sequence from hydroxide geometry, lattice structure, local coordination, and saline-electrolyte performance shows how Ni(OH)₂/CNT connects hydroxide active sites with two-electron electrocatalysis in chloride-containing media. In another Ni hydroxide-derived system, Zhang et al.^[172] investigated 1,4-Benzenedicarboxylate (BDC) ligand-functionalized Ni(OH)₂ derived from ultrathin NiBDC. The top view of BDC-functionalized Ni(OH)₂ presents the local ligand-modified hydroxide structure [Figure 8E]. The HRTEM image of PR-utn-NiBDC then records the reconstructed nanosheet structure and lattice information [Figure 8F]. The WT of the k³-weighted Ni K-edge EXAFS signals of utn-NiBDC further describes the local coordination state of Ni after ligand-related structural regulation [Figure 8G]. Finally, the polarization curves of utn-NiBDC, utn-Ni(OH)₂, and XC-72, together with the detected HO₂^- currents on the ring electrode at a fixed potential of 1.2 V vs. RHE, compare their oxygen-reduction behavior and peroxide-generation response under RRDE conditions [Figure 8H]^[172].

Figure 8. (A) Geometries of Ni(OH)₂; blue, pink, and white spheres represent Ni, O, and H atoms, respectively. (B) TEM image of Ni(OH)₂/CNT; under high resolution, the lattice fringes show a planar spacing of 0.236 nm, which can be ascribed to the (101) facet of Ni(OH)₂. (C) WT of the Ni K³-weighted EXAFS signals of Ni(OH)₂/CNT. (D) LSV polarization curves of Ni(OH)₂/CNT in NaCl solutions with different concentrations and Faradaic efficiency of Ni(OH)₂/CNT in 1 M NaCl. (A-D) is reprinted with permission from Ref.^[171]. Copyright © 2025 Springer Nature. (E) Top view of BDC ligand-functionalized Ni(OH)₂; pink, red and green spheres represent H, O and Ni atoms, respectively. (F) HRTEM image of PR-utn-NiBDC. (G) WT of the k³-weighted Ni K-edge EXAFS signals of utn-NiBDC. (H) Polarization curves of utn-NiBDC, utn-Ni(OH)₂, and XC-72 recorded at 1,600 rpm and a scan rate of 5 mV s^-1, together with the detected HO₂^- currents on the ring electrode at a fixed potential of 1.2 V vs. RHE. (E-H) is reprinted with permission from Ref.^[172]. Copyright © 2025 Springer Nature. (I) Co-SCD-2 models; azure, yellow, red, gray, and light pink spheres represent Co, S, O, C, and H atoms, respectively. (J) HAADF-STEM image of Co-SCD-2 after the test. (K) LSV polarization curves of Co-SCD-1, Co-SCD-2, and Co-SCD-3. (L) Tafel slope curves of Co-SCD-1, Co-SCD-2, and Co-SCD-3. (I-L) is reprinted with permission from Ref.^[173]. Copyright © 2023 Wiley-VCH GmbH. (M) Schematic illustration of deficient [NiO₆]-[InO₆] motifs of NiOOH/In(OH)₃ heterostructures for E-NiIn and C-NiIn. (N) High-resolution STEM image. (O) Free-energy diagram for the two-electron oxygen reduction reaction on C-NiIn-Ov and E-NiIn-Ov at U = 0 V; inset: configuration of HOO* adsorbed on E-NiIn. (P) Oxygen reduction reaction LSV curves of E-NiIn and C-NiIn measured by RRDE. (M-P) is reprinted with permission from Ref.^[174]. Copyright © 2024 Wiley-VCH GmbH.

Hydroxide chemistry can also be extended to Co hydroxide clusters and reconstructed heterometallic oxyhydroxide interfaces. Qi et al.^[173] reported Co-SCD-2 as a Co hydroxide cluster system for two-electron oxygen reduction reaction. The Co-SCD-2 model first presents the local cluster configuration of the Co-based catalytic structure [Figure 8I]. The HAADF-STEM image of Co-SCD-2 after the test then provides structural information on the Co-containing species after electrochemical operation [Figure 8J]. On this basis, the LSV polarization curves of Co-SCD-1, Co-SCD-2, and Co-SCD-3 compare their oxygen-reduction responses [Figure 8K]. The corresponding Tafel slope curves further evaluate the kinetic differences among these Co-SCD catalysts [Figure 8L]^[173]. This progression from cluster model, post-test structural characterization, polarization behavior, and kinetic comparison illustrates how Co hydroxide cluster structures can be related to two-electron oxygen reduction reaction activity. In reconstructed heterometallic systems, Yu et al.^[174] studied NiOOH/In(OH)₃ heterostructures derived from Ni-In precatalysts. The schematic illustration of deficient [NiO₆]-[InO₆] motifs compares the interfacial structures of E-NiIn and C-NiIn, providing a model for understanding oxygen-vacancy-related Ni-In coordination [Figure 8M]. The high-resolution STEM image then records the local atomic structure of the reconstructed heterostructure [Figure 8N]. The free-energy diagram for the two-electron oxygen reduction reaction on C-NiIn-Ov and E-NiIn-Ov at U = 0 V further compares the reaction energetics, with the inset showing the configuration of HOO* adsorbed on E-NiIn [Figure 8O]. Finally, the oxygen reduction reaction LSV curves of E-NiIn and C-NiIn measured by RRDE record their disk-current responses, while the ring-current behavior is used to evaluate peroxide-related reaction performance [Figure 8P]^[174].

Metal sulfides and phosphides

Metal sulfides and phosphides constitute an important non-oxide catalyst family for two-electron electrocatalysis because they introduce a fundamentally different anion chemistry into the catalytic interface, thereby reshaping how charge is distributed, how substrates are polarized, and how partially transformed intermediates are stabilized during turnover. In contrast to oxides and hydroxides, where oxygen-based ligands strongly influence ionicity, proton responsiveness, and lattice redox behavior, sulfides and phosphides are built from softer and more polarizable anionic frameworks^[175]. This chemical distinction has profound consequences for two-electron selectivity. Sulfur and phosphorus generally form bonds with transition metals that are less ionic and often more covalent than metal-oxygen linkages, which modifies the electronic density around the metal center and changes the nature of adsorbate interaction from the outset^[176]. A more polarizable lattice can facilitate substrate activation by allowing flexible charge redistribution during adsorption, yet it may also weaken the overly rigid stabilization of oxygenated or oxyanion intermediates that often drives overconversion on more strongly binding surfaces. For two-electron processes, this balance is highly advantageous because selective partial conversion typically requires a catalyst that can induce the first productive electron-transfer event without locking the product-forming intermediate into an excessively deep surface-bound state^[177]. In this sense, sulfides and phosphides frequently operate as chemically intermediate platforms between metallic and oxide-like behavior. They retain sufficient electronic conductivity and redox accessibility to support efficient interfacial charge transfer, while their non-oxide lattice chemistry imposes a distinct adsorption environment that can favor controlled redox termination over complete reduction or oxidation. Their catalytic significance is therefore not simply that they are alternative compounds of familiar metals, but that the presence of S or P changes the entire bonding logic of the interface, from the polarity of the active site to the degree of covalency involved in intermediate stabilization and release^[178].

This altered bonding logic becomes even more consequential when considering the structural and mechanistic flexibility of sulfide and phosphide surfaces under electrochemical conditions. The metal center in these materials is embedded within an anion framework that can respond to bias, solvation, and local redox demand in ways that differ substantially from oxygen-based lattices. Sulfur- and phosphorus-containing environments can modify the electron density and oxidation-state adaptability of the catalytic metal, but they also introduce a surface chemistry in which ligand softness, local coordination asymmetry, and partial surface reconstruction may selectively reshape the free-energy landscape of partial redox conversion^[179]. This is especially relevant in two-electron electrocatalysis, where deeper pathways often require prolonged retention of an intermediate, stronger bond cleavage capability, or a more highly oxidizing or reducing local state than that needed for selective product release. Sulfides and phosphides can suppress these undesired trajectories either by attenuating the adsorption strength of overreactive intermediates or by favoring adsorption geometries that are compatible with product formation but less favorable for continued transformation^[180,181]. At the same time, their surfaces are often chemically less static than idealized bulk structures suggest. Under operation, local sulfur or phosphorus environments may reorganize, partially oxidize, or become heterogeneously solvated, generating catalytically relevant interfaces that are best understood as dynamic chalcogenide- or phosphide-derived states rather than purely stoichiometric solids. The value of this class in two-electron electrocatalysis lies precisely in that dynamic balance. The interface can remain electronically competent for substrate activation while avoiding the strongly oxygen-affine or excessively rigid binding environments that frequently push reactions toward multielectron manifolds. From a broader materials-chemistry perspective, metal sulfides and phosphides therefore expand the design space of two-electron electrocatalysis by demonstrating how non-oxide inorganic frameworks can encode selective redox behavior through softer anion fields, more covalent metal-anion interactions, and structurally adaptive surfaces that favor controlled arrest at chemically defined two-electron endpoints^[182,183].

Metal sulfides and phosphides are important material systems in two-electron electrocatalysis because metal-S and metal-P bonding can regulate the electronic structure of metal centers and influence oxygen-intermediate adsorption. In sulfide-based systems, Zhang et al.^[184] reported exfoliated NiPS₃ nanosheets for H₂O₂ electrosynthesis from seawater. The energetically optimized structure of NiPS₃ first presents the stable atomic configuration of the layered metal phosphorus trisulfide framework, providing a theoretical structural basis for understanding the catalytic environment of Ni sites [Figure 9A]. The high-resolution TEM image of NiPS₃ then shows clear lattice information of the nanosheet structure, confirming the crystallinity of the sulfide catalyst [Figure 9B]. At the local coordination level, the EXAFS-related analysis shows two major peaks located at 2.52 and 3.47 A, which are attributed to the scattering of Ni-S and Ni-P paths, respectively, consistent with the Ni K-edge WT-EXAFS observations [Figure 9C]. In terms of catalytic kinetics, the Tafel plots compare FePS₃, CoPS₃, NiPS₃, ZnPS₃, NiS, and NiPSe₃, showing the kinetic differences among metal phosphorus sulfides and related sulfide catalysts [Figure 9D]^[184]. Through the progression from optimized structure, high-resolution lattice characterization, local coordination analysis, and kinetic comparison, these results indicate that the Ni-S/Ni-P coordination environment of NiPS₃ is closely associated with its two-electron oxygen reduction reaction behavior. Che et al.^[185] further used CdS as another sulfide model and introduced carbon-shell engineering. The final structural model of C_0.01@CdS nanorods presents the structural configuration of carbon-coated CdS nanorods, showing the composite structure in which the carbon layer covers the CdS nanorod surface [Figure 9E]. The HRTEM image of C_0.01@CdS nanorods then confirms the retained crystalline structure after carbon coating [Figure 9F]. Based on this structure, the charge-density difference of C@CdS samples further describes the interfacial electronic redistribution between CdS and the carbon shell [Figure 9G]. Finally, the H₂O₂ yield of pure CdS and C@CdS nanorods compares their peroxide-production performance [Figure 9H]^[185].

Figure 9. (A) Energetically optimized structure of NiPS₃. (B) High-resolution TEM image of NiPS₃. (C) EXAFS-related analysis of NiPS₃, where the two major peaks located at 2.52 and 3.47 A are attributed to the scattering of Ni-S and Ni-P paths, respectively, in accordance with the observations in the Ni K-edge WT-EXAFS. (D) Tafel plots of FePS₃, CoPS₃, NiPS₃, ZnPS₃, NiS, and NiPSe₃. (A-D) is reprinted with permission from Ref.^[184]. Copyright © 2025 Springer Nature. (E) Final structural model of C_0.01@CdS nanorods in the schematic of pure CdS and C_0.01@CdS nanorods. (F) HRTEM image of C_0.01@CdS nanorods. (G) Charge density difference for C@CdS samples; blue and yellow colors denote electron depletion and accumulation areas, respectively. (H) H₂O₂ yield of pure CdS and C@CdS nanorods. (E-H) is reprinted with permission from Ref.^[185]. Copyright © 2025 Wiley-VCH GmbH. (I) *OOH-adsorbed MoP (1010) phosphorus-terminated pillar surface. (J) High-resolution TEM image of a MoP pillar cross-section. (K) Double-layer capacitance determined from the scan-rate-dependent capacitive current densities of MoP pillars and MoP nanoplates. (L) H₂O₂ yield rates and Faradaic efficiencies of MoP pillars at potentials from 0.1 to 0.5 V vs. RHE. (I-L) is reprinted with permission from Ref.^[186]. Copyright © 2025 Wiley-VCH GmbH. (M) Cubic crystal structure of Co₉S₈ generated using VESTA software. (N) TEM image of popcorn-shaped Co₉S₈ nanocrystals synthesized under inert conditions. (O) Mass-transfer-corrected Tafel plots for the two-electron oxygen reduction reaction. (P) Mass activities of Dandelion NCs and Popcorn NCs for the two-electron oxygen reduction reaction at different potentials. (O) Mass activity for the two-electron oxygen reduction reaction of the catalysts. (P) Mass-transfer-corrected Tafel plots of kinetic current densities for the two-electron oxygen reduction reaction; the kinetic current densities were calculated based on the Koutecky-Levich equation. (M-P) is reprinted with permission from Ref.^[187]. Copyright © 2024 John Wiley and Sons.

In phosphide and ligand-regulated sulfide systems, Kim et al.^[186] synthesized facet-controlled MoP single crystals and investigated their catalytic behavior in two-electron oxygen reduction reaction for H₂O₂ production. The *OOH-adsorbed MoP (1010) phosphorus-terminated pillar surface first presents the adsorption configuration of the key oxygen intermediate on a specific phosphide facet, providing a structural basis for understanding facet-dependent catalytic behavior [Figure 9I]. The high-resolution TEM image of the pillar cross-section then shows the crystalline quality and lattice structure of the MoP pillar [Figure 9J]. In the catalytic evaluation, the scan-rate-dependent capacitive current densities give double-layer capacitances of 9.52 and 7.04 μF cm^-2 for the MoP pillars and MoP nanoplates, respectively, indicating a larger electrochemically accessible surface area for the pillar architecture [Figure 9K]. The H₂O₂ yield rates and Faradaic efficiencies measured from 0.1 to 0.5 V vs. RHE further demonstrate the potential-dependent peroxide-production performance of the MoP pillars [Figure 9L]. From *OOH adsorption configuration, lattice characterization, H₂O₂ production performance, and electrochemical surface analysis, this study indicates that exposed phosphide facets play a key role in regulating two-electron oxygen reduction reaction behavior. In another sulfide system, Kim et al.^[187] reported ligand-driven electron-deficient Co₉S₈ nanocrystals. The cubic crystal structure of Co₉S₈ generated using visualization for electronic and structural analysis (VESTA) first presents the pentlandite lattice and Co/S atomic arrangement [Figure 9M]. The TEM image of popcorn-shaped Co₉S₈ nanocrystals synthesized under inert conditions then provides the representative nanoscale morphology of the catalyst [Figure 9N]. The mass-transfer-corrected Tafel plots subsequently reveal faster two-electron ORR kinetics for Dandelion NCs, with a Tafel slope of 81.7 mV dec^-1 compared with 118.8 mV dec^-1 for Popcorn NCs [Figure 9O]. Consistently, the potential-dependent mass activities further demonstrate the substantially higher two-electron ORR activity of Dandelion NCs across the investigated potential range [Figure 9P]^[187].

Metal carbides and nitrides

Metal carbides and nitrides form another chemically distinctive catalyst family for two-electron electrocatalysis because the incorporation of carbon or nitrogen into a metal-centered lattice creates a bonding framework that differs fundamentally from those of metals, oxides, hydroxides, sulfides, or phosphides^[188,189]. Their catalytic significance arises from the fact that interstitial or strongly coordinated C and N atoms do not behave merely as inert lattice occupants. Instead, they reshape the electronic structure of the metal sublattice, modify metal-metal spacing, alter charge density distribution, and establish a hybrid bonding regime that combines metallic conductivity with substantial covalent character. This combination is highly relevant to two-electron chemistry. Selective partial conversion requires an interface that can activate relatively stable substrates while avoiding the deep trapping of intermediates that would promote further reduction or oxidation^[190]. Carbides and nitrides are often well positioned in this respect because their metal centers are electronically perturbed by neighboring C or N atoms in a way that can emulate certain aspects of noble-metal-like surface behavior while retaining access to chemically richer adsorption states. In carbides, the presence of carbon frequently induces strong metal-carbon hybridization and modifies the d-state distribution of the transition metal, which can weaken or rebalance adsorption of oxygenated intermediates relative to the parent metal^[191]. In nitrides, the higher electronegativity of nitrogen and the often more polarized metal-nitrogen bonding can further alter the electron density and local electrophilicity of surface metal atoms^[192-194]. The resulting surface chemistry is neither purely metallic nor conventionally ionic. It is best described as electronically reconstituted, with adsorption properties emerging from the coupled influence of conductive metal frameworks and strongly bound nonmetal sublattices. For two-electron electrocatalysis, this is advantageous because it creates a chemically differentiated surface on which the first productive electron-transfer event can remain accessible, while the subsequent transformation steps that drive multielectron conversion may become less favorable due to modified adsorbate covalency, altered bond polarization, or reduced propensity for excessive intermediate stabilization.

A second feature that makes carbides and nitrides important in this context is the way their lattice chemistry influences structural persistence and surface reactivity under electrochemical bias. The strongly integrated metal-carbon and metal-nitrogen frameworks often provide a degree of structural rigidity that differs from the more labile behavior of hydroxides or some sulfide- and phosphide-based systems, yet this rigidity does not imply chemical inertness. Instead, these materials can present surfaces in which partial oxidation, termination-dependent reconstruction, and near-surface heterogeneity generate catalytically active states that remain constrained by the underlying carbide or nitride scaffold. This is especially valuable for two-electron pathways, because selectivity often requires a surface that is reactive enough to engage the substrate but not so dynamically transformed that it continuously evolves toward structures favoring deeper redox chemistry. Carbides and nitrides can provide precisely such an intermediate regime. Their surfaces may support directional adsorption and efficient charge transfer, while the underlying lattice resists the formation of excessively oxygen-affine or overhydrogenating environments that would erase the kinetic distinction between two-electron and multielectron channels^[195]. At the same time, carbon- or nitrogen-modified metal centers can alter the balance between product desorption and readsorption, which is a decisive issue whenever the desired two-electron product is chemically vulnerable to further interfacial conversion. The broader materials-chemistry importance of this class therefore lies in its demonstration that selective electrocatalysis can be engineered not only by changing the surface element itself, but by embedding that element within a non-oxide covalent-metallic lattice that redefines its chemical behavior. Metal carbides and nitrides thus expand the conceptual space of two-electron electrocatalysis by offering catalyst platforms in which electronic reconstitution, lattice robustness, and chemically moderated adsorption cooperate to favor controlled arrest at a defined redox level rather than unrestricted descent into deeper reaction manifolds^[196].

Metal carbides and nitrides have emerged as important material platforms for two-electron electrocatalysis because their high conductivity and stable metal-nonmetal bonding can be coupled with surface functionalization, lattice regulation, and interfacial reconstruction to tune key intermediate adsorption. In nitride-based systems, Liu et al.^[197] reported triazolate-functionalized ZrN for air-fed H₂O₂ electrosynthesis. The HR-TEM image of T-ZrN displays the crystalline structure of the functionalized zirconium nitride, showing that the nitride lattice remains well defined after triazolate modification [Figure 10A]. The model of aqueous O₂ conversion on T-ZrN further illustrates how O₂ adsorbs and transforms on the functionalized surface, reflecting the regulatory effect of the triazolate ligand on the local oxygen-adsorption environment [Figure 10B]. To examine its response to different gas atmospheres, LSV curves of T-ZrN with O₂, air, Ar, N₂, or CO₂ flowing were compared to evaluate the electrochemical behavior under different feeding conditions, indicating that the main catalytic response originates from oxygen reduction [Figure 10C]. The H₂O₂ yield and Faradaic efficiency of T-ZrN under different current densities with O₂ or air flowing show that the catalyst can maintain peroxide production under air-fed and high-current-density conditions [Figure 10D]^[197]. This work demonstrates that triazolate modification can regulate O₂ adsorption on ZrN, converting a stable nitride surface into an air-fed two-electron oxygen reduction reaction catalytic interface. Different from this surface-ligand strategy, Li et al.^[198] developed ordered Ni₃ZnC_0.7/C from the perspective of intermetallic lattice regulation. The HR-TEM image of Ni₃ZnC_0.7/C presents the crystalline structure of carbon-supported ordered Ni-Zn-C nanoparticles, providing morphological and structural evidence for the catalyst [Figure 10E]. The charge-density difference of Ni₃ZnC_0.7/C(111) with *OOH adsorption reveals electronic redistribution around the oxygenated intermediate from a side-view perspective, indicating that the ordered lattice can alter the local adsorption environment of *OOH [Figure 10F]. The activity volcano plot based on *OOH adsorption free energy further shows that moderate *OOH adsorption is essential for high two-electron oxygen reduction reaction selectivity [Figure 10G]. The H₂O₂ yield rate and Faradaic efficiency at corresponding potentials then verify the potential-dependent H₂O₂ electrosynthesis performance of Ni₃ZnC_0.7/C [Figure 10H]^[198].

Figure 10. (A) HR-TEM image of T-ZrN. (B) Structural model of the conversion of aqueous O₂ on T-ZrN. (C) LSV curves of T-ZrN with O₂, air, Ar, N₂, or CO₂ flowing. (D) H₂O₂ yield and Faradaic efficiency of T-ZrN under different current densities with O₂ or air flowing. (A-D) is reprinted with permission from Ref.^[197]. Copyright © 2025 American Chemical Society. (E) HR-TEM image of Ni₃ZnC_0.7/C. (F) Charge density difference of Ni₃ZnC_0.7/C (111) with *OOH adsorption, side view. (G) Activity volcano plot for the two-electron oxygen reduction reaction based on *OOH adsorption free energy. (H) H₂O₂ yield rate and Faradaic efficiency of Ni₃ZnC_0.7/C at the corresponding potentials. (E-H) is reprinted with permission from Ref.^[198]. Copyright © 2026 Elsevier B.V. (I) HR-TEM images of Ni₃N/MX. (J) Charge density distribution of Ni₃N/Ti₃CN. (K) Disparity in charge distribution between *OOH on NiOOH/Ni₃N and NiO/Ni₃N. (L) H₂O₂ yield and Faradaic efficiency of Ni₃N/MX under O₂ flowing at different potentials; the test time under each potential is 1 h. (I-L) is reprinted with permission from Ref.^[199]. Copyright © 2025 Elsevier B.V. (M) HRHAADF-STEM image of the FeN/Fe₃N interface. (N) Wavelet transforms for the k³-weighted EXAFS signals of FeN/Fe₃N. (O) Normalized Fe K-edge EXAFS spectra; inset: the local FeN structure showing tetrahedral coordination and the Fe₃N structure showing octahedral coordination. (P) Calculated Gibbs free-energy diagrams for carbon dioxide reduction reaction on FeN, Fe₃N, and FeN/Fe₃N surfaces. (M-P) is reprinted with permission from Ref.^[200]. Copyright © 2023 Springer Nature.

If T-ZrN and Ni₃ZnC_0.7/C emphasize surface modification and ordered lattice regulation, Ni₃N/MX and FeN/Fe₃N further reveal the importance of structural evolution during reaction and heterointerface effects. Liu et al.^[199] reported Ni₃N/MX, in which Ni₃N undergoes surface reconstruction under oxygen reduction reaction conditions to form NiOOH-related active species. The HR-TEM image of Ni₃N/MX shows the lattice structure of the nickel nitride phase, providing a structural basis for understanding subsequent electronic modulation and reconstruction behavior [Figure 10I]. At the interfacial electronic level, the charge-density distribution of Ni₃N/Ti₃CN reveals electronic interaction between the nitride and Ti₃CN, suggesting that the conductive substrate can influence the electronic state of the Ni-based surface through charge transfer [Figure 10J]. With attention to the reconstructed surface, the charge-distribution disparity between *OOH on NiOOH/Ni₃N and NiO/Ni₃N shows that different oxide/oxyhydroxide surface states lead to distinct *OOH electronic environments and adsorption behaviors [Figure 10K]. Accordingly, the H₂O₂ yield and Faradaic efficiency of Ni₃N/MX under O₂ flowing at different potentials demonstrate that the reconstructed Ni-based surface can promote H₂O₂ generation in the two-electron oxygen reduction reaction [Figure 10L]^[199]. This system suggests that the activity of nitrides may not originate solely from the initial nitride phase; the reconstructed surface generated during operation can also become a decisive selectivity-determining structure. In contrast to the reconstruction-centered Ni₃N/MX system, Yin et al.^[200] constructed FeN/Fe₃N heterostructured nanoparticles to emphasize the role of nitride-nitride interfaces in the two-electron reduction of CO₂ to CO. The HRHAADF-STEM image of the FeN/Fe₃N interface directly presents the atomic arrangement at the junction between the two nitride phases, offering visual evidence for the interfacial structure [Figure 10M]. The wavelet transforms for the k³-weighted EXAFS signals of FeN/Fe₃N further record the local coordination features around Fe sites, reflecting the coexistence of different Fe-N coordination environments at the interface [Figure 10N]. The normalized Fe K-edge EXAFS spectra compare the local structural differences among FeN/Fe₃N, FeN, and Fe₃N, while the inset distinguishes tetrahedral FeN coordination from octahedral Fe₃N coordination [Figure 10O]. In terms of reaction energetics, the calculated Gibbs free-energy diagrams for carbon dioxide reduction reaction on FeN, Fe₃N, and FeN/Fe₃N surfaces show that the heterointerface provides a more favorable pathway for *COOH formation and CO generation [Figure 10P]^[200].

Beyond conventional catalyst families, emerging materials such as MOFs, COFs, and SACs further expand the structural design space for two-electron electrocatalysis by providing highly tunable coordination environments and electronically distinct active sites. MOFs and COFs offer controllable pore structures, ligand environments, and spatially defined catalytic centers, enabling regulation of substrate confinement, local solvation, charge transfer, and intermediate stabilization at the molecular level. SACs, in contrast, provide atomically dispersed metal centers whose coordination number, ligand identity, oxidation state, and electronic coupling with the support can be precisely tailored. These structural features allow the adsorption configuration and electronic activation of key intermediates to be selectively modulated, which is particularly relevant for distinguishing desired two-electron pathways from deeper multielectron conversion. However, the high structural tunability of these materials does not inherently guarantee enhanced two-electron selectivity; excessive confinement, overly strong coordination, or insufficient structural stability may instead promote intermediate retention and subsequent reaction. Their effectiveness therefore depends on precise control of the local coordination and interfacial environment to establish an appropriate balance between substrate activation, intermediate stabilization, and product release.