Energy-efficient anodic reactions for sustainable hydrogen production via water electrolysis
Abstract
Overall water splitting is considered as an effective technique for hydrogen (H2) production; however, it usually requires large operating voltage mainly due to the high equilibrium potential of the anodic oxygen evolution reaction (OER). Replacing OER with energy-saving anode reactions not only reduces the operating voltage for H2 production but also generates high-value-added chemicals or purifies wastewater. This review article provides an overview of the fundamental reaction principles of overall water splitting and typical energy-saving alternative anode reactions, including methanol oxidation, hydrazine oxidation, and urea oxidation reactions. Then, the preparation methods, regulation strategies, and composition/structure-performance relations of advanced catalysts for these energy-efficient H2 generation technologies are discussed. Finally, we propose the underlying challenges and perspectives for this promising field.
Keywords
INTRODUCTION
Hydrogen (H2) is regarded as a promising alternative to fossil fuels due to its high energy density and pollution-free features[1,2]. Traditional H2 production methods, such as methanol autothermal reforming, coal gasification, and methanol steam reforming (CH3OH + H2O → CO2 + 3H2), usually require high temperature and high pressure[3-6]. Moreover, these technologies are often associated with high CO2 and/or CO emissions, resulting in unsatisfactory H2 purity and environmental problems[7-11]. In contrast, water electrolysis powered by wind, solar, and electric energy is a green and sustainable technology for the production of high-purity H2[12]. Generally, water electrolysis systems consist of an anode where the oxygen evolution reaction (OER) takes place and a cathode where the hydrogen evolution reaction (HER) occurs[7]. The theoretical potential for water electrolysis is 1.23 V [vs. reversible hydrogen electrode (RHE)]; however, the actual operating voltage usually exceeds 1.5 V due to the inherently sluggish dynamics of the multi-proton coupled electron transfer process in OER[13-15]. In addition, the O2 produced in the anodic chamber may lead to the explosive O2/H2 mixture with the generated H2 through the gas crossover[16-18].
Replacing OER with other thermodynamically more favorable anodic oxidation reactions can not only reduce the energy consumption of H2 production and avoid potential explosion risk but also may purify wastewater or produce high-value-added chemicals simultaneously[19-21]. For instance, coupling of the urea oxidation reaction (UOR) with low thermodynamic potential (0.37 V vs. RHE) and HER in electrolyzers can not only greatly reduce the hydrogen-producing overpotential but also provide a feasible strategy to treat urea-containing wastewater[22-24]. Combining methanol oxidation reactions (MOR) with HER enables energy-saving H2 production while generating value-added products such as formic acid and formate[25-27]. Unfortunately, these reactions generally involve complicated multiple electron transfer processes that suffer from slow kinetics and require high-performance catalysts to reduce the energy barriers.
Noble metal-based catalysts (e.g., Pd, Pt, and Ru) show prominent activity for these reactions; however, their large-scale commercialization is subject to the high cost, scarcity, and insufficient stability[28,29]. Alloying noble metals with other inexpensive active components or loading them onto specific carriers such as porous carbon and oxides has been demonstrated to be effective in reducing the dosage of noble metals and improving activity[30-32]. On the other hand, great efforts have been devoted to the development of high-performance non-noble metal catalysts[33-35]. For instance, MoS2 nanosheet@CoNi-zeolitic imidazole frameworks, NiSe-carbon nanotubes (NiSe-CNTs), and Mo-doped Co4N were developed to catalyze MOR[36-38]. Ni-Cu alloy nanotubes, Cu-Ni nitride, and CoP-Co2P were demonstrated to have superior hydrazine oxidation reaction (HzOR) activity[39-41]. Nevertheless, the performance of the non-precious metal-based catalysts developed to date is still not adequate for practical applications. In addition, the energy-saving anodic oxidation reaction and HER go through different reaction paths, which poses a great challenge to the development of high-performance bifunctional catalysts that can reduce the overall cell voltage for H2 production[42,43]. Therefore, more work should be done to understand the basic reaction principles and design bifunctional catalysts with high activity and stability to advance this emerging field.
Recently, many excellent review papers have summarized the progress of small molecule conversion-assisted H2 production[8,24,44,45]. For example, Li et al. summarized the progress of engineering principles of Ni-based heterostructured catalysts for urea electrolysis[46]. Qiao[24] and Zhu[45]et al. summarized the design of catalysts for specific small molecule conversion coupled H2 evolution systems. Xu et al. focused on small-molecule (such as alcohols, aldehydes, amines, and carbohydrates) selection and catalyst design strategies in their recent review[21]. Nevertheless, the systematic summary of the fundamentals of representative energy-saving anodic reactions, as well as the design principles and structure-performance relations to electrocatalysts for these technologies, has rarely been realized.
In this review, we first summarize the reaction principles of OER and HER in water splitting and typical energy-saving anodic reactions, including MOR, HzOR, and UOR. This may help readers understand the elementary steps of these reactions and the interactions of intermediates with active species, thereby stimulating a deeper understanding of the design principles of high-performance catalysts. Then, the preparation methods, regulation strategies, and composition/structure-performance relations of catalysts for energy-efficient H2 generation technologies, including methanol oxidation-assisted H2 production (MOAHP), HzOR-assisted H2 production (HOAHP), urea oxidation-assisted H2 production (UOAHP), and other biomass conversion-assisted H2 production are systematically discussed [Figure 1]. This has rarely been realized in other similar reviews and is instructive for researchers to design and synthesize novel efficient catalysts for energy-saving H2 production. Lastly, possible challenges and some personal insights in this fascinating field are briefly proposed.
Figure 1. Schematic illustration of the cell configurations for MOAHP, HzOAHP, UOAHP, and other energy-saving anodic reaction-assisted H2 production. GEOR: Glycerol oxidation reaction; GOR: glucose oxidation reactions; HER: hydrogen evolution reaction; HzOAHP: hydrazine oxidation-assisted H2 production; HzOR: hydrazine oxidation reaction; MOAHP: methanol oxidation-assisted H2 production; MOR: methanol oxidation reaction; UOAHP: urea oxidation-assisted H2 production.
FUNDAMENTALS OF ENERGY-SAVING ANODIC REACTIONS-ASSISTED H2 PRODUCTION
Two half-reactions, i.e., OER at the anode and HER at the cathode, occur simultaneously in a conventional overall water splitting (OWS) device. In different electrolytes, such as acidic, neutral, and alkaline, OER and HER undergo different reaction pathways.
In acidic electrolytes, H+ acts as the reactant of HER, and the following reaction occurs (Equation 1).
The elementary steps of HER in acidic media are as shown in Figure 2A: First, a proton accepts an electron and adsorbs to the active site (Volmer step). Then, two adsorbed hydrogens combine (Tafel step), or the adsorbed hydrogen takes up an electron and reacts with another proton (Heyrovsky step)[27,47].
Figure 2. The mechanism for (A) HER and (B) OER in acidic conditions. HER: Hydrogen evolution reaction; OER: oxygen evolution reaction.
OER in acidic electrolytes is shown in Equation (2). The possible elementary steps are displayed in Figure 2B.
HER in neutral or alkaline electrolytes is described in Equation (3). The possible elementary steps are shown in Figure 3A.
Figure 3. The mechanism for (A) HER and (B) OER in neutral and alkaline medium. HER: Hydrogen evolution reaction; OER: oxygen evolution reaction.
OER in neutral or alkaline electrolytes is described in Equation (4). The possible elementary steps are described in Figure 3B.
The equilibrium potential for OER is 1.23 V (vs. RHE), and the standard potential for HER is 0 V; thus, the thermodynamic potential for OWS is 1.23 V. According to the above equations, OER involves a complex four-electron transfer process, which is inherently sluggish than HER. Therefore, it can be inferred that OER is the bottleneck of OWS in both thermodynamics and kinetics. Tremendous efforts have been made to reduce the overpotential of OER, including the development of noble metal Ir, Ru-based catalysts, or transition metal (Co, Ni, Fe, Cu, etc.)-based alloys, oxides/hydroxides, phosphides, sulfides, and selenides. In addition, a number of active site regulation strategies, such as the construction of single atomic sites, chemical doping, heterostructuring, and ligand-induced transformation, have been developed to tune the electronic structure of active sites and/or to break the scaling relation of oxygen-containing intermediates (HOads, Oads, and HOOads) to accelerate OER processes. Unfortunately, a voltage higher than 1.5 V is still generally required for the actual OER. Alternatively, recent studies have demonstrated that the addition of specific reactants, such as methanol, hydrazine, urea, and glucose, to the electrolyte can effectively reduce the overall potential for H2 production.
Methanol possesses a much lower oxidation potential than OER. Coupling the MOR with HER was demonstrated to be an effective way for energy-saving H2 production. Generally, the MOR in alkaline can be described by Equation (5)[48-54]. The main reaction path of methanol oxidation to CO2 can be divided into a direct pathway [Equation (6)] and an indirect pathway [Equation (7)], depending on the nature of catalysts[55]. For instance, Zeng et al. verified that the introduction of Fe and Cu modulates the Pt lattice stress, which will alter the selectivity of direct and indirect pathways for MOR. As a result, the PtCu0.3Fe0.7 has a higher CO2 yield than PtCu0.5Fe0.5, and the latter generates more formic acid and methyl formate than the former[56]. Li et al. reported a high-entropy alloy composed of Pt18Ni26Fe15Co14Cu27 that exhibits excellent MOR and HER activities. This is due to the synergistic effect between multiple active sites optimizing the adsorption energy of key intermediates in the catalytic process [Figure 4A-D]. Moreover, the alloy catalyst shows a high energy barrier for CO formation, resulting in its excellent resistance to methanol poisoning[57].
Figure 4. The energetic pathway of (A) MOR and (B) CO poisoning on the surface of Pt18Ni26Fe15Co14Cu27; The (C) bonding structure and (D) PDOS of the Pt18Ni26Fe15Co14Cu27[57]; The free-energy profiles for (E) HER and (F) HzOR on Ni3N-Co3N and the contrast samples[58]. HER: Hydrogen evolution reaction; HzOR: hydrazine oxidation reaction; MOR: methanol oxidation reaction; PDOS: projected density of state.
Hydrazine is another widely used reactant for energy-saving H2 production because HzOR possesses a low standard potential of -0.33 V (vs. RHE) compared to 1.23 V of OER. In addition, electrolyzers coupled with HzOR and HER can avoid the explosion risk of H2/O2 in the conventional OER-HER electrolyzer since the product of HzOR is green stable nitrogen (N2). The equation for the HzOR is described as follows:
The possible elementary steps for the HzOR are shown in Equations (9-12).
The density functional theory (DFT) calculation results of Qian et al. in Figure 4E and F confirm that the rate-determining step of HzOR on the (001) surface of Ni3N is [N2H3]ads-M → [N2H2]ads-M with free energy difference of 0.53 eV, while that on the (001) surface of Co3N is the [N2H2]ads-M → [N2H]ads-M process with free energy difference of 1.40 eV[58]. After coupling Ni3N and Co3N, the rate-determining step is still the deprotonation of [N2H2]ads-M, but the free energy difference is reduced to 0.19 eV. As a result, Ni3N-Co3N heterostructures show better HzOR performance than Ni3N and Co3N. In addition, the Ni3N-Co3N also exhibits excellent HER activity, allowing a two-electrode HzOR-HER electrolyzer with a low cell voltage of 0.76 V at 400 mA·cm-2. Recently, numerous HzOR catalysts have been developed, such as Ru-doped FeP4, CuPt and CuPd nanoalloy, and single atom Ru on WS2[59-62]. However, more efforts need to be made to improve HER/HzOR bifunctional catalytic performance with an in-depth understanding of catalyst design principles and active site regulation strategies.
Urea is a common raw material in industrial wastewater and domestic sewage. A low equilibrium potential of 0.37 V makes UOR another potential alternative to OER for energy-saving H2 production. Moreover, the UOR-HER electrolyzer provides a potential solution for the purification of urea-containing wastewater during H2 production. In general, the oxidation products of urea in alkaline electrolytes are CO2, H2O, and N2, and the reaction equation is described as follows.
According to the deprotonation order of urea, UOR can be divided into the following two typical reaction mechanisms [Equations (14-15)][63,64].
In these two reaction mechanisms, the UOR both undergoes a complex 6-electron transfer process, which requires rationally designed catalysts to reduce the energetics of the reaction. Precious metals, such as Ru and Ir, and non-precious metal catalysts, such as Ni, Co, and Fe-based catalysts, were demonstrated to have satisfactory UOR performance. Moreover, recent experimental and theoretical studies have demonstrated that the UOR pathway can be regulated by the coordination environment and/or geometric structure of the active sites. Xu et al. demonstrated that the UOR on NiSe2 followed Equation (14), with the dehydrogenation of [CO(NH2·NH)]ads to [CO(NH2·N)]ads as the rate-determining step[65]. After coupling NiSe2 with Ni3Se4, the rate-determining step becomes [CO(NH2·NH2)]ads → [CO(NH2·NH)]ads with lower reaction free energy [Figure 5A and B]. In another example, UOR follows the mechanism in Equation (15) on the Ni-oxyhydroxide surface, with [COOH]ads → [CO2]ads as the rate-determining step
Although the energy-efficient anodic reactions discussed above, such as glucose oxidation reactions (GOR) and UOR, have low equilibrium potentials, they involve complex multi-electron transfer processes and multiple reaction intermediates. Due to inherent scaling relationships, the energetics of the O- or
ADVANCED ELECTROCATALYSTS FOR ENERGY-SAVING H2 PRODUCTION SYSTEMS
Electrocatalysts for MOAHP
Among kinds of small molecule oxidations, MOR has attracted huge attention because of its ultra-low oxidation potential of 0.016 V vs. RHE. Due to the advantages of methanol, including its low cost and ease of storage and transfer, MOR is used in fuel cells[67,68]. Impressed, using MOR instead of OER, which has an oxidation potential of 1.23 V, is an economical idea to achieve low energy consumption while efficiently obtaining hydrogen energy. Noble metal-based catalysts are highly active for MOAHP[69]. Over the decades, a great deal of design strategies for noble metal catalysts have been proposed. Zhang et al. employed a selective Pt nanoplate as an active site to create a catalyst to achieve MOAHP[70]. They prepare vertical graphene nanosheet arrays with Pt nanoplates decorated grown on the carbon cloth (Pt200-VGNSAs/CC). Benefited from the three-dimensional (3D) structure, large surface area, and conductivity from arrays, they achieved high mass activity of 1,050 mA·mg-1, 0.38 V vs. Ag/AgCl for MOR, and overpotential of 60 mV @ 10 mA·cm-2 for HER. Similarly, Feng et al. choose PtTex as a template to prepare a new class of Pt superstructures (SSs) via electrochemical leaching of Te[71]. Different distorted degrees among H-/M-/W-Pt SSs/C were caused by differences in Te content. The distorted surface in materials can effectively mitigate the detrimental influence of agglomeration/degradation. Thus, H-Pt SSs/C showed a mass activity of
Figure 6. (A) MD calculation of H-Pt SSs/C. The right side is the atomic structure after 2,000 ps, the top is the area of nondetective and strained regions, and the bottom is the area of single atom layer nanopits[71]; (B) DFT calculation of OH- absorption energy for NiO after Pt introduction[72]; (C) Calculated results of energy for MOR processes of Pt, Pt5P2, and Pt/Pt5P2[73]; (D) LSV curve of the
Although noble metals show good performance of MOAHP, adding a second composite can not only optimize the stability of catalysts but also decrease the cost of noble metals. Thus, Ma et al. attained a
Noble metal catalysts have very significant electrochemical performance, but their practical applications are often limited by their high cost and complex preparation technology[29]. Forming an alloy of noble metals with other affordable metals can reduce the containing of noble metals, which helps reduce the cost. Meanwhile, the alloying strategy decreases the possibility of poisoning, leading to deactivation by changing the electronic structure. Considering this, Ren et al. constructed PtCo alloy nanoparticles and encapsulated them with N-doped nanorod arrays (Pt2Co8@NC)[75]. High specific surface area and abundant active sites endow the Pt2Co8@NC with good activity. The MOR catalytic performance included a forward anodic peak current density of 2.17 mA·cm-2, a retention rate of 94.4% after 100 cycles, and an overpotential of 12 mV for HER @10 mA·cm-2. Jiang et al. prepared core-shell structure Pd@RhPd nanodendrites (NDs) by galvanic replacement reacting Pd NDs and rhodium trichloride[76]. They found that the content of Rh in Pd@RhPd NDs determined the MOR catalytic activity. While the introduced Rh could improve the HER activity of the material, its content had little effect on HER activity. The Pd@Rh0.07Pd ND-based MOAHP device can achieve a current density of 10 mA·cm-2 at 0.813V. Yin et al. synthesized a highly active catalyst for crimp-perforated PdLn bimetallene with abundant defects via a solvothermal method[77]. The crimp-perforated structures endow PdLn bimetallene with high surface energy and expanded active sites. The constructed MOAHP device with PdLn bimetallene could achieve a current density of 100 mA·cm-2 at a low voltage of 1.26 V. In addition, as a special alloy species, high-entropy alloys have been widely used in the preparation of various catalysts and are also used in MOAHP devices. Wang et al. constructed a rambutan-like Au@PdRu catalyst with a core-shell structure[78]. The unique structure provided more exposed active sites for the material. Therefore, Au@PdRu displayed the mass activity of 1.56 A·mg-1Pd for MOR, and MOAHP devices of 0.88 V reached 10 mA·cm-2, while for OWS, it was 1.69 V. Differently, Sarno et al. using non-noble metal components to prepare the catalysts. They first proposed using MoS2-modified PtRh alloy through a wet chemical method[79]. The modified PtRh/MoS2 showed better stability than the unmodified one, with no obvious decay observed after 2,000/600 cycles for HER/MOR. They used the If/Ib ratio (If is forward peak current density, and Ib is backward peak current density) to quantify the poison tolerance of the catalyst, which was 2.37 V for PtRh/MoS2. This great performance could be attributed to the abundant edges of MoS2 nanosheets, providing an additional surface for stabilizing PtRh.
Lu et al. prepared a sea urchin-like heterogeneous material encapsulating CoPt3@Co2P with Co embedded in N2-doped carbon nanotubes (CoPt3@Co2P/Co@NCNT)[80]. The DFT calculation told the original of the catalytic performance improvement, electrons transferred from CoPt3 to Co2P assisting by the heterogeneous interfaces. This process would generate the electron hole region in CoPt3, which benefited MOR processes, and the electron-rich Co2P would optimize H* absorption energy of HER processes. Thus, CoPt3@Co2P/Co@NCNT exhibited MOR catalytic activity of 2,981 mA·mg-1Pt and HER overpotential of
In addition to noble metal catalysts, non-noble metal catalysts with good stability and low cost have also received attention in recent years. Many studies have demonstrated that non-noble metal catalysts can exhibit comparable activity to noble metal catalysts through rational structural/component designs. As shown above, MoS2 nanosheets have an abundant edge structure, which is conducive to catalysis. Thus, Liu et al. constructed CoNi-ZIF composites embedded with MoS2 nanosheets [MoS2@CoNi-ZIF (3-1)][37]. The MoS2@CoNi-ZIF (3-1) showed a mass activity of 430.08 mA·g-1 for MOR and could work stable for 3,000 s. The HER performance achieved an overpotential of 80 mV to reach 10 mA·cm-2 and maintained 97.5% after a 48-h cycle. In addition to MoS2, Peng et al. selected MoSe2 and NiSe to form a hetero-structured NiSe/MoSe2 grown on carbon cloth (NMS/CC, Figure 7A)[81]. The catalyst could achieve 100 mA·cm-2 at 1.38 V for MOR, and the Tafel slope of OER was ~ 14 times that of MOR. At the same time, NMS/CC could cycle stably for 120 h, as measured by the I-t test for MOR. Impressed, the NMS/CC-based MOAHP device could be powered by a 1.5 V solar battery.
Figure 7. (A) Simulated heterogeneous interface in NMS/CC[81]; (B) Stability of h-NiSe/CNTs/CC at 1.62 V[36]; (C) DFT calculation of h-NiSe/CNTs/CC during HzOR process[36]; (D) Ion chromatograms of Cl- for initial seawater, after SMOAHP and SOWS with catalysts of NiFe2O4/NF[86]. CC: Carbon cloth; CNTs: carbon nanotubes; DFT: density functional theory; HzOR: hydrazine oxidation reaction; NMS: NiSe/MoSe2; NF: NiFe2O4; SMOAHP: methanol oxidation-assisted H2 production in seawater; SOWS: overall water splitting in seawater.
Zhao et al. also selected NiSe as the active material and prepared the highly dispersed hollow h-NiSe/CNTs/CC heterogeneous catalyst via the one-pot method[36]. Interestingly, they chose the methanol oxidation route that produces formate, a high-value product, rather than conventional CO2. In detail, benefiting from the 3D CNT network and highly dispersed NiSe crystals, the MOAHP device constructed by h-NiSe/CNTs/CC could arrive 345 mA·cm-2 current density, high Faraday efficiency of 95% @1.62V, and nice stability, which can improve the H2 production efficiency ~ six times [Figure 7B]. Subsequently, they further studied the mechanism of h-NiSe/CNTs by DFT calculation. The results showed that the SeOx and NiOOH produced during the catalytic process can optimize the d-band center to achieve the selective oxidation of methanol and avoid further oxidation to CO2 [Figure 7C]. Choosing the MOR route without CO2/CO production or any gas production not only ensures the purity of H2 produced but also achieves higher economic benefits through the formation of formate/formate salts[82-85]. Therefore, Du et al. also opted for the CO-free MOR route but chose to use formate as a product by designing a NiFe2O4 catalyst (NiFe2O4/NF) and using alkaline seawater as electrolyte[86]. The NiFe2O4/NF achieved a current density of > 800 mA·cm-2 and a high Faraday efficiency (96% for HER and 95% for MOR) in alkaline seawater. Impressively, the material showed great chlorine corrosion resistance and could cycle stably for more than 6 h at a large current density of 700 mA·cm-2. Importantly, as shown in Figure 7D, utilizing MOR instead of OER actually avoids the chlorine evolution reaction (ClER).
Electrocatalysts for HOAHP
HzOR shows a small overpotential of -0.33 V vs. RHE. The H2 production device constructed by the combination of HzOR and HER can achieve the purpose of low consumption and high economic value[87]. Jia et al. designed N-doped nanobowls (Rh/N-CBs) grown with ultrafine Rh nanoparticles[88]. When the Rh/N-CBs were used for HOAHP devices, a current density of 20 mA·cm-2 at a low cell voltage of 0.2 V was achieved. DFT calculations revealed that the bonding between pyrrole-N and Rh in N-CBs resulted in near-zero adsorption energy of hydrogen, which endowed the Rh/N-CBs with good catalytic performance [Figure 8A]. Thus, when N-doped, the Rh site will strongly bond with N, which can tune the electron structure. Zhang et al. reported the preparation of Rh2S3 with N-doped carbon catalysts and applied Rh2S3/NC to HOAHP devices in a wide pH range[89]. As shown in Figure 8B, the DFT calculation demonstrated that the great performance not only came from N-doping but also from the longer N-N bonds on the Rh2S3 surface. Compared to Pt, during HzOR, adjacent S atoms can provide abundant charges to stabilize the Rh site, leading to an improvement in electrocatalysis performance. Therefore, Rh2S3/NC had lower energy of H adsorption and NHNH adsorption dehydrogenation. Benefited from these, Rh2S3/NC showed low HER overpotentials of 38, 46, and 21 mV in alkaline, neutral, and acidic electrolytes (@10 mA·cm-2) and saving energy consumptions of 93.3%, 85.2%, and 78.3% compared to OWS, respectively. Yang et al. designed a Ru nanocluster-modified mesoporous nanosphere (Ru20@mONC) and applied it to a wide pH range HAOHP device[90]. Ru20@mONC demonstrated better electrocatalysis performance, achieving current densities of
Figure 8. (A) Adsorption energy of hydrogen for Rh/N-CBs and Rh/CBs[88]; (B) Schematic diagram of N2H4* adsorption mode on Pt and Rh2S3 surfaces[89]; (C) Synthesis scheme of Au1Pt8 alloy[91]; (D) TEM images of Ni(OH)2 layers and Ni nanosheets (inset)[92]; (E) The charge density difference of P, W-Co3N NWA/NF. Top-view (top) and side-view (bottom), cyan/yellow demonstrate charge depletion/accumulation[93]; (F) The calculation of Ni NSA and Ni-C HNSA of HzOR processes[95]. HNSA: Hybrid nanosheets; HzOR: hydrazine oxidation reaction; N-CBs: nitrogen-doped carbon nanobowls; NF: NiFe2O4; NSA: nanoarrays; NWA: nanowire array; TEM: transmission electron microscopy.
The alloying strategy was also useful for HzOAHP catalysts design. Yu et al. prepared a Au1Pt8 alloy via pulsed laser irradiation and sonochemical processes [Figure 8C][91]. During testing, Au1Pt8 showed excellent HER performance, achieving a 26 mV overpotential to reach 10 mA·cm-2 in alkaline electrolytes and
Non-noble metal-based catalysts also showed great performance in HzAOHP catalysts. Kuang et al. prepared ultra-thin Ni nanoarrays (Ni-NSA) through in-situ topological reduction of Ni(OH)2 nanosheets and superhydrophobic arrangement as the carrier[92]. When applied in HzOR, it showed specific activity (227.6 mA·cm-2) and high mass activity (406.4 mA·mg-1cata) at 0.25 V, which is 1.81 and 3.25 times higher than that of Pt/C. As shown in Figure 8D, the great performance can be attributed to the ultra-thin nature of Ni-NSA, which consists of just ten atomic layers. As in many previous studies, transition metal nitrides (TMNs) exhibit excellent electrical conductivity, low resistance, and excellent stability in acid/base environments and are commonly used as catalysts. Therefore, Wang et al. designed a double-component TMN heterostructure catalyst Cu1Ni2-N[40]. The structure of TMNs and 3D structures of the carrier [carbon fiber cloth (CFC)] gave Cu1Ni2-N good catalytic performance. It showed a 71.4 mV overpotential for HER and 0.5 mV for HzOR at 10 mA·cm-2. The HOAHP device constructed by Cu1Ni2-N not only achieved
In order to maximize the utilization of resources, many electrolytic hydrogen energy devices chose seawater as the electrolyte[96-98]. However, for traditional OWS, the OER potential approaches the ClER potential
Figure 9. (A) TEM image of l-Rh metallene[100]; (B) Cycle stability of MIL-(IrNiFe)@NF-based HOAHP devices at 0.69 V in seawater[101]; (C) The DOS change after Ru-doped of Ru-FeP4/IF[59]; (D) Electricity expense of Fe-CoNiP@NC-based HOAHP and OWS devices[102]; (E) TEM image of MoNi@NF[103]; (F) The scheme and optical images of Mo-Ni3N/Ni/NF-based HOAHP with driving by used AAA batteries[104]; (G) Optical images of comparison of wastewater degradation with Fe/F-Ni2P@NC as catalysts at different times[105]. DOS: Density of states; HOAHP: hydrazine oxidation-assisted H2 production; l-Rh metallene: low-crystalline Rh metallene; OWS: overall water splitting; TEM: transmission electron microscopy.
In addition to realizing H2 production from SHOAHP devices without harmful gas generation, MOR has many applications in the reuse of wastewater or used batteries. Liu et al. formed heterostructures between Mo-doped Ni3N and porous nanosheets to obtain Mo-Ni3N/Ni/NF catalysts, which enabled HOAHP devices to achieve 10 mA·cm-2 at 55 mV[104]. Surprisingly, they built an environmentally friendly and green production system of Mo-Ni3N/Ni/NF by using waste 1.5 V AAA batteries driving the HOAHP device, as shown in Figure 9F. As wastewater containing N2H4 is difficult to handle due to its high salinity, Zhang
Electrocatalysts for UOAHP
UOR can convert urea into CO2 and N2 with a low theoretical voltage (0.37 V vs. RHE). Coupling UOR with HER provides an energy-saving route for H2 production[45,106-108]. Nowadays, transition metals with low prices and abundant reserves are widely considered[109-112]. Currently, many researchers have adopted a series of different strategies to prepare UOR catalysts with excellent properties. One common strategy is to modify transition metal sites with oxygen, sulfur, phosphorus, and other non-metallic elements[113,114]. For example, Hu et al. successfully synthesized a nickel nitride bead nanosphere array (Ni3N/NF) grown on nickel foam[115], and this material can be used as high-performance bifunctional catalysts for UOR and HER. The current density of the two-electrode cell constructed with this catalyst can reach 100 mA·cm-2 at 1.42 V, while the corresponding voltage of the cell constructed with Pt/C and IrO2 is 1.60 V, indicating that Ni3N/NF is superior to precious metals.
The combination of two or more kinds of active sites into a multi-component catalyst is another feasible strategy[116,117]. Bimetallic synergies can effectively improve the catalytic activity. For example, Sha et al. have in-situ grown NiCoP on carbon cloth (NiCoP/CC) in the shape of a prickly leaf[118]. Due to the synergistic effect of Ni and Co and its unique hierarchical structure, the catalyst showed excellent electrocatalytic activity. The urea electrolytic cell assembled with NiCoP/CC as anodes and cathodes at the same time has a voltage of 1.42 V at the current density of 10 mA·cm-2, which is 160 mV lower than that of the electrolytic cell without urea. Wang et al. prepared layered coral-like Ni-Mo sulfide (HC-NiMoS/Ti) on Ti grids through a simple hydrothermal and vulcanization process[119]. Due to the unique layered coral-like nanostructure, abundant active sites, fast charge transfer rate, and the synergy between multiple components, the electrocatalytic performance of HC-NiMoS/Ti has been greatly improved. The cell voltage of this catalyst is only 1.59 V when reaching a current density of 10 mA·cm-2 in 1.0 M KOH + 0.5 M urea.
In addition, element doping can be used to further improve the catalytic performance of single-component or multi-component catalysts. This strategy can effectively regulate the electron configuration state inside the catalyst, which plays a great role in the improvement of material properties[120,121]. For example, Xie et al. synthesized N2-doped Co9S8/Ni3S2 hybrid nanosheet arrays (N-Co9S8/Ni3S2/NF) on nickel foam substrates through electrodeposition combined with ammonification reaction, and the diagram of their synthesis process is shown in Figure 10A[122]. On the one hand, the unique two-dimensional porous nanosheet structure of the catalyst can expose more heterogeneous interfaces between Co9S8 and Ni3S2 so as to provide abundant reactive sites and accelerate the electron transport rate. On the other hand, the doping of N2 regulates the internal electronic structure of the catalyst and further enhances its electrocatalytic activity.
Figure 10. (A) Synthesis path diagram of N-Co9S8/Ni3S2/NF[122]; The ball-and-bat model of H2O adsorbed on (B)
In another example, Li et al. prepared abnormal metastable hcp-CoNi-N/C with metal Co and non-metal N double doping by using a new N-doped induced phase transfer strategy[124]. The catalyst has outstanding UOR performance, and the potential is only 1.31 V at the current density of 10 mA·cm-2. Through experimental analysis and theoretical calculation [Figure 11], it is proved that the electron density of
Figure 11. Schematic diagram of calculated state density of (A) fcc-Ni; (B) hcp-Ni-N; and (C) hcp-CoNi-N; Charge density difference analysis model of (D) hcp-Ni-N; and (E) hcp-CoNi-N, where yellow represents electron gain and cyan represents electron loss[124].
Apart from nickel-based catalysts, other transition metal-based catalysts can also obtain satisfactory UOR properties through rational morphology, structure design, and chemical regulation. For example, Hou et al. prepared N2-doped ferric phosphorus trisulfide nanosheets (FePS3), which showed excellent HER and UOR bi-functional catalytic activities[9]. As shown in Figure 12A, in 1.0 M KOH, the overpotential for HER of
Figure 12. (A) The polarization curves for HER of N-FePS3, FePS3, and Pt/C in 1.0 M KOH; (B) The polarization curves for UOR and the polarization curves for urea electrolysis of N-FePS3 and FePS3 in 1.0 M KOH + 0.33 M glucose; (C) The polarization curves for urea electrolysis of N-FePS3 and FePS3[23]; (D) The adsorption energy of urea on CuO (111) and Ni-CuO (111); (E) Reaction free spectrum of UOR catalyzed by CuO and Ni-CuO[125]. HER: Hydrogen evolution reaction; UOR: Urea oxidation reaction.
Yu et al. designed a novel Fe-Co0.85Se/FeCo layered double hydroxide (LDH) heterostructure electrocatalyst using a simple in-situ selenization method[126]. The catalyst showed excellent bi-functional catalytic performance. UOR and HER only needed 1.48 V and -0.274 V, respectively, to reach a high current density of 500 mA·cm-2. Moreover, the urea-assisted integral electrolysis H2 production system assembled by the catalyst needs only 1.57 V to reach the current density of 300 mA·cm-2. Finally, the reaction mechanism of UOR and HER was further discussed by in-situ Fourier infrared spectroscopy [Figure 13A-C] and DFT
Figure 13. (A) In-situ FTIR spectra for Fe-Co0.85Se/FeCo LDH in different constant potentials (1.1-1.7 V vs. RHE); (B) Under 1.45 V vs. RHE, the In-situ time-resolved FTIR measurements of Fe-Co0.85Se/FeCo LDH; (C) Diagram of possible intermediates in the critical steps for UOR; (D) Hydrogen adsorption on FeCo LDH, Fe-Co0.85Se, and Fe-Co0.85Se/FeCo LDH; (E) The diagram of free energy for UOR on FeCo LDH, Fe-Co0.85Se, and Fe-Co0.85Se/FeCo LDH[126]. LDH: Layered double hydroxide; RHE: reversible hydrogen electrode; UOR: urea oxidation reaction.
In recent years, there have been considerable reports on the application of metal organic frame (MOF)-derived materials in UOR. This is because these materials have unique advantages such as adjustable structure, controllable composition, and high porosity, so they have great application potential as efficient electrocatalysts for UOR and HER[127-129]. For example, Zhang et al. successfully synthesized a Fe-doped nickel-based MOF nanosheet array catalyst (FeNi-MOF NSs) through the one-step hydrothermal method. In this catalyst, the Fe dopant regulated the electronic structure of MOF[130]. To reach the current density of 10 and 100 mA·cm-2, the overpotentials for UOR of the catalytic were 131 and 155 mV, respectively. The voltage for the electrolytic cell of the dual-electrode system assembled by FeNi-MOF NSs was only 1.431 V at 10 mA·cm-2. Qin et al. have grown the nanosheet array structure of oxygen-rich vacancy and vanadium-doped Ni(OH)2 catalyst [Ovac-V-Ni(OH)2] on nickel foam, which was used to efficiently oxidize urea to assist H2 production[66]. The working potential for UOR of Ovac-V-Ni(OH)2 is only 1.47 V at 100 mA·cm-2. The good performance of the catalyst is due to the synergistic effect of V doping and oxygen vacancy.
Electrocatalysts for other energy-saving H2 production systems
Glucose is a widely distributed biomass in nature, which can be converted into gluconic acid, gluconic acid, formic acid, and other compounds with high commercial value. In recent years, due to the advantages of the low theoretical potential of GOR and high added value of products, the H2 production method of GOR-coupled HER has attracted the attention of researchers. Previously, GOR was generally used in glucose sensors and glucose fuel cells, and a considerable number of GOR materials were precious metal catalysts[131-135]. Precious metal catalysts have excellent catalytic performance and outstanding chemical stability, but their high costs and low reserves have limited their wide application. This has led researchers to explore non-precious metals with affordable prices and abundant reserves, especially the transition metals such as nickel, cobalt, and iron. Recently, cobalt-based catalysts have shown outstanding advantages in the field of GOR. For example, Wu et al. used the defect heterogeneity of MOF to prepare core-shell Co@CoO heterojunction[136]. The DFT calculation showed that there was spontaneous electron transfer from Co to CoO during the catalytic process, resulting in the d-band center of the catalyst moving up the Volmer-Heyrovsky path, and the electronic behavior and adsorption/desorption energy of the reaction intermediates were optimized, thus significantly improving the catalytic performance of the catalyst [Figure 14]. At the current density of 20 mA·cm-2, The GOR voltage of Co@CoO is 1.12 V, which is 0.42 V lower than OER. The glucose electrolytic system assembled by Co@CoO only needs 1.34 V to produce
Figure 14. (A) Local charge density difference of the Co@CoO interface. (Yellow and cyan mean electron accumulation and electron depletion, respectively); (B) The calculated state density of Co, CoO, and Co@CoO; Schottky models of semiconductor CoO and metal Co interface (C) before and (D) after contact; (E) Along the reaction coordinate, the relative energy curves of Co, CoO, and Co@CoO and the corresponding Volmer-Heyrovsky process at the Co@CoO interface[136].
In addition, it has been reported that cobalt is used as a dopant to improve the GOR performance. For example, Zhang et al. synthesized Co@NCNT by embedding cobalt nanoparticles into N-doped carbon nanotubes, which enables Co@NCNT to have HER, ORR, and GOR multifunctional catalytic performances[137]. For example, Li et al. used biomass-based ternary deep eutectic solvent (DES) as a precursor and prepared N2-doped porous carbon-supported cobalt nanoparticles (Co@NPC)[138]. The catalyst only required a small battery voltage of 1.56 V to provide a current density of 10 mA·cm-2, which was 180 mV lower than the overall water decomposition. Moreover, glucose can be efficiently converted into lactic acid, formic acid, gluconic acid, and other substances. Another example is Lin et al., who designed a cobalt-doped ultra-thin nickel-cobalt hydroxide nanosheet [Co0.5Ni0.5(OH)2 NS] on carbon cloth, whose initial catalytic GOR potential was 1.17 V at the current density of 10 mA·cm-2[139]. The cell voltage is only 1.56 V for glucose oxidation-assisted H2 production at the current density of 100 mA·cm-2.
Besides cobalt-based catalysts, nickel-based and iron-based catalysts have also been reported as high-performance GOR catalysts. For example, Liu et al. recently reported nano-structured NiFe oxide (NiFeOx) and nitride (NiFeNx) catalysts[140]. The two-electrode glucose electrolytic cells with NiFeOx-NF as anodes and NiFeNx-NF as cathodes can provide a high current density of 200 mA·cm-2 at 1.48 V and can run stably for 24 h. In addition, 87% and 83% gluconic acid and gluconic acid can be obtained by the reaction of the electrolytic cell. The special feature of this work is that the possible reaction pathways of glucose oxidation are also analyzed by in-situ infrared spectroscopy, nuclear magnetic resonance, and other characterization. Du et al. prepared a ferric phosphide catalyst (Fe2P/SSM), which was grown on stainless steel mesh[141]. The cell voltage of a two-electrode glucose electrolytic cell synthesized by this catalyst and Pt/C was 300 mV, lower than that of a cell split by water alone.
In addition, many other small molecule oxidations coupled with HER for H2 production have also been reported, such as glycerol oxidation (GEOR) and 5-hydroxymethylfurfural oxidation reactions (HMFOR). For example, Xu et al. prepared N-doped carbon-coated Ni-Mo-N nanowire arrays on nickel foam substrates (NC/Ni-Mo-N/NF) by hydrothermal methods and calcining processes [Figure 15A][142]. Due to surface modification, electronic structure optimization, and interfacial synergies, NC/Ni-Mo-N/NF showed excellent HER and GEOR properties under alkaline conditions. Li et al. reported a nickel-molybdenum nitride nanoplate catalyst (Ni-Mo-N/CFCS) for oxidizing glycerol to formic acid and producing H2[143]. When it reached 10 mA·cm-2, the GEOR voltage was only 1.30 V, and the voltage of the two-electrode system was only 1.36 V. The Faraday efficiency of formic acid and H2 was up to 95.0% and 99.7%, respectively. In this work, the authors also demonstrated that formic acid was produced from glycerol using isotopic labeling [Figure 15B]. Zhou et al. prepared a Co4N@CeO2 heterojunction catalyst. CeO2 was introduced into the Co4N system as an “electronic pump” to optimize the internal electronic structure of the catalyst. The Co4N@CeO2 showed excellent multifunctional catalytic activities of HER, OER, and HMFOR
Figure 15. (A) Scheme illustrating the synthesis of the NC/Ni-Mo-N/NF[142]; (B) 1H NMR spectra for the electro-oxidation of 2-13C glycerol and 1, 3-13C glycerol[143]; (C) Schematic diagram of the principle of CeO2 as “electronic pump” in promoting multifunctional catalytic performances of Co4N@CeO2 catalysts[144]. HER: Hydrogen evolution reaction; HMFOR: 5-hydroxymethylfurfural oxidation reaction; NMR: nuclear magnetic resonance; OER: oxygen evolution reaction.
Zhao et al. presented the pioneering integration of biomass electroreforming and green H2 production, where a chitin oxidation reaction (COR) was employed as a substitute for OER in conventional OWS
Coal is an abundant and cost-effective source of heavy carbon. The traditional coal gasification to produce H2 has serious pollution and poor economic benefits. Researchers are endeavoring to establish a coal-assisted electrolytic water H2 production system. This approach not only enables the clean utilization of coal but also reduces electricity consumption in H2 production. Coughlin and Farooqu reported the electrocatalytic H2 production of coal-water systems for the first time[146]. They introduced a coal slurry into the anode solution; the cathode led to H2, while CO2 was detected at the anode after a long reaction. Later, the researchers introduced a variety of charge transfer carriers, such as Fe2+/Fe3+, Ce4+/Ce3+, V5+/V2+, Mn3+/
Precious metals Pt and Ru and non-precious metals Fe, Co, Ni, and Mo-based materials have been widely investigated in the field of energy-saving reaction-assisted H2 production, and these systems exhibit cell voltages much lower than the OER-HER couple. Nevertheless, an in-depth understanding of the design principles of electrocatalysts for these reactions, including improving their activity and stability at high current densities and selectivity for specific high value-added products, is urgently needed for practical applications of energy-efficient H2 generation technologies, but it is rarely achieved.
CONCLUSION
Water electrolysis, as a green H2 production technology, has gained significant attention in recent decades. However, its high operating voltage has undoubtedly increased the energy consumption of H2 production. In this review, we reviewed the recent advances in energy-saving anodic reactions to replace OER to reduce the energy consumption of H2 production, including MOR, HzOR, UOR, GOR, and HMFOR[148-152]. Additionally, these reactions can be used to purify wastewater or to generate high-value-added chemicals simultaneously. Although these reactions possess low equilibrium potentials, they generally involve multi-electron transfer processes with slow kinetics. For example, urea oxidation requires an equilibrium potential of only 0.37 V, much lower than 1.23 V of the OER, yet contains complex six e- transfer processes to produce N2 and CO2. After highlighting the fundamental reaction principles of several typical energy-efficient anodic reaction-assisted H2 production systems, the preparation methods, regulatory strategies, and composition/structure-performance relations of advanced catalysts for these systems are introduced[153-163]. In addition, the performance of these energy-saving H2 production systems is summarized in Table 1.
Comparison of the performance of energy-efficient H2 production systems
| Catalysts | Electrolyte | HER performance | Performance of anodic reactions | Performance of cell | Stability | Ref. | |
| MOAHP | Co-doped Rh | 1 M KOH + 1 M CH3OH | E10 = 2 mV | 889 mA·mg-1 | E10 = 1.545 V | 12 h @ -0.1 V (HER) | [74] |
| 1 M KOH (HER) | 14 h @ 1.4 V (device) | ||||||
| NMS/CC | 1 M KOH + 1 M CH3OH | E10 = 106 mV | E100 = 1.38 V | E50 = 1.69 V | 120 h @ 1.4 V (MOR) | [81] | |
| 24 h @ 1.8 V (device) | |||||||
| PdIn bimetallene | 1 KOH + 1 M CH3OH | E10 = 171 mV | 2.27 A·mg-1Pd | E100 = 1.26 V | 3,600 s @ -0.2 V (MOR) | [77] | |
| 15 h @ 10 mA·cm-2 (HER) | |||||||
| 15 h @ 100 mA·cm-2 (device) | |||||||
| Pt-CoTe/NC-800 | 0.5 M H2SO4 + 1 M CH3OH | E10 = 28 mV | / | E10 = 0.68 V | 22.9 mA·cm-2 after 3,600 s (MOR) | [55] | |
| 30 h @ 10 mA·cm-2 (HER) | |||||||
| 18 h @ 0.68 V (device) | |||||||
| NiFe2O4/NF | 1 M KOH + 0.5 M CH3OH | E50 = 234 mV | E100 = 1.43 V | E100 = 1.68 V | 15 h @ -0.6 V (HER) 7 h @ 2 V (seawater device) | [86] | |
| E100 = 1.74 V (seawater) | |||||||
| HOAHP | Ni(Cu)/NF | 1.0 M KOH + 0.5 M N2H4 | / | E50 = 204 mV | E100 = 0.41 mV | 10 h @ 100 mA·cm-2 (device) | [39] |
| Cu1Ni2-N | 1.0 M KOH + 0.5 M N2H4 (HzOR and device) | E10 = 71.4 mV | E10 = 0.5 V | E10 = 0.24 V | 35 h @ 10 mA·cm-2 (HzOR) | [40] | |
| 70 h @ 10 mA·cm-2 (device) | |||||||
| 1.0 M KOH (HER) | 60 h @ 10 mA·cm-2 (HER) | ||||||
| Rh/NCBs | 1.0 M KOH + 0.5 M N2H4 (HzOR and device) | E10 = 77 mV | E10 = 72 mV | E10 = 0.149 V | 6,000 s @ 0.03 V (HZOR) | [88] | |
| 71% after 6,000 s (HER) | |||||||
| 6,000 s @ 20 mA·cm-2 (device) | |||||||
| 1.0 M KOH (HER) | |||||||
| Ni3N-Co3N PNAs/NF | 1.0 M KOH + 0.5 M N2H4 (HzOR and device) | E10 = -43 mV | E10 = -83 mV | E10 = 0.071 V | 40 h @ -0.105 V (HER) | [58] | |
| 1.0 M KOH (HER) | 40 h @ -0.04 V (HzOR) | ||||||
| 20 h @ -0.14 V (device) | |||||||
| Fe-CoNiP@NC | 1 M KOH + 0.5 M N2H4 seawater | E1000 = 0.28 V | E1000 = 0.49 V | E1000 = 0.56 V | 100 h @ 500 mA·cm-2 (device) | [102] | |
| 1 M KOH seawater (HER) | |||||||
| UOAHP | N-Co9S8/Ni3S2/NF | 1 M KOH + 0.5 M urea | E10 = 111 mV | E10 = 1.37 V | E10 = 1.4 V | 20 h @ 20 mA·cm-2 (HER) | [122] |
| 20 h @ 20 mA·cm-2 (UOR) | |||||||
| 50 h @ 1.77 V (device) | |||||||
| N-FePS3 | 1 M KOH + 0.33 M urea | E10 = 267 mV | E10 = 756 mV | E10 = 1.26 V | 36 h @ 50 mA·cm-2 (device) | [9] | |
| 1 M KOH (HER) | |||||||
| Fe-Co0.85Se/FeCo LDH | 1 M KOH + 0.5 M urea | E500 = 0.274 V | E500 = 1.48 V | E300 = 1.57 V | 50 h @ 10 mA·cm-2 (HER) | [126] | |
| 60 h @ 10 mA·cm-2 (UOR) | |||||||
| 60 h @ 100 mA·cm-2 (device) | |||||||
| FeNi-MOF NSs | 1 M KOH + 0.33 M urea | E100 = 288 mV | E100 = 155 mV | E100 = 1.85 V | 93.6% after 10 h @ 1.48 V (device) | [130] | |
| Ovac-V-Ni(OH)2 | 1 M KOH + 0.33 M urea | / | E100 = 1.47 V | E10 = 1.5 V | 110 h @ 1.37V (UOR) | [66] | |
| Others | Fe2P/SSM | 1 M KOH + 0.5M glucose | / | E10 = 1.35 V | E10 = 1.22 V | 24 h @ 10 mA·cm-2 (device) | [141] |
| Ni-Mo-N/CFC | 1 M KOH + 0.1 M glycerol | E10 = 43 mV | E10 = 1.3 V | E10 = 1.36 V | 11 h @ 10 mA·cm-2 (device) | [143] | |
| Co4N@CeO2 | 1 M KOH + 0.3 M HMF | E100 = 167 mV | E100 = 1.32 V | E10 = 1.33 V | / | [144] |
OUTLOOK
Although many high-performance noble and non-noble metal-based catalysts have been developed for these H2 production systems, several challenges remain in this emerging and fascinating field. First, to reduce the overall operating voltage for H2 production, the electrocatalysts are expected to have high intrinsic activity for both the anodic and cathodic reactions. However, the anodic reaction (e.g., MOR, UOR, and HzOR) and the HER occurring at the cathode undergo varying catalytic pathways and involve different reaction intermediates (e.g., CH2O* and CHO* for MOR, N2H3* and N2H2* for HzOR) adsorption/desorption at the active sites[164-169]. This poses a great challenge to the design and synthesis of highly active bifunctional catalysts. Modulation of the type, coordination environment, and geometric structure of active sites regulating its energetics with reaction intermediates can enhance intrinsic bifunctional catalytic activity. Strategies for precise atomic-scale regulation of catalysts need to be further developed to meet the needs of practical applications of energy-saving H2 production technologies. Additionally, factors such as dimensions, morphology, and pore structure also have an important impact on the accessibility of active sites and mass transfer during catalysis, which also need to be taken into account for the design of high-performance catalysts. Consequently, catalyst design and synthesis based on traditional synthetic science requires a great deal of trial and error. Using machine learning (ML) to predict the optimized active site species and coordination environment/geometry combined with high-throughput experimental techniques is expected to improve the efficiency of developing high-performance bifunctional catalysts for energy-saving H2 production.
Another concern is the surface rearrangement or reconfiguration of catalysts during the catalytic process. For example, transition metal oxides and hydroxides in alkaline electrolytes are sometimes partially converted to (oxy)hydroxides in alkaline electrolytes. Advanced in-situ and operando characterization techniques, such as in situ ambient-pressure X-ray photoelectron spectroscopy and operando Raman spectra, are highly in demand for investigating the true catalytically active species and exploring the structure-activity relationship. In addition, H2-producing systems, coupled with other anodic reactions, sometimes produce products that dissolve into the electrolyte. For example, the HMFOR-HER cell may produce 2,5-furandicarboxylic acid, and the GOR-HER cell may produce gluconic acid and arabinose. To enhance the economics and atomic utilization of H2-producing systems, it is crucial to carefully tune the composition and structure of catalysts to improve the selectivity of specific products. Moreover, improving these energy-saving H2-producing devices to efficiently separate specific products from the electrolyzer is still a big challenge.
Finally, in order to fully unravel the intricate relationship between the atomic and electronic structures of catalysts and their corresponding catalytic activity in HER electrocatalysts, it is imperative to undertake extensive and meticulous DFT simulations. These simulations offer powerful tools to explore the energetics and kinetics of the reaction, providing invaluable insights into the fundamental mechanisms and reaction pathways involved. However, it is currently uncommon for researchers to consider the solvation effect or pH gradient or both in the DFT model, resulting in simulation outcomes that deviate from actual experimental data. In particular, in some cases, it even contradicts the experimental findings. This discrepancy between theoretical predictions and experimental observations can lead to a lack of consistency and hinder the elucidation of the HER mechanism. To overcome this limitation, it is crucial to enhance the accuracy of DFT simulations by considering the influence of solvation and pH gradients, enabling a more comprehensive understanding of the underlying processes and facilitating the development of robust and reliable theoretical models for HER electrocatalysis. In addition, by examining various catalyst compositions, surface configurations, and reaction conditions, the DFT simulations can elucidate the intricate interplay between catalyst structures, surface reactivity, and overall catalytic performance, paving the way for the rational design and optimization of highly efficient and sustainable electrocatalysts for HER electrolysis. Furthermore, we must also establish precise experimental and DFT calculation methods to identify the contributions of other effects (e.g., structural strain and defect engineering) on activity.
DECLARATIONS
Authors’ contributions
Prepared and revised the draft: Liu W, Niu X, Tang J, Liu Q, Luo J, Liu X, Zhou Y
All authors contributed to the discussion of the draft.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LZ21E020003), the National Natural Science Foundation of China (22275166 and 22075211), and the Zhejiang Province Key Research and Development Project (2023 C01191).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2023.
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Liu W, Niu X, Tang J, Liu Q, Luo J, Liu X, Zhou Y. Energy-efficient anodic reactions for sustainable hydrogen production via water electrolysis. Chem Synth 2023;3:44. http://dx.doi.org/10.20517/cs.2023.28
AMA Style
Liu W, Niu X, Tang J, Liu Q, Luo J, Liu X, Zhou Y. Energy-efficient anodic reactions for sustainable hydrogen production via water electrolysis. Chemical Synthesis. 2023; 3(4): 44. http://dx.doi.org/10.20517/cs.2023.28
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Liu, Wenxian, Xinxin Niu, Jiawei Tang, Qian Liu, Jun Luo, Xijun Liu, Yingtang Zhou. 2023. "Energy-efficient anodic reactions for sustainable hydrogen production via water electrolysis" Chemical Synthesis. 3, no.4: 44. http://dx.doi.org/10.20517/cs.2023.28
ACS Style
Liu, W.; Niu X.; Tang J.; Liu Q.; Luo J.; Liu X.; Zhou Y. Energy-efficient anodic reactions for sustainable hydrogen production via water electrolysis. Chem. Synth. 2023, 3, 44. http://dx.doi.org/10.20517/cs.2023.28
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