Advanced catalysis for efficient hydrogen production
0 Abstract
Hydrogen energy, characterized by its cleanliness, high energy density, and zero carbon dioxide emissions, is increasingly recognized as a promising alternative to traditional fossil fuels. From this perspective, the highly efficient generation of hydrogen is of paramount importance for mitigating the urgent global energy shortage and environmental challenges. In contrast to conventional fossil fuel-based hydrogen production methods, advanced catalytic technologies that generate hydrogen from water or other renewable resources offer environmentally friendly and sustainable alternatives. In this context, this review exclusively examines the fundamental principles of representative catalytic hydrogen production technologies, encompassing electrocatalytic, piezocatalytic, pyrocatalytic, photocatalytic, and their synergistic catalytic methods at first. Secondly, the latest advancements in the above-mentioned catalytic hydrogen evolution pathways are scrutinized from the perspectives of material composition, structure designs, and hydrogen generation yields. Finally, a comprehensive summary and future outlook for the advancement and practical applications of catalytic hydrogen production technologies are provided. This in-depth review aims to offer both theoretical insights and experimental guidance to researchers in the fields of catalysis, environmental science, energy research, and related areas.
Keywords
INTRODUCTION
The deterioration of the environment and the depletion of fossil fuels have underscored the urgent need for developing renewable and clean sources[1]. Hydrogen energy, owing to its versatile and efficient attributes, high energy density characteristic, as well as clean and low-carbon nature, is regarded as a rising star in future energy[2,3]. At present, the production of hydrogen primarily relies on the decomposition of conventional fossil fuels, a process that results in the emission of significant amounts of carbon dioxide, thereby posing a substantial environmental challenge. In this context, advanced catalytic technologies for hydrogen production that are independent of fossil fuels have emerged, encompassing electrocatalytic, piezocatalytic, pyrocatalytic, photocatalytic, and their synergistic catalytic hydrogen production methods, and significant progress have been observed in recent decades[4]. For example, electrocatalytic hydrogen production technology lies in the conversion of electrical energy into chemical energy through electrocatalysts on the electrodes, achieving the rapid decomposition of water to produce hydrogen and oxygen[5]. Piezocatalytic hydrogen production is a process that harnesses the built-in electric field generated by piezoelectric materials under mechanical stress to facilitate the water splitting reaction, thereby producing hydrogen in the absence of external power sources[6]. Pyrocatalytic hydrogen production is realized in the condition of temperature fluctuations in which pyroelectric materials generate pyroelectric polarization charges and consequently catalyze the decomposition of water to yield hydrogen[7]. Hydrogen production via photocatalytic technology is accomplished using semiconductor photocatalysts that can generate electron-hole pairs upon light irradiation. These pairs subsequently engage in redox reactions with water, thereby facilitating the generation of hydrogen[8]. Moreover, plenty of catalytic materials with subtle structure and composition designs have been developed, and fascinating synergistic catalytic hydrogen production strategies have emerged in recent years. Therefore, it is imperative to systematically scrutinize the research progress in this field.
Through an extensive review of the literature, we have identified that existing studies predominantly focus on the advancements within a single specific technology, photocatalytic, electrocatalytic, piezoelectric, pyroelectric, or photoelectrocatalytic technology. In these reviews, hydrogen production frequently emerges as a key application area for the specific technology. For example, Teng et al. elaborated various coupling methods for photocatalytic hydrogen production, whereas the coupling among photocatalysis with other catalytic methods was not discussed[9]. Djellabi et al. summarized the advancements in multifunctional XTiO3 perovskite-type oxides that exhibit photocatalytic and piezoelectric properties and their applications in environmental remediation and energy production[10]. Zhang et al. reviewed the progress and applications of pyroelectric materials in the fields of pyroelectric sensing and hydrogen production[11]. Although these papers have underscored recent advancements in specialized catalytic materials and their applications across energy, environmental, and related domains, the interconnections among piezocatalytic, photocatalytic, pyrocatalytic, and electrocatalytic hydrogen production processes are not scrutinized. In existing reviews, there is an absence of thorough comparative analyses and comprehensive summaries regarding the aforementioned catalytic technologies for hydrogen production.
Hence, this review aims to provide a systematic overview of recent advancements in the field of catalytic hydrogen production. The fundamental principles of advanced electrocatalytic, piezocatalytic, pyrocatalytic, and photocatalytic hydrogen production, as well as their synergistic effects, have been systematically examined, as shown in Figure 1. The representative catalysts with tailored composition, structure, and morphology designed for highly efficient hydrogen generation are mentioned. Furthermore, this paper provides a comprehensive summary and offers a forward-looking perspective on future directions from the viewpoints of mechanism investigation, material design, and energy conversion efficiency to facilitate the development and application of hydrogen production technologies.
Figure 1. Schematic representation of advanced catalytic technologies for hydrogen production.
ELECTROCATALYTIC HYDROGEN PRODUCTION
Basic principle of electrocatalysis
Electrocatalytic hydrogen production technology mainly involves water electrolysis for hydrogen generation, which lies in the conversion of electrical energy into chemical energy through electrocatalysts on the electrodes, thereby achieving the decomposition of water to produce hydrogen and oxygen. This process includes two half-reactions: the hydrogen evolution reaction (HER) occurring at the cathode and the oxygen evolution reaction (OER) occurring at the anode[12]. The reaction process of the HER differs in acidic and alkaline electrolytes, yet both conditions entail a two-electron transfer mechanism and a
Cathode (reduction reaction): 4H2O + 4e- → 2H2 ↑ + 4OH-;
Anode (oxidation reaction): 4OH- - 4e- → 2H2O + O2 ↑;
Total reaction formula: 2H2O → 2H2 ↑ + O2 ↑.
Under alkaline conditions, the kinetics of HER is generally slower than that under acidic conditions due to the lower concentration of protons in alkaline media. Therefore, the HER process requires the dissociation of water near the catalyst surface to obtain protons, and the reaction pathway of HER may proceed via the Volmer-Tafel mechanism or the Volmer-Heyrovsky mechanism. As a result, the activity and exchange current density of catalysts in alkaline solutions are lower compared to those observed under acidic conditions. The corresponding HER and OER, which involve the transfer of protons (H+), can be described as follows:
Cathode (reduction reaction): 4H+ + 4e- → 2H2 ↑;
Anode (oxidation reaction): 2H2O - 4e- → O2 ↑ + 4H+;
Total reaction formula: 2H2O → 2H2 ↑ + O2 ↑.
Advancements in electrocatalytic hydrogen production
Owing to its high current density, stability, relatively low equipment costs, and ease of operation, electrocatalytic HER has garnered significant attention, and plenty of fascinating electrocatalysts exhibiting high HER performance have been developed, which can be mainly classified into the following five types: precious metal-based, transition metal-based, non-metal-based catalysts, as well as metal-organic frameworks-based and covalent organic frameworks-based catalysts. Some of the typical electrocatalysts for hydrogen production are summarized in Table 1.
Summary of typical catalytic H2 production methods and corresponding representative catalysts with specific hydrogen evolution efficiencies
| Methods | Materials | Experimental conditions | H2 production efficiency | Ref. |
| Electrocatalysis | 2D MoSe2 | 0.5 M H2SO4 solution | 105 mV (vs. RHE) at 10 mA cm -2 | [15] |
| P-mAuRh | 1 M KOH containing 0.33 M urea, loading 0.5 mg cm-2 | 1.47 V (vs. RHE) at 100 mA cm -2 | [17] | |
| Pt7IrNB | N2-saturated 0.5 M H2SO4 solution, a conventional three-electrode | 20 mV (vs. RHE) at 10 mA cm-2 | [18] | |
| CoxFe1-xP | 1 M KOH solution, a modified glassy carbon rotation disk electrode (5 mm diameter) | 294 mV (vs. RHE) at 10 mA cm-2 | [21] | |
| Piezocatalysis | Ti3C2 | 0.5 M Na2SO4 solution, ultrasonic cleaner (45 kHz, 600 W) | 184 μmol g-1 h-1 | [24] |
| UiO-66(Zr)-F4 | 110 W ultrasonic vibration (40 kHz) | 178.5 μmol g-1 h-1 | [25] | |
| R3c LiNbO3-ZnSnO3 | 150 W Xe lamp, 250 W ultrasonic vibration (40 kHz) | 3,453.1 μmol g-1 h-1 | [22] | |
| Pyrocatalysis | Hexagonal CdS | 25~55 ℃ heating-cooling cycles | 4.3 μmol g -1 cycle-1 | [26] |
| PZT sheet | 0.5 M KOH electrolyte, 43~46 °C heating-cooling cycles | 0.654 μmol h-1 | [27] | |
| Photocatalysis | MC-CN | loaded with Rh and Cr2O3 as the cocatalyst, λ > 420 nm | 57 μmol g-1 h-1 | [28] |
| 1D/2D CoS1.097@ZIS | 420 nm and 780 nm cutoff filters, excitation wavelength of 320 nm | 2,632.33 μmol g-1 h-1 | [29] | |
| Photoelectrocatalysis | Cu1/TiO2 | 0.1% v/v concentration of triethanolamine, 300 W xenon lamp with an AM 1.5 G filter | 9.6 μmol h-1 | [30] |
| HOFs@Fe3+ | 20 min under visible light irradiation | 21.55 mmol g-1 h-1 | [31] | |
| In2O3/ZnIn2S4 | H2PtCl6 added to produce Pt as co-catalyst, visible light irradiation | 2.18 mmol g-1 h-1 | [32] | |
| Piezo-photocatalysis | Pt/ZnIn2S4/BaTiO3 | 50 mL of 5-Hydroxymethylfurfural aqueous solution (0.1 M), 300 W xenon lamp with a 420 nm cutoff filter, 100 W ultrasonication | 1,335.3 μmol g-1 h-1 | [62] |
| SrTiO3/BaTiO3 nanofiber | 0.5 M Na2SO4 solution, 300 W xenon lamp with intensity of 100 mW⋅cm-2, 90 W ultrasonication | 1,950.2 μmol g-l h-l | [63] | |
| Pyro-photocatalysis | BaTiO3 nanorods | 0.1 M Na2SO4 and 0.1 M potassium phosphate, 20~50 °C heating-cooling cycles, 300 W xenon lamp with an AM 1.5G filter | 13.44 μmol cm-2 | [65] |
| NaNbO3 films | 20~50 ℃ heating-cooling cycles, visible light irradiation | 4.57 μmol h-1 | [66] | |
| Piezo-pyro-photocatalysis | PVDF/Fe3O4@g-C3N4 | magnetic stirring 200 rpm, 20~68 °C heating-cooling cycles, xenon lamp irradiation | 42.3 μmol h-1 | [64] |
Transition metal-based electrocatalysts
Transition metal-based catalysts, possessing advantages such as metallic-like properties, abundant resources, tunable composition, and high cost-effectiveness, have emerged as highly attractive alkaline electrocatalytic materials, including nickel-based materials (e.g., nickel metal, nickel-based alloys,
MoSe2, as a representative low-dimensional TMD, has aroused intensive attention for electrocatalytic hydrogen production. MoSe2 that exists in a stable 2H phase exhibits semiconductor properties, and the electrocatalytic performance can be observed at its flaky edges, significantly restricting its overall HER performance. To solve this problem, Zhang et al. developed a strategy to comprehensively activate TMDs by inserting two-dimensional MoSe2 and two-dimensional CoP, where the unique interlayer structure increased the active surface area by ten times and the boosted interface enabled rapid ion/electron transport, leading to improved conductivity. As shown in Figure 2A, a notable Tafel slope of 162 mV dec-1 was observed over MoSe2/CoP hybrid, which was much smaller than pristine MoSe2 and CoP, suggesting its improved reaction kinetics of HER. Furthermore, the MoSe2/CoP intercalated hybrids exhibited a remarkably lower charge transfer impedance (6.4 Ω) than those detected over MoSe2 (34 Ω) and porous CoP nanosheets (17.6 Ω) [Figure 2B], which further endowed MoSe2/CoP with an increased current intensity of 10 mA cm-2 and a lowered overpotential of 120 mV [vs. reversible hydrogen electrode (RHE)]
Figure 2. Electrocatalytic hydrogen production. (A) Tafel slope, (B) electrochemical impedance spectra recorded at a biased potential of -160 mV vs. RHE, and (C) overpotentials of MoSe2, CoP, and exfoliated colloidal MoSe2/CoP nanosheets. Figure A-C are quoted with permission from Zhang et al.[15]; (D) Schematic diagram of the fabrication process of the P-mAuRh film/NF, and (E) the corresponding current density of the prepared samples under different potentials. Figure D and E are quoted with permission from Zhang et al.[17]; (F) Schematic representation of the synthesis mechanism of hollow porous PtIr nanobowls, and (G) the corresponding overpotentials of the prepared samples at different current densities. Figure F and G are quoted with permission from Zhang et al.[18]. RHE: Reversible hydrogen electrode.
Non-metal-based electrocatalysts
Non-metal-based catalysts predominately refer to carbon-based electrocatalysts, such as graphene, carbon nanotubes (CNTs), and porous carbon materials. The unique physicochemical properties, e.g., excellent conductivity, a large specific surface area, and good chemical stability, impart these non-metal-based catalysts with great potential in water electrolysis for hydrogen production. For example, Chen et al. demonstrated that the defects on reduced graphene oxide (rGO) over CoP/rGO with platinum-like activity can not only serve as nucleation sites for the uniform growth of CoP nanoparticles, but also generate more isolated electrons, altering the local charge state around the defects, optimizing the Gibbs free energy absorption and thus improving HER activity[1]. This defect-engineered rGO offered a viable alternative to precious metals for electrocatalytic hydrogen production. In addition, Zhang et al. reported a self-supported P-mAuRh film/NF membrane [Figure 2D], which functions as both an anode and cathode catalyst, demonstrating the capability to effectively drive urea-assisted overall water electrolysis[17]. As Figure 2E shows, the constructed P-mAuRh film/NF sample exhibited much higher current density under the same potential than those detected over mAuRh film/NF, NF, Pt/CNF, and RuO2/NF, highlighting the great hydrogen evolution capability of P-mAuRh film/NF. The enhanced electrocatalytic overall water splitting performance was attributed to the synergistic effect of the mesoporous film nanostructure, the optimized bimetallic composition, and the electronic interactions between metals and non-metals. The previous studies indicated that non-metal-based electrocatalysts play a crucial role in facilitating both OER and HER catalysis.
Precious metal-based electrocatalysts
Precious metal-based electrocatalysts primarily consist of Pt, Ir, Pd, Ru, and Rh. Among these, platinum exhibits significantly superior HER performance, attributed to the free energy of hydrogen adsorption intermediates on Pt being closest to the ideal value of zero. In response to the growing demand for environmental protection and renewable energy, the development of electrocatalysts for hydrogen production with superior activity and stability has garnered considerable attention. Recently, Zhang developed a self-template strategy to synthesize hollow and porous bowl-shaped Pt-Ir alloy nanocrystals, which are composed of ultra-ruthenium nanoparticles with an average size of 2.4 nanometers
Metal-organic frameworks and covalent organic frameworks
In addition to the aforementioned electrocatalysts, materials developed based on metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) represent two emerging and promising branches in electrocatalytic hydrogen production, attributed to their high specific surface area, tunable porosity, and structural diversity[19]. These composite materials not only preserve the inherent advantages of carbon-based materials but also exhibit characteristics unique to non-carbon materials, thereby significantly enhancing electrocatalytic hydrogen generation performance. Moreover, it is reported that various strategies can be employed to improve the catalytic performance of the original MOFs, such as increasing the conductivity of MOFs by constructing with conductive particles or substrates, reducing the thickness of MOF nanosheets, introducing structure defects, and so on[20]. For instance, Zhang et al. synthesized a subtle MOF electrocatalyst CoxFe1-X with metallic-like properties due to the presence of phosphorus in the compound, which significantly improved the conductivity of the prepared MOF catalyst. As a consequence, a decreased overpotential of 1.51 V (vs. RHE from 105 to 0.01 Hz with 5 mV) was required for HER[21]. These findings highlight that the designability of structures within MOFs and COFs offers a promising avenue to enhance the HER performance of these two classes of electrocatalytic materials, and it is essential to optimize the defects, surface and conjugate ligands, metal nodes, and functional linkers of MOFs and COFs for realizing high-efficient HER.
Although electrocatalytic hydrogen production is emerging as a promising energy carrier with numerous advantages, it still faces significant challenges compared to traditional fossil fuels. The primary hurdles revolve around the dual constraints of energy conversion efficiency and economic viability. Most currently employed electrocatalysts rely heavily on precious metals such as platinum and iridium, which substantially increase costs and hinder the large-scale deployment of this technology. Additionally, the degradation of catalytic active sites under prolonged exposure to high potentials and highly corrosive environments remains unresolved, impacting the overall system longevity. Addressing these issues necessitates a focus on optimizing material synthesis for large-scale production while ensuring the uniformity and stability of catalysts. At first, a multiscale collaborative design approach is essential, emphasizing material structure regulation, precise synthesis processes, and scalability for mass production. Catalyst design should prioritize the development of robust active site anchoring mechanisms, leveraging strong interactions or confinement effects between the support and active components to mitigate particle migration and agglomeration. Concurrently, interface engineering can be employed to optimize electronic structures, thereby enhancing intrinsic catalytic activity. During synthesis, dynamic control over nucleation, growth, and loading stages must be rigorously enforced, guided by theoretical simulations to fine-tune process parameters and ensure the uniform distribution of active sites. For large-scale production, the development of continuous and modular synthesis routes is critical. This can be achieved through innovative reactor design and process optimization to ensure batch consistency. Enhancing stability requires a dual focus on chemical stability and mechanical durability. Incorporating self-healing functionalities or anti-corrosion interface layers can help mitigate the effects of extreme potentials and corrosive media during electrolysis. Additionally, establishing accelerated aging testing systems will enable the quantification of lifespan decay patterns, providing insights for further improvements. Ultimately, a cross-scale collaborative innovation strategy, spanning materials, processes, and equipment, can achieve comprehensive optimization from atomic-level activity regulation to macro-engineering manufacturing. This holistic approach will balance the cost-effectiveness of large-scale production with the long-term reliability of catalytic systems, paving the way for the widespread adoption of electrocatalytic hydrogen production.
PIEZOCATALYTIC HYDROGEN PRODUCTION
Basic fundamentals of piezocatalytic hydrogen production
There are two basic theories describing the piezocatalytic hydrogen production process: charge screening effect and energy band theory. The former theory attributes the role of the excited electrons and holes of piezoelectric materials under the applied mechanical force, and these electrons and holes then take part in redox reactions with water and oxygen to generate hydrogen [Figure 3A]. The later theory highlights piezocatalytic hydrogen evolution to the screening charges absorbed on the surface of piezoelectric materials, which participate in redox reactions with oxygen and water, giving access to the formation of hydrogen [Figure 3B]. In the above process, the piezoelectric field formed on the surface of piezoelectric material will lead to energy band bending, thereby promoting electrons and holes migration, facilitating hydrogen evolution efficiency. In both theories, the piezocatalytic hydrogen production rates are predominantly dependent on the piezoelectric potential output, which is related to the piezoelectric property of piezocatalytic material and the applied mechanical force.
Figure 3. Schematic diagrams illustrating piezocataytic hydrogen production process based on (A) charge screening effect and (B) energy band theory; (C) Schematic diagram of piezocatalytic hydrogen production process over Bi2Fe4O9 nanosheets. This figure is quoted with permission from Du et al.[6]; (D) Schematic diagram presenting the crystal structures of (i) pristine ZnSnO3 and ZnSnO3 NWs with (ii) moderated and (iii) excessive oxygen vacancies for hydrogen production. This figure is quoted with permission from Wang et al.[22]; (E) Schematic illustration of the piezocatalytic hydorgen production process of BaMoO4-BaWO4-Ag. This figure is quoted with permission from Kuru et al.[23]; (F) Schematic diagram illustrating the mechanism of piezocatalytic degradation of bisphenol A (BPA) and hydrogen production by Ti3C2/SAMs-NH2, and (G) the corresponding H2 production rates. Figure F and G are quoted with permission from Wang et al.[24].
Progress in piezocatalytic hydrogen generation
Compared with electrocatalytic technology, piezocatalytic hydrogen production technology offers a novel approach to generating green hydrogen by harnessing environmental mechanical energy sources, eliminating the need for additional electrical energy input. It makes the direct conversion from mechanical energy into chemical energy possible, and thus has sparked significant research interest. In recent years, significant advancements have been made in the fundamental investigations of piezocatalytic processes, piezocatalytic material modifications to achieve partial and overall water splitting, and the development of strategies for performance enhancement. Some typical piezocatalysts with fascinating hydrogen generation performance are summarized in Table 1.
It is widely reported that piezoelectric materials can be defined into three types - inorganic piezoelectric, organic piezoelectric, and organic-inorganic piezoelectric composites. At current stage, piezoelectric materials reported for hydrogen production are focused on inorganic piezocatalytic materials attributed to their higher electromechanical coupling coefficient. For instance, Du et al. investigated the piezocatalytic hydrogen production performance of Bi2Fe4O9 and found that Bi2Fe4O9 nanosheets with a centrosymmetric structure demonstrated exceptional piezoelectric catalytic performance in the degradation of
Although inorganic piezoelectric materials exhibit potential in hydrogen evolution, traditional inorganic piezoelectric materials frequently encounter limitations, including a wide bandgap and inadequate charge carrier transport, which impede the enhancement of their hydrogen evolution efficiency. In this context, polar materials with spontaneous polarization (Ps) have aroused intensive attention for realizing piezocatalytic hydrogen evolution. For example, Wang et al. constructed a polar self-assembled
Although piezoelectric systems hold immense potential for capturing environmental mechanical energy and converting it into hydrogen energy, significant challenges must be overcome to transition from laboratory research to real-world applications. For instance, the current energy conversion efficiency under mechanical field conditions is less than 5%, far below the theoretical maximum efficiency of over 30%. This gap severely limits the practical application of piezocatalytic technology for hydrogen evolution. Additionally, persistent issues such as fatigue degradation under prolonged cyclic mechanical loads and scalability constraints in hybrid energy harvesting architectures remain unresolved. Addressing these challenges requires the development of piezocatalysts that exhibit long-term stability, cost-effectiveness, and high energy conversion efficiency. Furthermore, the stability of hydrogen evolution in piezocatalytic technology is significantly influenced by mechanical forces, such as ultrasonication and ocean waves. Ultrasonication requires external energy input, while ocean waves exhibit inherent variability, leading to an unstable hydrogen generation process compared to the hydrogen production pathways based on fossil fuels. Therefore, it is imperative to foster collaboration in infrastructure sharing and supply chain optimization to support large-scale hydrogen production initiatives. These strategies are essential for advancing the practical deployment of piezoelectric energy harvesting technologies and unlocking their full potential.
PYROCATALYTIC HYDROGEN PRODUCTION
Basic principle of pyrocatalytic hydrogen generation
The basic principle of pyrocatalytic hydrogen evolution refers to the process that pyroelectric materials generate positive and negative polarization charges under environmental temperature fluctuations, and the absorbed screening charged take part in redox reactions with water and oxygen to form hydrogen [Figure 4A]. This process is similar to piezocatalysis, except that the stimulus is changed from the applied force to the environmental temperature fluctuations. Furthermore, the pyrocatalytic hydrogen production efficiency greatly relies on the generation of pyroelectric polarization charges of pyroelectric materials, depending on the pyroelectric property of pyroelectrics and temperature heating and cooling rates.
Figure 4. Pyrocatalytic hydrogen production. (A) Schematic diagram showing the basic principle of pyrocatalytic hydrogen evolution; (B) The energy levels of CdS nanorod catalysts and the HOMO energy level of 2MBI, and (C) pyrocatalytic hydrogen production pathway over the prepared CdS-2MBI sample under cold-hot alternation excitation. Figure B and C are quoted with permission from Zhang et al.[26]; (D) Schematic diagram of PZT sheet generating pyroelectricity and splitting water into hydrogen. This figure is quoted with permission from Zhang et al.[27]; (E) Schematic illustration of pyrocatalytic hydrogen generation of Au/BaTiO3 nanoparticles driven by surface plasmon local heating, and (F) the corresponding normalized H2 production rates (normalized to the production rate of Au/BaTiO3 NPs) of different samples under the irradiation by a 532 nm nanosecond laser. Figure E and F are quoted with permission from You et al.[28]. HOMO: Highest occupied molecular orbital; PZT: PbZrTiO3.
Advancements in pyrocatalytic hydrogen production
Although pyrocatalytic hydrogen production presents a routine to directly convert temperature into chemical energy and temperature fluctuations are ubiquitous in the environment, the temperature heating or cooling rates are too small to effectively drive pyrocatalytic hydrogen production. At present, the temperature fluctuations employed for driving pyrocatalysis are regulated through controlled heating and cooling systems, severely hindering the practical applications of pyrocatalytic technology in hydrogen production. In this context, significant efforts have been devoted to the design and optimization of the composition, crystal phase, and morphology of pyrocatalytic materials, and plenty of pyrocatalytic materials have been developed for hydrogen production. Some representatives are listed in Table 1.
To realize high-efficient pyrocatalytic hydrogen production, Zhang et al. employed organic molecule
Furthermore, You et al. introduced a localized plasmonic heat source to rapidly and efficiently heat the coke pyrocatalytic material itself by in-situ growing Au nanoparticles on three-dimensional coral-like BaTiO3 nanoparticles[28]. The constructed plasmonic metal/coking pyrocatalytic composite material showcased a rapid temperature heating rate of 100 K within a distance of around 1 μm of an Au nanoparticle (around
The core challenge currently facing pyroelectric catalytic hydrogen production technology lies in the
PHOTOCATALYTIC HYDROGEN PRODUCTION
Basic principle of photocatalytic hydrogen evolution
Photocatalytic principle is developed based on semiconductors. During the photocatalytic hydrogen evolution process, as depicted in Figure 5A, the semiconductor is irradiated by the photons with energy higher or equal to the energy bandgap of semiconductor photocatalysts, and electrons are excited from the valence band (VB) to the CB, generating the same number of holes on the VB. Some of these electrons and holes separate and then migrate to the surface, where the absorbed water undergoes reduction with electrons, producing hydrogen. The other part of electrons and holes get recombined inside or on the surface of photocatalyst, restricting photocatalytic hydrogen production efficiency. It can be inferred that the photocatalytic hydrogen production rate is predominately determined by the light absorption rate, carrier separation and transfer rate, and surface reduction reaction rate.
Figure 5. (A) Schematic diagram presenting the principle of photocatalytic hydrogen production; (B) Energy band diagram and charger transfer behavior of meso-TiO2/Cu3Mo2O9/CuO. This figure is quoted with permission from Chen et al.[30]; (C) Schematic diagram elucidating the photocatalytic H2 production process over TiO2/(MoP/CdS). This figure is quoted with permission from Wang et al.[32]; (D) Mechanism diagram of HOFs@Fe3+ for photocatalytic H2 production. This figure is quoted with permission from Yang et al.[31]; (E) Schematic diagram of photocatalytic H2 evolution over ZrC@ZIS core-shell heterostructure upon near-infrared light (NIR) irradiation. This figure is quoted with permission from Shi et al.[29]; (F) Mechanism of photocatalytic H2 generation over NCDs/CdS. This figure is quoted with permission from Shi et al.[25]; (G) Schematic representation of the structure-performance correlation in MC-CN. This figure is quoted with permission from You et al.[28]. MC-CN: Monomolecular carbohydrazide-carbon nitride; ZIS: ZnIn2S4.
Progress in photocatalytic hydrogen production
Based on the photocatalytic hydrogen production process, plenty of strategies have been proposed for improving hydrogen generation efficiency, focusing on increasing light absorption efficiency, suppressing photoexcited carrier recombination, and accelerating surface reaction kinetics. A variety of photocatalysts have been developed to split water into hydrogen, and some typical representatives are summarized in Table 1. These photocatalysts can be classified into three categories based on their energy bandgap characteristics: ultraviolet-light-responsive photocatalysts with wide bandgaps, visible-light-responsive photocatalysts featuring medium bandgaps or heterojunction structures, and infrared-light-responsive photocatalysts characterized by narrow bandgaps or hybrid compositions.
Ultraviolet-light-responsive photocatalysts
Photocatalysts that respond to ultraviolet (UV) light are predominantly composed of wide bandgap semiconductor materials, such as TiO2 and ZnO. Despite UV light having higher energy compared to visible and infrared light, it constitutes less than 5% of the solar spectrum. Therefore, the conversion efficiency from solar energy to chemical hydrogen over photocatalysts responsive to UV light is severely limited[29]. To address this challenge and circumvent the inherent limitations of wide bandgap photocatalysts, several methodologies focusing on enhancing carrier separation and facilitating carrier transfer have been proposed. For example, Chen et al. conducted an in-depth investigation into the impact of electron transfer on the photocatalytic hydrogen production performance of composite catalyst Cu3@TiO2[30]. They demonstrated that the interactions among TiO2, copper molybdate, and the newly formed CuO constituted a dual-channel electron transfer system (p-n heterojunction and co-catalyst), which significantly increased the transfer rate of photoexcited electrons, giving access to a high hydrogen evolution rate of 9.6 μmol h-1 [Figure 5B]. Owing to the restricted solar light utilization efficiency of wide bandgap photocatalysts, UV light-responsive photocatalysts need to be modified through element doping, defect engineering, or by integration with other narrow bandgap photocatalysts for realizing high-efficient hydrogen production.
Visible-light-responsive photocatalysts
Visible light constitutes approximately 43% of the solar spectrum’s energy[31]. Therefore, the development of photocatalytic materials that can effectively respond to visible light is essential for enhancing photocatalytic efficiency. In this context, various strategies, such as defect engineering, localized surface plasmon resonance, element doping, heterostructure construction, and sensitization, have been proposed to regulate the energy band structure of photocatalysts, and obvious achievements have been witnessed. For example, Wang et al. employed a non-precious metal co-catalyst molybdenum phosphide (MOP) to modify TiO2 and CdS, forming a TiO2/MOP/CdS film, which was then crafted on polymethyl methacrylate (PMMA) acrylic glass [Figure 5C][32]. It exhibited a high photocatalytic hydrogen evolution efficiency of 35.5 mmol g-1 h-1 and long-term stability of up to 150 h under visible light irradiation. Shi et al. constructed a hierarchical 1D/2D CoS1.097@ZnIn2S4 photocatalyst, where 2D ZnIn2S4 nanosheets and 1D flower-like CoS1.097 nanoparticles acted as the shell and core, respectively[29]. The intimate interfacial contact within the structure served as an effective collector for electron transfer, which facilitated the rapid transfer and separation of photogenerated charge carriers and accelerated proton reduction on the surface, consequently enhancing hydrogen production efficiency. Huang et al. utilized elemental red phosphorus (RP) to decorate the surface of TiO2 hollow spheres to broaden the light absorption range of TiO2, and the obtained TiO2@RP heterostructure presented a high hydrogen production rate of 215.5 μmol g-1 h-1 under the simulated sunlight irradiation, which is 11.1 times higher than detected over TiO2[33]. The increased photocatalytic hydrogen efficiency was attributed to the enhanced light absorption due to the hollow sphere structure and RP photosensitizer and promoted carrier separation caused by the interface heterostructure. These findings demonstrate structure design and heterojunction construction can not only increase light absorption rate but also facilitate carrier separation, leading to strikingly enhanced photocatalytic hydrogen production activity.
Furthermore, to simultaneously address environmental pollution and energy shortage issues through photocatalytic technology, bifunctional catalytic materials are developed. Yang et al. constructed a porous nanomaterial (HOFS@Fe3+) where non-noble metal ions Fe3+ were anchored on nanorod-shaped
Infrared-light-responsive photocatalysts
Infrared light accounts for more than 50% of solar energy, which has long been neglected in the field of photocatalysis due to its low photon energy. Therefore, the development of infrared light-responsive photocatalysts is expected to enhance the quantum efficiency of photocatalytic hydrogen production. In recent decades, several methodologies have been proposed to extend the light absorption range of photocatalysts from UV or visible light to infrared light. These methods include integrating wide-bandgap photocatalysts with upconversion luminescence materials, depositing plasmonic nanoparticles on the surfaces of photocatalysts, and introducing various functional materials, among others.
For example, Shi et al.[29] developed an efficient photothermal auxiliary system ZrC@ZnIn2S4 within a
Compared to other hydrogen production technologies, photocatalytic technology faces significant challenges in terms of stability and efficiency[34-36]. These challenges primarily stem from the instability of solar light irradiation, the low energy density of solar energy, and the inadequate stability and efficiency of photocatalysts. The low efficiency of photocatalytic hydrogen evolution can be primarily attributed to inadequate light energy capture, suboptimal charge carrier dynamics, and insufficient surface reaction efficiency. These issues stem from a narrow absorption spectrum and low energy utilization efficiency, rapid recombination of photogenerated electron-hole pairs, and inappropriate energy band positions, respectively. To address these challenges, several strategies can be employed. From a material science perspective, band engineering and microstructure design are crucial to broadening the spectral response range. Introducing gradient energy levels or built-in electric fields within photocatalysts is desired to facilitate the directed separation of charge carriers. Surface modification with co-catalysts or the creation of atomic-level active sites is a promising modulation strategy to lower the reaction energy barrier, accelerate proton adsorption and reduction processes, and inhibit reverse reactions by establishing selective transport pathways. Furthermore, the enhanced stability of photocatalysts can be achieved by optimizing the catalyst surface passivation layer or designing core-shell structures to protect against electrolyte corrosion. Additionally, coupling photothermal synergistic effects or employing external electric field assistance can improve overall energy conversion efficiency. Developing dynamic monitoring methods enables real-time adjustment of reaction conditions, balancing the effects of light intensity, temperature, and mass transfer on hydrogen production performance. By integrating these approaches, the efficiency, stability, and scalability of photocatalytic hydrogen production can be significantly improved, paving the way for more sustainable and economically viable hydrogen generation technologies.
PHOTOELECTROCATALYTIC HYDROGEN PRODUCTION
Basic principle of photoelectrocatalytic hydrogen evolution
Photoelectrocatalysis is a process that occurs under the simultaneous influence of light radiation and an applied bias voltage[37-41]. As displayed in Figure 6A, under the irradiation of light with photon energy greater than or equal to the bandgap energy of the semiconductor, the electrons of semiconductor photocatalyst (photoelectrode) in the VB are excited to the CB, forming photogenerated electron-hole pairs. The photoexcited electrons and holes are spatially separated and transferred toward the edge of the CB and VB, respectively, driven by the applied bias voltage. Subsequently, the separated electrons and holes migrate to the surface of the semiconductor and exclusively flow through an external circuit to the cathode and remain on the surface of the photoanode. As a consequence, holes participate in oxidation reactions (oxidizing water into oxygen and protons) at the photoanode, and electrons take part in reduction reactions (reducing protons into hydrogen) at the cathode. Compared with photocatalysis, photoelectrocatalytic hydrogen production demonstrates superior stability and efficiency as a result of enhanced carrier separation and transfer. The primary chemical reactions occurring during photoelectrocatalytic hydrogen production can be described by the following equations:
Figure 6. Photoelectrocatalytic hydrogen production. (A) Schematic representation of the principle of photoelectrocatalysis;
Semiconductor + hν → e- (conduction band) + h+ (valence band);
Oxidation reaction (OER, photoanode surface):
Acidic conditions: 2H2O + 4h+ → O2 ↑ + 4H+ (Eo = 1.23 V vs. RHE);
Alkaline conditions: 4OH- + 4h+ → O2 ↑ + 2H2O (Eo = 0.40 V vs. RHE);
Reduction reaction (HER, cathode surface):
Acidic conditions: 2H+ + 2e- → H2 ↑ (Eo = 0 V vs. RHE);
Alkaline conditions: 2H2O + 2e- → H2 ↑ + 2OH- (Eo = -0.83 V vs. RHE);
Total reaction (water splitting): 2H2O → 2H2 ↑ + O2 ↑ (ΔG0 = +237.1 kJ/mol).
Recent advancements in photoelectrocatalytic hydrogen production
The development of photoelectrocatalytic hydrogen evolution is intricately linked to photocatalytic and electrocatalytic hydrogen production technologies[42], encompassing mechanism elucidation, material characterization, and practical implementation. Photoelectrocatalytic hydrogen production demonstrates more superior stability and efficiency than photocatalysis, which is resulted from the enhanced carrier separation and transfer rates in the presence of bias voltage. Compared to electrocatalysis, photoelectrocatalytic hydrogen production requires a lower input of electrical energy, offering a more sustainable approach to hydrogen generation. Consequently, this field has garnered increasing attention, leading to the development of numerous advanced photoelectrocatalytic materials and systems, which can be classified into two categories according to the utilization of the applied electric source, routine and
Routine photoelectrocatalytic hydrogen production system
Photoelectrocatalytic materials with heterojunction structures demonstrate superior performance in
Zhang et al. developed a high-performance photocathode heterostructure system for solar WO3@RuSe2 hydrogen evolution, which showcased a short-circuit photocurrent density of 36 mA cm-2 and a half-cell solar hydrogen conversion efficiency of 9.43%[46]. To get deep insight into the obvious photoelectrocatalytic performance, theoretical simulation calculation centering on the hydrogen generation reaction pathway was performed. The calculated result is displayed in Figure 6G. It indicated that Gibbs free energy for H* adsorption over WO3@RuSe2 (0.15 eV) was much lower than that observed over pristine WO3 and RuSe2, implying the more superior capability of WO3@RuSe2 in hydrogen production. Based on this, a possible hydrogen generation pathway was proposed, as depicted in Figure 6H. In detail, the charge redistribution between p-Si and heterostructure WO3@RuSe2 facilitated the bending of the d-band center of WO3@RuSe2 heterostructure toward the Fermi level, enabling H* adsorption. This significantly inhibited the interfacial charge recombination, thereby improving overall efficiency. In addition, Bashiri et al. developed a layered modified SrTiO3@TiO2@Fe2O3 nanorod heterostructure, which exhibited a maximum photocurrent density of 0.63 mA cm-2 and a photoconversion efficiency of 0.69%, leading to the highest hydrogen production rate of 716 μmol cm-2 under visible light irradiation [Figure 6I][47]. The enhanced hydrogen evolution performance was attributed to the efficient separation of electrons and holes in a wide-narrow bandgap composite catalyst system. These results further demonstrate that the heterojunction structure featuring a built-in electric field and a broad light absorption range is beneficial for improving carrier separation and light utilization, thereby boosting catalytic hydrogen production efficiency.
Self-driven photoelectrocatalytic hydrogen production
Photovoltaic devices and nanogenerators present an innovative approach to hydrogen production, wherein the bias voltage required for photoelectrocatalytic hydrogen evolution is self-generated under light irradiation, eliminating the need for an external power supply. The first kind of self-driven photoelectrocatalytic hydrogen production system consists of two main devices, photovoltaic cell and electrolyzer, in which the electricity generated by the photovoltaic cell drives the water splitting reaction in the electrolyzer under light irradiation. Furthermore, the second type of self-driven photoelectrocatalytic hydrogen production system utilizes piezoelectric, pyroelectric, or triboelectric nanogenerators to supply electricity and drive water splitting reactions at the electrode surface under light irradiation.
Based on the first kind of hydrogen production system, Kim et al. developed a hetero-tandem organic optoelectronic device consisting of large (PM6:IT-M) and small bandgap (PM6:Y6) bulk heterojunctions, which presented an open-circuit voltage of 1.84 V and short current of 9.81 mA cm-2, achieving an outstanding power conversion efficiency of 11.7% [Figure 7A][48]. Notably, when the fabricated organic optoelectronic device was integrated with an alkaline electrolyzer comprising NiFeOx(OH)y and Pt (functioning as the oxygen evolution catalyst and hydrogen evolution catalyst), remarkable
Figure 7. Self-driven photoelectrocatalytic H2 production. (A) Schematic representative of the organic optoelectronic device for H2 evolution. This figure is quoted with permission from Kim et al.[48]; (B) Schematic structure illustration of PTB7-Th/EH-IDTBR photocathode and (C) the corresponding H2 evolution rate. Figure A-C are quoted with permission from Bashiri et al.[47]; (D) Schematic diagram of the UDC RF-Pulsed-TENG system, and (E) the corresponding H2 generation rate and Faraday efficiency. Figure D and E are quoted with permission from Gong et al.[50]; (F) Illustration of biomechanical energy conversion into H2 fuel based on the integrated CSTENG and electrochemical reactor, and (G) the continuous clapping converted into electricity for H2 evolution. Figure F and G are quoted with permission from Ghosh et al.[51]; (H) Schematic diagram of the structure of ID-TENG, and (I) the corresponding
As for the second type of hydrogen production system, Gong et al.[50] fabricated an integrated system consisting of triboelectric nanogenerator (TENG) and electrochemical reactor, which was capable of generating green hydrogen at the cathode and formic acid at the anode driven by unidirectional
Compared to photocatalysis and electrocatalysis, photoelectrocatalytic hydrogen production technology offers significant advantages, including enhanced carrier separation efficiency and reduced reliance on external energy inputs[50]. Despite its promise, several challenges hinder the large-scale practical application of this technology. A primary issue is the multidimensional mismatch between light absorption, charge separation, and surface reaction kinetics. The limited light response range of semiconductor materials restricts the generation of sufficient photogenerated charge carriers, while the high rate of charge recombination in bulk and at interfaces far exceeds the effective migration rate, resulting in substantial energy loss as heat. Furthermore, the lack of bifunctional catalytic activity at the electrochemical interface significantly limits overall efficiency. Additionally, photoelectrodes are susceptible to photo-corrosion or structural degradation under the combined stresses of intense light exposure, electrolyte corrosion, and applied bias, making it difficult to achieve the long-term operational stability required for practical applications. To overcome these challenges, several strategies can be employed. For instance, designing gradient band structures or constructing heterojunctions can broaden the spectral absorption range and create built-in electric fields to facilitate the separation of photogenerated electrons and holes. Modulating atomic-level active sites can lower the energy barrier for surface reactions, while introducing selective transport layers can suppress side reactions, thereby improving charge transfer efficiency between the photoelectrode and the electrolyte. Moreover, constructing conductive protective layers or adaptive passivation films can enhance corrosion resistance without compromising catalytic activity. Optimizing the multi-field coupling mechanisms of light, electricity, and heat, starting from the intrinsic fatigue resistance of materials, can provide insights into lifespan prediction and failure protection strategies through advanced multiscale simulation and modeling. By integrating these approaches, the efficiency, stability, and scalability of photoelectrocatalytic hydrogen production would be significantly improved.
SYNERGISTIC CATALYTIC HYDROGEN PRODUCTION
Synergistic catalytic hydrogen production is the coupling among different catalysis, which can be classified into piezo-pyrocatalysis, piezo-photocatalysis, pyro-photocatalysis, and piezo-pyro-photocatalysis. In contrast to one specific catalysis, synergistic catalytic hydrogen production offers more advantages in terms of efficiency, flexibility, energy utilization efficiency, economic viability, and environmental adaptability. The recent advancements in the above-mentioned synergistic catalysis for hydrogen production will be exclusively discussed below.
Piezo-pyrocatalytic hydrogen production
To achieve synergistic hydrogen production through the integration of piezocatalysis and pyrocatalysis, it is crucial to develop ferroelectric catalytic materials that exhibit spontaneous polarization along with high sensitivity to both applied mechanical stress and temperature variations. In this process, environmental mechanical and thermal energy can be harnessed to split water into hydrogen, with the efficiency of hydrogen production being highly dependent on the piezoelectric and pyroelectric potential outputs. From this perspective, strategies aimed at enhancing the piezoelectric and pyroelectric properties are essential for improving the performance of piezo-pyrocatalytic hydrogen production. Therefore, the crystallographic orientation within the piezo-pyrocatalyst, a critical factor that determines the piezoelectric and pyroelectric responses to applied mechanical and thermal stimuli, the electron transfer pathways during catalytic reactions and the exposure of active sites, must be precisely controlled to achieve highly efficient hydrogen evolution[53-55]. For example, Zhang et al. proposed that the crystallographic orientation of hexagonal-phase CdS, governed by its non-centrosymmetric structure (space group P63mc), critically dictated its spontaneous polarization behavior, and thus impacted its pyroelectric effect under thermal fluctuations[26]. Previous studies have shown that surface modification of piezo-pyrocatalysts significantly enhanced interfacial charge transfer efficiency, resulting in an increased hydrogen evolution rate. Therefore, a rational material design approach integrating structural considerations with interfacial modifications to enhance thermoelectric conversion efficiency and catalytic activity is highly desirable.
Despite its promising potential, the investigation of piezo-pyrocatalytic technology remains in its infancy[56], primarily owing to the intrinsic limitations of available materials. On one hand, the intricate coupling mechanism between piezoelectricity and pyroelectricity remains unclear, which poses significant challenges in optimizing the composition, crystal structure, and phase structures of ferroelectric catalysts. On the other hand, the development of theoretical simulation calculations for the coupling among mechanical force, temperature fluctuations, piezoelectricity, and pyroelectricity lacks depth, thereby limiting its ability to guide the exploration and optimization of conditions for achieving highly efficient synergistic
Piezo-photocatalytic hydrogen production
Piezo-photocatalytic hydrogen production can be achieved over piezoelectric photocatalytic semiconductor or piezoelectric-photocatalytic composites under the stimuli of mechanical stress and light irradiation[57]. From the perspective of energy band theory, as depicted in Figure 8A[58], the piezoelectric field formed at the ideal interface between the piezoelectric material and photocatalyst induces either upward or downward energy band bending. This phenomenon, driven by the piezo-phototronic effect, further optimizes carrier separation and transfer dynamics, consequently boosting the efficiency of photocatalytic hydrogen evolution. Specifically, when subjected to simultaneous light irradiation and mechanical strain, a piezoelectric field is generated on the surface of the piezoelectric material. This field acts as a potent driving force, directing photoexcited electrons and holes to migrate in opposite directions, thereby significantly enhancing carrier separation and transfer efficiency [Figure 8B]. Moreover, the induced piezoelectric field not only enhances photocatalytic activity but also enables piezocatalytic hydrogen production through the piezocatalysis mechanism. Consequently, compared to standalone piezocatalytic or photocatalytic hydrogen production technologies, the integrated piezo-photocatalytic approach demonstrates superior performance in terms of energy conversion efficiency, owing to the synergistic interplay between these two catalytic mechanisms.
Figure 8. Synergistic catalysis for H2 production. Schematic diagram of (A) energy band bending of catalyst and (B) carrier behavior within catalyst during piezo-photocatalysis. Figure A and B are quoted with permission from Han et al.[58]; (C) Experimental and simulated piezo-photocatalytic H2 production rate and selectivity of Pt/ZnIn2S4/BaTiO3 heterojunctions, and (D) corresponding H2 evolution mechanism. Figure C and D are quoted with permission from Wang et al.[59]; (E) Schematic illustration of the piezopotential switch mechanisms, and (F) corresponding H2 production rates under ultrasonication-light (U-L) and stirring-light irradiation (O-L) with the working current of 0.8 and 0.2 A, respectively. Figure E and F are quoted with permission from Jin et al.[61]; (G) Schematic representative of the synergistic pyro-photocatalytic water splitting over BaTiO3 photoanode, and (H) corresponding
Based on the above merits of piezo-photocatalysis, plenty of piezo-photocatalysts have been developed. For example, Wang et al.[59] prepared barium strontium titanate (Ba0.7Sr0.3TiO3) nanorod arrays (BST NRs) on a glass substrate as a recoverable catalyst for investigating their piezo-photocatalytic hydrogen production performance. The results suggested that the hydrogen generation efficiency of polarized BST NRs observed under light irradiation in the presence of ultrasonic vibration (411.5 mmol g-1 h-1) was 1.7 times higher than that in the absence of ultrasonic vibration (photocatalysis), strongly demonstrating the significance of synergistic piezo-photocatalysis for efficient hydrogen production[60]. In addition, Yang fabricated a novel piezo-photocatalyst platinum/zinc indium sulfate barium (Pt/ZIS/BaTiO3) with a unique
Furthermore, Jin et al. studied the impact of piezoelectric potential generated by quartz/TiO2 particles of different sizes under ultrasonic vibration on hydrogen production efficiency through experiments and COMSOL simulation[61]. Specifically, the hydrogen production enhancement factor (the ratio of hydrogen production rate under the combined influence of ultrasound and light to that under light alone) was significantly increased over quartz/TiO2 when the size of quartz was decreased from 150 (with a potential output of 1.125 V) to 40 μm (corresponding to 0.3 V) upon the working current of 0.8 A [Figure 8E]. They attributed this phenomenon to the fact that the weak piezoelectric potential (< 1 V) played a positive role in enhancing piezo-photocatalytic hydrogen production performance by promoting the separation of photogenerated charge carriers [Figure 8F]. Conversely, the strong piezoelectric potential (> 1 V) caused the energy bands to bend downwards, reducing the electron reduction capability and thus inhibiting hydrogen production[60]. Moreover, a notably increased hydrogen production enhancement factor of 3.9 was achieved over 150 μm quartz/TiO2 sample under the working current of 0.2 A, which was much 10 times higher than detected under 0.8 A [Figure 8E], further demonstrating the modulation role of piezoelectric potential in adjusting piezo-photocatalytic hydrogen evolution performance. This study corroborated that the hydrogen production efficiency was predominantly determined by the piezoelectric potential output, which was significantly dependent on the grain size of piezoelectric quartz and working currents. Therefore, it is necessary to optimize the grain size and piezoelectric property of piezoelectric component within
Pyro-photocatalytic hydrogen production
Pyro-photocatalytic synergistic hydrogen production process can be achieved over pyroelectric photocatalyst or pyroelectric-photocatalytic composite materials under the co-stimulation of light irradiation and temperature fluctuations, and the pyro-photocatalytic hydrogen production efficiency is predominately modulated by the pyroelectric and photocatalytic properties of the catalyst. The fundamental principle of pyro-photocatalysis is analogous to that of piezo-photocatalysis, with the key difference being that the mechanical strain stimulus is replaced by temperature fluctuations. To achieve high-performance pyro-photocatalytic hydrogen production, it is necessary to optimize the composition of pyro-photocatalyst and the stimuli conditions[62], especially for temperature fluctuations. Thakur et al. constructed the pyroelectric photocatalytic semiconductor BaTiO3 and investigated its pyro-photocatalytic hydrogen production efficiency [Figure 8G][57]. The results indicated that under the stimulation of hot-cool thermal cycles and light irradiation, an efficient overall water splitting reaction with a stoichiometric ratio of 2:1 between H2 and O2 was achieved, demonstrating the great potential of pyro-photocatalysis in hydrogen production [Figure 8H].
To realize self-driven synergistic pyro-photocatalytic hydrogen evolution, Dai et al., for the first time, constructed a photothermal-pyroelectric-photocatalytic multifunctional composite fiber Polyvinylidene fluoride-hexafluoropropylene/carbon nanotube/CdS (PVDF-HFP/CNT/CdS)[7]. In this structure, the photothermal material CNT that was responsive to infrared light was heated under full-spectrum irradiation, and subsequently heated the pyroelectric material
Piezo-pyro-photocatalytic hydrogen production
By coupling the piezoelectric, pyroelectric, and photocatalytic effects, hydrogen production can be achieved in the conditions of mechanical stress[59], temperature fluctuation, and light irradiation[63]. In this process, three types of energy sources are utilized for hydrogen evolution, which will greatly increase the energy conversion efficiency from natural energy to chemical counterpart. In addition, the piezoelectric field
Based on the above merits, Wei et al. constructed magnetically driven PVDF/Fe3O4@g-C3N4 helical
SUMMARY AND OUTLOOK
In summary, hydrogen, characterized by its high energy density, zero carbon dioxide emissions, and environmentally friendly attributes, is considered a promising alternative to traditional fossil fuels, and the development of efficient hydrogen production technology is of great significance for realizing a hydrogen economy in the future. In this context, this review systematically scrutinized the recent advancements in electrocatalytic, piezocatalytic, pyrocatalytic, photocatalytic, photoelectrocatalytic, and their synergistic catalytic hydrogen production processes. The discussion focuses on fundamental principles, catalyst material composition, structural design, functional engineering, energy utilization efficiency, and carrier behavior modulation, among other aspects. Furthermore, the relationships and distinctions between the aforementioned catalytic hydrogen production processes are explored, which would provide theoretical insights and experimental guidance for the beginner in the field of catalytic hydrogen production.
Despite significant progress in the investigation of catalytic mechanisms and the exploration of catalysts, numerous challenges persist in both the theoretical research and practical implementation of hydrogen production. At first, despite the significant potential of synergistic catalysis in enhancing hydrogen production, the underlying mechanisms of piezo-pyrocatalysis, piezo-photocatalysis, pyro-photocatalysis, and piezo-pyro-photocatalysis remain poorly understood. For instance, the coupling mechanism between pyroelectric polarization charges and piezoelectric polarization charges requires investigation. It is important to examine whether there is competition between photoexcited carriers and
Secondly, the current structure and functionality of the developed catalytic materials are relatively rudimentary, which poses significant limitations on the application of synergistic catalytic hydrogen production technology for converting natural low-density energy into chemical hydrogen energy. For example, the piezoelectric properties of most piezoelectric-photocatalytic semiconductors or
Finally, the preparation of catalysts, particularly for heterojunction catalysts and multifunctional composites, typically involves a lengthy and intricate process that is not conducive to large-scale fabrication. This severely extends the production cycle of catalysts, thereby resulting in high manufacturing costs and hindering the practical application of catalytic hydrogen production technology. From the application point of view, developing straightforward yet robust methodologies for the preparation of catalysts that exhibit high-efficiency hydrogen production, excellent stability, and cost-effectiveness is greatly desired.
DECLARATIONS
Authors’ contributions
Conceptualization, writing-reviewing and editing, supervision, funding acquisition. Dai, B.
Investigation, writing-original draft, writing-reviewing and editing: Zhou, C.
Investigation, writing-reviewing and editing: Yin, H.
Visualization, writing-reviewing and editing: Kong, R.
Visualization, writing-reviewing and editing: Wang, H.
Writing-reviewing and editing, supervision, funding acquisition: Xie, Y.
Availability of data and materials
The datasets supporting the findings of this study are available from the corresponding authors upon reasonable request.
Financial support and sponsorship
This work was financially supported by the National Natural Science Foundation of China (Nos. 22202106, 61974071, and 82301104), China Postdoctoral Science Foundation (No. 2023M731772), National Key Research and Development Program of China (Nos. 2021YFA1202904, 2017YFA0205302), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, No. YX030003), and the Jiangsu Provincial Key Research and Development Program (No. BE2018732).
Ethical approval and consent to participate
Not applicable.
Conflicts of interest
The authors declared that there are no conflicts of interest.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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