Nonprecious Bimetallic Iron–Molybdenum Sulfide Electrocatalysts for the Hydrogen Evolution Reaction in Proton Exchange Membrane Electrolyzers

Crystalline Phase Engineering to Modulate the Interfacial Interaction of the Ruthenium/Molybdenum Carbide for Acidic Hydrogen Evolution

Ruthenium (Ru) is an ideal substitute to commercial Pt/C for the acidic hydrogen evolution reaction (HER), but it still suffers from undesirable activity due to the strong adsorption free energy of H* (ΔGH*). Herein, we propose crystalline phase engineering by loading Ru clusters on precisely prepared cubic and hexagonal molybdenum carbide (α-MoC/β-Mo2C) supports to modulate the interfacial interactions and achieve high HER activity. Advanced spectroscopies demonstrate that Ru on β-Mo2C shows a lower valence state and withdraws more electrons from the support than that of Ru on α-MoC, indicative of a strong interfacial interaction. Density functional theory reveals that the ΔGH* of Ru/β-Mo2C approaches 0 eV, illuminating an enhancement mechanism at the Ru/β-Mo2C interface. The resultant Ru/β-Mo2C exhibits an encouraging performance in a proton exchange membrane water electrolyzer with a low cell voltage (1.58 V@ 1.0 A cm-2) and long stability (500 h@ 1.0 A cm-2).

Corrosion resistance and earth-abundance FeS-based heterojunction catalyst for seawater splitting at industrial grade density

The strategic progression toward highly efficient transition metal electrocatalytic electrodes is crucial to achieving efficiency and long-term stability in hydrogen production from authentic seawater sources. This work reports the development of a self-supporting, heterogeneous and corrosion-resistant iron sulfur-based catalytic electrode via a streamlined, one-step process involving sulfide etching and electroless plating on an iron foam substrate (IF). This new electrode, named NiS-FeS@IF, involves a nanostructured NiS-FeS catalytic material that combines in situ, resulting in a thin, ultrathin nanospherical layer on the IF. This construction has low overpotentials of merely 322 mV for the hydrogen evolution reaction (HER) and 563 mV for the oxygen evolution reaction (OER) with a current density of 500 mA cm-2 in alkaline simulated seawater electrolytes. Importantly, the NiS-FeS@IF electrode enduring more than 500 h at an industrial grade high current density of 1 A cm-2 without noteworthy performance deterioration. The unique and uniformly dispersed morphology of NiS-FeS facilitates intensified interfacial electron transfer, optimizes active site exposure and provides efficient channels for the rapid release and mass transfer of gas bubbles. This work introduces a novel approach for the facile preparation of efficient electrode materials.

Triple roles of Ni(OH)2 promoting the electrocatalytic activity and stability of Ni3S4@Ni(OH)2 in anion exchange membrane water electrolyzers

Developing high performance and durable electrocatalysts is crucial for the practical application of large-scale water splitting under high current density. Here, we constructed a Mott-Schottky heterojunction bifunctional electrocatalyst coating of Ni3S4 with Ni(OH)2 thin film supported on Ni foam substrate (Ni3S4@Ni(OH)2) for anion exchange membrane water electrolyzers (AEMWEs). Remarkably, the η500 is as low as 274.6 mV toward the hydrogen evolution reaction and 423.8 mV toward the oxygen evolution reaction. AEMWEs deliver a stable performance that achieves current densities of 500 and 1000 mA cm-2 at a cell voltage of 1.84 and 1.95 V, respectively. In particular, the Ni3S4@Ni(OH)2 exhibits durable stability for 100 h at 500 mA cm-2 without significant degradation and uses 0.75 kW·h of electricity less than commercial Ni foam electrode to produce each standard cubic meter of hydrogen gas at 500 mA cm-2. The excellent performance is ascribed to the triple roles of Ni(OH)2, which prevent the inner Ni3S4 from decomposing during the reaction process, promoting the dissociation of water and formation of adsorbed hydrogen intermediate and accelerating electron transfer ability due to the Mott-Schottky heterojunction between Ni(OH)2 and Ni3S4. This work sheds light on the development of advanced bifunctional electrocatalysts based on non-precious transition metals for AEMWEs.

Oxygen evolution reaction on MoS2/C rods-robust and highly active electrocatalyst

Recently, water oxidation or oxygen evolution reaction (OER) in electrocatalysis has attracted huge attention due to its prime role in water splitting, rechargeable metal-air batteries, and fuel cells. Here, we demonstrate a facile and scalable fabrication method of a rod-like structure composed of molybdenum disulfide and carbon (MoS2/C) from parent 2D MoS2. This novel composite, induced via the chemical vapor deposition (CVD) process, exhibits superior oxygen evolution performance (overpotential = 132 mV at 10 mA cm-2and Tafel slope = 55.6 mV dec-1) in an alkaline medium. Additionally, stability tests of the obtained structures at 10 mA cm-2during 10 h followed by 20 mA cm-2during 5 h and 50 mA cm-2during 2.5 h have been performed and clearly prove that MoS2/C can be successfully used as robust noble-metal-free electrocatalysts. The promoted activity of the rods is ascribed to the abundance of active surface (ECSA) of the catalyst induced due to the curvature effect during the reshaping of the composite from 2D precursor (MoS2) in the CVD process. Moreover, the presence of Fe species contributes to the observed excellent OER performance. FeOOH, Fe2O3, and Fe3O4are known to possess favorable electrocatalytic properties, including high catalytic activity and stability, which facilitate the electrocatalytic reaction. Additionally, Fe-based species like Fe7C3and FeMo2S5offer synergistic effects with MoS2, leading to improved catalytic activity and durability due to their unique electronic structure and surface properties. Additionally, turnover frequency (TOF) (58 1/s at the current density of 10 mA cm-2), as a direct indicator of intrinsic activity, indicates the efficiency of this catalyst in OER. Based onex situanalyzes (XPS, XRD, Raman) of the electrocatalyst the possible reaction mechanism is explored and discussed in great detail showing that MoS2, carbon, and iron oxide are the main active species of the reaction.

High-performance alkaline water electrolyzers based on Ru-perturbed Cu nanoplatelets cathode

Alkaline electrolyzers generally produce hydrogen at current densities below 0.5 A/cm2. Here, we design a cost-effective and robust cathode, consisting of electrodeposited Ru nanoparticles (mass loading ~ 53 µg/cm2) on vertically oriented Cu nanoplatelet arrays grown on metallic meshes. Such cathode is coupled with an anode based on stacked stainless steel meshes, which outperform NiFe hydroxide catalysts. Our electrolyzers exhibit current densities as high as 1 A/cm2 at 1.69 V and 3.6 A/cm2 at 2 V, reaching the performances of proton-exchange membrane electrolyzers. Also, our electrolyzers stably operate in continuous (1 A/cm2 for over 300 h) and intermittent modes. A total production cost of US$2.09/kgH2 is foreseen for a 1 MW plant (30-year lifetime) based on the proposed electrode technology, meeting the worldwide targets (US$2-2.5/kgH2). Hence, the use of a small amount of Ru in cathodes (~0.04 gRu per kW) is a promising strategy to solve the dichotomy between the capital and operational expenditures of conventional alkaline electrolyzers for high-throughput operation, while facing the scarcity issues of Pt-group metals.

Efficient acidic hydrogen evolution in proton exchange membrane electrolyzers over a sulfur-doped marcasite-type electrocatalyst

Large-scale deployment of proton exchange membrane (PEM) water electrolyzers has to overcome a cost barrier resulting from the exclusive adoption of platinum group metal (PGM) catalysts. Ideally, carbon-supported platinum used at cathode should be replaced with PGM-free catalysts, but they often undergo insufficient activity and stability subjecting to corrosive acidic conditions. Inspired by marcasite existed under acidic environments in nature, we report a sulfur doping-driven structural transformation from pyrite-type cobalt diselenide to pure marcasite counterpart. The resultant catalyst drives hydrogen evolution reaction with low overpotential of 67 millivolts at 10 milliamperes per square centimeter and exhibits no degradation after 1000 hours of testing in acid. Moreover, a PEM electrolyzer with this catalyst as cathode runs stably over 410 hours at 1 ampere per square centimeter and 60°C. The marked properties arise from sulfur doping that not only triggers formation of acid-resistant marcasite structure but also tailors electronic states (e.g., work function) for improved hydrogen diffusion and electrocatalysis.

Research Progress of Hydrogen Production Technology and Related Catalysts by Electrolysis of Water

As a clean and renewable energy source for sustainable development, hydrogen energy has gained a lot of attention from the general public and researchers. Hydrogen production by electrolysis of water is the most important approach to producing hydrogen, and it is also the main way to realize carbon neutrality. In this paper, the main technologies of hydrogen production by electrolysis of water are discussed in detail; their characteristics, advantages, and disadvantages are analyzed; and the selection criteria and design criteria of catalysts are presented. The catalysts used in various hydrogen production technologies and their characteristics are emphatically expounded, aiming at optimizing the existing catalyst system and developing new high-performance, high-stability, and low-cost catalysts. Finally, the problems and solutions in the practical design of catalysts are discussed and explored.

Ultrafast synthesis of halogen-doped Ru-based electrocatalysts with electronic regulation for hydrogen generation in acidic and alkaline media

Developing a facile and time-saving method for preparing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts can accelerate the practical applications of hydrogen energy. In this study, halogen (X = F, Cl, Br and I) doped Ru-RuO₂ on carbon cloth (CC) (X-Ru-RuO₂/MCC) was synthesized via an ultrafast microwave-assisted method for 30 s. Particularly, the doped Br (Br-Ru-RuO₂/MCC) significantly improved the electrocatalytic performances of the catalyst through the regulation of electronic structures. Then, the Br-Ru-RuO₂/MCC catalyst featured HER overpotentials of 44 mV and 77 mV in 1.0 M KOH and 0.5 M H₂SO₄, and the OER overpotential of 300 mV at 10 mA cm^-2 in 1.0 M KOH. This study provides a novel method for developing of halogen-doped catalysts.

Exploring the Sub-nanoscale Structure of Cobalt Molybdenum Sulfide and the Role of a Cobalt Promoter in Catalytic Hydrogen Evolution

Cobalt-promoted molybdenum sulfide (CoMoS) is known as a promising catalyst for H₂ evolution reaction and hydrogen desulfurization reaction. This material exhibits superior catalytic activity as compared to its pristine molybdenum sulfide counterpart. However, revealing the actual structure of cobalt-promoted molybdenum sulfide as well as the plausible contribution of a cobalt promoter is still challenging, especially when the material has an amorphous nature. Herein, we report, for the first time, on the use of positron annihilation spectroscopy (PAS), being a nondestructive nuclear radiation-based method, to visualize the position of a Co promoter within the structure of MoS at the atomic scale, which is inaccessible by conventional characterization tools. It is found that at low concentrations, a Co atom occupies preferably the Mo-vacancies, thus generating the ternary phase CoMoS whose structure is composed of a Co-S-Mo building block. Increasing the Co concentration, e.g., a Co/Mo molar ratio of higher than 1.12/1, leads to the occupation of both Mo-vacancies and S-vacancies by Co. In this case, secondary phases such as MoS and CoS are also produced together with the CoMoS one. Combining the PAS and electrochemical analyses, we highlight the important contribution of a Co promoter to enhancing the catalytic H₂ evolution activity. Having more Co promoter in the Mo-vacancies promotes the H₂ evolution rate, whereas having Co in the S-vacancies causes a drop in H₂ evolution ability. Furthermore, the occupation of Co to the S-vacancies leads also to the destabilization of the CoMoS catalyst, resulting in a rapid degradation of catalytic activity.

Water Electrolysis toward Elevated Temperature: Advances, Challenges and Frontiers

Since severe global warming and related climate issues have been caused by the extensive utilization of fossil fuels, the vigorous development of renewable resources is needed, and transformation into stable chemical energy is required to overcome the detriment of their fluctuations as energy sources. As an environmentally friendly and efficient energy carrier, hydrogen can be employed in various industries and produced directly by renewable energy (called green hydrogen). Nevertheless, large-scale green hydrogen production by water electrolysis is prohibited by its uncompetitive cost caused by a high specific energy demand and electricity expenses, which can be overcome by enhancing the corresponding thermodynamics and kinetics at elevated working temperatures. In the present review, the effects of temperature variation are primarily introduced from the perspective of electrolysis cells. Following an increasing order of working temperature, multidimensional evaluations considering materials and structures, performance, degradation mechanisms and mitigation strategies as well as electrolysis in stacks and systems are presented based on elevated temperature alkaline electrolysis cells and polymer electrolyte membrane electrolysis cells (ET-AECs and ET-PEMECs), elevated temperature ionic conductors (ET-ICs), protonic ceramic electrolysis cells (PCECs) and solid oxide electrolysis cells (SOECs).

Spark Ablation for the Fabrication of PEM Water Electrolysis Catalyst-Coated Membranes

Proton-exchange-membrane (PEM) electrolyzers represent a promising technology for sustainable hydrogen production, owing to their efficiency and load flexibility. However, the acidic nature of PEM demands the use of platinum-group metal-electrocatalysts. Apart from the associated high capital costs, the scarcity of Ir hinders the large-scale implementation of the technology. Since low-cost replacements for Ir are not available at present, there is an urgent need to engineer catalyst-coated membranes (CCMs) with homogeneous catalyst layers at low Ir loadings. Efforts to realize this mainly rely on the development of advanced Ir nanostructures with maximized dispersion via wet chemistry routes. This study demonstrates the potential of an alternative vapor-based process, based on spark ablation and impaction, to fabricate efficient and durable Ir- and Pt-coated membranes. Our results indicate that spark-ablation CCMs can reduce the Ir demand by up to five times compared to commercial CCMs, without a compromise in activity. The durability of spark-ablation CCMs has been investigated by applying constant and dynamic load profiles for 150 h, indicating different degradation mechanisms for each case without major pitfalls. At constant load, an initial degradation in performance was observed during the first 30 h, but a stable degradation rate of 0.05 mV h−1 was sustained during the rest of the test. The present results, together with manufacturing aspects related to simplicity, costs and environmental footprint, suggest the high potential of spark ablation having practical applications in CCM manufacturing.

High-current density alkaline electrolyzers: The role of Nafion binder content in the catalyst coatings and techno-economic analysis

We report high-current density operating alkaline (water) electrolyzers (AELs) based on platinum on Vulcan (Pt/C) cathodes and stainless-steel anodes. By optimizing the binder (Nafion ionomer) and Pt mass loading (mPt) content in the catalysts coating at the cathode side, the AEL can operate at the following (current density, voltage, energy efficiency -based on the hydrogen higher heating value-) conditions (1.0 A cm-2, 1.68 V, 87.8%) (2.0 A cm-2, 1.85 V, 79.9%) (7.0 A cm-2, 2.38 V, 62.3%). The optimal amount of binder content (25 wt%) also ensures stable AEL performances, as proved through dedicated intermittent (ON-OFF) accelerated stress tests and continuous operation at 1 A cm-2, for which a nearly zero average voltage increase rate was measured over 335 h. The designed AELs can therefore reach proton-exchange membrane electrolyzer-like performance, without relying on the use of scarce anode catalysts, namely, iridium. Contrary to common opinions, our preliminary techno-economic analysis shows that the Pt/C cathode-enabled high-current density operation of single cell AELs can also reduce substantially the impact of capital expenditures (CAPEX) on the overall cost of the green hydrogen, leading CAPEX to operating expenses (OPEX) cost ratio <10% for single cell current densities ≥0.8 A cm-2. Thus, we estimate a hydrogen production cost as low as $2.06 kgH2 -1 for a 30 years-lifetime 1 MW-scale AEL plant using Pt/C cathodes with mPt of 150 μg cm-2 and operating at single cell current densities of 0.6-0.8 A cm-2. Thus, Pt/C cathodes enable the realization of AELs that can efficiently operate at high current densities, leading to low OPEX while even benefiting the CAPEX due to their superior plant compactness compared to traditional AELs.

Heterostructured V-Doped Ni2 P/Ni12 P5 Electrocatalysts for Hydrogen Evolution in Anion Exchange Membrane Water Electrolyzers

Regulating the electronic structure and intrinsic activity of catalysts' active sites with optimal hydrogen intermediates adsorption is crucial to enhancing the hydrogen evolution reaction (HER) in alkaline media. Herein, a heterostructured V-doped Ni₂ P/Ni₁₂ P₅ (V-Ni₂ P/Ni₁₂ P₅ ) electrocatalyst is  fabricated through a hydrothermal treatment and controllable phosphidation process. In comparison with pure-phase V-Ni₂ P, in/ex situ characterizations and theoretical calculations reveal a redistribution of electrons and active sites in V-Ni₂ P/Ni₁₂ P₅ due to the V doping and heterointerfaces effect. The strong coupling between Ni₂ P and Ni₁₂ P₅ at the interface leads to an increased electron density at interfacial Ni sites while depleting at P sites, with V-doping further promoting the electron accumulation at Ni sites. This is accompanied by the change of active sites from the anionic P sites to the interfacial Ni-V bridge sites in V-Ni₂ P/Ni₁₂ P₅ . Benefiting from the interface electronic structure, increased number of active sites, and optimized H-adsorption energy, the V-Ni₂ P/Ni₁₂ P₅ exhibits an overpotential of 62 mV to deliver 10 mA cm^-2 and excellent long-term stability for HER. The V-Ni₂ P/Ni₁₂ P₅ catalyst is applied for anion exchange membrane water electrolysis to deliver superior performance with a current density of 500 mA cm^-2 at a cell voltage of 1.79 V and excellent durability.

Hierarchical Metal Sulfides Heterostructure as Superior Bifunctional Electrode for Overall Water Splitting

The development of highly active bifunctional electrocatalysts for overall water splitting is of significant importance, but huge challenges remain. The key element depends on engineering the electronic structure and surface properties of material to achieve improved catalytic activity. Herein, a hierarchical nanowire array of metal sulfides heterostructure on nickel foam (FeCoNiS_x /NF) was designed as a novel type of hybrid electrocatalyst for overall water splitting. The hybrid structure endowed plenty of catalytic active sites, strong electronic interactions, and high interfacial charge transferability, leading to superior bifunctional performance. As a result, the FeCoNiS_x /NF catalyst delivered low overpotentials of 97 and 260 mV at the current density of 50 mA cm^-2 for hydrogen and oxygen evolution reactions, respectively. Moreover, the FeCoNiS_x /NF-based water electrolyzer exhibited a small potential of 1.57 V for a high current density of 50 mA cm^-2 . These results indicate the promising application potential of FeCoNiS_x /NF electrode for hydrogen generation. This work provides a new approach to develop robust hybrid materials as the highly active electrode for electrocatalytic water splitting.

Rational Design of Better Hydrogen Evolution Electrocatalysts for Water Splitting: A Review

The excessive dependence on fossil fuels contributes to the majority of CO₂ emissions, influencing on the climate change. One promising alternative to fossil fuels is green hydrogen, which can be produced through water electrolysis from renewable electricity. However, the variety and complexity of hydrogen evolution electrocatalysts currently studied increases the difficulty in the integration of catalytic theory, catalyst design and preparation, and characterization methods. Herein, this review first highlights design principles for hydrogen evolution reaction (HER) electrocatalysts, presenting the thermodynamics, kinetics, and related electronic and structural descriptors for HER. Second, the reasonable design, preparation, mechanistic understanding, and performance enhancement of electrocatalysts are deeply discussed based on intrinsic and extrinsic effects. Third, recent advancements in the electrocatalytic water splitting technology are further discussed briefly. Finally, the challenges and perspectives of the development of highly efficient hydrogen evolution electrocatalysts for water splitting are proposed.

Epitaxial Growth of High-Energy Copper Facets for Promoting Hydrogen Evolution Reaction

Copper is known as a conductive metal but an inert catalyst for the hydrogen evolution reaction due to its inappropriate electronic structure. In this work, an active copper catalyst is prepared with high-energy surfaces by adopting the friction stir welding (FSW) technique. FSW can mix the immiscible Fe and Cu materials homogenously and heat them to a high temperature. Resultantly, α-Fe transforms into γ-Fe, and low-energy γ-Fe (100) and (110) surfaces induce the epitaxial growth of high-energy Cu (110) and (100) planes, respectively. After the removal of γ-Fe by acid etching, the copper electrode exposes high-energy surface and exhibits excellent acidic HER activity, even being superior to Pt foil at high current densities (>66 mA cm^-2 ). Density functional theory calculation reveals that the high-energy surface favors the adsorption of hydrogen intermediate, thus accelerating the hydrogen evolution reaction. The epitaxial growth induced by FSW opens a new avenue toward engineering high-performance catalysts. In addition, FSW makes it possible to massively fabricate low-cost catalyst, which is advantageous to industrial application.

Polar hydrogen species mediated nitroarenes selective reduction to anilines over an [FeMo]Sx catalyst

We herein present an efficient approach for the chemoselective synthesis of arylamines from nitroarenes and hydrazine over an iron-molybdenum sulfide catalyst ([FeMo]S_x). The heterogeneous hydrogen transfer reduction can be efficiently carried out at 30 °C and provides anilines with 95-99% selectivities. The in situ gas product analysis demonstrates that [FeMo]S_x can catalyze the decomposition of N₂H₄ to H* species, not H₂. Combining with the kinetic analysis of the aniline generation rates from nitrobenzene and intermediates, the nitro group reduction to the nitroso group is confirmed to be the rate-determining step. The positive slope of Hammett's equation suggests that the critical intermediate in the rate-determining step is in the negative state, which suggests that the active H* should be in polar states (H^δ- and H^δ+). These findings will provide a novel route for the synthesis of substituted anilines and broaden the application of MoS_x catalysts under mild conditions.

Synergistic mechanism of Ni(OH)2/NiMoS heterostructure electrocatalyst with crystalline/amorphous interfaces for efficient hydrogen evolution over all pH ranges

Designing and fabricating efficient electrocatalysts is a practical step toward the commercial application of the efficient hydrogen evolution reaction (HER) over all pH ranges. Herein, novel Ti@Ni(OH)₂-NiMoS heterostructure with interface between crystalline Ni(OH)₂ and amorphous NiMoS was rationally designed and fabricated on Ti mesh (denoted as Ti@Ni(OH)₂-NiMoS). Acid etching and calcination experiments helped in accurate elucidation of the synergistic mechanism as well as the vital role on crystalline Ni(OH)₂ and amorphous NiMoS. In acidic solutions, the HER performance of Ti@Ni(OH)₂-NiMoS was mainly attributed to the amorphous NiMoS. In neutral, alkaline, and natural seawater solutions, the HER performance was mainly determined by the synergistic interface behaviors between the Ni(OH)₂ and NiMoS. The crystalline Ni(OH)₂ accelerated water dissociation kinetics, while the amorphous NiMoS provided abundant active sites and allowed for fast electron transfer rates. To deliver current densities of 10 mA·cm^-2 in acidic, neutral, alkaline, and natural seawater solutions, the Ti@Ni(OH)₂-NiMoS required overpotentials of 138, 198, 180 and 371 mV, respectively. This paper provides general guidelines for designing efficient electrocatalyst with crystalline/amorphous interfaces for efficient hydrogen evolution over all-pH ranges.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.