Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Efficient photocatalytic overall water vapor splitting over amorphous Ni(OH)2/Ni2B heterojunctions

Developing efficient photocatalysts for solar-driven overall water vapor splitting is crucial for sustainable hydrogen production. However, current photocatalytic efficiencies are limited by inadequate hygroscopicity, sluggish proton transport, and rapid recombination of photogenerated electron-hole pairs. Here, we report the successful synthesis of an amorphous Ni(OH)2/Ni2B heterojunction material with a core-shell structure by regulating the reducing environment during the formation of nickel boride. This material exhibits highly efficient overall water vapor splitting performance without any cocatalysts. Under simulated solar irradiation, the optimized sample achieves a hydrogen production rate of 976 μmol·g-1·h-1, with near-stoichiometric evolution of hydrogen and oxygen, an apparent quantum yield of 5.4 %, and a solar-to-hydrogen conversion efficiency of 3.8 %. The enhanced performance is attributed to the unique amorphous/amorphous heterojunction structure that promotes effective charge separation, the abundant surface hydroxyl groups that improve proton transport and hygroscopicity, and the formation of photo-induced frustrated Lewis pairs (FLPs). Our findings shed light on the critical role of amorphous structures and surface chemistry in boosting photocatalytic activity, paving the way for the rational design of advanced photocatalysts for overall water vapor splitting.

From Grotthuss Transfer to Conductivity: Machine Learning Molecular Dynamics of Aqueous KOH

Accurate conductivity predictions of KOH(aq) are crucial for electrolysis applications. OH- is transferred in water by the Grotthuss transfer mechanism, thereby increasing its mobility compared to that of other ions. Classical and ab initio molecular dynamics struggle to capture this enhanced mobility due to limitations in computational costs or in capturing chemical reactions. Most studies to date have provided only qualitative descriptions of the structure during Grotthuss transfer, without quantitative results for the transfer rate and the resulting transport properties. Here, machine learning molecular dynamics is used to investigate 50,000 transfer events. Analysis confirmed earlier works that Grotthuss transfer requires a reduction in accepted and a slight increase in donated hydrogen bonds to the hydroxide, indicating that hydrogen-bond rearrangements are rate-limiting. The computed self-diffusion coefficients and electrical conductivities are consistent with experiments for a wide temperature range, outperforming classical interatomic force fields and earlier AIMD simulations.

3D-Printed Metal Electrodes with Enhanced Bubble Removal for Efficient Water Electrolysis

Improving the water electrolysis efficiency at high current densities is constrained by the structure of available foam and mesh electrodes, which suffer from internal bubble entrapment. Herein, we used laser powder bed fusion-based 3D printing to fabricate Schwarz Diamond (SD) structure nickel electrodes for water electrolysis. After loading with NiMoFeOx as the oxygen evolution reaction catalyst and MoNi4-MoO2 as the hydrogen evolution reaction catalyst, the anion exchange membrane water electrolyzer utilizing SD nickel electrodes achieved a current density of 1 A cm-2 at 1.74 V, outperforming conventional nickel foam and mesh electrode-based electrolyzers in the same conditions and demonstrated durable operation for more than 1000 h. In-situ observations of bubble evolution in the electrolyzer and single-frequency impedance spectra reveal that the 3D-printed SD structure exhibits highly efficient bubble/liquid transport. The present study investigates the potential of 3D printing technology in the fabrication of metallic porous electrodes for efficient water electrolysis.

Crystal facet-induced reconstruction of MoN-supported Co pre-catalysts for optimized active sites and enhanced alkaline hydrogen evolution

The self-reconstruction of electrocatalysts during the cathodic hydrogen evolution reaction (HER) has garnered significant interest due to its impact on microstructure and electrocatalytic efficiency. Understanding the mechanisms driving this transformation is crucial for the development of high-performance HER pre-catalysts. In this study, an efficient Co(OH)2 (001)/MoN (002) heterostructured catalyst is fabricated through the self-reconstruction of the Co/MoN pre-catalyst and the mechanism of facet-induced reconstruction is investigated in detail. This Co/MoN pre-catalyst exhibits an impressive 58 % reduction in overpotential at a constant current density of 100 mA cm-2 over 5 h. It ultimately achieves a low overpotential of 339 mV at 1 A cm-2, outperforming commercial Pt/C under similar current conditions, while maintaining high current activity with 99.4 % retention after 110 h of continuous electrolysis. Operando characterizations and theoretical simulations reveal that metallic Co dissolves rapidly under bias as H+ ions infiltrate the interstitial spaces, and the dissolved Co2+ ions preferentially deposit as Co(OH)2 nanosheets. This deposition aligns with the (001) facet of Co(OH)2 and the prominent (002) plane of the MoN matrix through lattice matching, exhibiting a very low interfacial formation energy. Density-functional theory analysis reveals that the alignment of the crystal facets between Co(OH)2(001) and MoN (002) enhances electron transfer and modulates the interface to boost the water dissociation and hydrogen adsorption activity and kinetics. Our results underscore the importance of precise control over the reconstruction process for cathodic HER and facilitate the development of advanced transition metal-based electrocatalysts for industrial alkaline hydrogen production.

Simple and Scalable Introduction of Single-Atom Mn on RuO2 Electrocatalysts for Oxygen Evolution Reaction with Long-Term Activity and Stability

Electrochemical oxygen evolution reaction (OER) is the bottleneck for realizing renewable powered green hydrogen production through water splitting due to the challenges of electrode stability under harsh oxidative environments and electrolytes with extreme acidity and basicity. Here, we introduce a single-atom manganese-incorporated ruthenium oxide electrocatalyst via a facile impregnation approach for catalyzing the OER across a wide pH range, while solving the stability issues of RuO2. The modified catalyst maintains stability for over 1000 h, delivering a current density of 10 mA cm-2 at a 213 mV overpotential in acid (pH 0), 570 mV in potassium bicarbonate (pH 8.8), and 293 mV in alkaline media (pH 14), demonstrating exceptional durability under various conditions. When used as an anode for realistic water-splitting systems, Mn-modified RuO2 performs at 1000 mA cm-2 with a voltage of 1.69 V (Nafion 212 membrane) for proton-exchange membrane water electrolysis, and 1.84 V (UTP 220 diaphragm) for alkaline water electrolysis, exhibiting low degradation and verifying its substantial potential for practical applications.

Atomic-Scale Confined Synthesis of Ultrathin W2C Nanowires in Single-Wall Carbon Nanotubes for the High-Performance Hydrogen Evolution Reaction

Phase-pure ultrafine W2C nanostructures are promising electrocatalysts but face synthesis challenges due to unclear formation mechanisms and harsh thermodynamics. Here, we reveal the formation mechanism of ultrathin W2C nanowires (NWs) confined in the cavity of single-wall carbon nanotubes (SWCNTs) at the atomic scale by combined in situ transmission electron microscopy and density functional theory calculations. It was found that the hollow core of SWCNTs can control the phase, axial orientation, and diameter of W2C NWs. Leveraging this mechanism, we synthesized SWCNT-encapsulated W2C NWs, WS2-W2C heterostructures, and WS2 NWs (1D@1D), which assembled into free-standing hybrid films. The integrated W2C NWs@SWCNT membrane was primarily tested, exhibiting a low overpotential of 44 mV to reach a current density of 10 mA cm-2 and outstanding durability (500 h at a high current density of 250 mA cm-2 in acidic conditions).

Eccentric Corrosion-Induced Formation of γ-NiFeOOH and γ-NiCoOOH on NiFeCo Alloy for Enhanced OER

An indirect, swift, and easy method of enhancing the oxygen evolution reaction (OER) performance of an economically viable Fe-rich NiFeCo (NFC) alloy has been developed. This approach leverages the anodic potential sweeps applied indirectly to the counter electrode (CE) when one does cathodic hydrogen evolution reaction (HER) on by potential sweeping at the working electrode (WE). In this method, NFC was intentionally corroded indirectly by using it as a CE for the potential sweeping HER experiment done with a Pt WE. The indirectly corroded NFC (NFC_IC) featuring mostly γ-NiFeOOH and γ-NiCoOOH entities on the surface was able to begin the OER at an onset overpotential of 250 mV and reach the benchmark of 10 mA cm-2 at 290 mV, which is 60 and 55 mV lesser than that of the bare NFC and RuO2, respectively, all with exceptionally faster kinetics, as evidenced by a relatively smaller Tafel slope of 30 mV dec-1. These insights into designing a trimetallic alloy-based OER electrocatalyst have opened a previously unknown avenue in the development of advanced self-supported OER electrodes for better and efficient H2 production via water electrolysis.

Structure and Dissociation of Water at the Electrode-Solution Interface Studied by In Situ Vibrational Spectroscopic Techniques

In aqueous electrochemistry, water in contact with charged surfaces is ubiquitous and indispensable, dictating the binding of solutes to electrode surfaces as well as the transport process of protons and electrons in the interfacial region. A comprehensive understanding of the structure and dissociation of interfacial water at the molecular level is extremely important yet challenging, given its critical role in various physical, chemical, and biological processes. In situ vibrational spectroscopic techniques serve as a powerful tool for acquiring the molecular structure of electrode surfaces and probing interfacial reaction mechanisms in real time. In this review, we briefly summarize the latest advances in the electric double layer model and the experimental methods employed at the electrode-solution interface. Particular emphasis is placed on in situ vibrational spectroscopic techniques that have unveiled new insights into the molecular structure of interfacial water across diverse electrode surfaces under ambient conditions. And then, it also provides an overview of recent progress on the subtle relationship between the structure of interfacial water and its dissociation activity, aiming to provide novel insights into the fields of electrochemistry, energy and catalysis.

Built-In Electric Field in Freestanding Hydroxide/Sulfide Heterostructures for Industrially Relevant Oxygen Evolution

Alkaline water electrolysis (AWE), as a premier technology to massively produce green hydrogen, hinges on outstanding oxygen evolution reaction (OER) electrodes with high activity and robust stability under high current densities. However, it is often challenged by issues such as catalytic layer shedding, ion dissolution, and inefficient bubble desorption. Herein, a scalable corrosion-electrodeposition method is presented to synthesize nickel-iron layered double hydroxide (NiFe-LDH)/Ni3S2 heterostructures on nickel mesh, tailored to meet the stringent requirements of industrial AWE. The study underscores the critical role of the built-in electric field (BEF) in optimizing electronic properties, curtailing Fe leaching, and enhancing mass transfer. The resultant NiFe-LDH/Ni3S2 heterostructure manifests remarkable OER performance, with ultra-low overpotentials of 202 mV at 10 mA cm-2 and 290 mV at 800 mA cm-2 in 1.0 m KOH at 25 °C, alongside superior steady-state stability and resistance to reverse current under fluctuating conditions. Furthermore, the performance is further validated in an alkaline electrolyzer, achieving a large current density of 800 mA cm-2 at a cell voltage of 1.908 V, while maintaining excellent stability. This work offers a blueprint for the design of efficient OER electrodes for industrially relevant AWE applications.

Advanced Nickel-Based Gas Diffusion Anode for Zero-Gap Anion-Exchange Membrane Water Electrolyzers

Electrolytic hydrogen production using abundant water and renewable electricity is a key step toward achieving a carbon-neutral economy. Anion-exchange membrane water electrolyzers (AEMWE) present an opportunity to enhance sustainability and reduce the costs of green hydrogen technology. This study focuses on reducing electrical losses in the AEMWE by designing an improved anode catalyst layer. The approach involves modifying nickel foam by using a microporous nickel ink. This modification not only smooths the nickel foam to prevent membrane punctures during compression assembly but also enhances the utilization of the mesoporous NiO (mesoNiO) catalyst in the anode process, namely, the oxygen evolution reaction (OER). The anode leverages a mechanism where both the mesoNiO catalyst and the nickel powder layer participate in the OER, hosting a NiOOH intermediate formed through surface oxidation. By optimization of the mass loading, the design achieves a balance between smooth membrane-electrode contact, reduced kinetic losses during the OER, and efficient ionic transport. As a result, the optimized AEMWE reaches a competitive current density of 2.6 A cm-2 at a cell voltage of 2 V, comparable to the performance of state-of-the-art proton-exchange membrane water electrolyzers. These findings highlight that fluorocarbon membrane-free, zero-gap water electrolyzers with platinum-free anodes can deliver significant advancements in green hydrogen technology. This promising performance encourages further research toward catalyst-free water electrolyzers as the next step in sustainable hydrogen production.

Interfacial effects on metal-organic frameworks for boosting electrocatalytic reactions

Metal-organic framework (MOF) materials exhibit great potential in the field of electrocatalysis due to their high specific surface area, tunable pore structures, and abundant active sites. However, further enhancement of their electrocatalytic performance is often limited by factors such as electron transport efficiency, accessibility of active sites, and interfacial reaction kinetics. Interface engineering strategies have been proposed as a promising strategy for modifying MOF-based catalysts for optimizing their catalytic performance. Significant progress has been made in recent years. Based on this, this review summarizes recent developments in interface modification to enhance MOF materials, focusing on the unique effects induced by the interfacial modification of MOF materials, such as optimizing electron transport and conductivity, increasing the exposure of active sites, improving mass transfer of reactants/products, and stabilizing interfacial structures. Additionally, the applications of various types of MOF-based composite materials for promoting electrocatalytic performance that induced by interfacial effects are also manifested. Finally, the challenges and perspectives of this interesting field are also discussed to offer guidance for the future design of more advanced MOF-based electrocatalysts.

Electrochemical Hydrogen Production Coupling with the Upgrading of Organic and Inorganic Chemicals

Electrocatalytic water splitting powered by renewable energy is a green and sustainable method for producing high-purity H2. However, in conventional water electrolysis, the anodic oxygen evolution reaction (OER) involves a four-electron transfer process with inherently sluggish kinetics, which severely limits the overall efficiency of water splitting. Recently, replacing OER with thermodynamically favorable oxidation reactions, coupled with the hydrogen evolution reaction, has garnered significant attention and achieved remarkable progress. This strategy not only offers a promising route for energy-saving H₂ production but also enables the simultaneous synthesis of high-value-added products or the removal of pollutants at the anode. Researchers successfully demonstrate the upgrading of numerous organic and inorganic alternatives through this approach. In this review, the latest advances in the coupling of electrocatalytic H2 production and the upgrading of organic and inorganic alternative chemicals are summarized. What's more, the optimization strategy of catalysts, structure-performance relationship, and catalytic mechanism of various reactions are well discussed in each part. Finally, the current challenges and future prospects in this field are outlined, aiming to inspire further innovative breakthroughs in this exciting area of research.

Vacancy Engineering Strategy Releases the Electrocatalytic Oxygen Evolution Reaction Activity of High-Entropy Oxides

The sluggish kinetics of oxygen evolution reaction (OER) poses a great challenge to the industrial promotion of electrocatalytic water splitting and zinc-air battery. Herein, we demonstrate that the kinetic limitation of the OER imposed by a conventional adsorbate evolution mechanism can be successfully overcome through activating lattice oxygen in the electrocatalyst. For example, incorporating aluminum (Al) into high-entropy oxides (HEO) remarkably enhances the oxygen vacancy concentration, facilitates the generation of reactive oxygen species, and promotes the deprotonation during the electrochemical OER process, thereby boosting the kinetic reaction. This defect engineering strategy effectively decreases the energy barrier associated with the lattice oxygen oxidation and optimizes the configurational entropy of HEO, resulting in remarkable structural stability. Consequently, the developed HEO decorated with Al (HEO-Al) achieves an overpotential of ∼206 mV at 10 mA cm-2 in water electrolysis and a power density (∼20 mW cm-2) in rechargeable zinc-air battery, with long-term stability of 100 h, realizing an optimal balance between electrocatalytic activity and stability. More importantly, the performances of HEO-Al are significantly superior to those of the HEO counterpart (∼260 mV, ∼1.5 mW cm-2) and commercial ruthenium oxide (∼359 mV, ∼5 mW cm-2), showing great competitiveness and application prospect. These results offer essential inspiration for other electrochemical applications dominated by the OER at the same time.

Nickel and Ferrocene as Catalyst Candidates to Promote an Effective Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is the main bottleneck in water splitting and other key technologies due to its slow kinetics. The development of low-cost, highly active, stable, and more efficient electrocatalysts as an alternative to the commonly expensive and scarce Ir- and Ru-based catalysts used is necessary. The present work reports the preparation of OER catalysts by a straightforward and easy method based on the synergistic effect of Ni/Ferrocene combination that leads to high current densities, lower overpotential, high stability, and low Tafel slope in alkaline conditions. The optimized Ni/Ferrocene catalyst demonstrates exceptional performance, providing a constant potential of ∼1.51 and ∼1.65 V and overpotentials of ∼0.278 and ∼0.420 V for more than 26 and 46 h at current densities of 10 and 100 mA·cm-2, respectively. It represents an important and significant advance in obtaining electrocatalyst materials for OER anodes without the need for previous and tedious synthetic procedures. Only commercial chemicals with low amounts of metals are employed, thus facilitating that this Ni/ferrocene configuration could be scaled up to real devices.

Electron donation from carbon support enhances the activity and stability of ultrasmall ruthenium dioxide nanoparticles in acidic oxygen evolution reaction

Developing non-iridium (Ir)-based electrocatalysts with good stability and activity for acid oxygen evolution reaction (OER) is of great importance for electrocatalytic water splitting. Ruthenium dioxide (RuO2), which has lower price and higher OER activity, has been recognized as an attractive alternative to Ir-based electrocatalyst for acidic OER. However, the stability of most Ru-based electrocatalysts faces a great challenge in acidic condition. Here, a highly stable and active RuO2-based catalyst, tiny RuO2 nanoparticles inlaid onto carbon support (RuO2/C), is successfully prepared for acidic OER. Such a structure can efficiently inhibit the over-growth of RuO2 nanoparticles and prevent the agglomeration of RuO2 nanoparticles. Moreover, it is found that carbon support donate electron to RuO2 nanoparticles, which enhances the OER activity and stability of RuO2 during acidic OER. The RuO2/C exhibits an impressive OER performance with a low overpotential (197 mV at 10 mA cm-2) and low degradation rate (0.035 mV h-1) over a 450-h stability test in 0.5 M H2SO4, which are much better than the commercial Ir/C, RuO2 and the reported Ru-based electrocatalysts. This work provides an efficient strategy to simultaneously improve both stability and activity of Ru-based catalysts for acidic water oxidation.

Tailoring Mesopores on Ultrathin Hollow Carbon Nanoarchitecture with N2O2 Coordinated Ni Single-Atom Catalysts for Hydrogen Evolution

Single-atom catalysts (SACs) offer exceptional atomic utilization and catalytic efficiency, particularly in the hydrogen evolution reaction (HER), where effective mass transport and electronic structure control are critical. However, many SACs suffer from suboptimal hydrogen adsorption energies and limited synergy with the support matrix, which restrict their intrinsic activity and durability. Overcoming these limitations requires an integrated strategy that simultaneously optimizes both the atomic coordination environment and the support architecture. Here, we present a dual-template strategy for synthesizing ultrathin mesoporous hollow carbon (MHC) with tunable mesopores, which enhances ion transport and structural accessibility. Ni single atoms are stabilized within the MHC framework via a tailored N2O2 coordination environment, which fine-tunes the electronic structure of Ni and facilitates efficient hydrogen adsorption and HER kinetics. This coordination environment and the hierarchical porous framework collectively enhance HER activity, significantly reducing the overpotential to 68 mV at 10 mA cm-2 and resulting in remarkable mass activity (5 A mgNi-1 at 50 mV) and enhanced durability over 5000 cycles. Spectroscopic analyses and density functional theory calculations reveal that the N2O2 coordination fine-tunes the electronic structure of Ni, promoting efficient hydrogen adsorption and evolution. These findings highlight the synergistic effects of atomic-level Ni dispersion and tailored support, offering a robust strategy for fabricating single-atom electrocatalysts for sustainable hydrogen production.

Topology-Sensitive Spin-Selecting Super-Exchange Interactions at Half-Antiperovskites with Orderly-Oxidized Semimetals for Acidic Water Oxidation

Developing inexpensive catalysts for acidic water oxidation plays a critical role in energy conversions and storages, but which have to trade off their catalytic activity and electrochemical stability. Different from traditional nano-engineering, herein we focus onto manipulating the spin-dependent topological quantum interactions to facilitate acidic water splitting at half-antiperovskite (Ni2Co1In2S2) with orderly-oxidized Kagome lattices, in which spatial wave functions of the triangular metal units (M3) are spontaneously converted to the anti-symmetry feature due to the symmetry breaking from controllable bridged-oxygen decorations (M3O), leading to a spin-selecting-dependent electronic reconfiguration. This topology-sensitive symmetry breaking makes the super-exchange interactions among neighboring metal sites switch to the indirect out-plane configuration (M─O─M) from the direct in-plane form (M─M), meanwhile the spin-dependent transformation from antiferromagnetic states to ferromagnetic states causes these metal sites to demonstrate a semi-metallic property for optimizing their bonding interactions with reactants. As a result, the capacity of carrier migration and intermediate diffusion in the inner Helmholtz region is significantly improved, which makes mass activity increase to 20.5 A g-1 at an overpotential of ≈356 mV, demonstrating an obvious superiority over the other state-of-the-art catalysts without noble metals. This work provides a new insight for designing topology-dependent catalysts to better understand spin-related reaction kinetics.

Co3O4-RuO2/Ti3C2Tx MXene Electrocatalysts for Oxygen Evolution Reaction in Acidic and Alkaline Media

MXene 2D materials and non-noble transition metal oxide nanoparticles have been proposed as novel pH-universal platforms for oxygen evolution reaction (OER), owing to the enhancement of active site exposures and conductivity. Herein, Co3O4-RuO2 /Ti3C2Tx/carbon cloths (CRMC) were assembled in a facile way as an efficient OER platform through a hydrothermal process. The Co3O4-RuO2/Ti3C2Tx demonstrated prominent OER catalytic performance under acidic and alkaline conditions, which showed overpotential values of 195 and 247 mV at 10 mA cm-2 with Tafel slopes of 93 and 97 mVdec-1, respectively. The experimental results demonstrated that the electron transfer from Co3O4-RuO2 to Ti3C2Tx/carbon cloth played a remarkable role in promoting OER catalytic activity. Further OER characterization indicated that the enhanced multi-electron reaction kinetics are attributed to Co and Ru acting as the primary active places for O2 adsorption and activation, which facilitated the generation of *OOH intermediate.

Versatile Decal-Transfer Method for Fabricating and Analyzing Microporous Layers in Polymer Electrolyte Membrane Water Electrolysis

Polymer electrolyte membrane water electrolysis (PEMWE) is hindered by the reliance on expensive iridium-based catalysts. To address this economic challenge, minimizing iridium usage while maintaining performance and durability is imperative. Achieving this goal requires enhanced catalyst utilization through improved electron, ion, and mass transport within the anode. Recent research has increasingly emphasized the development of microporous layers (MPLs) as a key strategy for enhancing the interface between the porous transport layer (PTL) and the catalyst layer (CL). However, standardized methodologies for MPL design and fabrication remain elusive. In this study, a decal-transfer method is presented as an effective method for introducing a uniform, thin MPL at the CL/PTL interface. By varying the MPL properties, including pore size, thickness, and back-layer structure, two-phase transport phenomena are investigated and established guidelines for optimal MPL design. The findings reveal that smaller micrometer-scale pores in the MPL enhance catalyst utilization and strengthen water capillary force, thereby reducing kinetic and transport overpotentials. Moreover, it is demonstrated that, unless the back layer hinders the in-plane mass transport beneath the flow field, its structural configuration has minimal influence on electrolysis performance. These results underscore the importance of the CL/PTL interface in determining the overall efficiency of PEMWE systems.

Perchlorate Fusion-Hydrothermal Synthesis of Nano-Crystalline IrO2: Leveraging Stability and Oxygen Evolution Activity

Iridium oxides are the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in proton-exchange-membrane water electrolyzers (PEMWEs), but their high cost and scarcity necessitate improved utilization. Crystalline rutile-type iridium dioxide (IrO2) offers superior stability under acidic OER conditions compared to amorphous iridium oxide (IrOx). However, the higher synthesis temperatures required for crystalline phase formation result in lower OER activity due to the loss in active surface area. Herein, a novel perchlorate fusion-hydrothermal (PFHT) synthesis method to produce nano-crystalline rutile-type IrO2 with enhanced OER performance is presented. This low-temperature approach involves calcination at a mild temperature (300 °C) in the presence of a strong oxidizing agent, sodium perchlorate (NaClO4), followed by hydrothermal treatment at 180 °C, yielding small (≈2 nm) rutile-type IrO2 nanoparticles with high mass-specific OER activity, achieving 95 A gIr -1 at 1.525 VRHE in ex situ glass-cell testing. Most importantly, the catalyst displays superior stability under harsh accelerated stress test conditions compared to commercial iridium oxides. The exceptional activity of the catalyst is confirmed with in situ PEMWE single-cell evaluations. This demonstrates that the PFHT synthesis method leverages the superior intrinsic properties of nano-crystalline IrO2, effectively overcoming the typical trade-offs between OER activity and catalyst stability.

Ferroelectric Polarization Effects of Single-Atom Catalysts on Water Oxidation

The oxygen evolution reaction (OER) performance of single-atom catalysts (SACs) heavily depends on their substrates. However, heterojunctions with traditional substrate materials often fail to provide the desired dynamic interface effects. Here, through a systematic study of the ferroelectric heterostructure In2Se3/C-N-M, the feasibility of using ferroelectric materials to achieve dynamic optimization of the OER activity on SACs is demonstrated. The ferroelectric In2Se3 is confirmed to be an effective substrate for improving the stability of various SACs, supported by theoretical results of their negative formation energy and positive dissolution potential. Activity analysis indicates that among these In2Se3/C-N-M systems, the In2Se3/C-N-Ir can achieve near-ideal catalytic activities through polarization switching. It can unprecedentedly catalyze OER via a hybrid pathway of adsorbate evolution mechanism and O-O coupling mechanism under different pH conditions (from pH = 1 to pH = 13). Machine learning models have been developed to conduct feature analysis and make ultrafast predictions of OER activity, which identify that the interfacial charge transfer triggered by ferroelectric polarization is the key to fine-tuning the OER performance of SACs. This work provides a theoretical framework that utilizes ferroelectric polarization as a powerful approach to navigate the design of efficient SACs.

Identifying and Quantifying Loss Sources in Anion-Exchange Membrane Water Electrolyzers

Anion-exchange membrane (AEM) water electrolyzers (AEMWEs) have gained significant attention for their ability to utilize precious-metal-free catalysts and environmentally friendly fluorine-free hydrocarbon polymeric membranes. In this study, we identify and quantify the sources of performance losses in operando AEMWEs using an innovative approach based on electrochemical impedance spectroscopy and MATLAB-based impedance spectroscopy genetic programming. Using this approach, we move beyond conventional equivalent circuit models to develop a proper and analytical model of the distribution function of relaxation times (DFRT), enabling a deeper analysis of Faradaic and non-Faradaic processes. We apply this framework to isolate the critical processes-ohmic, ionic transport, charge transfer, and mass transfer-across various conditions, including KOH concentration, dry cathode operation mode with different anode electrolytes (KOH, K2CO3, and pure water), cell temperature, and membrane type. Our results indicate a considerable performance reduction as the KOH concentration in the anode decreases, primarily due to the relatively high ionic transport resistance. Our observations show that the performance of dry cathode operation with KOH in the anode yields a comparable performance to dual-side electrolyte feeding due to sufficient water back-diffusion from the anode, which efficiently maintains cathode hydration. Conversely, using pure water as an electrolyte in the anode with a dry cathode significantly increases cell resistances and compromises ionic transport, underscoring the urgent need for highly conductive ionomeric materials and strategies. These insights indicate that using DFRT to evaluate the AEMWE operation by separating and associating the electrochemical phenomena could simplify system design while enabling more efficient generation of dry, pure hydrogen and advancing the technology toward commercial application.

CNT-Supported RuNi Composites Enable High Round-Trip Efficiency in Regenerative Fuel Cells

Regenerative fuel cells hold significant potential for efficient, large-scale energy storage by reversibly converting electrical energy into hydrogen and vice versa, making them essential for leveraging intermittent renewable energy sources. However, their practical implementation is hindered by the unsatisfactory efficiency. Addressing this challenge requires the development of cost-effective electrocatalysts. In this study, a carbon nanotube (CNT)-supported RuNi composite with low Ru loading is developed as an efficient and stable catalyst for alkaline hydrogen and oxygen electrocatalysis, including hydrogen evolution, oxygen evolution, hydrogen oxidation, and oxygen reduction reaction. Furthermore, a regenerative fuel cell using this catalyst composite is assembled and evaluated under practical relevant conditions. As anticipated, the system exhibits outstanding performance in both the electrolyzer and fuel cell modes. Specifically, it achieves a low cell voltage of 1.64 V to achieve a current density of 1 A cm- 2 for the electrolyzer mode and delivers a high output voltage of 0.52 V at the same current density in fuel cell mode, resulting in a round-trip efficiency (RTE) of 31.6% without further optimization. The multifunctionality, high activity, and impressive RTE resulted by using the RuNi catalyst composites underscore its potential as a single catalyst for regenerative fuel cells.

Ionomer Interphase Layers Enable Efficient Anion-Exchange Membrane Water Electrolyzer Operation at Low pH

Anion-exchange membrane water electrolysis (AEMWE) is an emerging green hydrogen technology. As of today, AEM water electrolyzers operate using highly alkaline electrolytes. Design strategies to operate AEMWE systems sustainably under lower alkalinity toward pure water conditions have become a scientific priority. Under low-alkaline conditions, the alkaline-exchange ionomer (AEI) is, in addition to the AEM, the key ion-transport medium inside the AEMWE cell. While prior work addressed ion transport and the ionomer-catalyst interface at the anode in low-pH AEMWEs, a thorough investigation at the cathode side, including different AEI architectures, received limited attention. In this contribution, we explore the impact of AEI architectures in AEMWE cathodes using an ionomer and a membrane that are both commercially available. We demonstrate separate ionomer top layer (ITL) interphases placed between the cathode catalyst layer and the membrane as the most effective strategy toward high cell performance under low pH feeding. ITLs enabled performance benefits even at pH 14, which leads us to perceive their mechanistic role as an ion-transport buffer enabling ready ion migration from the cathode to the anode. Our insights on the ITL architecture will aid the design of AEMWE cells for sustained efficient operation under pure water feeds.

Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water Electrolysis at Industrial-Scale Current Density

The development of non-noble metal-based hydrogen evolving reaction (HER) electrocatalysts operating under high current density plays a critical role in the large-scale application of anion exchange membrane water electrolysis (AEM-WE). Herein, a porous and hybrid MoS2/Ni3S2 is synthesized on nickel foam (NF) via a one-step hydrothermal method and studied its reconstruction process during alkaline HER conditions. Experimental results indicated that the MoS2 underwent an oxidative dissolution followed by a dynamic equilibrium between dissolution and redeposition of the amorphous MoOx during HER. Meanwhile, S-vacancy-rich Ni3S2 (A-Ni3S2) is exposed and acts as the real active site for HER. The obtained MoOx/A-Ni3S2 catalyst exhibited high catalytic performance in three-electrode systems and single-cell AEM-WE. Finally, for a long-term durability test in the AEM electrolyzer, a dry cathode method is applied to suppress the Mo species leaching from the MoOx/A-Ni3S2 electrode. Remarkably, the device assembled by MoOx/A-Ni3S2 as the cathode catalyst and NiFe as the anode catalyst demonstrated a high stability of 2500 h at 2 A cm-2 and 40 °C with a small aging rate of 30 µV h-1.

Environmentally Friendly and Earth-Abundant Self-Healing Electrocatalyst Systems for Durable and Efficient Acidic Water Splitting

Electrochemical water splitting under acidic conditions is an efficient route for green hydrogen production from renewable electricity. Its implementation on a globally relevant scale is hindered by the lack of abundant and low-cost electrocatalysts for the oxygen evolution reaction that can operate stably and efficiently under highly acidic anodic conditions. Here, we report the design of stable and efficient acidic OER electrocatalysts consisting of a self-healing bismuth (Bi)-based matrix hosting transition metal active sites. Comprehensive structural performance investigation of Co- and Ni-BiOx electrodes provides insights into the role of the electrolyte composition and pH in the self-healing mechanism under anodic conditions. Our best-performing [Co-Bi]Ox and [Ni-Bi]Ox anodes achieve over 200 h of continuous electrolysis at a catalytic current of 10 mA cm-2 with an overpotential of 590 and 670 mV at a pH of 1 in a 0.1 M H2SO4 electrolyte. Notably, while the [Bi]Ox matrix did not contribute to the catalytic activity, it was essential to stabilize the active Co and Ni sites during the acidic OER. Our findings provide a promising strategy for the engineering of earth-abundant materials for efficient acidic water splitting, as an alternative to the use of poorly scalable and expensive noble metal catalysts.

Energy-Efficient Hydrogen Generation via Peroxide-Mediated Electrocatalytic Pathways

Hydrogen (H2) production through water electrolysis is a promising route for sustainable energy storage. However, conventional water electrolysis faces several challenges, such as large thermodynamic potential gaps and sluggish oxygen evolution kinetics, which lead to high electricity consumption and limitations in H2 storage and transportation. A promising approach to overcoming these hurdles is hybrid water electrolysis, which integrates alternative, thermodynamically favorable reactions at the anode to enhance efficiency. In this study, we explore how peroxide redox electrocatalysis can address critical barriers in sustainable H2 production, storage, and transport. By leveraging a cost-effective and highly efficient peroxide redox electrocatalyst, we demonstrate various electrolysis configurations that significantly reduce the required cell voltage-from the standard 1.23 V down to -0.06 V, highlighting its potential for scalable and economically viable electrolysis methodologies for H2 production.

Dynamic and interconnected influence of dissolved iron on the performance of alkaline water electrolysis

Dissolved iron (Fe) species is an intriguing player in the overall alkaline water electrolysis (AWE) system, considered both as a poison that needs to be avoided and as a precursor for enhancing the water splitting activity. Here, we unveil the intricate mechanisms governing the Fe influence on practical AWE systems, by measuring the dynamic changes in cell voltage and overpotential of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The dissolved Fe will deposit on the cathode, which significantly enhances the HER activity of bare Ni mesh (BN) while showing negligible impact on the porous RANEY® Ni mesh (RN). The dissolved Fe will also improve the OER activity of the BN by a mechanism based on an equilibrium between leaching and incorporation of Fe onto the oxide layer of the anode. The continuous deposition of Fe on the cathode will gradually deplete the electrolyte of dissolved Fe, which will in turn push the anode surface equilibrium towards low density of active Fe sites thus to a decrease of OER activity. Inspired by the above results, by optimizing the addition of Fe(iii) salt into the system, an impressively low cell voltage of 1.95 V for a water splitting current density of 0.4 A cm-2 was achieved for a simple, cheap and robust BN cathode//BN anode zero-gap assembly. This performance is equivalent to a power consumption around 19.3% lower compared to the system without Fe(iii) addition.

Role of Water in Green Carbon Science

Within the context of green chemistry, the concept of green carbon science emphasizes carbon balance and recycling to address the challenge of achieving carbon neutrality. The fundamental processes in this field are oxidation and reduction, which often involve simple molecules such as CO2, CO, CH4, CHx, and H2O. Water plays a critical role in nearly all oxidation-reduction processes, and thus, it is a central focus of research in green carbon science. Water can act as a direct source of dihydrogen in reduction reactions or participate in oxidation reactions, frequently involving O-O coupling to produce hydrogen peroxide or dioxygen. At the atomic level, this coupling involves the statistically unfavorable proximity of two atoms, requiring optimization through a catalytic process influenced by two types of factors, as described by the authors. Extrinsic factors are related to geometrical and electronic criteria associated with the catalytic metal, involving its d-orbitals (or bands in the case of zerovalent metals and electrodes). Intrinsic factors are related to the coupling of oxygen atoms via their p-orbitals. At the mesoscopic or microscopic scale, the reaction medium typically consists of mixtures of lipophilic and hydrophilic phases with water, which may exist under supercritical conditions or as suspensions of microdroplets. These reactions predominantly occur at phase interfaces. A comprehensive understanding of the phenomena across these scales could facilitate improvements and even lead to the development of novel conversion processes.

Topological Control Over Porphyrin-Based Covalent Organic Frameworks for Elucidating Electron Transfer Characteristics

Two-dimensional covalent organic frameworks (2D COFs) have emerged as promising functional materials due to their programmable architectures and tunable functionalities. Nevertheless, the structural diversity of porphyrin-based 2D COFs remains restricted by the prevalent use of sql topology, hindering comprehensive structure-property exploration. Herein, we systematically designed and synthesized porphyrinic 2D COFs featuring distinct sql and bex topological configurations. Comprehensive structural characterization confirmed precise control over lattice geometries, revealing monoporous structure in sql topology versus biporous architecture in bex topology. Electrochemical investigations uncovered topology-governed electron transport characteristics, with the unique coordination geometry of bex topology exhibiting enhanced electron transfer efficiency. Band structure analysis demonstrated that topological configuration and chemical composition collectively modulate electronic structures. Inspired by these findings, we developed nickel-incorporated bex-COFs for electrocatalytic oxygen evolution. The optimized Ni-BBFPP-TAPP-COF with bex topology demonstrated remarkable catalytic performance, achieving a low overpotential of 342 mV at 10 mA cm-2, which surpasses most reported porphyrin-based electrocatalysts. This study not only significantly expands the structural repertoire of porphyrinic COFs but also establishes explicit correlations between topological engineering and electrocatalytic performance, providing fundamental design principles for advanced energy conversion materials.

Combined effects of electrode morphology and electrolyte composition on single H2 gas bubble detachment during hydrogen evolution reaction

During the hydrogen evolution reaction, H2 gas bubbles form on the electrode surface, significantly affecting electrochemical processes, particularly at high current densities. While promoting bubble detachment has been shown to enhance the current density, the mechanisms governing gas bubble detachment at the electrochemical interface remain poorly understood. In this study, we investigated the interplay between electrode surface morphology and electrolyte composition on single H2 gas bubble detachment during hydrogen evolution reaction (HER). Using well-defined Pt microelectrodes as model systems, we systematically modify and enhance their surface roughness through mechanical polishing to investigate these effects in detail. By modulating the Marangoni effect through variations in electrolyte composition and applied potential, we identified two distinct detachment behaviours. When the Marangoni force acts towards the electrodes, H2 gas bubbles are positioned closer to the electrode surface and exhibit roughness-dependent detachment, with smaller bubbles detaching earlier on rougher surfaces. Conversely, when the Marangoni force is directed away from the electrode, H2 gas bubbles are located farther from the electrode surface and show roughness-independent detachment sizes. These findings highlight the importance of considering both electrode and electrolyte effects to optimize gas bubble detachment during electrochemical reactions.

Lamellar Nanoporous Intermetallic Cobalt-Titanium Multisite Electrocatalyst with Extraordinary Activity and Durability for the Hydrogen Evolution Reaction

Constructing well-defined multisites with high activity and durability is crucial for the development of highly efficient electrocatalysts toward multiple-intermediate reactions. Here we report negative mixing enthalpy caused intermetallic cobalt-titanium (Co3Ti) nanoprecipitates on a lamellar hierarchical nanoporous cobalt skeleton as a high-performance nonprecious multisite electrocatalyst for an alkaline hydrogen evolution reaction. The intermetallic Co3Ti as a robust multisite substantially boosts the reaction kinetics of water dissociation and hydrogen adsorption/combination by unisonous adsorptions of hydrogen and hydroxyl intermediates with proper binding energies. By virtue of a bicontinuous and hierarchical nanoporous cobalt skeleton that enables sufficiently accessible Co3Ti multisites and facilitates electron transfer and ion/molecule transportation, a self-supported nanoporous cobalt-titanium heterogeneous electrode exhibits extraordinary electrocatalytic activity and durability toward the hydrogen evolution reaction in 1 M KOH. It reaches a current density of as high as ∼3.31 A cm-2 at a low overpotential of 200 mV and maintains exceptional stability at ∼1.33 A cm-2 for >1000 h.

State-of-the-art and perspectives of hydrogen generation from waste plastics

Waste plastic utilization and hydrogen production present significant economic and social challenges but also offer opportunities for research and innovation. This review provides a comprehensive analysis of the latest advancements and innovations in hydrogen generation coupled with waste plastic recycling. It explores various strategies, including pyrolysis, gasification, aqueous phase reforming, photoreforming, and electrocatalysis. Pyrolysis and gasification in combination with catalytic reforming or water gas-shift are currently the most feasible and scalable technologies for hydrogen generation from waste plastics, with pyrolysis operating in an oxygen-free environment and gasification in the presence of steam, though both require high energy inputs. Aqueous phase reforming operates at moderate temperatures and pressures, making it suitable for oxygenated plastics, but it faces challenges related to feedstock limitations, catalyst costs and deactivation. Photoreforming and electrocatalytic reforming are emerging, sustainable methods that use sunlight and electricity, respectively, to convert plastics into hydrogen. Still, they suffer from low efficiency, scalability issues, and limitations to specific plastic types like oxygenated polymers. The challenges and solutions to commercializing plastic-to-hydrogen technologies, drawing on global industrial case studies have been outlined. Maximizing hydrogen productivity and selectivity, minimizing energy consumption, and ensuring stable operation and scaleup of plastic recycling are crucial parameters for achieving commercial viability.

NiFe-based arrays with manganese dioxide enhance chloride blocking for durable alkaline seawater oxidation

Seawater splitting is increasingly recognized as a promising technique for hydrogen production, while the lack of good electrocatalysts and detrimental chlorine chemistry may hinder further development of this technology. Here, the interfacial engineering of manganese dioxide nanoparticles decorated on NiFe layered double hydroxide supported on nickel foam (MnO2@NiFe LDH/NF) is reported, which works as a robust catalyst for alkaline seawater oxidation. Density functional theory calculations and experiment findings reveal that MnO2@NiFe LDH/NF can selectively enrich OH- and repel Cl- in oxygen evolution reaction (OER). MnO2@NiFe LDH/NF attains a current density of 1000 mA cm-2 in alkaline seawater with an ultralow overpotential of only 313 mV. Furthermore, it can maintain stability at 1500 mA cm-2 over 600 h. Further phosphidation of MnO2@NiFe LDH/NF can create MnOx@NiFeP/NF used in efficient hydrogen evolution reaction. Moreover, an anion exchange membrane electrolyzer with MnO2@NiFe LDH/NF as the anode and MnOx@NiFeP/NF as the cathode was also capable of seawater splitting at 500 mA cm-2 for 100 h. This work offers light to develop effective and long-lasting electrocatalysts for seawater splitting.

Carbon-Supported Single Fe/Co/Ni Atom Catalysts for Water Oxidation: Unveiling the Dynamic Active Sites

Extensive research has been conducted on carbon-supported single-atom catalysts (SACs) for electrochemical applications, owing to their outstanding conductivity and high metal atom utilization. The atomic dispersion of active sites provides an ideal platform to investigate the structure-performance correlations. Despite this, the development of straightforward and scalable synthesis methods, along with the tracking of the dynamic active sites under catalytic conditions, remains a significant challenge. Herein, we introduce a biomass-inspired coordination confinement strategy to construct a series of carbon-supported SACs, incorporating various metal elements, such as Fe, Co, and Ni. We have systematically characterized their electronic and geometric structure using various spectroscopic and microscopic techniques. Through in situ X-ray absorption spectroscopy (XAS), atomic scanning transmission electron microscopy (STEM), and electron paramagnetic resonance (EPR) analyses, it is demonstrated that the single atoms undergo structural rearrangement to form amorphous (oxy)hydroxide clusters during oxygen evolution reaction (OER), where the newly formed oxygen-bridged dual metal M─O─M or M─O─M' (M/M' = Fe, Co, Ni) moieties within these clusters play key role in the OER performance. This work provides essential insights into tracking the actual active sites of SACs during electrochemical OER.

Operando Characterization of Porous Nickel Foam Water Splitting Electrodes Using Near-Ambient Pressure X-ray Photoelectron Spectroscopy

This study presents a practical approach for characterizing industrial water-splitting nickel foam electrodes under both cathodic and anodic conditions by employing synchrotron radiation near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The in situ studies reveal quantitatively reduced and oxidized Ni species on the electrode surface by recording the Ni 2p3/2 signals after cycling the potential in cathodic and anodic regions, respectively. Operando studies demonstrate that a stable electrolyte film forms, allowing the probing of the solid/liquid interface under applied potentials. We attribute this stability to capillary forces within the porous structure of the foam, which enables the monitoring of surface deprotonation under anodic potentials and surface protonation under cathodic potentials. Given that the most common industrial alkaline water electrolyzer electrodes are based on nickel foams similar to the samples measured in this study, the demonstrated method offers a valuable approach for fundamental NAP-XPS examination directly on industrially employed electrodes.

Enhancing the oxygen evolution reaction by tuning the electrode-electrolyte interface in nickel-based electrocatalysts

A comprehensive understanding of the electrode-electrolyte interface in energy conversion systems remains challenging due to the complex and multifaceted nature of interfacial processes. This complexity hinders the development of more efficient electrocatalysts. In this work, we propose a hybrid approach to the theoretical description of the OER process on nickel-iron-based oxyhydroxides (γ-Ni1-xFexOOH) electrodes in alkaline media as a model system. Multiple reaction pathways represented by the single- and dual-site mechanisms were investigated by taking into account the realistic structure of the catalyst, the doping, and the solvation effects using a simple and computationally feasible strategy. Accounting for the variable solvation effects considerably affects the predicted overpotential in a roughly linear relationship between overpotential and dielectric constant. By incorporating quantum chemical simulations with kinetic modeling, we demonstrate that tuning the local solvation environment can significantly enhance the OER activity, opening new routine ways for elucidation of the emerging issues of OER processes on transition metal oxide surfaces and design of cost-effective, efficient electrocatalytic systems.

Enhance Water Electrolysis for Green Hydrogen Production with Material Engineering: A Review

Water electrolysis, a traditional and highly technology, is gaining significant attention due to the growing demand for renewable energy resources. It stands as a promising solution for energy conversion, offer substantial benefits in environmental protection and sustainable development efforts. The aim of this research is to provide a concise review of the current state-of-the-art in the field of water electrolysis, focusing on the principles of water splitting fundamental, recent advancements in catalytic materials, various advanced characterization methods and emerging electrolysis technology improvements. Moreover, the paper delves into the development trends of catalysts engineering for water electrolysis, providing insight on how to enhance the catalytic performance. With the advancement of technology and the reduction of costs, hydrogen production through water electrolysis is expected to assume a more significant role in future energy ecosystem. This paper not only synthesizes existing knowledge but also highlights emerging opportunities and potential advancements in this field, offering a clear roadmap for further research and innovation.

Platinum-Nickel Oxide Cluster-Cluster Heterostructure Enabling Fast Hydrogen Evolution for Anion Exchange Membrane Water Electrolyzers

Carbon black has been extensively employed as the support for noble metal catalysts for electrocatalysis applications. However, the nearly catalytic inertness and weak interaction with metal species of carbon black are two major obstacles that hinder the further improvement of the catalytic performance. Herein, we report a surface functionalization strategy by decorating transition metal oxide clusters on the commercial carbon black to offer specific catalytic activity and enhanced interaction with metal species. In the case of NiOx cluster-decorated carbon black, a strongly coupled cluster-cluster heterostructure consisting of Pt clusters and NiOx clusters (Pt-NiOx/C) is formed and delivers greatly enhanced alkaline hydrogen evolution kinetics. The NiOx clusters can not only accelerate the hydrogen evolution process as the co-catalyst, but also optimize the adsorption of H intermediates on Pt and stabilize the Pt clusters. Notably, the anion exchange membrane water electrolyzer with Pt-NiOx/C as the cathode catalyst (with a loading of only 50 μgPt cm-2) delivers the most competitive electrochemical performance reported to date, requiring only 1.90 V to reach a current density of 2 A cm-2. The results demonstrate the significance of surface functionalization of carbonaceous supports toward the development of advanced electrocatalysts.

Leveraging data mining, active learning, and domain adaptation for efficient discovery of advanced oxygen evolution electrocatalysts

Developing advanced catalysts for acidic oxygen evolution reaction (OER) is crucial for sustainable hydrogen production. This study presents a multistage machine learning (ML) approach to streamline the discovery and optimization of complex multimetallic catalysts. Our method integrates data mining, active learning, and domain adaptation throughout the materials discovery process. Unlike traditional trial-and-error methods, this approach systematically narrows the exploration space using domain knowledge with minimized reliance on subjective intuition. Then, the active learning module efficiently refines element composition and synthesis conditions through iterative experimental feedback. The process culminated in the discovery of a promising Ru-Mn-Ca-Pr oxide catalyst. Our workflow also enhances theoretical simulations with domain adaptation strategy, providing deeper mechanistic insights aligned with experimental findings. By leveraging diverse data sources and multiple ML strategies, we demonstrate an efficient pathway for electrocatalyst discovery and optimization. This comprehensive, data-driven approach represents a paradigm shift and potentially benchmark in electrocatalysts research.

Multifunctional High-Entropy Alloys and Oxides for Self-Powered Electrocatalytic Nitrate Reduction to Ammonia

High-entropy alloys (HEAs) show high activities toward oxygen reduction reaction (ORR), Zn-air batteries (ZABs) and nitrate reduction reaction (NO3 -RR). In this work, FeNiCoMnRh HEA supported by N-doped carbon frameworks is prepared and showed excellent ORR performance with a half-wave potential (E1/2) of 0.89 V versus RHE, limiting diffusion current (jL) of 5.6 mA cm-2 and better current stability. The HEA-assembled ZAB exhibited a high-power density of 103.8 mW cm-2 with a specific capacity of 790 mAh gZn -1. Also, its oxides presented 77% Faraday efficiency (FE) for ammonia production at -0.3 V versus RHE. Accordingly, our designed ZAB was employed to drive NO3 -RR to construct a self-powered system, which provides an attractive route for low-energy sewage treatment and environmentally friendly preparation of ammonia.

Corrosion-resistant single-atom catalysts for direct seawater electrolysis

Direct seawater electrolysis (DSE) for hydrogen production is an appealing method for renewable energy storage. However, DSE faces challenges such as slow reaction kinetics, impurities, the competing chlorine evolution reaction at the anode, and membrane fouling, making it more complex than freshwater electrolysis. Therefore, developing catalysts with excellent stability under corrosion and fulfilling activity is vital to the advancement of DSE. Single-atom catalysts (SACs) with excellent tunability, high selectivity and high active sites demonstrate considerable potential for use in the electrolysis of seawater. In this review, we present the anodic and cathodic reaction mechanisms that occur during seawater cracking. Subsequently, to meet the challenges of DSE, rational strategies for modulating SACs are explored, including axial ligand engineering, carrier effects and protective layer coverage. Then, the application of in-situ characterization techniques and theoretical calculations to SACs is discussed with the aim of elucidating the intrinsic factors responsible for their efficient electrocatalysis. Finally, the process of scaling up monoatomic catalysts for the electrolysis of seawater is described, and some prospective insights are provided.

Layered double hydroxide-derived bimetallic-MOF as a promising platform: Urea-coupled water oxidation and supercapattery-driven water electrolyzer

Developing a two-dimensional (2D) ultrathin metal-organic framework plays a significant role in energy conversion and storage systems. This work introduced a facile strategy for engineering ultrathin NiMn-MOF nanosheets on Ni foam (NF) via in situ conversion from NiMn-layered double hydroxide (LDH). The as-synthesized LDH-derived NiMn-MOF (LDH-D NiMn-MOF) nanosheet exhibited an overpotential of 350 mV to drive a current density of 100 mA cm-2 during oxygen evolution reaction (OER) owing to its better redox activity, hierarchical architecture, and intercalating ability. The similar effective catalytic trend was noticed during the urea-assisted water oxidation process. The developed catalyst required only a potential of 1.39 V vs. RHE at 100 mA cm-2 towards urea oxidation reaction (UOR). Moreover, the urea-assisted overall water-splitting voltage was found to be 1.5 V at the current density of 10 mA cm-2. Furthermore, the same catalyst was explored as an energy-storage material for supercapattery application with an aerial specific capacity value of 2613.9 mC cm-2 at 1 mA cm-2 which was found to be 1.5 times higher than NiMn-LDH (1724.3 mC cm-2). Additionally, an aqueous asymmetric supercapattery device was fabricated which demonstrated the best electrochemical performance and provided a maximum energy density of 64.1 Wh kg-1 at a power density of 493 W kg-1 with 77.8 percent capacity retention after a continuous run of 8000 cycles at 10 mA cm-2 current density. Hence, the multifaceted properties of energy conversion and storage of LDH-D NiMn-MOF outline its performance in real-world applications.

Electrocatalytic Water Splitting in Isoindigo-Based Covalent Organic Frameworks

Developing a low-cost, robust, and high-performance electrocatalyst capable of efficiently performing both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) under both basic and acidic conditions is a major challenge. This area of research has attracted much attention in recent decades due to its importance in energy storage and conversion. Herein, we report the synthesis of two imine-linked isoindigo-based covalent organic networks I-TTA and I-TG (I=Isoindigo, TTA=4,4',4''-(1,3,5-triazine-2,4,6-triyl)-trianiline, TG=triamino-guanidinium hydrochloride salt). By introducing two amine core units with different planarity, such as triazine and ionic guanidinium units, we control the morphology, crystallinity, and corresponding electrocatalytic properties of the materials. The combination of isoindigo dialdehyde with a planar triazine core, leads to the formation of thin, highly crystalline, planar two dimensional (2D) nanosheets covalent organic framework (COF), I-TTA whereas its combination with ionic non-planar guanidinium core leads to an amorphous covalent organic polymer (COP), I-TG with a fibrous morphology. The sheet-like crystalline I-TTA COF shows better electrocatalytic activity compared to the amorphous fibrous I-TG COP. I-TTA exhibits a current density of 10 mA cm-2 at an overpotential of ~134 mV for HER (in 0.5 M H2SO4) and ~283 mV for OER (in 1 M KOH). The electrocatalytic activity of the I-TTA COF in the OER exceeds that of other metal-free COFs. The catalytic activity is maintained even after 24 hours of chronoamperometry and 500 cycles of cyclic voltammetry (CV) at high scan rates.

Support-free iridium hydroxide for high-efficiency proton-exchange membrane water electrolysis

The large-scale implementation of proton-exchange membrane water electrolyzers relies on high-performance membrane-electrode assemblies that use minimal iridium (Ir). In this study, we present a support-free Ir catalyst developed through a metal-oxide-based molecular self-assembly strategy. The unique self-assembly of densely isolated single IrO6H8 octahedra leads to the formation of μm-sized hierarchically porous Ir hydroxide particles. The support-free Ir catalyst exhibits a high turnover frequency of 5.31 s⁻¹ at 1.52 V in the membrane-electrode assembly. In the corresponding proton-exchange membrane water electrolyzer, notable performance with a cell voltage of less than 1.75 V at 4.0 A cm⁻² (Ir loading of 0.375 mg cm⁻²) is achieved. This metal-oxide-based molecular self-assembly strategy may provide a general approach for the development of advanced support-free catalysts for high-performance membrane-electrode assemblies.

Enhanced Alkaline Water Electrolysis by the Rational Decoration of RuOx with the In Situ-Grown CoFe Nanolayer

Rational engineering of the surfaces of heterogeneous catalysts (especially the surfaces of supported metals) can endow intriguing catalytic functionalities for electrochemical reactions. However, it often requires complicated steps, and even if it does not, breaking the trade-off between activity and stability is quite challenging. Herein, we present a strategy for reconstructing supported catalysts via in situ growth of metallic nanolayers from the perovskite oxide support. When Ru-coated LaFe0.9Co0.1O3 is thermally reduced, the CoFe nanoalloy spontaneously migrates onto the Ru and greatly increases the physicochemical stability of Ru in alkaline water electrolysis. Benefiting from an 81% reduction in Ru dissolution after decoration, it operates for over 200 h without noticeable degradation. Furthermore, the underlying Ru modifies the electronic structure and surface adsorption properties of the CoFe overlayer toward reaction intermediates, synergistically catalyzing both the oxygen evolution reaction and the hydrogen evolution reaction. Specifically, the mass activity of the oxygen evolution reaction is 64.1 times greater than that of commercial RuO2. Our work highlights a way to protect inherently unstable Ru from dissolution while allowing it to influence surface kinetics from the subsurface sites in heterogeneous catalysts.

Partially Interstitial Silicon-Implanted Ruthenium as an Efficient Electrocatalyst for Alkaline Hydrogen Evolution

To enhance the alkaline hydrogen evolution reaction (HER), it is crucial, yet challenging, to fundamentally understand and rationally modulate potential catalytic sites. In this study, we confirm that despite calculating a low water dissociation energy barrier and an appropriate H adsorption free energy (ΔG*H) at Ru-top sites, metallic Ru exhibits a relatively inferior activity for the alkaline HER. This is primarily because the Ru-top sites, which are potential H adsorption sites, are recessive catalytic sites, compared with the adjacent Ru-hollow sites that have a strong ΔG*H. To promote the transformation of Ru-top sites from recessive to dominant catalytic sites, interstitial Si atoms are implanted into the hollow sites. However, complete interstitial implantation leads to a high water dissociation energy barrier at the RuSi intermetallic surface. Thus, we present a partial interstitial incorporation strategy to form a Ru-RuSi heterostructure that not only converts the Ru-top sites from recessive to dominant catalytic sites but also preserves the low water dissociation energy barrier at the Ru surface. Moreover, the spontaneously formed built-in electric fields bidirectionally optimize the adsorption ability of the Ru sites, thereby greatly reducing the thermodynamic energy barrier and enhancing the alkaline HER.

Tuning the electronic-state of metal cobalt/cobalt iron alloy hetero-interface embedded in nitrogen-doped carbon nanotube arrays for boosting electrocatalytic overall water splitting

Maximizing the utilization of active sites and tuning the electronic-state are crucial yet extremely challenging in enhancing the ability of alloy-based catalysts to catalyze hydrogen and oxygen evolution reactions (HER and OER). Here, the 3D self-supported N-doped carbon nanotube arrays (NCNTAs) was synthesized on Ni foam by the drop-casting and calcination method, where the metal Co and Co7Fe3 alloy were enclosed at the NCNT tip (denoted as Co/Co7Fe3@NCNT/NF). The Co/Co7Fe3 hetero-interface formation led to changes in the electronic state, which can optimize the adsorption free energy of reaction intermediates and thereby boost the intrinsic catalytic performance. The well-dispersed carbon nanotube arrays with superhydrophilic and superaerophobic characteristic promotes electrolyte permeation and bubbles escape. Therefore, the optimized Co/Co7Fe3-10@NCNT/NF exhibits superior bifunctional activities with overpotential of 93 and 174 mV at 10 mA cm-2 for HER and OER, respectively. For overall water splitting (OWS), the assembled dual electrode device with Co/Co7Fe3-10@NCNT/NF only requires a low voltage of 1.56 V to achieve 10 mA cm-2 and stabilizes for 24 h at 100 mA cm-2. The result underscores the importance of hetero-interface electronic effect and carbon nanotube arrays in catalytic water splitting, providing valuable insights for the design of more advanced bifunctional electrocatalysts for OWS.

Elucidating the Effect of the Catalyst Layer Morphology on the Growth and Detachment of Bubbles in Water Electrolysis via Lattice Boltzmann Modeling

The performance of water electrolysis is profoundly influenced by the behavior of the gas bubbles generated at the electrode surface. In order to improve the efficiency of bubble transportation, various nanostructures for the catalyst layer (CL), such as nanorods (NRs) or nanoparticles (NPs), have been proposed. However, there is still a lack of complete understanding about the relationship between the catalyst layer morphology and bubble evolution, which has a considerable impact on bubble transport. This study examines the effect of the catalyst layer morphology on the growth and detachment of bubbles in water electrolysis by employing the Lattice Boltzmann (LB) model. The performance of three different catalyst layer morphologies, namely, nanorods, nanoparticles, and hierarchical nanostructures, on bubble dynamics is estimated. The results suggest that the catalyst layer morphology is characterized by two factors: the bubble contact area and the electrochemically active surface area, which varies the bubble diameter, detachment time, and, subsequently, the bubble coverage. As a result, the energy efficiency of water electrolysis is influenced, aligned with experimental data that show a voltage differential of 148 mV at a current density of 0.65 A/cm2. This study highlights the significance of improving the structure of the catalyst layer to boost the efficiency of water electrolysis systems.