Low pH electrolytic water splitting using earth-abundant metastable catalysts that self-assemble in situ

Optimal Coatings of Co3O4 Anodes for Acidic Water Electrooxidation

Implementation of proton-exchange membrane water electrolyzers for large-scale sustainable hydrogen production requires the replacement of scarce noble-metal anode electrocatalysts with low-cost alternatives. However, such earth-abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale-thin oxide layer, namely Al2O3, SiO2, TiO2, SnO2, and HfO2, prepared by atomic layer deposition, on the stability and catalytic activity of low-cost and active but insufficiently stable Co3O4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co3O4 following the order of HfO2 > SnO2 > TiO2 > Al2O3, SiO2. An optimal HfO2 layer thickness of 12 nm enhances the Co3O4 anode durability by more than threefold, achieving over 42 h of continuous electrolysis at 10 mA cm-2 in 1 m H2SO4 electrolyte. Density functional theory is used to investigate the superior performance of HfO2, revealing a major role of the HfO2|Co3O4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth-abundant materials for low-pH OER catalysts with improved performance from earth-abundant materials for efficient hydrogen production.

Non-Noble-Metal-Based Electrocatalysts for Acidic Oxygen Evolution Reaction: Recent Progress, Challenges, and Perspectives

The oxygen evolution reaction (OER) plays a pivotal role in diverse renewable energy storage and conversion technologies, including water electrolysis, electrochemical CO2 reduction, nitrogen fixation, and metal-air batteries. Among various water electrolysis techniques, proton exchange membrane (PEM)-based water electrolysis devices offer numerous advantages, including high current densities, exceptional chemical stability, excellent proton conductivity, and high-purity H2. Nevertheless, the prohibitive cost associated with Ir/Ru-based OER electrocatalysts poses a significant barrier to the broad-scale application of PEM-based water splitting. Consequently, it is crucial to advance the development of non-noble metal OER catalysis substance with high acid-activity and stability, thereby fostering their widespread integration into PEM water electrolyzers (PEMWEs). In this review, a comprehensive analysis of the acidic OER mechanism, encompassing the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) and oxide path mechanism (OPM) is offered. Subsequently, a systematic summary of recently reported noble-metal-free catalysts including transition metal-based, carbon-based and other types of catalysts is provided. Additionally, a comprehensive compilation of in situ/operando characterization techniques is provided, serving as invaluable tools for furnishing experimental evidence to comprehend the catalytic mechanism. Finally, the present challenges and future research directions concerning precious-metal-free acidic OER are comprehensively summarized and discussed in this review.

Heterointerface MnO2/RuO2 with rich oxygen vacancies for enhanced oxygen evolution in acidic media

The design and synthesis of oxygen evolution reaction (OER) electrocatalysts that operate efficiently and stably under acidic conditions are important for the preparation of green hydrogen energy. The low intrinsic catalytic activity and poor acid resistance of commercial RuO2 limit its further development, and the construction of heterointerface structures is the most promising strategy to break through the intrinsic activity limitation of electrocatalysts. Herein, we synthesized spherical and oxygen vacancy-rich heterointerface MnO2/RuO2 using morphology control, which promoted the kinetics of the oxygen evolution reaction with the interaction between oxygen vacancies and the oxide heterointerface. MnO2/RuO2 was reported to be an acidic OER catalyst with excellent performance and stability, requiring only an ultra-low overpotential of 181 mV in 0.5 M H2SO4 to achieve a current density of 10 mA cm-2. The catalyst activity remained essentially unchanged in a 140 h stability test with an ultra-high mass activity (858.9 A g-1@ 1.5 V), which was far superior to commercial RuO2 and most previously reported noble metal-based acidic OER catalysts. The experimental results showed that the effect of more oxygen vacancies and the heterointerfaces of manganese ruthenium oxides broke the intrinsic activity limitation, provided more active sites for the OER, accelerated reaction kinetics, and improved the stability of the catalyst. The excellent performance of the catalyst suggests that MnO2/RuO2 provides a new idea for the design and study of heterointerfaces in metal oxide nanomaterials.

The Influence of Ions on the Electrochemical Stability of Aqueous Electrolytes

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.

Electrocatalytic water oxidation on CuO-Cu2O modulated cobalt-manganese layered double hydroxide

Layered double hydroxides (LDH) are potential electrocatalysts to address the sluggish oxygen evolution reaction (OER) of water splitting. In this work, copper oxide (CuO/Cu2O) nanoparticles are integrated with cobalt-manganese layered double hydroxide (CoMn-LDH) to enhance their performance towards OER. The catalyst is synthesized by growing CoMn-LDH nanosheets in the presence of CuO/Cu2O nanoparticles that were obtained by the calcination of the copper containing metal-organic framework (HKUST-1). The synthesized CoMn-LDH@CuO/Cu2O electrocatalyst shows excellent activity towards OER with an overpotential of 297 mV at a catalytic current density of 10 mA cm-2 and have a Tafel slope value of 89 mV dec-1. Moreover, a slight decrease in the performance parameters is observed until the 15 h of continuous operation. We propose that the conductive strength of CuO/Cu2O and its synergistic effect with the CoMn-LDH are responsible for the improved OER performance of the desired electrocatalyst.

Energy-saving H2 production from a hybrid acid/alkali electrolyzer assisted by anodic glycerol oxidation

Water electrolysis is a promising technology for efficient hydrogen production, but it has been heavily hindered by the sluggish kinetics and high potential of the anodic oxygen evolution reaction (OER). Replacing the OER with the glycerol oxidation reaction (GOR) at the anode is recognized as a potential strategy to address this issue. In this work, the self-supported electrocatalytic electrode of Cu-Cu₂O nanoclusters on carbon cloth (Cu-Cu₂O/CC) is fabricated for the electrocatalysis of the GOR, which has high activity towards the GOR, reaching 10 mA cm^-2 at an applied voltage of 1.21 V, and shows high selectivity for formate production with a faradaic efficiency (FE) of over 80% in a wide potential range. Moreover, a hybrid acid/alkali electrolyzer is assembled by coupling the Cu-Cu₂O/CC anode for the GOR in an alkaline electrolyte with commercial Pt/C as the cathode for the hydrogen evolution reaction (HER) in an acid electrolyte. The dual-electrolyte electrolytic cell only requires an applied voltage of 0.59 V to reach 10 mA cm^-2 with a FE of ∼100% for H₂ and 97% for formate production. This work provides a facile strategy for the application of glycerol upgradation in energy-saving water electrolysis systems.

FeNi LDH/V2CTx/NF as Self-Supported Bifunctional Electrocatalyst for Highly Effective Overall Water Splitting

The development of bifunctional electrocatalysts with efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is still a key challenge at the current stage. Herein, FeNi LDH/V2CTx/nickel foam (NF) self-supported bifunctional electrode was prepared via deposition of FeNi LDH on V2CTx/NF substrate by hydrothermal method. Strong interfacial interaction between V2CTx/NF and FeNi LDH effectively prevented the aggregation of FeNi LDH, thus exposing more catalytic active sites, which improved electrical conductivity of the nanohybrids and structural stability. The results indicated that the prepared FeNi LDH/V2CTx/NF required 222 mV and 151 mV overpotential for OER and HER in 1 M KOH to provide 10 mA cm-2, respectively. Besides, the FeNi LDH/V2CTx/NF electrocatalysts were applied to overall water splitting, which achieved a current density of 10 mA cm-2 at 1.74 V. This work provides ideas for improving the electrocatalytic performance of electrocatalysts through simple synthesis strategies, structural adjustment, use of conductive substrates and formation of hierarchical structures.

Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4

Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) able to work in acidic working conditions are elusive. While many first-row transition metal oxides are competitive in alkaline media, most of them just dissolve or become inactive at high proton concentrations where hydrogen evolution is preferred. Only noble-metal catalysts, such as IrO₂, are fast and stable enough in acidic media. Herein, we report the excellent activity and long-term stability of Co₃O₄-based anodes in 1 M H₂SO₄ (pH 0.1) when processed in a partially hydrophobic carbon-based protecting matrix. These Co₃O₄@C composites reliably drive O₂ evolution a 10 mA cm^-2 current density for >40 h without appearance of performance fatigue, successfully passing benchmarking protocols without incorporating noble metals. Our strategy opens an alternative venue towards fast, energy efficient acid-media water oxidation electrodes.

Nanoscale TiO2 Coatings Improve the Stability of an Earth-Abundant Cobalt Oxide Catalyst during Acidic Water Oxidation

The large-scale deployment of proton-exchange membrane water electrolyzers for high-throughput sustainable hydrogen production requires transition from precious noble metal anode electrocatalysts to low-cost earth-abundant materials. However, such materials are commonly insufficiently stable and/or catalytically inactive at low pH, and positive potentials required to maintain high rates of the anodic oxygen evolution reaction (OER). To address this, we explore the effects of a dielectric nanoscale-thin layer, constituted of amorphous TiO2, on the stability and electrocatalytic activity of nanostructured OER anodes based on low-cost Co3O4. We demonstrate a direct correlation between the OER performance and the thickness of the atomic layer deposited TiO2 layers. An optimal TiO2 layer thickness of 4.4 nm enhances the anode lifetime by a factor of ca. 3, achieving 80 h of continuous electrolysis at pH near zero, while preserving high OER catalytic activity of the bare Co3O4 surface. Thinner and thicker TiO2 layers decrease the stability and activity, respectively. This is attributed to the pitting of the TiO2 layer at the optimal thickness, which allows for access to the catalytically active Co3O4 surface while stabilizing it against corrosion. These insights provide directions for the engineering of active and stable composite earth-abundant materials for acidic water splitting for high-throughput hydrogen production.

Why Did Nature Choose Manganese over Cobalt to Make Oxygen Photosynthetically on the Earth?

All contemporary oxygenic phototrophs─from primitive cyanobacteria to complex multicellular plants─split water using a single invariant cluster comprising Mn₄CaO₅ (the water oxidation catalyst) as the catalyst within photosystem II, the universal oxygenic reaction center of natural photosynthesis. This cluster is unstable outside of PSII and can be reconstituted, both in vivo and in vitro, using elemental aqueous ions and light, via photoassembly. Here, we demonstrate the first functional substitution of manganese in any oxygenic reaction center by in vitro photoassembly. Following complete removal of inorganic cofactors from cyanobacterial photosystem II microcrystal (PSIIX), photoassembly with free cobalt (Co²⁺), calcium (Ca²⁺), and water (OH^-) restores O₂ evolution activity. Photoassembly occurs at least threefold faster using Co²⁺ versus Mn²⁺ due to a higher quantum yield for PSIIX-mediated charge separation (P*): Co²⁺ → P* → Co³⁺Q_A^-. However, this kinetic preference for Co²⁺ over native Mn²⁺ during photoassembly is offset by significantly poorer catalytic activity (∼25% of the activity with Mn²⁺) and ∼3- to 30-fold faster photoinactivation rate. The resulting reconstituted Co-PSIIX oxidizes water by the standard four-flash photocycle, although they produce 4-fold less O₂ per PSII, suggested to arise from faster charge recombination (Co³⁺Q_A ← Co⁴⁺Q_A^-) in the catalytic cycle. The faster photoinactivation of reconstituted Co-PSIIX occurs under anaerobic conditions during the catalytic cycle, suggesting direct photodamage without the involvement of O₂. Manganese offers two advantages for oxygenic phototrophs, which may explain its exclusive retention throughout Darwinian evolution: significantly slower charge recombination (Mn³⁺Q_A ← Mn⁴⁺Q_A^-) permits more water oxidation at low and fluctuating solar irradiation (greater net energy conversion) and much greater tolerance to photodamage at high light intensities (Mn⁴⁺ is less oxidizing than Co⁴⁺). Future work to identify the chemical nature of the intermediates will be needed for further interpretation.

Self-healing oxygen evolution catalysts

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Large scale sustainable energy storage by water splitting benefits from performing the oxygen evolution reaction under a variety of conditions. Here, the authors discuss self-healing catalysis as a new tool in the design of stable and functionally active catalysts in acidic to basic solutions, and a variety of water sources

Molten salt method synthesis of multivalent cobalt and oxygen vacancy modified Nitrogen-doped MXene as highly efficient hydrogen and oxygen Evolution reaction electrocatalysts

Nitrogen-doped Ti₃C₂T_y MXene with multivalent cobalt and oxygen vacancy (Vo) modification was obtained by using molten salt method and greatly improved electrocatalytic performance. The structural properties of MXene and the valence state of cobalt were adjusted by controlling the molten salt temperature. When the molten salt treatment temperature was 377 °C, the obtained 377-CoO_xN_1-x-Ti₃C₂T_y maintained the chemical structure of MXene well, and also has high Co²⁺ content and Vo content. Electrochemical test results showed that 377-CoO_xN_1-x-Ti₃C₂T_y had the lowest Hydrogen Evolution Reaction (HER) overpotential of 87.73 mV and good electrocatalytic stability. X-ray Photoelectron Spectroscopy (XPS) results and Density Functional Theory (DFT) calculations showed that the introduction of polyvalent cobalt and Vo in the nitrogen-doped Ti₃C₂T_y structure effectively reduced the energy barrier of the electrocatalytic reaction of MXene.

Kinetic 2D Crystals via Topochemical Approach

The designing of novel materials is a fascinating and innovative pathway in materials science. Particularly, novel layered compounds have tremendous influence in various research fields. Advanced fundamental studies covering various aspects, including reactants and synthetic methods, are required to obtain novel layered materials with unique physical and chemical properties. Among the promising synthetic techniques, topochemical approaches have afforded the platform for widening the extent of novel 2D materials. Notably, the synthesis of binary layered materials is considered as a major scientific breakthrough after the synthesis of graphene as they exhibit a wide spectrum of material properties with varied potential applicability. In this review, a comprehensive overview of the progress in the development of metastable layered compounds is presented. The various metastable layered compounds synthesized from layered ternary bulk materials through topochemical approaches are listed, followed by the descriptions of their mechanisms, structural analyses, characterizations, and potential applications. Finally, an essential research direction concerning the synthesis of new materials is indicated, wherein the possible application of topochemical approaches in unprecedented areas is explored.

Progress of Nonprecious-Metal-Based Electrocatalysts for Oxygen Evolution in Acidic Media

Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious-metal-free electrocatalysts for water oxidation in acidic media is attractive for the large-scale application of PEM electrolyzers. In recent years, various types of precious-metal-free catalysts such as carbon-based materials, earth-abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious-metal-free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed.

Stable Acidic Water Oxidation with a Cobalt-Iron-Lead Oxide Catalyst Operating via a Cobalt-Selective Self-Healing Mechanism

The instability and expense of anodes for water electrolyzers with acidic electrolytes can be overcome through the implementation of a cobalt-iron-lead oxide electrocatalyst, [Co-Fe-Pb]O_x , that is self-healing in the presence of dissolved metal precursors. However, the latter requirement is pernicious for the membrane and especially the cathode half-reaction since Pb²⁺ and Fe³⁺ precursors poison the state-of-the-art platinum H₂ evolving catalyst. To address this, we demonstrate the invariably stable operation of [Co-Fe-Pb]O_x in acidic solutions through a cobalt-selective self-healing mechanism without the addition of Pb²⁺ and Fe³⁺ and investigate the kinetics of the process. Soft X-ray absorption spectroscopy reveals that low concentrations of Co²⁺ in the solution stabilize the catalytically active Co(Fe) sites. The highly promising performance of this system is showcased by steady water electrooxidation at 80±1 °C and 10 mA cm^-2 , using a flat electrode, at an overpotential of 0.56±0.01 V on a one-week timescale.

Modifying redox properties and local bonding of Co3O4 by CeO2 enhances oxygen evolution catalysis in acid

Developing efficient and stable earth-abundant electrocatalysts for acidic oxygen evolution reaction is the bottleneck for water splitting using proton exchange membrane electrolyzers. Here, we show that nanocrystalline CeO₂ in a Co₃O₄/CeO₂ nanocomposite can modify the redox properties of Co₃O₄ and enhances its intrinsic oxygen evolution reaction activity, and combine electrochemical and structural characterizations including kinetic isotope effect, pH- and temperature-dependence, in situ Raman and ex situ X-ray absorption spectroscopy analyses to understand the origin. The local bonding environment of Co₃O₄ can be modified after the introduction of nanocrystalline CeO₂, which allows the Co^III species to be easily oxidized into catalytically active Co^IV species, bypassing the potential-determining surface reconstruction process. Co₃O₄/CeO₂ displays a comparable stability to Co₃O₄ thus breaks the activity/stability tradeoff. This work not only establishes an efficient earth-abundant catalysts for acidic oxygen evolution reaction, but also provides strategies for designing more active catalysts for other reactions.

Developing efficient and stable earth-abundant electrocatalysts for acidic oxygen evolution reaction is challenging. Here, the authors modify the local bonding environment of Co₃O₄ by CeO2 nanocrystallites to regulate the redox properties, thus enhance the catalytic activity.

Highly robust, novel aluminum counter cation-based monophosphate tungsten bronze electro-catalysts for oxygen evolution in acidic solution

This study describes the successful synthesis of novel bronze with a low tungsten oxidation state for the efficient electro-catalytic oxidation of water. An extraordinarily robust monophosphate tungsten bronze (MPTB)-modified graphite anode was successfully fabricated for the oxygen evolution reaction (OER) at a thermodynamic potential of 1.23 V in H2SO4 acidic solution. Several Al, Cr and Fe counter-cation-based MPTBs were synthesized by the solution combustion method. Novel Al-based MPTBs calcined at 700 °C in O2 (AlO7) showed almost zero onset overpotential, high current density, high turnover frequency for OER and steady catalysis in repeated use even after 30 weeks. The orthorhombic AlO7 comprising crystallites of 9.89 nm and an indirect band gap (1.89 eV), is an unusually stable MPTB that contains 98% W5+ state stabilized with the Al3+ counter cation. The catalysis decreases as the ratio of W5+ : W6+ in MPTBs decreases and [410] and [601] facets play main roles in the first H2O association and nucleophilic attack of the second H2O molecule on the catalyst surface. Thus, MPTBs can be non-noble metal anode materials for robust acidic H2O electrolyzers.

Copolymer of Phenylene and Thiophene toward a Visible-Light-Driven Photocatalytic Oxygen Reduction to Hydrogen Peroxide

π-Conjugated polymers including polythiophenes are emerging as promising electrode materials for (photo)electrochemical reactions, such as water reduction to H2 production and oxygen (O2) reduction to hydrogen peroxide (H2O2) production. In the current work, a copolymer of phenylene and thiophene is designed, where the phenylene ring lowers the highest occupied molecular orbital level of the polymer of visible-light-harvesting thiophene entities and works as a robust catalytic site for the O2 reduction to H2O2 production. The very high onset potential of the copolymer for O2 reduction (+1.53 V vs RHE, pH 12) allows a H2O2 production setup with a traditional water-oxidation catalyst, manganese oxide (MnO x ), as the anode. MnO x is deposited on one face of a conducting plate, and visible-light illumination of the copolymer layer formed on the other face aids steady O2 reduction to H2O2 with no bias assistance and a complete photocatalytic conversion rate of 14 000 mg (H2O2) gphotocat -1 h-1 or ≈0.2 mg (H2O2) cm-2 h-1.

Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO4-Based Microrod Arrays in Neutral Media through Alkaline Activation

We present that activation of CoMoO₄-based microrod arrays in KOH (1.0 M, 2 h) allows us to significantly improve their electrochemical hydrogen evolution performance in phosphate buffer solution (1.0 M, pH = 7.1). The activation mechanism originates from the conversion of the surface layer of CoMoO₄ to Co(OH)₂ nanosheets, together with the release of Mo₃O₁₀^2- ions into the activation solution. Our experimental and calculated results suggest that the Co(OH)₂ nanosheets on the surface of the CoMoO₄-based microrod arrays show the ability to improve water molecule disassociation and stabilize the catalytic activity of the two-component catalysts by decreasing their overpotentials in the hydrogen evolution reaction. When extending this strategy to activate P-doped CoMoO₄ with a low hydrogen absorption free energy, we report the synthesis of a new class of superior neutral electrochemical hydrogen evolution catalysts of P-doped CoMoO₄-Co(OH)₂ microrod arrays. We show that a low overpotential of about 30 mV (obtained from bulk electrolysis) is required to deliver a current density of 10 mA cm^-2 in the neutral media. By making use of our catalyst and NiFe double hydroxide as cathodic and anodic electrodes, respectively, we fabricated a two-electrode electrolysis device for neutral overall water splitting. Our results showed a low cell voltage of 1.78 V (obtained from bulk electrolysis) that is needed for delivering a current density of about 10 mA cm^-2 in the neutral electrolyte, even outperforming the state-of-the-art catalyst combination of Pt/C∥RuO₂ in terms of catalytic activity and stability. These findings suggest that our strategy may be utilized as a facile but useful strategy toward the activation of molybdate catalysts to improve their HER performance in both basic and neutral media.

Coupling amorphous cobalt hydroxide nanoflakes on Sr2Fe1.5Mo0.5O5+δ perovskite nanofibers to induce bifunctionality for water splitting

Developing cost-effective, stable and environmentally friendly catalysts is of prime importance for the commercial application of overall water splitting. Perovskite oxides have emerged as one of the promising bifunctional catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, their bifunctional activity, especially towards HER, is still not meeting the anticipated energy efficiency. Herein, we highlight a facile and efficient surface modification approach for boosting the bifunctionality of perovskites for overall water splitting. The construction of amorphous cobalt hydroxide (Co(OH)2) on the Sr2Fe1.5Mo0.5O6-δ (SFM) surface is conducted via an atomic layer deposition (ALD) technology. The optimized crystalline core-amorphous shell structure only needs 384 mV to reach a current density of 10 mA cm-2 for the OER and 322 mV at -10 mA cm-2 for the HER in alkaline media. The optimized catalytic activity is probably due to the unique structure and the synergistic effect between Co(OH)2 and SFM, resulting in the large electrochemical surface area, abundant oxygen vacancies and fast electron transfer. The cell assembled with Co(OH)2/SFM-NF as both cathode and anode electrodes delivers a low voltage of 1.60 V to achieve 10 mA cm-2 and remarkable stability over 68 h in practical operation, offering a viable alternative for overall water splitting.

Phytic acid-guided ultra-thin N,P co-doped carbon coated carbon nanotubes for efficient all-pH electrocatalytic hydrogen evolution

Nanostructure engineering of heteroatom-doped carbon catalysts can greatly enhance their electrocatalytic activity by increasing the accessible active sites and beneficial physical properties (e.g., surface area, conductivity, etc.). Herein, we successfully constructed ultra-thin N,P co-doped carbon (NPC) on the surface of multi-walled carbon nanotubes (CNT) by using phytic acid (PA) as a "guide". The rich phosphate groups in PA allow them to be covalently modified on the surface of CNT by the condensation reaction and to further attract large aniline monomers through acid-base interactions, resulting in the uniform and tight bonding between polyaniline and CNT after the polymerization process. During the subsequent thermal reaction, PA also serves as a self-sacrificial dopant for the formation of ultra-thin NPC and the doping amount of P in NPC can be easily adjusted by changing the amount of PA. Due to the abundance of active sites, large electrochemically active surface area and rapid electron transfer, the developed CNT@NPC presents remarkable electrocatalytic activities for the hydrogen evolution reaction (HER) with an overpotential of 167, 440 and 304 mV to reach a current density of 10 mA cm^-2 in acidic, neutral, and alkaline electrolytes, respectively. In particular, its acidic HER activity exceeds that of most reported metal-free electrocatalysts and is comparable to that of some excellent transition metal-based catalysts. The approach proposed here is of potential importance for the preparation of ideal heteroatom-doped carbon/nanocarbon composites for use in a variety of future energy conversion systems.

Bifunctional earth-abundant phosphate/phosphide catalysts prepared via atomic layer deposition for electrocatalytic water splitting

The development of active and stable earth-abundant catalysts for hydrogen and oxygen evolution is one of the requirements for successful production of solar fuels. Atomic Layer Deposition (ALD) is a proven technique for conformal coating of structured (photo)electrode surfaces with such electrocatalyst materials. Here, we show that ALD can be used for the deposition of iron and cobalt phosphate electrocatalysts. A PE-ALD process was developed to obtain cobalt phosphate films without the need for a phosphidation step. The cobalt phosphate material acts as a bifunctional catalyst, able to also perform hydrogen evolution after either a thermal or electrochemical reduction step.

Electrodeposition of amorphous molybdenum sulfide thin film for electrochemical hydrogen evolution reaction

Amorphous molybdenum sulfide (MoS_x) is a highly active noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). The MoS_x was prepared by electrochemical deposition at room temperature. Low-cost precursors of Mo and S were adopted to synthesize thiomolybdates solution as the electrolyte. It replaces the expensive (NH)₂MoS₄ and avoid the poison gas (H₂S) to generate or employ. The (MoO₂S₂)²⁻, (MoOS₃)²⁻ and (MoS₄)²⁻ ions were determined by UV–VIS spectroscopy. The electrodeposition of MoS_x was confirmed with XRD, XPS and SEM. The electrocatalyst activity was measured by polarization curve. The electrolyte contained (MoO₂S₂)²⁻ ion and (MoOS₃)²⁻ ion electrodeposit the MoS_x thin film displays a relatively high activity for HER with low overpotential of 211 mV at a current density of 10 mA cm⁻², a relatively high current density of 21.03 mA cm⁻² at η = 250 mV, a small Tafel slope of 55 mV dec⁻¹. The added sodium dodecyl sulfate (SDS) can efficient improve the stability of the MoS_x film catalyst.

Influence of pH Modification on Catalytic Activities of Metal-Doped IrO2 Nanoparticles

The effects of pH variation on the catalytic activity of IrO₂ nanoparticles (NPs) doped with Cr (an early transition metal) or Ni (a late transition metal) depending on the amount of defect structures on the NP surfaces were analyzed. It was found that both Cr@IrO₂ and Ni@IrO₂ NPs, fabricated under basic conditions (pH = 13.5) denoted as Cr@IrO₂-B and Ni@IrO₂-B, respectively, were the best catalysts among the eight tested ones. Moreover, it was confirmed that variation in pH resulted in the changes in the surface area (defect structure), which were considered to be responsible for the changes in the catalytic properties of these NPs. For the oxygen evolution reaction, these NPs exhibited relatively smaller overpotential (η) values than other tested Cr@IrO₂- and Ni@IrO₂-containing NPs. Furthermore, methylene blue degradation analysis and OH radical formation experiments by benzoic acid showed the same trend. Thus, we confirmed that the catalytic activity of transition metals doped IrO₂ NPs fabricated under basic conditions can be improved.

Discovery of Anion Insertion Electrochemistry in Layered Hydroxide Nanomaterials

Electrode materials which undergo anion insertion are a void in the materials innovation landscape and a missing link to energy efficient electrochemical desalination. In recent years layered hydroxides (LHs) have been studied for a range of electrochemical applications, but to date have not been considered as electrode materials for anion insertion electrochemistry. Here, we show reversible anion insertion in a LH for the first time using Co and Co-V layer hydroxides. By pairing in situ synchrotron and quartz crystal microbalance measurements with a computational unified electrochemical band-diagram description, we reveal a previously undescribed anion-insertion mechanism occurring in Co and Co-V LHs. This proof of concept study demonstrates reversible electrochemical anion insertion in LHs without significant material optimization. These results coupled with our foundational understanding of anion insertion electrochemistry establishes LHs as a materials platform for anion insertion electrochemistry with the potential for future application to electrochemical desalination.

One-dimensional CoS2-MoS2 nano-flakes decorated MoO2 sub-micro-wires for synergistically enhanced hydrogen evolution

CoS2-MoS2 nanoflakes decorated MoO2 (CoMoOS) hybrid submicro-wires with rich active interfaces were synthesized via the sulfuration of CoMoO4. They showed excellent activity while synergistically catalyzing the hydrogen evolultion reaction (HER) in basic media by promoting both the water dissociation and hydrogen absorption steps. Thus, the CoMoOS catalysts only needed 123 mV to achieve 10 mA cm-2 with a small Tafel slope in alkaline solutions, and required 1.68 V to obtain the same current density when assembled into an alkaline electrolyser.

Earth-Abundant Electrocatalysts in Proton Exchange Membrane Electrolyzers

In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

Boosting Overall Water Splitting via FeOOH Nanoflake-Decorated PrBa0.5Sr0.5Co2O5+δ Nanorods

The development of an efficient, robust, and low-cost catalyst for water electrolysis is critically important for renewable energy conversion. Herein, we achieve a significant improvement in electrocatalytic activity for both the oxygen-evolution reaction (OER) and the hydrogen-evolution reaction (HER) by constructing a novel hierarchical PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC)@FeOOH catalyst. The optimized PBSC@FeOOH-20 catalyst consisted of layered perovskite PBSC nanorods and 20 nm thick amorphous FeOOH nanoflakes exhibiting an excellent electrocatalytic activity for the OER and the HER in 0.1 M KOH media, delivering a current density of 10 mA cm^-2 at overpotentials of 390 mV for the OER and 280 mV for the HER, respectively. The substantially enhanced performance is probably attributed to the hierarchical nanostructure, the good charge-transfer capability, and the strong electronic interactions of FeOOH and PBSC. Importantly, an alkaline electrolyzer-integrated PBSC@FeOOH-20 catalyst as both the anode and cathode shows a highly active overall water splitting with a low voltage of 1.638 V at 10 mA cm^-2 and high stability during continuous operation. This study provides new insights into exploring efficient bifunctional catalysts for overall water splitting, and it suggests that the rational design of the oxyhydroxide/perovskite heterostructure shows great potential as a promising type of electrocatalysts.

Stable Water Oxidation in Acid Using Manganese-Modified TiO2 Protective Coatings

Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water-splitting devices and other fuel-forming electro- and photocatalytic processes. Because water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the photoactive component and hence loss in device performance and lifetime. In this work, we show that thin films of manganese-modified titania, (Ti,Mn)O _x, topped with an iridium catalyst, can be used in a coating stabilization scheme for acid-stable water oxidation. We achieved a device lifetime of more than 100 h in pH = 0 acid. We successfully grew (Ti,Mn)O _x coatings with uniform elemental distributions over a wide range of manganese compositions using atomic layer deposition (ALD), and using X-ray photoelectron spectroscopy, we show that (Ti,Mn)O _x films grown in this manner give rise to closer-to-valence-band Fermi levels, which can be further tuned with annealing. In contrast to the normally n-type or intrinsic TiO₂ coatings, annealed (Ti,Mn)O _x films can make direct charge transfer to a Fe(CN)₆^3-/4- redox couple dissolved in aqueous electrolytes. Using the Fe(CN)₆^3-/4- redox, we further demonstrated anodic charge transfer through the (Ti,Mn)O _x films to high work function metals, such as iridium and gold, which is not previously possible with ALD-grown TiO₂. We correlated changes in the crystallinity (amorphous to rutile TiO₂) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)O _x films to their hole conductivity and electrochemical stability in acid. Finally, by combining (Ti,Mn)O _x coatings with iridium, an acid-stable water-oxidation anode, using acid-sensitive conductive fluorine-doped tin oxides, was achieved.

Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

Electronic Structure Tuning in Ni3FeN/r-GO Aerogel toward Bifunctional Electrocatalyst for Overall Water Splitting

Searching for the highly active, stable, and high-efficiency bifunctional electrocatalysts for overall water splitting, e.g., for both oxygen evolution (OER) and hydrogen evolution (HER), is paramount in terms of bringing future renewable energy systems and energy conversion processes to reality. Herein, three-dimensional (3D) Ni₃FeN nanoparticles/reduced graphene oxide (r-GO) aerogel electrocatalysts were fabricated using precursors of (Ni,Fe)/r-GO alginate hydrogels through an ion-exchange process, followed by a convenient one-step nitrogenization treatment in NH₃ at 700 °C. The resultant materials exhibited excellent electrocatalytic performance for OER and HER in alkaline media, with only small overpotentials of 270 and 94 mV at a current density of 10 mA cm^-2, respectively. The good performance was attributed to abundant active sites and high electrical conductivity of the bimetallic nitrides and efficient mass transport of the 3D r-GO aerogel framework. Furthermore, an alkaline electrolyzer was set up using Ni₃FeN/r-GO as both the cathode and the anode, which achieved a 10 mA cm^-2 current density at 1.60 V with durability of 100 h for overall water splitting. Density functional theory calculations support that Ni₃FeN (111)/r-GO is more favorable for overall water splitting since the surface electronic structure of Ni₃FeN is tuned by transferring electrons from Ni₃FeN cluster to the r-GO through interaction of two metal species. Thus, the currently developed Ni₃FeN/r-GO with superior water-splitting performance may potentially serve as a material for use in industrial alkaline water electrolyzers.

Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media

Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO₂ catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm^-2. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

Design of template-stabilized active and earth-abundant oxygen evolution catalysts in acid

Oxygen evolution reaction (OER) catalysts that are earth-abundant and are active and stable in acid are unknown. Active catalysts derived from Co and Ni oxides dissolve at low pH, whereas acid stable systems such as Mn oxides (MnO x ) display poor OER activity. We now demonstrate a rational approach for the design of earth-abundant catalysts that are stable and active in acid by treating activity and stability as decoupled elements of mixed metal oxides. Manganese serves as a stabilizing structural element for catalytically active Co centers in CoMnO x films. In acidic solutions (pH 2.5), CoMnO x exhibits the OER activity of electrodeposited Co oxide (CoO x ) with a Tafel slope of 70-80 mV per decade while also retaining the long-term acid stability of MnO x films for OER at 0.1 mA cm-2. Driving OER at greater current densities in this system is not viable because at high anodic potentials, Mn oxides convert to and dissolve as permanganate. However, by exploiting the decoupled design of the catalyst, the stabilizing structural element may be optimized independently of the Co active sites. By screening potential-pH diagrams, we replaced Mn with Pb to prepare CoFePbO x films that maintained the high OER activity of CoO x at pH 2.5 while exhibiting long-term acid stability at higher current densities (at 1 mA cm-2 for over 50 h at pH 2.0). Under these acidic conditions, CoFePbO x exhibits OER activity that approaches noble metal oxides, thus establishing the viability of decoupling functionality in mixed metal catalysts for designing active, acid-stable, and earth-abundant OER catalysts.