Anion insertion enhanced electrodeposition of robust metal hydroxide/oxide electrodes for oxygen evolution

Regulating interfacial microenvironment via anion adsorption to boost oxygen evolution reaction

The often-overlooked anions in raw materials have been less explored in terms of their promotion of the oxygen evolution reaction (OER) from the perspective of interfacial microenvironment regulation. In this study, we obtained the catalyst (CoOOH-NO3-) by a one-step electrochemical reconstruction method, in which anion adsorption onto the active catalyst can optimize the interfacial microenvironment to promote OER under alkaline conditions. This is because the issue of excessive OH- adsorption within the inner compact layer of CoOOH can be ameliorated by the adsorption of anions, making it easier for active sites to branch OH-, thereby regulating the interfacial microenvironment. It was validated through a series of experiments that after tuning the interfacial microenvironment, reduction in the contact angle on the electrode surface facilitates the release of O2, promotes Co transformation to a higher oxidation state (Co(IV)) and lowers the onset potential for the reaction. Furthermore, the one-step synthesis method, along with the strategy of microenvironment regulation, applies to various metal salts (chlorides, acetates). Our research introduces a non-chemical synthesis method for the direct use of metal salts, providing a rational approach and insight for understanding how anion adsorption regulates interfacial microenvironment to enhance catalytic activity.

Coupling CoNi-LDH and Fe2N Nanoparticles on N-Doped Biomass-Derived Carbon as Oxygen Electrocatalyst for Rechargeable Zn-Air Batteries

The engineering of structure and composition is crucial and challenging for modulating electrocatalytic activity. Herein, a bifunctional electrocatalyst, CoNi@Fe2N/C, is designed and prepared by coupling CoNi-layered double hydroxides (CoNi-LDH) nanosheets and Fe2N nanoparticles on 3D N-doped biomass-derived carbon through pyrolysis and electric field induction. CoNi@Fe2N/C delivers remarkable bifunctional electrocatalytic activity with a narrow overpotential gap (ΔE) of only 0.63 V for ORR and OER, surpassing commercial Pt/C+RuO2. Additionally, the aqueous rechargeable zinc-air batteries (RZABs) assembled with CoNi@Fe2N/C as a catalyst on the air electrode show a high peak power density (177.4 mW cm-2) and excellent durability (over 360 h). The high stability of 3D biomass-derived carbon assures the high durability of catalysts. Assistance of electric field induction ensures the uniform dispersion of CoNi-LDH nanosheets on 3D N-doped biomass-derived carbon. The interaction at the interface between two components appropriately modulates the electronic structure of hybrids via charge transfer to promote reaction kinetics. This work will provide a practical avenue to create ORR/ OER bifunctional biomass-derived catalytically active carbon hybrid catalysts for efficient RZABs.

Zincate Ion Enables M(II)-Vacancy NiFe Layered Double Hydroxide for Stable Seawater Electrolysis at 3 A cm-2

Seawater electrolysis offers a sustainable route for hydrogen production. Operating at high current densities can improve the energy efficiency but requires anodes that can sustain high oxygen evolution reaction (OER) activity, selectivity, and stability against negative effects of Cl-. Herein, NiFeZn layered double hydroxide (NiFeZn-LDH) demonstrates remarkable OER performance, requiring only 220 mV overpotential to achieve 10 mA cm-2, and maintaining 100% selective seawater oxidation to oxygen for 500 h at an unprecedented current density of 3 A cm-2, with minimal degradation. Through comprehensive characterizations, it is found that the dissolution of the amphoteric Zn-site and the following formation of Zn2+ vacancies are key to the excellent OER activity. The free Zn2+ in electrolyte converts to Zn(OH)4 2- and adsorbs onto the electrode, facilitating the OH- nucleophilic attack by disrupting the hydrogen bond network at the electrochemical interface. Furthermore, the steric hindrance of Zn(OH)4 2- suppresses the Cl- competing adsorption, ensuring 100% OER selectivity and long-term stability. As a result, an industrial-scale electrolyzer with NiFeZn-LDH as the anode operates stably for over 700 h in a saturated NaCl electrolyte, consuming only 4.26 Nm-3 H2. This work demonstrates the feasibility of developing energy-efficient, highly stable seawater electrolyzers that outperform conventional water electrolyzers.

Strategies for industrial-grade seawater electrolysis: from electrocatalysts and device design to techno-economic analysis

Seawater electrolysis offers a promising route for sustainable hydrogen production, utilizing abundant seawater to meet global energy demands while addressing environmental concerns. However, challenges such as inefficiencies, high costs, and reliance on noble metal catalysts hinder its practical implementation. This review examines the fundamental mechanisms of seawater electrolysis, focusing on the hydrogen and oxygen evolution reactions (HER and OER) at industrial-scale current densities. Key strategies for catalyst design, including interfacial engineering, structural optimization, and improved mass and electron transport, to enhance efficiency and stability are discussed. Additionally, device architecture and techno-economic considerations are explored to facilitate scalable, cost-effective deployment. By providing insights into advanced materials and system innovations, this review outlines pathways for integrating seawater electrolysis into large-scale sustainable energy solutions.

Obtaining materials from local sources: surface modification engineering enabled substrates for water splitting

The preparation of an efficient electrode is the key to achieving efficient overall water-splitting for H2 production. Substrate surface modification engineering (SSME) provides a feasible method for preparing self-supported electrodes with high active site utilization, fast mass transport, and a simple fabrication process. This review summarizes and discusses the recent advances in preparing transition-metal-based HER/OER electrocatalysts via SSME. We first highlight the description and advantages of SSME, followed by the detailed introduction of electrocatalysts prepared via the SSME, such as hydroxides, oxyhydroxides, chalcogenides, phosphides, and borides. Finally, we provide the challenges and perspectives. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.

FeHf Binary Hydroxide/Oxide Nanostructures as Catalysts for Oxygen Evolution

We present pulsed electrodeposition (PED) of FeHf binary hydroxide/oxide (FeHf-BH) nanocomposites from aqueous electrolyte baths containing NO3 - ions. The deposition was carried out on a graphite foil at room temperature. This study, for the first time, demonstrated a controlled variation of Fe (5.9-49.9 avg. at. %) and Hf (2.4-58.7 avg. at. %) in the deposited materials. We showed the high scalability of FeHf-BH deposition by tuning the PED parameters. The morphology, composition, chemical structure, and oxidation states of metals in the materials were investigated by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The deposited materials consist of agglomerated nanoparticles sized 50-150 nm. Thermal annealing studies revealed improved crystallinity, with the appearance of thermodynamically stable oxide phases of Fe3O4, Fe2O3, and HfO2 in the composites. The oxygen evolution activity of the materials was analyzed in an alkaline medium based on the Hf content. The optimized material containing 11.9 avg. at. % Hf demonstrated an OER onset potential of 1.63 V vs RHE, Tafel slope of 47 mV dec-1, and required only 470 mV overpotential to reach a 50 mA cm-2 OER current. These PED strategies of designing FeHf-BH materials may open an avenue for designing other catalytically active and stable multimetallic hydroxides/oxides composites.

Exploring the properties, types, and performance of atomic site catalysts in electrochemical hydrogen evolution reactions

Atomic site catalysts (ASCs) have recently gained prominence for their potential in the electrochemical hydrogen evolution reaction (HER) due to their exceptional activity, selectivity, and stability. ASCs with individual atoms dispersed on a support material, offer expanded surface areas and increased mass efficiency. This is because each atom in these catalysts serves as an active site, which enhances their catalytic activity. This review is focused on providing a detailed analysis of ASCs in the context of the HER. It will delve into their properties, types, and performance to provide a comprehensive understanding of their role in electrochemical HER processes. The introduction part underscores HER's significance in transitioning to sustainable energy sources and emphasizes the need for innovative catalysts like ASCs. The fundamentals of the HER section emphasizes the importance of understanding the HER and highlights the key role that catalysts play in HER. The review also explores the properties of ASCs with a specific emphasis on their atomic structure and categorizes the types based on their composition and structure. Within each category of ASCs, the review discusses their potential as catalysts for the HER. The performance section focuses on a thorough evaluation of ASCs in terms of their activity, selectivity, and stability in HER. The performance section assesses ASCs in terms of activity, selectivity, and stability, delving into reaction mechanisms via experimental and theoretical approaches, including density functional theory (DFT) studies. The review concludes by addressing ASC-related challenges in HER and proposing future research directions, aiming to inspire further innovation in sustainable catalysts for electrochemical HER.

Electrocatalytic Conversion of Glucose into Renewable Formic Acid Using "Electron-Withdrawing" MoO3 Support under Mild Conditions

Electrocatalysis is a sustainable and effective approach to produce value-added chemical commodities from biomass, where highly effective catalyst is required. Since transition metal hydroxide is a feasible catalyst for electrochemical biomass conversion, rational optimization of its electrocatalytic activity is highly desired. Herein, electrocatalytic activity of glucose oxidation is significantly optimized by reducing the electron density at Ni active sites, which is achieved by depositing Ni(OH)2 at "electron-withdrawing" MoO3 support (Ni(OH)2MoO3-x). As results, the formation of active sites (NiOOH) and the adsorption of glucose are simultaneously facilitated in Ni(OH)2MoO3-x, which effectively converts glucose to formic acid (FA) with remarkable yield and Faraday efficiency (≈90.5 and 98%, respectively), far superior to conventional β-Ni(OH)2 catalyst (≈22.5 and 58.9%, respectively). In addition to a novel strategy for efficient FA production from glucose, this work offers valuable insights into the rational optimization of electrocatalytic oxidation of biomass-based substrates.

Construction of Self-healing Active Centers over Amorphous Ni2Fe(OH)x Nanosheets for Enhanced Oxygen Evolution Performance

Developing efficient nickel-iron-based electrocatalysts for the oxygen evolution reaction (OER) still remains a challenge for long-term application in water electrolysis. Herein, amorphous Ni2Fe hydroxide nanosheets with self-healing active centers supported on stainless steel mesh (a-Ni2Fe(OH)x/SSM) are fabricated using a simple electrochemical deposition strategy. In situ Raman evidence shows that the amorphous structure of a-Ni2Fe(OH)x/SSM exhibits strong self-healing ability, efficiently promoting the rapid interconversion between the γ-NiOOH intermediate and Ni-based hydroxides during electrocatalysis. As a result, the as-obtained a-Ni2Fe(OH)x/SSM exhibits a low overpotential of 247 mV at 100 mA cm-2 and robust electrochemical stability for 200 h. Moreover, an anion-exchange membrane electrolyzer for water splitting is assembled utilizing a-Ni2Fe(OH)x/SSM as the anode, and current densities of 500 and 1000 mA cm-2 are achieved at 1.62 and 1.72 V, respectively, with stable performance at 500 mA cm-2 over 30 h. This work provides a facile approach for designing amorphous catalysts for various useful reactions.

Site-specific synergy in heterogeneous single atoms for efficient oxygen evolution

Heterogeneous single-atom systems demonstrate potential to break performance limitations of single-atom catalysts through synergy interactions. The synergy in heterogeneous single atoms strongly dependes on their anchoring sites. Herein, we reveal the site-specific synergy in heterogeneous single atoms for oxygen evolution. The RuTIrV/CoOOH is fabricated by anchoring Ru single atoms onto three-fold facial center cubic hollow sites and Ir single atoms onto oxygen vacancy sites on CoOOH. Moreover, IrTRuV/CoOOH is also prepared by switching the anchoring sites of single atoms. Electrochemical measurements demonstrate the RuTIrV/CoOOH exhibits enhanced OER performance compared to IrTRuV/CoOOH. In-situ spectroscopic and mechanistic studies indicate that Ru single atoms at three-fold facial center cubic hollow sites serve as adsorption sites for key reaction intermediates, while Ir single atoms at oxygen vacancy sites stabilize the *OOH intermediates via hydrogen bonding interactions. This work discloses the correlation between the synergy in heterogeneous single atoms and their anchoring sites.

Insight into the structural reconstruction of alkaline water oxidation electrocatalysts

The oxygen-evolution reaction (OER) is an indispensable component of various energy storage and conversion electrocatalytic systems. However, the slow reaction kinetics have forced the development of advanced, efficient, and inexpensive OER electrocatalysts to break through the bottleneck of its application. Recently, the structural reconstruction of precatalysts has provided a promising avenue to boost the catalytic activity of electrocatalysts. Structural reconstruction implies atomic rearrangement and composition change of the pristine catalytic materials, which is a very complex process. Therefore, it is very crucial to have a deep understanding of the reconstruction chemical process and then modulate the reconstruction by deliberate design of electrochemical conditions and precatalysts. However, a systematic review of the structural reconstruction process, research methods, influencing factors and structure-performance relationship remains elusive, significantly impeding the further developments of efficient electrocatalysts based on structural reconstruction chemistry. This critical review is dedicated to providing a deep insight into the structural reconstruction during alkaline water oxidation, comprehensively summarizing the basic research methods to understand the structural evolution process and various factors affecting the structural reconstruction process, and providing a reference and basis for regulating the dynamic reconstruction. Moreover, the impact of reconstruction on the structure and performance is also covered. Finally, challenges and perspectives for the future study on structural reconstruction are discussed. This review will offer future guidelines for the rational development of state-of-the-art OER electrocatalysts.

Enhanced Glycerol Electrooxidation Capability of NiO by Suppressing the Accumulation of Ni4+ Sites

Glycerol electrooxidation (GOR), as a typical nucleophilic biomass oxidation reaction, provides a promising anodic alternative for coupling green hydrogen generation at the cathode. However, the challenges of identifying active sites and elucidating reaction mechanisms greatly limit the design of high-performance catalysts. Herein, we use NiO and Ni/NiO as model catalysts to investigate glycerol oxidation. Electrochemical measurements and operando spectroscopic studies uncovered that Ni2+/Ni3+ species are the true active sites of NiO for GOR at lower potentials. However, the Ni2+/Ni3+ species formed on the NiO surface were easily converted to Ni4+ species (NiO2) at higher potentials, which not only contributed to the overoxidation of glycerol electrolysis products but also worked as the main active sites of the competitive oxygen evolution reaction (OER), resulting in the rapid decay of Faradic efficiencies (FEs) at high potentials. Interestingly, for Ni/NiO, only Ni3+ species were formed on the surface. Experimental and density functional theory (DFT) investigations indicated that due to the relatively lower average valence state of Ni in Ni/NiO and strong electronic interaction on the Ni/NiO interface, the surface reconstruction of Ni/NiO was effectively manipulated. Only Ni/NiO → NiOOH (Ni3+) transformation was observed, and the formation of Ni4+ species was greatly suppressed. As a result, Ni/NiO delivered superior GOR activity, and the FE did not drop apparently at high potentials. This work offers mechanistic insight into how to identify and maintain the true active sites of catalytic materials for value-added nucleophile electrooxidation reactions.

Cerium Oxide-Induced Synchronous Lattice Oxygen Activation and Accelerated Deprotonation Kinetics in Cobalt (oxy)Hydroxide for Robust Water Oxidation

Theoretically, triggering the lattice oxygen mechanism (LOM) of the catalysts during the alkaline oxygen evolution reaction (OER) can effectively break through the thermodynamic limitations, while following this path, the rate of simultaneous deprotonation also determines the overall kinetics. A cerium oxide units-modified cobalt (oxy)hydroxide nanocomposite of CeO2-CoOOH/NF is proposed, where the Ce(4f)-O(2p)-Co (3d) coupling with sites interaction mediates the Co─O Mott-Hubbard splitting state to trigger efficient LOM. Meanwhile, the 4f orbital electron-rich state near the Fermi level is favorable for proceeding the electron-involved deprotonation behavior. All these empower CeO2-CoOOH/NF with considerable OER activity, which delivers an overpotential of 249 mV at 10 mA cm-2, and coupling with commercial Pt/C in anion exchange membrane water electrolyze (AEMWE) to realize energy-saving hydrogen production. This work is instructive for the design of high-performance OER catalysts through controlling the electron orbitals hybridization state of the catalysts to synchronously accelerate the kinetics of each link in OER.

High-Valence Metals Accelerate the Reaction Kinetics for Boosting Water Oxidation

The transition metal with high valence state in oxyhydroxides can accelerate the reaction kinetics, enabling highly intrinsic OER activity. However, the formation of high-valence transition-metal ions is thermodynamically unfavorable in most cases. Here, a novel strategy is proposed to realize the purpose and reveal the mechanism by constructing amorphous phase and incorporating of elements with the characteristic of Lewis acid or variable charge state. A model catalyst, CeO2-NiFeOxHy, is presented to achieve the modulation of valence state of active site (Ni2+→Ni3+→Ni4+) for improved OER, leading to dominant active sites with high valence state. The CeO2-NiFeOxHy electrode exhibits superior OER performance with overpotential of 214 and 659 mV at 10 and 500 mA cm-2, respectively (without IR correction), and high stability, which are much better than those of NiOxHy, NiFeOxHy and CeO2-NiOxHy. These findings provide an effective strategy to achieve the active metals with high-valence state for highly efficient OER.

Operando Studies Redirect Spatiotemporal Restructuration of Model Coordinated Oxides in Electrochemical Oxidation

Tetrahedral, pyramidal, and octahedral metal-oxygen coordinated ligands are fundamental components in all metal-oxide structures. Understanding the impacts of their spatiotemporal behaviors during electrochemical oxidation is crucial for diverse applications, yet remains unsolved due to challenges in designing model oxides and conducting operando characterizations. Herein, combining a suite of advanced operando characterizations and systematic computations, a link between oxygen-evolving performance and operational structural properties is established on model oxides. Compared with tetrahedral and octahedral structures, pyramidal structure is more susceptible to OH- attack due to its pristine unsaturated and asymmetric features and constant single-electron occupancy on the active z2 orbital during reaction, leading to surface-to-bulk restructuration into active amorphous high-valence CoOOHx with edge-sharing configurations. This is accompanied by ion leaching to create nanoscale space, following a leaching tendency of Sr2+ > Ba2+ > La3+ > Y3+. Operando soft X-ray absorption spectroscopy demonstrates a harder non-uniform dehydrogenation process over time (Co3+OOH → Co3+/4+OOHx → Co4+OO) because of the enhanced CoO covalency with higher energy barriers. Lattice oxygen participates in active CoOOHx formation but sacrifices stability. To address this activity-stability trade-off, an ion-tuning strategy is proposed to simultaneously enhance both activity and stability in electrode and device.

Optimizing the stability of NiFeOOH via oxyanion intercalation for water oxidation at large current densities

In alkaline water splitting, transition metals (Ni, Fe) have received extensive attention, and NiFe-oxyhydroxide (NiFeOOH) is regarded as an exceptionally active electrocatalysts for oxygen evolution reaction (OER). However, maintaining the long-term stability of NiFeOOH at high current densities is challenging due to Fe segregation and catalyst degradation. Herein, this study proposes an approach to enhancing the stability of the Ni/Fe-O covalent bond by intercalating oxyanions (NO3-, PO43-, SO42-, and SeO42-) into the NiFeOOH substrate, improving its resistance to bond breakage. And the NiFeOOH-NO3- electrocatalyst was found to be optimal, achieving an overpotential of 311 mV and stable performance at 1 A cm-2 for several hundred hours. Consequently, NiFeOOH-NO3- exhibited a significantly improved OER stability, with a mere 3.33 % stability attenuation after 100 h, compared to 13.19 % for pristine NiFeOOH. Notably, the presence of NO3- in NiFeOOH effectively mitigates Fe segregation, leading to a fourfold enhancement in long-term stability relative to that of NiFeOOH without NO3- modification. Theoretical calculations show that the introduction of NO3- effectively shifts metal 3d band centers of NiFeOOH closer to the Fermi level. It is suggested that the oxyanions lead to increased strength of the Ni/Fe-O bonds, thereby inhibiting the dissolution of Fe and enhancing the stability of NiFeOOH phase. This research represents a significant advance in controlling Fe segregation to stabilize NiFe-based electrocatalysts for high-current-density water oxidation.

Electrodeposited Nitrate-Intercalated NiFeCe-Based (Oxy)hydroxide Heterostructure as a Competent Electrocatalyst for Overall Water Splitting

Electrochemical water splitting is a promising method for the generation of "green hydrogen", a renewable and sustainable energy source. However, the complex, multistep synthesis processes, often involving hazardous or expensive chemicals, limit its broader adoption. Herein, a nitrate (NO3-) anion-intercalated nickel-iron-cerium mixed-metal (oxy)hydroxide heterostructure electrocatalyst is fabricated on nickel foam (NiFeCeOxHy@NF) via a simple electrodeposition method followed by cyclic voltammetry activation to enhance its surface properties. The NiFeCeOxHy@NF electrocatalyst exhibited a low overpotential of 72 and 186 mV at 10 mA cm-2 for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1.0 M KOH. In a two-electrode system, the NiFeCeOxHy@NF obtained a low voltage of 1.47 V at 10 mA cm-2 in 1.0 M KOH with robust stability. Results revealed that the notable activity of the NiFeCeOxHy@NF catalyst is primarily due to (i) hierarchical nanosheet morphology, which provides a large surface area and abundant active sites; (ii) NO3- anion intercalation enhances electrode stability and eliminates the need for binders while simultaneously promoting a strong catalyst-substrate adhesion, resulting in decreased electrode resistance and accelerated reaction kinetics; and (iii) the unique superhydrophilic surface properties facilitate electrolyte penetration through capillary action and minimize gas bubble formation by reducing interfacial tension.

Electrochemical reconfiguration of iron-modified Ni3S2 surface induced oxygen vacancies to immobilize sulfate for enhanced oxygen evolution reaction

There is an urgent need for highly active, durable, and low-cost electrocatalysts to overcome the shortcomings of high overpotential in the oxygen evolution reaction (OER) process. In this work, the nickel-iron hydroxysulfate rich in sulfate and oxygen vacancies (SO42-@Fe-NiOOH-Ov/NiS) is legitimately constructed. SO42-@Fe-NiOOH-Ov/NiS only requires a low overpotentials of 190 mV and 232 mV at 10 mA cm-2 and 100 mA cm-2 current densities in 1 M KOH, with excellent stability for 200 h at 100 mA cm-2 current density. In situ Raman spectroscopy and Fourier transform infrared spectroscopy demonstrated the stable adsorption of more SO42- on the surface of catalyst. Density functional theory calculations testify surface reconstruction, doped Fe and oxygen vacancies significantly reduced the adsorption energy of sulfate on the surface. More importantly, the formation of *OOH to O2 is facilitated by the highly hydrogen bonding between SO42- and *OOH, accelerating the OER process.

Doping Ti into RuO2 to Accelerate Bridged-Oxygen-Assisted Deprotonation for Acidic Oxygen Evolution Reaction

The development of efficient and durable electrocatalysts for the acidic oxygen evolution reaction (OER) is essential for advancing renewable hydrogen energy technology. However, the slow deprotonation kinetics of oxo-intermediates, involving the four proton-coupled electron steps, hinder the acidic OER progress. Herein, a RuTiOx solid solution electrocatalyst is investigated, which features bridged oxygen (Obri) sites that act as proton acceptors, accelerating the deprotonation of oxo-intermediates. Electrochemical tests, infrared spectroscopy, and density functional theory results reveal that the moderate proton adsorption energy on Obri sites facilitates fast deprotonation kinetics through the adsorbate evolution mechanism. This process effectively prevents the over-oxidation and deactivation of Ru sites caused by the lattice oxygen mechanism. Consequently, RuTiOx shows a low overpotential of 198 mV at 10 mA cm-2 geo and performance exceeding 1400 h at 50 mA cm-2 geo with negligible deactivation. These insights into the OER mechanism and the structure-function relationship are crucial for the advancement of catalytic systems.

The Interfacial Ni/Fe─O─Y Bonds Contribute to High-Efficiency Water Splitting

Developing economical and efficient electrocatalysts is critical for hydrogen energy industrialization through water electrolysis. Herein, a novel dual-site synergistic NiFe/Y2O3 hybrid with abundant interfacial Ni/Fe─O─Y bonds is designed by density functional theory (DFT) simulations. In situ Raman spectra combined with DFT calculations reveal that the interfacial Ni/Fe─O─Y units greatly promote H2O dissociation and optimize the adsorption of both H* and oxygen species, achieving excellent activity and durability for hydrogen evolution reaction. As expected, NiFe/Y2O3 exhibits a low overpotential of 27 mV at 10 mA cm-2 and robust stability of over 200 h at 1000 mA cm-2, and also outstanding water splitting performance with a low cell voltage of 1.64 V at 100 mA cm-2, showing significant potential for real-world applications.

Transition metals-based electrocatalysts on super-flat substrate for perovskite photovoltaic hydrogen production with 13.75% solar to hydrogen efficiency

Harnessing the inexhaustible solar energy for water splitting is regarded one of the most promising strategies for hydrogen production. However, sluggish kinetics of oxygen evolution reaction (OER) and expensive photovoltaics have hindered commercial viability. Here, an adhesive-free electrodeposition process is developed for in-situ preparation of earth-abundant electrocatalysts on super-flat indium tin oxide (ITO) substrate. NiFe hydroxide exhibited prominent OER performance, achieving an ultra-low overpotential of 236 mV at 10 mA/cm2 in alkaline solution. With the superior OER activity, we achieved an unassisted solar water splitting by series connected perovskite solar cells (PSCs) of 2 cm2 aperture area with NiFe/ITO//Pt electrodes, yielding overall solar to hydrogen (STH) efficiency of 13.75 %. Furthermore, we upscaled the monolithic facility to utilize perovskite solar module for large-scale hydrogen production and maintained an approximate operating current of 20 mA. This creative strategy contributes to the decrease of industrial manufacturing expenses for perovskite-based photovoltaic-electrochemical (PV-EC) hydrogen production, further accelerating the conversion and utilization of carbon-free energy.

Out-of-plane coordination of iridium single atoms with organic molecules and cobalt-iron hydroxides to boost oxygen evolution reaction

Advancements in single-atom-based catalysts are crucial for enhancing oxygen evolution reaction (OER) performance while reducing precious metal usage. A comprehensive understanding of underlying mechanisms will expedite this progress further. Here we report Ir single atoms coordinated out-of-plane with dimethylimidazole (MI) on CoFe hydroxide (Ir1/(Co,Fe)-OH/MI). This Ir1/(Co,Fe)-OH/MI catalyst, which was prepared using a simple immersion method, delivers ultralow overpotentials of 179 mV at a current density of 10 mA cm-2 and 257 mV at 600 mA cm-2 as well as an ultra-small Tafel slope of 24 mV dec-1. Furthermore, Ir1/(Co,Fe)-OH/MI has a total mass activity exceeding that of commercial IrO2 by a factor of 58.4. Ab initio simulations indicate that the coordination of MI leads to electron redistribution around the Ir sites. This causes a positive shift in the d-band centre at adjacent Ir and Co sites, facilitating an optimal energy pathway for OER.

Heterostructure Fe7S8/Mn(OH)2 of incomplete sulfurization induces Mn atoms with high density of states for enhancing oxygen evolution reaction and supercapacitor electrochemical performance

Heterostructures and the introduction of heterogeneous elements have been regarded as effective strategies to promote electrochemical performance. Herein, sulfur species are introduced by a simple hydrothermal vulcanization method, which constructs the open heterostructure Fe7S8/Mn(OH)2 as a bifunctional material. The open cordyceps-like morphology can make the material contact more sufficiently with the electrolyte, exposing a large number of reaction sites. Furthermore, the introduction of the heterogeneous element S successfully constructs a heterogeneous interface, the interface-modulated composite material consists of Mn atoms contributing the main density of states (DOS) near the Fermi energy level from the density functional theory (DFT) calculations, which optimizes the adsorption energy of oxygen-containing intermediates during the oxygen evolution reaction (OER) process and reduces the reaction energy barrier, being conducive to the improvement of the material's electrochemical properties. As predicted, the Fe7S8/Mn(OH)2 material exhibits remarkable electrochemical properties, such as an overpotential of 202 mV at 10 mA cm-2 for the oxygen evolution reaction and even a specific capacitance of 2198 F g-1 at 1 A g-1. This work provides new insights into the role of introducing sulfur species and controlling the structure of the material, and exemplifies novel design ideas for developing bifunctional materials for energy storage and conversion.

Engineering electronic structures and oxygen vacancies of manganese-doped nickel molybdate porous nanosheets for efficient oxygen evolution reaction

The design strategy of designing effective local electronic structures of active sites to improve the oxygen evolution reaction (OER) performance is the key to the success of sustainable alkaline water electrolysis processes. Herein, a series of manganese-doped nickel molybdate porous nanosheets with rich oxygen vacancies on the nickel foam (Mn-NiMoO4/NF PNSs) synthesized by the facile hydrothermal and following annealing routes are used as high-efficiency and robust catalysts towards OER. By virtue of unique nanosheets architectures, more exposed active site, rich oxygen vacancies, tailored electronic structures, and improved electrical conductivity induced by Mn incorporation, as predicted, the optimized Mn0.10-NiMoO4/NF PNSs catalyst exhibits superior the OER performance with a low overpotential of 211 mV at 10 mA‧cm-2, a small Tafel slope of 41.7 mV‧dec-1, and an excellent stability for 100 h operated at 100 mA‧cm-2 in 1.0 M KOH electrolyte. The in-situ Raman measurements reveal the surface dynamic reconstruction. Besides, the results of density functional theory (DFT) calculations unveil the reaction mechanism. This study provides an effective design strategy via Mn incorporation to synergistically engineer electronic structures and oxygen vacancies of metal oxides for efficiently boosting the OER performance.

Novel electrochemical process for recycling of valuable metals from spent lithium-ion batteries

Effective recovery of Li, Co, Ni and Mn from cathode materials of spent lithium-ion batteries (LIBs) has become a global concern. In this study, electrolysis of copper sulfate to produce sulfuric acid and electrons were utilized to recover Li, Co, Ni and Mn from spent LIBs. The obtained results showed that 93 % of Ni, 91 % of Co, 89 % of Mn and 94 % of Li were leached and 99 % of Cu was deposited during leaching process by adopting the 0.225 mol/L of copper sulfate with a solid/liquid ratio of 15 g/L at a current density of 50 mA/m2 and 80 °C for 4.5 h. Then, a current efficiency of 72 % for the cathode and 30 % for the anode was achieved at a current density of 40 mA/m2, 70 °C and pH 2.5 during electrodeposition process. The Ni-Co deposition followed the principle of anomalous codeposition and the complete deposition time of Co, Ni and Mn were 3 h, 9 h and 10 h, respectively. Eventually, the Ni, Co, Mn, Li and Cu can be recovered as Ni-Co alloy, MnO2 and Li2CO3 and Cu metals with the corresponding recovery rates of 99.40 %, 91.00 %, 90.68 %, 85.59 % and 89.55 %, respectively. This study proposes a promising strategy for recycling cathode materials from spent LIBs without addition of chemical reductants and acids.

In Situ Anodic Transition and Cathodic Contamination Affect the Overall Voltage of Alkaline Water Electrolysis

NiFe (oxy)hydroxide has been widely used as a benchmark anodic catalyst for oxygen evolution reactions (OERs) in alkaline water electrolysis devices; however, the energy saving actually takes contributions from both the anodic OER and cathodic hydrogen evolution reaction (HER). In this work, we observe the catalytic promotion upon the in situ-derived NiFe (oxy)hydroxide from the NiFe alloy monolithic electrode and also point out that the coupled nickel cathode is contaminated, leading to the loss of HER activity and a reduction in overall efficiency. It is found that Ni2+ and Fe3+ cations are inevitably detached from the anode into the electrolyte and electrodeposited on the nickel cathode after the three-month industrial simulation. This research presents the significant enhancement of the oxygen evolution catalysis using an in situ aging process and emphasizes that the catalytic application should not only be isolated on the half reaction, but a reasonable coupled electrode match to get rid of the contamination from the electrolyte is also of great significance to sufficiently present the intrinsic catalytic yielding for the real application.

Anchoring Ni(OH)2-CeOx Heterostructure on FeOOH-Modified Nickel-Mesh for Efficient Alkaline Water-Splitting Performance with Improved Stability under Quasi-Industrial Conditions

Developing low-cost and industrially viable electrode materials for efficient water-splitting performance and constructing intrinsically active materials with abundant active sites is still challenging. In this study, a self-supported porous network Ni(OH)2-CeOx heterostructure layer on a FeOOH-modified Ni-mesh (NiCe/Fe@NM) electrode is successfully prepared by a facile, scalable two-electrode electrodeposition strategy for overall alkaline water splitting. The optimized NiCe0.05/Fe@NM catalyst reaches a current density of 100 mA cm-2 at an overpotential of 163 and 262 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1.0 m KOH with excellent stability. Additionally, NiCe0.05/Fe@NM demonstrates exceptional HER performance in alkaline seawater, requiring only 148 mV overpotential at 100 mA cm-2. Under real water splitting conditions, NiCe0.05/Fe@NM requires only 1.701 V to achieve 100 mA cm-2 with robust stability over 1000 h in an alkaline medium. The remarkable water-splitting performance and stability of the NiCe0.05/Fe@NM catalyst result from a synergistic combination of factors, including well-optimized surface and electronic structures facilitated by an optimal Ce ratio, rapid reaction kinetics, a superhydrophilic/superaerophobic interface, and enhanced intrinsic catalytic activity. This study presents a simple two-electrode electrodeposition method for the scalable production of self-supported electrocatalysts, paving the way for their practical application in industrial water-splitting processes.

The interfacial synergy of hierarchical FeCoNiP@FeNi-LDH heterojunction for efficient alkaline water splitting

In response to the growing demand for clean, green, and sustainable energy sources, the development of cost-effective and durable high-activity overall water splitting electrocatalysts is urgently needed. In this study, the heterogeneous structure formed by the combination of FeCoNiP and FeNi-LDH was homogeneously dispersed onto CuO nanowires generated by in-situ oxidation of copper foam as a substrate using an electrodeposition method. This multilevel structure exhibits excellent bifunctional properties as an electrode material in alkaline solutions, for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) only 206 mV and 147 mV overpotentials are needed to achieve a current density of 100 mA cm-2 respectively. Full water electrolysis is thus enabled to take place at such a low cell voltage as 1.64 V to reach the current density of 100 mA cm-2, which exhibits a long-term stability of 30 h. These improved electrocatalytic performances stem from the construction of multilevel structures. The X-ray photoelectron spectroscopy suggests that strong electron transfer occurs between heterogeneous structures, thus facilitating the OER and HER process. The dispersion of CuO nanowires not only increases the electrochemically active surface areas but also improves the overall hydrophilic and aerophobic properties. This work highlights the positive effect of multilevel structure in the design of more efficient electrocatalysts and provides a reference for the preparation of other low-cost, high-activity bifunctional electrocatalysts.

Structure-Activity Relationships in Oxygen Electrocatalysis

Oxygen electrocatalysis, as the pivotal circle of many green energy technologies, sets off a worldwide research boom in full swing, while its large kinetic obstacles require remarkable catalysts to break through. Here, based on summarizing reaction mechanisms and in situ characterizations, the structure-activity relationships of oxygen electrocatalysts are emphatically overviewed, including the influence of geometric morphology and chemical structures on the electrocatalytic performances. Subsequently, experimental/theoretical research is combined with device applications to comprehensively summarize the cutting-edge oxygen electrocatalysts according to various material categories. Finally, future challenges are forecasted from the perspective of catalyst development and device applications, favoring researchers to promote the industrialization of oxygen electrocatalysis at an early date.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Aging Ni(OH)2 on 3C-SiC Photoanodes to Achieve a High Photovoltage of 1.1 V and Enhanced Stability for Solar Water Splitting in Strongly Alkaline Solutions

Photoelectrochemical (PEC) water splitting is a promising approach to directly convert solar energy to renewable and storable hydrogen. However, the very low photovoltage and serious corrosion of semiconductor photoelectrodes in strongly acidic or alkaline electrolytes needed for water splitting severely impede the practical application of this technology. In this work, we demonstrate a facile approach to fabricate a high-photovoltage, stable photoanode by depositing Ni(OH)2 cocatalyst on cubic silicon carbide (3C-SiC), followed by aging in 1.0 M NaOH at room temperature for 40 h without applying electrochemical bias. The aged 3C-SiC/Ni(OH)2 photoanode achieves a record-high photovoltage of 1.10 V, an ultralow onset potential of 0.10 V vs the reversible hydrogen electrode, and enhanced stability for PEC water splitting in the strongly alkaline solution (pH = 13.6). This aged photoanode also exhibits excellent in-air stability, demonstrating identical PEC water-splitting performance for more than 400 days. We find that the aged Ni(OH)2 dramatically promotes the hole transport at the photoanode/electrolyte interface, thus significantly enhancing the photovoltage and overall PEC performance. Furthermore, the oxygen evolution reaction (OER) activity and the phase transitions of the Ni(OH)2 electrocatalyst before and after aging are systematically investigated. We find that the aging process is critical for the formation of the relatively stable and highly active Fe-doped β-NiOOH, which accounts for the enhanced OER activity and stability of the PEC water splitting. This work provides a simple and effective approach to fabricate high-photovoltage and stable photoanodes, bringing new premise toward solar fuel development.

Interfacial engineering layered bimetallic oxyhydroxides for efficient oxygen evolution reaction

Transition metal-based oxyhydroxides (MOOH) have garnered significant attention as promising catalyst for the Oxygen Evolution Reaction (OER). However, the direct synthesis of MOOH poses challenges due to the instability of trivalent cobalt and nickel salts, attrivuted to their high oxidation states. In this study, theoretical computations predicted that Co(OH)2 nanosheets are exclusively formed on carbon structures, owing to the stronger binding energy between CoOOH and CC compared to Co(OH)2. Furthermore, the presence of FeOOH interface reduces the binding energy between CoOOH and carbon structure. Experiment evidence confirms that CoOOH can be directly synthesized through controlled epitaxial growth on an FeOOH interface using a hydrothermal method. Moreover, the in-situ doping of iron leads to the formation of high-quality Fe0.35Co0.65OOH with exceptional OER performance, displaying a low overpotential of 240 mV at 10 mA cm-2 and a small Tafel slope of 43 mV dec-1. Density functional theory (DFT) calculations uncover the substantial enhancement of oxygen-containing species adsorption abilities by Fe0.35Co0.65OOH, resulting in improved OER activity. This work presents a promising strategy for the efficient preparation of layered cobalt oxyhydroxides, enabling efficient energy conversion and storage.

Boron modification promoting electrochemical surface reconstruction of NiFe-LDH for efficient and stable freshwater/seawater oxidation catalysis

Transition metal-based electrocatalysts generally take place surface reconstruction in alkaline conditions, but little is known about how to improve the reconstruction to a highly active oxyhydroxide surface for an efficient and stable oxygen evolution reaction (OER). Herein, we develop a strategy to accelerate surface reconstruction by combining boron modification and cyclic voltammetry (CV) activation. Density functional theory calculations and in-situ/ex-situ characterizations indicate that both B-doping and electrochemical activation can reduce the energy barrier and contribute to the surface evolution into highly active oxyhydroxides. The formed oxyhydroxide active phase can tune the electronic configuration and boost the OER process. The reconstructed catalyst of CV-B-NiFe-LDH displays excellent alkaline OER performance in freshwater, simulated seawater, and natural seawater with low overpotentials at 100 mA cm-2 (η100: 219, 236, and 255 mV, respectively) and good durability. This catalyst also presents outstanding Cl- corrosion resistance in alkalized seawater electrolytes. The CV-B-NiFe-LDH||Pt/C electrolyzer reveals prominent performance for alkalized freshwater/seawater splitting. This study provides a guideline for developing advanced OER electrocatalysts by promoting surface reconstruction.

Neighbouring Synergy in High-Density Single Ir Atoms on CoGaOOH for Efficient Alkaline Electrocatalytic Oxygen Evolution

The catalytic performance of single-atom catalysts was strictly limited by isolated single-atom sites. Fabricating high-density single atoms to realize the synergetic interaction in neighbouring single atoms could optimize the adsorption behaviors of reaction intermediates, which exhibited great potential to break performance limitations and deepen mechanistic understanding of electrocatalysis. However, the catalytic behavior governed by neighbouring single atoms is particularly elusive and has yet to be understood. Herein, we revealed that the synergetic interaction in neighbouring single atoms contributes to superior performance for oxygen evolution relative to isolated Ir single atoms. Neighbouring single atoms was achieved by fabricating high-density single atoms to narrow the distance between single atoms. Electrochemical measurements demonstrated that the Nei-Ir1/CoGaOOH with neighbouring Ir single atoms exhibited a low overpotential of 170 mV at a current density of 10 mA cm-2, and long-durable stability over 2000 h for oxygen evolution. Mechanistic studies revealed that neighbouring single atoms synergetic stabilized the *OOH intermediates via extra hydrogen bonding interactions, thus significantly reducing the reaction energy barriers, as compared to isolated Ir single atoms. The discovery of the synergetic interaction in neighbouring single atoms could offer guidance for the development of efficient electrocatalysts, thus accelerating the world's transition to sustainable energy.

Energy-Saving Electrochemical Hydrogen Production Coupled with Biomass-Derived Isobutanol Upgrading

The widespread application of electrochemical hydrogen production faces significant challenges, primarily attributed to the high overpotential of the oxygen evolution reaction (OER) in conventional water electrolysis. To address this issue, an effective strategy involves substituting OER with the value-added oxidation of biomass feedstock, reducing the energy requirements for electrochemical hydrogen production while simultaneously upgrading the biomass. Herein, we introduce an electrocatalytic approach for the value-added oxidation of isobutanol, a high energy density bio-fuel, coupled with hydrogen production. This approach offers a sustainable route to produce the valuable fine chemical isobutyric acid under mild condition. The electrodeposited Ni(OH)2 electrocatalyst exhibits exceptional electrocatalytic activity and durability for the electro-oxidation of isobutanol, achieving an impressive faradaic efficiency of up to 92.4 % for isobutyric acid at 1.45 V vs. RHE. Mechanistic insights reveal that side reactions predominantly stem from the oxidative C-C cleavage of isobutyraldehyde intermediate, forming by-products including formic acid and acetone. Furthermore, we demonstrate the electro-oxidation of isobutanol coupled with hydrogen production in a two-electrode undivided cell, notably reducing the electrolysis voltage by approximately 180 mV at 40 mA cm-2. Overall, this work represents a significant step towards improving the cost-effectiveness of hydrogen production and advancing the conversion of bio-fuels.

Bi-Interlayer Strategy for Modulating NiCoP-Based Heterostructure toward High-Performance Aqueous Energy Storage Devices

Nickel-cobalt (NiCo) phosphides (NCPs) possess high electrochemical activity, which makes them promising candidates for electrode materials in aqueous energy storage devices, such as supercapacitors and zinc (Zn) batteries. However, the actual specific capacitance and rate capability of NCPs require further improvement, which can be achieved through reasonable heterostructural design and loading conditions of active materials on substrates. Herein, novel hierarchical Bi-NCP heterogeneous structures with built-in electric fields consisting of bismuth (Bi) interlayers (electrodeposited on carbon cloth (CC)) are designed and fabricated to ensure the formation of uniform high-load layered active materials for efficient charge and ion transport. The resulting CC/Bi-NCP electrodes show a uniform, continuous, and high mass loading (>3.5 mg) with a superior capacitance reaching 1200 F g-1 at 1 A g-1 and 4129 mF cm-2 at 1 mA cm-2 combined with high-rate capability and durable cyclic stability. Moreover, assembled hybrid supercapacitors (HSCs), supercapatteries, and alkaline Zn-ion (AZBs) batteries constructed using these electrodes deliver high energy densities of 64.4, 81.8, and 319.1 Wh kg-1, respectively. Overall, the constructed NCPs with excellent aqueous energy storage performance have the potential for the development of novel transition metal-based heterostructure electrodes for advanced energy devices.

Balance between FeIV-NiIV synergy and Lattice Oxygen Contribution for Accelerating Water Oxidation

Hydrogen obtained from electrochemical water splitting is the most promising clean energy carrier, which is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Thus, the development of an efficient OER electrocatalyst using nonprecious 3d transition elements is desirable. Multielement synergistic effect and lattice oxygen oxidation are two well-known mechanisms to enhance the OER activity of catalysts. The latter is generally related to the high valence state of 3d transition elements leading to structural destabilization under the OER condition. We have found that Al doping in nanosheet Ni-Fe hydroxide exhibits 2-fold advantage: (1) a strong enhanced OER activity from 277 mV to 238 mV at 10 mA cm-2 as the Ni valence state increases from Ni3.58+ to Ni3.79+ observed from in situ X-ray absorption spectra. (2) Operational stability is strengthened, while weakness is expected since the increased NiIV content with 3d8L2 (L denotes O 2p hole) would lead to structural instability. This contradiction is attributed to a reduced lattice oxygen contribution to the OER upon Al doping, as verified through in situ Raman spectroscopy, while the enhanced OER activity is interpreted as an enormous gain in exchange energy of FeIV-NiIV, facilitated by their intersite hopping. This study reveals a mechanism of Fe-Ni synergy effect to enhance OER activity and simultaneously to strengthen operational stability by suppressing the contribution of lattice oxygen.

Benchmarking the Intrinsic Activity of Transition Metal Oxides for the Oxygen Evolution Reaction with Advanced Nanoelectrodes

The intrinsic activity assessment of transition metal oxides (TMOs) as key electrocatalysts for the oxygen evolution reaction (OER) has not been standardized due to uncertainties regarding their structure and composition, difficulties in accurately measuring their electrochemically active surface area (ECSA), and deficiencies in mass-transfer (MT) rates in conventional measurements. To address these issues, we utilized an electrodeposition-thermal annealing method to precisely synthesize single-particle TMOs with well-defined structure and composition. Concurrently, we engineered low roughness, spherical surfaces for individual particles, enabling precise measurement of their ECSA. Furthermore, by constructing a conductor-core semiconductor-shell structure, we evaluated the inherent OER activity of perovskite-type semiconductor materials, broadening the scope beyond just conductive TMOs. Finally, using single-particle nanoelectrode technique, we systematically measured individual TMO particles of various sizes for OER, overcoming MT limitations seen in conventional approaches. These improvements have led us to propose a precise and reliable approach to evaluating the intrinsic activity of TMOs, not only validating the accuracy of theoretical calculations but also revealing a strong correlation of OER activity on the melting point of TMOs. This discovery holds significant importance for future high-throughput material research and applications, offering valuable insights in electrocatalysis.

Bio-Inspired Superhydrophilic Self-Assembled Coronavirus-Like Pt-WC/CNT for Hydrogen Evolution Reaction

This study presents a novel approach to enhance the catalytic activity of composite materials by promoting active surface exposure and improving hydrogen transfer performance. Through a self-assembly route involving tailored gas-solid and galvanic replacement reactions, Pt-WC/CNT catalysts with superhydrophilicity and coronavirus-like structure are synthesized. These unique structural features contribute to a remarkable enhancement in the electrocatalytic performance of the hydrogen evolution reaction (HER). Notably, the Pt-WC/CNT catalyst exhibits an outstanding intrinsic activity and efficient bubble transfer properties, leading to a high turnover frequency of 34.97 H2·s-1 at an overpotential of 100 mV. This value is 4.8 times higher than that achieved by commercial Pt/C catalysts (7.30 H2·s-1), establishing Pt-WC/CNT as one of the most active catalysts reported to date. Moreover, the combination of gas-solid and galvanic replacement reactions in the synthesis process offers a scalable route for the production of Pt-loading controllable composite catalysts, thus challenging the dominance of commercial Pt/C catalysts.

Regulating Lateral Adsorbate Interaction for Efficient Electroreforming of Bio-polyols

Tailoring the selectivity at the electrode-electrolyte interface is one of the greatest challenges for heterogeneous electrocatalysis, and complementary strategies to catalyst structural designs need to be developed. Herein, we proposed a new strategy of controlling the electrocatalytic pathways by lateral adsorbate interaction for the bio-polyol oxidation. Redox-innocent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) anion possesses the alcoholic property that facilely adsorbs on the nickel oxyhydroxide catalyst, but is resistant to oxidation due to the electron-withdrawing trifluoromethyl groups. The alien HFIP adsorbents can compete with bio-polyols and form a mixed adsorbate layer that creates lateral adsorbate interaction via hydrogen bonding, which achieved a >2-fold enhancement of the oxalate selectivity to 55 % for the representative glycerol oxidation and can be extended to various bio-polyol substrates. Through in situ spectroscopic analysis and DFT calculation on the glycerol oxidation, we reveal that the hydrogen-bonded adsorbate interaction can effectively tune the adsorption energies and tailor the oxidation capabilities toward the targeted products. This work offers an additional perspective of tuning electrocatalytic reactions via introducing redox-innocent adsorbates to create lateral adsorbate interactions.

New perspective crosslinking electrochemistry and other research fields: beyond electrochemical reactors

Over the years, electrochemical reactors have evolved significantly, with modern reactors now able to achieve a high current density and power output in compact sizes. This leap in performance has not only greatly accelerated the rate of electrochemical reactions but also had a broader impact on the environment. Traditional research perspectives, focused primarily on the internal working systems of reactors, possibly overlook the potential of electrochemical systems in regulating their surrounding environment. A novel research perspective considering the interaction between electrochemical processes and their environmental context as a unified subject of study has gradually emerged alongside the dramatic development of electrochemical techniques. This viewpoint introduces a paradigm shift: electrochemical reactors are not isolated entities but rather are integral parts that interact with their surroundings. Correspondingly, this calls for an innovative research methodology that goes beyond studying the electrochemical processes in isolation. Rather, it integrates the design of the electrochemical system with its specific application environment, ensuring seamless integration for optimal performance under various practical conditions. Therefore, performance metrics should include not only the basic parameters of the electrochemical reactions but also the adaptability of the electrochemical system in real-world scenarios beyond the laboratory. By focusing on environmental integration and application-driven design, the applications of electrochemical technology can be more effectively leveraged. This perspective is exemplified by an electrochemical system based on coupled cathodic oxygen reduction and anodic oxygen evolution reactions. By adopting this new research paradigm, the applications of this electrochemical system can be extended to fields like medical treatment, food science, and microbial fermentation, with an emphasis on tailored designs for these specific application fields. This comprehensive and systematic new research approach aims to fully explore the potential applications of electrochemical technology and foster interdisciplinary collaboration in the electrochemical field.

From Atomic-Level Synthesis to Device-Scale Reactors: A Multiscale Approach to Water Electrolysis

ConspectusThe development of an advanced energy conversion system for water electrolysis with high efficiency and durability is of great significance for a hydrogen-powered society. This progress relies on the fabrication of electrocatalysts with superior electrochemical performance. Despite decades of advancements in exploring high-performance noble and non-noble metal electrocatalysts, several challenges persist at both the micro- and macrolevels in the field of water electrolysis.At the microlevel, which encompasses electrocatalyst synthesis and characterization, design strategies for high-performance electrocatalysts have primarily focused on interface chemical engineering. However, comprehensive understanding and investigation of interface chemical engineering across various length scales, from micrometers to atomic scales, are still lacking. This deficiency hampers the rational design of catalysts with optimal performance. Under harsh reaction conditions, such as high bias potential and highly acidic or alkaline media, the surface of catalyst materials is susceptible to undergoing "reconstruction", deviating from what is observed through ex situ characterization techniques postsynthesis. Conventional ex situ characterization methods do not provide an accurate depiction of the catalyst's structural evolution during the electrocatalytic reaction, hindering the exploration of the catalytic mechanism.At the macrolevel, pertaining to catalysis-performance evaluation systems and devices, traditional laboratory settings employ a conventional three-electrode or two-electrode system to assess the catalytic performance of electrocatalysts. However, this approach does not accurately simulate hydrogen production under realistic industrial conditions, such as elevated temperatures (60-70 °C), high current densities exceeding 0.5 A cm-2, and flowing electrolytes. To address this limitation, it is crucial to develop testing equipment and methodologies that replicate the actual industrial conditions.In this Account, we propose a multiscale research framework for water electrolysis, spanning from microscale synthesis to macroscale scaled reactor design. Our approach focuses on the design and evaluation of high-performance HER/OER (hydrogen evolution reaction/oxygen evolution reaction) electrocatalysts, incorporating the following strategies: Leveraging principles of interface chemical engineering across various length scales (micrometers, nanometers, and atoms) enables the design of catalyst materials that enhance both activity and durability. This approach provides a comprehensive understanding of the intricate interplay between the catalyst structure and activity, implementing in situ/operando characterization techniques to monitor dynamic interfacial reactions and surface reconstruction processes. This facilitates a profound exploration of catalytic reaction mechanisms, offering insights into the catalyst's structural evolution during the electrocatalytic reaction. We construct a laboratory-scale membrane electrode assembly (MEA) electrochemical reactor capable of operating at high current densities (>1 A cm-2) to evaluate the electrocatalytic performance under simulated industrial conditions. This ensures objective and authentic assessments of the catalyst application potential. Throughout the following sections, we illustrate the application of interface chemical engineering on different length scales in designing diverse electrocatalyst materials. We rely on in situ characterization techniques to gain a profound understanding of the mechanisms behind the HER and OER. Additionally, we describe the development of both acidic and alkaline MEA electrochemical reactors to enhance the precision of electrocatalytic performance evaluation. Finally, we provide a concise overview of the challenges and opportunities in this field.

External Field-Responsive Ternary Non-Noble Metal Oxygen Electrocatalyst for Rechargeable Zinc-Air Batteries

Despite the increasing effort in advancing oxygen electrocatalysts for zinc-air batteries (ZABs), the performance development gradually reaches a plateau via only ameliorating the electrocatalyst materials. Herein, a new class of external field-responsive electrocatalyst comprising Ni0.5Mn0.5Fe2O4 stably dispersed on N-doped Ketjenblack (Ni0.5Mn0.5Fe2O4/N-KB) is developed via polymer-assisted strategy for practical ZABs. Briefly, the activity indicator ΔE is significantly decreased to 0.618 V upon photothermal assistance, far exceeding most reported electrocatalysts (generally >0.680 V). As a result, the photothermal electrocatalyst possesses comprehensive merits of excellent power density (319 mW cm-2), ultralong lifespan (5163 cycles at 25 mA cm-2), and outstanding rate performance (100 mA cm-2) for liquid ZABs, and superb temperature and deformation adaptability for flexible ZABs. Such improvement is attributed to the photothermal-heating-enabled synergy of promoted electrical conductivity, reactant-molecule motion, active area, and surface reconstruction, as revealed by operando Raman and simulation. The findings open vast possibilities toward more-energy-efficient energy applications.

SOx Functionalized NiOOH Nanosheets Embedded in Ni(OH)2 Microarray for High-Efficiency Seawater Oxidation

A nano-micro heterostructure has been established to address the challenges of selectivity, stress, pitting corrosion, and long-term durability of anodes in unpurified seawater. The heterostructure comprised NiOOH nanosheets embedded within a high surface area Ni(OH)2 microarray, and the surface structure is further functionalized with sulfate (SOx). This cation-selective protective layer impedes chloride (Cl-) diffusion and abstracts H from reaction intermediates, leading to enhanced selectivity and corrosion resistance of the anode. The multilevel porosity within the randomly oriented nanosheets and the underlying support provide short diffusion channels for ions and mass migration, ensuring efficient ion transport and long-term structural and mechanical durability of the active sites, even at high current density. Remarkably, the catalyst requires a small input voltage of 400 mV to deliver a current density of 1 A cm-2 and maintains it for over 168 h without noticeable degradation or hypochlorite formation. Spectroscopic analysis and density functional theory (DFT) calculations reveal that the Ni electronic structure in the +3 valence state, its strong structural interaction with the underlying microarray, and the functionality of SOx significantly reduce the required potential for O-O coupling.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Tailoring the Brønsted acidity of Ti-OH species by regulating Pt-TiO2 interaction

Bifunctional catalysts comprising metal and acid sites are commonly used for many reactions. Interfacial acid sites impact intermediate reactions more than other sites. However, controlling the type and amounts of interfacial acid sites by regulating metal-support interaction (MSI) via traditional methods is difficult. Thus, the influence of MSI on interfacial acid sites remains unclear. We prepared Pt-mTiO2/α-Al2O3 (m represents the cycle number of TiO2) catalysts via atomic layer deposition (ALD). New Brønsted acid sites were generated via Pt-TiO2 interaction, and the acidity was precisely regulated by regulating Pt-TiO2 interaction by changing the TiO2 nanolayer thickness. We chose levulinic acid (LA) hydrogenation as a model reaction. The catalytic activity varied with the TiO2 nanolayer thickness and was linearly correlated with the Ti-OH species (Brønsted acid) content. Pt-40TiO2/α-Al2O3, with the highest acid site content of 0.486 mmol/g, exhibited the best catalytic activity. Hydrogen spillover and water dissociation at the Pt-TiO2 interface promoted Ti-OH species generation.

Cu Promoted the Dynamic Evolution of Ni-Based Catalysts for Polyethylene Terephthalate Plastic Upcycling

Upcycling plastic wastes into value-added chemicals is a promising approach to put end-of-life plastic wastes back into their ecocycle. As one of the polyesters that is used daily, polyethylene terephthalate (PET) plastic waste is employed here as the model substrate. Herein, a nickel (Ni)-based catalyst was prepared via electrochemically depositing copper (Cu) species on Ni foam (NiCu/NF). The NiCu/NF formed Cu/CuO and Ni/NiO/Ni(OH)2 core-shell structures before electrolysis and reconstructed into NiOOH and CuOOH/Cu(OH)2 active species during the ethylene glycol (EG) oxidation. After oxidation, the Cu and Ni species evolved into more reduced species. An indirect mechanism was identified as the main EG oxidation (EGOR) mechanism. In EGOR, NiCu60s/NF catalyst exhibited an optimal Faradaic efficiency (FE, 95.8%) and yield rate (0.70 mmol cm-2 h-1) for formate production. Also, over 80% FE of formate was achieved when a commercial PET plastic powder hydrolysate was applied. Furthermore, commercial PET plastic water bottle waste was employed as a substrate for electrocatalytic upcycling, and pure terephthalic acid (TPA) was recovered only after 1 h electrolysis. Lastly, density functional theory (DFT) calculation revealed that the key role of Cu was significantly reducing the Gibbs free-energy barrier (ΔG) of EGOR's rate-determining step (RDS), promoting catalysts' dynamic evolution, and facilitating the C-C bond cleavage.

Electrochemical hydrogenation and oxidation of organic species involving water

Fossil fuel-driven thermochemical hydrogenation and oxidation using high-pressure H2 and O2 are still popular but energy-intensive CO2-emitting processes. At present, developing renewable energy-powered electrochemical technologies, especially those using clean, safe and easy-to-handle reducing agents and oxidants for organic hydrogenation and oxidation reactions, is urgently needed. Water is an ideal carrier of hydrogen and oxygen. Electrochemistry provides a powerful route to drive water splitting under ambient conditions. Thus, electrochemical hydrogenation and oxidation transformations involving water as the hydrogen source and oxidant, respectively, have been developed to be mild and efficient tools to synthesize organic hydrogenated and oxidized products. In this Review, we highlight the advances in water-participating electrochemical hydrogenation and oxidation reactions of representative organic molecules. Typical electrode materials, performance metrics and key characterization techniques are firstly introduced. General electrocatalyst design principles and controlling the microenvironment for promoting hydrogenation and oxygenation reactions involving water are summarized. Furthermore, paired hydrogenation and oxidation reactions are briefly introduced before finally discussing the challenges and future opportunities of this research field.

Cross-linked α-Ni(OH)2 nanosheets with a Ni3+-rich structure for accelerating electrochemical oxidation of 5-hydroxymethylfurfural

Electrochemical oxidation of biomass-based 5-hydroxymethylfurfural (HMF) is an effective approach for achieving the high-value products of 2,5-furandicarboxylic acid (FDCA). However, the restricted formation of high-valence metal active species for electrocatalysts results in a sluggish kinetic process of HMF oxidation reaction (HMFOR). Herein, we fabricated the Ni3+-rich cross-linked α-Ni(OH)2 nanosheets for accelerating the HMFOR through an anion-mediated strategy. It is identified that the Cl- ions with strong penetrability replace a portion of lattice oxygen atoms in α-Ni(OH)2 to form Ni-Cl bonds, contributing to breaking the inherent lattice order and generating a special Ni3+-rich structure. Owing to the promoted adsorption and accelerated oxidation of hydroxyl and aldehyde groups by the affluent Ni3+ active species, the large oxidation current density of 116.5 mA cm-2 and HMFOR kinetic constant of 0.067 min-1 has been achieved at 1.45 V (vs. RHE). By analyzing the oxidation products, the FDCA yield and Faradic efficiency are both higher than 99.25 % and 99.36 % for five successive determinations. Therefore, this work provides an insightful anion-mediated strategy for designing high-performance electrocatalysts for biomass conversion application.

Zn-Leaching Induced Rapid Self-Reconstruction of NiFe-Layered Double Hydroxides for Boosted Oxygen Evolution Reaction

Optimizing the active centers through reconstruction is recognized as the key to construct high-performance oxygen evolution reaction (OER) catalysts. Herein, a simple and rapid in situ leaching strategy to promote the self-reconstruction of NiFe-layered double hydroxides (LDHs) catalysts is reported. The trace Zn dopants are introduced in advance by a facile and one-step hydrothermal method, followed by leaching over the electrochemical activation process, which can remarkably reduce the formation potential of NiFeOOH active centers to enable the deeper self-reconstruction for the formation of abundant highly active centers. Moreover, the self-restructured NiFeOOH-VZn cannot only significantly lower the dehydrogenation energy barrier for the transformation from Ni(OH)2 to NiOOH, but also decrease the free energy barrier of rate determining step for the *OH converted to *O through a deprotonation process, thus significantly boosting the OER behaviors. As a proof of concept, the obtained NiFeOOH-VZn catalyst just requires a low overpotential of 240 mV at 10 mA cm-2, and delivers robust stability at 50 mA cm-2 over 120 h, which outperforms the benchmark of noble metal RuO2 and those of most non-noble metal catalysts ever reported.