Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Advances in Electrocatalyzed Water Oxidation by Molecular Complexes of First Row Transition Metals

Energy transition toward sustainable, alternative, and affordable solutions is likely to be one of the major challenges of the anthropocene era. The oxygen evolution reaction (OER) is a pivotal electrocatalytic process essential for advancing renewable energy conversion and storage technologies, including water splitting, artificial photosynthesis, metal-air batteries, and fuel cells. Electrocatalytic pathways can significantly reduce the overall energy requirements of these devices, particularly focusing on the energy demands associated with water splitting for hydrogen production. This review, after introducing the state of the art in heterogeneous catalysis, will be devoted to the description of molecular water oxidation electrocatalysts (MWOCs), focusing on the recent advancements on catalysts composed of various metals, including Mn, Co, Cu, Ni, and Fe, in combination with a range of mono- and multidentate ligands. Critical insights are presented and discussed to provide readers with suggestions for ligand design in assisted catalysis. These observations aim to identify synergistic solutions that could enhance technological maturity by reducing energy absorption while improving stability and efficiency.

Multifunctional porphyrinic metal-organic framework-based nanoplatform regulating reactive oxygen species achieves efficient imaging-guided cascaded nanocatalytic therapy

The integration of reactive oxygen species (ROS) related photodynamic therapy (PDT) with the strategy of reshaping the tumor microenvironment (TME) has emerged as a potential approach for nanodiagnostic and therapeutic interventions. However, the therapeutic efficacy based on ROS treatments may be hindered by intracellular antioxidants such as glutathione (GSH) and tumor hypoxia. To address these challenges, a nanoplatform based on GSH-responsive multifunctional porphyrinic metal-organic framework (PCN-224@Au@MnO2@HA, PAMH) was proposed. It was developed through a layer-by-layer in-situ growth method. This method avoids the need for high-temperature calcination and complex modification processes while improving the stability of PCN-224 in a phosphate-rich environment. GSH depletion leads to oxidation-reduction imbalance in TME. With the inactivation of GSH peroxidase 4 (GPX4), the content of hydrogen peroxide (H2O2) increases, ultimately triggering lipid peroxidation (LPO) and promoting ferroptosis. The catalase-like activity of Au nanozymes facilitates the generation of oxygen (O2), thereby mitigating tumor hypoxia and downregulating hypoxia-inducing factors (HIF-1α). Due to the presence of porphyrin ligands in PCN-224, the generated O2 can be further converted to toxic singlet oxygen (1O2) under laser irradiation. Additionally, the platform allows near-infrared (NIR) fluorescence imaging, providing real-time information on intracellular GSH changes during PDT and ferroptosis. The PAMH nanoplatform has shown effective inhibition of tumor growth in subcutaneous models via both intravenous and intratumoral injection, indicating its potential in modulating reactive oxygen/sulfur species and reshaping TME, thereby facilitating imaging-guided cascaded nanocatalytic therapy.

Strategies for Enhancing Stability in Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) holds significant promise as a sustainable approach to address global energy challenges and reduce carbon emissions. However, achieving long-term stability in terms of catalytic performance remains a critical hurdle for large-scale commercial deployment. This mini-review provides a comprehensive exploration of the key factors influencing CO2RR stability, encompassing catalyst design, electrode architecture, electrolyzer optimization, and operational conditions. We examine how catalyst degradation occurs through mechanisms such as valence changes, elemental dissolution, structural reconfiguration, and active site poisoning and propose targeted strategies for improvement, including doping, alloying, and substrate engineering. Additionally, advancements in electrode design, such as structural modifications and membrane enhancements, are highlighted for their role in improving stability. Operational parameters such as temperature, pressure, and electrolyte composition also play crucial roles in extending the lifespan of the reaction. By addressing these diverse factors, this review aims to offer a deeper understanding of the determinants of long-term stability in the CO2RR, laying the groundwork for the development of robust, scalable technologies for efficient carbon dioxide conversion.

A Review of Surface Reconstruction and Transformation of 3d Transition-Metal (oxy)Hydroxides and Spinel-Type Oxides during the Oxygen Evolution Reaction

Developing efficient and sustainable electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing energy conversion and storage technologies. 3d transition-metal (oxy)hydroxides and spinel-type oxides have emerged as promising candidates due to their structural flexibility, oxygen redox activity, and abundance in earth's crust. However, their OER performance can be changed dynamically during the reaction due to surface reconstruction and transformation. Essentially, multiple elementary processes occur simultaneously, whereby the electrocatalyst surfaces undergo substantial changes during OER. A better understanding of these elementary processes and how they affect the electrocatalytic performance is essential for the OER electrocatalyst design. This review aims to critically assess these processes, including oxidation, surface amorphization, transformation, cation dissolution, redeposition, and facet and electrolyte effects on the OER performance. The review begins with an overview of the electrocatalysts' structure, redox couples, and common issues associated with electrochemical measurements of 3d transition-metal (oxy)hydroxides and spinels, followed by recent advancements in understanding the elementary processes involved in OER. The challenges and new perspectives are presented at last, potentially shedding light on advancing the rational design of next-generation OER electrocatalysts for sustainable energy conversion and storage applications.

Oxygen evolution reaction on NiFe-LDH/(Ni,Fe)OOH: theoretical insights into the effects of electronic structure and spin-state evolution

NiFe-layered double hydroxides (NiFe-LDH) have emerged as promising oxygen evolution reaction (OER) catalysts in alkaline medium, but their commercial applications are limited due to the decrease in their activity as the electrolyte becomes less alkaline. Thus, a comprehensive understanding of the OER mechanism of NiFe-LDH in alkaline medium is desirable for the rational design of new catalysts with improved performances. Especially, their spin-related factors have rarely been systematically investigated during the OER (diamagnetic H2O → paramagnetic O2). Herein, we simulated the OER performance of NiFe-LDH and (Ni,Fe)OOH as NiFe-LDH underwent surface-reconstruction and formed (Ni,Fe)OOH under alkaline conditions. Results demonstrated an enhanced OER performance on (Ni,Fe)OOH, and the Fe active site of NiFe-LDH on losing 3H (namely, NiFe(OH)2 - 3H) showed the lowest overpotential for OER because the d-orbital electron of the Fe atom shifted up to the Fermi level. Notably, the electronic interaction between Fe and OOH induced a change in the spin state of Fe, which further decreased the overpotential for the OER. Thus, the overpotential of the Fe site on NiFe(OH)2 - 3H decreased from 0.55 eV to 0.46 eV. The density of states (DOS) analysis revealed that the spin flip of Fe promoted the formation of bonding states between Fe and OOH, endowing the catalyst with a better OER performance. Our findings can help pave the way for the development of high-performance OER catalysts at the spintronic level.

NiIr Nanowire Assembles as an Efficient Electrocatalyst Towards Oxygen Evolution Reaction in Both Acid and Alkaline Media

Oxygen evolution reaction (OER) is the rate-limiting step in water electrolysis due to its sluggish kinetic, and it is challenging to develop an OER catalyst that could work efficiently in both acid and alkaline environment. Herein, NiIr nanowire assembles (NAs) with unique nanoflower morphology were prepared by a facile hydrothermal method. As a result, the NiIr NAs exhibited superior OER activity in both acid and alkaline media. Specifically, in 0.1 M HClO4, NiIr NAs presented a superior electrocatalytic performance with a low overpotential of merely 242 mV at 10 mA cm-2 and a Tafel slope of only 58.1 mV dec-1, surpassing that of commercial IrO2 and pure Ir NAs. And it achieved a significantly higher mass activity of 148.40 A/g at -1.5 V versus RHE. In 1.0 M KOH, NiIr NAs has an overpotential of 291 mV at 10 mA cm-2 and a Tafel slope of 42.1 mV dec-1. Such remarkable activity makes the NiIr NAs among the best of recently reported representative Ir-based OER electrocatalysts. Density functional theory (DFT) calculations confirmed alloying effect promotes surface bonding of NiIr with oxygen-containing reactants, resulting in excellent catalytic properties.

Construction of "Metal Defect/Oxygen Defect Junction" in ZnFe2O4-NiCo2O4 Heterostructures for Enhancing Electrocatalytic Oxygen Evolution

Defect engineering is a promising approach to improve the conductivity and increase the active sites of transition metal oxides used as catalysts for the oxygen evolution reaction (OER). However, when metal defects and oxygen defects coexist closely within the same crystal, their compensating charges can diminish the benefits of both defect structures on the catalyst's local electronic structure. To address this limitation, a novel strategy that employs the heterostructure interface of ZnFe2O4-NiCo2O4 to spatially separate the metal defects from the oxygen defects is proposed. This configuration positions the two types of defects on opposite sides of the heterojunction interface, creating a unique structure termed the "metal-defect/oxygen-defect junction". Physical characterization and simulations reveal that this configuration enhances electron transfer at the heterostructure interface, increases the oxidation state of Fe on the catalyst surface, and boosts bulk charge carrier concentration. These improvements enhance active site performance, facilitating hydroxyl adsorption and deprotonation, thereby reducing the overpotential required for the OER.

Thermodynamic Assessment of Sacrificial Oxidant Potential, H2O/O2 Potential, and Rate-Overpotential Relationship to Examine Catalytic Water Oxidation in Nonaqueous Solvents

The water oxidation reaction (WOR), which is pivotal to storing energy in chemical bonds, requires a catalyst to overcome its inherent kinetic barrier. In bulk solutions, sacrificial oxidants (SOs) can regenerate the catalysts to ensure that the homogeneous WOR can be operated with long-term consistent performance. To implement this strategy for organic WOR systems, we modified four common SOs with tetra-n-butylammonium ([NBu4]+)─[NBu4]2[Ce(NO3)6], [NBu4][IO4], [NBu4][HSO5], and [NBu4]2[S2O8]─and examined their chemical stability and electrochemical behaviors in various organic solvents. We also derived the organic-solvent-associated redox potential of H2O/O2 in organic media (EH2O/O2(org)) using open-circuit potential measurements of the H+/H2 redox couple and the related thermochemical cycle. Our findings indicate that the EH2O/O2(org) varies with solvent identity and can be adjusted by changing the [H2O], [acid], and [base] levels; thus, the SO should be carefully selected for WOR, because the innate redox potentials of SOs are not always higher than EH2O/O2(org) under the studied conditions. Lastly, we obtained catalyst-performance-related insights via a rate-overpotential free-energy relationship by calculating the overpotentials of previously studied WOR systems in organic media.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn-Teller-Active Dopants for Improved Electrocatalytic Oxygen Evolution

Triggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen-oxygen bond. High-valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre-catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn-Teller (J-T) distortion induced structural instability, we incorporate high-spin Mn3+ (t 2 g 3 e g 1 ${{t}_{2g}^{3}{e}_{g}^{1}}$ ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn-doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH-dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O-labelled catalysts. In general, this work not only offers the introducing J-T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

Electrocatalytic hydrogenation of 1-octene as non-activated model substrate with neutral water as H-donor is reported, using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a kobs of 87 s-1. Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl- and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]- (2), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [(tBuPCP)Ir(H)2] (3) in 88 % FE from the reaction of 2 with H2O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro-hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H2 formation. Even in strongly alkaline conditions (Bu4NOH added), the electro-hydrogenation of 1-octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H2O activation, reminiscent to transfer hydrogenation pathways, instead of classical H+ reduction. DFT calculations confirmed oxidative addition of H2O as a key step in this context.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Bifunctional Near-Neutral Electrolyte Enhances Oxygen Evolution Reaction

Performance of electrocatalytic reactions depends on not only the composition and structure of the active sites, but also their local environment, including the surrounding electrolyte. In this work, we demonstrate that BF2 (OH)2 - anion is the key fluoroborate species formed in the mixed KBi/KF (KBi=potassium borate) electrolyte to enhance the rate of the oxygen evolution reaction (OER) at near-neutral pH. Through a combination of electrokinetic and in situ spectroscopic studies, we show that the mixed KBi/KF electrolyte promotes the OER via two pathways: 1) stabilizing the interfacial pH during the proton-producing reaction with its high buffering capacity; and 2) activating the interfacial water via strong hydrogen bonds with F-containing species. With the KBi/KF electrolyte, electrodeposited Co(OH)2 is able to achieve 100 mA/cm2 at 1.74 V, which is among the highest reported activities with earth-abundant electrocatalysts at near neutral conditions. These findings highlight the potential of leveraging electrolyte-engineering for improving the electrochemical performance of the OER.

Oxygen-vacancy-rich Co3O4@Fe-B-O heterostructure for efficient oxygen evolution reaction in alkaline and neutral media

Developing highly efficient OER catalysts is essential for producing hydrogen from water electrolysis to compensate for conventional fossil fuel shortages. Here, the oxygen-vacancy-rich heterostructure grown on the Ni foam (NF) (Co₃O₄@Fe-B-O/NF) is fabricated. The synergistic effect between Co₃O₄ and Fe-B-O has been proven effectively modulate the electronic structure and produce highly active interface sites, ultimately leading to enhanced electrocatalytic activity. Co₃O₄@Fe-B-O/NFrequiresan overpotential of 237 mV to drive 20 mA cm^-2 in 1 M KOH, and 384 mV to drive 10 mA cm^-2 in 0.1 M PBS, superior to most catalysts currently used. Moreover, Co₃O₄@Fe-B-O/NF as an oxygen evolution reaction (OER) electrode shows great potential in overall water splitting and CO₂ reduction reaction (CO₂RR). This work may provide effective ideas for designing efficient oxide catalysts.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Ambiguities and best practices in the determination of active sites and real surface area of monometallic electrocatalytic interfaces

Determining the number of electrocatalytically accessible sites (ECAS) and real surface area (RSA) for any given electrocatalyst precisely is important in energy conversion electrocatalysis as these are directly used in the determination of intrinsic activity markers. For monometallic electrocatalysts and electrocatalysts of just one type of active site, there believed to be ways of making precise determination of ECAS and RSA using underpotential deposition (UPD), stripping, and redox-charge integration employing transient voltammetric sweeping techniques. This transient nature of sweeping techniques makes the determination of ECAS and RSA relatively less reliable. This study is directed at examining the effects of scan rate in the determination of ECAS and RSA taking Ni(OH)₂/CC and Pt wire as model catalytic electrodes. The results suggest that the scan rate and the determined ECAS and RSA values are inversely related and the lowest possible scan rate set experiment was witnessed to give the highest possible ECAS or RSA values with LSV/CV.

Interface strong-coupled 3D Mo-NiS@Ni-Fe LDH flower-cluster as exceptionally efficient electrocatalyst for water splitting in wide pH range

It is crucial to create a bifunctional catalyst with high efficiency and low cost for electrochemical water splitting under alkaline and neutral pH conditions. This study investigated the in-situ creation of ultrafine Mo-NiS and NiFe LDH nanosheets as an effective and stable electrocatalyst with a three-dimensional (3D) flower-cluster hierarchical structure (Mo-NiS@NiFe LDH). The strong interfacial connection between Mo-NiS and NiFe LDH enhances the formation of metal higher chemical states in the material, optimizes the electronic structure, increases OH^- adsorption capacity improves electron transfer/mass diffusion, and promotes O₂/H₂ gas release. As a result, at 10 mA cm^-2, Mo-NiS@NiFe LDH/NF demonstrates the outstanding bifunctional electrocatalytic activity of just 107 mV (HER, hydrogen evolution reaction) and 184 mV (hydrogen evolution reaction) (OER, oxygen evolution reaction). The catalytic performance is remarkably stable after 72 h of continuous operation in 1 M KOH at high current densities (300 mA cm^-2). More interestingly, in the overall water splitting system, the cell voltages for anode and cathode in both alkaline and neutral electrolytes for Mo-NiS@NiFe LDH/NF are only 1.54 V (alkaline) and 2.06 V (neutral) at 10 mA cm^-2. These results demonstrated that the bifunctional electrocatalyst design concept is a viable solution for water splitting in both alkaline and neutral systems.

High Current Density Oxygen Evolution in Carbonate Buffered Solution Achieved by Active Site Densification and Electrolyte Engineering

High current density reaching 1 A cm^-2 for efficient oxygen evolution reaction (OER) was demonstrated by interactively optimizing electrolyte and electrode at non-extreme pH levels. Careful electrolyte assessment revealed that the state-of-the-art nickel-iron oxide electrocatalyst in alkaline solution maintained its high OER performance with a small Tafel slope in K-carbonate solution at pH 10.5 at 353 K. The OER performance was improved when Cu or Au was introduced into the FeO_x -modified nanostructured Ni electrode as the third element during the preparation of electrode by electrodeposition. The resultant OER achieved 1 A cm^-2 at 1.53 V vs. reversible hydrogen electrode (RHE) stably for 90 h, comparable to those in extreme alkaline conditions. Constant Tafel slopes, apparent activation energy, and the same signatures from operando X-ray absorption spectroscopy among these samples suggested that this improvement seems solely correlated with enhanced electrochemical surface area caused by adding the third element.

Development of Electrolyzer Using NiCo(OH)2 Layered Double Hydroxide Catalyst for Efficient Water Oxidation Reaction

Over the past decade, layered double hydroxides (LDH) have been the subject of extensive investigations owing to their remarkable water splitting catalytic activity. Stability and porosity are several of the features of LDH which help them to serve as efficient oxygen evolution reaction (OER) catalysts. Based on these considerations, we synthesized NiCo(OH)₂ LDH and probed its OER electrocatalytic performance. The synthesized catalyst was subjected to X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy for structural analysis and investigation of its surface morphology, surface composition, and oxidation states. The LDH-NiCo(OH)₂ was anchored over the FTO surface and the fabricated electrode was found to exhibit a much lower OER onset potential of 265 mV, a much higher current density of 300 mAcm⁻² and a smaller Tafel slope of 41 mVdec⁻¹. Moreover, the designed catalyst was found to be stable up to 2500 repeated voltametric scans. These figures of merit regarding the structure and performance of the designed LDH are expected to provide useful insights into the fundamental understanding of the OER catalysts and their mechanisms of action, thus enabling the more rational design of cost effective and highly efficient electrocatalysts for use in water splitting.

Intramolecular hydroxyl nucleophilic attack pathway by a polymeric water oxidation catalyst with single cobalt sites

Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, π-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

Magnetic-Field-Induced Strain Enhances Electrocatalysis of FeCo Alloys on Anode Catalysts for Water Splitting

In water splitting, the oxygen evolution reaction (OER) performance of transition metal alloy catalysts needs to be further improved. To solve this problem, the method of an external magnetic field was used to improve the OER catalytic performance of the alloy catalyst. In this paper, FeCo alloys with different composition ratios were prepared by an arc melting method, and OER catalysts with different compositions were obtained by annealing treatment. Under the action of a magnetic field, all three groups of catalysts showed a better catalytic performance than those without a magnetic field. The overpotentials of Fe35Co65, Fe22Co78 and Fe15Co85 at a current density of 20 mA cm−2 were reduced by 12 mV, 6 mV and 2 mV, respectively. It is found that, due to the magnetostrictive properties of FeCo alloys, the catalyst itself will generate strain under the action of a magnetic field, and the existence of strain may be the main reason for the enhanced OER performance of the magnetic field. Therefore, this work provides a new idea for the development of magnetic material catalysts and a magnetic field to improve the performance of catalysts.

Research progress in improving the oxygen evolution reaction by adjusting the 3d electronic structure of transition metal catalysts

As a clean and renewable energy carrier, hydrogen (H₂) has become an attractive alternative to dwindling fossil fuels. The key to realizing hydrogen-based energy systems is to develop efficient and economical hydrogen production methods. The water electrolysis technique has the advantages of cleanliness, sustainability, and high efficiency, which can be applied to large-scale hydrogen production. However, the electrocatalytic oxygen evolution reaction (OER) at the anode plays a decisive role in the efficiency of hydrogen evolution during water splitting. Generally, noble metal catalysts (such as ruthenium and iridium) are considered to exhibit the best OER performance; however, they exhibit disadvantages such as high costs, limited reserves, and poor stability. Therefore, the research on highly efficient non-noble metal catalysts that can replace their noble metal counterparts has always been important. This review presents the recent advances in the preparation of high-performance OER electrocatalysts by regulating the electronic structure of 3d transition metals. First, we introduce the reaction mechanism of water splitting and the OER, which reveals the high requirement of the complex four-electron process of the OER. Second, the electron transfer mode and development progress of highly active transition metal electrocatalysts are used to summarize the research situation of transition metal OER catalysts in water splitting. Finally, the future development direction and challenges of transition metal catalysts are prospected based on the current research progress.

Electrocatalytic Water Splitting: From Harsh and Mild Conditions to Natural Seawater

Electrocatalytic water splitting is regarded as the most effective pathway to generate green energy-hydrogen-which is considered as one of the most promising clean energy solutions to the world's energy crisis and climate change mitigation. Although electrocatalytic water splitting has been proposed for decades, large-scale industrial hydrogen production is hindered by high electricity cost, capital investment, and electrolysis media. Harsh conditions (strong acid/alkaline) are widely used in electrocatalytic mechanism studies, and excellent catalytic activities and efficiencies have been achieved. However, the practical application of electrocatalytic water splitting in harsh conditions encounters several obstacles, such as corrosion issues, catalyst stability, and membrane technical difficulties. Thus, the research on water splitting in mild conditions (neutral/near neutral), even in natural seawater, has aroused increasing attention. However, the mechanism in mild conditions or natural seawater is not clear. Herein, different conditions in electrocatalytic water splitting are reviewed and the effects and proposed mechanisms in the three conditions are summarized. Then, a comparison of the reaction process and the effects of the ions in different electrolytes are presented. Finally, the challenges and opportunities associated with direct electrocatalytic natural seawater splitting and the perspective are presented to promote the progress of hydrogen production by water splitting.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Anode Catalysts in CO2 Electrolysis: Challenges and Untapped Opportunities

The field of electrochemical carbon dioxide reduction has developed rapidly during recent years. At the same time, the role of the anodic half-reaction has received considerably less attention. In this Perspective, we scrutinize the reports on the best-performing CO2 electrolyzer cells from the past 5 years, to shed light on the role of the anodic oxygen evolution catalyst. We analyze how different cell architectures provide different local chemical environments at the anode surface, which in turn determines the pool of applicable anode catalysts. We uncover the factors that led to either a strikingly high current density operation or an exceptionally long lifetime. On the basis of our analysis, we provide a set of criteria that have to be fulfilled by an anode catalyst to achieve high performance. Finally, we provide an outlook on using alternative anode reactions (alcohol oxidation is discussed as an example), resulting in high-value products and higher energy efficiency for the overall process.

A Fe-doped Co-oxide Electrocatalyst Synthesized Through Post-Modification Method Toward Advanced Water Oxidation

In the context of the ever-increasing energy crisis, electrocatalytic water splitting has attracted widespread attention as an effective means to provide clean energy. However, the oxygen evolution reaction (OER), which...

Degradation of antibiotics, organic matters and ammonia during secondary wastewater treatment using boron-doped diamond electro-oxidation combined with ceramic ultrafiltration

In this study, a BDD electrolytic oxidation-ceramic membrane ultrafiltration (EO-CM) system for the removals of antibiotics, organic matters and ammonia in wastewater was evaluated. Sulfamethazine (SMZ) was degraded following a pseudo first-order kinetics. The removal rate of SMZ improved with the increase of electro-oxidation time (0-60 min) and current density (5-30 mA/cm²). During the BDD electro-oxidation process, H₂O₂ and hydroxyl radicals (•OH) were generated which were detected by N, N-diethyl-p-phenylenediamine (DPD) method and electron paramagnetic resonance spectroscopy (EPR), respectively. Chemical oxygen demand (COD) was able to be removed by EO and CM processes, in which proteins and humic acids were regarded as the main removed components measured using excitation-emission matrix (EEM) technique. Moreover, BDD electro-oxidation pretreatment could make the CM process maintain a high water flux and significantly control the membrane fouling and relieve transmembrane pollution. In addition, the removal of ammonia was enhanced with the increase of chloride ions (Cl^-) in wastewater during EO process due to the generation of active chlorine (i.e., ClO^-, HClO, or Cl₂) from the oxidation of Cl^-. Chloramine and nitrogen were produced in the oxidation of ammonia by active chlorine. Overall, the results of this study suggest that BDD EO-CM system is a promising process for removing antibiotics, organic matters and ammonia.

Recent Advances in the Catalytic Applications of Lanthanide-Oxo Clusters

Lanthanide-oxo/hydroxo clusters (LOCs) in this mini-review refer to polynuclear complexes featuring a polyhedral metal-oxo/hydroxo cluster core of lanthanide ions exclusively or with coexisting 3d metal ions. We summarize herein the recent works using this unique family of cluster complexes for catalysis; this aspect of research stands in stark contrast to their extensively studied synthetic and structural chemistry as well as the much-researched magnetic properties. Following a brief introduction of the synthetic strategies for these clusters, pertinent results from available literature reports are surveyed and discussed according to the types of catalyzed reactions. Particular attention was paid to the selection of a cluster catalyst for a specific type of reactions as well as the corresponding reaction mechanism. To the end, the advantages and challenges in utilizing LOCs as multifunctional catalysts are summarized, and possible future research directions are proposed.

Local Chemical Environment Governs Anode Processes in CO2 Electrolyzers

A major goal within the CO₂ electrolysis community is to replace the generally used Ir anode catalyst with a more abundant material, which is stable and active for water oxidation under process conditions. Ni is widely applied in alkaline water electrolysis, and it has been considered as a potential anode catalyst in CO₂ electrolysis. Here we compare the operation of electrolyzer cells with Ir and Ni anodes and demonstrate that, while Ir is stable under process conditions, the degradation of Ni leads to a rapid cell failure. This is caused by two parallel mechanisms: (i) a pH decrease of the anolyte to a near neutral value and (ii) the local chemical environment developing at the anode (i.e., high carbonate concentration). The latter is detrimental for zero-gap electrolyzer cells only, but the first mechanism is universal, occurring in any kind of CO₂ electrolyzer after prolonged operation with recirculated anolyte.

A conductive polyacrylamide hydrogel enabled by dispersion-enhanced MXene@chitosan assembly for highly stretchable and sensitive wearable skin

MXene is recognized as an ideal material for sensitive wearable strain sensors because of its unique advantages of conductivity, hydrophilicity and mechanical properties. However, conventional hydrogel sensors utilizing MXene as a conductive material inevitably encounter the excessive accumulation of MXene nanosheets during the process of synthesis, which limits the electron transmission, reduces the conductivity, and concurrently weakens the mechanical capability and sensitivity of sensors. Herein, we construct a dispersion-enhanced MXene hydrogel (DEMH) through a chitosan-induced self-assembly strategy for the first time. Charge transfer is carried out through the flow of a material or a collection of material microstructures, and thus the highly interconnected 3D MXene@Chitosan network provides fast transport channels for electrons, and the DEMH exhibits excellent conductivity and sensibility simultaneously. Besides, the electrostatic self-assembly between MXene and chitosan, and the supramolecular interactions between MXene, chitosan and polyacrylamide chain segment result in excellent mechanical strength (of up to 1900%) and flexibility of DEMH. Furthermore, the introduction of chitosan which possesses a high density of positively charged groups and MXene with semiconducting properties also endows sensor versatility, such as self-adhesion properties and antibacterial activity. This work develops a simple and cut-price strategy for combining MXene unaggregated into a hydrogel as a sensor with high conductivity, sensibility and flexibility. A simple and inexpensive strategy for avoiding self-stacking of two-dimensional conductive materials is proposed, which paves the way for a broad range of applications in electronic skin, human motion detection and intelligent devices.

Amorphous Yolk-Shelled ZIF-67@Co3(PO4)2 as Nonprecious Bifunctional Catalysts for Boosting Overall Water Splitting

It is challenging to generate inexpensive and noble metal-free catalysts for efficient overall water splitting (OWS). To achieve this goal, suitable tuning of the structure and composition of electrocatalytic materials is a promising approach that has attracted much attention in recent years. Herein, novel hybrid amorphous ZIF-67@Co₃(PO₄)₂ electrocatalysts with yolk-shell structures were prepared using a reflux method. It is demonstrated that yolk-shelled ZIF-67@Co₃(PO₄)₂ is not only an active catalyst for the hydrogen evolution reaction (HER) but also an efficient catalyst for the oxygen evolution reaction (OER). The optimized composite electrode showed superior performance with low overpotentials of 73 and 334 mV @ 10 mA·cm^-2 toward HER and OER, respectively, and a low potential of 1.62 V @ 10 mA·cm^-2 and 1.66 V @ 30 mA·cm^-2 in a practical OWS test under alkaline conditions. N-O bonds were formed to connect the two components of ZIF-67 and Co₃(PO₄)₂ in the composite ZIF-67@Co₃(PO₄)₂, which indicates that the two components are synergistic but not isolated, and this synergistic effect may be one of the important reasons to boost the oxygen and hydrogen evolution performances of the hybrid. Based on experimental data, the high electrocatalytic performance was inferred to be related to the unique structure of ZIF-67, tuning the ability of Co₃(PO₄)₂ and synergism between ZIF-67 and Co₃(PO₄)₂. The preparation strategy reported herein can be extended for the rational design and synthesis of cheap, active, and long-lasting bifunctional electrocatalysts for OWS and other renewable energy devices.

MnO2 Electrocatalysts Coordinating Alcohol Oxidation for Ultra-Durable Hydrogen and Chemical Productions in Acidic Solutions

Electrocatalytic hydrogen production under acidic conditions is of great importance for industrialization in comparison to that in alkaline media, which, unfortunately, still remains challenging due to the lack of earth-abundant, cost-effective and highly active anodic electrocatalysts that can be used durably under strongly acidic conditions. Here we report an unexpected finding that manganese oxide, a kind of common non-noble catalysts easily soluble in acidic solutions, can be applied as a highly efficient and extremely durable anodic electrocatalyst for hydrogen production from an acidic aqueous solution of alcohols. Particularly in a glycerol solution, a potential of as low as 1.36 V (vs. RHE) is needed at 10 mA cm^-2 , which is 270 mV lower than that of oxygen evolution reaction (OER), to oxidize glycerol into value-added chemicals such as formic acid, without oxygen production. To our surprise, the manganese oxide exhibits extremely high stability for electrocatalytic hydrogen production in coupling with glycerol oxidation for longer than 865 hours compared to shorter than 10 h for OER. Moreover, the effect of the addition of glycerol on the electrochemical durability has been probed via in situ Raman spectroscopic analysis and density functional theory (DFT) calculations. This work demonstrates that acid-unstable metal oxide electrocatalysts can be used robustly in acidic media under the presence of certain substances for electrochemical purposes, such as hydrogen production.

Synthesis and Electronic Modulation of Nanostructured Layered Double Hydroxides for Efficient Electrochemical Oxygen Evolution

Water electrolysis is considered to be one of the most promising technologies to produce clean fuels. However, its extensive realization critically depends on the progress in cost-effective and high-powered oxygen evolution reaction (OER) electrocatalysts. As a member of the big family of two-dimensional (2D) materials, nanostructured layered double hydroxides (nLDHs) have made significant processes and continuous breakthroughs for OER electrocatalysis. In this Review, the advancements in designing nLDHs for OER in recent years were discussed with a unique focus on their electronic modulations and in situ analysis on catalytic processes. After a brief discussion on different synthetic methodologies of nLDHs, including "bottom-up" and "top-down" approaches, the general strategies to enhance the catalytic performances of nLDHs reported so far were summarized, including compositional substitution, heteroatom doping, vacancy engineering, and amorphous/crystalline engineering. Furthermore, the in situ OER processes and mechanism analysis on engineering efficient nLDHs electrocatalysts were discussed. Finally, the research trends, perspectives, and challenges on designing nLDHs were also carefully outlined. This progress Review may offer enlightening experimental/theoretical guidance for designing highly catalytic active nLDHs and provide new directions to promote their future prosperity for practical utilization in water splitting.

Fast microwave-assisted preparation of nickel-copper-chromium-layered double hydroxide as an excellent electrocatalyst for water oxidation

Herein, we describe a simple microwave method for the doping of Cu2+ into NiCr-LDH and the preparation of ternary Ni2.25Cu0.75Cr-LDH as a superior electrocatalyst for water oxidation in a neutral solution. The obtained Ni2.25Cu0.75Cr-LDH was characterized by XRD, DRS, TEM and FE-SEM techniques. The results showed that Ni2.25Cu0.75Cr-LDH was formed with a size of 30 nm. In order to examine the water oxidation activity of Ni2.25Cu0.75Cr-LDH, the as-prepared samples were used as an electrocatalyst-modified carbon paste electrode in neutral solution. The electrochemical results revealed that the optimized Ni2.25Cu0.75Cr-LDH presented extraordinary water oxidation activity with a low onset potential of 1.40 V (vs. RHE) and an overpotential of 170 mV compared to other molar ratios (Ni2.5Cu0. 5Cr-LDH), (Ni2CuCr-LDH), and bimetallic (CuCr-LDH), and even outperformed NiCr-LDH. Also, a small Tafel slope of 31 mV and high durability of 14 h could be obtained for Ni2.25Cu0.75Cr-LDH. The excellent OER could be assigned to the decreased band gap energy and increased charge transfer at Ni2.25Cu0.75Cr-LDH. Therefore, Ni2.25Cu0.75Cr-LDH is a promising water oxidation catalyst owing to its improved charge transfer ability.

Ruthenium-doped NiFe-based metal-organic framework nanoparticles as highly efficient catalysts for the oxygen evolution reaction

Developing highly efficient and stable electrocatalysts toward the oxygen evolution reaction (OER) is essential for large-scale sustainable energy conversion and storage technologies. Herein, we design and synthesize a ruthenium (Ru) doped NiFe bimetallic metal-organic framework (MOF) deposited on the nickel foam (Ru-NiFe-MOF/NF) by a facile one-pot hydrothermal reaction. Ru-NiFe-MOF/NF exhibits favourable electrocatalytic OER activity in alkaline solution, and requires a low overpotential of 205 mV to achieve 10 mA cm^-2, a small Tafel slope of 50 mV dec^-1, and long-term electrochemical stability over 100 h. This work demonstrates the rational nano-architectural design and synthesis of predominantly efficient and robust cation-doped MOF-derived materials for energy catalysis and beyond.