High-Voltage-Enabled Stable Cobalt Species Deposition on MnO2 for Water Oxidation in Acid

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

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

Enabling Unconventional "Alternating-Distal" N2 Reduction Pathway for Efficient Ammonia Electrosynthesis

The general understanding on the reaction path is that the electrocatalytic N2 reduction follows either individual associative alternating or distal pathways, where efficient N2 activation and selective NH3 production are very challenging. Herein, an unconventional "alternating-distal" pathway was achieved by shifting the "*NHNH2→*NH2NH2" to "*NHNH2→*NH + NH3" step to boost NH3 synthesis with an amorphous CeMnOx electrocatalyst. In this unconventional process, N2 activation was realized through π back donation on the Mn site, while the Mn/Ce dual active sites could regulate the intermediate configurations to avoid the nitrogen-containing by-product formation. Such "alternating-distal" pathway was affirmed by in situ spectroscopic analyses and theoretical calculations. In a neutral media, an average ammonia production rate of 82.8 µg h-1 mg-1 and an outstanding Faradaic efficiency of 37.3% were attained. This work validated an unconventional mechanism in electrocatalytic ammonia synthesis, which might be extended to other catalytic process with multiple possible reaction paths.

Robust Oxygen Evolution on Ni-Doped MoO3: Overcoming Activity-Stability Trade-Off in Alkaline Water Splitting

Electrochemical water splitting using earth-abundant materials is crucial for enabling green hydrogen production and energy storage. In recent years, molybdenum trioxide (MoO3), a semiconducting material, has been proposed as a candidate catalyst for the oxygen evolution reaction (OER). Here, we advance nickel (Ni) doping of MoO3 as a strategy to increase the activity and stability of the material during alkaline electrochemical water splitting, thereby overcoming the typical activity-stability trade-off encountered with OER catalysts. The instability of MoO3 in alkaline media can be mitigated by doping with Ni, whose oxide is stable under such conditions. Using density functional theory (DFT) with Hubbard corrections, we show that Ni doping reduces the thermodynamic OER overpotential on the MoO3 basal plane to 0.64 V. Experiments demonstrate that Ni-doped MoO3 requires an overpotential of 0.34 V for an OER current density of 10 mA/cm2 (and 0.56 V at 100 mA/cm2), as opposed to a value of 0.40 V for pure MoO3. Further, Ni-doped MoO3 exhibits a lower Tafel slope of 74.8 mV/dec, compared to 98.3 mV/dec for the pristine material under alkaline conditions. While Mo leaches in alkaline conditions, X-ray photoelectron spectroscopy reveals enhanced stability with Ni doping. Overall, our work advances Ni-doped MoO3 as a promising water-splitting electrocatalyst and provides new insights into its OER mechanism and stability in alkaline media. More generally, the work sheds light on choosing a dopant to increase a material's activity and stability, which will also find applications in other catalytic materials.

Proton Exchange Membrane Water Splitting: Advances in Electrode Structure and Mass-Charge Transport Optimization

Proton exchange membrane water electrolysis (PEMWE) represents a promising technology for renewable hydrogen production. However, the large-scale commercialization of PEMWE faces challenges due to the need for acid oxygen evolution reaction (OER) catalysts with long-term stability and corrosion-resistant membrane electrode assemblies (MEA). This review thoroughly examines the deactivation mechanisms of acidic OER and crucial factors affecting assembly instability in complex reaction environments, including catalyst degradation, dynamic behavior at the MEA triple-phase boundary, and equipment failures. Targeted solutions are proposed, including catalyst improvements, optimized MEA designs, and operational strategies. Finally, the review highlights perspectives on strict activity/stability evaluation standards, in situ/operando characteristics, and practical electrolyzer optimization. These insights emphasize the interrelationship between catalysts, MEAs, activity, and stability, offering new guidance for accelerating the commercialization of PEMWE catalysts and systems.

Stable High-Entropy Alloy AlCoCrFeNi2.1 with Anti-Dealloying Effect for Enhanced Oxygen Evolution Performance

The catalytic performance of oxygen evolution reaction (OER) catalysts is influenced by their elemental composition. Aluminum (Al) offers abundant active sites due to its high oxidation affinity, which makes it unstable in both acidic and alkaline environments. We used the gas atomization method (GAM) to prepare the aluminum-containing single-phase high-entropy alloy AlxCoCrFeNi2.1 (x = 0, 0.1, 0.3, 0.5, 1). Besides, changing the aluminum content in the HEAs can control the alloy particle size. The GAM can enable the HEAs with different Al contents to present the body-centered cubic (BCC) structure, avoiding alloy phase separation caused by other component contents in the smelting method. As predicted by theory, as aluminum content increases, the AlCoCrFeNi2.1 shows the best OER performance (overpotential ≈313 mV for 1000 h at 100 mA·cm-2). Furthermore, through the precise weight detection system, COMSOL simulations, and Density Functional Theory (DFT) calculations, we have further demonstrated the superiority of AlCoCrFeNi2.1 catalysis. Overall, this work provides a streamlined way to slow down the dissolution of oxygen catalysts for water electrolysis and contributes a controllable choice for more element-dependent scenarios.

Advances in Oxygen Evolution Reaction Electrocatalysts via Direct Oxygen-Oxygen Radical Coupling Pathway

Oxygen evolution reaction (OER) is a cornerstone of various electrochemical energy conversion and storage systems, including water splitting, CO2/N2 reduction, reversible fuel cells, and rechargeable metal-air batteries. OER typically proceeds through three primary mechanisms: adsorbate evolution mechanism (AEM), lattice oxygen oxidation mechanism (LOM), and oxide path mechanism (OPM). Unlike AEM and LOM, the OPM proceeds via direct oxygen-oxygen radical coupling that can bypass linear scaling relationships of reaction intermediates in AEM and avoid catalyst structural collapse in LOM, thereby enabling enhanced catalytic activity and stability. Despite its unique advantage, electrocatalysts that can drive OER via OPM remain nascent and are increasingly recognized as critical. This review discusses recent advances in OPM-based OER electrocatalysts. It starts by analyzing three reaction mechanisms that guide the design of electrocatalysts. Then, several types of novel materials, including atomic ensembles, metal oxides, perovskite oxides, and molecular complexes, are highlighted. Afterward, operando characterization techniques used to monitor the dynamic evolution of active sites and reaction intermediates are examined. The review concludes by discussing several research directions to advance OPM-based OER electrocatalysts toward practical applications.

Noble-metal-free catalysts for the oxygen evolution reaction in acids

Oxygen evolution catalysts are critical components of proton exchange membrane water electrolysers (PEMWEs), playing a decisive role in determining both the performance and cost of these devices. Non-noble metal-based oxygen evolution catalysts have recently drawn significant attention as potential alternatives to expensive noble metal catalysts. This review systematically summarizes the mechanism of non-noble metal catalysts for the oxygen evolution reaction in acids with respect to their activity and stability, incorporating theoretical calculations and the Pourbaix diagram. Advanced in situ techniques are highlighted as powerful tools for probing intermediate evolution and valence changes and further elucidating the catalytic mechanism. Furthermore, key strategies for enhancing catalytic activity and durability, such as elemental doping, the support effect, surface protection and novel phase design, are discussed. Finally, this review provides insights into the remaining challenges and emerging opportunities for advancing practical oxygen evolution catalysts in PEMWEs.

Regulating Ru-O Bond and Oxygen Vacancies of RuO2 by Ta Doping for Electrocatalytic Oxygen Evolution in Acid Media

Proton exchange membrane water electrolysis (PEMWE) is considered an ideal green hydrogen production technology with promising application prospects. However, the development of efficient and stable acid electroanalytic oxygen electrocatalysts is still a challenging bottleneck. This progress is achieved by adopting a strategic approach with the introduction of the high valence metal Ta to regulate the electronic configuration of RuO2 by manipulating its local microenvironment to optimize the stability and activity of the electrocatalysts. The Ta-RuO2 catalysts are notable for their excellent electrocatalytic activity, as evidenced by an overpotential of only 202 mV at 10 mA cm-2, which significantly exceeds that of homemade RuO2 and commercial RuO2. Furthermore, the Ta-RuO2 catalyst exhibits exceptional stability with negligible potential reduction observed after 50 h of electrolysis. Theoretical calculations show that the asymmetric configuration of Ru-O-Ta breaks the thermodynamic activity limitations usually associated with adsorption evolution, weakening the energy barrier for the formation of the OOH* formation. The strategic approach presented in this study provides an important reference for the development of a stable active center for acid water splitting.

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.

Enriched Oxygen Coverage Localized within Ir Atomic Grids for Enhanced Oxygen Evolution Electrocatalysis

Inefficient active site utilization of oxygen evolution reaction (OER) catalysts have limited the energy efficiency of proton exchange membrane (PEM) water electrolysis. Here, an atomic grid structure is demonstrated composed of high-density Ir sites (≈10 atoms per nm2) on reactive MnO2-x support which mediates oxygen coverage-enhanced OER process. Experimental characterizations verify the low-valent Mn species with decreased oxygen coordination in MnO2-x exert a pivotal impact in the enriched oxygen coverage on the surface during OER process, and the distributed Ir atomic grids, where highly electrophilic Ir─O(II-δ)- bonds proceed rapidly, render intense nucleophilic attack of oxygen radicals. Thereby, this metal-support cooperation achieves ultra-low overpotentials of 166 mV at 10 mA cm-2 and 283 mV at 500 mA cm-2, together with a striking mass activity which is 380 times higher than commercial IrO2 at 1.53 V. Moreover, its high OER performance also markedly surpasses the commercial Ir black catalyst in PEM electrolyzers with long-term stability.

Modulating the Electronic Structure of Cobalt-Vanadium Bimetal Catalysts for High-Stable Anion Exchange Membrane Water Electrolyzer

Modulating the electronic structure of catalysts to effectively couple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential for developing high-efficiency anion exchange membrane water electrolyzer (AEMWE). Herein, a coral-like nanoarray composed of nanosheets through the synergistic layering effect of cobalt and the 1D guiding of vanadium is synthesized, which promotes extensive contact between the active sites and electrolyte. The HER and OER activities can be enhanced by modulating the electronic structure through nitridation and phosphorization, respectively, enhancing the strength of metal-H bond to optimize hydrogen adsorption and facilitating the proton transfer to improve the transformation of oxygen-containing intermediates. Resultantly, the AEMWE achieves a current density of 500 mA cm-2 at 1.76 V for 1000 h in 1.0 M KOH at 70 °C. The energy consumption is 4.21 kWh Nm-3 with the producing hydrogen cost of $0.93 per kg H2. Operando synchrotron radiation and Bode phase angle analyses reveal that during the high-energy consumed OER, the dissolution of vanadium species transforms distorted Co-O octahedral into regular octahedral structures, accompanied by a shortening of the Co-Co bond length. This structural evolution facilitates the formation of oxygen intermediates, thus accelerating the reaction kinetics.

Towards bridging thermo/electrocatalytic CO oxidation: from nanoparticles to single atoms

Proton exchange membrane fuel cells (PEMFCs), as a feasible alternative to replace the traditional fossil fuel-based energy converter, contribute significantly to the global sustainability agenda. At the PEMFC anode, given the high exchange current density, Pt/C is deemed the catalyst-of-choice to ensure that the hydrogen oxidation reaction (HOR) occurs at a sufficiently fast pace. The high performance of Pt/C, however, can only be achieved under the premise that high purity hydrogen is used. For instance, in the presence of trace level carbon monoxide, a typical contaminant during H2 production, Pt is severely deactivated by CO surface blockage. Addressing the poisoning issue necessitates for either developing anti-poisoning electrocatalysts or using pre-purified H2 obtained via a thermo-catalysis route. In other words, the CO poisoning issue can be addressed by either thermal-catalysis from the H2 supply side or electrocatalysis at the user side, respectively. In spite of the distinction between thermo-catalysis and electro-catalysis, there are high similarities between the two routes. Essentially, a reduction in the kinetic barrier for the combination of CO to oxygen containing intermediates is required in both techniques. Therefore, bridging electrocatalysis and thermocatalysis might offer new insight into the development of cutting edge catalysts to solve the poisoning issue, which, however, stands as an underexplored frontier in catalysis science. This review provides a critical appraisal of the recent advancements in preferential CO oxidation (CO-PROX) thermocatalysts and anti-poisoning HOR electrocatalysts, aiming to bridge the gap in cognition between the two routes. First, we discuss the differences in thermal/electrocatalysis, CO oxidation mechanisms, and anti-CO poisoning strategies. Second, we comprehensively summarize the progress of supported and unsupported CO-tolerant catalysts based on the timeline of development (nanoparticles to clusters to single atoms), focusing on metal-support interactions and interface reactivity. Third, we elucidate the stability issue and theoretical understanding of CO-tolerant electrocatalysts, which are critical factors for the rational design of high-performance catalysts. Finally, we underscore the imminent challenges in bridging thermal/electrocatalytic CO oxidation, with theory, materials, and the mechanism as the three main weapons to gain a more in-depth understanding. We anticipate that this review will contribute to the cognition of both thermocatalysis and electrocatalysis.

Inverted loading strategy regulates the Mn-OV-Ce sites for efficient fenton-like catalysis

Regulating interfacial active sites to improve peroxymonosulfate (PMS) activation efficiency is a hot topic in the heterogeneous catalysis field. In this study, we develop an inverted loading strategy to engineer asymmetric Mn-OV-Ce sites for PMS activation. Mn3O4@CeO2 prepared by loading CeO2 nanoparticles onto Mn3O4 nanorods exhibits the highest catalytic activity and stability, which is due to the formation of more oxygen vacancies (OV) at the Mn-OV-Ce sites, and the surface CeO2 layer effectively inhibits corrosion by preventing the loss of manganese ion active species into the solution. In situ characterizations and density functional theory (DFT) studies have revealed effective bimetallic redox cycles at asymmetric Mn-OV-Ce active sites, which promote surface charge transfer, enhance the adsorption reaction activity of active species toward pollutants, and favor PMS activation to generate (•OH, SO4•-, O2•- and 1O2) active species. This study provides a brand-new perspective for engineering the interfacial behavior of PMS activation.

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design

Water electrolysis driven by "green electricity" is an ideal technology to realize energy conversion and store renewable energy into hydrogen. With the development of proton exchange membrane (PEM), water electrolysis in acidic media suitable for many situations with an outstanding advantage of high gas purity has attracted significant attention. Compared with hydrogen evolution reaction (HER) in water electrolysis, oxygen evolution reaction (OER) is a kinetic sluggish process that needs a higher overpotential. Especially in acidic media, OER process poses higher requirements for the electrocatalysts, such as high efficiency, high stability and low costs. This review focuses on the acidic OER electrocatalysis, reaction mechanisms, and critical parameters used to evaluate performance. Especially the modification strategies applied in the design and construction of new-type electrocatalysts are also summarized. The characteristics of traditional noble metal-based electrocatalysts and the noble metal-free electrocatalysts developed in recent decades are compared and discussed. Finally, the current challenges for the most promising acidic OER electrocatalysts are presented, together with a perspective for future water electrolysis.

Understanding the catalytic performances of metal-doped Ta2O5 catalysts for acidic oxygen evolution reaction with computations

The design of stable and active alternative catalysts to iridium oxide for the anodic oxygen evolution reaction (OER) has been a long pursuit in acidic water splitting. Tantalum pentoxide (Ta2O5) has the merit of great acidic stability but poor OER performance, yet strategies to improve its intrinsic OER activity are highly desirable. Herein, by using density functional theory (DFT) calculations combined with aqueous stability assessment from surface Pourbaix diagrams, we systematically evaluated the OER activity and acidic stability of 14 different metal-doped Ta2O5 catalysts. Apart from the experimentally reported Ir-doped Ta2O5, we computationally identified Ru- and Nb-doped Ta2O5 catalysts as another two candidates with reasonably high stability and activity in acidic OER. Our study also underscores the essence of considering stable surface states of catalysts under working conditions before a reasonable activity trend can be computationally achieved.

Straightforward electrochemical synthesis of a Co3O4 nanopetal/ZnO nanoplate p-n junction for photoelectrochemical water splitting

Hydrogen production through photoelectrochemical (PEC) reactions is an innovative and promising approach to producing clean energy. The PEC working electrode of a Co3O4/ZnO-based p-n heterojunction was prepared by a straightforward electrochemical deposition with different deposition times onto an FTO (Fluorine-doped Tin Oxide) glass substrate. The successful synthesis of the materials was confirmed through analysis using XRD, FTIR, SEM-EDX, DRS, and PL techniques. Mott-Schottky plots and some characterization studies also checked the determination of the formation of the p-n junction. Co3O4/ZnO/FTO with a Co3O4 deposition time of 2 minutes exhibited the lowest onset potential of 0.82 V and the lowest overpotential of 470 mV at a current density of 10 mA cm -2. Furthermore, the photo-conversion efficiency of the Co3O4/ZnO/FTO sample showed 1.4 times higher current density than the ZnO/FTO sample. A mechanism is also proposed to enhance the Co3O4/ZnO/FTO electrode photo-electrocatalytic activity involved in the water-splitting reaction. The Co3O4/ZnO/FTO electrode shows significant potential as a promising PEC electrode to produce hydrogen.

Recycled Cathodes in Rechargeable Aqueous Batteries as Ready-Made Electrodes for Oxygen Evolution Catalysis

The development of a low-cost and efficient oxygen evolution reaction (OER) electrode is of critical importance for water electrolysis technologies. The general approach to achieving a high-efficiency OER electrode is to regulate catalytic material structures by synthetic control. Here we reported an orthogonal approach to obtaining the OER electrode without intentional design and synthesis, namely, recycling MnO2 cathodes from failed rechargeable aqueous batteries and investigating them as ready-made catalytic electrodes. The recycled MnO2 cathode showed very little Zn2+ storage capacity but surprisingly high OER activity with a low overpotential of 307 mV at 10 mA cm-2 and a small Tafel slope of 77.9 mV dec-1, comparable to the state-of-the-art RuO2 catalyst (310 mV, 86.9 mV dec-1). In situ electrochemical and theoretical studies jointly revealed that the accelerated OER kinetics of the recycled MnO2 electrode was attributed to the enlarged active surface area of MnO2 and optimized electronic structure of Mn sites. This work suggests failed battery cathodes as successful catalysis electrodes for sustainable energy development.

Fabrication of Fe-Zr, Co-Zr, and Ni-Zr Catalysts to Boost CNTs Synthesis from Plastic Wastes and the Electrocatalytic Oxygen Evolution Reaction

The efficient conversion of plastic wastes to high-value carbon materials like carbon nanotubes (CNTs) is one important issue about the rational recycling, reduction, and reuse of solid wastes. Herein, Fe-, Co-, and Ni-Zr catalysts were prepared and used for CNTs synthesis from polyethylene (PE) waste via a two-stage reaction system. At the same time, the effects of the PE/catalyst ratio and reaction temperature on CNTs synthesis have been studied. Compared with Co-Zr and Ni-Zr, Fe-Zr exhibited the best activity in CNTs synthesis from PE, and it achieved the highest CNTs yield of 806.3 mg/g (per gram of Fe-Zr) at 800 °C with a PE/catalyst ratio of 4. Furthermore, the obtained Fe-Zr/CNTs composite exhibited a low overpotential of 267 mV for the electrocatalytic oxygen evolution reaction (OER) at 20 mA/cm2 in 1 M KOH electrolyte solution, which was 21 mV lower than commercial RuO2 (288 mV) and 50 mV lower than Fe-Zr (317 mV). It was deduced that the in situ growth of CNTs reduced the charge transfer resistance and improved the electron transport efficiency of the Fe-Zr/CNTs composite, leading to superior activity in the electrocatalytic OER. This work provided detailed information for the preparation of the metal/CNTs composite from plastic wastes, which contributed positively to alleviate the environment and energy crisis.

Universal Synthesis of Single-Atom Catalysts by Direct Thermal Decomposition of Molten Salts for Boosting Acidic Water Splitting

Single-atom catalysts (SACs) are considered prominent materials in the field of catalysis due to their high metal atom utilization and selectivity. However, the wide-ranging applications of SACs remain a significant challenge due to their complex preparation processes. Here, a universal strategy is reported to prepare a series of noble metal single atoms on different non-noble metal oxides through a facile one-step thermal decomposition of molten salts. By using a mixture of non-noble metal nitrate and a small-amount noble metal chloride as the precursor, noble metal single atoms can be easily introduced into the non-noble metal oxide lattice owing to the cation exchange in the in situ formed molten salt, followed by the thermal decomposition of nitrate anions during the heating process. Analyses using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure spectroscopy confirm the formation of the finely dispersed single atoms. Specially, the as-synthesized Ir single atoms (10.97 wt%) and Pt single atoms (4.60 wt%) on the Co3O4 support demonstrate outstanding electrocatalytic activities for oxygen evolution reaction and hydrogen evolution reaction, respectively.

Study on Water Splitting of the 214-Type Perovskite Oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga)

We present a study on the electrocatalysis of 214-type perovskite oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga) with semiconducting-like behavior synthesized using the sol-gel method. Among these five catalysts, PrSrCoO4 exhibits the optimal electrochemical performance in both the oxygen evolution reaction and the hydrogen evolution reaction, mainly due to its larger electrical conductivity, mass activity, and turnover frequency. Importantly, the weak dependency of LSV curves in a KOH solution with different pH values, revealing the adsorbate evolving mechanism in PrSrCoO4, and the density functional theory (DFT) calculations indicate that PrSrCoO4 has a smaller Gibbs free energy and a higher density of states near the Fermi level, which accelerates the electrochemical water splitting. The mutual substitution of different rare-earth elements will change the unit-cell parameters, regulate the electronic states of catalytic active site Co ions, and further affect their catalytic performance. Furthermore, the magnetic results indicate strong spin-orbit coupling in the electroactive sites of Co ions in SmSrCoO4 and EuSrCoO4, whereas the magnetic moments of Co ions in the other three catalysts mainly arise from the spin itself. Our experimental results expand the electrochemical applications of 214-type perovskite oxides and provide a good platform for a deeper understanding of their catalytic mechanisms.

Reconstructing Hydrogen-Bond Network for Efficient Acidic Oxygen Evolution

Developing highly active oxygen evolution reaction (OER) catalysts in acidic conditions is a pressing demand for proton-exchange membrane water electrolysis. Manipulating proton character at the electrified interface, as the crux of all proton-coupled electrochemical reactions, is highly desirable but elusive. Herein we present a promising protocol, which reconstructs a connected hydrogen-bond network between the catalyst-electrolyte interface by coupling hydrophilic units to boost acidic OER activity. Modelling on N-doped-carbon-layer clothed Mn-doped-Co3O4 (Mn-Co3O4@CN), we unravel that the hydrogen-bond interaction between CN units and H2O molecule not only drags the free water to enrich the surface of Mn-Co3O4 but also serves as a channel to promote the dehydrogenation process. Meanwhile, the modulated local charge of the Co sites from CN units/Mn dopant lowers the OER barrier. Therefore, Mn-Co3O4@CN surpasses RuO2 at high current density (100 mA cm-2 @ ~538 mV).

Surface oxidation/spin state determines oxygen evolution reaction activity of cobalt-based catalysts in acidic environment

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism.

Atomically dispersed silver atoms incorporated in spinel cobalt oxide (Co3O4) for boosting oxygen evolution reaction

Incorporating noble metal single atoms into lattice of spinel cobalt oxide (Co3O4) is an attractive way to fabricate oxygen evolution reaction (OER) electrocatalysts because of the high activity and economic benefit. The commonly used high valence noble metal dopants such as ruthenium, iridium and rhodium tend to supersede Co3+ at octahedral site of Co3O4 and result in great activity, the origins of admirable activity were also wildly investigated. However, bare explorations on doping noble metal single atom into tetrahedral site of Co3O4 to construct OER catalyst have been reported, corresponding catalytic activity and mechanism remain mystery. Here, a promising structure that tetrahedrally substituent Ag single atom embedded in Co3O4 nanoparticles on the surface of carbon nanotube (Ag-Co3O4/CNT) was presented, and its performance in OER was probed. The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) demonstrate the successful embeddedness of atomical Ag atom in Co3O4 lattice, the resultant electronic interaction is conducive to promote charge transfer for OER. Theoretical calculations further disclose that atomical Ag dopant prefers to replace tetrahedral Co2+ rather than octahedral Co3+. The substitution Ag acts as the active site through Ag-Co bridge and facilitates the desorption process, which improves the turnover frequency (TOF) and boosts the intrinsic activity of Ag-Co3O4/CNT. Benefiting from the essentials above, Ag-Co3O4/CNT displays remarkable activity (236 mV@10 mA cm-2) and robust stability for alkaline OER. This finding offers a potential direction for the design of noble metal single atom involved Co3O4 based OER electrocatalysts.

Bimetallic metal-organic framework-derived bamboo-like N-doped carbon nanotube-encapsulated Ni-doped MoC nanoparticles for water oxidation

Molybdenum carbide materials with unique electronic structures have received special attention as water-splitting catalysts, but their structural stability in the alkaline water electrolysis process is not satisfactory. This study reports an in situ pyrolysis method for preparing NiMo-based metal-organic framework (MOF)-derived chain-mail oxygen evolution reaction (OER) electrocatalysts and bamboo-like N-doped carbon nanotube (NCNT)-encapsulated Ni-doped MoC nanoparticles (NiMoC-NCNTs). The NCNTs can provide chain mail shells to protect the inner highly reactive Ni-doped MoC cores from electrochemical corrosion by the alkaline electrolyte and regulate their catalytic properties through charge redistribution. Benefiting from high N-doping with abundant pyridinic moieties and abundant active sites of the periodic bamboo-like nodes, the as-prepared NiMoC-NCNTs display an outstanding activity for the OER with an overpotential of 310 mV at 10 mA cm-2 and a superior long-term stability of 50 h. Density functional theory calculations reveal that the excellent electrocatalytic activity of NiMoC-NCNTs comes from the electron transfer from NiMoC nanoparticles to NCNTs, resulting in a decrease in the local work function at the carbon surface and optimized free efficiencies of OER intermediates on C sites. This work provides an effective approach to improve the structural stability of fragile catalysts by equipping them with carbon-based chain.

The Promising Seesaw Relationship Between Activity and Stability of Ru-Based Electrocatalysts for Acid Oxygen Evolution and Proton Exchange Membrane Water Electrolysis

The development of electrocatalysts that are not reliant on iridium for efficient acid-oxygen evolution is a critical step towards the proton exchange membrane water electrolysis (PEMWE) and green hydrogen industry. Ruthenium-based electrocatalysts have garnered widespread attention due to their remarkable catalytic activity and lower commercial price. However, the challenge lies in balancing the seesaw relationship between activity and stability of these electrocatalysts during the acid-oxygen evolution reaction (OER). This review delves into the progress made in Ru-based electrocatalysts with regards to acid OER and PEMWE applications. It highlights the significance of customizing the acidic OER mechanism of Ru-based electrocatalysts through the coordination of adsorption evolution mechanism (AEM) and lattice oxygen oxidation mechanism (LOM) to attain the ideal activity and stability relationship. The promising tradeoffs between the activity and stability of different Ru-based electrocatalysts, including Ru metals and alloys, Ru single-atomic materials, Ru oxides, and derived complexes, and Ru-based heterojunctions, as well as their applicability to PEMWE systems, are discussed in detail. Furthermore, this paper offers insights on in situ control of Ru active sites, dynamic catalytic mechanism, and commercial application of PEMWE. Based on three-way relationship between cost, activity, and stability, the perspectives and development are provided.

Modulation of Phase Transition in Cobalt Selenide with Simultaneous Construction of Heterojunctions for Highly-Efficient Oxygen Electrocatalysis in Zinc-Air Battery

Phase transformation of cobalt selenide (CoSe2 ) can effectively modulate its intrinsic electrocatalytic activity. However, enhancing electroconductivity and catalytic activity/stability of CoSe2 still remains challenging. Heterostructure engineering may be feasible to optimize interfacial properties to promote the kinetics of oxygen electrocatalysis on a CoSe2 -based catalyst. Herein, a heterostructure consisting of CoSe2 and cobalt nitride (CoN) embedded in a hollow carbon cage is designed via a simultaneous phase/interface engineering strategy. Notably, the phase transition of orthorhombic-CoSe2 to cubic-CoSe2 (c-CoSe2 ) accompanied by in situ CoN formation is realized to build the c-CoSe2 /CoN heterointerface, which exhibits excellent/highly stable activities for oxygen reduction/evolution reactions (ORR/OER). Notably, heterostructure can modulate the local coordination environment and increase Co-Se/N bond lengths. Theoretical calculations show that Co-site (c-CoSe2 ) with an electronic state near Fermi energy level is the main active site for ORR/OER.Energetical tailoring of the d-orbital electronic structure of the Co atom of c-CoSe2 in heterostructure by in situ CoN incorporation lowers thermodynamic barriers for ORR/OER. Attractively, a zinc-air battery with a c-CoSe2 -CoN cathode displays excellent cycling stability (250 h) and charge/discharge voltage loss (0.953/0.96 V). It highlights that heterointerface engineering provides an option for modulating the bifunctional activity of metal selenides with controlled phase transformation.

RuO2 Catalysts for Electrocatalytic Oxygen Evolution in Acidic Media: Mechanism, Activity Promotion Strategy and Research Progress

Proton Exchange Membrane Water Electrolysis (PEMWE) under acidic conditions outperforms alkaline water electrolysis in terms of less resistance loss, higher current density, and higher produced hydrogen purity, which make it more economical in long-term applications. However, the efficiency of PEMWE is severely limited by the slow kinetics of anodic oxygen evolution reaction (OER), poor catalyst stability, and high cost. Therefore, researchers in the past decade have made great efforts to explore cheap, efficient, and stable electrode materials. Among them, the RuO2 electrocatalyst has been proved to be a major promising alternative to Ir-based catalysts and the most promising OER catalyst owing to its excellent electrocatalytic activity and high pH adaptability. In this review, we elaborate two reaction mechanisms of OER (lattice oxygen mechanism and adsorbate evolution mechanism), comprehensively summarize and discuss the recently reported RuO2-based OER electrocatalysts under acidic conditions, and propose many advanced modification strategies to further improve the activity and stability of RuO2-based electrocatalytic OER. Finally, we provide suggestions for overcoming the challenges faced by RuO2 electrocatalysts in practical applications and make prospects for future research. This review provides perspectives and guidance for the rational design of highly active and stable acidic OER electrocatalysts based on PEMWE.

Ambient γ-Rays-Mediated Noble-Metal Deposition on Defect-Rich Manganese Oxide for Glycerol-Assisted H2 Evolution at Industrial-Level Current Density

Developing novel synthesis technologies is crucial to expanding bifunctional electrocatalysts for energy-saving hydrogen production. Herein, we report an ambient and controllable γ-ray radiation reduction to synthesize a series of noble metal nanoparticles anchored on defect-rich manganese oxides (M@MnO2-x , M=Ru, Pt, Pd, Ir) for glycerol-assisted H2 evolution. Benefiting from the strong penetrability of γ-rays, nanoparticles and defect supports are formed simultaneously and bridged by metal-oxygen bonds, guaranteeing structural stability and active site exposure. The special Ru-O-Mn bonds activate the Ru and Mn sites in Ru@MnO2-x through strong interfacial coordination, driving glycerol electrolysis at low overpotential. Furthermore, only a low cell voltage of 1.68 V is required to achieve 0.5 A cm-2 in a continuous-flow electrolyzer system along with excellent stability. In situ spectroscopic analysis reveals that the strong interfacial coordination in Ru@MnO2-x balances the competitive adsorption of glycerol and OH* on the catalyst surface. Theoretical calculations further demonstrate that the defect-rich MnO2 support promotes the dissociation of H2 O, while the defect-regulated Ru sites promote deprotonation and hydrogen desorption, synergistically enhancing glycerol-assisted hydrogen production.

Interface Engineering a High Content of Co3+ Sites on Co3O4 Nanoparticles to Boost Acidic Oxygen Evolution

Non-noble metal oxides have emerged as potential candidate electrocatalysts for acidic oxygen evolution reactions (OERs) due to their earth abundance; however, improving their catalytic activity and stability simultaneously in strong acidic electrolytes is still a major challenge. In this work, we report Co3O4@carbon core-shell nanoparticles on 2D graphite sheets (Co3O4@C-GS) as mixed-dimensional hybrid electrocatalysts for acidic OER. The obtained Co3O4@C-GS catalyst exhibits a low overpotential of 350 mV and maintains stability for 20 h at a current density of 10 mA cm-2 in H2SO4 (pH = 1) electrolyte. X-ray photoelectron and X-ray absorption spectroscopies illustrate that the higher content of Co3+ sites boosts acidic OER. Operando Raman spectroscopy reveals that the catalytic stability of Co3O4@C nanoparticles during the acidic OER is enhanced by the introduction of graphite sheets. This interface engineering of non-noble metal sites with high valence states provides an efficient approach to boost the catalytic activity and enhance the stability of noble-metal-free electrocatalysts for acidic OER.

Fabrication of highly dispersed Pd-Mn3O4 catalyst for efficient catalytic propane total oxidation

Adjusting the interaction between dual active components for enhancing volatile organic compounds (VOCs) degradation is an effective but still challenging means of air pollution control. Herein, a limited pyrolysis oxidation strategy was adopted to prepare Pd-Mn3O4 spinel catalysts with uniform morphology and active component dispersion. Among these, 1.08Pd-Mn3O4 presented the highest catalytic efficiency with a T90 value of 240 °C, which was 94 °C lower than that of Mn3O4. Characterization and density functional theory (DFT) calculation results revealed that the strong metal-support interaction (SMSI) effect between Pd and Mn3O4 promoted the redistribution of surface charges, thus strengthening the oxidation-reduction ability of the active sites. Moreover, the SMSI effect led to a better migration of surface oxygen species, and boosted the generation of active surface oxygen species. Simultaneously, the Pd catalyst further reduced the energy barrier in the initial stage of the dehydrogenation of propane. Overall, this study provided a novel design strategy for dual active components catalysts with SMSI effect and extended the application of these catalysts in the important field of VOCs elimination.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.