Uncovering Interfacial Oxygen-Bridged Binuclear Metal Centers of Heterogenized Molecular Catalyst for Water Electrolysis

The success of different heterogeneous strategies of organometallic catalysts has been demonstrated to achieve high selectivity and activity in photo/electrocatalysis. However, yielding their catalytic mechanisms at complex molecule-electrode and electrochemical interfaces remains a great challenge. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy is employed to elucidate the dynamic process, interfacial structure, and intermediates of copper hydroxide-2-2' bipyridine on Au electrode ((bpy)Cu(OH)2/Au) during the oxygen evolution reaction (OER). Direct Raman molecular evidences reveal that the interfacial (bpy)Cu(OH)2 oxidizes into Cu(III) and bridges to Au atoms via oxygenated species, forming (bpy)Cu(III)O2-Au with oxygen-bridged binuclear metal centers of Cu(III)-O-Au for the OER. As the potential further increases, Cu(III)-O-Au combines with surface hydroxyl groups (*OH) to form the important intermediate of Cu(III)-OOH-Au, which then turns into Cu(III)-OO-Au to release O2. Furthermore, in situ electrochemical impedance spectroscopy proves that the Cu(III)-O-Au has lower resistance and faster mass transport of hydroxy to enhance OER. Theoretical calculations reveal that the formation of Cu(III)-O-Au significantly modify the elementary reaction steps of the OER, resulting in a lower potential-determining step of ≈0.58 V than that of bare Au. This work provides new insights into the OER mechanism of immobilized-molecule catalysts for the development and application of renewable energy conversion devices.

DFT-metadynamics insights on the origin of the oxygen evolution kinetics at the (100)-WSe2 surface

Water oxidation or oxygen evolution reaction (OER) in electrochemical cells is considered to be a major bottleneck in the way of hydrogen production by electro-synthesis, mainly due to a sluggish kinetics that characterizes the OER steps. Layered transition metal dichalcogenides, such as WSe2, are emerging as promising non-precious electrocatalysts for water splitting due to their excellent activity and stability. This paper aims to shed light on the (100) WSe2-aqueous interface in catalyzing the slow kinetics of the OER in the context of water splitting electro-catalysis. We employ state-of-the-art DFT-metadynamics to explore reaction mechanisms, activation free energies, and catalytic sites. This study reveals an energetically preferred water-assisted OER mechanism, where proton transfer is facilitated by the surrounding aqueous environment. Our findings not only provide insights into the OER process but also offer a design strategy for optimizing WSe2-based catalysts and a modeling protocol for future DFT-based OER investigations.

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.

Dynamic restructuring of electrocatalysts in the activation of small molecules: challenges and opportunities

Electrochemical activation of small molecules plays an essential role in sustainable electrosynthesis, environmental technologies, energy storage and conversion. The dynamic structural changes of catalysts during the course of electrochemical reactions pose challenges in the study of reaction kinetics and the design of potent catalysts. This short review aims to provide a balanced view of in situ restructuring of electrocatalysts, including its fundamental thermodynamic origins and how these compare to those in thermal and photocatalysis, and highlighting both the positive and negative impacts of in situ restructuring on the electrocatalyst performance. To this end, examples of in situ electrocatalyst restructuring within a focused scope of reactions (i.e. electrochemical CO2 reduction, hydrogen evolution, oxygen reduction and evolution, and dinitrogen and nitrate reduction) are used to demonstrate how restructuring can benefit or adversely affect the desired process outcome. Prospects of manipulating in situ restructuring towards an energy-efficient and durable electrocatalytic process are discussed. The practicality of pulse electrolysis on an industrial scale is questioned, and the need for genius schemes, such as self-healing catalysis, is emphasized.

Alkene Epoxidation and Oxygen Evolution Reactions Compete for Reactive Surface Oxygen Atoms on Gold Anodes

Rates and selectivities for the partial oxidation of organic molecules on reactive electrodes depend on the identity and prevalence of reactive and spectator species. Here, we investigate the mechanism for the epoxidation of 1-hexene (C6H12) with reactive oxygen species formed by electrochemical oxidation of water (H2O) on gold (Au) in an aqueous acetonitrile (CH3CN) electrolyte. Cyclic voltammetry measurements demonstrate that oxygen (O2) evolution competes with C6H12 epoxidation, and the Au surface must oxidize before either reaction occurs. In situ Raman spectroscopy reveals reactive oxygen species and spectators (CH3CN) on the active anode as well as species within the electrochemical double layer. The Faradaic efficiencies toward epoxidation and the ratios of epoxide formation to O2 evolution rates increase linearly with the concentration of C6H12 and depend inversely on the concentration of H2O, which agree with analytical expressions that describe rates for reaction between C6H12 and chemisorbed oxygen atoms (O*) and exclude proposals for other forms of reactive oxygen (e.g., O2*, OOH*, OH*). These findings show that the epoxidation and O2 evolution reactions share a set of common steps that form O* through electrochemical H2O activation but then diverge. Subsequently, epoxides form when O* reacts with C6H12 through a non-Faradaic process, whereas O2 evolves when O* reacts with H2O through a Faradaic process to form OOH*, which then deprotonates. These differences lead to distinct changes in rates in response to electrode potential, and hence, disparate Tafel slopes. Collectively, these results provide a self-consistent mechanism for C6H12 epoxidation that involves reactive O*.

Disentangling the Pitfalls of Rotating Disk Electrode-Based OER Stability Assessment: Bubble Blockage or Substrate Passivation?

Oxygen evolution reaction (OER) catalyst stability metrics derived from aqueous model systems (AMSs) prove valuable only if they are transferable to technical membrane electrode assembly (MEA) settings. Currently, there is consensus that stability data derived from ubiquitous rotating disk electrode (RDE)-based investigations substantially overestimate material degradation mainly due to the nonideal inertness of catalyst-backing electrode materials as well as bubble shielding of the catalyst by evolved oxygen. Despite the independently developed understanding of these two processes, their interplay and relative impact on intrinsic and operational material stability have not yet been established. Herein, we employ an inverted RDE-based approach coupled with online gas chromatographic quantification that exploits buoyancy and anode hydrophilicity existing under operating acidic OER conditions and excludes the influence of bubble retention on the surface of the catalyst. This approach thus allows us to dissect the degradation process occurring during the RDE-based OER stability studies. We demonstrate that the stability discrepancy between galvanostatic nanoparticle (NP)-based RDE and MEA data does not originate from the accumulation of bubbles in the catalyst layer during water oxidation but from the utilization of corrosion-prone substrate materials in the AMS. Moreover, we provide mechanistic insights into the degradation process and devise experimental measures to mitigate or circumvent RDE-related limitations when the technique is to be applied to an OER catalyst stability assessment. These findings should facilitate the transferability between AMS and MEA approaches and promote further development of the latter.

Electrochemical and Non-Electrochemical Pathways in the Electrocatalytic Oxidation of Monosaccharides and Related Sugar Alcohols into Valuable Products

In this contribution, we review the electrochemical upgrading of saccharides (e.g., glucose) and sugar alcohols (e.g., glycerol) on metal and metal-oxide electrodes by drawing conclusions on common trends and differences between these two important classes of biobased compounds. For this purpose, we critically review the literature on the electrocatalytic oxidation of saccharides and sugar alcohols, seeking trends in the effect of reaction conditions and electrocatalyst design on the selectivity for the oxidation of specific functional groups toward value-added compounds. Importantly, we highlight and discuss the competition between electrochemical and non-electrochemical pathways. This is a crucial and yet often neglected aspect that should be taken into account and optimized for achieving the efficient electrocatalytic conversion of monosaccharides and related sugar alcohols into valuable products, which is a target of growing interest in the context of the electrification of the chemical industry combined with the utilization of renewable feedstock.

In Situ STM Study of Roughening of Au(111) Single-Crystal Electrode in Sulfuric Acid Solution during Oxidation-Reduction Cycles

Oxidation-reduction cycles (ORCs) on Au(111) in 0.1 M sulfuric acid solution change the electrode morphology due to the formation of many new nanosized islands. With increasing the cycle number, the roughness of the surface increases due to the formation of multiatomic-step adatom islands and pits. The final roughness value is a function of the applied potential window, number of ORCs, scan rate, electrolyte concentration, and any applied delay time. In a first experiment, the roughening was tracked by recording the STM images in 11 steps during 200 ORCs. The results show the formation of pyramidal islands and a linear correlation between the roughness amplitude and the cycle number. In a second experiment, the 200 cycles were studied in 38 steps, while after each step, two images were recorded with a 3 min delay by holding the potential in the double-layer window. This leads to a lower roughness increase due to the high surface mobility of the Au surface atoms, which smoothens the surface during the delay time. Finally, the oxidation-reduction charge density per cycle shows an inverse correlation with surface roughness due to the (111) terrace showing a higher surface oxidation charge than the other sites and facets. Each delay causes a strong increase in the oxidation charge which is a consequence of surface smoothening during the delays leading to an enhancement of the (111) related oxidation charge.

Visualization of Electrooxidation on Palladium Single Crystal Surfaces via In Situ Raman Spectroscopy

The electrooxidation of catalyst surfaces is across various electrocatalytic reactions, directly impacting their activity, stability and selectivity. Precisely characterizing the electrooxidation on well-defined surfaces is essential to understanding electrocatalytic reactions comprehensively. Herein, we employed in situ Raman spectroscopy to monitor the electrooxidation process of palladium single crystal. Our findings reveal that the Pd surface's initial electrooxidation process involves forming *OH intermediate and ClO4 - ions facilitate the deprotonation process, leading to the formation of PdOx. Subsequently, under deep electrooxidation potential range, the oxygen atoms within PdOx contribute to creating surface-bound peroxide species, ultimately resulting in oxygen generation. The adsorption strength of *OH and the coverage of ClO4 - can be adjusted by the controllable electronic effect, resulting in different oxidation rates. This study offers valuable insights into elucidating the electrooxidation mechanisms underlying a range of electrocatalytic reactions, thereby contributing to the rational design of catalysts.

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

Powering the Future by Iron Sulfide Type Material (FexSy) Based Electrochemical Materials for Water Splitting and Energy Storage Applications: A Review

Water electrolysis is among the recent alternatives for generating clean fuels (hydrogen). It is an efficient way to produce pure hydrogen at a rapid pace with no unwanted by-products. Effective and cheap water-splitting electrocatalysts with enhanced activity, specificity, and stability are currently widely studied. In this regard, noble metal-free transition metal-based catalysts are of high interest. Iron sulfide (FeS) is one of the essential electrocatalysts for water splitting because of its unique structural and electrochemical features. This article discusses the significance of FeS and its nanocomposites as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and overall water splitting. FeS and its nanocomposites have been studied also for energy storage in the form of electrode materials in supercapacitors and lithium- (LIBs) and sodium-ion batteries (SIBs). The structural and electrochemical characteristics of FeS and its nanocomposites, as well as the synthesis processes, are discussed in this work. This discussion correlates these features with the requirements for electrocatalysts in overall water splitting and its associated reactions. As a result, this study provides a road map for researchers seeking economically viable, environmentally friendly, and efficient electrochemical materials in the fields of green energy production and storage.

Na Substitution Steering RuO6 Unit in Ruthenium Pyrochlores for Enhanced Oxygen Evolution in Acid

Although Ruthenium-based pyrochlore oxides can function as promising catalysts for acidic water oxidation, their limitations in terms of stability and activity still need to be addressed for further application in practical conditions. In this work, the possibility to enhance both oxygen evolution reaction activity and durability of Gd2Ru2O7- δ through partial replacement with Na+ in Gd3+ sites is first offered, leading to the electronic and geometric regulation of active center RuO6. Na+ triggers the emergence of Ru<4+ and the electron rearrangement of active-centered RuO6. Specifically, Ru ions with a negative d-band center after Na+ doping exhibit weaker adsorption energies of *O and result in the conversion of the rate-limiting step from *O/*OOH to *OH/O*, reducing energy barriers for boosting activities. Therefore, the NaxGd2- xRu2O7- δ requires a low overpotential of 260 mV at 10 mA cm-2 in 0.1 m HClO4 electrolyte. Moreover, the higher formation energy of Ru vacancy and less distorted RuO6 enable the as-prepared NaxGd2- xRu2O7- δ to operate steadily at 10 mA cm-2 for 300 h and multi-current chronopotentiometry with current densities from 20 to 100 mA cm-2 for 60 h in acidic proton exchange membrane electrolyzer, respectively.

Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Capturing critical gem-diol intermediates and hydride transfer for anodic hydrogen production from 5-hydroxymethylfurfural

The non-classical anodic H2 production from 5-hydroxymethylfurfural (HMF) is very appealing for energy-saving H2 production with value-added chemical conversion due to the low working potential (~0.1 V vs RHE). However, the reaction mechanism is still not clear due to the lack of direct evidence for the critical intermediates. Herein, the detailed mechanisms are explored in-depth using in situ Raman and Infrared spectroscopy, isotope tracking, and density functional theory calculations. The HMF is observed to form two unique inter-convertible gem-diol intermediates in an alkaline medium: 5-(Dihydroxymethyl)furan-2-methanol anion (DHMFM-) and dianion (DHMFM2-). The DHMFM2- is easily oxidized to produce H2 via H- transfer, whereas the DHMFM- is readily oxidized to produce H2O via H+ transfer. The increases in potential considerably facilitate the DHMFM- oxidation rate, shifting the DHMFM- ↔ DHMFM2- equilibrium towards DHMFM- and therefore diminishing anodic H2 production until it terminates. This work captures the critical intermediate DHMFM2- leading to hydrogen production from aldehyde, unraveling a key point for designing higher performing systems.

Structural Evolution of Ultrathin SrFeO3-δ Films during Oxygen Evolution Reaction Revealed by In Situ Electrochemical Stress Measurements

We report the electrochemical stress analysis of SrFeO3-δ (SFO) films deposited on Au substrates during oxygen evolution reactions (OERs). Our in situ analysis of Au reveals conversion reactions from Au to Au(OH)3, AuOOH, and AuOx during the OER. Au reactions cause a monotonic compressive stress on surfaces assigned to the formation of Au hydroxides and oxides. Electrochemical stress analysis of SrFeO3-δ/Au shows a dramatically different behavior during the OER, which we attribute to structural evolutions and conversion reactions, such as the conversion of SFO to iron (oxy)hydroxides. Interestingly, electrochemical stress analysis of SrFeO3-δ/Au shows a tensile trend, which evolves with cycling history. Electrochemical stress analysis of SFO films before the onset of the OER shows in situ changes, which cause tensile stresses when cycling to 1.2 V. We attribute these stresses to the formation of Fe2+δOδ(OH)2-δ (0 ≤ δ ≤ 1.5)-type materials where δ approaches 1.5 at higher potentials. At potentials higher than 1.2 V and during OER, surface stress response is rather stable, which we assign to the full conversion of SFO to iron (oxy)hydroxides. This analysis provides insight into the reaction mechanism and details of in situ structural changes of iron perovskites during the OER in alkaline environments.

SERS Sensing of Dopamine with Fe(III)-Sensitized Nanogaps in Recleanable AuNP Monolayer Films

Sensing of neurotransmitters (NTs) down to nm concentrations is demonstrated by utilizing self-assembled monolayers of plasmonic 60 nm Au nanoparticles in close-packed arrays immobilized onto glass substrates. Multiplicative surface-enhanced Raman spectroscopy enhancements are achieved by integrating Fe(III) sensitizers into the precisely-defined <1 nm nanogaps, to target dopamine (DA) sensing. The transparent glass substrates allow for efficient access from both sides of the monolayer aggregate films by fluid and light, allowing repeated sensing in different analytes. Repeated reusability after analyte sensing is shown through oxygen plasma cleaning protocols, which restore pristine conditions for the nanogaps. Examining binding competition in multiplexed sensing of two catecholamine NTs, DA and epinephrine, reveals their bidentate binding and their interactions. These systems are promising for widespread microfluidic integration enabling a wide range of continuous biofluid monitoring for applications in precision health.

Active oxygen species mediate the iron-promoting electrocatalysis of oxygen evolution reaction on metal oxyhydroxides

Iron is an extraordinary promoter to impose nickel/cobalt (hydr)oxides as the most active oxygen evolution reaction catalysts, whereas the synergistic effect is actively debated. Here, we unveil that active oxygen species mediate a strong electrochemical interaction between iron oxides (FeOxHy) and the supporting metal oxyhydroxides. Our survey on the electrochemical behavior of nine supporting metal oxyhydroxides (M(O)OH) uncovers that FeOxHy synergistically promotes substrates that can produce active oxygen species exclusively. Tafel slopes correlate with the presence and kind of oxygen species. Moreover, the oxygen evolution reaction onset potentials of FeOxHy@M(O)OH coincide with the emerging potentials of active oxygen species, whereas large potential gaps are present for intact M(O)OH. Chemical probe experiments suggest that active oxygen species could act as proton acceptors and/or mediators for proton transfer and/or diffusion in cooperative catalysis. This discovery offers a new insight to understand the synergistic catalysis of Fe-based oxygen evolution reaction electrocatalysts.

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys

Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal-liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron-based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni-Cr-Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr-containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni-Cr-Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.

Unveiling the Hidden Energy Profiles of the Oxygen Evolution Reaction via Machine Learning Analyses

The oxygen evolution reaction (OER) is a crucial electrochemical process for hydrogen production in water electrolysis. However, due to the involvement of multiple proton-coupled electron transfer steps, it is challenging to identify the specific elementary reaction that limits the rate of the OER. Here we employed a machine-learning-based approach to extract the reaction pathway exhaustively from experimental data. Genetic algorithms were applied to search for thermodynamic and kinetic parameters using the current-electrochemical potential relationship of the OER. Interestingly, analysis of the datasets revealed the energy state distributions of reaction intermediates, which likely originated in the interactions among intermediates or the distribution of multiple sites. Through our exhaustive analyses, we successfully uncovered the hidden energy profiles of the OER. This approach can reveal the reaction pathway to activate for efficient hydrogen production, which facilitates the design of catalysts.

How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper

Copper is a unique catalyst for the electrochemical CO₂ reduction reaction (CO2RR) as it can produce multi-carbon products, such as ethylene and propanol. As practical electrolyzers will likely operate at elevated temperatures, the effect of reaction temperature on the product distribution and activity of CO2RR on copper is important to elucidate. In this study, we have performed electrolysis experiments at different reaction temperatures and potentials. We show that there are two distinct temperature regimes. From 18 up to ∼48 °C, C2+ products are produced with higher Faradaic efficiency, while methane and formic acid selectivity decreases and hydrogen selectivity stays approximately constant. From 48 to 70 °C, it was found that HER dominates and the activity of CO2RR decreases. Moreover, the CO2RR products produced in this higher temperature range are mainly the C1 products, namely, CO and HCOOH. We argue that CO surface coverage, local pH, and kinetics play an important role in the lower-temperature regime, while the second regime appears most likely to be related to structural changes in the copper surface.

Ligand effect on switching the rate-determining step of water oxidation in atomically precise metal nanoclusters

The ligand effects of atomically precise metal nanoclusters on electrocatalysis kinetics have been rarely revealed. Herein, we employ atomically precise Au₂₅ nanoclusters with different ligands (i.e., para-mercaptobenzoic acid, 6-mercaptohexanoic acid, and homocysteine) as paradigm electrocatalysts to demonstrate oxygen evolution reaction rate-determining step switching through ligand engineering. Au₂₅ nanoclusters capped by para-mercaptobenzoic acid exhibit a better performance with nearly 4 times higher than that of Au₂₅ NCs capped by other two ligands. We deduce that para-mercaptobenzoic acid with a stronger electron-withdrawing ability establishes more partial positive charges on Au(I) (i.e., active sites) for facilitating feasible adsorption of OH^- in alkaline media. X-ray photo-electron spectroscopy and theoretical study indicate a profound electron transfer from Au(I) to para-mercaptobenzoic acid. The Tafel slope and in situ Raman spectroscopy suggest different ligands trigger different rate-determining step for these Au₂₅ nanoclusters. The mechanistic insights reported here can add to the acceptance of atomically precise metal nanoclusters as effective electrocatalysts.

In Situ Spectroscopic Identification of the Electron-Transfer Intermediates of Photoelectrochemical Proton-Coupled Electron Transfer of Water Oxidation on Au

Experimental elucidation of the decoupling of electron and proton transfer at a molecular level is essential for thoroughly understanding the kinetics of heterogeneous (photo)electrochemical proton-coupled electron transfer water oxidation. Here we illustrate the electron-transfer intermediates of positively charged surface oxygenated species on Au (Au-OH⁺) and their correlations with the rate of water oxidation by in situ microphotoelectrochemical surface-enhanced Raman spectroscopy (SERS) and ambient-pressure X-ray photoelectron spectroscopy. At the intermediate stage of water oxidation, a characteristic blue shift of the vibration of Au-OH species in laser-power-density-dependent measurements was assigned to the light-induced production of Au-OH⁺ in water oxidation. The photothermal effect was excluded according to the vibrational frequencies of Au-OH species as the temperature was increased in a variable-temperature SERS measurement. Density functional theory calculations evidenced that the frequency blue shift is from the positively charged Au-OH species. The photocurrent-dependent frequency blue shift indicated that Au-OH⁺ is the key electron-transfer intermediate in water oxidation by decoupled electron and proton transfer.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.

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

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

The low overpotential regime of acidic water oxidation part II: trends in metal and oxygen stability numbers

The operating conditions of low pH and high potential at the anodes of polymer electrolyte membrane electrolysers restrict the choice of catalysts for the oxygen evolution reaction (OER) to oxides based on the rare metals iridium or ruthenium. In this work, we investigate the stability of both the metal atoms and, by quantitative and highly sensitive 18O isotope labelling experiments, the oxygen atoms in a series of RuO x and IrO x electrocatalysts during the OER in the mechanistically interesting low overpotential regime. We show that materials based on RuO x have a higher dissolution rate than the rate of incorporation of labelled oxygen from the catalyst into the O2 evolved ("labelled OER"), while for IrO x -based catalysts the two rates are comparable. On amorphous RuO x , metal dissolution and labelled OER are found to have distinct Tafel slopes. These observations together lead us to a full mechanistic picture in which dissolution and labelled OER are side processes to the main electrocatalytic cycle. We emphasize the importance of quantitative analysis and point out that since less than 0.2% of evolved oxygen contains an oxygen atom originating from the catalyst itself, lattice oxygen evolution is at most a negligible contribution to overall OER activity for RuO x and IrO x in acidic electrolyte.

Heterointerface Created on Au-Cluster-Loaded Unilamellar Hydroxide Electrocatalysts as a Highly Active Site for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical element for all sorts of reactions that use water as a hydrogen source, such as hydrogen evolution and electrochemical CO₂ reduction, and novel design principles that provide highly active sites on OER electrocatalysts push the limits of their practical applications. Herein, Au-cluster loading on unilamellar exfoliated layered double hydroxide (ULDH) electrocatalysts for the OER is demonstrated to fabricate a heterointerface between Au clusters and ULDHs as an active site, which is accompanied by the oxidation state modulation of the active site and interfacial direct OO coupling ("interfacial DOOC"). The Au-cluster-loaded ULDHs exhibit excellent activities for the OER with an overpotential of 189 mV at 10 mA cm^-2 . X-ray absorption fine structure measurements reveal that charge transfer from the Au clusters to ULDHs modifies the oxidation states of trivalent metal ions, which can be active sites on the ULDHs. The present study, supported by highly sensitive spectroscopy combining reflection absorption infrared spectroscopy and modulation-excitation spectroscopy and density functional theory calculations, indicates that active sites at the interface between the Au clusters and ULDHs promote a novel OER mechanism through interfacial DOOC, thereby achieving outstanding catalytic performance.

Phase Segregation in Cobalt Iron Oxide Nanowires toward Enhanced Oxygen Evolution Reaction Activity

The impact of reduction post-treatment and phase segregation of cobalt iron oxide nanowires on their electrochemical oxygen evolution reaction (OER) activity is investigated. A series of cobalt iron oxide spinel nanowires are prepared via the nanocasting route using ordered mesoporous silica as a hard template. The replicated oxides are selectively reduced through a mild reduction that results in phase transformation as well as the formation of grain boundaries. The detailed structural analyses, including the ⁵⁷Fe isotope-enriched Mössbauer study, validated the formation of iron oxide clusters supported by ordered mesoporous CoO nanowires after the reduction process. This affects the OER activity significantly, whereby the overpotential at 10 mA/cm² decreases from 378 to 339 mV and the current density at 1.7 V vs RHE increases by twofold from 150 to 315 mA/cm². In situ Raman microscopy revealed that the surfaces of reduced CoO were oxidized to cobalt with a higher oxidation state upon solvation in the KOH electrolyte. The implementation of external potential bias led to the formation of an oxyhydroxide intermediate and a disordered-spinel phase. The interactions of iron clusters with cobalt oxide at the phase boundaries were found to be beneficial to enhance the charge transfer of the cobalt oxide and boost the overall OER activity by reaching a Faradaic efficiency of up to 96%. All in all, the post-reduction and phase segregation of cobalt iron oxide play an important role as a precatalyst for the OER.

Inter-relationships between Oxygen Evolution and Iridium Dissolution Mechanisms

The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir-based catalysts is, therefore, crucial for a scale-up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state-of-the-art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.

Plasmonic Photoelectrochemistry: In View of Hot Carriers

Utilizing plasmon-generated hot carriers to drive chemical reactions has emerged as a popular topic in solar photocatalysis. However, a complete description of the underlying mechanism of hot-carrier transfer in photochemical processes remains elusive, particularly for those involving hot holes. Photoelectrochemistry enables to localize hot holes on photoanodes and hot electrons on photocathodes and thus offers an approach to separately explore the hole-transfer dynamics and electron-transfer dynamics. This review summarizes a comprehensive understanding of both hot-hole and hot-electron transfers from photoelectrochemical studies on plasmonic electrodes. Additionally, working principles and applications of spectroelectrochemistry are discussed for plasmonic materials. It is concluded that photoelectrochemistry provides a powerful toolbox to gain mechanistic insights into plasmonic photocatalysis.

Electrochemically Deposited Amorphous Cobalt-Nickel-Doped Copper Oxide as an Efficient Electrocatalyst toward Water Oxidation Reaction

Production of hydrogen through water splitting is one of the green and the most practical solutions to cope with the energy crisis and greenhouse effect. However, oxygen evolution reaction (OER) being a sluggish step, the use of precious metal-based catalysts is the main impediment toward the viability of water splitting. In this work, amorphous copper oxide and doped binary- and ternary-metal oxides (containing CoII, NiII, and CuII) have been prepared on the surface of fluorine-doped tin oxide by a facile electrodeposition route followed by thermal treatment. The fabricated electrodes have been employed as efficient binder-free OER electrocatalysts possessing a high electrochemical surface area due to their amorphous nature. The cobalt-nickel-doped copper oxide (ternary-metal oxide)-based electrode showed promising OER activity with a high current density of 100 mA cm-2 at 1.65 V versus RHE that escalates to 313 mA cm-2 at 1.76 V in alkaline media at pH 14. The high activity of the ternary-metal oxide-based electrode was further supported by a smaller semicircle in the Nyquist plot. Furthermore, all metal-oxide-based electrodes offered high stability when tested for continuous production of oxygen for 50 h. This work highlights the synthesis of efficient and cost-effective amorphous metal-based oxide catalysts to execute electrocatalytic OER employing an electrodeposition approach.

Electrosorptive removal of organic water constituents by positively charged electrically conductive UF membranes

Negatively charged electrically conductive ultrafiltration (UF) membranes have been intensively investigated for fouling mitigation and rejection enhancement in recent years. This study reports the novel approach of applying positive charge (+2.5 V cell potential) to a conductive membrane to induce electrosorption of negatively charged substances onto the membrane. Subsequently, desorption of negatively charged substances is achieved by changing the potential periodically (e.g., after 30 min) to negative charge (-2.5 V cell potential). For this purpose, sputter deposition of ultra-thin gold layers (40 nm) is used to generate electrically conductive gold-polymer-gold flat sheet membranes by coating the active and the support layer of two commercial polymer UF membranes (polyethersulfone UP150, polyamide M5). When M5 membrane was charged positively during filtration (+2.5 V), Suwannee River NOM, Hohloh lake NOM, humic acid and Brilliant Blue ionic dye showed removal rates of 70 %, 75% and 93% and 99%, respectively. Whereas, when no potential was applied (0 V) removal rates were only 1 - 5 %. When a positive potential was applied to the active membrane layer and a negative potential was applied to the support layer (cell potential 2.5 V), a significant increase of flux with 25 L/(m² h) was observed due to the induction of electro-osmosis. Electrosorption was only observed for M5 membrane (ζ: +13 mV, pH 7) and not with UP150 membrane (ζ: -29 mV, pH 7). Due to a low current density of 1.1 A/m² at a flux of 100 L/(m² h), the additional energy consumption of electrosorption and desorption process was low with 0.03 kWh per m³ of permeate. This study delivered the proof of concept for the novel process of electrosorptive UF with energy consumption between microfiltration and ultrafiltration but NOM removal rates of nanofiltration membranes.

General Design Concept for Single-Atom Catalysts toward Heterogeneous Catalysis

As a new and popular material, single-atom catalysts (SACs) exhibit excellent activity, selectivity, and stability for numerous important reactions, and show great potential in heterogeneous catalysis due to their high atom utilization efficiency and the controllable characteristics of the active sites. The composition and coordination would determine the geometric and electronic structures of SACs, and thus greatly influence the catalytic performance. Based on atom economy, rational design and controllable synthesis of SACs have become central tasks in the fields of low-cost and green catalysis. Herein, an introduction to the recent progress in the precise synthesis of SACs including the regulation of the coordination structure and the choice of different systems is presented. Thereafter, the potentials of SACs in different applications are comprehensively summarized and discussed. Furthermore, a detailed discussion of the recent developments regarding the large-scale preparation of SACs is provided, including the major issues and prospects for industrialization. Finally, the main challenges and opportunities of rapid large-scale industrialization of SACs are briefly discussed.

Defect-Engineered Nanozyme-Linked Receptors

Though nanozymes are successfully applied in various areas, the increasing demands facilitate the exploitation of nanozymes possessing higher activity and more functions. Natural enzyme-linked receptors (ELRs) are critical components for signal transductions in vivo by expressing activity variations after binding with ligands. Inspired by this, the defect-engineered carbon nitrides (DCN) are reported to serve as nanozyme-linked receptors (NLRs). For one thing, cyano defects increase the enzyme-like activity by a factor of 109.5. For another, DCN-based NLRs are constructed by employing cyano groups as receptors, and variable outputs are ensued upon the addition of ion ligands. Significantly, both the cascade effect and electronic effect are demonstrated to contribute to this phenomenon. Finally, NLRs are used for pattern recognition of metal ions, indicating the signal transduction ability of NLRs as well. This work not only provides great promise of defect engineering in nanozymes, but also contributes to the design of artificial ELRs.