Conduction and Reactivity in Heterogeneous-Molecular Catalysis: New Insights in Water Oxidation Catalysis by Phosphate Cobalt Oxide Films

Atomic Insights into the Competitive Edge of Nanosheets Splitting Water

The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxohydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with the atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain the performance. However, it is not fully understood how activity and stability are related at the atomic level, hampering rational design. Herein, we provide simple design rules (Figure 12) derived from the literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that IrOOH signals the chemical stability of crystalline IOHs while surpassing the activity of amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3Δ-O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e., μ1-O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.

Indispensable Nafion Ionomer for High-Efficiency and Stable Oxygen Evolution Reaction in Alkaline Media

Addressing the challenge of sluggish kinetics and limited stability in alkaline oxygen evolution reactions, recent exploration of novel electrochemical catalysts offers improved prospects. To expedite the assessment of these catalysts, a half-cell rotating disk electrode is often favored for its simplicity. However, the actual catalyst performance strongly depends on the fabricated catalyst layers, which encounter mass transport overpotentials. We systematically investigate the role and sequence of electrode drop-casting methods onto a glassy carbon electrode regarding the efficiency of the oxygen evolution reaction. The catalyst layer without Nafion experiences nearly 50% activity loss post stability test, while those with Nafion exhibit less than 5% activity loss. Additionally, the sequence of application of the catalyst and Nafion also shows a significant effect on catalyst stability. The catalyst activity increases by roughly 20% after the stability test when the catalyst layer is coated first with an ionomer layer, followed by drop-casting the catalysts. Based on the half-cell results, the Nafion ionomer not only acts as a binder in the catalyst layer but also enhances the interfacial interaction between the catalyst and electrolyte, promoting performance and stability. This study provides new insights into the efficient and accurate evaluation of electrocatalyst performance and stability as well as the role of Nafion ionomer in the catalyst layer.

Adjusting the valence band center of Co-Ni-bimetallic sulfides through lattice expansion and stacking faults triggered by strain engineering to boost oxygen evolution reaction

Stress engineering can improve catalytic performance by straining the catalyst lattice. An electrocatalyst, Co₃S₄/Ni₃S₂-10%Mo@NC, was prepared with abundant lattice distortion to boost oxygen evolution reaction (OER). With the assistance of the intramolecular steric hindrance effect of metal-organic frameworks, slow dissolution by MoO₄^2- of the Ni substrate and recrystallization of Ni²⁺ was observed in the process of Co(OH)F crystal growth with mild temperature and short time reaction. The lattice expansion and stacking faults created defects inside the Co₃S₄ crystal, improved the material conductivity, optimized the valence band electron distribution of the material, and promoted the rapid conversion of the reaction intermediates. The presence of reactive intermediates of the OER under catalytic conditions was investigated using operando Raman spectroscopy. The electrocatalysts exhibited super high performance, a current density of 10 mA cm^-2 at an overpotential of 164 mV and 100 mA cm^-2 at 223 mV, which were comparable to those of integrated RuO₂. Our work for the first time demonstrates that the dissolution-recrystallization triggered by strain engineering is a good modulation approach to adjust the structure and surface activity of catalyst, suggesting promising industrial application.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode

The integration of heterogeneous electrocatalysis and molecular catalysis is a promising approach to designing new catalysts for the oxygen evolution reaction (OER) and other processes. We recently showed that the electrostatic potential drop across the double layer contributes to the driving force for electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. Here, we report high current densities and low onset potentials for water oxidation attained using a metal-free voltage-assisted molecular catalyst (TEMPO). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine faradic efficiencies for the generation of H₂O₂ and O₂. The same catalyst was employed for efficient oxidations of butanol, ethanol, glycerol, and H₂O₂. DFT calculations show that the applied voltage alters the electrostatic potential drop between TEMPO and the reactant as well as chemical bonding between them, thereby increasing the reaction rate. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts for OER and alcohol oxidations.

Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

Nanocrystalline or amorphous cobalt oxyhydroxides (CoCat) are promising electrocatalysts for the oxygen evolution reaction (OER). While having the same short-range order, CoCat phases possess different electrocatalytic properties. This phenomenon is not conclusively understood, as multiple interdependent parameters affect the OER activity simultaneously. Herein, a layered cobalt borophosphate precatalyst, Co(H₂ O)₂ [B₂ P₂ O₈ (OH)₂ ]·H₂ O, is fully reconstructed into two different CoCat phases. In contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex situ and in situ investigations of the two borophosphate derived CoCats, as well as the prominent CoP_i and CoB_i identify differences in the Tafel slope/range, buffer binding and content, long-range order, number of accessible edge sites, redox activity, and morphology. Considering and interconnecting these aspects together with proton mass-transport limitations, a comprehensive picture is provided explaining the different OER activities. The most decisive factors are the buffers used for reconstruction, the number of edge sites that are not inhibited by irreversibly bonded buffers, and the morphology. With this acquired knowledge, an optimized OER system is realized operating in near-neutral potassium borate medium at 1.62 ± 0.03 V_RHE yielding 250 mA cm^-2 at 65 °C for 1 month without degrading performance.

Ba2Bi2[Zn(PO4)4]: ionic-isolation-activated first barium bismuth zincophosphate with unprecedented ∞[Zn(PO4)4]10− chains and balanced overall performance

The combination of ion-mixing and anionic condensation has activated the first barium bismuth zincophosphate of Ba2Bi2[Zn(PO4)4] with unprecedented ∞[Zn(PO4)4]10− chains and balanced overall performance.

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

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

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

Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation

The development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm-2 at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co2+ species to Co3+ species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Ag nanoparticles decorated urchin-like cobalt carbonate hydroxide composites for highly efficient oxygen evolution reaction

Herein, a novel composite of small amounts of Ag nanoparticles (NPs) decorated urchin-like cobalt carbonate hydroxide hydrate (CCHH) was developed for highly-efficient alkaline oxygen evolution reaction (OER). Not only can Ag colloids, as template agents, modify the morphologies of urchin-like CCHH microspheres to expose more active sites available, but also the supported Ag NPs formed by Ag colloids can transfer the electron to CCHH surfaces, accelerating the transformation of surface Co^II to Co^III/Co^IV (proton-coupled electron transfer (PCET) process). The urchin-like Ag/CCHH (0.013 mmol) precatalyst (before cyclic voltammetry (CV) activation) exhibits a better OER performance (a low overpotential of 273 mV at 10 mA cm^-2 and small Tafel slope of 65 mV dec^-1) as compared with commercial RuO₂. Furthermore, the dynamic surface self-reconstruction (surface CO₃ ^2- and OH ^- exchange) can further enhance the activities of Ag/CCHH precatalysts. Consequently, the optimal Ag/CCHH (0.013 mmol) catalyst presents a superior activity (a lower overpotential of 267 mV at 10 mA cm^-2 and markedly reduced Tafel slope to 56 mV dec^-1) along with an excellent stability after CV cycles. The study provides a feasible strategy to fully realize the low overpotential of CCHH-based OER electrocatalysts.

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

The solubility of molecular transition metal complexes can vary widely across different redox states, leaving these compounds vulnerable to electron transfer-initiated heterogenization processes in which oxidation or reduction of the soluble form of the redox couple generates insoluble molecular deposits. These insoluble species can precipitate as suspended nanoparticles in solution or, under electrochemical conditions, as an electrode-adsorbed material. While this electrochemically-driven solubility cycling is technically reversible, the reverse electron transfer to regenerate the soluble redox couple state is a practical challenge if sluggish electron transfer kinetics result in a loss of electronic communication between the molecular deposits and the electrode. In this work, we present an example of this electrochemically-driven solubility cycling, report a novel strategy for catalytically enhancing the oxidation of the insoluble material using homogeneous redox mediators, and develop the theoretical framework for analysing and digitally simulating the action of a homogeneous catalyst on a heterogeneous substrate via cyclic voltammetry. First, a mix of electrochemical and spectroscopic methods are used to characterize an example of this electrochemically-driven solubility cycling which is based on the two-electron reduction of homogeneous [Ni(PPh 2NPh 2)2(CH3CN)]2+ (PPh 2NPh 2 = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane). The limited solubility of the doubly-reduced product in acetonitrile leads to precipitation and deposition of molecular [Ni(PPh 2NPh 2)2]. While direct oxidation of this heterogeneous [Ni(PPh 2NPh 2)2] at the electrode surface is possible, this electron transfer is kinetically limited. We demonstrate how a freely diffusing redox mediator (ferrocene) - which shuttles electrons between the electrode and the molecular material - can be used to overcome these slow electron transfer kinetics, enabling catalytic regeneration of soluble [Ni(PPh 2NPh 2)2]2+. Finally, mathematical models are developed that describe the current-potential response for a generic EC' mechanism involving a homogeneous catalyst and surface-adsorbed substrate. This novel strategy has the potential to enable reversible redox chemistry for heterogeneous, molecular deposits that are adsorbed on the electrode or suspended as nanoparticles in solution.

The NHx Group Induced Formation of 3D α-Co(OH)2 Curly Nanosheet Aggregates as Efficient Oxygen Evolution Electrocatalysts

Recently, the curly structure attracts researchers' attention due to the strain effect, electronic effect, and improved surface area, which exhibits enhanced electrocatalytic activity. However, the synthesis of metastable curved structures is very difficult. Herein, a simple room temperature coprecipitation method is proposed to synthesize 3D cobalt (Co) hydroxide (α-Co(OH)₂ ) electrocatalysts that consist of curly 2D nanosheets. The formation process of curly nanosheets is elaborated systematically and the results demonstrate that the NH_x group has great effect on the formation of curly structure. Combining the advantage of 2D curly nanosheet and 3D aggregate structure, the as-prepared α-Co(OH)₂ curly nanosheet aggregates show the best water oxidation activity with an overpotential of 269 mV at j = 10 mA cm^-2 in 1.0 m KOH. The electrocatalytic process studies demonstrate that the formation of Co^IV O species is the rate-determining step. Theoretical calculations further confirm the beneficial effect of the bent structure on the conductivity, the adsorption of OH^- and the formation of OOH* species.

Operando Raman spectroscopy tracks oxidation-state changes in an amorphous Co oxide material for electrocatalysis of the oxygen evolution reaction

Transition metal oxides are of high interest in both energy storage (batteries) and production of non-fossil fuels by (photo)electrocatalysis. Their functionally crucial charge (oxidation state) changes and electrocatalytic properties are best investigated under electrochemical operation conditions. We established operando Raman spectroscopy for investigation of the atomic structure and oxidation state of a non-crystalline, hydrated, and phosphate-containing Co oxide material (CoCat), which is an electrocatalyst for the oxygen evolution reaction (OER) at neutral pH and is structurally similar to LiCoO₂ of batteries. Raman spectra were collected at various sub-catalytic and catalytic electric potentials. ²H labeling suggests Co oxidation coupled to Co-OH deprotonation at catalytic potentials. ¹⁸O labeling supports O-O bond formation starting from terminally coordinated oxygen species. Two broad bands around 877 cm^-1 and 1077 cm^-1 are assigned to CoCat-internal H₂PO₄ ^-. Raman peaks corresponding to terminal oxide (Co=O) or reactive oxygen species were not detectable; 1000-1200 cm^-1 bands were instead assigned to two-phonon Raman scattering. At an increasingly positive potential, the intensity of the Raman bands decreased, which is unexpected and explained by self-absorption relating to CoCat electrochromism. A red-shift of the Co-O Raman bands with increasing potentials was described by four Gaussian bands of potential-dependent amplitudes. By linear combination of Raman band amplitudes, we can follow individually the Co(2+/3+) and Co(3+/4+) redox transitions, whereas previously published x-ray absorption spectroscopy analysis could determine only the averaged Co oxidation state. Our results show how electrochemical operando Raman spectroscopy can be employed as a potent analytical tool in mechanistic investigations on OER catalysis.

Photocatalytic hydrogen evolution over nickel cobalt bimetallic phosphate anchored graphitic carbon nitrides by regulation of the d-band electronic structure

The electronic regulation function played by the Ni/Co molar ratio in nickel cobalt phosphate co-catalysts is vital for photocatalytic hydrogen evolution efficiency.

Modulation of crystal water in cobalt phosphate for promoted water oxidation

Cobalt phosphate tetrahydrate was identified as the optimal Co-Pi phase for electrocatalytic oxygen evolution reaction.

Molecular Control of Heterogeneous Electrocatalysis through Graphite Conjugation

The efficient interconversion of electrical and chemical energy requires catalysts capable of accelerating multielectron reactions at or near electrified interfaces. These reactions can be performed at metallic surface sites on heterogeneous electrocatalysts or through redox mediation at molecular electrocatalysts. The relative ease of synthesis and characterization for homogeneous catalysts has allowed for molecular-level control over the active site and permitted systematic tuning of activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts, because they typically exhibit a distribution of active site geometries and local electronic structures, which are challenging to modify with molecular precision. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states that distribute the redox burden of multielectron transformations, enabling more efficient catalysis. We envisioned that we could combine the attractive properties of molecular and heterogeneous catalysts by integrating tunable molecular active sites into the delocalized band states of a conductive solid. The Surendranath group has developed a class of electrocatalysts in which molecules are strongly electronically coupled to graphitic electrodes through a conductive, aromatic pyrazine linkage such that they behave like metallic surface active sites. In this Account, we discuss the dual role of these graphite-conjugated catalysts (GCCs) as a platform with which to answer molecular-level questions of metallic active sites and as a tool with which to fundamentally alter the mechanism and enhance the performance of molecular active sites. We begin by describing the electrochemical and spectroscopic studies that demonstrated that GCC sites behave like metallic active sites rather than simply as redox mediators attached to electrode surfaces. We then discuss how electrochemical studies of a series of graphite-conjugated acids enabled the construction of a molecular model for the thermochemistry of proton-coupled electron transfer reactions at GCC sites based on the pK_a of the molecular analogue of the conjugated site and the potential of zero free charge of the electrode. In the final section, we discuss the effects of graphite conjugation on the mechanism and rate of oxygen reduction, hydrogen evolution, and carbon dioxide reduction catalysis across four different GCC platforms involving N-heterocycle, organometallic, and metalloporphyrin active sites. We discuss how molecular-level tuning at graphite-conjugated active sites directly correlates to changes in catalytic activity for the oxygen reduction reaction. We demonstrate that graphite-conjugated porphyrins show enhanced catalytic oxygen reduction activity over amide-linked porphyrins. Lastly, we describe how catalysis at graphite-conjugated sites proceeds through mechanisms involving concerted electron transfer and substrate activation, in stark contrast to the mechanisms observed for molecular analogues. Overall, we showcase how GCCs provide a rich platform for controlling heterogeneous catalysis at the molecular level.

Microfluidic electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering

Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Å spatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 µm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometre-scale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.

Nature of Electronic Conduction in "Pseudocapacitive" Films: Transition from the Insulator State to Band-Conduction

The transition between the insulator state and the band-conducting state is investigated by means of cyclic voltammetry in cobalt oxide porous film electrodes in phosphate-buffered solutions. It is shown that a proton-coupled faradaic oxidative process starting in the insulator region eventually builds an ohmic conduction mode upon anodic polarization. This model allows one to understand the origin of the authentic capacitive behavior of conductive metal oxide films rather than the so-called "pseudocapacitive" behavior. The particular example of cobalt oxide serves to illustrate the way in which, more generally, the behavior of "pseudocapacitors", long ascribed to the superposition of faradaic reactions, is in fact that of true capacitors, once band-conduction has been established upon oxidation of the material.

Stabilization of reactive Co4O4 cubane oxygen-evolution catalysts within porous frameworks

A major challenge to the implementation of artificial photosynthesis (AP), in which fuels are produced from abundant materials (water and carbon dioxide) in an electrochemical cell through the action of sunlight, is the discovery of active, inexpensive, safe, and stable catalysts for the oxygen evolution reaction (OER). Multimetallic molecular catalysts, inspired by the natural photosynthetic enzyme, can provide important guidance for catalyst design, but the necessary mechanistic understanding has been elusive. In particular, fundamental transformations for reactive intermediates are difficult to observe, and well-defined molecular models of such species are highly prone to decomposition by intermolecular aggregation. Here, we present a general strategy for stabilization of the molecular cobalt-oxo cubane core (Co₄O₄) by immobilizing it as part of metal-organic frameworks, thus preventing intermolecular pathways of catalyst decomposition. These materials retain the OER activity and mechanism of the molecular Co₄O₄ analog yet demonstrate unprecedented long-term stability at pH 14. The organic linkers of the framework allow for chemical fine-tuning of activity and stability and, perhaps most importantly, provide "matrix isolation" that allows for observation and stabilization of intermediates in the water-splitting pathway.

Graphite-Conjugated Acids Reveal a Molecular Framework for Proton-Coupled Electron Transfer at Electrode Surfaces

Proton-coupled electron-transfer (PCET) steps play a key role in energy conversion reactions. Molecular PCET reactions are well-described by "square schemes" in which the overall thermochemistry of the reaction is broken into its constituent proton-transfer and electron-transfer components. Although this description has been essential for understanding molecular PCET, no such framework exists for PCET reactions that take place at electrode surfaces. Herein, we develop a molecular square scheme framework for interfacial PCET by investigating the electrochemistry of molecularly well-defined acid/base sites conjugated to graphitic electrodes. Using cyclic voltammetry, we first demonstrate that, irrespective of the redox properties of the corresponding molecular analogue, proton transfer to graphite-conjugated acid/base sites is coupled to electron transfer. We then show that the thermochemistry of surface PCET events can be described by the pK _a of the molecular analogue and the potential of zero free charge (zero-field reduction potential) of the electrode. This work provides a general framework for analyzing and predicting the thermochemistry of interfacial PCET reactions.

Energy storage: pseudocapacitance in prospect

This question and its implications are discussed in detail.

The two main types of charge storage devices – batteries and double layer charging capacitors – can be unambiguously distinguished from one another by the shape and scan rate dependence of their cyclic voltammetric current–potential (CV) responses. This is not the case with “pseudocapacitors” and with the notion of “pseudocapacitance”, as originally put forward by Conway et al. After insisting on the necessity of precisely defining “pseudocapacitance” as involving faradaic processes and having, at the same time, a capacitive signature, we discuss the modelling of “pseudocapacitive” responses, revisiting Conway's derivations and analysing critically the other contributions to the subject, leading unmistakably to the conclusion that “pseudocapacitors” are actually true capacitors and that “pseudocapacitance” is a basically incorrect notion. Taking cobalt oxide films as a tutorial example, we describe the way in which a (true) electrical double layer is built upon oxidation of the film in its insulating state up to an ohmic conducting state. The lessons drawn at this occasion are used to re-examine the classical oxides, RuO₂, MnO₂, TiO₂, Nb₂O₅ and other examples of putative “pseudocapacitive” materials. Addressing the dynamics of charge storage—a key issue in the practice of power of the energy storage device—it is shown that ohmic potential drop in the pores is the governing factor rather than counter-ion diffusion as often asserted, based on incorrect diagnosis by means of scan rate variations in CV studies.

Modelling the (Essential) Role of Proton Transport by Electrolyte Bases for Electrochemical Water Oxidation at Near-Neutral pH

The oxygen-evolution reaction (OER) in the near-neutral pH-regime is of high interest, e.g., for coupling of OER and CO2-reduction in the production of non-fossil fuels. A simple model is proposed that assumes equal proton activities in the catalyst film and the near-surface electrolyte. Equations are derived that describe the limitations relating to proton transport mediated by fluxes of molecular “buffer bases” in the electrolyte. The model explains (1) the need for buffer bases in near-neutral OER and (2) the pH dependence of the catalytic current at high overpotentials. The latter is determined by the concentration of unprotonated buffer bases times an effective diffusion constant, which can be estimated for simple cell geometries from tabulated diffusion coefficients. The model predicts (3) a macroscopic region of increased pH close to the OER electrode and at intermediate overpotentials, (4) a Tafel slope that depends on the reciprocal buffer capacity; both predictions are awaiting experimental verification. The suggested first-order model captures and predicts major trends of OER in the near-neutral pH, without accounting for proton-transport limitations at the catalyst–electrolyte interface and within the catalyst material, but the full quantitative agreement may require refinements. The suggested model also may be applicable to further electrocatalytic processes.

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Quasi-single-crystalline CoO hexagrams with abundant defects for highly efficient electrocatalytic water oxidation

Defects and structural long-range ordering have been recognized as two crucial characters for advanced electrocatalysts. However, these two features have rarely been achieved together. Herein, we report the preparation of single-crystalline CoO hexagrams and demonstrate their exceptional electrocatalytic properties for water oxidation. The quasi-single-crystalline CoO hexagrams, prepared at the critical phase transition point of β-Co(OH)2/Co(OH)F hexagrams, possess both abundant oxygen vacancies as defects and structural long-range ordering. The matching between the b-axis of Co(OH)F crystals and the a-axis of β-Co(OH)2 crystals is critical for the formation of the CoO hexagram single crystals. The quasi-single-crystalline CoO hexagrams with abundant defects are highly efficient for water oxidation by delivering a 10 mA cm-2 current density at a low overpotential of 269 mV in a 1 M KOH aqueous solution.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO₂-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

Catalysis of the Oxygen Evolution Reaction by 4-10 nm Cobalt Nanoparticles

Electrolysis of water is key technology, not only for clean energy production, but to ensure a continued supply of hydrogen beyond fossil resources, essential to the manufacture of many chemical goods other than fuels. Cobalt nanomaterials have been widely identified as a promising candidate for the anode (oxygen evolution) reaction in this process, but much work has focused on applied materials or electrode design. Given the importance of oxidation state changes Co(III) → Co(IV) in the accepted reaction mechanism, in this work we look at size effects in small (4–10 nm) cobalt nanoparticles, where the ease of oxidation for lower cobalt oxidation states is known to change with particle size. To discriminate between geometric and chemical effects we have compared the catalysts in this study to others in the literature by turnover frequency (widely used in other areas of catalysis), in addition to the more commonly employed performance metric of the overpotential required to produce a current density of 10 mA cm⁻². Comparisons are drawn to key examples of using well defined nanomaterials (where the surface are of cobalt sites can be estimated). This has enabled an estimated intrinsic turnover rate of ~ 1 O₂ molecule per surface Co atom per second at an overpotential of 500 mV in the oxygen evolution reaction under typical alkaline reaction conditions (pH 14.0) to be identified.

Engineering iridium-based metal organic frameworks towards electrocatalytic water oxidation

Bipyridine-incorporated metal organic frameworks were used to fabricate solid iridium-bipyridine catalysts, which were coated onto the surface of a glassy carbon electrode and showed electrical water oxidation activity with the onset potential of 0.87 V versus NHE.

Nickel oxide–polypyrrole nanocomposite electrode materials for electrocatalytic water oxidation

Electrochemically prepared nickel oxide nanoparticles entrapped into a polymer matrix as efficient material for O₂ evolution.

In Situ Coupling Strategy for the Preparation of FeCo Alloys and Co4 N Hybrid for Highly Efficient Oxygen Evolution

An in situ coupling approach is developed to create a new highly efficient and durable cobalt-based electrocatalyst for the oxygen evolution reaction (OER). Using a novel cyclotetramerization, a task-specific bimetallic phthalocyanine-based nanoporous organic framework is successfully built as a precursor for the carbonization synthesis of a nonprecious OER electrocatalyst. The resultant material exhibits an excellent OER activity with a low overpotential of 280 mV at a current density of 10 mA cm^-2 and high durability in an alkaline medium. This impressive result ranks among the best from known Co-based OER catalysts under the same conditions. The simultaneous installation of multiple diverse cobalt-based active sites, including FeCo alloys and Co₄ N nanoparticles, plays a critical role in achieving this promising OER performance. This innovative approach not only enables high-performance OER activity to be achieved but simultaneously provides a means to control the surface features, thereby tuning the catalytic property of the material.

In situ evolution of highly dispersed amorphous CoOx clusters for oxygen evolution reaction

Electrocatalytic water splitting is a key technique to produce hydrogen fuels, which can be considered as an efficient strategy to store renewable energy. Oxygen evolution reaction (OER) that occurs at the anode side requires a four-electron transfer under highly oxidizing conditions. OER has a large overpotential and therefore determines the overall efficiency. Certain electrocatalysts can efficiently help to improve the reaction kinetics. Owing to the high cost of precious metals such as Pt, Ru, and Ir, non-precious metal oxide catalysts have been vigorously investigated under alkaline conditions. Herein, we synthesized novel highly dispersed amorphous CoO_xfor the first time in the form of a cluster favorable to enhance the OER activity using a facile method via the air dielectric barrier discharge (DBD) plasma. Compared with the pristine biopolymer-cobalt complex, the amorphous CoO_x cluster exhibits a much higher current density and a lower overpotential for OER, e.g., the overpotential of 290 mV at 10 mA cm^-2 and the overpotential of only 350 mV at 300 mA cm^-1. The excellent electrocatalytic OER activity was attributed to the unsaturated catalytic sites on the amorphous CoO_x cluster. In addition, we studied the reaction mechanism, and it was observed that pure O₂ DBD plasma could lead to the evolution of crystalline CoO_x; however, the presence of N₂ and O₂ in DBD plasma could ensure the facile evolution of amorphous CoO_x clusters. This study provides a new strategy to design amorphous materials for electrocatalysis and beyond.

Bimetallic Nickel-Substituted Cobalt-Borate Nanowire Array: An Earth-Abundant Water Oxidation Electrocatalyst with Superior Activity and Durability at Near Neutral pH

There is an urgent demand to develop earth-abundant electrocatalysts for efficient and durable water oxidation under mild conditions. A nickel-substituted cobalt-borate nanowire array is developed on carbon cloth (Ni-Co-Bi/CC) via oxidative polarization of NiCo₂ S₄ nanoarray in potassium borate (K-Bi). As a bimetallic electrocatalyst for water oxidation, such Ni-Co-Bi/CC is superior in catalytic activity and durability in 0.1 m K-Bi (pH: 9.2), with a turnover frequency of 0.33 mol O₂ s^-1 at the overpotential of 500 mV and nearly 100% Faradaic efficiency. To drive a geometrical catalytic current density of 10 mA cm^-2 , it only needs overpotential of 388 mV, 34 mV less than that for Co-Bi/CC, outperforming reported non-noble-metal catalysts operating under benign conditions. Notably, its activity is maintained over 80 000 s. Density functional theory calculations suggest that the O* to OOH* conversion is the rate-determining step and Ni substitution decreases the free energy on Co-Bi from 2.092 to 1.986 eV.

Hierarchical Co(OH)F Superstructure Built by Low-Dimensional Substructures for Electrocatalytic Water Oxidation

The development of new materials/structures for efficient electrocatalytic water oxidation, which is a key reaction in realizing artificial photosynthesis, is an ongoing challenge. Herein, a Co(OH)F material as a new electrocatalyst for the oxygen evolution reaction (OER) is reported. The as-prepared 3D Co(OH)F microspheres are built by 2D nanoflake building blocks, which are further woven by 1D nanorod foundations. Weaving and building the substructures (1D nanorods and 2D nanoflakes) provides high structural void porosity with sufficient interior space in the resulting 3D material. The hierarchical structure of this Co(OH)F material combines the merits of all material dimensions in heterogeneous catalysis. The anisotropic low-dimensional (1D and 2D) substructures possess the advantages of a high surface-to-volume ratio and fast charge transport. The interconnectivity of the nanorods is also beneficial for charge transport. The high-dimensional (3D) architecture results in sufficient active sites per the projected electrode surface area and is favorable for efficient mass diffusion during catalysis. A low overpotential of 313 mV is required to drive an OER current density of 10 mA cm^-2 on a simple glassy carbon (GC) working electrode in a 1.0 m KOH aqueous solution.

Cyclic voltammetry modeling of proton transport effects on redox charge storage in conductive materials: application to a TiO2 mesoporous film

Cyclic voltammetry is a particularly useful tool for characterizing charge accumulation in conductive materials. A simple model is presented to evaluate proton transport effects on charge storage in conductive materials associated with a redox process coupled with proton insertion in the bulk material from an aqueous buffered solution, a situation frequently encountered in metal oxide materials. The interplay between proton transport inside and outside the materials is described using a formulation of the problem through introduction of dimensionless variables that allows defining the minimum number of parameters governing the cyclic voltammetry response with consideration of a simple description of the system geometry. This approach is illustrated by analysis of proton insertion in a mesoporous TiO₂ film.

Ultrathin Co3O4 nanofilm as an efficient bifunctional catalyst for oxygen evolution and reduction reaction in rechargeable zinc-air batteries

Two-dimensional (2D) nanocatalysts with a large specific surface area and efficient charge conductivity are promising candidates for catalyzing the sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which are at the heart of various electrochemical energy conversion and storage technologies. Here, we report the synthesis of an ultrathin Co₃O₄ nanofilm with a thickness of nearly 1.8 nm via a surfactant- and template-free facile hydrothermal route. The proposed synthesis strategy can be extended to the preparation of 2D Ni_xCo_3-xO₄ and Fe_xCo_3-xO₄ nanostructures. The synthesized Co₃O₄ nanofilm exhibited bifunctional activity that was superior to that of the counterpart Co₃O₄ nanoparticles, including a lower overpotential and higher reduction and evolution current densities, and demonstrated faster catalytic kinetics over the 2D nanofilm surface. In comparison with precious metal-based catalysts, to achieve an OER current density of 40 mA cm^-2 the overpotential of the nanofilm (461 mV) was lower than that of RuO₂ (526 mV), whereas the ORR on the nanofilm proceeded via a dominant 4e^- transfer mechanism, which is similar to that of commercial carbon-supported Pt (Pt/C). The Co₃O₄ nanofilm enabled the assembly of rechargeable Zn-air batteries with a lower overpotential (0.72 V), higher round-trip efficiency (62.7%), and a longer cycle lifetime (175 cycles). The remarkable bifunctional activity contributes to an increase in the number of electrochemically active sites, a large interfacial contact area with the electrolyte, and the enrichment of Co³⁺ ions on the surface, which facilitates the adsorption and activation of oxygen-containing species. This study should shed light on the future development of new electroactive materials with optimized 2D nanostructures to enhance the overall bifunctional ORR/OER performance of rechargeable metal-air batteries.

Synthetic control and empirical prediction of redox potentials for Co4O4 cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation

The oxo-cobalt cubane unit [Co4O4] is of interest as a homogeneous oxygen-evolution reaction (OER) catalyst, and as a functional mimic of heterogeneous cobalt oxide OER catalysts. The synthesis of several new cubanes allows evaluation of redox potentials for the [Co4O4] cluster, which are highly sensitive to the ligand environment and span a remarkable range of 1.42 V. The [CoIII4O4]4+/[CoIII3CoIVO4]5+ and [CoIII3CoIVO4]5+/[CoIII2CoIV2O4]6+ redox potentials are reliably predicted by the pKas of the ligands. Hydrogen bonding is also shown to significantly raise the redox potentials, by ∼500 mV. The potential-pKa correlation is used to evaluate the feasibility of various proposed OER catalytic intermediates, including high-valent Co-oxo species. The synthetic methods and structure-reactivity relationships developed by these studies should better guide the design of new cubane-based OER catalysts.

Cobalt-Borate Nanoarray: An Efficient and Durable Electrocatalyst for Water Oxidation under Benign Conditions

The development of efficient earth-abundant electrocatalysts for oxygen evolution reaction (OER) under benign conditions is still urgent and challenging. Herein, we report the electrochemical generation of novel Co-Bi nanoarray on carbon cloth (Co-Bi NA/CC) from CoS₂ nanoarray precursor. As a three-dimensional anode, such Co-Bi NA/CC exhibits excellent electrocatalytic performance for OER with the overpotential requirement of 411 mV to drive 10 mA cm^-2. Notably, this electrode also demonstrates outstanding long-term electrochemical durability for 20 h.