Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film.

Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.

Perspectivas y aplicaciones reales del grafeno después de 16 años de su descubrimiento / Perspectives and real applications of graphene at 16 years after its breakthrough / Perspectivas e aplicações reais do grafeno após 16 anos de sua descoberta

Resumen A 16 años del gran descubrimiento del grafeno los focos de atención vuelven a estar en este material con el reporte de su comportamiento superconductor dependiendo del apilado de sus capas. Sin embargo, su nombre durante estos últimos años no solo se ha relacionado a la superconductividad, sino que ha sido relacionado con una diversidad muy amplia de aplicaciones, en disciplinas muy diversas, entre las que cabe mencionar: materiales opto-electrónicos, electrodos para catálisis, dispositivos para tratamiento de desechos, biosensores, entre otros. Esto ha hecho que un gran número de grupos de investigación se hayan interesado no solo en estudiar sus propiedades, sino también en investigar nuevos métodos sintéticos que puedan ser escalables a niveles industriales, sin perder sus propiedades electrónicas y mecánicas. A pesar de los numerosos estudios y los recursos invertidos en grafeno no todas las aplicaciones han llegado a ser una realidad, en esta revisión se muestran algunas de las más exitosas.

Abstract 16 years after the great discovery of graphene, the focus and attention are again on this material after the report of its superconducting behavior depending on the stacking of its layers. The graphene has not only been related to superconductivity but has also been related to a wide diversity of applications, in very diverse disciplines. Among them, we can mention: Opto-electronic materials, electrodes for catalysis, devices for waste-water treatment, biosensors, batteries, and solar cells. This has caused a large number of research groups to be interested not only in the study of its properties, but also in the research of new synthetic methods that can be scaled to industrial levels, without losing its electronic and mechanical properties. Despite numerous studies and resources invested in graphene, not all applications have become a reality, some of the most successful are shown in this review.

Resumo 16 anos após a grande descoberta do grafeno, o foco e as atenções voltam a ser neste material com o relato de seu comportamento supercondutor em função do empilhamento de suas camadas. No entanto, seu nome nos últimos anos não tem sido apenas relacionado à supercondutividade, mas tem sido relacionado a uma diversidade muito ampla de aplicações, em disciplinas muito diversas. Entre eles podemos citar: materiais optoeletrônicos, eletrodos para catálise, dispositivos para tratamento de águas residuais, biossensores, baterias e células solares. Isso fez com que um grande número de grupos de pesquisa se interessassem não apenas em estudar suas propriedades, mas muitas pesquisas também foram feitas na geração de métodos sintéticos que pudessem ser dimensionados para níveis industriais, sem perder suas propriedades eletrônicas e mecânicas. Apesar dos inúmeros estudos e recursos investidos em grafeno, nem todas as aplicações se tornaram realidade, algumas das mais bem-sucedidas são apresentadas nesta revisão.

Structural and functional role of anions in electrochemical water oxidation probed by arsenate incorporation into cobalt-oxide materials.

Direct (photo)electrochemical production of non-fossil fuels from water and CO2 requires water-oxidation catalysis at near-neutral pH in the presence of appropriate anions that serve as proton acceptors. We investigate the largely enigmatic structural role of anions in water oxidation for the prominent cobalt-phosphate catalyst (CoCat), an amorphous and hydrated oxide material. Co3([(P/As)O]4)2·8H2O served, in conjunction with phosphate-arsenate exchange, as a synthetic model system. Its structural transformation was induced by prolonged operation at catalytic potentials and probed by X-ray absorption spectroscopy not only at the metal (Co), but for the first time also at the anion (As) K-edge. For initially isostructural microcrystals, anion exchange determined the amorphization process and final structure. Comparison to amorphous electrodeposited Co oxide revealed that in CoCat, the arsenate binds not only at oxide-layer edges, but also arsenic substitutes cobalt positions within the layered-oxide structure in an unusual AsO6 coordination. Our results show that in water oxidation catalysis at near-neutral pH, anion type and exchange dynamics correlate with the catalyst structure and redox properties.

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 CoII/III and CoIII/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 CoIV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Spectroscopic identification of active sites for the oxygen evolution reaction on iron-cobalt oxides.

The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-µ-oxo bridged cobalt-cobalt, di-µ-oxo bridged cobalt-iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-µ-oxo bridged cobalt-cobalt and di-µ-oxo bridged iron-cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.

Electrosynthesis of Biomimetic Manganese-Calcium Oxides for Water Oxidation Catalysis--Atomic Structure and Functionality.

Water-oxidizing calcium-manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.

Heterogeneous water oxidation: surface activity versus amorphization activation in cobalt phosphate catalysts.

Is water oxidation catalyzed at the surface or within the bulk volume of solid oxide materials? This question is addressed for cobalt phosphate catalysts deposited on inert electrodes, namely crystallites of pakhomovskyite (Co3(PO4)2⋅8 H2O, Pak) and phosphate-containing Co oxide (CoCat). X-ray spectroscopy reveals that oxidizing potentials transform the crystalline Pak slowly (5-8 h) but completely into the amorphous CoCat. Electrochemical analysis supports high-TOF surface activity in Pak, whereas its amorphization results in dominating volume activity of the thereby formed CoCat material. In the directly electrodeposited CoCat, volume catalysis prevails, but not at very low levels of the amorphous material, implying high-TOF catalysis at surface sites. A complete picture of heterogeneous water oxidation requires insight in catalysis at the electrolyte-exposed "outer surface", within a hydrated, amorphous volume phase, and modes and kinetics of restructuring upon operation.

Screen-printed calcium-birnessite electrodes for water oxidation at neutral pH and an "electrochemical harriman series".

A mild screen-printing method was developed to coat conductive oxide surfaces (here: fluorine-doped tin oxide) with micrometer-thick layers of presynthesized calcium manganese oxide (Ca-birnessite) particles. After optimization steps concerning the printing process and layer thickness, electrodes were obtained that could be used as corrosion-stable water-oxidizing anodes at pH 7 to yield current densities of 1 mA cm(-2) at an overpotential of less than 500 mV. Analyses of the electrode coatings of optimal thickness (≈10 µm) indicated that composition, oxide phase, and morphology of the synthetic Ca-birnessite particles were hardly affected by the screen-printing procedure. However, a more detailed analysis by X-ray absorption spectroscopy revealed small modifications of both the Mn redox state and the structure at the atomic level, which could affect functional properties such as proton conductivity. Furthermore, the versatile new screen-printing method was used for a comparative study of various transition-metal oxides concerning electrochemical water oxidation under "artificial leaf conditions" (neutral pH, fairly low overpotential and current density), for which a general activity ranking of RuO2 >Co3 O4 ≈(Ca)MnOx ≈NiO was observed. Within the group of screened manganese oxides, Ca-birnessite performed better than "Mn-only materials" such as Mn2 O3 and MnO2 .

Water oxidation by amorphous cobalt-based oxides: volume activity and proton transfer to electrolyte bases.

Water oxidation in the neutral pH regime catalyzed by amorphous transition-metal oxides is of high interest in energy science. Crucial determinants of electrocatalytic activity were investigated for a cobalt-based oxide film electrodeposited at various thicknesses on inert electrodes. For water oxidation at low current densities, the turnover frequency (TOF) per cobalt ion of the bulk material stayed fully constant for variation of the thickness of the oxide film by a factor of 100 (from about 15 nm to 1.5 µm). Thickness variation changed neither the nanostructure of the outer film surface nor the atomic structure of the oxide catalyst significantly. These findings imply catalytic activity of the bulk hydrated oxide material. Nonclassical dependence on pH was observed. For buffered electrolytes with pKa values of the buffer base ranging from 4.7 (acetate) to 10.3 (hydrogen carbonate), the catalytic activity reflected the protonation state of the buffer base in the electrolyte solution directly and not the intrinsic catalytic properties of the oxide itself. It is proposed that catalysis of water oxidation occurs within the bulk hydrated oxide film at the margins of cobalt oxide fragments of molecular dimensions. At high current densities, the availability of a proton-accepting base at the catalyst-electrolyte interface controls the rate of water oxidation. The reported findings may be of general relevance for water oxidation catalyzed at moderate pH by amorphous transition-metal oxides.