Search

We analyze the stability of anodically electrodeposited cobalt hydroxide films during operation as electrocatalysts for water oxidation and show that the stability considerations of these films are...

Co encapsulated in N-doped carbon nanotubes (Co-NCNT) catalysts are of high interest as bifunctional electrocatalyst material for both efficient oxygen evolution and reduction (OER/ORR) in applications of rechargeable metal-air batteries. Up to now, the role played by the functional metal species in OER/ORR is still insufficiently understood. The main focus of our research is to shed light on the mechanistic role of the Co species that serve as active sites in the bi-functional Co-NCNT catalysts. It is found that S700 exhibits an outstanding OER/ORR activity. We thus hypothesize that CoII and CoIII clusters predominately function as active sites in the OER and ORR processes, respectively. Furthermore, OER/ORR activity for Co-NCNT catalyst primarily correlates to eg occupation. A near-unity occupancy of the eg orbital of S700 is revealed to be the cause for the maximum intrinsic OER/ORR activity, which provides guidelines for the design of highly active catalysts.

Nickel-based catalysts accomplish the direct conversion of urea to pure hydrogen via electrochemical oxidation; yet mechanistic understanding is lacking. Synthesizing a series of carbon fiber paper (CFP) supported Ni(OH)2/MnO2 catalysts, we explored relevant redox transitions and catalysis of both UOR (urea oxidation reaction, in KOH-with-urea) and OER (oxygen evolution reaction, in KOH). Cyclic Voltammetry (CV) in KOH-only solution demonstrated a more cathodic transformation from Ni(III/IV) to Ni(II) compared with that in KOH-with-urea solution. The water oxidation overpotential was shifted to higher values (from 0.48 to 0.53 VRHE) as the Mn:Ni atom ratio increases in CFP-NiMn films. In contrast, a higher Mn content results in higher UOR activity and lower onset potential in KOH solution containing urea (1.395–1.375 VRHE). Quasi in-situ, freeze-quench X-ray absorption spectroscopy (XAS) at the Ni and Mn K-edges was employed to uncover oxidation state changes as well as structural transformations at the atomic level showing that CFP-Ni(OH)2 underwent oxidation state changes by about 1.15 e− and 0.21 e− per Ni ion during OER and UOR processes, respectively, versus only 0.71 e− and 0.07 e− per Ni ion in CFP-NiMn2.4. Mn incorporation can stabilize the Ni in lower valent states in a mixed NiMn catalyst without significant changes in oxidation state and structure. The here investigated, readily synthesized CFP-NiMn films exhibit opposite activity trends in KOH and KOH-with-urea electrolytes: Mn incorporation depresses water oxidation, but it promotes the urea oxidation process. We propose that the water oxidation rate (OER) is positively correlated with the capacity for accumulation of Ni and Mn oxidation equivalents, while the urea oxidation (UOR) rate is negatively correlated with this capacity. Our work offers a mechanistic guideline for designing and synthesizing nonprecious metal-coupled Ni-based catalysts with appropriate redox-properties for urea-oxidation applications.

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. Universidad de Costa Rica/[]/UCR/Costa Rica Consejo Nacional para Investigaciones Científicas y Tecnológicas/[]/CONICIT/Costa Rica Ministerio de Ciencia, Tecnología y Telecomunicaciones/[]/MICITT/Costa Rica German Federal Ministry of Education and Research/[]/BMBF/Alemania Deutsche Forschungsgemeinschaft/[]/DFG/Alemania UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Electroquímica y Energía Química (CELEQ) UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA)

Neodymium oxide (Nd2O3) nanoparticles were chemically embedded on a state-of-the art carbon felt (CF) by a precipitation method in non-aqueous solution. Different Nd2O3 loadings were chosen and the obtained electrocatalyst-loaded felts tested for application as electrode in all-vanadium redox flow batteries. Cyclic voltammetry (CV) studies confirmed that Nd2O3 has a catalytic effect towards both redox couples, V4+/V5+ at the positive and V2+/V3+ at the negative side. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) demonstrated only minor particle agglomeration and high dispersion of the particles on the fibres. Charge/discharge profiles revealed an enhanced performance with higher discharge capacity and higher energy efficiency for the modified felts when compared to a thermally activated CF. For instance, after 50 consecutive charge/discharge cycles the energy efficiency of the Nd2O3 modified carbon felt (Nd2O3-CF) was reduced only by 3% compared to a 12% irreversible loss observed for the thermally activated CF. After exchanging the electrolyte after 50 cycles, the felts retained their original performance indicating that less degradation occurred in the modified felts than in the industrial standard and that they maintained their oxygen-donating functionalities on the surface as compared to thermally activated CF.

The dramatic change in electrochemical behavior of nickel (oxy)hydroxide films upon incorporation of Fe ions provides an opportunity to establish effective electrocatalyst design principles. We characterize a photochemically deposited series of Fe-Ni (oxy)hydroxides by X-ray absorption spectroscopy and track the voltage- and composition-dependence of structural motifs. We observe a trigonal distortion in di-m-hydroxo bridged NiII-NiII motifs that is preserved following a symmetric contraction of Ni-O bond lengths when oxidized to di-m-oxo NiIV-NiIV. Incorporation of Fe ions into the structure generates di-m-hydroxo NiII-FeIII motifs in which Ni-Fe distances are dependent on nickel oxidation state, but Fe-O bond lengths are not. This asymmetry minimizes the trigonal distortion in di-m-hydroxo NiII-FeIII motifs and neighboring di-m-hydroxo NiII-NiII sites in the reduced state, but exacerbates it in the oxidized state. We attribute both the Fe-induced anodic shift in nickel-based redox peaks and the improved ability to catalyze the oxygen evolution reaction to this inversion in geometric distortions. Spectroelectrochemical experiments reveal a previously unreported change in optical absorbance at ca. 1.5 V vs. RHE in Fe-containing samples. We attribute this feature to oxidation of nickel ions in di-m-hydroxo NiII-FeIII motifs, which we propose is the process relevant to catalytic oxygen evolution.

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., Optimization of electrocatalysts requires an understanding of all active reaction sites. Here, the authors combine X-ray absorption spectroscopy and electrochemistry to identify cobalt atoms with different coordination geometries and probe their contribution to electrocatalytic water oxidation.

CO2 reduction is of significant interest for the production of nonfossil fuels. The reactivity of eight Cu foams with substantially different morphologies was comprehensively investigated by analysis of the product spectrum and in situ electrochemical spectroscopies (X-ray absorption near edge structure, extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and Raman spectroscopy). The approach provided new insight into the reactivity determinants: The morphology, stable Cu oxide phases, and *CO poisoning of the H2 formation reaction are not decisive; the electrochemically active surface area influences the reactivity trends; macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at the CuCat surfaces (operando Raman spectroscopy). H2 and CH4 formation was suppressed by macroscopic buffer alkalization, whereas CO and C2 H4 formation still proceeded through a largely pH-independent mechanism. C2 H4 was formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.