Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been ...most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C
products, selectivities (> 400 mA cm
) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu
dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C
H
was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH
and ethanol. We demonstrate evidence that adsorbed methyl species, *CH
, serve as common intermediates of both CH
H and CH
CH
OH and possibly of other CH
-R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO
electrolysis on Cu catalysts.
In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, ...and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn sub(4)Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca.+3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di- mu -oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of Mn super(III) sites and prevented formation of well-ordered and unreactive Mn super(IV)O sub(2). Structure-function relations in Mn-based water-oxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use.
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.
Modification of the Co-oxo cores of cobalt-polyoxometalate water oxidation catalysts is detectable by X-ray absorption spectroscopy (XAS) as demonstrated by comparison of Na10Co4(H2O)2(PW9O34)2 (1) ...and Na17((Co(H2O))Co2PW9O34)2(PW6O26) (2). XAS reveals the integrity of 1 uncompromised by oxidant-driven water oxidation, which proceeds without formation of catalytic cobalt oxide.
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.
It's what's inside that counts! Amorphous oxides are a high‐interest material class in energy science. Rather than at the outer surface, catalysis of water oxidation by an amorphous cobalt oxide takes place inside the hydrated oxide material. Unprotonated buffer molecules of the electrolyte solution are likely to pick up protons at the surface of the catalyst material (see picture).
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.
The complete transformation during catalytic operation of crystalline and surface‐active Co3(PO4)2⋅8 H2O into amorphous and volume‐active cobalt oxide reveals basic features of heterogeneous water oxidation catalysis, which is discussed as a convolution of three phenomena: surface catalysis, volume catalysis, and restructuring of the material under operation.
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 C2H4 formation still proceeded through a largely pH‐independent mechanism. C2H4 was formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.
Inner values matter! Eight Cu foams with substantially different morphologies were investigated by analyzing the product spectrum. Electrochemical in situ spectroscopies revealed that the reactivity was controlled by internal surface area.
For the production of nonfossil fuels, water oxidation by inexpensive cobalt‐based catalysts is of high interest. Films for the electrocatalysis of water oxidation were obtained by oxidative ...self‐assembly (electrodeposition) from aqueous solutions containing, apart from Co, either K, Li or Ca with either a phosphate, acetate or chloride anion. X‐ray absorption spectroscopy (XAS) at the Co K‐edge revealed clusters of edge‐sharing CoO6 octahedra in all films, but the size or structural disorder of the Co‐oxido clusters differed. Whereas potassium binding is largely unspecific, CaCo3O4 cubanes, which resemble the CaMn3O4 cubane of the biological catalyst in oxygenic photosynthesis, may form, as suggested by XAS at the Ca K‐edge. Cyclic voltammograms in a potassium phosphate buffer at pH 7 revealed that no specific combination of anions and redox‐inactive cations is required for catalytic water oxidation. However, the anion type modulates not only the size (or order) of the Co‐oxido clusters, but also electrodeposition rates, redox potentials, the capacity for oxidative charging, and catalytic currents. On these grounds, structure–activity relations are discussed.
Cobalt crowds crack it up: The catalytic activity of a cobalt‐oxido film for water oxidation may be inversely proportional to atomic order, determined by the size of contiguous CoxOy clusters in the amorphous material. The redox‐inert cations and anions in CoCat modulate redox properties and catalytic activity without modifying the basic structural motif of Co‐oxido clusters.
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.
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