The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable and efficient systems for the conversion and storage of renewable energy sources. The ...production of hydrogen through water splitting seems a promising and appealing solution. We found that a robust nanoparticulate electrocatalytic material, H(2)-CoCat, can be electrochemically prepared from cobalt salts in a phosphate buffer. This material consists of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte and mediates H(2) evolution from neutral aqueous buffer at modest overpotentials. Remarkably, it can be converted on anodic equilibration into the previously described amorphous cobalt oxide film (O(2)-CoCat or CoPi) catalysing O(2) evolution. The switch between the two catalytic forms is fully reversible and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode. After deposition, the noble-metal-free coating thus functions as a robust, bifunctional and switchable catalyst.
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.
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‐MnIV oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of MnIII ions at catalytic potentials is identified as a prerequisite for catalytic activity.
Catalyst Ca‐n do: Water‐oxidizing calcium–manganese oxides mimic the inorganic core of the biological catalyst. The oxides are electrodeposited and modified by active‐site creation through annealing at moderate temperatures. The ability to undergo redox transitions and the presence of a minority fraction of MnIII ions at catalytic potentials facilitates water‐oxidation catalysis.
The atomic structure of water-oxidizing nanoparticles (10–60nm) formed from cobalt(II) salts and methylenediphosphonate (M2P) is investigated. These amorphous nanoparticles are of high interest for ...production of solar fuels. They facilitate water oxidation in a directly light-driven process using Ru(bpy)32+ (bpy=2,2’-bipyridine) as a photosensitizer and persulfate (S2O82−) as an electron acceptor. By X-ray absorption spectroscopy (XAS) at the cobalt K-edge, cobalt L-edge and oxygen K-edge, we investigate the light-driven transition from the CoII/M2P precursor to the active catalyst, which is a layered cobalt(III) oxide with structural similarities to water-oxidizing electrocatalysts. The M2P ligand likely binds at the periphery of the nanoparticles, preventing their further agglomeration during the catalytic reaction. This system opens a possibility to link the catalytically active nanoparticles via a covalent bridge to a photosensitizer and build an artificial photosynthetic system for direct utilization of solar energy for fuel production without production of electricity as an intermediate step.
This article is part of a Special Issue entitled: Photosynthetic and Biomimetic Hydrogen Production.
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► Atomic structure of water-oxidizing Co oxide/methylenediphosphonate nanoparticles by X-ray absorption spectroscopy. ► Methylenediphosphonate prevents particle agglomeration during the catalytic reaction. ► Water is oxidized and oxygen is produced in a process directly driven by light. ► System shows prospects for direct linking of a photosensitizer.
The atomic structure of an electrodeposited Ni catalyst film is dominated by extensive di-μ-oxido bridging between Ni(III/IV) ions, as revealed by X-ray absorption spectroscopy. The structure is ...surprisingly similar to that of an analogous Co-based film and colloidal Mn-based catalysts. Structural requirements for water oxidation are discussed.
Go with the CO: The functionalization of multiwalled carbon nanotubes with molecular complexes through π–π stacking produces robust, noble‐metal‐free electrocatalytic nanomaterials for H2 evolution ...and uptake. The catalysts are compatible with the conditions encountered in classical proton‐exchange membrane devices and are tolerant of the common pollutant CO, thus offering significant advantages over traditional Pt‐based 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.
Während der Katalyse wandelt sich kristallines Co3(PO4)2⋅8 H2O mit aktiver Oberfläche vollständig in amorphes Cobaltoxid mit aktivem Volumen um. Grundlegende Merkmale der heterogenen Wasseroxidationskatalyse werden anhand dreier zusammenhängender Phänomene diskutiert: Katalyse an der Oberfläche, Katalyse im Volumen und Strukturumwandlung eines Materials während des Betriebs.
The development of new energy storage technologies is central to solving the challenges facing the widespread use of renewable energies. An option is the reduction of carbon dioxide (CO2) into ...carbon-based products which can be achieved within an electrochemical cell. Future developments of such processes depend on the availability of cheap and selective catalysts at the electrode. Here we show that a unique well-characterized active electrode material can be simply prepared via electrodeposition from a molecular copper complex precursor. The best performances, namely activity (150 mV onset overpotential and 1 mA cm−2 current density at 540 mV overpotential), selectivity (90% faradaic yield) and stability for electrocatalytic reduction of CO2 into formic acid in DMF/H2O (97 : 3 v/v) have been obtained with the Cu(cyclam)(ClO4)2 complex (cyclam = 1,4,8,11-tetraazacyclotetradecane) as the precursor. Remarkably the organic ligand of the Cu precursor remains part of the composite material and the electrocatalytic activity is greatly dependent on the nature of that organic component.
The development of new energy storage technologies is central to solving the challenges facing the widespread use of renewable energies. An option is the reduction of carbon dioxide (CO
2
) into ...carbon-based products which can be achieved within an electrochemical cell. Future developments of such processes depend on the availability of cheap and selective catalysts at the electrode. Here we show that a unique well-characterized active electrode material can be simply prepared
via
electrodeposition from a molecular copper complex precursor. The best performances, namely activity (150 mV onset overpotential and 1 mA cm
−2
current density at 540 mV overpotential), selectivity (90% faradaic yield) and stability for electrocatalytic reduction of CO
2
into formic acid in DMF/H
2
O (97 : 3 v/v) have been obtained with the Cu(cyclam)(ClO
4
)
2
complex (cyclam = 1,4,8,11-tetraazacyclotetradecane) as the precursor. Remarkably the organic ligand of the Cu precursor remains part of the composite material and the electrocatalytic activity is greatly dependent on the nature of that organic component.
The development of new energy storage technologies is central to solving the challenges facing the widespread use of renewable energies.
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