Mathias Wiechen

To better support researchers and contribute constructively to international scientific discourse, Angewandte Chemie has launched two opinion‐based article categories with the support of the German Chemical Society (GDCh) and the journal's governing bodies. Scientific Perspectives outline best practice in the chemical sciences while Viewpoint Articles address the culture of the chemical community.

To better support researchers and contribute constructively to international scientific discourse, Angewandte Chemie has launched two opinion‐based article categories with the support of the German Chemical Society (GDCh) and the journal's governing bodies. Scientific Perspectives outline best practice in the chemical sciences while Viewpoint Articles address the culture of the chemical community.

A solar-to-fuel conversion efficiency of 22% was achieved by using concentrated solar power to run a matched electrolyser based on Earth-abundant materials.

Efficient catalysis of water oxidation represents one of the major challenges en route to efficient sunlight‐driven water splitting. Cobalt oxides (CoO_x) have been widely investigated as water oxidation catalysts, although the incorporation of these materials into photoelectrochemical devices has been hindered by a lack of transparency. Herein, the electrosynthesis of transparent CoO_x catalyst films is described by utilizing cobalt(II) aminopolycarboxylate complexes as precursors to the oxide. These complexes allow control over the deposition rate and morphology to enable the production of thin, catalytic CoO_x films on a conductive substrate, which can be exploited in integrated photoelectrochemical devices. Notably, under a bias of 1.0 V (vs. Ag/AgCl), the film deposited from [Co(NTA)(OH₂)₂]⁻ (NTA=nitrilotriacetate) decreased the transmission by only 10 % at λ=500 nm, but still generated >80 % of the water oxidation current produced by a [Co(OH₂)₆]²⁺‐derived oxide film whose transmission was only 40 % at λ=500 nm.

The Oxygen Evolving Complex (OEC) in photosystem II, a cluster that contains four manganese and one calcium ions bridged by five oxygen atoms in a distorted chair like arrangement, carries out the biological oxidation of water during photosynthesis. Since this is the only cluster established in biological water oxidation catalysis, efforts have been made to develop synthetic systems that mimic its structure, properties and water oxidation activity. This perspective provides a brief overview of the current structural and mechanistic understanding of the OEC in photosystem II. It then compares the structural features of this complex with those of manganese oxide water oxidation catalysts and discusses structure–function relationships that inform the development of new catalysts. The identified features should be considered when endeavouring to design manganese oxide, and other metal oxide, catalysts with optimal activity that can ultimately be integrated into photo-electrochemical devices to achieve solar water-splitting.

Layered manganese oxides from the birnessite mineral family have been identified as promising heterogeneous compounds for water-oxidation catalysis (WOC), a key reaction for the conversion of renewable energy into storable fuels. High catalytic rates were especially observed for birnessites which contain calcium as part of their structures. With the aim to systematically improve the catalytic performance of such oxide materials, we used a flexible synthetic route to prepare three series of calcium birnessites, where we varied the calcium concentrations, the ripening times of the original precipitates and the temperature of the heat treatment following the initial synthetic steps (tempering) during the preparation process. The products were carefully analysed by a number of analytical techniques and then probed for WOC activity using the Ce⁴⁺-system. We find that our set of twenty closely related manganese oxides shows large, but somewhat systematic alterations in catalytic rates, indicating the importance of synthesis parameters for maximum catalytic performance. The catalyst of the series for which the highest water-oxidation rate was found is a birnessite of medium calcium content (Ca : Mn ratio 0.2 : 1) that had been subjected to a tempering temperature of 400 °C. On the basis of the detailed analysis of the results, a WOC reaction scheme for birnessites is proposed to explain the observed trends in reactivity.

Chara corallina freshwater algae produce brown deposits of manganese oxides on their cell wall surfaces when growing in manganese‐rich media. We report on the formation, topology, composition, atomic structure, and catalytic activities of these biogenic manganese oxides (BMOs). The deposits are volcano shaped and exhibit 3–5 μm craters in their centers. Microfocus X‐ray irradiation and detection of characteristic X‐ray fluorescence lines allowed elemental mapping at 5 μm spatial resolution and the identification of the volcano‐shaped deposits as a Mn–Ca oxide. X‐ray absorption spectroscopy (XAS) revealed a high‐valent Mn^III/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K. On the basis of the XAS data, the oxides were identified as members of the birnessite family of layered manganese oxides containing di‐μ‐oxido‐bridged Mn^III/IVO₆ octahedra as central building units. The deposits share structural motifs with synthetic water‐oxidizing Mn–Ca oxides and with the Mn₄Ca complex of photosystem II, the biological water‐oxidation catalyst. Model reactions demonstrate low, but clearly detectable, activity of the manganese deposits for water‐oxidation catalysis. The biogenic manganese oxides on the cell walls of Chara corallina thus represent an intriguing object to study how manganese‐based catalysts for water oxidation are formed in a biological environment. The formation of BMOs in relation to cellular ion transport and the possibility of BMOs to fulfill a detoxification function in plants were also examined.