Visible-Light-Driven Water Oxidation Catalysts WOC

Lead by Prof. Greta Patzke

The construction of stable and efficient WOCs in artificial photosynthesis remains a major challenge due to the complex four-electron-transfer processes involved in the oxidation of water to afford O2. This requires the development of flexible, stable and recyclable water oxidation catalysts (WOCs) for overall water splitting systems. Low-cost elements, scalable and facile synthetic procedures and chemical robustness against side reactions or shortcuts are our key development criteria for WOCs. Concerning homogeneous WOCs for solution-based processes, we focus on polyoxometalates (POMs) with catalytic cores made of readily accessible elements, especially Co and Mn. The latter is particularly interesting due to its bio-mimetic potential with respect to nature's photosystem II. Starting from our previous expertise in POM research, we routinely access a wide spectrum of POM types for continuous catalytic performance tests through targeted modification of their building block concept (Fig. 1). Main general challenges in POM research range from structure-activity relationships over interactions with the photosensitizer components to long-time stability issues. We pursue a comprehensive approach: (1) influence of POM structure on WOC activity is investigated with systematic screening experiments, (2) POMs and their photosensitizer complexes are studied in detail with a wide spetrum of analytical methods and (3) systematic WOC stability tests are performed with modern techniques (including XAS and related methods) to achieve further insight into catalytically active species. Analogous investigations are performed on powerful WOCs with cubane cores and flexible organic ligands.

Figure 1: POM design parameters (modified from: Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich (2013) 158(1/2): 23–36).

In parallel, we investigate oxide-based heterogeneous WOCs with special emphasis on spinel systems. Recently, we have studied the influence of matrix elements and Mn-doping on cobalt spinel WOCs in detail (Fig. 2). Based on our experience in nanomaterials fabrication, we optimize the influence of synthetic history, morphology and chemical composition on the spinel WOC performance. Step by step, we track the key oxide WOC design parameters and compare them to POM-WOCs. Special emphasis is placed on potential interface and metal leaching processes which might link homogeneous and heterogeneous WOCs in new and unexpected ways.

Figure 2: Co/Mn/Ga-spinel WOC nanoparticles (taken from: RSC Advances 2012 (2) 3076).