Amorphous Co-oxo and Mn-oxo catalysts for electrochemical water splitting: An ab initio point of view.

Photosynthetic processes represent the most important source of energy produced by biological systems. A crucial role in this task is played by small clusters containing transition metals (M), embedded into a biochemical environment. For instance, the MnCa-oxo complex contained into the photosystem(II) promotes the water splitting to dioxygen, protons and "energized" electrons; the latter are used for carbon dioxide reduction and chemical energy storage (e.g., sugar formation). Inspired by such natural processes, the goal of artificial photosynthesis is to develop simplified "artificial leaves" which can be exploited in a large-scale generation of fuels (e.g., hydrogen) directly from sunlight [1]. The step involving water oxidation is considered to be a main bottleneck, hampering progress in the development of applicable technologies. Several synthetic catalysts have been recently proposed, containing different M-oxo structures, but earth-abundant Fe, Ni, Mn, Co amorphous oxides likely represent the most promising route toward technologically relevant and low cost photo- electrolytic cells [2]. A thorough understanding of the water oxidation reaction promoted by amorphous M oxides requires detailed information about the atomistic texture and electronic properties of the catalysts. Only on these grounds more detailed investigations can be approached in future work: the lacking of such insight represents the main hurdle on the way toward the development of mechanistic models which go beyond a speculative level. The amorphous character of these hydrated oxides largely excludes a full characterization of the atomic structure by conventional X-ray diffraction methods. Therefore the main source of atomic-level structural information is X-ray absorption spectroscopy, which is not limited by the need for crystalline order. However, the thereby accessible information is insufficient: the XAFS spectra can facilitate determination of precise values of key distances, but do not result in complete 3D-models including atomic structure, protonation and oxidation- state localization. We obtained major progress toward the solution of this problem by combining the information content of XAFS data with the predictive power of theoretical simulations. We present here an account of our DFT-based investigation of the structural, electronic and catalytic properties of Co-based and Mn-based amorphous catalysts. Our theoretical models have been developed in a close comparison with the results of XAFS measurement collected by H. Dau and co-workers [3]. The close convergence of theoretical and experimental results has given impulse to further investigations providing deeper insight into the oxygen evolution reaction mechanism promoted by amorphous oxides [4]. References: [1] Nocera, D. G. The artificial leaf. Acc. Chem. Res. 2012, 45, 767-776 [2] Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co 2+. Science 2008, 321, 1072-1075; Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G. Structure-Activity Correlations in a Nickel- Borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 6801-6809; Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.; Fischer A.; Dau, H. Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide. Energy Environ. Sci. 2012, 5, 7081-7089; Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette C. P. Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340, 60-63. [3] Mattioli, G.; Risch, M.; Amore Bonapasta, A.; Dau, H.; Guidoni, L. Protonation states in a cobalt-oxide catalyst for water oxidation: fine comparison of ab initio molecular dynamics and X-ray absorption spectroscopy results. Phys. Chem. Chem. Phys. 2011, 13, 15437-15441; Mattioli, G.; Zaharieva, I.; Dau, H.; Guidoni, L. Atomistic Texture of Amorphous Manganese Oxides for Electrochemical Water Splitting: Fine Comparison of Ab Initio Calculations and XAS Measurements. 2015 (in preparation). [4] Mattioli, G.; Giannozzi, P.; Amore Bonapasta, A.; Guidoni, L. Reaction Pathways for Oxygen Evolution Promoted by Cobalt Catalyst. J. Am. Chem. Soc. 2013, 135, 15353-15363; Risch, M.; Ringleb, F.; Kohlhoff, M.; Bogdanoff, P.; Chernev, P.; Zaharieva, I.; Dau, H. Water oxidation by amorphous cobalt-based oxides: in situ tracking of redox transitions and mode of catalysis. Energy Environ. Sci. 2015, DOI:10.1039/C4EE03004D.