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The Joint Center for Artificial Photosynthesis (JCAP) was designed to cover the breadth of research from basic science to prototyping and scale-up, and to benefit from the synergies between basic and applied research. This chapter provides a review of the experimental research on electronic structure effects in electrocatalysis carried out in the Heterogeneous Catalysis group.

All oxygen and fuel forming reactions in water splitting and carbon dioxide reduction have in common that they require electrodes that not only provide for electron transfer to or from the reactants, but that also provide catalytic activity. It is not sufficient to align photoelectrode band edges with the redox potentials of a multi-electron reaction. Without a catalyst, single-electron transfers would result in highly unstable intermediates, e.g., OH, whose generation requires much higher electrochemical potentials than the thermodynamic potential for the final products. The role of the catalyst is to allow for the formation of more stable intermediates that are chemically bonded to its surface. It is the chemical bonding between catalyst and intermediates that partially compensates the energy required to dissociate the starting molecules, H2O or CO2. The energy of each such catalyst-intermediate bond is the key criterion that needs to be optimized in order to accelerate the reaction rate. It must be noted here that optimization does not mean merely increasing, but instead actually fine-tuning bond strengths in between too weak and too strong interactions, since every catalytic process requires a sequence of both bond-breaking as well as bond-making steps. Too weak adsorption of intermediates fails to promote the bond-breaking steps at a sufficient rate, while too strong adsorption prevents the reaction from proceeding through bond-making steps, which compete energetically with the catalyst-intermediate bonds. A breakthrough towards understanding existing and predicting new catalyst materials was made with the d-band model, which describes the surface reactivity of transition metals and their alloys as a result of their valence band structures and their interaction with reactant molecular levels.1,2  Knowledge of adsorption energies, e.g. thermodynamic parameters of stable intermediates, which in many cases can be measured experimentally, is sufficient for a prediction of trends in catalytic reaction rates, due to the Brønsted–Evans–Polanyi relation between activation energy and reaction energy.3  For electrocatalytic reactions, the influence of applied electrochemical potential can be added by means of the “computational hydrogen electrode”.4–7 

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