Our drive towards a sustainable economy powered by renewable energy systems features two remarkable elemental materials, Si and Pt. Decades of fundamental and applied research have unraveled the complexity of their bulk and surface properties, which have led to astonishing improvements in efficiency and costs of key technologies such as photovoltaics (PV), hydrogen fuel cells and water electrolysers. Towards 2050 and beyond, these technologies face enormous challenges in terms of integration and scalability, requiring new more complex materials. In this presentation, I will highlight recent examples of our research into: 1. Multicomponent transition metal oxide electrocatalysts and photoelectrodes, focusing on correlations between composition, electronic structure, and activity towards oxygen electrocatalysis [1-5]. 2. Solution-processed CuZnSn(S,Se)4 thin film solar cells, discussing how bulk and surface defects can affect device performance [6-8]. I will share some of the general principles we have learnt about correlating material structure and performance from studying these two very different families of compounds.