1 Design principles for hydrogen evolution reaction catalyst materials Dusan Strmcnik*, Pietro Papa Lopes, Bostjan Genorio, Vojislav R. Stamenkovic and Nenad M. Markovic Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 *corresponding author: strmcnik@anl.gov Abstract Design and synthesis of active, stable and cost-effective materials for efficient hydrogen production (hydrogen evolution reaction, HER) is of paramount importance for the successful deployment of hydrogen-based alternative energy technologies. The HER, seemingly one of the simplest electrochemical reactions, has served for decades to bridge the gap between fundamental electrocatalysis and practical catalyst design. However, there are still many open questions that need to be answered before it would be possible to claim that design principles of catalyst materials are fully developed for the efficient hydrogen production. In this review, by summarizing key results for the HER on well-characterized electrochemical interfaces in acidic and alkaline media, we have broadened our understanding of the HER in the whole range of pH by considering three main parameters: the nature of the proton donor (H 3 O + in acid and H 2 O in alkaline), the energy of adsorption of H ad and OH ad , and the presence of spectator species. Simply by considering these three parameters we show that great deal has already been learned and new trends are beginning to emerge, giving some predictive ability with respect to the nature of electrochemical interface and electrocatalytic activity of the HER. Introduction For more than 70 years, there has been a substantial academic interest and technological development effort directed towards fundamental understanding of the hydrogen evolution reaction (HER) and the development of materials for efficient hydrogen production in acid (2H + + 2e - ↔ H 2 ) © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/