Theory of Hydrogen Electrocatalysis

WOLFGANG SCHMICKLER; PAOLA QUAINO; ELISABETH SANTOS

Nice

Congreso; The 61st Annual Meeting of the International Society of Electrochemistry. Electrochemistry from Biology to Physics; 2010

International Society of Electrochemistry - ISE

Hydrogen evolution has always been considered one of the most importantelectrochemical reactions. Particularly during the second half of the last century, mucheffort has been spent, and sometimes wasted, in trying to understand, why its ratevaries by over six orders of magnitude on the various metals, and what makes a goodcatalyst. Event today, with the much improved methods of quantum chemistry, astraightforward treatment by density functional theory (DFT) or similar methods isimpossible, because the proton is the most strongly solvated ion, with a hydrationenergy of the order of 11 eV, and a proper treatment of solvation effects requires aprohibitively large number of paticlesWe have therefore combined DFT with ideas from Marcus theory and theAnderson-Newns model to construct a theory which describes both the Volmer and theHeyrowsky reaction. It allows the calculations of free energy surfaces as a function ofthe positions of the reacting particles and a generalized solvent coordinate, whichindicates the state of the solvent. Our theory shows, that catalysis requires a metal dband,which spans the Fermi level and interacts strongly with the 1s orbital ofhydrogen. We have applied our model to a number of cases, compared it with thepredictions of the volcano plot, and explain why the latter does not hold. In particularwe demonstrate, why rhenium is an excellent catalyst, and nickel merely mediocre.The Heyrowsky reaction shows an important interplay between hydrogen repulsionat large and attraction at short distances. We have performed model calculations forAg(111), where experimental data from our own group show that the Heyrowskyreaction is the second step, and estimate its energy of activation and potentialdependence.