Response to “Comment on ‘Trends in the Exchange Current for Hydrogen Evolution’ [J. Electrochem. Soc., 152, J23 (2005)]”
J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov, and U. Stimming, “Response to ‘Comment on Trends in the Exchange Current for Hydrogen Evolution’ [J. Electrochem. Soc., 152, J23 (2005)]”, Journal of The Electrochemical Society 153(12), L33 (2006). https://doi.org/10.1149/1.2358292
This summary was written by a language model from the paper and has not been checked line by line. Rely on the published paper for details.
Question
This one-page letter is the authors’ reply to a published comment by Schmickler and Trasatti on their 2005 paper about trends in the hydrogen evolution reaction (HER) exchange current, (Nørskov et al. 2005). The comment questioned the novelty of the HER volcano concept, the treatment of hydrogen binding on metals such as Au and Ni, and aspects of the simple kinetic model. The reply sets out what the authors consider the actual contribution of the original paper and defends its main conclusions (No̸rskov et al. 2006).
Approach
The response is a short argumentative letter rather than a new study. It contains no new calculations or measurements. Instead it restates the method of the original work (computing hydrogen adsorption free energies with density functional theory and using them as the descriptor on a volcano plot), points to subsequent applications of that method by other groups and by the authors, and appeals to electronic structure arguments based on the d-band picture of metal–hydrogen bonding (No̸rskov et al. 2006).
Key findings
The authors concede that volcano plots of HER rate against hydrogen binding energy were introduced decades earlier, and they acknowledge that they should have cited the pioneering work of Parsons (Trans. Faraday Soc., 1958). They describe their own contribution as modest: a practical route to the hydrogen adsorption free energy via DFT, whose main value is that it makes predictions for new systems straightforward. As evidence of that predictive value they cite work by Kibler and Kolb rationalizing HER trends on Pd overlayers on various substrates, and their own related work explaining why the active sites of hydrogenase and nitrogenase enzymes evolve hydrogen well and predicting that MoS2 nanoparticles should be reasonable HER catalysts (No̸rskov et al. 2006).
On the substantive chemistry, the authors state that the calculated adsorption energy databases do not support the idea, raised in the comment, that the metal–H bond on Au is as strong as on Ni. They attribute the large difference to electronic structure: Au has filled d bands, whereas Ni has partly empty d bands, which leaves the H–Ni antibonding states partly empty and the bond stronger. They argue it is hard to see how H–H interactions on the surface could raise the free energy of adsorbed H on Ni to well above the chemical potential of the HER reactants and products (No̸rskov et al. 2006).
Regarding the kinetic model, the authors explain that it was introduced only to show how a volcano with its maximum near a hydrogen adsorption free energy of zero can arise, and that the original paper explicitly focused on trends across metals arising from thermochemistry. They acknowledge that more sophisticated models give the same trend and that their model leaves room for improvement as molecular-level insight grows. They maintain that a near-zero hydrogen adsorption free energy is a necessary but not sufficient condition for a good HER catalyst, and they note that, for their fully symmetric model, it is correct that all rates are equal at equilibrium (No̸rskov et al. 2006).
Significance
The letter clarifies how the 2005 hydrogen evolution paper should be read: not as the origin of the HER volcano, but as a way of supplying the hydrogen binding descriptor from first principles so that trends can be computed and new catalysts screened. That framing, together with the necessary-but-not-sufficient criterion on the hydrogen adsorption free energy, became a standard reference point for computational electrocatalyst design. The exchange also records an early defense of using d-band filling to explain differences in hydrogen binding between noble and transition metals.