This article reports a study of electrochemical cycling effects on hydrogen evolution catalytic activity of gold nanoparticles in sulfuric acid. We found that after cycling within the double layer regime, up to 0.64 V vs RHE, there is a dramatic effect on the HER activity of gold nanoparticles: 128 mV drop of the overpotential, a decrease of 23 mV/dec for the Tafel slope as well as a nearly twenty times increase of the turn-over frequency (TOF). This enhanced activity is tentatively assigned to the formation of stepped like structures by the consecutive electrochemical cycling in the double layer region.

A good understanding of the correlation between electronic properties and catalytic performance is vital to the rational design of active oxygen evolution reaction (OER) catalysts. Here, a volcano-shaped correlation between the OER activity and the Cr substitution amount was found for spinel oxides ZnFe2-xCrxO4 (x = 0-2), in which Zn2+ resides in tetrahedral (Td) sites while Fe3+ and Cr3+ occupy octahedral (Oh) sites. Such a relationship probably is because Cr substitution tunes the eg occupancy of Oh-site transition metals (TMOh) via the Oh-Oh superexchange effect. Density functional calculations further revealed the hybridization degree between the TMOh 3d and the O 2p states, and a volcano-shaped trend was also found in the variation of TMOh 3d-O 2p hybridization with the amount of Cr substitution. The good correlation between the OER activity and the hybridization highlights the important role of metal-oxygen hybridization in determining the OER activity of these spinel oxides.

Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxide_FM/oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1^st dehydrogenation to reduce the barrier for subsequent O-O coupling.