The Effect of Interlayer Anion Grafting on Water Oxidation Electrocatalysis: A Comparative Study of Ni‐ and Co‐Based Brucite‐Type Layered Hydroxides, Layered Double Hydroxides and Hydroxynitrate Salts

The urge for carbon‐neutral green energy conversion and storage technologies has invoked the resurgence of interest in applying brucite‐type materials as low‐cost oxygen evolution reaction (OER) electrocatalysts in basic media. Transition metal layered hydroxides belonging to the brucite‐type structure family have been shown to display remarkable electrochemical activity. Recent studies on the earth‐abundant Fe3+ containing mössbauerite and Fe3+ rich Co−Fe layered oxyhydroxide carbonates have suggested that grafted interlayer anions might play a key role in OER catalysis. To probe the effect of such interlayer anion grafting in brucite‐like layered hydroxides, we report here a systematic study on the electrocatalytic performance of three distinct Ni and Co brucite‐type layered structures, namely, (i) brucite‐type M(OH)2 without any interlayer anions, (ii) LDHs with free interlayer anions, and (iii) hydroxynitrate salts with grafted interlayer anions. The electrochemical results indeed show that grafting has an evident impact on the electronic structure and the observed OER activity. Ni‐ and Co‐hydroxynitrate salts with grafted anions display notably earlier formations of the electrocatalytically active species. Particularly Co‐hydroxynitrate salts exhibit lower overpotentials at 10 mA cm−2 (η=0.34 V) and medium current densities of 100 mA cm−2 (η=0.40 V) compared to the corresponding brucite‐type hydroxides and LDH materials. Grafting of interlayer anions to brucite‐type layered structures impacts electrocatalytic performance. Three Ni and Co compounds are investigated: (i) brucite‐type M(OH)2 (no interlayer anions), (ii) LDHs (free interlayer anions), and (iii) hydroxynitrate salts (grafted interlayer anions). Substitution of OH− by NO3− allows for larger layer separation and reduced electron density at the metal center contributing to earlier onset of OER activity.