Earth-Abundant Electrocatalysts for the Oxygen Evolution Reaction Supported on Layered Zirconium Phosphate Nanomaterials

Abstract

Recently, we have demonstrated improved electrocatalytic activity of layered zirconium phosphate (ZrP) nanomaterials loaded with earth-abundant metal ions suitable as catalysts for the oxygen evolution reaction (OER). We compared the catalytic efficiency of ZrP nanoparticles ion-exchanged with transition metals (Fe, Ni, Co) in the interior of the layers as well as in the external surface to a system in which the metal catalysts are confined exclusively on the external ZrP surface. Linear sweep voltammetry revealed that the electrocatalytic systems with metal catalysts on the external ZrP nanomaterials’ surface had improved OER activity compared to systems with metals intercalated in the ZrP interlayers. This result prompted us to study a system of exfoliated ZrP particles which provide only external surfaces to the electrocatalytic metal ions. A comparison between adsorbed Co or Ni catalysts on ZrP nanoparticles and those same catalysts on exfoliated ZrP nanoplatelets proved that the later systems were more active, with diminished overpotentials and reduced Tafel plot slopes. Comparison between Co and Ni catalyst on ZrP particles with different morphologies (hexagonal platelets, rods, cubes, and spheres) revealed that the more active Co catalysts are those on hexagonal ZrP platelets, whereas the best Ni catalysts are those on ZrP spheres. More recently, efficient OER catalysts were obtained with cobalt porphyrin molecular electrocatalysts both intercalated in ZrP and adsorbed on exfoliated ZrP nanoplatelets. Mixed metal NiFe-intercalated ZrP electrocatalysts at 90% Fe metal content proved to have superior OER electrocatalytic performance (decreased overpotentials, increased mass activities, reduced Tafel slopes) compared to adsorbed counterparts. We are exploring OER activities of other mixed-metal catalysts on ZrP, bifunctional catalysts, and operando synchrotron X-ray absorption spectroscopy studies to elucidate the nature of the active species.