Linking Lattice Strain and Electron Transfer Kinetics in Crystalline Layered Double Hydroxides

The improved electrocatalytic property of disordered metal hydroxides relative to that of crystalline layered double hydroxides remains a poorly understood phenomenon. We use a hydrothermally synthesized series of mixed metal hydroxides to study these composition-dependent structural similarities and electrochemical behavior differences. X-ray diffraction, X-ray absorption spectroscopy, and Raman spectroscopy provide complementary perspectives on the structure of the FexNi1-x(OH)(2) series. These techniques reveal near quantitative incorporation of Fe(III) into the Ni(OH)(2) lattice at low Fe-content but also that Fe(III) is distributed into a contaminating iron oxide phase and a non-traditional coordination environment atop the layered double hydroxide structure as the Fe-content increases. Systematic lattice contraction is observed with increasing Fe-content, similar to structurally disordered analogues, but the electrochemical behavior is markedly different. The characteristic anodic shift of pre-catalytic redox peaks does not occur, and electron transfer kinetics exhibit a much more gradual improvement. Measured Tafel slopes are found to possess a linear relationship with the O-Ni-O bond angles within the lattice across the full composition series. The asymmetric Marcus-Hush theory is used to explain this unexpected result, where Fe(III) ions systematically introduce a lattice strain that alters the reaction coordinate for the nickel oxidation reactions.