Electro-Oxidation Reaction of Methanol over La_2-SrNi_1-(Mn/Fe/Co)O_4+δ Ruddlesden-Popper Oxides

Solution combustion-synthesized Ruddlesden-Popper oxides La_1.4Sr_0.6Ni_0.9(Mn/Fe/Co)_0.1O_4+δ were explored for the methanol electro-oxidation reaction. With optimal doping of Sr²⁺ in the A site and Co²⁺ in the B site, Ni³⁺ with t_2g⁶ $$ configuration in La_1.4Sr_0.6Ni_0.9Co_0.1O_4+δ exhibited a tetragonal distortion with compression in axial bonds and elongation in equatorial bonds. This structural modification fostered an augmented overlap of $$ orbitals with axial O 2p orbitals, leading to a heightened density of states at the Fermi level. Consequently, this facilitated not only elevated electrical conductivity but also a noteworthy reduction in the charge transfer resistance. These effects collectively contributed to the exceptional methanol oxidation activity of La_1.4Sr_0.6Ni_0.9Co_0.1O_4+δ, as evidenced by an impressive current density of 21.4 mA cm^-2 and retention of 95% of initial current density even after 10 h of prolonged reaction. The presence of Ni³⁺ further played a pivotal role in the creation of NiOOH, a crucial intermediate species, facilitated by the presence of surface oxygen vacancies. These factors synergistically enabled efficient methanol oxidation. In summary, our present study not only yields substantial insights but also paves the way for a novel avenue to fine-tune the activity of Ruddlesden-Popper oxides for the successful electro-oxidation of methanol.