Pore size and electronic tuning in cerium-doped CoFe-LDH for the oxygen evolution reaction

Abstract

A series of cerium-doped CoFe-layered double hydroxide (LDH) materials were synthesized using a co-precipitation method, and they were utilized for the oxygen evolution reaction. Among all the materials, CoFeCe2, having the highest cerium doping, exhibited a remarkable mass activity of 294.15 A g⁻¹, turnover frequency (TOF) of 1.82 s⁻¹, and TOF_EIS of 126.76 s⁻¹, and these activity parameters were 6.5–8 times higher than those of undoped CoFe-LDH (CoFeCe0). Electrochemically active surface area (ECSA) measurements suggested a massive increase in roughness factor (21 times) for CoFeCe2 compared to undoped CoFeCe0. BET results revealed that cerium doping resulted in mostly a narrow mesoporous distribution, which helped to increase the surface area and pore volume. Employing the mechanism of LDH formation reported earlier, we proposed that the cerium doping minimized the interparticle electrostatic repulsion during LDH synthesis and hence, an ordered LDH nanosheet with shallow pore distribution (mostly narrow mesopores) was observed for CoFeCe2, while the pore distribution was very wide for undoped LDH (CoFeCe0) due to the formation of disordered LDH nanosheets. XPS results provided evidence that the cerium doping resulted in electron transfer from Co²⁺ to Ce⁴⁺ to generate more Co³⁺ in the synthesized LDH material, which acted as active sites for water oxidation and helped to enhance water oxidation. The high ionic radius of the cerium ion also resulted in lattice defects that decreased the material's crystallinity and increased the catalytic reactivity by providing more active sites. Furthermore, during water oxidation, the unsymmetrical electron occupancy in the t_2g orbitals of Co²⁺ leads to a dynamic Jahn–Teller distortion, which results in the lengthening of the Co–O bonds, and further facilitates the formation of O–O bonds during the OER process.