Ceria Nanodots Anchored on ZIF67-Derived Framework By Atomic Layer Deposition for Efficient Bifunctional Oxygen Electrocatalysis

Abstract

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are the two most important reactions for electrocatalytic activities, has been widely studied for sustainable and renewable electrochemical applications, including fuel cells[1] , [2], metal-air batteries[3] and water electrolysis[4]. For the development of new catalysts alternative to noble metal-based materials, metal organic frameworks (MOFs) are being actively used as the precursor structure. ZIF-67, a kind of MOF structure made of cobalt nitrate and 2-methylimidazole ligands, has proved to be a good candidate for ORR catalysis due to their rich of Co-N-C active sites. However, ZIF-67 tends to aggregate during the pyrolysis process by breaking C-H or C-O bonds.In this study, atomic layer deposition (ALD) was applied to coat an atomically thin cerium oxide (CeO2, ceria) coatings the MOF structure, which is expected to incur synergistic catalytic activity and suppress the thermal agglomeration. ALD is a self-limiting surface chemistry process, and is able to create highly conformally layer-by-layer thin film on any complex 3D structure with controllable thickness at an atomic level.[5] Ceria has shown good synergistic effect with other metals for OER performance due to its ability for electron exchange and ion reduction.[6] We demonstrates a facile synthesis of Co/Ce co-doped porous carbon nanomaterials by pyrolyzing the ceria-coated ZIF structure. The resultant samples showed uniform crystal structure which are in well accordance with ZIF-67 precursors. The as-prepared samples also showed enhanced electrocatalytic activities for both ORR and OER comparable to that of noble metal benchmarks (Pt/C and IrO2) and in terms of onset potential, half-wave potential, and current density. A sample prepared by performing 15 cycles of ceria ALD followed a pyrolysis at 700 °C shows the highest onset potential (0.95 V versus RHE) for ORR and superior stability in comparison to commercialized Pt/C. Meanwhile, a sample with 5 cycles of ceria ALD exhibits an excellent OER performance (1.48 V at 10 mA cm-2) and maintained a good OER durability performance (from 1.48 V to 1.50 V after 2,000 potentiodynamic cycles).