(A) study on ni-mo catalyst for hydrogen oxidation reaction in anion exchange membrane fuel cell

Abstract

Anion exchange membrane fuel cell(AEMFC) is a fuel cell which tried to solve slow oxygen reduction kinetics of Nafion-based polymer electrolyte membrane fuel cell and conventional alkaline fuel cell which use liquid electrolyte. However, currently developed membranes for AEMFC are not stable and durable enough to be commercialized, and significantly decreased hydrogen oxidation kinetics limits the total cell performance. To gain high catalytic activity with low cost, several tries were done for hydrogen oxidation reaction in alkaline media. One of the effective way to reducing cost is applying transition metal catalysts which is adundant and much cheaper than platinum-group-metal catalysts(PGM). Several studies reported that nickel has hydrogen oxidation activity in alkaline media. Early researches tried to increase surface area of nickel by using Raney-nickel catalyst. However, it could not overcome the calytic activity of PGM catalyst because activity of nickel was significantly low. Recently, some articles are reporting that synergetic effect of nickel and PGM catalysts enhance the HOR activity, but PGM catalyst must be used. In this study, method for changing electronic structure of nickel was used to reduce the cost of catalyst. NiMo catalyst was synthesized by electrodeposition which nickel was base material and molybdenum was modifying material. $NiCl_2, Na_2MoO_4$, sodium citrate was used. Galvanostatic electrodeposition with current density of $-50 mA/cm^2$ was applied on mirror-polished copper substrate. Flat, clean, and thin film of NiMo catalyst was synthesized upon copper substrate. Crystal structure of catalyst was containing both crystalline and amorphous structure. $Ni_{2.3}Mo$ catalyst, which had the largest area of (111) plane, showed best activity among synthesized NiMo catalysts. Furthermore, $Ni_{2.3}Mo$ showed similar activity to Pt/C catalyst, implies that it can be the promising catalyst for anode of AEMFC. Further experiments of durability and stability gave advantages to $Ni_{2.3}Mo$ catalyst in contrast to Pt/C catalyst. Cycling durability and contamination resistance against alkyl ammonium group were higher than Pt/C. Also, $Ni_{2.3}Mo$ catalyst can endure at higher overpotential than previously reported nickel catalysts. Enhancement in catalytic activity was from weakened hydrogen binding energy of nickel. From hydrogen temperature programmed desorption spectra, it was confirmed that NiMo has lower hydrogen binding energy than nickel, which is origin of the high HOR acitivity of NiMo catalyst.