Development of porous cobalt-iron-phosphorous electrocatalyst for oxygen and hydrogen evolution reaction in alkaline water electrolysis

Abstract

Recently, hydrogen energy has been attracting attention due to global warming and depletion of fossil fuels. This energy can be produced by water electrolysis without fossil fuel. However, in the actual water electrolysis reaction, the overpotential is higher than the theoretical voltage (1.23 V) for decomposing water, so efficiency of the electrolysis dissolution reaction is lowered. Therefore, researches on electrochemical catalysts that reduce the overpotential applied to both electrodes have been continuously carried out. Transition metal phosphide catalysts, which are attracting attention in water electrolysis system, are being studied as catalysts for the hydrogen evolution reaction (HER) in an acid environment and the oxygen evolution reaction (OER) in an alkaline environment. Particularly, it is known binary transition metal phosphide materials such as Co-Fe-P have excellent catalytic performance. In the existing research, most of the catalysts were synthesized via multi-step processes. It has high cost, time-consumption and use of toxic chemical problems. It must overcome to economically manufacture electrodes for OER and HER in alkaline electrolytic water electrolysis system. In this study, Co-Fe-P material was developed with high catalytic activity in an alkaline environment by using electrodeposition. The active catalyst was prepared without a complicated process by Cu foam structure with high porosity. When the Co-Fe-P film catalysts were synthesized, the Fe content of the catalyst increased with increasing $FeCl_2$ concentration in bath. XRD analysis showed that all of the Co-Fe-P film catalysts had an amorphous structure. The catalytic activities of the Co-Fe-P film catalysts in which the Co/Fe ratio was the only variable were evaluated in a 1 M KOH solution. When the Co/Fe ratio was close to 1:1, Co-Fe-P film catalyst showed 330 mV (overpotential at $10mA/cm^2$), which is superior to the conventional Co-P catalyst and Ir/C catalyst. The improved performance of Co-Fe-P was caused by low charge transfer resistance and transformation of valence electron. In the XPS analysis, the binding energy of Co-Fe-P showed a positive charge transfer of Co and negative charge transfer of Fe and P due to a partial charge separation. On the basis of Co-Fe-P film, a porous Co-Fe-P foam catalyst was fabricated by optimized Co-Fe-P electrodeposition solution on the Cu foam structure. It was electrodeposited well without clogging the pore, and thickness was about 15 nm by TEM. At XRD analysis of Co-Fe-P foam catalyst, it was confirmed that the prepared Co-Fe-P foam catalyst had the same amorphous structure as the Co-Fe-P film catalyst. The OER activity of Co-Fe-P foam catalyst in the 1 M KOH was 294 mV and the HER activity was 73 mV. It has been analyzed to be a substitute for other noble metal catalysts. In addition, HER activity of the Co-Fe-P foam catalyst in the 0.5 M H2SO4 was 70 mV, which was superior to other catalysts. The improved performance of Co-Fe-P foam was due to low charge transfer resistance and high electrochemical active surface area (ECSA) caused by highly porous foam structure. The ECSA of Co-Fe-P foam increased to 94 times higher than that of Co-Fe-P film.