Para-Polybenzimidazole for High Temperature Alklaine Water Electrolysis

Abstract

Due to limited storage capacity and environmental resources is a trend of increasing interest in new energy sources. Among various new energy source, Hydrogen has attracted attention as a future energy source because it emits very little environmental pollutants. In addition, hydrogen has high energy density per mass of 143 kJ/g, which is about 3 times higher than of fossil fuels. So, hydrogen is widely used in energy production, storage, utilization. Hydrogen production methods include reforming using fossil fuels, and using non-fossil fuels such as electricity and heat. Reforming emits many environmental pollutants but the method using electricity, heat, and etc. don’t emit environmental pollutants. However, other methods, except electricity, are still need to research. The method using electricity is largely divided into alkaline water electrolysis (AWE), polymer electrolyte water electrolysis (PEMWE), anion exchange membrane water electrolysis (AEMWE). The PEMWE has advantages such as various operating conditions, small cell design, and high performance, but has a large disadvantage of expensive due to using high cost component. The anion exchange membrane water electrolysis has advantages of other water electrolysis and complements disadvantages. But anion exchange membrane has low stability and conductivity, so it needs to research. AWE has advantages such as non-PGM catalyst, low price and long-term stability. However, there is a disadvantage in that gas crossover, and low current densities are easier than other water electrolysis. But, zero gap assembly can compensate for the disadvantages of the AWE.At present, there are anion exchange membranes used in water electrolysis which is PBI, PEEK, PVA, and etc. Especially, PBI showed a high performance and stability in high temperature, alkaline condition. In addition, it showed excellent stability even at high temperature. Therefore, PBI is expected to be applied to high temperature steam process due to its excellent high temperature stability.In this study, the water uptake, thickness changes were measured in 10 wt% potassium hydroxide (KOH) to make the chemical stability of the PBI. It was measured for about a month, and there was no significant change in water uptake and thickness, but it was confirmed that cracks occurred on the surface. PBI was used to develop electrolytic MEAs that can operate in high temperature and alkaline conditions. Electrodes were fabricated catalyst slurry. Catalyst slurry were prepared by catalyst, H2O, ionomer and IPA. The catalyst used was IrO2 as anode and Pt/C as cathode, respectively. Both anode and cathode were used PFTE as ionomer. The catalyst to ionomer ratio was 0.091 to 1. The resulting catalyst slurry was sonicated in water bath for 40 min. Electrodes were fabricated by catalyst coated substrate (CCS) method on porous transport layers (PTLs). The cathode catalyst loading as 0.95 to 1 mg/cm2 and anode catalyst loading as 2 mg/cm2, respectively. The prepared electrode was heated at 350 oC for 10 min to disperse PTFE. The electrode size was 6.25 cm2. Single cell tests were operated 80, and 120 ℃, 1.5 bar. Electrolyte was 10 wt% Potassium hydroxide (KOH) which was injected into the anode side. The flow rate was 20 ml/min. Electrochemical analysis was performed using LSV, and EIS. LSV was measured by increasing 10 mV per second from 1.0 to 2.0 V, and EIS was measured in the range of 50 kHz to 50 mHz at 1.6 V. MEA containing PBI membrane is showed about 700 mA/cm2 at 2.0 V. Compared to the previous research results (80 oC), it was applied to high temperature alkaline water electrolysis and exhibited high performance. In addition, high stability of the PBI membrane was maintained even in long term operation for more than 200 h in 120 ℃, 1.5 bar.