Benchmarking protocols for proton exchange membrane water electrolyser

Abstract

Fossil fuel-based energy resources covers nearly 84 percent of the global primary energy consumption. However, the dependence on fossil fuel is no longer sustainable as these fuels account for more than 84 percent of anthropogenic greenhouse gas emissions. Renewable energy sources are the most promising alternatives to reduce anthropogenic greenhouse gas emissions. However, issues such as fluctuating and intermittent energy supply associated with these technologies require the use of energy storage. Proton exchange membrane water electrolysis (PEMWE) can be coupled with renewable energy sources to store excess energy in the form of hydrogen. However, the widespread commercial use of PEMWE is still impeded by its high operating cost, short durability and low system efficiency. As a part of National Hydrogen and Fuel Cells Technologies Flagship project in South Africa, the HySA Catalysis Centre of Competence has been tasked to establish South Africa as one of the main global exporters of electrolyser technologies. This entails but not limited to the development of manufacturing processes for electrocatalysts and other components for PEMWE systems. Towards meeting these objectives, in this project a fabrication method for PEMWE catalyst coated membranes (CCMs) was developed in house, using the Mayer rod coating technique. Anode catalyst layers were coated onto a polytetrafluoroethylene (PTFE, Teflon™) substrate from which it was transferred to a membrane via decal transfer, making up the 3-layer CCM used for electrochemical evaluation in an electrolyser cell. Several coating parameters were investigated to obtain uniform catalyst layers. It was found that the water to isopropyl alcohol mixture ratio of 3:1 and catalyst ink solid content of 30 wt% showed the most uniform catalyst layer surface structure and improved attachement to the substrate. A catalyst ink mixing time of 24 hours provided the most uniform distribution of the catalyst nanoparticle aggregate sizes. For the addition of pore forming additives to the catalyst ink suspension, complete transfer of the catalyst layer and removal of the pore forming additives was achieved with a hot-pressing pressure of as low as 500 Kg/cm2 for 3 minutes. Additionally, reliable electrochemical evaluation protocols to assess the performance of the fabricated CCMs were also developed. Several operating test parameters were investigated, where it was found that a compression of 4 kN and a water flow rate of 0.1 L/min provided better temperature control and improved overall CCM performance. Furthermore, from the porous transport layer (PTL) investigation, it was found that titanium powder sintered PTLs on both the anode and cathode sides provided a better overall electrolyser cell performance at high current density operation. Also investigated, were the effects of different electrolyser cell conditioning and evaluation measurement parameters on the overall CCM performance and it was found that the shortest cell conditioning time of 5 min provided the lower performance while cell conditioning times of 15 min, 30 min and 45 min showed no significant differences in their results. The addition of the open circuit voltage (OCV) step and the halving of the current-voltage measurement interval time from 5 min to 2.5 min provided the best CCM performance and improved significantly the cell performance profile at 1000 mA cm-2 over time. Finally, the effect of pore forming additives to the anode catalyst ink on the anode catalyst layer morphology and overall PEMWE cell performance was investigated. Ammonium carbonate and ammonium bicarbonate with varying weight ratios were used as pore forming additives in the catalyst ink formulation and subsequently removed during the decal transfer process. The investigation was conducted on both Nafion 212 and Nafion 115 membranes. From the investigation of pore forming additives effects, the physical characterisation of the anode catalyst layers data showed that an increase of porosity (pore size, pore quantity and pore distribution) in the anode catalyst layer was achieved in this study. The addition of pore forming substances increased the quantity of pores in the catalyst layer by 2.5-fold (from 30% to 74 ± 1% of the total catalyst layer volume) and 1.3-fold (from 30% to 45% ± 0.5% of the total catalyst layer volume) for pore forming materials to catalyst weight ratios of 1:1 and 1:10, respectively for the Nafion 212 CCMs samples. ~1.75-fold (from 40 % to 70% of the total catalyst layer volume) and ~1.5-fold (from 40% to around 60% of the total catalyst layer volume) for 1:1 and 1:10 ammonium bicarbonate to IrOx-TiO2 weight ratio, respectively for Nafion 115 CCMs samples. The electrochemical performance evaluation of the PEMWE cell showed that the addition of pore forming additives to the anode electrode catalyst ink formulation allowed for the reduction of iridium catalyst loading while improving the electrochemical performance. The iridium catalyst loading reduction of up to 45% was achived while improving the overall cell perfomance, with CCM of 1.31 mgIr cm-2 performing with 1.89 V at 1 A cm-2 and 0.72 mgIr cm-2 performing with 1.82 V at 1.0 A cm-2. Furthermore, the electrochemical evaluation tests showed that the CCMs with a high number of pores in their catalyst layers had the best catalyst utilisation compared to CCMs without pore formers.