Water Management in a High Current Density PEM Electrolysis Cell

Abstract

Water electrolysis and fuel cell systems are storage technologies with great potential capacities but suffer from high costs. Noble metals are used for electrocatalysts and separate plants are needed to work in both operation modes. The running costs are also subject of optimization. This includes costs for water pumps, gas storage next to the main topic efficiency. This work deals with the issue of investment costs as well as running costs. Requirements for industrial use is next to long-term stability an increased current density. This allows relatively small plants to produce high amounts of hydrogen gas. For this, high efficiency as well as optimizing the transport issues of water and gas inside the membrane electrode assemblies is needed. The running costs include the costs for pumps and deionized water, needed to prevent degradation of the membrane and the electrocatalyst. First, thin membranes for 4 cm2 active area are tested for their suitability for water electrolysis. Nafion® 211 and Nafion® XL are examined on their mechanical stability. While the former does regularly break at increased contact pressures - needed to reduce the ohmic contact resistance between electrode and membrane -, the reinforced Nafion® XL is properly suited for this and does not break even at high contact pressures of 60 bar and elongated operation of several days and repeated start-up and humidification changes before failing. The Nafion® XL is then optimized for their electrocatalyst amount which is iridiumdioxide. Loadings from 0.26 mg/cm2 to 0.94 mg/cm2 are tested. While the highest amount of electrocatalyst shows the highest efficiency with 1.69 V at 1 A/cm2, the efficiency increase per loading can be a major factor when trying to reduce the electrocatalyst amount. Even a low loading of 0.44 mg/cm2 proves to show good results while needing less costly electrocatalyst. Two analyses are conducted to examine the water effects inside the cell. First, the electro-osmotic drag coefficient - the amount of water dragged by protons through the membrane - is analyzed at current densities up to 5 A/cm2. Higher current densities do seem to hinder water molecules to be transported and require a lower electro-osmotic drag coefficient to be accounted for, even though the total transported amount is increasing. Second, the stability depending on the fed water is examined. Different amounts of water are tested for stable operation of water electrolysis. It is found that for lower current densities of up to 2 A/cm2, the optimal stoichiometry is around 10 or higher. At stoichiometries below this level, the current density is not stable at constant voltages. Also, higher stoichiometries are necessary for stable operation at higher current densities. The effects of start-up show inertia of the system and require further investigation.