Overview of PEM Hydrogen Production by Water Electrolysis I

Overview of PEM Hydrogen Production by Water Electrolysis I

Hydrogen is a clean and flexible energy carrier that can be used to provide electricity and heat. Hydrogen-fueled vehicles and stationary power generation are zero-emission technologies. Hydrogen can be produced both from traditional fossil fuels and from carbon-free energy sources, both of which are used to store energy and provide responsive management for the grid.

Currently only 4% of hydrogen is produced by electrolysis, mainly using low-cost preparation methods such as gas reforming of natural gas or refinery gas. However, in the future, renewable energy sources (RES) will account for a significant part of the produced electricity. Electrolysis is considered the cleanest way to produce hydrogen using renewable energy.

An emerging application for electrolysers is in the “power to gas” sector. Hydrogen produced by electrolysers connected to RES is injected into the gas network. This approach allows using gas pipelines as large “storage tanks” and avoids building new infrastructure. The amount of hydrogen injected depends on the regulations of each country. This issue can be solved by methanation, where hydrogen and carbon monoxide/carbon dioxide are converted into sustainable methane. The hydrogen stored in the natural gas infrastructure can be used for heating, transport or reconverted into electricity. Refueling stations with on-site hydrogen production are another application for electrolysers.

The main advantages of PEM electrolysis over alkaline electrolysis are higher safety and reliability, since no corrosive electrolyte is used. In addition, the possibility of operating at high pressure differences across the membrane avoids oxygen compression. Due to the solid and thin membranes, PEM electrolysis has a faster ion transport than alkaline electrolysis. Liquid electrolytes have a greater inertia in terms of ion transport. Alkaline electrolyzers react slowly when the electrolyzer is operated under fluctuating conditions and have difficulty starting up after shutdown. In addition, the technology can be operated at higher current densities than alkaline electrolyzers.

Catalyst
Expensive noble materials are usually used as electrocatalysts in PEM electrolysis. Palladium or platinum at the cathode for hydrogen evolution reaction (HER) and iridium or ruthenium oxide at the anode for oxygen evolution reaction (OER) are most commonly used. IrO2 exhibits higher corrosion resistance than RuO2, but it shows poor OER activity. RuO2 performs well in the low overpotential range, but stability issues hinder practical applications. The stability of RuO2 can be slightly improved by using binary IrO2–RuO2 solid solutions. The use of small particle size     (2–3 nm) IrO2 can reduce the noble metal loading while maintaining similar performance. Conductivity, electrocatalytic activity, and stability are challenging aspects of non-noble metal catalysts.

Proton Exchange Membrane
In PEM electrolysis, perfluorosulfonic acid membranes (PFSA) are used as solid electrolytes. Important properties of PEM electrolyser membranes are low crossover, the ability to work at high temperatures (>100°C) and high mechanical resistance. Crossover in PEMWE can damage the  membrane and lead to stack failure. The reaction of hydrogen and oxygen is very exothermic and causes local heating, which over time can damage the membrane. This problem is particularly important when the electrolyser operates at high pressure (up to 350 bar). The possibility of operating at high pressure allows to reduce the mechanical energy required for gas pressurization.

In these applications, a low level of crossover is necessary and requires an appropriate polymer film thickness. Another important mechanical property of polymer films is tear resistance. In fact, during the stack assembly process, large stresses are generated, especially between the electrode edges and the gaskets. Good tensile properties and low tear propagation resistance are key properties of polymer membranes in proton exchange membrane electrolyzers. Typically, composite or reinforced membranes are used to operate at high pressures and temperatures. PEM electrolyzers operate at high temperatures (>100°C), which reduces the Gibb free energy change and improves the reaction kinetics. In addition, their low cost makes them a real and attractive option for PEM electrolyzers.