Hydrogen is an important part of any discussion on sustainability and reduction in emissions across major energy sectors. In addition to being a feedstock and process gas for many industrial processes, hydrogen is emerging as a fuel alternative for transportation applications. Renewable sources of hydrogen are therefore required to increase in capacity. Low-temperature electrolysis of water is currently the most mature method for carbon-free hydrogen generation and is reaching relevant scales to impact the energy landscape. However, costs still need to be reduced to be economical with traditional hydrogen sources. Operating cost reductions are enabled by the recent availability of low-cost sources of renewable energy, and the potential exists for a large reduction in capital cost withmaterial and manufacturing optimization. This article focuses on the current status and development needs by component for the low-temperature electrolysis options.

Energy efficient and cost efficient water electrolysis is essential for the large scale implementation of renewable energy. The two commercial low temperature electrolyzer technologies each suffer from serious drawbacks. The proton exchange membrane (PEM) electrolyzers remain expensive and depend strongly on the scarce metal iridium. The alkaline electrolyzers suffer from a large footprint due to low rate capability. Here we present an approach to make an alkaline electrolyzer perform like a PEM electrolyzer by means of an ion-solvating membrane. A long lasting effort to replace the state-of-the-art thick porous diaphragm by an anion exchange membrane has not proven successful. The ion-solvating membrane represents a third way. Demonstration cells based on KOH doped polybenzimidazole membranes and nickel based electrodes exhibited 1700 mA cm⁻² at 1.8 V. This is far exceeding what has previously been achieved with membranes in alkaline environments without platinum group metal catalysts, and is comparable to state-of-the-art PEM electrolyzers.

Hydrogen produced in a polymer electrolyte membrane (PEM) electrolyzer must be stored under high pressure. It is discussed whether the gas should be compressed in subsequent gas compressors or by the electrolyzer. While gas compressor stages can be reduced in the case of electrochemical compression, safety problems arise for thin membranes due to the undesired permeation of hydrogen across the membrane to the oxygen side, forming an explosive gas. In this study, a PEM system is modeled to evaluate the membrane-specific total system efficiency. The optimum efficiency is given depending on the external heat requirement, permeation, cell pressure, current density, and membrane thickness. It shows that the heat requirement and hydrogen permeation dominate the maximum efficiency below 1.6 V, while, above, the cell polarization is decisive. In addition, a pressure-optimized cell operation is introduced by which the optimum cathode pressure is set as a function of current density and membrane thickness. This approach indicates that thin membranes do not provide increased safety issues compared to thick membranes. However, operating an N212-based system instead of an N117-based one can generate twice the amount of hydrogen at the same system efficiency while only one compressor stage must be added.