Proton exchange membrane (PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE)[3] that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes.

In terms of sustainability and environmental impact, PEM electrolysis is considered as a promising technique for high purity and efficient hydrogen production since it emits only oxygen as a by-product without any carbon emissions.

[11] A thorough review of the historical performance from the early research to that of today can be found in chronological order with many of the operating conditions in the 2013 review by Carmo et al.[1] One of the largest advantages to PEM electrolysis is its ability to operate at high current densities.

The polymer electrolyte allows the PEM electrolyzer to operate with a very thin membrane (~100-200 μm) while still allowing high pressures, resulting in low ohmic losses, primarily caused by the conduction of protons across the membrane (0.1 S/cm) and a compressed hydrogen output.

[1] Maintaining a high gas purity is important for storage safety and for the direct usage in a fuel cell.

[13] An electrolyzer is an electrochemical device to convert electricity and water into hydrogen and oxygen, these gases can then be used as a means to store energy for later use.

The PEM electrolyzer utilizes a solid polymer electrolyte (SPE) to conduct protons from the anode to the cathode while insulating the electrodes electrically.

[14] The actual value for open circuit voltage of an operating electrolyzer will lie between the 1.23 V and 1.48 V depending on how the cell/stack design utilizes the thermal energy inputs.

This is however quite difficult to determine or measure because an operating electrolyzer also experiences other voltage losses from internal electrical resistances, proton conductivity, mass transport through the cell and catalyst utilization to name a few.

Here the supplied electrons and the protons that have conducted through the membrane are combined to create gaseous hydrogen.

Assuming that the maximum amount of heat energy (48.6 kJ/mol) is supplied to the reaction, the reversible cell voltage

The calculation of cell voltage assuming no irreversibilities exist and all of the thermal energy is utilized by the reaction is referred to as the lower heating value (LHV).

[1] A PEM electrolysis system's performance can be compared by plotting overpotential versus cell current density.

This essentially results in a curve that represents the power per square centimeter of cell area required to produce hydrogen and oxygen.

The figure below is the result of a simulation from the Forschungszentrum Jülich of a 25 cm2 single cell PEM electrolyzer under thermoneutral operation depicting the primary sources of voltage loss and their contributions for a range of current densities.

Ohmic losses are an electrical overpotential introduced to the electrolysis process by the internal resistance of the cell components.

The amount of heat energy that can be recaptured is dependent on many aspects of system operation and cell design.

The proton conductivity of the PEM is very dependent on the hydration, temperature, heat treatment, and ionic state of the membrane.

[15] Thus, safety hazards due to explosive anodic mixtures hydrogen in oxygen can result.

As the process operates at 80 °C for PEM electrolysers the waste heat can be redirected through the system to create the steam, resulting in a higher overall electrical efficiency.

[18][19] The efficiency of PEM electrolysis is expected to reach 82-86%[20] before 2030, while also maintaining durability as progress in this area continues at a pace.