PEM electrolysis

Polymer electrolyte membrane electrolysis
Diagram of PEM electrolysis reactions.
Typical Materials
Type of Electrolysis:	PEM Electrolysis
Style of membrane/diaphragm	Solid polymer
Bipolar/separator plate material	Titanium or gold and platinum coated titanium
Catalyst material on the anode	Iridium
Catalyst material on the cathode	Platinum
Anode PTL material	Titanium
Cathode PTL material	Carbon paper/carbon fleece
State-of-the-art Operating Ranges
Cell temperature	50-80C
Stack pressure	<30 bar
Current density	0.6-2.0 A/cm2
Cell voltage	1.75-2.20 V
Power density	to 4.4 mW/cm2
Part-load range	0-10%
Spec. energy consumption stack	4.2-5.6 kWh/Nm3
Spec. energy consumption system	4.5-7.5 kWh/Nm3
Cell voltage efficiency	57-69%
System hydrogen production rate	30 Nm3/h
Lifetime stack	<60,000 h
Acceptable degradation rate	<14 µV/h
System Lifetime	10-20 a

Polymer electrolyte membrane (PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the issues of partial load, low current density, and low pressure operation currently plaguing the alkaline electrolyzer.

However, a recent scientific comparison showed that state-or-the-art alkaline water electrolysis shows competitive or even better efficiencies than PEM water electrolysis. This comparison moreover showed that many of the advantages such as gas purities or high current densities that were ascribed to PEM water electrolysis are also achievable by alkaline water electrolysis. Electrolysis is an important technology for the production of hydrogen to be used as an energy carrier.

With fast dynamic response times, large operational ranges and high efficiencies water electrolysis is a promising technology for energy storage coupled with renewable energy sources.

The use of a PEM for electrolysis was first introduced in the 1960s by General Electric, developed to overcome the drawbacks to the alkaline electrolysis technology. The initial performances yielded 1.88 V at 1.0 A/cm² which was, compared to the alkaline electrolysis technology of that time, very efficient. In the late 1970s the alkaline electrolyzers were reporting performances around 2.06 V at 0.215 A/cm², thus prompting a sudden interest in the late 1970s and early 1980s in polymer electrolytes for water electrolysis.

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.

One of the largest advantages to PEM electrolysis is its ability to operate at high current densities. This can result in reduced operational costs, especially for systems coupled with very dynamic energy sources such as wind and solar, where sudden spikes in energy input would otherwise result in uncaptured energy. The polymer electrolyte allows the PEM electrolyzer to operate with a very thin membrane (~100-200μm) 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.

The polymer electrolyte membrane, due to its solid structure, exhibits a low gas crossover rate resulting in very high product gas purity. Maintaining a high gas purity is important for storage safety and for the direct usage in a fuel cell. The safety limits for H₂ in O₂ are at standard conditions 4 mol-% H₂ in O₂.