Some more recent applications for ionomers include being used as ion-selective membranes in a variety of electrical and energy applications. Examples include the cation exchange membrane for fuel cells, which allow only protons or specific ions to cross the membrane, a polymer electrolyte membrane (PEM) water electrolyzer to optimize the uniform coating of the catalyst on membrane surfaces, a redox flow battery separator, electrodialysis, where ions are transported between solutions using the ionomer membrane, and electrochemical hydrogen compressors to increase the strength of the membrane against the pressure differentials that can occur within the compressor.