As the required compression power for water is less than that for hydrogen-gas the water is pumped up to a high-pressure, in the other approach differential pressure is used. There is also an importance for the electrolyser stacks to be able to accept a fluctuating electrical input, such as that found with renewable energy. This then enables the ability to help with grid balancing and energy storage.
Ultrahigh-pressure electrolysis is high-pressure electrolysis operating at 34–69 megapascals (5,000–10,000 psi). At ultra-high pressures the water solubility and cross-permeation across the membrane of H2 and O2 is affecting hydrogen purity, modified PEMs are used to reduce cross-permeation in combination with catalytic H2/O2 recombiners to maintain H2 levels in O2 and O2 levels in H2 at values compatible with hydrogen safety requirements.
The US DOE believes that high-pressure electrolysis, supported by ongoing research and development, will contribute to the enabling and acceptance of technologies where hydrogen is the energy carrier between renewable energy resources and clean energy consumers.
High-pressure electrolysis is being investigated by the DOE for efficient production of hydrogen from water. The target total in 2005 is $4.75 per gge H2 at an efficiency of 64%. The total goal for the DOE in 2010 is $2.85 per gge H2 at an efficiency of 75%. As of 2005 the DOE provided a total of $1,563,882 worth of funding for research.
Mitsubishi is pursuing such technology with its High-pressure hydrogen energy generator (HHEG) project.