Pure water supply to an electrolyser is essential. Electrolysis splits water molecules to oxygen and hydrogen. The cost of water supply for a typical green hydrogen scheme will be insignificant compared to the cost of the renewable power and will generally be only 1% or 2% of the total operating cost.

However, the consequences of water supply issues are unacceptable. Impurities such as calcium ions in the water will rapidly damage a PEM electrolyser membrane due to the interaction with the catalyst coating. Alkaline electrolysers also have sensitivities to poisons in the water.

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