Editing Electrolysis (section)

=== Electrolysis of seawater ===
A 2020 study investigated direct electrolysis of seawater, alkaline electrolysis, [[Proton-exchange membrane|proton-exchange membrane electrolysis]], and [[Solid oxide electrolyser cell|solid oxide electrolysis]].<ref>{{Cite journal |last1=d’Amore-Domenech |first1=Rafael |last2=Santiago |first2=Óscar |last3=Leo |first3=Teresa J. |date=2020 |title=Multicriteria analysis of seawater electrolysis technologies for green hydrogen production at sea |url=https://linkinghub.elsevier.com/retrieve/pii/S1364032120304573 |journal=Renewable and Sustainable Energy Reviews |language=en |volume=133 |pages=110166 |doi=10.1016/j.rser.2020.110166|bibcode=2020RSERv.13310166D |s2cid=224843343 }}</ref> Direct electrolysis of seawater follows known processes, forming an electrolysis cell in which the seawater acts as the electrolyte to allow for the reaction at the [[anode]], {{chem2 | 2 Cl-(aq) -> Cl2(g) + 2e-}} and the reaction at the [[cathode]], {{chem2 | 2 H2O(l) + 2 e- -> H2(g) + 2OH-(aq) }}. The inclusion of [[magnesium]] and [[calcium]] [[ion]]s in the seawater makes the production of alkali [[hydroxide]]s possible that could form scales in the  electrolyser cell, cutting down on lifespan and increasing the need for maintenance. The alkaline electrolysers operate with the following reactions at the anode, {{chem2 | 2 OH-(aq) -> 1/2 O2(g) + H2O(l) + 2 e- }}and cathode, {{chem2 | 2 H2O(l) + 2 e- -> H2(g) + 2 OH-(aq) }}, and use high base solutions as electrolytes, operating at {{Convert|60-90|C|F|abbr=on}} and need additional separators to ensure the gas phase hydrogen and oxygen remain separate. The electrolyte can easily get contaminated, but the alkaline  electrolyser can operate under pressure to improve energy consumption. The electrodes can be made of inexpensive materials and there's no requirement for an expensive catalyst in the design.   

Many alternatives to the simple electrolyzer above are there.  There are micro-electrolyzer designs that are able to eliminate the separator requirement by designing the internal flow to separate the gases autonomously.  See for example US12116679B2 where the operating pressure is increased to the point of Chlorine liquefaction so that sea water electrolyzer can proceed in a locally alkaline electrolytic fluid.   Removing separators allows operating at very high temperatures.  The structural design allows for operations at upto 700 bar thereby eliminating the need for Hydrogen compressors.   

Proton-exchange membrane electrolysers operate with the reactions at the anode, {{chem2 | H2O(l) -> 1/2 O2(g) + 2 H+(aq) + 2 e- }} and cathode, {{chem2 | 2 H+(aq) + 2 e- -> H2(g) }}, at temperatures of {{Convert|60-80|C|F|abbr=on}}, using a solid polymer electrolyte and requiring higher costs of processing to allow the solid electrolyte to touch uniformly to the electrodes. Similar to the alkaline electrolyser, the proton exchange membrane  electrolyser can operate at higher pressures, reducing the energy costs required to compress the hydrogen gas afterward, but the proton exchange membrane  electrolyser also benefits from rapid response times to changes in power requirements or demands and not needing maintenance, at the cost of having a faster inherent degradation rate and being the most vulnerable to impurities in the water. 

Solid oxide electrolysers run the reactions {{chem2 | O2-(g) -> 1/2 O2(g) + 2 e- }} at the anode and {{chem2 | H2O(g) + 2 e- -> H2(g) + O2-(g) }} at the cathode. The solid oxide electrolysers require high temperatures ({{Convert|700-1000|C|F|abbr=on}}) to operate, generating superheated steam. They suffer from degradation when turned off, making it a more inflexible hydrogen generation technology. In a selected series of [[Multiple-criteria decision analysis|multiple-criteria decision-analysis]] comparisons in which the highest priority was placed on economic operation costs followed equally by environmental and social criteria, it was found that the proton exchange membrane electrolyser offered the most suitable combination of values (e.g., investment cost, maintenance, and operation cost, resistance to impurities, specific energy for hydrogen production at sea, risk of environmental impact, etc.), followed by the alkaline electrolyser, with the alkaline electrolyser being the most economically feasible, but more hazardous in terms of safety and environmental concerns due to the need for basic electrolyte solutions as opposed to the solid polymers used in proton-exchange membranes. Due to the methods conducted in multiple-criteria decision analysis, non-objective weights are applied to the various factors, and so multiple methods of decision analysis were performed simultaneously to examine the electrolysers in a way that minimizes the effects of bias on the performance conclusions.