Sustainable Hydrogen Production from Seawater Electrolysis

Electrochemistry enables direct conversion between electrical and chemical energy with high efficiency, and is a key to achieving net zero. An exciting electrochemical technology is the hydrogen-oxygen (H2-O2) fuel cell that produces electricity at high efficiency with only clean water as the byproduct. A green and sustainable route for H2 production to support this technology is water electrolysis using renewable or excess electricity; however, it is an energetically uphill process involving the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Whilst the 2-electron HER is relatively facile, the 4-electron OER is particularly sluggish and requires noble metals (Ir, Ru) as catalysts under acidic conditions. Nevertheless, recently significant progress has been made (including some adventurous work by the applicant and their collaborators) towards more efficient OER under alkaline conditions, where non-noble metal catalysts such as transition metal (Ni, Fe) layered double hydroxides (LDHs) were effectively used.

Notably, for water electrolysis to be used to store (as H2) a substantial portion of the world's energy, water distribution issues will arise as vast amounts of purified water will be needed. On the other hand, seawater is the most abundant aqueous electrolyte feedstock on Earth, but its implementation in the water-splitting process presents many challenges, especially for the anodic reaction. The most serious challenges in seawater electrolysis are posed by the chloride anions (around 3% NaCl in seawater by weight). Under acidic conditions, the OER equilibrium potential (1.23 V) is only slightly (130 mV) lower than that (1.36 V) of the chlorine evolution reaction (ClER); and OER as a 4-electron reaction requires a high overpotential while ClER is a facile 2-electron reaction with a kinetic advantage, thus CIER can compete with OER.

However, in alkaline conditions, the equilibrium potential of OER is significantly shifted lower, e.g., 0.40 V at pH=14; while that of ClER does not change so much (1.36 V) but now the hypochlorite (ClO-) formation from chloride oxidation reaction (ClOR) must be considered as the latter has a relatively lower equilibrium potential of 0.88 V at pH=14; clearly now there is a significant difference of 480 mV in potential domain for OER to work before ClOR occurs.
Within the above context, this exciting project aims to draw together the nascent work on new catalysts (including surface structures and layers) for the OER anode and HER cathode, the anion exchange membrane, membrane-electrode-assembly and reactor system development, in order to determine the feasibility of formulating low-cost and high performance (active and durable) electrodes and membrane-electrode-assemblies (MEAs) for a cost-effective and scalable seawater electrolyser for sustainable hydrogen production with the maximum resource and energy efficiencies.

The proposed work is highly ambitious and high risk, as seawater electrolysis is very attractive but extremely challenging, ranging from competitive chloride oxidation to corrosive environments, which require highly selective electrocatalysts together with good stability at material level, and well-engineered electrodes and interfaces to facilitate mass transport (gas bubble removal) to enable high current density to be sustained at reactor level. However, if this feasibility research is successful, it will be extremely rewarding as it opens a new paradigm for low cost, large scale, and truly sustainable green hydrogen production for delivering sustainable net zero for the UK and beyond.