Electrolytic hydrogen production is gaining commercial ground as a technology that can efficiently and economically link the electricity with the fuels and chemicals branch of the energy system, while offering a means for large-scale and long-term energy storage. Amongst the different electrolysis technologies, alkaline electrolysis (AE) stands out as the most well established for large-scale electrolytic hydrogen production, with commercially available multi-MW units combined in plants of 100s of MW and operated for decades. Besides proven reliability and availability, a key advantage of AE over alternative technologies when it comes to large-scale deployment is the relatively abundant and inexpensive materials it relies on. Nevertheless, AE suffers from relatively poor performance in terms of production rate and efficiency when compared to proton exchange membrane electrolysis (PEME) and solid oxide electrolysis (SOE). This contribution discusses the main issues leading to the inferior AE performance and approaches to overcome them.

One of the main reasons is associated with the sluggish hydrogen evolution reaction (HER) kinetics in alkaline environment [1]. Improvements in HER catalysts, promoting the rate of H₂O dissociation, have reduced the HER kinetics difference between alkaline and acidic environment. Furthermore, the far lower price of these catalysts (e.g. Ni, Ni_1-xMo_x) compared to Pt, allow for much higher catalyst loadings, which can circumvent this challenge in conjunction with the much higher ionic conductivity of concentrated aqueous KOH as compared to PEME and SOE electrolytes. Taking full advantage of this opportunity requires a careful optimization of the AE electrode microstructure to achieve both a high electrochemically active surface area in close proximity to the separator as well as macro-porosity to enable gas evolution with minimal blocking of the active area. This was attempted here by applying high surface area catalytic coatings of Ni_1-xMo_x on porous conducting supports with varying macro-pore structure.

Raising the operating temperature offers an additional means to drastically improve performance, as both ionic transport and reaction kinetics are exponentially activated with temperature [2,3]. The development of a corrosion resistant ceramic separator [4] has enabled alkaline electrolysis cells operating at 200-250 °C and 20-50 bar [5,6], showing pronounced thermal activation, and achieving a current density of up to 3.75 A cm^-2 at a cell voltage of 1.75 V at 200 °C and 20 bar [7]. The feasibility and promise of this concept, as well as the challenges that lie ahead are also discussed.

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[4] F. Allebrod, C. Chatzichristodoulou, P. L. Mollerup and M. B. Mogensen, Int. J. Hydrogen Energy, 2012, 37, 16505-16514.

[5] F. Allebrod, C. Chatzichristodoulou and M. B. Mogensen, J. Power Sources, 2013, 229, 22–31.

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[7] C. Chatzichristodoulou, F. Allebrod and M. B. Mogensen, J. Electrochem. Soc., 2016, 163, F3036-F3040.