(Invited) Advanced Alkaline Electrolysis Cells for the Production of Sustainable Fuels and Chemicals
Authors: Chatzichristodoulou Christodoulos , Kraglund Mikkel Rykær , Georgolamprou Xanthi , Pitscheider Simon , Seselj Nedjeljko , Gellrich Florian , Gadea Christophe , Khajavi Peyman , Kiebach Ragnar , Frandsen Henrik Lund , Jensen Jens Oluf , Mogensen Mogens Bjerg 
Published: May 2020

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).
One of the main reasons is associated with the sluggish hydrogen evolution reaction (HER) kinetics in alkaline environment [1]. Recent improvements in HER catalysts, have reduced the HER kinetics difference between alkaline and acidic environment. Furthermore, the far lower price of these catalysts (e.g. Ni, Ni1-xMox) 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 and Ni1-xMox on porous conducting supports with varying macro-pore structure. Furthermore, a finite element multi-physics simulation model was employed to provide further insight and guidance to the microstructural optimization effort.
Raising the operating temperature offers an additional means to drastically improve performance, as both ionic transport and reaction kinetics are strongly activated with temperature [2]. The development of a corrosion resistant ceramic separator [3] has enabled a novel concept of alkaline electrolysis cells operating at 200-250 °C and 20-50 bar [4,5], 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 [6]. The feasibility and promise of this concept, as well as the challenges that lie ahead are also discussed.
[1] V. R. Stamenkovic, D. Strmcnik, P. P. Lopes and N. M. Markovic, Nature Materials, 2017, 16, 57–69.
[2] M. H. Miles, G. Kissel, P. W. T. Lu and S. J. Srinivasan, J. Electrochem. Soc., 1976, 123, 332-336.
[3] F. Allebrod, C. Chatzichristodoulou, P. L. Mollerup and M. B. Mogensen, Int. J. Hydrogen Energy, 2012, 37, 16505-16514.
[4] F. Allebrod, C. Chatzichristodoulou and M. B. Mogensen, J. Power Sources, 2013, 229, 22–31.
[5] F. Allebrod, C. Chatzichristodoulou and M. B. Mogensen, J. Power Sources, 2014, 255, 394-403.
[6] C. Chatzichristodoulou, F. Allebrod and M. B. Mogensen, J. Electrochem. Soc., 2016, 163, F3036-F3040.