The need for energy efficient as well as cost efficient electrolyzers is obvious in the transition to a sustainable energy system. The PEM electrolyzer has recently gained high popularity with outstanding rate capability, efficiency and lifetime. The key to the success of the PEM electrolyzer is the thin proton exchange membrane, which allows for low internal resistance in a zero-gap configuration. The down-side of it is high cost due in part to noble metal catalysts and expensive coatings of cell components to limit contact resistance in the acidic environment. The extensive use of noble metals is not only costly; especially the need for iridium for the anode catalyst most likely limits large scale roll-out due to the low availability of this element.
The alkaline electrolyzer, on the other hand, does not depend on noble metals. It represents a proven and robust technology, but in its traditional form, it suffers from low rate capability, and a bulky construction with a footprint an order of magnitude larger than that of the PEM electrolyzer. An alkaline ion conducting membrane with properties comparable to those of the proton exchange membrane could change that and make the alkaline system very competitive. The game-changer anion exchange membrane is the dream and the search for a suitable material is ongoing (often as fuel cell research). However, the materials still lack in terms of either conductivity or durability when compared to proton exchange membranes.
We propose the use of an ion-solvating membrane instead. It has no side chains terminated by quaternary ammonium ions, but a polymer backbone with affinity to aqueous KOH, with which the membrane is imbibed. The principle is similar to that of polybenzimidazole (PBI) imbibed with phosphoric acid in high-temperature PEM fuel cells[1], and the work was initiated using meta-PBI imbibed with aqueous KOH [2-6] .
With this system, a performance of 2-3 A cm-2 at 2 V has been demonstrated without the use of noble metals [7]. With the ongoing development of the electrodes, further improvement is expected. The long term stability of the membrane has not been proven, and meta-PBI is, although surprisingly stable in hot 6 M KOH, not stable enough for long term operation. Other ion-solvating membrane materials within the PBI family and apart are being studied.
[1] Q. Li, J. O. Jensen, R. F. Savinell and N. J. Bjerrum. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Progress in Polymer Science 34 449-477 (2009)
[2] D. Aili, M. K. Hansen, R. F. Renzaho, Q. Li, E. Christensen, J. O. Jensen and N. J. Bjerrum. Heterogeneous anion conducting membranes based on linear and crosslinked KOH doped polybenzimidazole for alkaline water electrolysis. J. Membrane Sci. 447 424–432 (2013)
[3] J. O. Jensen, D. Aili, M. K. Hansen, Q. Li, N. J. Bjerrum and E. Christensen. A Stability Study of Alkali Doped PBI Membranes for Alkaline Electrolyzer Cells. ECS Trans. 64 (3) 1175-1184 (2014)
[4] D. Aili, K. J. Jankova, Q. Li, N. J. Bjerrum, J. O. Jensen. The stability of poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole) membranes in aqueous potassium hydroxide. J. Membrane Sci. 492, 422-429 (2015)
[5] M. R. Kraglund, D. Aili, K. Jankova, E. Christensen, Q. Li, and J. O. Jensen. Zero-gap alkaline water electrolysis using ion-solvating polymer electrolyte membranes at reduced KOH concentrations. J. Electrochem. Soc. 163 (11) (2016) F3125-F3131
[6] D. Aili, A. G. Wright, M. R. Kraglund, K. Jankova, S. Holdcroft and J. O. Jensen. Towards a stable ion-solvating polymer electrolyte for advanced alkaline water electrolysis. J. Mater. Chem. A. 5 (10) (2017) 5055-5066
[7] M. R. Kraglund, M. Carmo et al. Advanced alkaline polymer electrolyte membrane electrolysis using ion-solvating membranes. In manuscript.