Anode-Fed Anion Exchange Membrane Electrolyzers for Hydrogen Generation Tolerant to Anion Contaminants

Anion exchange membrane water electrolysis has the ability to produce green hydrogen with high voltage efficiencies at low capital cost with zero CO2 emissions. The alkaline environment of these devices allows for the use of economical metal catalysts and anion exchange membranes (AEMs) that conduct hydroxide ions and are less expensive than their proton exchange membrane counterparts. However, research has shown that anion contamination of hydroxide exchange membranes can lead to significant performance losses in anion exchange membrane fuel cells (AEMFCs), particularly when air fed to the oxygen-reducing cathode contains CO2.1 A similar contamination effect occurs in anion exchange membrane electrolyzers (AEMELs), when dissolved CO2 in the electrolyte reacts to form carbonate (CO3 2-) and bicarbonate (HCO3 2-) anions which compete with the hydroxide (OH-) ions that must be conducted through the AEM and ionomer. The presence of these and other anion contaminants can lower the ionic conductivity of the cell. Under high current density operation, an AEMEL undergoes a self-purging process that uses an ionic potential gradient to push hydroxide and anion contaminants through the membrane to the anode. This creates a significant pH gradient between cathode and anode that can lead to concentration polarization which further lowers performance.2 In this work, we use 1-D CO2 transport modeling and experiments to show how altering the electrolyte feed method allows for CO2-tolerant AEMEL operation in several different electrolytes. The unique advantages of AEMELs over AEMFCs and proton exchange membrane electrolyzers (PEMELs) are that (1) this self-purge occurs during normal operation, and (2) they allow for flexibility in the location of the potentially contaminated water feed. In PEMELs, water is intuitively fed to the anode, and cation contaminants are purged through the entire MEA to the cathode. Although water in AEMELs is intuitively fed to the cathode, where it is consumed in the hydrogen evolution reaction, water can diffuse easily through the membrane, allowing for it to be fed as an anolyte to the oxygen-evolving side of the cell. This allows for better contaminant rejection because anions can be concentrated mostly on the anode side of the electrolyzer. The model described in this paper predicts that an anode-fed AEMEL can more easily purge CO2 without contaminating as much of the AEM or inducing as high of a pH gradient due to more rapid self-purging of anions. We find experimentally that AEMELs with DI water anolyte are more tolerant of forced CO2 contamination than those with DI water catholytes, which is likely one of the major reasons for superior anode-feed performance. Furthermore, electrolyzer operation at high current densities can lead to voltage recoveries greater than 200 mV due to self-purging of anions. Although supporting electrolytes such as potassium hydroxide can mitigate catholyte contamination, the anion self-purging shows that electrolyzer operation in DI water and even tap water (containing fluoride, chloride, and nitrates) can improve when employing an anode feed. 1. Y. Zheng et al., Energy Environ. Sci., 12, 2806–2819 (2019). 2. B. P. Setzler, L. Shi, T. Wang, and Y. Yan, in ECS Meeting Abstracts, vol. MA2019-01, p. 1824–1824, IOP Publishing (2019) https://iopscience.iop.org/article/10.1149/MA2019-01/34/1824.