From the development of anion exchange membrane fuel cells it is known that airborne CO₂ is easily absorbed in this type of membranes and has a detrimental effect on the conductivity of the membrane¹. It was also found, however, that at sufficiently high current densities carbonate and hydrogen carbonate ions are flushed out of the membrane and ion conduction occurs predominantly via hydroxyl ions¹. This behavior led to the idea of using AEM based cells for gas separation for two different type of applications. So, during the first waves of COVID-19 medical oxygen became a scarce resource. Local production of oxygen by separation from air is an interesting approach to overcome this issue. This should be feasible using an anion exchange membrane cell with an air side oxygen reduction electrode and an oxygen evolution electrode. The hope is that in combination of these two reactions the required cell voltage can be kept much below typical water electrolysis voltages so that operation with simple PV cells or modules is possible. At the same time the question is if current densities can be achieved at which the OH^- transport and thus the anodic release of O₂ dominates, as an enrichment of CO₂ must be avoided for this application. On the opposite there are worries about the effect of increasing CO₂ concentration in occupied confined rooms like vehicles. Here a device that removes CO₂ from air without consuming much oxygen is desirable. The removal of CO₂ from the air supply to AEM fuel cells by an additional cell placed upstream of the cathode inlet and downstream of the anode outlet was demonstrated by Matz at al.². As for CO₂ separation only, no hydrogen is available for the anode reaction here again the operation against an oxygen evolution electrode is required, optimized, however, towards the CO₂ transport minimizing O₂ transport.

In this study for one type of AEM membrane and ionomer, namely Fumapem FAA3 and Fumion (both Fumatech BWT, Germany) the effect of different cathode electrodes and electrode materials as well as the effect of operation conditions towards the selectivity of oxygen or CO₂ transport were investigated. For that purpose, test set-ups were used were the cathode side was supplied with synthetic air, compressed air or synthetic air with added CO₂ and the anode side with an inert gas N₂ or helium. Nass spectrometry and dedicated CO₂ concentration sensors were used to determine the anodic release of O₂ and CO₂ as well as the reduction of O₂ and CO₂ concentrations on the cathode side.

First measurements of the oxygen transfer using a cell with a silver cathode (Oxag, Gaskatel, Germany) and a nickel foam anode (Gaskatel, Germany) it was found that the oxygen transfer had a promising faradaic efficiency exceeding 80% (cf. Fig. 1). The cell voltage of 1.5 V at 90 mA cm^-2 was however still too high for the application. Replacing the Oxag silver electrode by an industrial ORR electrode for alkaline cells did not yield an improvement (cf. fig 2). Increasing humidification to 100 % r.H. improved the current density (cf. Fig 3). Using high humidification conditions, the CO₂ effect was studied by switching from synthetic air to pressured air and back and 160 mA cm^-2. A rapid increase of the anodic CO₂ concentration to values exceeding 2000 ppm was observed (cf. Fig. 4). Comparing CO₂ transfer for cells with Oxag cathode and the commercial ORR cathode again showed that the latter would not improve the situation. It proves however, that the CO₂ transfer rate can be influenced by the cathode catalyst. Regarding the CO₂ separation test with synthetic air with added 3.85 Vol% of CO₂ as cathode feed have shown that under such conditions, mainly CO₂ is transported (cf. Fig. 5). In total it will be shown that CO₂ separation with state-of-the-art materials is already possible. The use of O₂ separation regarding onside production of medical O₂ today fails due to still too high CO₂ selectivity. The fact that CO₂ selectivity seems to be dependent on the cathode catalyst may allow further optimization also valuable for AEMFC fuel cell developments.

Acknowledgment: The work received financial support in parts from the Fraunhofer-Gesellschaft by the Demo-medVer project part project e³C-O₂ as part of the Fraunhofer cs. Corona program.

1. M. Inaba, Y. Matsui, M. Saito, A. Tasaka, K. Fukuta, S. Watanabe and H. Yanagi, Electrochemistry, 79, 322–325 (2011).

2. S. Matz, B. Setzler, C. M. Weiss, L. Shi, S. Gottesfeld and Y. Yan, Meet. Abstr., MA2020-02(34), 2227 (2020).