Anion-Exchange Membrane Fuel Cells with Improved CO₂ Tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption

Abstract

Over the last few decades, because of the significant development of anion exchange membranes, increasing efforts have been devoted the realization of anion exchange membrane fuel cells (AEMFCs) that operate with the supply of hydrogen generated on-site. In this paper, ammonia was selected as a hydrogen source, following which the effect of conceivable impurities, unreacted NH and atmospheric CO, on the performance of AEMFCs was established. As expected, we show that these impurities worsen the performance of AEMFCs significantly. Furthermore, with the help of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, it was revealed that the degradation of the cell performance was primarily due to the inhibition of the hydrogen oxidation reaction (HOR). This is attributed to the active site occupation by CO-related adspecies derived from (bi)carbonate adspecies. Interestingly, this degradation in the HOR activity is suppressed in the presence of both NH and HCO because of the bicarbonate ion consumption reaction induced by the existence of NH. Further analysis using in situ ATR-IR and electrochemical methods revealed that the poisonous CO-related adspecies were completely removed under NH-HCO conditions, accompanied by the improvement in HOR activity. Finally, a fuel cell test was conducted by using the practical AEMFC with the supply of NH-contained H gas to the anode and ambient air to the cathode. The result confirmed the validity of this positive effect of NH-HCO coexistence on CO-tolerence of AEMFCs. The cell performance achieved nearly 95% of that without any impurity in the fuels. These results clearly show the impact of the chemically induced bicarbonate ion consumption reaction on the realization of highly CO-tolerent AEMFCs.

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

“…300,302,309 Further, recent studies point to the use of small concentrations of NH 3 (<7%) in the anode H 2 feed led to marginal improvements in the CO 2 tolerance. 310 This was possibly due to the ability of NH 3 to capture HCO 3 − and convert it into a weakly adsorbing anion of the form H 2 NCO 2 − . This led to a local drop in the concentration of the HCO 3 − anion and improved the anode HOR activity.…”

“…As shown in Figure c, a direct carbonate cycle corresponds to the case where the working anion is CO 3 2– and no hydroxide anions are formed during AAEM operation; rather cathodic ORR involves direct formation of CO 3 2– which is transported to the anode followed by direct reaction with H 2 to form H 2 O. This is an ingenious design, but given the lower mobility of the carbonate anions compared to hydroxide anions, it is highly unlikely that the performance of AEM fuel cells operating on direct carbonate cycle could match those operating on direct hydroxide cycle at comparable conditions, unless the HOR and ORR reaction kinetics under the direct carbonate cycle is significantly better which appears not to be the case. ,, Further, recent studies point to the use of small concentrations of NH 3 (<7%) in the anode H 2 feed led to marginal improvements in the CO 2 tolerance . This was possibly due to the ability of NH 3 to capture HCO 3 – and convert it into a weakly adsorbing anion of the form H 2 NCO 2 – .…”

A review of the synthesis and characterization of anion exchange membranes

“…Reduced AEMFC/AEMWE performance is attributed to this carbonation reaction which increases membrane resistance and also enables the adsorption of carbonates on the anode catalyst layer [16,39,41]. CO 2 exposure to the AEM results in the carbonation reaction converting the ion-conducting group, a hydroxide ion, in the AEM to a larger carbonate ion which is four to five times less conductive compared to hydroxide [39,42].…”

“…Modelling work by Krewer et al [44] suggests that operating AEMFC at current densities greater than 1 A/cm 2 can significantly improve CO 2 tolerance; however, this is awaiting experimental validation. Additionally, recent work by Katayama et al [41] has investigated feeding a gas blend (e.g. ammoniahydrogen) at the anode to facilitate a HCO 3 consumption reaction and improving AEMFC CO 2 tolerance.…”

“…ammoniahydrogen) at the anode to facilitate a HCO 3 consumption reaction and improving AEMFC CO 2 tolerance. Katayama et al [41] suggest that low CO 2 tolerance in AEMFC is primarily due to carbonate species adsorbing on the hydrogen oxidation reaction catalyst at the anode, so by facilitating HCO 3 consumption at the anode, it removes the adsorbed species and frees the catalyst to perform its function, thus retaining AEMFC performance. As Krewer et al and Katayama et al have shown, the area of AEMFC/AEMWE CO 2 tolerance is rapidly evolving and shows great research potential to understand the carbonation mechanisms and mitigation strategies.…”