Direct sodium borohydride fuel cells (DBFCs) present challenging electrochemistry due to the complexity of the electrocatalytic reactions at the cathode and anode plus the lack of a suitable membrane. The complex 8-electron oxidation reaction for sodium borohydride (NaBH₄) to borate (BO₂) at the anode is most efficient in alkaline electrolyte. However, the reduction of peroxide (H₂O₂) at the cathode is best carried out in acid electrolyte. The selection of whether to use an acidic or alkaline membrane has not been clear. Most researchers have used a perfluorosulfonic acid proton exchange membrane (PEM) as the membrane separator in the cells, but this becomes highly resistive when it becomes exchanged with sodium ions from the NaBH₄, serving as a cation exchange membrane (CEM). The standard membrane for DBFCs has been Nafion 117, which is 7-mils thick. This relatively thick membrane has been chosen by DBFC researchers to improve the separation between the acidic cathode electrolyte and the alkaline anode electrolyte. We determine herein how H₂O₂-DBFCs performance is improved when made with a commercial 0.8-mil-thick anion exchange membrane (AEM) or a 0.8-.mil-thick CEM, in comparison to cells with 7-mil-thick CEMs.

The H₂O₂-DBFCs design is based on a well-characterized flow-cell design (1). Both the anode and cathode catalyst layers are prepared with carbon-black supported Pt electrocatalyst (Pt/CB, 50 wt. % Pt). The compartments are separated by either a Na-exchanged CEM (Nafion® 117 or Nafion HP) or AEM (Fumasep FAA-3-20). The electrochemical performance of the cell and its impedance are measured at an operating cell temperatures of 25 and 60 ° C.

H₂O₂-DBFCs with the 0.8-mil-thick AEM have a marked improvement of the current density of the standard cells with the 7-mil-thick Nafion 117 membrane below 0.8 V at 25 ° C (Figure 1a). There is also a performance gain over the cell with the Nafion 117 membrane when the0.8-mil-thick Nafion HP is used, suggesting that the standard cells are highly limited by the resistance of the membranes. The trend changes at the more relevant operation condition of 60 ° C (Figure 1b). The H₂O₂-DBFCs with the Nafion HP membrane are the best performers with a maximum power density of ~600 mW cm^-2, followed by the cells with the AEM (~500 mW cm^-2), and then the Nafion 117 (~360 mW cm^-2).

The presentation will discuss these results, in combination with electrochemical impedance spectroscopy results, to reconcile how the thinner membranes improve cell performance and also affect the charge-transfer reactions at the electrodes.

References:

1. M. E. Hjelm, Y. Garsany, R. W. Atkinson III, R. O. Stroman, K. E. Swider-Lyons, C. Lafforgue and M. Chatenet, ECS Trans, 80, 1033 (2017).