Comparison of Direct Sodium Borohydride Fuel Cells with Cation and Anion Exchange Membranes

Abstract

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.