(Invited) Efficient pH-Gradient-Enabled Microscale Bipolar Interfaces in High Performance Direct Borohydride Fuel Cells

Abstract

Electrochemical energy conversion devices usually operate at uniform pH or over a narrow pH range. The uniform pH environment of these devices frequently hampers the facility of one of the half-cell reactions. In proton exchange membrane fuel cells (PEMFCs)/ proton exchange membrane water electrolyzers (PEMWEs), where both the anode and cathode operate at acidic pH, the H2 oxidation/evolution reaction is quite facile while the O2 reduction/evolution reaction is sluggish. In anion exchange membrane fuel cells (AEMFCs)/ anion exchange membrane water electrolyzers (AEMWEs), the converse is true. Moreover, the chemical stability of some reactants depends strongly on the pH. Direct borohydride fuel cells (DBFCs), operating with NaBH4 as a fuel and H2O2 as the oxidant have attracted a lot of attention due to their high theoretical cell voltages and tactical advantages, particularly for defense-related applications. DBFCs have a high energy density of 9.3 kWh kg-1 and specific capacity of 5.67 kAh kg-1 based on NaBH4 oxidation in alkaline media [1], although the practical energy density will be lowered upon accounting for the added weight of KOH solution required to stabilise NaBH4. Using liquid hydrogen peroxide as the oxidant, the DBFC is a safe and attractive low temperature power source for unmanned underwater vehicles (UUVs) as they exhibit excellent energy and power density, are safe, given the high flash point of NaBH4, and do not release any gaseous effluents, enabling operation at neutral buoyancy [2]. One challenge for this system is the separation between the anolyte fuel (1.5M NaBH4 in 3M KOH) and catholyte oxidant streams (15% H2O2 in 1.5M H2SO4), while another is the high instability of NaBH4 in acidic or even neutral pH [3]. Herein, a pH-gradient-enabled microscale bipolar interface (PMBI) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The PMBI employed to separate anolyte and catholyte was based on an anion exchange membrane (AEM) interface in conjunction with a cation exchange membrane (Nafion®) separator. The DBFC with the PMBI yielded a current density of 330 mA/cm2 at 1.5 V and a promising peak power density of 630 mW cm-2 at 1 V in a 25-cm2 active area cell [4]. Using a recessed planar electrode (RPE) in conjunction with TEM, we show that the PMBI maintained a sharp local pH gradient (0.82 pH units/nm on average) at the electrocatalytic reaction site.