An alkaline direct borohydride fuel cell with hydrogen peroxide as oxidant

Abstract

A novel alkaline direct borohydride fuel cell (ADBFC) using varying concentrations of hydrogen peroxide as oxidant and sodium borohydride with sodium hydroxide, each of differing concentration, as fuel is reported. A peak power density of ca. 150 mW cm⁻² at a cell voltage of 540 mV can be achieved from the optimized ADBFC operating at 70 °C.

Introduction

Proton exchange membrane fuel cells (PEFCs) are subject to problems of carbon monoxide poisoning of anode [1], [2], [3] when using a reformer, and hydrogen storage when using a direct fuel. Therefore, alternative hydrogen-carrying liquid fuels, such as methanol, have attracted attention for fuelling PEFCs directly with methanol [4], [5], [6]. Such cells are referred to as direct methanol fuel cells (DMFCs). This use of methanol highly simplifies the engineering problems at the front end of the fuel cell and hence reduces system complexity and lowers cost [7]. Nevertheless, DMFCs have limitations of low open-circuit potential, low electrochemical activity, and methanol cross-over [8].

Attempts are being made to overcome the above limitations by using other hydrogen-carrying liquid fuels such as borohydrides [9], [10], [11], [12], [13], [14], [15], [16], e.g., sodium borohydride it at has a capacity of 5.67 Ah g⁻¹ and a hydrogen content of about 11 wt.%. Amendola et al. [12], [13] were the first to report an OH⁻-ion conducting anion exchange membrane-based borohydride–air fuel cell, which had a power density close to 60 mW cm⁻² at 70 °C. It was found that the cell suffered from borohydride cross-over as BH₄⁻-ions could pass through the anion-exchange membrane. In additions, it is mandatory to scrub carbon dioxide from the air inlet of such a fuel cell to avoid carbonate fouling. In order to mitigate borohydride cross-over, Suda [15] and Li et al. [16], [17] adopted a fuel-cell structure with a Nafion membrane as the electrolyte in order to separate the fuel from the cathode. But even in the borohydride-air cell reported by Suda [15], it would be compulsory to scrub carbon dioxide, not only to avoid carbonate fouling but also to prevent accumulation of alkali in the cathode pores so as to facilitate oxidant flux [15].

This communication describes an alkaline direct borohydride fuel cell (ADBFC) that uses hydrogen peroxide as oxidant to prevent carbonate fouling of the cathode. The use of hydrogen peroxide as oxidant in fuel cells also extends their operation to locations with limited air convection, e.g., underwater applications.