Development of a High-Performance Ammonium Formate Fuel Cell

Abstract

Direct liquid fuel cells fueled by molecules such as methanol, ethanol and formate have received many attentions as proton exchange membrane fuel cells, primarily because of its high energy density, simple structure, small fuel cartridge, and ease storage and transport of fuels. Among them, formate, mainly referring to sodium formate or potassium formate, has drawn increasing interest due to the following advantageous characteristics: a high theoretical voltage of 1.45 V, facile and complete formate oxidation in alkaline media, and easy handle in storage and transportation. However, the cation ion (Na+ or K+) in the fuel solution does not contribute to electricity generation but only increase the extra weight load of the fuel cell system, thus lowering the energy density of formate to 2.1 kWh L-1. In this work, ammonium formate is adopted as the fuel, which not only retains the advantages of formate, but also substitutes the Na+/K+ with ammonium (NH4 +) that can be simultaneously oxidized to release electrons, i.e., electricity generation. Hence, the energy density ammonium formate is upgraded to 3.2 kWh L-1. Although promising, the development of ammonium formate fuel cell has been hindered by technical challenges related to the oxidation of ammonium formate, such as sluggish kinetics and electrode degradation. To tackle these issues, a bimetallic Pt-Pd/C electrocatalyst is synthesized by a facial chemical reduction method using sodium borohydride as agent. The Pt-Pd/C electrocatalyst exhibits better electrocatalytic activity for ammonium formate oxidation than pure Pt/C and Pd/C. Then, an ammonium formate fuel cell is developed and tested, which is composed of a Pt-Pd/C anode, a single-atom Fe/C cathode, and an anion exchange membrane (Figure 1). This fuel cell yields an open-circuit voltage (OCV) of 0.88 V and a peak power density (PPD) of 60.7 mW cm-2 at 60oC. When the hydrogen peroxide is used as oxidant, the OCV and PPD are boosted to 1.31 V and 99.6 mW cm-2, respectively. Further increasing the operating temperature to 90oC leads to a PPD of as high as 187.6 mW cm-2. In addition, a mathematical model incorporating mass/charge transport and electrochemical reactions is developed to shed light on the voltage losses and provide guidance for optimizing structural design and operating parameters.AcknowledgementThis work was supported by a grant the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. N_PolyU559/21) and The Hong Kong Polytechnic University Centrally Funded Postdoctoral Fellowship Scheme (Project No. 1-YXAZ).Figure 1 Schematic of an ammonium formate fuel cell. Figure 1