Effect of anion functional groups on the conductivity and performance of anion exchange polymer membrane fuel cells

Abstract

A study of the effect of anion functional group on the conductivity and performance of alkaline anion exchange membrane fuel cells (AAEMFCs) is reported.

Membranes and ionomer were characterised in terms of ionic conductivity and separate anode, cathode and cell performances. TMA functionalised (LDPE-co-VBC) or PVBC offered the highest conductivity amongst the various selections of amine/sulphide-based functional groups with conductivities values up to 0.25 (in plane) and 0.043 S cm⁻¹ (through plane). Sulphide-based groups showed lower stability with temperature in comparison to amine-based groups. The increase in length of the chain in the aryl group attached to the nitrogen or sulphur led to lower conductivity. The high OH⁻ conductivity in membranes functionalised with TMA is reflected by its low activation energy of 12 kJ mol^−1; close to the reported value for H⁺ conductivity in Nafion.

The ionomer functional groups affected oxygen permeability, the activation energy and the exchange current density for oxygen reduction. TMA functionalised ionomer provided an improved medium for the oxygen reduction reaction with exchange current density some 300 times higher than that with DMS. Anode flooding appeared to severely restrict cell performance and was determined by the type of functional group used for the ionomer. TMA functionalized electrodes showed superior cell performance with a current density of 0.722 A cm⁻² at 0.6 V and a peak power density of 478 mW cm⁻².

Introduction

Solid (cation-free) OH⁻ ion conducting polymer AEMs could hold the key answer to many of the limitations of Proton Electrolyte Membrane Fuel Cell (PEMFC). AEMs exhibit several advantages over PEMFCs including: the oxygen reduction reaction (ORR) is faster under alkaline conditions than in acidic conditions therefore providing lower activation losses [1], non-precious metal catalysts (NPMCs) can be used quite effectively [2], [3], increased number of inexpensive materials for cell components due to less corrosive environment [4]. Other major issues with PEMFCs, of water management, crossover and cathode flooding are potentially addressed in Alkaline Electrolyte Membrane Fuel Cells (AEMFCs) by water and ion transport away from the cathode to the anode, mitigating crossover and flooding problems [5].

A large number of electro-active polymers have been studied to prepare modified electrodes such as chloromethylstyrene [6], [7], 2,4,5-trichlorophenyl acrylate [8], polyacrylamides [9], quaternized poly(ether sulfone) PES [10], [11], [12], quaternized poly(2,6-dimethyl-1,4-phenylene oxide) PPO [13], quaternized poly(phthalazinone ether sulfone ketone) PPESK [14], and quaternized poly(phenylene) [15], [16]. Poly(Chloromethyl Styrene) or Poly(Vinylbenzyl Chloride) PVBC is one of the widely used as base polymer for anion exchange membranes [17], [18], [19].

Alkaline electrolyte ionomers are solid polymer electrolyte membranes that contain positive ionic functional groups (e.g. quaternary ammonium (QA) functional groups such as poly – N⁺CH₃) and mobile negatively charged anions (e.g., usually OH⁻). While quaternary ammonium based functional groups are the most commonly used for anion exchange ionomers, other functional groups include tertiary and mixed amine, organic sulfides [20], phosphonic, secondary phosphate, and carboxylic groups.

Ammonium groups were thought to have a higher thermal and chemical stability compared to phosphonium or sulfonium groups [21]. However, it was proved that phosphonium groups have demonstrated their potential and seem to be more stable towards attack by the hydroxide ion than the more conventional quaternary ammonium [22].

Park et al. [23] investigated the effect of the length of alkyl chain of the diamines on membrane properties such as ion conductivity and thermal characteristics. Poly(sulfone) and MEAs were aminated by mixing amine agents of trimethylamine (TMA) as a monoamine and various diamines such as N,N,N′N′-tetramethylmethanediamine (TMMDA), N,N,N′N′-tetramethylethylenediamine (TMEDA), N,N,N′N′-tetramethyl-1,3-propandiamine (TMPDA), N,N,N′N′-tetramethyl-1,4-butanediamine (TMBDA) and N,N,N′N′-tetramethyl-1,6-hexanediamine (TMHDA). They concluded that mixing TMA and TMHDA (with longer alkyl chain) showed better hydroxyl ion conductivity and thermal stability than those aminated by a diamine with peak power densities of 30 mW cm⁻² with air using 0.5 mg cm⁻² Pt/C at the anode and the cathode, respectively.

Comparative analysis of the alkaline stability of AAEM prepared with trimethyl, triethyl, tri-n-propyl- and tri-n-butyl ammonium groups showed that as the chain length of alkyl groups bonded to ammonium groups increased the loss of ion-exchange capacity was significant [24].

Komkova et al. [25] prepared a series of anion exchange membranes from chloromethylated polysulfone and aliphatic diamine compounds. They showed that the quaternization with diamines with long aliphatic chain of the alkyl groups bonded to amine nitrogen require lower excess of diamine to produce membrane with low electrical resistance and high perm-selectivity. It was generally shown that bi-quaternization with diamine was more preferable than mono-quaternization. However, an exception was found for the di-amine with bulky substitute at the amine nitrogen.

We have reported [7] an increase of around 60% of the power density at 0.4 V when TMA was used instead of TMHDA at 60 °C.

In this work, the effect of functional groups has been studied in terms of OH⁻ conductivity and ionomer suitability for fuel cell electrodes.