Abstract
Polymer electrolyte fuel cells (PEFCs) are considered to be one of the prospective power sources for automobile and domestic use due to their outstanding energy conversion efficiency and compactness. Recently, anion-exchange membrane fuel cells (AEMFCs) are promising due to the practicablity of eliminating the need for platinum-group metal catalysts, and employing non-precious metal electrocatalysts such as nickel, cobalt, silver are feasible, conducing a potentially low-cost technology. Also, an alkaline environment can inherently enhance the electrochemical kinetics of reaction such as the oxygen reduction reaction (ORR) on cathode, enabling higher power density and energy conversion efficiency. Hence, AMEFCs are attracting strong interest as the substitutes of proton-exchange membrane fuel cells requiring platinum-group metal catalysts[i]. However, the AEMFCs are still suffer from low performance and low OH- conductivities. In order to enhance the performance and durability, it's required to enhance the ionic conductivity of anion-exchange electrolyte for both membrane and electrocatalyst. Typically, the fabrication of anion-exchange electrolyte is required for both polymer main chains and side chains which include cationic functional groups and charged anions[ii]. In order to enhance the anion conductivity, increasing the cationic density of anion-exchange electrolyte was considered to be one of the promising method for designing anion-exchange electrolyte with superior performance[iii].However, the critical drawback of anion-exchange electrolyte with high cationic density of side chains is their water solubility, due to their hydrophilicity of ionic moieties, which may cause the loss of anion-exchange electrolyte by leaching in water generated in the humidified AMEFCs cell.
To overcome this issue, approach of hydrophobic part was introduced by condensation copolymerization to inhibit their water solubility and leaching, while, the reduction of ionic conductivity has been found. Also, another approach of crosslinking by directly casting on the electrode to insolubilize the electrolyte was reported[iv], while the uniform modification of the surface of the electrocatalyst is probably difficult.
In our previous studies, we suggested an anchoring of the anion-exchange electrolyte on a polymer backbone, which can prevent the leaching of the anion-exchange electrolyte with high cationic density, uniformly and individually wrap on the surface of carbon supporting materials. We chose multi-walled carbon nanotubes (MWNT) as the carbon supporting material since they provide an outstanding electrical conductivity and lower impurities[v]. We used polybenzimidazole (PBI) as polymer backbone of anion-exchange electrolyte since we found that PBI can uniformly and individually wrap MWNT based on π-π interactions. Since high yield chemical modification of PBI on MWNT has already being reported, this strategy is expected to homogeneously decorate supporting material by anion-exchange electrolyte, which can probably offering a high ionic conductivity. Furthermore, PBI was chosen because Pt nanoparticles can be loaded onto the PBI wrapped MWNT (MWNT/PBI) with a homogeneous distribution in high yield.
Based on this strategy, we studied on grafting approach by anchoring 1,4-diazabicyclo[2,2,2]octane (DABCO) polymer moieties as anion-exchange electrolyte onto MWNT/PBI (MWNT/PBI-g-DABCO), and loaded Pt nanoparticles onto MWNT/PBI-g-DABCO to fabricated MWNT/PBI-g-DABCO/Pt as an electrocatalyst for AEMFCs. This approach enabled to developed anion-exchange electrolyte for AEMFCs with high content of cationic moieties and water insolubility[vi].
In this study, instead of grafting approach, we designed a new approach to introduce cationic group by methylation of PBI, due to the advantages of convenient synthesis, high potential of ionic conductivity[vii], and excellent durability under alkaline environment[viii]. Hence, we fabricated methylated polybenzimidazole (mPBI) as electrolyte for AEMFCs electrocatalyst as well as the membrane (Fig. 1). Single cell test of membrane electrode assembly (MEA) using mPBI as electrocatalyst wrapped on MWNT was performed using H2 and conditioned air as the fuel for anode and cathode, respectively, and power density of 24.6 mWcm-2was obtained.
[i] Z. Chen et al., Int. J. Hydrogen energy 2014, 39, 18405.
[ii] K. Nijmeijer et al., J. Membrane Sci. 2011, 377, 1.
[iii] M. A. Hickner et al., Macromolecules 2013, 46, 9270.
[iv] Park et al., Int. J. Hydrogen Energy 2014, 39, 16556.
[v] N. Nakashima et al., Small. 2009, 5, No. 6, 735.
[vi] N. Nakashima. Submitted.
[vii] Sun et al., Int. J. Hydrogen Energy 2011, 36, 11955.
[viii] S. Holdcroft et al. J. Am. Chem. Soc. 2012, 134, 10753.
Figure 1