Performance of Pd/Graphene Nanocomposite Catalysts in H2/O2 Anion Exchange Membrane Fuel Cells

Abstract

With the recent developments of high-performance anion-exchange membranes (AEM), there has been an increased interest towards studying various electrochemical reactions such as ethanol electrooxidation, hydrogen oxidation reactions and oxygen reduction reactions in AEM fuel cells (AEMFC). This increased attention has been attributed to the faster electro oxidation kinetics, increased stability and lower poisoning of the catalysts in alkaline media. And since AEMFCs are not limited to platinum - unlike in proton exchange membrane fuel cells (PEMFCs) with highly acidic environments - intensive efforts towards developing alternatives such as palladium (Pd) and Pd-based alloys are currently underway.1 , 2 Various studies have shown the enhanced stability and higher activities of Pd-based catalysts compared to Pt in alkaline medium using potentiodynamic and potentiostatic techniques. However, there are only a limited number of studies that have investigated the performance of Pd-based electrocatalysts in AEMFCs. Also, majority of these studies have demonstrated relatively low performances in AEMFCs, mainly due the complications associated with forming the triple-phase boundary (TPB) structure in the membrane layer, where reactions are taking place in the electrolyte, gaseous fuel and electrode interface. The density of the TPB, along with the intrinsic activities of the catalysts can play an important role in determining the overall performance of AEMFCs. Moreover, Pd-based are usually synthesized using surfactants, organic stabilizers or reducing agents, that can get adsorbed onto the surface of Pd and inhibit ionomer-catalyst-fuel interactions in the TPB. To mitigate the limitations associated with TPB structure and subpar catalyst activities, macroporous three-dimensional Graphene nanosheet (3D-GNS) supports with controlled morphologies and porosities were fabricated by utilizing silica sacrificial templates.3 The sacrificial templates were then etched to leave a network of porous channels within its matrix, and utilized as supports for Pd nanoparticles. The porous structure of these highly graphitized 3D-GNS supports can facilitate mass-transport kinetics by enabling the ionomer and polymer electrolyte getting the reactants to get catalyzed by Pd nanoparticles. Also, to enhance ionomer-nanoparticle interactions, the Pd nanoparticles of an average size of 3-5 nm were synthesized using a previously established surfactant-free soft alcohol reduction method.4 In this study, anion exchange catalyst-coated membranes (CCMs) were prepared using Pd nanoparticles supported on both commercial carbon blacks (Vulcan) and 3D-Graphene supports as both cathode and anode catalyst in H2/O2 fed AEMFCs. The Pd catalyst inks were prepared by mixing the dispersed Pd-catalyst in an optimized solution of isopropyl alcohol and quaternary ammonium-functionalized AS4 ionomer. The inks were then applied onto an anion exchange membrane (A201, Tokuyama) with an active area of 5 cm2. The CCMs was then hydrated in 0.5 M KOH for 24 hours, followed by rinsing in de-ionized water for 24 hours. It was then sandwiched between gas diffusion layers and annealed with gaskets by pressing under 500 psi for 5 minutes. The cell was then assembled and activated by operating at 0.3V at 60ºC for 10 minutes under humidified O2 and H2, followed by the measuring the polarization curves. Preliminary results show that Pd nanoparticles synthesized using surfactant-free method SARM and deposited on commercial carbon supports have and OCV of 0.95 V and a peak power output of 200 mWcm-2. These results already show the promising potential of using surfactant-free non-alloyed monometallic Pd nanoparticles. Further tests will also be performed to show the effect of porous 3D-Graphene supports towards modifying the TPB interphase and its effects on MEA performance in both Ethanol/O2 and H2/O2 fed AEMFCs. The results from this study will not only contribute towards the development of Pt-free oxidative and reductive electrocatalysts, but also lead to further advancements in AEMFC technology that will enable it to operate with other fuels such as methanol and hydrazine.