Applications of Novel Materials in Direct Formic Acid Fuel Cells

Abstract

The aim of this study was to employ different materials within a direct formic acid fuel cell (DFAFC) to address three key challenges hampering its performance, dehydration of the proton exchange membrane (PEM), catalyst deactivation and catalyst support degradation. The employment of different sulfonated tetrafluoroethylene based fluoropolymer membranes of the Nafion brand in the configurations 117, 115, 211 and XL provided an opportunity to address the dehydration affecting the performance of the fuel cell by understanding the behaviour of all the membranes under DFAFC operating conditions. An image analysis protocol was first developed to observe the effect of formic acid over the PEMs whereas electrochemical analysis provided detail on power generation as well as de effect of the dehydration over the membranes under DFAFC operation. The results demonstrated that Nafion not only displayed the lowest dimensional change (10%) compared to the other membranes but also exhibit the highest performance at room temperature reaching power densities of 146 mW/cm2 with almost no dehydration observable outperforming the reported 110 mW/cm2 obtained under similar operating conditions. Catalyst related challenges were addressed first by the development of graphene oxide, catalyst support, through a more environmentally friendly approach to the current chemical exfoliation method (Hummers method) by the employment of two different methods of electrochemical exfoliation of graphite. Analytical techniques such as Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) presented information related to the structure of the materials as well as their chemical composition to verify the potential use of the diverse materials synthesised for catalyst application. Among the materials synthesised the material produced by a two-step exfoliation in ammonium sulphate presented the highest degree of defects in the graphene Sp2 structure as well as the highest content of oxygen functional groups, particularly in C-O-C groups, indicating that the dispersibility of the material in polar solvents was significantly high a characteristic highly necessary for subsequent employment as catalyst support. The synthesis of Pd supported catalysts for electrocatalytic oxidation of formic acid was conducted through a green method using l-ascorbic acid as a reduction agent on three different carbon supports, graphene oxide produced by the Hummers method (Pd/rGH), graphene oxide produced by a two-step exfoliation in ammonium nitrate (Pd/rEGO) and carbon black (Pd/CB). The reduction of the graphene oxide to reduce graphene oxide was confirmed through XPS, which also indicated the presence of metallic Pd(0) on the structure of the materials. Surface morphology was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques from which the last one indicated a mean particle size of 7-9 nm and a polycrystalline structure of the particles data also supported by X-ray diffraction (XRD) analysis. Electrochemical performance of all materials was evaluated through cyclic voltammetric (CV) techniques as well as for the electrooxidation of formic acid. Results of the materials synthesised indicated that the Pd/rEGO displayed good catalytic activity towards formic acid oxidation as well as high electrochemical active surface area (ECSA). Accelerated degradation tests (ADT) performed electrochemically as well as results obtained through Scanning electrochemical microscopy (SECM) also indicated that catalyst Pd/rEGO remained active even after 12 hours of constant degradation outperforming commercially available Pd supported on Vulcan XC-72 (Pd/XC-72) catalysts, demonstrating that the material is a promising catalyst for DFAFCs application.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Holmes (Supervisor) & James Winterburn (Supervisor)