Proton exchange membranes for fuel cells
This project aims to develop a new class of nanostructured proton exchange membranes (PEM) to apply to low temperature fuel cells (PEMFC) and direct methanol fuel cells (DMFC). The strategy for producing this new class of membranes is to exploit the unique properties of nanoscale particles of metal phosphates and silicates, hybridised with proton-conducting polymers. By producing proton-conducting materials with high proton-conductivity, low gas permeability and good thermal and mechanical integrity, this research will lead to great improvements in the efficiency, reliability and economics of both hydrogen and methanol fuel cells, promising a revolutionary clean energy supply particularly for transport vehicles and mobile devices.
Major achievements
Nanocomposite PEM membranes using a phosphorus silicate system were further developed to achieve optimised processing conditions.
Investigations on the optimised PEMs were conducted using a membrane electrode assembly (MEA) to obtain their electrochemical performance.
A new design for a MEA for hydrogen and direct methanol fuel cell applications (a mi-Cel) that enables dry PEM membranes to be used effectively in fuel cells was developed further.
A set of membrane electrode assemblies was constructed to test the various materials identified as promising for fuel cell applications from the previous years’ research.
Research highlights in 2007
Hydrolytically stable phosphorylated hybrid silicas
Based on the prior work on proton conducting sol–gel titanium phosphates, we have developed a new approach to the synthesis of fully immobilised phosphorus functionalised hybrid proton conductive gels using phosphonic acid grafting. The hybrid silicas with different amounts of phosphonic acid have been prepared and characterised using FTIR, XPS, BET, TGA and electrochemical techniques. The proton conductivity of the materials depended strongly on hydration, increasing by four orders of magnitude over the relative humidity (RH) range 20-100%, up to a maximum of 0.027 S/cm at 100 °C and 100% RH.
For these reported samples, proton conduction is believed to occur within a dynamic hydrogen bond network formed by functionalised P-OH groups and water molecules via the Grotthuss mechanism. However, the proton conductive sites (P-OH) are likely to be partially immobilised by strong protonic receptors (N atoms in amines), which reduces the free P-OH groups and restricts proton transfer. Hydration may cause a bonding structural rearrangement resulting in more free P-OH groups as active proton conductive sites and therefore greatly increased proton conductivity was observed. Stable solid acid proton conductors such as these have the potential to extend the operating temperature of PEM fuel cells beyond the glass transition temperatures of the current polymer technology.
Development of mi-Cel membrane and electrode Assembly
We also further tested a new design for testing solid PEM materials such as titanium phosphate in a cell called mi-Cel. A Membrane and Electrode Assembly (MEA) based on this design was fabricated and tested. The new system was found to function as designed and is currently being used for testing a number of new proton conducting membranes in hydrogen fuel cells.
Proton conductivity measurements were performed in a two-electrode AC impedance technique using a Solartron 1260 frequency response analyser. The sample pellet was placed between two gold-plated blocking electrodes in a home-made test cell, as shown in the figure below. The cell allowed the measurement of the conductivity up to 150°C with adjustable humidity. A variety of RH can be realised by controlling vapour pressure in the cell, which was generated by heating a small quantity of water on the bottom of the cell up to various temperatures. The water was stirred to keep a homogenous temperature while heating.
Highly Proton Conductive Phosphonic Acid Functionalised Silicas (PAFS)
Highly proton conductive silicas with phosphonic acid functional groups were synthesised from diethylphosphatoethyltriethoxysilane (DPTS) and tetraethoxysilane using a sol-gel method, followed by hydrolysis treatment of phosphonate groups. Samples with different contents of phosphonic acid have been prepared and characterised using Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric-differential thermal analysis, nitrogen sorption, transmission electron microscopy, and electrochemical techniques. The results show that various structures, either mesoporous or nonporous, are formed by varying the addition content of DPTS. Comparatively high thermal stability was evident when used as the fuel cell electrolyte.
The proton conductivity of the samples was measured systematically from room temperature to intermediate temperatures up to 150°C under varying relative humidity (RH). It was found that proton conduction was still water-dependent, but the conductivity at reduced humidity conditions has been much more enhanced than the materials with phosphonic acid functionalisation reported in earlier work. The sample with the largest incorporation of phosphonic acid exhibits the highest conductivity at 150°C ranging from 4.4×10-4 S cm-1 at 20% RH to 0.031 S cm-1 at 100% RH. In particular, it should be noted that the sample with a very high surface area shows a distinct improvement in conductivity at reduced hydration. Both the vehicle mechanism and the hopping mechanism are believed to contribute to the proton conduction. The proton transport was described as dependent on the density of functional phosphonic acid and the pore structure.
Future plans and directions
Further develop and optimise the highly proton conductive phosphonic acid functionalised silicas (PAFS) based on phosphorus silicas to achieve even better performance in proton conduction, and obtain optimised processing conditions. Detailed optimisation and engineering proof of the concept will be the major focus for the composite nanofunctionalised membranes for PEM and DMFC technologies. Future work will involve performance enhancement by improving and maintaining nanoparticle functionalisation, and reducing further methanol crossover, whilst improving membrane film technology.
