Tatsuhiro Okada

Direct methanol fuel cells (DMFCs), employing liquid methanol as a fuel, offer an attractive option in portable devices due to their simplicity in the system structure (easy storage and supply), no need for fuel reforming or humidification. For obtaining a higher power density, the membranes that show high proton conductivity, and at the same time, low methanol permeability are strongly desired. However, there is achieved only a little progress because of trade-off relations between these parameters. Also the membrane stability, particular to hydrolytic and chemical stability is recognised as a key factor that affects fuel cell performances. In the authors' recent work, they have been working on the design and the development of new families of cost-effective, readily prepared proton-conducting membranes based on chemically cross-linked PVA-PAMPS [poly(vinyl alcohol) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid)] composites. The authors have first introduced new concepts of secondary polymer chains such as "binary chemical cross-linking" or "hydrophobiciser" and the "stabiliser"effect. Also, the authors have established a new concept of PVA-PAMPS based semi-interpenetrating polymer networks (semi-IPNs) by incorporating plasticizer variants R (R = poly(ethylene glycol)(PEG), poly(ethylene glycol) methyl ether (PEGME), poly(ethylene glycol) dimethyl ether (PEGDE), poly(ethylene glycol) diglycidyl ether (PEGDCE)) and poly(ethylene glycol)bis(carboxymethyl)ether (PEGBCME) as the third components. Incorporation of the above concepts promoted not only the high proton conductivity , flexibility with low methanol permeability (1/3 - 1/2 of Nafion 117 membrane), but also the excellent hydrolytic and the oxidative stability of PVA-PAMPS composites. The membrane electrode assembly (MEA) fabricated with PVA-PAMPS composites has been successfully established, which showed the similar open circuit voltage (OCV) to that of Nafion 115, and a power density 52 mW cm-2 at 80oC. A striking feature of the long-term test was that no appreciable decay of the current density was observed during the whole operation time longer than 130 hours at 50oC, and so was the power density. This book is the first time that such long-term operation of DMFC was reported since PVA-PAMPS composite are all hydrocarbon membranes made simply of aliphatic skeletons. They are very different from the perfluorosulfonic membranes such as Nafion, or other reported membranes with aromatic skeletons. Therefore this affords the PVA-PAMPS composites unique structure compared to most of the proposed membranes, which suggests the good candidacy of PVA-PAMPS composites when they are intended for use in low temperature DMFCs.

In this book the authors focus on the ion and water transport characteristics in Nafion and other perfluorinated ionomer membranes that are recently attracting attention in various fields such as water electrolysis, mineral recovery, electrochemical devises and energy conversion. Methodology of measurements and data analysis is first presented that enables basic characterisation of transport parameters in the perfluorinated ionomer membranes. Cation exchange isotherm data are collected in binary cation systems, with the aim to see the behaviours of cationic species that exist with H+ in the membrane. Water transference coefficients, ionic transference numbers, ionic mobilities and other membrane transport parameters are measured in single and mixed counter cation systems using electrochemical methods. Diffusion coefficients of water and cations are also measured by pulsed-field-gradient spin-echo NMR (PGSE-NMR) at various temperatures in different kinds of perfluorinated ionomer membranes. The results are discussed in two perspectives. One is to predict the hydration state in perfluorosulfonated ionomer membranes in relation to the possible degradation of performances in fuel cells under contaminated conditions with foreign cations. An analytical formulation of membrane transport equations with proper boundary conditions is proposed, and using various parameters of membrane transport, a simple diagnosis of water dehydration problem is carried out. This analysis leads one to an effective control of fuel cell operation conditions, especially from viewpoint of proper water management. The others are to elucidate the ion and water transport mechanisms in the membrane in relation to polymer structures (e.g., different ion exchange capacity), and to propose a new design concept of polymer electrolyte membranes for fuel cell applications. Additionally for this purpose methanol and other alcohols are penetrated into the membrane, and alcohol permeability, membrane swelling, ionic conductivity and diffusion coefficients of water and CH3 are measured systematically for various kinds of membranes to cope with the problem of methanol crossover in direct methanol fuel cells (DMFCs). It is found that in order to realise a high ionic conductivity in the membrane, one should aim at a polymer structure through molecular design that takes into account the relative size of ions with a hydration shell against the size and atmosphere of ionic channels. For DMFC, a partially cross-linked polymer chain with high degree of hydrophilic ion transport paths based on phase-separated structures is recommended. Various possibilities of such polymer electrolytes are discussed.