a review article

1. FUEL CELLS
REVIEW:
A major challenge—some would argue, the major challenge facing our planet today—relates to
the problem of anthropogenic-driven climate change and its inextricable link to our global
society’s present and future energy needs.
Hydrogen and fuel cells are now widely regarded as one of the key energy solutions for the 21st
century. These technologies will contribute significantly to a reduction in environmental impact,
enhanced energy security (and diversity) and creation of new energy industries. Hydrogen and
fuel cells can be utilised in transportation, distributed heat and power generation, and energy
storage systems. However, the transition from a carbon-based (fossil fuel) energy system to a
hydrogen-based economy involves significant scientific, technological and socioeconomic
barriers to the implementation of hydrogen and fuel cells as clean energy technologies of the
future.
Global drivers for a sustainable energy vision of our future centre on the need to: reduce global
anthropogenic carbon dioxide (CO2) emissions and improve local (urban) air quality, ensure
security of energy supply and create a new industrial and technological energy base, crucial for
our economic prosperity.
Fuel cells are emerging as a leading alternative technology to the more polluting internal
combustion engines in vehicle and stationary distributed energy applications. In addition, the
future demand for portable electric power supplies is likely to exceed the capability of current
battery technology.
Proton Exchange Membrane (PEM) fuel cells have been developed extensively since their
introduction over thirty years ago.
Composite membranes offer the capability of using a wide variety of ionomeric polymers that
may be mechanically too weak to use as freestanding films. These thinner membranes can
replace thicker non‐ reinforced membranes, thereby increasing performance while
simultaneously increasing durability. However, additional advancements will be necessary to
meet aggressive operating conditions of higher temperatures and/or lower humidities, as well as
longer operating lifetimes demanded in both stationary and automotive applications. In this
paper, these challenges for fuel cell membranes are considered.
PEM membrane requirements are discussed in terms of two different parameters: temperature
and relative humidity. The effect of these two operating parameters on the proton conductivity of
PEM fuel cell membranes and the resulting effect on fuel cell performance are examined using
experimental observations.
Although many polymers have been investigated over the years, starting with phenol sulphonic
and polystyrene sulphonic acid polymers. Although these polymers alone are prone to
mechanical failure in use, the introduction of composite perfluorinated sulphonic acid based
polymers, have greatly increased the durability and lifetime of perfluorinated sulphonic acid
membranes. In these materials, the perfluorinated sulphonic acid polymer is micro-reinforced

2. with a thin, porous, and strong polymer, for example polytetrafluoroethylene, making the
composite membrane stronger and more durable without adversely affecting its other properties.
Yet even with these improvements, additional advancements will be necessary to meet the
aggressive operating conditions and requirements demanded of fuel cells in both stationary and
automotive conditions. We will examine the future of PEM membrane requirements in terms of
two different parameters: temperature and relative humidity.
The markets and applications today and for the foreseeable future segment into three areas:
automotive, stationary and portable. Each of these has different operating requirements, which in
turn lead to different membrane needs. The automotive market will require higher temperatures
and drier operating conditions (Gasteiger and Matthias, 2002; Masten and Bosco, 2003). In
stationary systems, on the other hand, the primary requirements will be durability, power output,
high efficiency, and long life under relatively constant operating conditions. The portable market
is perhaps the least well defined and hardest to predict.
Higher temperatures affect ionomers used in fuel cell membranes in many ways. Proposed
operating temperatures between 100 and 2008C may be above the glass transition temperature of
some current membrane materials and are approaching the melting temperature of others. The
mechanical properties are weaker: tensile strength, compression strength, and puncture resistance
all decrease at higher temperatures. Fatigue and relaxation set in over time. One approach to
overcome these mechanical limitations is through the use of a mechanical reinforcement.
The chemical stability is also compromised at higher temperatures as degradation reactions are
expected to accelerate at higher temperatures. A lower RH inside the cell has negative effects on
the ionic conductivity of the membrane, which directly leads to a decreased performance
(Cleghorn et al., 2003). Electro-osmotic drag and water diffusion need to be balanced to maintain
membrane hydration in all areas of the fuel cell. Finally, the interface between the membrane and
catalyst layer may become disconnected at lower RH operation. This results in a poor
connectivity of the ionic phase, which could lead to slow transport of protons across the
membrane–electrode interface.
New membrane materials must be developed to reach the targets of the fuel cell industry. The
ionomer needs to be optimized to provide sufficient conductivity at low RH. A mechanical
reinforcement will be useful to sustain the integrity of the membrane under high mechanical
forces during fuel cell operation. It will be necessary to understand the location and transport of
the water inside the fuel cell to assess the influence of material properties on performance and
durability. The combination of careful fuel cell experiments and mathematical simulation will be
valuable to the development of new and successful membrane materials, which will meet the
challenges of future operating requirements.
Edwards, Peter P., Vladimir L. Kuznetsov, William IF David, and Nigel P. Brandon.
"Hydrogen and fuel cells: towards a sustainable energy future." Energy policy 36, no. 12
(2008): 4356-4362.
Beuscher, U. , Cleghorn, S. J. and Johnson, W. B. (2005), Challenges for PEM fuel cell
membranes. Int. J. Energy Res., 29: 1103-1112.