(351g) Improving Performance in Alkaline Membrane Fuel Cells through Enhanced Water Management

Water management in anion
exchange membrane fuel cells (AEMFCs) is more complex than proton exchange
membrane fuel cells (PEMFCs).  In the PEM system, water is generated at the
cathode and otherwise only serves as the membrane proton transport medium. In
the hydroxide AEMFC, water is generated at the anode, consumed at the cathode,
and is greatly needed for transport of the larger hydroxide ion. The challenge
this introduces is the need to provide adequate water to maintain the membrane
humidity without flooding the catalyst or gas diffusion layers.¹ One method being used to achieve the necessary high membrane water
content is utilizing hydrophilic gas diffusion layers without microporous
layers.²  Others have used gas diffusion layers with a hydrophobic
coated microporous layer while raising the temperature of the humidifiers well
above the cell temperature.³ Gas stream dew points above 100%
relative humidity and back pressure are common approaches in increasing the
membrane water content.  However there is instability in the cell performance
and conditions that generate high performance levels are not easily
repeatable.  The problem is the line between proper membrane hydration and
flooded catalysts layers is very thin, and non-existent in some cases.

In this study, the influence of
the membrane, ionomer and gas diffusion layer as well as the flow rate and dew
points of the anode and cathode gases on AEMFC performance were explored.
Tokuyama A201 membranes and AS-4 ionomer were investigated alongside quaternary-ammonium-functionalized
radiation-grafted ETFE alkaline anion-exchange membranes and ionomers.⁴  Using a hydrophilic gas diffusion layer without a microporous layer
increased membrane hydration, but as expected increased the potential for
flooding. Manipulating the dew points led to the counter-intuitive discovery
that the cell performs better with the humidity higher at the anode than the
cathode, despite water generation and electro-osmotic drag towards that
electrode.  In fact, removing too much water from the anode caused instability
in the cell, while increasing the water at the anode decreased the membrane
resistivity. Back diffusion likely plays an important role in membrane hydration
and hydroxide transport through the membrane. A very high flow rate (1.0 L/min)
also increased cell performance, despite being several orders of magnitude
above the stoichiometric need.

Combining all areas of
improvement resulted in a very high performing AEMFC with a maximum current
density of 2.2 A/cm² (at 0.1 V) and max power density of 670 mW/cm²
(880 mW/cm² iR-corrected) with a membrane resistivity of 75 mOhms*cm²
(Figure 1a). Only a minor drop in the current was observed using air at the
cathode with the same 1.0 L/min flow rate as oxygen, giving max current density
of 1.7 A/cm² and a max power density of 580 mW/cm² with a
resistivity of 74 mOhms*cm² (Figure 1b). This near identical
behavior confirms that the amount of reactant present supplied by the higher
flow rate is not necessary, but the volumetric flow rate is needed for water
management. It is likely that the pressure drop along the single pass cell
hardware allows the gas to “jump the bar” only at very high flowrates, which
results in better water removal and limits cell flooding in the cell.

References:

1. T. D.
Myles, A. M. Kiss, K. N. Grew, A. A. Peracchio, G. J. Nelson and W. K. Chiu, J.Electrochem.Soc.,
158, 7 (2011).

2. R. B.
Kaspar, M. P. Letterio, J. A. Wittkopf, K. Gong, S. Gu and Y. Yan, J.Electrochem.Soc.,
162, 6 (2015).

3. M.
Mamlouk, J. Horsfall, C. Williams and K. Scott, Int J Hydrogen Energy.,
37, 16 (2012).

4. S. D.
Poynton, R. C. Slade, T. J. Omasta, W. E. Mustain, R. Escudero-Cid, P. Ocón and
J. R. Varcoe, Journal of Materials Chemistry A., 2, 14 (2014).