(511b) Nature-Inspired Flow-Fields and Water Management for PEM Fuel Cells

Nature-Inspired
Flow-Fields and Water Management for PEM Fuel Cells

Jason In Sung Cho^1,2, Toby
Neville^1,2, Panagiotis Trogadas¹, Billy Wu³,
Dan Brett^1,2 and Marc-Olivier Coppens¹

¹
Department of Chemical Engineering, University College London, London WC1E 7JE,
United Kingdom, m.coppens@ucl.ac.uk

²
Electrochemical Innovation Lab, Department of Chemical Engineering, University
College London, London WC1E 7JE, United Kingdom

³
Department of Mechanical Engineering, Imperial College London, UK

Flow-field design is crucial to polymer electrolyte membrane fuel cell (PEMFC) performance, since non-uniform
transport of species to and from the membrane electrode assembly (MEA) results in significant power losses^[1]. The long channels of conventional
serpentine flow-fields cause
large pressure drops between inlets and outlets, thus large parasitic energy
losses and low fuel cell performance^[2].

Here, a lung-inspired approach is used to
design flow fields guided by the structure of a lung. The
fractal geometry of the human lung has been shown to ensure uniform
distribution of air from a single outlet (trachea) to multiple outlets
(alveoli). Furthermore, the human lung transitions between
two flow regimes: 14-16 upper generations of branches dominated by convection,
and 7-9 lower generations of space-filling acini dominated by diffusion^[3,4].
The upper generations of branches are designed to slow down the gas flow to a
rate compatible with the rate in the diffusional regime (Pé ~ 1)^[5], resulting in uniform distribution of entropy production in both
regimes^[3,4].

By employing a three-dimensional
(3D)
fractal structure as flow field inlet channel, we aim to yield similar benefits
from replicating these characteristics of the human lung. The fractal pattern consists of repeating “H” shapes where daughter
“H’s” are located at the four tips of the parent “H”. The fractal geometry
obeys Murray’s law, much like the human lung, hereby leading to minimal
mechanical energy losses. Furthermore, the three-dimensional branching
structure provide uniform local conditions on the
surface of the catalyst layer as only the outlets of
the fractal inlet channel are exposed to the MEA.

Numerical simulations were conducted to
determine the number of generations required to achieve uniform reactant
distribution and minimal entropy production. The results reveal that
the ideal number of generations for minimum entropy production lies between N
= 5 and 7^[6]. Guided
by the simulation results, three flow-fields with N = 3, 4 and 5 (10 cm² surface area) were 3D printed via direct metal laser sintering
(DMLS), and
experimentally validated against conventional serpentine flow fields. The fractal flow fields with N = 4 and 5 generations showed ~20% and ~30% increase in performance and maximum
power density over serpentine flow fields above 0.8 A cm^-2 at 50% RH^[6]. At
fully humidified conditions, though, the performance of
fractal N = 5 flow-field
significantly deteriorates. The susceptibility towards
flooding of the high-generation fractal flow-fields is due to the lack of convective
gas flow requisite for effective liquid water removal, resulting in significant
water accumulation in the interdigitated outlet channels. This was recently confirmed
using neutron radiography in an in situ water visualisation study.

Another defining characteristic of the fractal approach
is scalability, which is an important feature in nature. Fractal flow-fields
can bridge multiple length scales by adding further generations, while
preserving the building units and microscopic function of the system. Larger,
3D printed fractal flow-fields (25 cm² surface area) with N =
4 are compared to conventional serpentine flow-field based PEMFCs. Performance results show that fractal and serpentine flow-field
based PEMFCs have similar polarization curves, which
is attributed to the significantly higher pressure drop (~ 25 kPa) of large
serpentine flow fields compared to fractal flow-fields.
However, such excessive pressure drop renders the use of a large scale
serpentine flow field prohibitive, thus favouring the fractal flow-field.

Lastly, a novel
water management strategy that draws inspiration from the passive directional
liquid water transport across the skin of the thorny devil is presented. The thorny devil is an Australian
lizard with ridged scales, which collects water from any part of its body by
simply touching water. The water is transported to the mouth across the skin
via capillary action. The phenomenon is based on geometric principles, namely
on a periodic pattern of interconnected capillary channels^[7]. Using a one-step processing
technique, we reproduced the passive water transport phenomenon through
fabrication of interconnected capillary channels on the surface of graphite
flow field. Preliminary results indicate more than a twofold improvement in
fuel cell power density with the integration of capillary channels onto the
surface of a parallel flow field, due to superior liquid water separation and
removal. Implementation of this water management
strategy could circumvent remaining problems of
high-generation fractal flow fields.

References

[1] Wu, H.-W., A review of recent
development: Transport and performance modeling of PEM fuel cells. Applied
Energy, 2016. 165: p. 81-106.

[2] Hsieh, S.-S., B.-S. Her, and Y.-J.
Huang, Effect of pressure drop in different flow fields on water accumulation
and current distribution for a micro PEM fuel cell. Energy Conversion and
Management, 2011. 52(2): p. 975-982.

[3] Kjelstrup, S., Coppens, M.-O., Pharoah,
J., and Pfeifer, P., Nature-inspired energy-and material-efficient design of a
polymer electrolyte membrane fuel cell. Energy & Fuels, 2010. 24(9):
p. 5097-5108.

[4] Trogadas, P., Ramani, V., Strasser, P.,
Fuller, T.F., and Coppens, M.-O., Hierarchically Structured Nanomaterials for
Electrochemical Energy Conversion. Angewandte Chemie International Edition, 2016.
55(1): p. 122-148.

[5] Coppens, M.-O., A nature-inspired
approach to reactor and catalysis engineering. Current Opinion in Chemical
Engineering, 2012. 1(3): p. 281-289.

[6] Trogadas, P., Cho, J.I.S., Neville,
T.P., Marquis, J., Wu, B., Brett, D.J.L., and Coppens, M.-O., A lung-inspired
approach to scalable and robust fuel cell design. Energy & Environmental
Science, 2018. 11(1): p. 136-143.

[7] Comanns, P., Effertz, C., Hischen, F.,
Staudt, K., Böhme, W., Baumgartner, Directional, passive liquid
transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’. Journal of The Royal Society Interface,
2015. 12(109).