A team at University College London (UCL) has used a lung-inspired approach to designing flow fields to improve significantly the performance of polymer electrolyte fuel cells. The researchers, with colleagues from Imperial College London and Rensselaer Polytechnic Institute in the US, used the fractal geometry of the lung as the model to design flow-fields of different branching generations, resulting in uniform reactant distribution across the electrodes and minimum entropy production of the whole system.
In an open-access paper in the RSC journal Energy & Environmental Science, the researchers report that 3D-printed, lung-inspired flow field based PEM fuel cells outperformed conventional serpentine flow field designs at 50% and 75% RH, exhibiting a ∼20% and ∼30% increase in performance (at current densities higher than 0.8 A cm−2) and maximum power density, respectively.
In terms of pressure drop, fractal flow-fields with N = 3 and 4 generations demonstrated ∼75% and ∼50% lower values than conventional serpentine flow-field design for all RH tested, reducing the power requirements for pressurization and recirculation of the reactants.
They also found that the positive effect of uniform reactant distribution is pronounced under extended current-hold measurements, in which lung-inspired flow field based fuel cells exhibited the lowest voltage decay (∼5 mV h−1). Further, the enhanced fuel cell performance and low pressure drop values of fractal flow field design are preserved at large scale (25 cm2), in which the excessive pressure drop of a large-scale serpentine flow field renders its use prohibitive.
Background. Some of the engineering issues hampering broader commercialization of PEM fuel cells, especially for automotive applications, are the high cost of the electrocatalyst, durability, and performance limitations associated with unoptimized flow-field designs.
Current PEM fuel cell designs typically use serpentine flow fields to transport species to and from the membrane electrode assembly. However, the long channels of the serpentine flow fields cause large pressure drops between inlets and outlets, resulting in parasitic energy losses and lowering fuel cell perfromance.
… poor flow-field design can lead to channels becoming clogged with liquid water and non-uniform reactant distribution. Such mass transport issues can lead to the accumulation of excess water in the pores of the gas diffusion layer (GDL) and reactant starvation, which, in turn, can lead to corrosion of carbonaceous support material, electrocatalyst sintering and facile membrane degradation, all of which are detrimental to fuel cell longevity. Reactions of bulk and crossover gases at each electrode result in the formation of harmful radicals, which significantly hinder the oxygen reduction kinetics and oxygen/air transport through the polymer electrolyte.
Thus far, there have been two prevalent strategies reported in the literature to overcome the uneven reactant distribution issue in PEFCs. The first approach is based on empirical alteration of the channel configurations (such as channel path length, land width, and land/channel ratio), whereas the second approach imitates the apparent structure of biological organisms. The consensus to the first strategy is that the utilization of flow-fields with wider rib spacing, narrower and shorter channels and path length improves reactant distribution. However, these modifications tend to result in lower membrane hydration and membrane conductivity, a higher pressure drop as well as ineffective water and heat management.
These drawbacks to the first strategy have led to the exploration of an alternative route, taking “inspiration” from biological systems. The term “inspiration” is purposely enclosed within parentheses, since all reports to date imitate the apparent structure of a natural fluid distribution system (such as leaves, lungs, veins, etc.) without being fundamentally grounded in the underlying physical phenomena. Lack of a formal mathematical description and a methodology to inform the design of such flow-fields leads to difficulties in reproducing those designs, optimizing their channel geometries and scaling them up.
The UCL approach. The UCL researchers used a more systematic approach to design their flow fields, guided by a mechanistic understanding of the structure of the lung.
The researchers used direct metal laser sintering (DMLS) to 3D print three flow fields with 3, 4 and 5 fractal generations, using H-shaped branching. The final generation of H-shaped branches of the N = 3 and N = 4 flow-fields were left open to create additional contact area between the gas channel and the GDL. However, due to fabrication limitations, only the tips of the fifth-generation H-shaped branches were open to the GDL for the N = 5 prototype.
To sum up, the obtained experimental results show that the proposed nature-inspired approach can be successfully used to resolve uneven reactant distribution issues in PEFCs. The defining characteristic of the fractal approach, though, is scalability, which is an important feature in nature. This characteristic makes the proposed nature-inspired approach stand out among other, bio-mimetic techniques reported in the literature, even though advancements in 3D printing technology via DMLS are required to mass produce large fractal flow fields with a high number of generations.—Trogadas et al.
P. Trogadas, J. I. S. Cho, T. P. Neville, J. Marquis, B. Wu, D. J. L. Brett and M.-O. Coppens (2017) “A lung-inspired approach to scalable and robust fuel cell design” Energy Environ. Sci., doi: 10.1039/C7EE02161E