CMU researchers rule out one potential cause of resistance in polymer electrolyte fuel cells; R&D guidance toward commercialization
14 September 2015
Researchers at Carnegie Mellon University have discovered how nanoscale polymer films limit future cost reductions in fuel cell electric vehicles. Moving forward, this result will direct research and development efforts to address the problem of the electrolyte acid’s interaction with the platinum instead of focusing on the oxygen transport properties. The goal will be to accelerate the commercialization of fuel cell electric vehicles. The results were reported in a paper in the ACS journal Langmuir.
Polymer electrolyte fuel cells (PEFCs) currently use platinum as the catalyst at the cathode, the site where the fuel cell combines oxygen (from the air) with protons and electrons extracted from the hydrogen fuel at the anode, producing the water exhaust. The large amount of platinum for the cathode forces the cost of the fuel cell system higher. The key to lowering the cost is to reduce the amount of platinum. But as industry and researchers attempt to do this, they encounter a previously-neglected resistance in the cathode that prevents further platinum reductions.
In trying to resolve this resistance, significant debate has emerged as to its origin. Researchers agree that polymer electrolyte films that are only tens of nanometers thick are in some way responsible. These films provide the proton transport for the electrochemical reaction and bind the electrode together.
There have been two main hypotheses to explain this resistance, one focusing on reduced oxygen permeability in nanoscale films and the other on electrolyte poisoning of the catalyst. Some researchers put forth that changes in the polymer electrolyte’s structure (as it’s made very thin) could restrict oxygen transport, causing unexpectedly high resistances. The second hypothesis is based on the electrolyte’s acid adsorbing, or “sticking,” to the platinum surface, blocking it from performing the desired reactions.
—Associate Professor of Mechanical Engineering Shawn Litster
Litster’s team set out to test these hypotheses by separating the platinum from the polymer and measuring the film’s oxygen transport resistance independently. Previous studies have measured the resistance while the polymer was in contact with the platinum catalyst. The team achieved this by supporting the thin polymer films on inert nano-porous polymer membranes.
Here, we present the first characterization of the thin-film O2 transport resistance in the absence of a polarized catalyst, using a nanoporous substrate that geometrically mimics the active catalyst particles. Through a parametric study of varying PFSA film thickness, as thin as 50 nm, we observe no enhanced gas transport resistance in thin films as a result of either interfacial effects or structural changes in the PFSA. Our results suggest that other effects, such as anion poisoning at the Pt catalyst, could be the source of the additional resistance observed at low Pt loading.
—Liu et al.
During their experiments, they found no dramatic change in the transport properties as they moved to films as thin as 30 nanometers. This provides strong evidence that the origin of the resistance is the electrolyte acid’s interaction with the platinum, Litster said.
By supporting thin PFSA films on inert, nanoporous supports, we have measured the oxygen transport resistance of films mimicking those present in PEFC cathodes. Our parametric study of the O2 transport resistance as a function of the film thickness showed no indication of an interfacial resistance. The observed resistance was roughly proportional to thickness, indicating no significant changes in the O2 diffusion coefficient at thicknesses as thin as 50 nm. This result differs significantly from characterizations of O2 transport in thin films with polarized Pt interfaces, which have shown significant interfacial resistances. This contrast suggests that an electrochemical effect, such as an interaction between the acid site of an ionomer and the polarized catalyst, may play a significant role in this apparent interfacial resistance. In addition to the immediate implications for PEFCs, these results are also an important new data set for the general understanding of thickness effects in polymer electrolyte films.
—Liu et al.
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Litster runs the Laboratory for Transport Phenomena in Energy Systems where his team researches sustainable energy technologies and their applications, including fuel cells, batteries, supercapacitors and CO2 capture technologies.
Resources
Hang Liu, William K. Epting, and Shawn Litster (2015) “Gas Transport Resistance in Polymer Electrolyte Thin Films on Oxygen Reduction Reaction Catalysts” Langmuir doi: 10.1021/acs.langmuir.5b02487
This could be another step on the road to lower cost more efficient future FCs.
The anti-FCs posters may have to change their mind by 2020 or so when FCEVs become competitive with Extended range BEVs, specially for cold weather operations.
Posted by: HarveyD | 16 September 2015 at 05:35 AM