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Green alternative to fluorinated membranes in PEM fuel cells

Polymer electrolyte membrane (PEM) fuel cells have high power density, low operational temperatures. If PEM runs on green hydrogen, it doesn’t even emit carbon. But their fabrication requires perfluorinated sulfonic acid (PFSA) polymers as an electrolyte separator membrane and as an ionomer in the electrode, which is quite expensive. Nafion® is the leading commercial PFSA polymer in the market. However, its manufacturing is costly as well as environmentally hazardous. Therefore, low-cost, environmentally friendly PFSA polymer substitutes are the primary goals for the fuel cell scientific community worldwide.

Researchers of the Texas A&M University and Kraton Performance Polymers Inc. experimented using NEXAR ™ polymer membranes in hydrogen fuel cells, studying different ion exchange capacities. NEXAR ™ polymer membranes are commercially available sulfonated pentablock terpolymers. They published the results in the Journal of Membrane Science . Previous studies showed that changing the ion exchange capacity, that is, the degree of sulfonation of NEXAR ™ membranes can alter the nanoscale morphology and significantly affect mechanical properties. This may influence fuel cell performance. Hence, this polymer may be used as a membrane alternative to Nafion® in fuel cells.

Experimental procedure

  1. Materials under consideration: three different variants of the polymer were taken up each with different Ion Exchange Capacities (IECs: 2.0, 1.5, and 1.0 meq / g), which were named NEXAR ™ -2.0, NEXAR ™ -1.5, and NEXAR ™ – 1.0 respectively.
  2. NEXAR ™ membrane preparation: NEXAR ™ membranes were fabricated by casting the NEXAR ™ solutions onto a silicon-coated Mylar PET film using an automatic film applicator under ambient conditions. Two different sizes were manufactured for measuring mechanical properties and conductivity.
  3. NEXAR ™ membrane characterization: mechanical properties using the size 25 mm (L) x 0.5 mm (W) membrane as test pieces were determined and for proton conductivity test pieces of size 30 mm (L) x 10 mm (W) were tested upon.
  4. Nafion® electrode fabrication: conventional Nafion® electrodes were also fabricated as controls to conduct simultaneous tests.
  5. NEXAR ™ electrode fabrication: NEXAR ™ electrodes were prepared in 2 ways for the 2-part study, each with a different composition.
  6. Electrode characterization: electrode profiling was done using a scanning electron microscopy (SEM).
  7. Membrane electrode assembly (MEA) and fuel cell tests: MEAs were fabricated by placing the membrane in between two catalyst-coated gas diffusion layers (anode and cathode) and heat pressing. The entire fuel cell assembly consisted of an MEA, two gaskets, and two flow plates placed between copper current collectors followed by end plates all held together by bolts. Fuel cell performance tests were conducted under ambient pressure with saturated (100% RH) anode and cathode flow rates of 0.43 L / min hydrogen and 1.02 L / min oxygen respectively.
  8. Electrochemical impedance spectroscopy (EIS): electrochemical impedance spectroscopy was performed after the fuel cell tests and the results analyzed.

Results

NEXAR ™ -2.0 and NEXAR ™ -1.5 had a similar proton conductivity at all temperatures, suggesting that there is a maximum limit in proton conductivity. On the contrary, NEXAR ™ membranes, when compared to Nafion® NR-212 membranes , have sufficient proton conductivity to translate into high power density hydrogen fuel cell performance.

However, NEXAR ™ -2.0 and NEXAR ™ -1.5 membranes (with Nafion® as Ionomer) did not exhibit expected fuel cell performance at all fuel cell operating conditions (temperature, pressure, voltage and humidity). Surprisingly, the NEXAR ™ -1.0 membrane (with Nafion® as Ionomer) showed expected fuel cell performance across all fuel cell operating conditions and comparable power densities to Nafion® , suggesting that NEXAR ™ -1.0 may be a viable alternative to Nafion® in hydrogen fuel cells.

During fuel cell operation the NEXAR ™ -1.0 / NEXAR ™ -1.0 membrane-ionomer was thermally and mechanically stable. These results were supported by the power density results, where MEAs with NEXAR ™ -1.0 membrane-ionomers performed better than all the other MEAs.

From the above-mentioned results it became evident that the NEXAR ™ -1.0 variant was the optimal contender to substitute current state-of-the-art PFSA polymers.

Further, to understand the impact of the NEXAR ™ -1.0 ionomer on fuel cell performance, the composition of the ionomer and solvent mixture ratios in the catalyst ink solution were modified and investigated. Results suggested that NEXAR ™ -1.0 as an ionomer behaves similarly to Nafion® ionomers in fuel cell electrodes.

SEM analysis suggested that the amount of ionomer has a significant impact on the binding of ionomer to the catalyst particles, and consequently on the catalyst layer morphology. Therefore, there is an optimum catalyst / ionomer ratio of 2/1 for the Pt / C ionomer using NEXAR ™ -1.0 in fuel cell electrodes.

Conclusions

Ultimately, NEXAR ™ -1.0 is a potentially commercially viable greener substitute to Nafion® as a membrane and ionomer in PEM Fuel cell applications due to its high conductivity, however; alternative block compositions may improve the properties of the polymer to minimize resistances within the fuel cell to match the performance of Nafion® .

Overall Nafion® / Nafion® MEAs still showed the highest fuel cell performance when overall performance was taken into account but alternative hydrocarbon-based polymer compositions for the NEXAR ™ Polymer might provide a future non-fluorinated polymer as a  Nafion® substitute for PEM fuel cells .

Way forward

More analysis is required to perhaps get an accurate approximation of what variant of the NEXAR ™ polymer might cut the mark, future research may be focused upon exploring variants of Ion Exchange Capacities ranging from say 1 meq / g to 1.5 meq / g. But for now, it can be said that NEXAR ™ polymer shows promise as a viable replacement as a non-fluorinated membrane, and perhaps further research with more iterations of mechanical specs as well as chemical specs of the material we might witness a breakthrough.

Reference: https://doi.org/10.1016/j.memsci.2021.119330 : Sulfonated pentablock terpolymers as membranes and ionomers in hydrogen fuel cells , Journal of Membrane Science, 2021, 119330

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Highly durable platinum-palladium-based alloy electrocatalyst for PEM fuel cells

To decrease the consumption of fossil fuel-derived energy for transportation, proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean power sources. Their performance, however, strongly depends on the efficiency and durability of the electrocatalyst used for the hydrogen and oxygen reactions occurring at the electrodes. Noble metals such as platinum and gold are still considered as the most efficient catalysts. At the same time, their high cost and scarcity are major road blocks for scale commercialization of these energy devices.

Various solutions of catalyst design are intensively investigated in order to make this technology economically successful. Searching for high catalyst activity and durability for fuel cells is in focus of current research and development. To date, state-of-the-art electrocatalysts are based on carbon materials with varying platinum loadings.

Ultra-high active platinum group metal (PGM) alloy catalyst

Although, recent research reported ultra-high activity of some metal alloy catalysts, problems still remain. Some of these issues are related to utilization of high atomic percentages of PGM (sometimes up to 75% Pt), poor durability and performance under industrial conditions. In search for new solutions, researchers of the State University of New York at Binghamton, USA, and their collaborators reported a new design in journal Nature Communication: a highly-durable alloy catalyst was obtained by alloying platinum and palladium at less than 50% with 3d-transition metals (Cu, Ni or Co) in ternary compositions.

They addressed the problem of severe de-alloying of conventional alloy catalysts under the operating conditions, resulting in declining performance. For the first time, dynamic re-alloying as a way to self-healing catalysts under realistic operating conditions has been demonstrated to improve fuel cell durability.

Alloy combination and composition

The wet-chemical method was used for synthesis of Pt20PdnCu80−n alloy nanoparticles with the desired platinum, palladium and copper percentages. The selected set of ternary alloy nanoparticles with tunable alloy combinations and compositions, contained a total content of platinum and palladium of less than 50%, keeping it lower than current PGM-based alloy catalysts. The incorporation of palladium into platinum nanomaterials enabled a lower degree of de-alloying and therefore better stability. Additionally, palladium is a good metal partner to platinum due to their catalytic synergy and their resistance to acid corrosion.

To reduce the need for platinum and palladium core catalysts, a third, non-noble transition metal played a central role in the catalytic synergy of alloying formation. Non-noble metals such as copper, cobalt, nickel or similar were used. The platinum-palladium alloy with base metals allowed the researchers to fine tune the thermodynamic stability of the catalysts.

Morphology and phase structure

The thermochemical treatment of carbon-supported nanoparticles was crucial for the structural optimization. The metal atoms in the catalytic nanoparticles were loosely packed with an expanded lattice constant. The oxidative and reductive treatments of the platinum-palladium alloy (PGM <50%) allowed a thermodynamically stable state in terms of alloying, re-alloying and lattice strains. The re-alloying process not only homogenized the inhomogeneous composition by inter-diffusion upon calcination of nanoparticles, but also provided an effective pathway for self-healing following de-alloying.

Single face-centered cubic type structures were observed in Pt20PdnCu80–n nanoparticles (n = 20, 40, 60, 80) nanoalloys. Copper-doping of platinum-palladium alloys reduced the lattice constant effectively, as shown by high energy X-ray diffraction. Maximized compressive strain and maximized activity of the Pt20Pd20Cu60 catalyst confirmed strong correlation between the lattice constants and the oxygen reduction activity.

The researchers demonstrated that the thermodynamically-stable Pt20Pd20Cu60/C catalyst exhibited not only the largest compressive strain after 20,000 cycles, but also high activity and high durability. The discovery that the alloy catalyst remains alloyed under fuel cell operating condition is in sharp contrast to the fully de-alloyed or phase-segregated platinum skin or platinum shell catalysts reported in almost all current literature.

The significance in understanding of the thermodynamic stability of the catalyst system is a potential paradigm shift of design, preparation, and processing of alloy electrocatalysts.

(Photo: Pixabay)