CRESCENDO reached the power density target of 0.42 W/cm2 with air-feed with a PGM-free cathode at the automotive relevant operation point of 0.6 V in the final weeks of the contract, in small area hardware in optimised operation conditions. This major achievement is on a par with the highest performing PGM-free catalysts internationally.
A catalyst available earlier in the project, and selected for up-scale based on its performance and its synthesis route suitable for scale-up in an industrial environment, was prepared at a scale some 100 times that of the laboratory preparation. Its successful upscale provided important learning on the requirements for large-scale preparations of such catalysts and underlined that significantly more resources would be required for scale-up in a future project. Improved kinetic performance of the scaled batch was observed in the rotating disk electrode and in membrane electrode assemblies (MEAs), however performance at higher current densities was not as high as the original lab-scale preparation. Multiple cathode constructions were developed for catalyst-coated membranes (CCMs) using the up-scaled catalyst for testing in 50 cm2 hardware. Complete MEAs were evaluated under different operation conditions including the European harmonised automotive conditions. Results from the fuel cell testing with the up-scaled catalyst lead to an estimated MEA cost of 476 €/kW. A different projection of 204 €/kW derives from reaching the 0.42 W/cm2 target, however this is still four times higher than the generally considered objective for automotive MEAs of 50 €/kW. Significant gain in performance is essential to reach this target, given the cost of the components of the MEA and stack other than that of the cathode catalyst. Improvements must come from uplifts in the catalyst activity, but also from redesigned catalyst textural properties to facilitate catalyst layer development, and from novel flow-field design.
CRESCENDO developed PGM-free (mainly carbon-embedded iron) cathode catalysts using a range of nitrogen-containing carbon sources and synthesis approaches, including trans-metalation and flash pyrolysis making use of metal organic frameworks, micro- and meso-pore activation of, and grafting onto, commercial carbon supports, as well as sulfur and phosphorus doping of the carbons. In parallel to these advances in new materials, the partners also developed methodologies to quantify the catalyst active site density and turnover frequency, and systematically applied them as a means to guide materials design for higher activity. This approach led to an internationally leading current density at 0.9 ViR-free of 41.3 mA/cm2 (single cell) by the end of the project. Detailed surface studies carried out on planar model systems contributed to the understanding of the influence of ultra-low amounts of platinum on the stabilization and catalytic activity of Fe-NC catalysts.
A bottleneck to the application of PGM-free catalysts in proton exchange membrane fuel cells remains their poor stability on operation. CRESCENDO's comprehensive assessment of organic antioxidants and inorganic oxides led to a focus on two PGM-free components that were demonstrated for the first time to exert a stabilising effect on PGM-free catalyst layers, leading to lower voltage loss on operation and significantly lower hydrogen peroxide yield (increased selectivity to 4-electron reduction reaction). This stabilising effect was however restricted in time and is not, currently, of practical applicability. However, as at present the only means of stabilising PGM-free catalysts on long-term fuel cell operation is by the addition of very low (0.5 wt% Pt) amounts of platinum, other means must be found for truly non-PGM cathodes, and the in-depth study of the mechanism of action of the above stabilising agents has led to useful learning for future work.
CRESCENDO had a double approach to addressing the challenge of impurity-tolerant anode catalyst development, a first with a nickel-based bio-inspired catalyst supported on carbon nanotubes, and a second with ultra-low platinum catalysts with high tolerance to carbon monoxide and hydrogen sulfide. Both approaches were highly successful. The final nickel-arginine type catalyst displayed an exceptionally high activity of 150 ± 20 mA/cm2 at 0.1 V and 55°C (32 ± 3 A/mg Ni), and half-cell measurements confirmed the high (50 ppm) tolerance to CO. The fuel cell performance of an MEA with an iron modified Pt/C anode catalyst of loading 0.08 mg Pt/cm2 was shown to be unchanged by 10,000 ppm carbon monoxide in the hydrogen feed, while floating electrode measurements also demonstrated the tolerance of this catalyst type to 1,000 ppb H2S.
In summary, CRESCENDO was a highly integrated and immensely productive collaboration, with enormous progress in all of its work areas: cathode catalyst – cathode catalyst layer – catalyst stabilisation – anode catalyst – complete cell. Although PGM-free-cathode MEAs are far from mature for automotive fuel cell application, the project has identified the gaps between the capabilities of current materials and catalyst layers, and industry needs, as well as ways forward to close that gap comprising increased site density and active site accessibility and design features for improved mass transport.
CRESCENDO has contributed to the scientific literature through fifteen publications in some of the highest impact journals, and was the focus of international attention in the field with its organisation in 2019 of the international conference "Towards Reduction of Critical Raw Materials in Fuel Cells and Electrolysers".
