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novel CATAlyst structures employing Pt at Ultra Low and zero loadings for auTomotive MEAs

Periodic Report Summary 2 - CATAPULT (novel CATAlyst structures employing Pt at Ultra Low and zero loadings for auTomotive MEAs)

Project Context and Objectives:
Conventional platinum nanoparticles supported on high surface area carbon must be replaced at the fuel cell cathode by radically new materials types to produce the decisive step-change increase in mass activity to the target 0.44 A/mg Pt (at 900 mVIR-free). The CATAPULT project addressed these challenges by developing: i) ultra-low platinum loading MEAs using catalysts with ultra-thin extended film platinum coatings; ii) non-PGM based catalysts and hybrid ultra-low Pt/non-PGM catalysts and catalyst layers.
Developing a game-changing approach to fuel cell catalysts offered the opportunity of providing a solution to the problem of catalyst support degradation that severely limits fuel cell lifetime. The use of fibrous structures as fuel cell catalyst supports is an emerging concept, which is particularly attractive from the point of view of their high surface area to volume ratio, as well as the void volume generated by their packing. Corrosion-resistant fibrous and tubular support structures were elaborated in CATAPULT by electrospinning, a versatile and up-scaleable approach allowing the fabrication of one dimensional mesostructured nanomaterials of controlled dimensions.
The CATAPULT project proposed to develop a radically new concept for PEM fuel cell catalysts wherein platinum is deposited as an extremely thin layer. In this approach, platinum is deposited as contiguous and conformal films, minimising sub-surface platinum and the contribution from edge and corner sites. As long as a sufficiently thin coating can be deposited, then it was considered possible to increase the platinum mass activity significantly beyond that which is possible with just further development of the current nano-particulate catalyst approach. In addition, considerable benefit with regard to durability was expected, since sintering is energetically unfavourable and the oxide-dissolution mechanism leading to Pt loss is significantly reduced. The extended platinum surfaces were elaborated using atomic layer deposition, allowing films to be built up monolayer by monolayer, on supports of any geometry. In an alternative approach, CATAPULT also aimed to investigate the electrochemical deposition of extended Pt and Pt alloy layers.
Increased fundamental understanding from theoretical modelling of the platinum surfaces most active for the oxygen reduction reaction and of the Pt-support binding energy, and how this depends on the number of Pt layers, as well as the impact of metal tie layers on lattice mis-match and binding energies, were designed to provide guidance to the strategies developed experimentally and to the down-selection of the new corrosion-resistant supports and their supported catalyst designs.
Our development of non-PGM catalysts was conceived so as to build on recent work highlighting the importance of the carbon/nitrogen support. In CATAPULT, tailored syntheses of metal-organic frameworks were planned to allow fine-tuning of the required properties for their use either sacrificially to generate the C/N support for non-PGM species, or directly as non-PGM catalysts. It was also intended to investigate the possible synergy of hybrid ultra-low Pt/non-PGM catalysts.
It was proposed to down-select catalysts for integration into novel electrode designs and into MEAs, and to evaluate MEAs according to protocols reproducing the stresses encountered in a drive cycle. CATAPULT targeted a catalyst mass activity of 0.44 A/mg Pt (at 900 mVIR-free) contributing to an MEA power density of ≥ 1.0 W/cm2 at 0.67 V (1.5 A/cm2, single cell) at beginning of life, and ≥ 0.9 W/cm2 at 0.64 V (1.4 A/cm2, single cell) at end of life. In the final part of the work, the aim was to scale-up the candidate MEA best satisfying performance and stability targets for further assessment. The results of the scale-up stage were designed as the key input to the techno-economic assessment, the output from which was to show whether the new catalyst concepts are compatible with the cost and durability targets.

Project Results:
The main results achieved in CATAPULT are listed below:
1. Corrosion resistant electronically conducting nanofibrous supports were prepared, the chemical composition of which was guided by modelling results generated in the first months of the project.
2. Oxide tie-layers were deposited on carbon nanofibre supports by atomic layer deposition. The corrosion resistance of the oxide-coated carbon support was increased dramatically, compared to un-coated carbon.
3. Pt was deposited by atomic layer deposition on fibrous supports. The mass activity of >0.5 A/mg Pt in RDE with these electrocatalysts exceeds the project target.
4. Innovative electrochemical methods of Pt deposition were successfully developed: over-potential deposition of thin Pt nano-islands on carbon nanofibres, microwave assisted galvanic displacement of nickel by Pt, and low temperature polyol deposition of Pt on Ni nanoparticles. The stability of the Pt on these supports in accelerated stress testing (voltage cycling) is greater than that of conventional Pt particles on conventional carbon.
5. The most mature catalyst was integrated into novel electrode designs, and the electrodes into MEAs of 50 cm2 active area.
6. Performance testing was carried out on these MEAs, and accelerated stress testing in conditions relevant to automotive applications.
7. Methods for electrochemical and chemical characterisation of fibrous electrocatalysts were successfully developed, and greater knowledge and understanding of design rules for effective catalyst layer development with Pt layer/fibrous support electrocatalysts was generated.
8. A rational programme of development of non-PGM catalysts derived from metal organic frameworks was implemented, and the structural properties of MOF components successfully related to the activity of the final catalyst.
9. A means of stabilising non-PGM catalysts to voltage decay during operation was discovered, based upon functionalisation with ultra-low amounts of Pt.
10. An internal technical assessment of results against conventional Pt nanoparticles on particulate C was carried out.
11. A voltage loss breakdown modelling tool was implemented to allow rational, objective assignment of voltage losses in MEAs.
12. A DFT-validated force-field model of the oxidative disruption of Pt crystal facets (111) was successfully developed, which showed that such facets would be unlikely to persist in real fuel cell operation.
13. Highly qualified personnel were trained, including BSc students (3), MSc students (3), PhD students (2) and post-docs (5), in addition to the training of two (6 months) student assistants.
14. A successful international conference was held 13-16 September 2016 on "Challenges for Zero Platinum for Oxygen Reduction", which gathered 170 international specialists.

Potential Impact:
CATAPULT aimed at developing novel approaches to catalyst and catalyst layer design able to achieve low Pt usage requirement, whilst simultaneously addressing degradation issues associated with current catalyst support corrosion and Pt nanoparticle stability. Lowering Pt loading is the one remaining critical materials technology development required for the MEA, and the impact of being able to demonstrate this in project CATAPULT has a major impact on the prospects for widespread commercialisation of fuel cell powered vehicles. Nearly all vehicle OEMs subscribe to the need to attain such Pt loadings, whilst maintaining the high peak power densities, and also high electrical efficiency (cell voltages) at part-load, lower current density operation. Achievement of ca. 0.1 g/kW Pt equates to a requirement of ca.10 g Pt per vehicle (for a peak power requirement of 95 kW), and this is regarded as a key target by the automotive industry. As well as being compatible with the ultimate stack cost targets, it is also important to recognise that current gasoline and diesel powered internal combustion engines (ICEs) for medium to large size passenger cars require the use of precious metals for exhaust emission clean-up catalysts, the quantities of which when normalised to the price of Pt, equate to around 10 g Pt per vehicle. It has often been cited by the OEMs that they would like to employ no more precious metal content in FCVs than currently used in ICEs. In this scenario, platinum availability goes from being a reasonable constraint with the OEMs to being no constraint at all.

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