Midtermreport

Project Context and Main Objectives

The electrochemical oxidation of reactants in fuel cells represents, from a thermodynamic point of view, a very efficient way to convert chemical energy into electrical energy. When using hydrogen as fuel, fuel cells represent a very attractive choice as power supply for electric vehicles, with zero local emissions and driving ranges around 500 km. However, the true efficiency is much lower than the thermodynamically possible one. In low temperature proton exchange membrane fuel cells (PEM FCs) this is mainly due to the electrode reactions and especially to the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. At present, there is demand for a significant increase in electrical efficiency and higher volumetric and gravimetric power densities of fuel cells. State-of-the-art catalysts for both anode- and cathode-side are based on noble metals, mainly Platinum. Especially in mass production, the platinum would significantly add to the total system cost. Also, the production of Pt is not sufficient for widespread implementation of the technology at current loadings. Finally, the lifetime of the fuel cells needs to be improved. The FCH JU has set the following technical targets in the 2011 call regarding performance and durability of PEM fuel cells: Pt loading below 0.15 g/kW, preferentially below 0.1 g/kW, at a BOL efficiency above 55%, BOL powers > 1 W cm-2 @ 1.5 A cm-2, and a lifetime above 5000 h. The aim of the CathCat project is to improve the performance and reduce the cost of PEM cathodes by development of new alloy catalysts based on Pt or Pd as one constituent and Rare Earth Elements as the second constituent. These alloys are known to form thick compressed Pt (or – possibly – Pd) overlayers during initial de-alloying, leading to a significant enhancement of the catalytic activity. Within the project, the possible materials are screened by DFT methods for stability and activity as well as by studies on model alloys based on polycrystalline bulk alloys, single crystalline surface alloys and on thin films. Methods for the preparation of nanoparticles of those materials are developed and up-scaled for MEA production. In parallel, new support materials are explored based on functionalized carbons, carbon nanotubes and oxides. The new support/catalyst combinations are transferred to the cathode side of MEAs that are tested for performance and durability. Benchmarking is done with respect to state of the art catalyst. Aside from low temperature PEMs, the materials are also tested for application in high temperature PEMs, using membranes from Advent technologies. The starting point of the research were significant advances in the theoretical understanding of the deciding factors determining the rate of the ORR at different pure metal and later also alloy surfaces. A number of Pt based alloys form after initial de-alloying Pt skin structures with an outer layer of Pt showing a different lattice constant compared to bulk Pt. This shifts the Pt d-band center and alters the binding energy of ORR intermediates (strain effect). If the skin layer is only one monolayer thick and the underlying layers have a different composition, then the electronic interaction between other elements and the Pt skin also can change this binding energy (ligand effect). While the focus of attention in literature was originally on alloys like Pt3Ni or Pt3Co, that show improved catalytic activity, but low stability and a strong tendency to dealloying, in later work some Pt rare earth alloys were shown to combine increased catalytic activity with enhanced stability, starting with Pt3Y and Pt3Sc, later also including Pt5Gd. These studies are expanded within CathCat, complemented by research on active support materials, ultimately aiming at improved MEAs made from these materials and innovative support materials that meet the targets above.

Description of the work performed and main results so far

At Danmarks Tekniske Universitet (DTU)-CAMD DFT calculations were carried out to predict activity and stability of highly active catalysts. The range of compositions of suitable Pt(Pd)-rare earth (RE) element alloys to be studied was selected. The focus was on catalysts with Pt:RE ratios of 3:1 and higher to prevent leaching out. Since a several layers thick Pt skin forms on Pt-RE alloys, ligand effect and f electrons had not to be considered. Experimental lattice parameters for the Pt alloys were taken to determine the strain, and the OH binding energy was modelled. The influence of surface reconstruction was discussed, and comparison to experimental activities made. All Pt-RE alloys exhibit activities higher than that of Pt. For studies on Pd, the scaling relations between the binding strength of different intermediates were reinvestigated specifically for strained Pd. This allows to predict the activity changes of Pd by alloying. The theoretical findings were confirmed at UniPd by experimental studies on a Pd model alloy. At DTU-CINF, studies on polycrystalline Pt-RE alloys were carried out. ORR RDE measurements of sputter-cleaned Pt5Gd showed a 5-fold increase in activity relative to Pt at 0.9 V in 0.1 M HClO4, and Pt5La and Pt5Ce more than a 3-fold enhancement. Angle resolved XPS (AR-XPS) was performed before and after testing for reconstructing the surface structure. Depth profiles of Pt5La and Pt5Ce after electrochemistry exhibited the formation of a thick Pt overlayer, as previously observed for Pt5Gd. The catalysts were very stable, losing less than 15% of their initial activity after 10 000 cycles between 0.6 V and 1.0 V. Further studies concerned UHV prepared Pt(111)-Y surface alloys, and mass-selected PtxY nanoparticles. The latter also demonstrated exceptional catalytic activity. These findings confirmed theoretical predictions. At Chalmers, substrates for investigating catalyst nanoparticles via indirect nanoplasmonic sensing were prepared and tested with Pt nanoparticles. A major challenge is the large scale synthesis of these alloys as nanoparticles for MEA fabrication. Efforts regarding the synthesis of Pt-rare earth alloys included solution-based methods, namely the carbonyl method and the water-in-oil route applied at Université de Poitiers (UP), electrochemical methods at TUM and gas phase reduction of precursors at elevated temperatures at University of Padova (UniPd). None of these routes has been fully successful yet, but interesting results and materials have been obtained. The water-in-oil route led to Y-/Gd-oxide modified Pt nanoparticles that showed an increased catalytic activity at 0.9 V. The Y-based catalyst was prepared in an amount sufficient for MEA preparation at Ion Power. In collaboration with FORTH, the catalyst was also studied on modified carbon nanotubes. The gas phase reduction at UniPd led to a catalyst outperforming pure Pt. At FORTH, a modified polyol process was applied for the preparation of Pt-Co catalyst. Depending on the exact nature of the carbon nanotube supports used, formation of a Pt3Co alloy was observed or not. Further work focused on advanced support materials: N-Ion implantation in HOPG did not improve the catalytic activity of Pd nanoparticles, as shown by UniPd and TUM. Therefore UniPd focussed on the development of functionalized mesoporous carbons as advanced support materials, and found improved catalytic activities for both Pd and Pt. They also explore graphene oxide based materials. At UP, TiO2 composites and mixed Ti-metal oxides (e.g. Ti-W-oxides) were studied. One of the latter oxides showed a significantly enhanced catalytic activity of supported metal nanoparticles. All the materials prepared were characterized with respect to ORR, composition and structure. Changes during electrochemical reactions were monitored. Benchmark measurements were carried out in half cell configuration and in low (Ion Power, Toyota, JRC) and high temperature (FORTH) MEAs.

Description of the expected final results and their potential impact and use

This project will exceed the state of the art both from a fundamental point of view as well as from the application point of view. New improved catalysts with increased activity and decreased Pt content will be developed, studied, made into NPs, supported onto advanced supports and manufactured into advanced MEAs. Both the catalysts and the MEAs will be tested with respect to performance and durability. Different strategies have in the meantime be devised to solve the problems with the large scale nanoparticle preparation from Pt-rare earth alloys. Several improved nanoparticular catalyst systems have already been developed within the project, and studies on new advantageous support materials have resulted in further improvements and new ideas. Together with the anticipated progress in the second half of the project, finally several (planned: 5+) MEAs with new cathode catalysts and advanced support materials types will have been developed, that outperform the benchmark MEAs and fulfil the FCH JU targets. The research in CathCat will allow a significant reduction and/or the replacement of Pt in MEAs and therefore enable a vast improvement in commercial cost of PEMFCs allowing for commercialization and wide range application in automotive industry. In addition the research has led and will further lead to an improved understanding of electrocatalysis and support/catalyst interactions that in turn can be applied to design new, even more powerful materials. Also the activities regarding the synthesis of nanoparticles and advanced support materials improve the portfolio of available techniques that can be applied for materials synthesis even for applications outside catalysis or electrocatalysis. The use of two different types of advanced membrane materials will provide the relevant data for the operation of the improved catalyst layers at a large range of operation temperatures, and warrants the transferability of the technology to new developments in the field of membranes and gas diffusion layers in the next years. It is expected that at the end of the project the technology and the catalyst design for improved MEAs have been developed and are available for stack testing and commercialisation in cooperation between the industrial partners in the consortium. The knowledge gained through CathCat has already resulted in eight publications, and several more will follow within the next months. The results will have economic, social and environmental impact. The development of novel catalyst materials through this project would lead to a reduction in fuel cell production costs, and to an increased lifetime, leading to an increased total cruising range of the fuel cell. This will help to enhance the competitiveness of the European fuel cell industry. Also the new synthesis methods developed in this project can be commercialized. Economic prosperity and quality of life depend crucially on the provision of secure clean energy at competitive prices. Fuel cells running on hydrogen derived from a renewable source could offer significant improvements to local air quality. Electric cars based on fuel cells with long life-span and low cost fuel cells could provide electromobility without the disadvantages of battery driven cars, like limited driving range, high costs and safety issues. CathCat has the prospect to improve the performance and reduce the cost of these emerging technologies, facilitating their market introduction. The fore-seen economic impact will create new options for employment within the industry. A hydrogen-based energy economy, if the hydrogen is produced from a sustainable source, has the potential to significantly reduce greenhouse gas emissions and therefore assist in combating the effects of climate change. The project also contributes to the education of young people in the field of electrocatalysis, electrochemistry and fuel cells, as the experimental work is mainly carried out by young postdocs and Ph.D. students.