Within the second year of CathCat, the project made significant progress. The theoretical studies on Pt-rare earth metal alloys were concluded, and additional studies were carried out regarding the behaviour of Pd-rare earth alloys. Corresponding studies on polycrystalline model alloys were carried out at DTU-CINF [1, 2] and UniPd, respectively. Preliminary studies regarding advanced optical characterization techniques were performed at CUT. Advanced oxide based support materials were fabricated and tested at UP-IC2MP [3, 4], and nitrogen-doped mesoporous carbon materials at UniPd [5]. Studies regarding the effect of particle size and support doping for the ORR at Pd nanoparticles supported on HOPG and N-HOPG were concluded at TUM and UniPd, and published [6, 7]. DTU-CINF successfully prepared Pt-rare earth nanoparticles using magnetron-sputtering and demonstrated their excellent electrocatalytic behaviour [8]. Chemical methods for the fabrication of Pt-Y alloys were applied at UP-IC2MP, but resulted in Pt-Y2O3 instead of alloys [9]. Interesting behaviour was found for deposition attempts in ionic liquids at TUM that however were not yet successful. More successful were the efforts using a solid state reduction technique at UniPd, as described below. Other techniques are currently under study. Benchmark catalysts were characterized by several partners under different conditions, especially using RDE experiments, in order to have a baseline for the tests of the new catalysts. Also benchmark MEAs were fabricated at IonP for testing at Toyota and JRC. Current-voltage curves, impedance measurements, electrochemical surface area determination and other data were recorded for the MEAs. At FORTH, catalysts for high temperature membranes were synthesized and tested in MEAs. The Pt-Y2O3 catalysts from UP-IC2MP were tested both in LT and HT PEM.
Details of the work at Technische Universität München
At TUM, the major emphasis was on studying the electrodeposition of rare earth elements from ionic liquids. Worldwide, there are currently activities regarding the electrodeposition of such metals for different reasons, like the fabrication of magnets and for metal electrowinning. For example, it has been reported in literature that La electrodeposition was successful from 1-octyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imid. At TUM, the electrodeposition of both La and Y from ionics liquids using different precursors was investigated. The electrochemical quartz crystal microbalance technique and ex-situ characterization methods demonstrated that indeed material had been deposited, but the nature of the deposit is still under study. In further work, the studies on the particle size effects for Pd nanoparticles supported on HOPG were concluded and submitted for publication. Finally, a 50 wt% Pt catalyst from Tanaka was characterized with an RDE at 25 and 60 °C both in sulphuric acid and perchloric acid, in order to generate benchmark data. The procedure applied was based on the literature [10]. Typical data are shown in Figure 1.
Figure 1. RDE experiments of a Pt benchmark catalyst dispersed on a glassy carbon electrode. The total mass of catalyst was 20 µg. At 0.9 V vs RHE, the mass activity was 260 mA mgPt-1.
Details of the work at Joint Research Centre of the European Commission
JRC intensively worked on general harmonized testing protocols for testing of fuel cells at the single cell level. They communicated with the other partners active in MEA fabrication and testing, and prepared single cell measurements of CathCat MEAs at JRC.
Details of the work at Université de Poitiers
At UP-IC2MP, the focus of the work was on the preparation of alloy nanoparticles and on the preparation and testing of advanced support materials. For the alloy nanoparticles, chemical routes like carbonyl route and the water-in-oil method were first tested, and up-scaled [9]. This work resulted in Pt nanoparticles on rare earth oxides instead of alloys. Therefore another approach is now under investigation, namely alkalide reduction for Pt@Gd NPs based on [11].
Alloyed Pd-Cu nanowires (NWs) in Pd/HKUST1/C prepared usingaMOF precursor as Cu source showed enhanced ORR activity with respect to pure Pd NWs in Pd/C and commercial Pd/C (ETEK) one. The improvement of ORR activity in acid medium should be associated with both NWs morphology and Pd-Cu alloy. Further investigations on this issue, ex. linear analysis of strain, stacking-fault, and crystallite size from pXRD patterns, XPS, in-situ infra-red spectroscopy are under way.
Platinum based nanoparticles deposited on different supports, TiO2-C, and Ti0.7Nb0.3O2 were synthesized by the carbonyl chemical and the photo-deposition route. UP’s results suggest that the best supports are TiO2-C with 20 and 50 wt% TiO2 loading for photo-deposition and carbonyl chemical route, respectively. These results open a possibility to study the interaction between Metal NPs and Ti0.7M0.3O2 supported on carbon.
Details of the work at DTU
DTU-CAMD concluded the theoretical studies regarding Pt-rare earth and Pd-rare earth alloys. Activity and stability of these catalysts were predicted. 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 specific crystallographic structure had to be considered. The theoretical findings then were able to confirm experimental data obtained at UniPd on a Pd model alloy. The results on Pd have been submitted for publication.
At DTU-CINF, further studies on Pt-rare earth model compounds were carried out. These included polycrystalline bulk samples as well as this surface films on single crystalline support. A joint publication with DTU-CAMD of the results of these studies is in preparation. In addition, further studies on mass-selected nanoparticles of these alloys confirming both their exceptional stability and activity have been carried out.
Details of the work at Chalmers University of Technology
Chalmers University of Technology is fabricating and investigating model samples for characterization with advanced techniques, especially with optical techniques. Along these lines, thin film deposition of platinum and its alloys is a key step to prepare model electrodes within this project. The thin films are used as-deposited and studied using electrochemistry in combination with optical spectroscopy. Alternatively, the thin films are patterned and studied using electrochemistry in combination with nanoplasmonic sensing. All samples are also physically characterized.
In previous reports it was shown that it is possible to form thin films of Pt-alloys using single-target co-sputtering. Clips of foil of the alloying material are attached onto the Pt-target and when sputtering, both Pt and the alloy material from the clips will be co-deposited. The number and area of the clips control the composition of the alloy. The samples are annealed in a controlled environment of a flow reactor, typically at 600 to 800 °C using 500 sccm flow of reducing gas to minimize risk for oxidation (4 % H2 in Ar).
Single-target co-sputtering was done using different fractions of the sputtering Pt-target covered by Tb and Y clips, respectively. The obtained Platinum alloy thin films were characterized by EDX in SEM. Their electrochemical properties will be examined to complete the subtask.
The methods for preparing thin films of alloys will also be used to fabricate arrays of nanoparticles of Pt-alloy in combination with Hole-Mask Colloidal Lithography (HCL). Previously, model catalysts with arrays of 120 nm Pt nanodisks made by HCL on fused silica were fabricated. To form arrays of nanodisks from sputtered thin films colloidal lithography (CL) combined with ion beam etching (IBE), is one option. In an attempt to develop samples compatible with direct nanoplasmonic sensing (DNPS), the method was evaluated on platinum thin films deposited on evaporated carbon on fused silica and on flourinated tin-oxide (FTO) on glass.
In order to combine measured optical and nanoplasmonic signals with electrochemistry on the model catalysts, an electrochemical window cell has been designed and manufactured. The optical setup has been designed to provide sufficiently stable optical signals over long periods of time. In addition, to guarantee that the changes in the optical signal from the sample really stems from the relevant processes occurring on its surface and not from, e.g. lamp fluctuations, a reference signal is useful. To be able to measure a reference signal with the same spectrometer and light sources as used for the sample signal, a beam-combiner in combination with a double-beam configuration was used (Figure 2). In this way the signal from the sample and the reference signal can be simultaneously recorded and subtracted from each other to account for drift in the setup. This is important to be able to perform reliable long-term measurements for studying e.g. corrosion where small signal changes over long time periods are expected.
Figure 2. Schematic image of the setup for combining electrochemical and optical characterization and using beam-combiner to in addition obtain a reference signal.
Optical measurements can be conducted in either transmission or reflection mode. Using the beam-combiner measurements of transmission and reflection can be made at the same time from the same sample if desired. Alternatively, reflection from both front and backsides can be measured simultaneously. In summary, the expanded possibilities of our setup achieved by integrating the beam-combiner enable collection of complementary optical characterization data in the same experiment, which is a step beyond the state-of-the-art.
Details of the work at University of Padova
At University of Padova the second year was devoted to the preparation of new scalable innovative carbon support and on the synthesis of Pt and Pt/Y nanoparticles. Mesoporous carbon (MC) materials are promising candidates as real catalyst supports in fuel cells and have recently attracted worldwide attention. Different types of doped mesoporous carbon (d-MC) were prepared depending on the starting precursors i.e. carbazole, 1,10 phenanthroline, phenothiazine, dibenzothiophene and indigo carmine. The precursors were dissolved in a suitable solvent and allowed to impregnate the silica template, which imprints the mesoporosity to the resulting d-MC. The specific BET surface area were determined being in the range 810−1160 m2 g−1. The XPS and elemental analyses confirmed that the resulting carbon material is doped with a high content of nitrogen and/or sulfur.
Nanoparticles from different metal salts were loaded on n-MC samples by a simple wet chemical reduction method or in solid state under N2/H2 atmosphere. An example of the morphology of the resulting samples is shown in Figure 3. The electrocatalytic performances for the whole batch of electrocatalysts toward ORR were tested by linear sweep voltammetry at RDE. In general, Pt@d-MC (nitrogen and sulphur doped) show better performances than the commercial standard Johnson Matthey 30 wt % Pt on Vulcan XC72R; in fact a positive shift up to about 50 mV exists in the half-wave potential of Pt@d-MC as compared to Pt@vulcan.
Figure 3. SEM image of the Pt@d-MC
Pt/Y NPs were prepared by solid state reduction in H2/N2 flow by starting from different Pt and Y salts and deposited on a commercial mesoporous substrate; among all the considered salt precursors PtCl2, K2PtCl6 afford always Pt/Y2O3 NPs notwithstanding the different reduction temperature, time reaction or reagent ratio. So far the best performing catalysts in term of half wave potential and specific electrochemical surface area are obtained starting from Pt(acac)2 as precursor. Pt/Y NPs prepared at 600°C results even better than the commercial standard Pt@vulcan 30% by 40 mV, notwithstanding a lower Pt content (24.5%). It is worth noting that all the investigated catalysts have yttrium in the form of Y2O3. Only in one case, it was observed by XPS analysis, the formation of 47% of Y in the form of PtY alloy and the remaining 53% in the form of Y2O3. The preliminary ORR characterization indicates that the sample with maximized amount of alloy PtY (with a Pt/C loading of ca 25%) is not as active as the one containing only the Y oxide, but that it anyway performs better than the commercial standard Pt@vulcan 20%.
Details of the work at Ion Power
Ion Power fabricated both benchmark MEAs based on a 50%Pt/C catalyst, as well as the first CathCat MEA based on the Pt/Y2O3 catalyst from Poitiers.
Details of the work at FORTH
At, FORTH, the focus is on high temperature PEM. Toward the development of new electrocatalysts for the high temperature PEMFCs, they continued with the synthesis of Pt3Co and Pt3Y using the polyol method; i.e. the reduction of precursor metal salts in an ethyleneglycol/water solution. The substrates are oxidized carbon nanotubes (ox.MWCNTs) and the same after functionalization with polar basic pyridine groups, (ox.MWCNT)-Py. The latter is specially designed for high temperature electrodes and aims at the increase of the active electrochemical interface through the interactions of the pyridine moieties with the proton conductor, the phosphoric acid. The aim is to understand the effect of the substrate on the Pt, Co or Y deposition, the formation of alloys between the components and the obtained catalyst morphology. So far, additional catalysts have been synthesized and are now being physicochemically characterized (XRD, TEM, ICP, XPS).
During the first half of the CathCat project, Forth created the “baseline” and gathered results with Pt based catalysts that will be used later on when comparing with catalysts developed from this project. More specifically commercial (Tanaka) 30% Pt/C and Pt on MWCNTs (oxidized and functionalized with pyridines) were used. As the next step, Forth prepared MEAs using the catalysts developed within the CathCat project at the cathode and performs in situ electrochemical measurements including evaluation of the ECSA with CO stripping, I-V plots and AC impedance spectroscopy. PtCo alloy catalysts showed high performance. On the other hand, so far, preliminary measurements with Pt-Y2O3 catalysts (no alloy formed) prepared by the University of Poitiers did not perform as expected according to the enhanced performance using the RDE setup. Further investigations are in progress.
Details of the work at Toyota
Toyota carried out MEA testing of small size MEA. Both benchmark MEAs as well as MEAs with a Pt-Y2O3 catalyst were tested under different operation conditions. Also durability measurements were carried out.
References
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