Summary of first year results

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Objectives

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 [1-4]. Within the project, the possible combinations are screened by DFT methods for stability and activity as well as by the use of model alloys based on single- and polycrystalline bulk 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 will be tested for performance and durability. Benchmarking will be done with respect to state of the art catalyst.

The starting point of the research within CathCat were significant advances in the theoretical understanding of the deciding factors determining the rate of the oxygen reduction reaction at different pure metal and later also alloy surfaces [5-9]. A number of Pt based alloys form after initial dealloying Pt skin structures with an outer layer of Pt showing a different lattice constant compared to bulk Pt [1, 3]. This leads to a shift in the d-band center of the Pt and therefore to a modulation in the binding energy of ORR intermediates [7] (strain effect) [1, 10]. 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 outer Pt skin also can change this binding energy (ligand effect) [1, 10]. While the focus of attention was originally on alloys like Pt3Ni or Pt3Co, that show improved catalytic activity, but low stability and a strong tendency to dealloying [1, 7], in later work some Pt rare earth alloys were shown to combine increased catalytic activity with enhanced stability, starting with Pt3Y and Pt3Sc [5, 11], later also including Pt5Gd [4]. These studies are expanded within CathCat, ultimatively aiming at improved MEAs made from these materials and innovative support materials.

 

The focus of the first year of research was to expand the fundamental theoretical and experimental studies to alloys with different rare earth elements and different conditions, and to work on the preparation of nanoparticles of Pt-Y and Pt-Gd alloys. A number of different approaches like vacuum based techniques for more fundamental studies, chemical and electrochemical techniques were explored, and tests of the ORR activity using conventional tests setups in three electrode configurations and aqueous electrolytes were carried out.

 

Fundamental Work

The theoretical work in the first year focused on the selection of the most promising alloy compositions and the calculation of activity, stability, and structural properties. The OH-binding energies for Pt alloys Pd alloys with a number of rare earth elements were calculated. As these alloys – at least for the Pt alloys – form a several layers thick overlayer of pure Pt, only the strain effect influences the results. For some alloys, an ORR activity much larger than for Pt is expected from the calculations.

 

At DTU, studies on polycrystalline Pt-rare earth alloys were carried out. Alloys of Pt with early transition metals or lanthanides, such as Pt3Y, Pt5Gd, Pt5La, Pt5Sm, Pt5Tm and Pt5Ce, present exceptionally negative alloying energy [2, 12]. Motivated by this, Malacrida and co-authors have studied Pt5La, Pt5Ce, and Pt5Gd polycrystalline samples as cathode electrocatalysts for proton exchange membrane fuel cells (PEMFCs). Rotating disk measurements show that sputter-cleaned, polycrystalline Pt5Gd shows a 5-fold increase in ORR [4], and all the previously mentioned alloys exhibit more than a 3-fold activity enhancement [2], relative to Pt at 0.9 V in 0.1 M HClO4.

In order to follow the chemical state of the catalytic surfaces, angle resolved XPS (AR-XPS) was performed before and after all the preparation and testing steps. By discerning the XPS spectra at different emission angles, AR-XPS is a powerful technique for reconstructing the surface structure of these catalysts. The presence of a Pt overlayer explains the increase in the Pt to La and Pt to Ce ratios, from the initial values measured during sputtering. This is particularly evident for higher angles due to the higher surface sensitivity. Depth profiles of Pt5La and Pt5Ce after electrochemistry were calculated from the data. Both catalysts exhibit the formation of a thick Pt overlayer, as had been previously observed for Pt5Gd [4]. The stability of the alloys under ORR conditions was tested by applying consecutive cycles from 0.6 to 1.0 V vs. RHE in an O2-saturated 0.1 M HClO4 electrolyte at 100 mV s−1 and 23 °C. Not only are the catalysts highly active, they are also very stable, losing less than 15% of their initial activity after 10 000 cycles between 0.6 V and 1.0 V.After 10 000 potential cycles in the above described conditions, the final specific activity of Pt5La, Pt5Ce and Pt5Gd is still more than 3 times higher than for pure Pt.

 

At Chalmers, Hole-Mask Colloidal Lithography (HCL) has been the work-horse for nanofabrication since 2007 [13]. It is based on electrostatic self-assembly when forming the mask and allows for efficient fabrication of quasi-random arrays of nanoparticles such as disks, ellipses or pairs on a support material of choice and covering large areas (cm2) homogeneously, if required. Typically, particle sizes between 20 nm and up to several 100 nm can easily be achieved and controlled, and any metal that either can be deposited by thermal evaporation or by sputtering can be used. Figure 1 shows arrays of 120 nm (left) or 50 nm (right) Pt nanodisk arrays fabricated by HCL.

 

     

 

Figure 1. SEM images of arrays of Pt catalyst nanodisks with 120 nm (left) and 50 nm (right) diameter.

 

Plasmonic nanoantennas create locally strongly enhanced electric fields in so-called hot spots. To place a relevant nano-object with high accuracy in such a hot spot it is crucial to fully capitalize on the potential of nanoantennas to probe and enhance processes at the nanoscale on an adjacent, e.g., catalyst nanoparticle. For this purpose at Chalmers the bottom-up and self-assembly-based Shrinking-Hole Colloidal Lithography (SHCL) [14, 15] nanofabrication method was developed, which provides (i) unique control of the size and position of subsequently deposited particles forming the nanoantenna itself, and (ii) allows delivery of catalytic nanoobjects consisting of a material of choice to the antenna hot spot – all in a single lithography step and, if desired, uniformly covering several cm2 of surface. The SHCL technique is characterized by high flexibility in terms of exploited materials, absence of contamination of the grown structures after fabrication or alteration of deposited material properties. This opens unique possibilities for the fabrication of model catalysts where one can probe oxidation/reduction processes, corrosion processes or the role of position and interplay of the noble metal catalyst and (oxidic or conducting, e.g. carbon) support via spill-over. If implemented on a conducting support like carbon or a transparent conducting oxide, this approach should be perfectly compatible with electrochemistry.

 

In order to simultaneously measure nanoplasmonic signal and electrochemistry on the model catalysts, an electrochemical window cell has been designed and manufactured. The cell is a flow cell where electrolyte is supplied from a gas bubbled reservoir. Optical measurements can be conducted in either transmission mode (i.e. collecting the light that passes through the sample) or reflection mode. To validate the setup, thin films of Pt and Pd have been measured with in-situ optical characterization. The technique will be developed further, e.g. to also analyze nanoplasmonic signals. Preliminary measurements on stability (not shown) indicate that this optical technique can offer valuable insight also into corrosion of fuel cell catalysts.

 

At University of Padova, a transfer system for the transfer between an electrochemical cell and UHV equipment for surface analysis and sample preparation was constructed. In addition electrochemical studies of Pt and Pd on undoped and doped carbon materials were carried out, including experiments on carbon model substrates. Nitrogen doping of HOPG was studied at two different implantation energies, followed by XPS analysis, and pure Pd nanoparticles were deposited on the samples both by vacuum deposition and electrochemical techniques. The stability of the particles and the electrocatalytic activity with respect to ORR were studied. No significant improvement of the electrocatalytic activity of these samples by nitrogen functionalization was found. Pd NPs were also deposited; following an electrochemical approach, on a nitrogen doped glassy carbon (N-GC) [16]. The N-GC samples were prepared by using both ion implantation and by electrochemical oxidation in a phosphate buffer containing 0.1 M carbamic acid. In the former case a nitrogen content of 15% was obtained whereas in the latter case only 5.7 % was observed. The deposition of Pd NPs on N-GC (Pd@N-GC) has been carried out by double step potential electrodeposition from a 1M HClO4 solution with 1 mM PdSO4. In the case of N-GC prepared via the two different approaches, the NPs are spherical in shape with average dimension of 20 nm and are uniformly distributed over the N-GC surface. On the opposite, the deposition of Pd on pure GC results in the formation of NPs of average dimension of 40 nm but also in the presence of NP clusters of 100-300 nm. It is clear that the electrodeposition of Pd on N-doped GC results in a better nucleation than on GC and this is probably the result of a much stronger interaction between defect sites of GC and Pd. The two differently doped GC surface and pristine GC loaded with Pd NPs were characterized by means of XPS. The surveys for all three samples clearly show similar features that account for the presence of Pd, carbon and oxygen, while nitrogen is present only in the case of Pd@N-GC. In Pd@N-GC, one can distinguish between different chemical defects; by deconvoluting the N 1s XPS peak, one can single out the presence of pyridinic (398.0 eV), –CºN terminal groups, pyrrolic, N graphitic defects and N+ ions trapped into carbon vacancies. A further component present at high binding energy can be assigned to the interaction between nitrogen and oxygen with the formation of NOx groups, though in a very limited amount. In the case of Pd@N-GC prepared via electrochemical doping aminic and pyrrolic groups are predominant with respect to other chemical defects such as pyridinic or graphitic nitrogen, that are however consistently present. In order to transfer the N-functionalization to materials suitable for fuel cells, mesoporous carbon materials were studied as well. At TU München, research on electrochemical Pd deposition on different carbon substrates was carried out, in part in collaboration with Padova [17, 18].

 

Nanoparticle Synthesis and Characterization

At DTU, research on size selected nanoclusters of alloy nanoparticles was carried out [19]. The synthesis of such nanoparticles was also carried out with chemical methods at University of Poitiers, and first results indicate a significant improvement of catalytic activity and good stability of some of these catalysts. At FORTH Institute, Pt-Co nanoparticles were made and supported on carbon nanotubes.

 

MEA Testing

MEAs for low temperature fuel cells are manufactured by Ion Power, while those for HT fuel cells are made by FORTH Institute. HT PEMFCs having certain advantages over the state-of-the-art low temperature fuel cells constitute a key research issue aiming at higher efficiencies, cost reduction and compactness. The state-of-the-art HT technology is based on phosphoric acid doped polymer membranes as the electrolyte. As such, FORTH Institute chose to work with Advent Technologies high temperature polymer electrolyte. For this type of cells, within the framework of previous FCH JU projects, a Pt based electrocatalyst with improved features was developed [20]. CNTs were used as the catalyst support due to their high specific surface area, unique electrical, mechanical and thermal properties, as well as the fact that CNTs have been reported to be more corrosion resistant than carbon black. Towards the development of an optimized electrocatalytic system, there are two important considerations; the deposition of fine Pt particles on the carbon support and the construction of an electrocatalytic layer where they can thoroughly participate in the electrocatalytic active network of a 3D structured electrochemical interface, thus aiming at the total utilization of Pt particles’ specific surface area. In this respect, the idea was to transfer the concept of the electrolyte into the catalytic layer (CL). This was accomplished by the covalent attachment and uniform distribution of polar pyridine moieties throughout the CL, which are expected to interact with phosphoric acid originating either by doping the electrode or from the electrolyte. Thus the acid–base interaction of the H3PO4 with the pyridines will ensure the uniform distribution of the acid so that a 3D proton ionic link will provide an active electrochemical interface with all deposited Pt particles. Only particles, which are connected to both electrolyte (PA) and current collector (CNTs) can contribute to the real surface area. The catalysts have been thoroughly characterized by relevant techniques. MEAs were homemade using these catalysts and new ones, and characterized for electrochemical active surface area and other properties. Finally fuel cell tests at 180°C were carried out, showing good performance.

 

 

References

[1] I.E.L. Stephens, A.S. Bondarenko, U. Gronbjerg, J. Rossmeisl, I. Chorkendorff, Understanding the electrocatalysis of oxygen reduction on platinum and its alloys, Energy & Environmental Science, 5 (2012) 6744-6762.

[2] P. Malacrida, M. Escudero-Escribano, A. Verdaguer-Casadevall, I.E.L. Stephens, I. Chorkendorff, Enhanced activity and stability of Pt-La and Pt-Ce alloys for oxygen electroreduction: the elucidation of the active surface phase, Journal of Materials Chemistry A, doi: DOI: 10.1039/C3TA14574C (2014).

[3] I.E.L. Stephens, A.S. Bondarenko, L. Bech, I. Chorkendorff, Oxygen Electroreduction Activity and X-Ray Photoelectron Spectroscopy of Platinum and Early Transition Metal Alloys, ChemCatChem, 4 (2012) 341-349.

[4] M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Grønbjerg, B.P. Knudsen, A.K. Jepsen, J. Rossmeisl, I.E.L. Stephens, I. Chorkendorff, Pt5Gd as a Highly Active and Stable Catalyst for Oxygen Electroreduction, Journal of the American Chemical Society, 134 (2012) 16476-16479.

[5] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Alloys of platinum and early transition metals as oxygen reduction electrocatalysts, Nature Chemistry, 1 (2009) 552-556.

[6] J.K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jonsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, Journal of Physical Chemistry B, 108 (2004) 17886-17892.

[7] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure, Angewandte Chemie-International Edition, 45 (2006) 2897 –2901.

[8] J. Rossmeisl, J.K. Nørskov, Electrochemistry on the computer: Understanding how to tailor the metal overlayers for the oxygen reduction reaction (A perspective on the article, ‘‘Improved oxygen reduction reactivity of platinum monolayers on transition metal surfaces”, by A.U. Nilekar and M. Mavrikakis), Surface Science, 602 (2008) 2337–2338.

[9] V. Tripković, E. Skúlason, S. Siahrostami, J.K. Nørskov, J. Rossmeisl, The oxygen reduction reaction mechanism on Pt(111) from density functional theory calculations, Electrochimica Acta, 55 (2010) 7975-7981.

[10] T. Bligaard, J.K. Nørskov, Ligand effects in heterogeneous catalysis and electrochemistry, Electrochimica Acta, 52 (2007) 5512-5516.

[11] S.J. Yoo, K.-S. Lee, S.J. Hwang, Y.-H. Cho, S.-K. Kim, J.W. Yun, Y.-E. Sung, T.-H. Lim, Pt3Y electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy, 37 (2012) 9758-9765.

[12] I.E.L. Stephens, J. Rossmeisl, M.E. Escribano, A. Verdaguer-Casadevall, P. Malacrida, U.G. Vej-Hansen, B.P. Knudsen, A.K. Jepsen, I. Chorkendorff, Platinum and palladium alloys suitable as fuel cell electrodes, Patent, WO2014005599A1, in, Danmarks Tekniske Universitet, Denmark, 2014, pp. 39pp.

[13] H. Fredriksson, Y. Alaverdyan, A. Dmitriev, C. Langhammer, D.S. Sutherland, M. Zäch, B. Kasemo, Hole–Mask Colloidal Lithography, Advanced Materials, 19 (2007) 4297-4302.

[14] B. Wickman, H. Fredriksson, S. Gustafsson, E. Olsson, B. Kasemo, Fabrication of poly- and single-crystalline platinum nanostructures using hole-mask colloidal lithography, electrodeposition and annealing, Nanotechnology, 22 (2011) 345302.

[15] S. Syrenova, C. Wadell, C. Langhammer, Shrinking-hole colloidal lithography - self-aligned nanofabrication of complex plasmonic nanoantennas, submitted (2014).

[16] L. Perini, C. Durante, M. Favaro, S. Agnoli, G. Granozzi, A. Gennaro, Electrocatalysis at palladium nanoparticles: Effect of the support nitrogen doping on the catalytic activation of carbonhalogen bond, Applied Catalysis B: Environmental, 144 (2014) 300-307.

[17] W. Ju, T. Brülle, M. Favaro, L. Perini, C. Durante, O. Schneider, U. Stimming, Palladium Nanoparticles Supported on HOPG: Preparation, Reactivity and Stability, Electrochim. Acta, in preparation (2014).

[18] W. Ju, M. Favaro, C. Durante, L. Perini, S. Agnoli, O. Schneider, U. Stimming, G. Granozzi, Pd Nanoparticles deposited on nitrogen-doped HOPG: New Insights into the Pd-catalyzed Oxygen Reduction Reaction, Electrochim. Acta, submitted (2014).

[19] P. Hernández-Fernández, I.E.L. Stephens, I. Chorkendorff, Mass-selected nanoalloys as model catalysts: PtxY nanoparticles for oxygen electroreduction, submitted (2014).

[20] A. Orfanidi, M.K. Daletou, S.G. Neophytides, Preparation and characterization of Pt on modified multi-wall carbon nanotubes to be used as electrocatalysts for high temperature fuel cell applications, Appl. Catal. B-Environ., 106 (2011) 379-389.