Novel Fuel Cell MEA Based on Pt-C Deposited by Magnetron Sputtering

Transcription

1 / ecst The Electrochemical Society Novel Fuel Cell MEA Based on Pt-C Deposited by Magnetron Sputtering A. Ostroverkh a, V. Johanek a, M. Dubau a, P. Kus a, K. Veltruska a, M. Vaclavu a, R. Fiala b, B. Smid a Y. Ostroverkh a and V.Matolin a a Department of Surface and Plasma Science, Charles University, Prague, 18000, Czech Republic b Central European Research Infrastructure Consortium, S.S km 163,5 in AREA Science Park, Basovizza, Trieste, Italy Based on the concept of single atom catalysis for the development of low-cost catalytic materials and supported metal nanostructures (most widely used type of heterogeneous catalyst in industrial processes) a novel Pt-C composite catalysts was developed for the proton exchange membrane fuel cell (PEMFC) application. Nanostructured Pt-C thin films catalysts with thickness range nm were deposited on carbon paper gas diffusion layer substrates covered with standard microporous layer (MPL) by magnetron sputtering method. Total Pt loading in the thin films varied in the range µg/cm 2 as determined by XPS. These thin film catalysts were examined for their activity at both sides of proton exchange membrane fuel cell. Thin film catalyst with total Pt loading of 19.9 µg/cm 2 giving maximum power of 0.93 W/cm 2 was checked during 450-h accelerated durability test. The average cell voltage decay of only 15.6 µv/h showed very high catalyst stability despite very low loading of noble metal. Introduction PEMFC is one of the strategic techniques in the concept of the renewable energy. However, several issues still need to be addressed and solved to reduce its cost, extend its lifetime and improve its efficiency (1). One of the main application barriers is the use of costly platinum (Pt) required as catalyst. Furthermore, this noble metal is listed by the European Commission as a critical material; its resources are limited and it is expected to be depleted by the end of the 21 st century regardless of introduction of the fuel cell vehicles (2). The latest developments in fuel cell technology may reduce the total Pt loading in PEMFC to 0.15 mg/cm 2 which is still above the target value (0.125 mg/cm 2 ) set for the year 2017 (3). The concept of single atom catalysis offers maximum noble metal efficiency for the development of low-cost catalytic materials. Lykhach and others deeply summarized the properties of new materials containing atomically dispersed Pt (4). Our ultimate goal is to achieve stable efficiency catalyst with highly dispersed Pt. The magnetron sputtering deposition was chosen as a very suitable method for the deposition of Pt-C composite thin-film catalysts due to its versatility and well-defined operation conditions. 225

2 Experimental Magnetron Sputtering. The platinum-doped amorphous carbon films have been prepared via DC magnetron sputtering of a graphite target (Goodfellow, 2 inch diameter, 5 mm thickness) on Sigracet 29BC substrate. Doping with Pt was achieved by placing Pt wires (Alfa Aesar, 0.5 mm diameter) on top of the graphite target. The process gas was pure argon (1 Pa) and the DC discharge power 60 W. Mass-spectrometry. For the corrosion test a quadrupole mass-spectrometry (QMS) Pfeiffer Prisma 200 was employed for reaction product detection via precision dosing valve. SEM. For the morphological investigation a scanning electron microscope MIRA3 from TESCAN was used. The measurement was carried out in the secondary-electronmode using primary electrons with 30keV energy. XPS. Concentration and oxidation state of Pt were evaluated by X-ray photoelectron spectroscopy (XPS). The fits of the XPS spectra were carried out using a Shirley background, energy separation between the Pt 4f doublet components fixed at 3.35 ev, and their mutual intensity ratio at 3/2. Fuel Cell Test. For the PEMFC testing we used semi-automated stations LeanCat FCS-4M-100W with active area of the cell 4cm 2. Results and Discussion Using the mass-spectrometry technique described in (5) along with the current dependence measurement, we demonstrated the higher stability of carbon doped by nitrogen (CNx) with respect to carbon etching as compared to amorphous carbon (a-c) when used as a platinum nanoparticle support. For this study we analyzed the etching resistance of composite Pt-C (200 nm thick) layers prepared by magnetron sputtering. The catalysts were deposited directly onto a Nafion membrane. No signal corresponding to a potential etching of the thin film was detected within the full range of applied voltages (see Fig. 1). 226

3 Figure 1. Dependence of carbon etch rate at the FC cathode of PEMFC on its potential for Pt nanoparticles supported on amorphous carbon (squares) and carbon nitride (circles), and for a composite Pt-C thin film (triangles). The main difference of our Pt-C catalytic layer compared to current state-of-the-art commercial electrodes for fuel cell application is demonstrated in Fig. 2. The advantage of the clean physical method of magnetron sputtering deposition is the absence of the hydrophilic ionomer compound. It allows to decrease the temperature required for the humidification of the feed gases (H2, O2). Figure 2. Model (left) and SEM cross-section view (right) of Pt distribution at PEMFC electrodes: commercial electrode with high Pt loading (top) compared to magnetron sputtered Pt-C thin film (bottom). 227

4 Nanostructured Pt-C thin films were deposited on a carbon paper gas diffusion layer (Sigracet 29 BC) substrates by magnetron sputtering with thickness ranging from 25 to 150 nm. The typical SEM top-view image of the Pt-C composite layer is shown in Fig. 3a. Total Pt loading in the thin films varied in the range µg/cm 2 as determined by XPS. Figure 3. As-deposited Pt-C thin film composite layer: SEM image (left); XPS spectrum of the Pt 4f level (right). In all the samples platinum was found by XPS predominantly in the metallic state, see Fig. 3b. We assume that the non-metallic Pt signal originates from post-deposition oxidation of the surface during the sample transfer through air, forming surface oxides and hydroxides via catalytic dissociation of ambient water molecules. A fuel cell with a membrane electrode assembly (MEA) based on Nafion 212 membrane with our Pt-C anode (Pt loading µg/cm 2 ) and a commercial state-ofthe-art Pt/C cathode (Pt loading 300µg/cm 2 ) generated output power density up to 1.0 W/cm 2, value comparable to MEA with commercial powder anode using much higher Pt loading µg/cm 2. This result demonstrates significant improvement in platinum utilization for hydrogen oxidation reaction (HOR) due to high dispersion of Pt provided by the magnetron sputtering deposition method. Similarly, a MEA with the commercial Pt/C anode and our Pt/C thin film (Pt loading µg/cm 2 ) as cathode yielded stable power density up to 0.5 W/cm 2, proving its high efficiency also for oxygen reduction reaction (ORR). The examples of results obtained for custom anode and cathode is presented in Fig. 4. The open voltage of V drops to about 0.4 V at the maximum output current of 2.5 A (anode) and 1.1 A (cathode). In both configurations the FC provides excellent stability within 24-hour time period of operation under heavy load (2 A constant current and 0.4 V constant voltage for anode and cathode, respectively) with only relatively small temporary oscillations and no detectable degradation over the whole period. 228

5 Figure 4. Output performance of the fuel cell with Pt-C custom electrode as anode with 18 µgpt/cm 2 (left) and as cathode with 11 µgpt/cm 2 (right). The insets show I-V curves measured before and after 24-hour period. Based on the above findings, a MEA with both magnetron sputtered Pt/C electrodes using total Pt loading 20 µg/cm 2 was tested with a 4 cm 2 cell operating at 70 C in H2/O2- feed regime for 24 hours. The platinum content on cathode and anode side was chosen 14.4 µg/cm 2 and 5.5 µg/cm 2, providing highest efficiency for HOR and ORR, respectively. With this setup the MEA efficiency was checked in a wide range of working pressures (regulated by means of pressure controllers at the FC outlets) between bar. We detected the power density increase with gas pressure following a dependence very close to linear in the whole measured range. The maximum output of 0.93 W/cm 2 was achieved with a unit at 4.25 bar. After verifying the excellent stability within 24-hour time period of operation under heavy load (1A/cm 2 ) we subjected the fuel cell to a 450-hour accelerated durability test with on-off cycling (Fig. 5). The average cell voltage decay as low as 15.6µV/h showed very high stability of the Pt-C catalyst despite the very low loading of platinum. Figure h accelerated durability test of PEMFC with magnetron sputtered Pt-C cathode and anode (14.4 µg/cm 2 and 5.5 µg/cm 2 Pt, respectively), performed at cycling 30 min at 0.4 A/cm 2 current load and 30 min open voltage. The cell back-pressure was kept at 3.0 bar. 229

6 Conclusions The magnetron sputtered Pt-C with fine metal dispersion proved to be a promising catalytic material with very high Pt utilization for hydrogen-feed PEMFC applications. Additional significant benefits of the Pt-C composite is its lower requirement with respect to feed humidity and its excellent stability under the working conditions. Acknowledgments This work was supported by the Czech Ministry of Education under grant LM References 1. C. Damour and M.Benne, Int. J. Hydrogen Energy, p.2371, 40 (2015). 2. A. Elshkaki, An analysis of future platinum resources, emissions and waste streams using a system dynamic model of its intentional and non-intentional flows and stocks, Resources Policy, p , 38 (2013). 3. Fuel Cell Technologies Office Multi-Year Research, Development, and Demonstration Plan, Section 3.4, U.S. Department of Energy, (2016). 4. Y. Lykhach, Cat. at Sci. Technol., Advance Article (2017). 5. V.Johanek, J. Anal. Methods in Chem., Article ID , 2016 (2016). 230