Magnetron sputtering a high-performance catalyst for ultra-low-Pt loading PEMFCs

Journal of Alloys and Compounds xxx (xxxx) xxx

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Magnetron sputtering a high-performance catalyst for ultra-low-Pt loading PEMFCs Kailin Fu a, Liuli Zeng a, Jiaming Liu a, Min Liu a, Shang Li a, Wei Guo a, *, Ying Gao b, **, Mu Pan a a b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, PR China College of Engineering and Applied Science, University of Cincinnati, Cincinnati, OH, 45221, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2019 Received in revised form 16 September 2019 Accepted 20 September 2019 Available online xxx

Magnetron sputtering deposition (MSD) is a physical method that has been extensively study for PEMFC applications because it is simple, flexible, inexpensive, high productive and allowing control of catalyst particle size. In this study, we first combine the advantages of carbon-supported Pt-type structures and MSD to prepare a carbon-sphere-supported Pt catalyst. Compare to conventional chemical methods, the Pt/C catalyst prepare by MSD has some characteristics such as easy to prepare, high Pt utilization, and non-polluting. The results demonstrate that the electrochemical surface area (ECSA) of the sputtered-Pt/ C is increased by 34% compare with the commercial-Pt/C with the same Pt loading. Moreover, the testing results for the single cell indicate that the rated power density of the sputtered-Pt/C catalyst is 26.7% higher than that of the commercial-Pt/C catalyst under H2/air test conditions. We are optimistic that this sputtered-Pt/C catalyst could have broad applications in commercial fuel cells. © 2019 Published by Elsevier B.V.

Keywords: Sputter deposition Ultrathin carbon film Proton exchange membrane fuel cell Carbon supported Pt High catalytic activity

1. Introduction It is well-know that the main bottleneck of the electrochemical performance of proton exchange membrane fuel cells (PEMFCs) is the sluggish oxygen reduction activity (ORR) kinetics of the catalyst. Therefore, the discovery and optimization of an efficient and low-cost ORR catalyst has been a consistent driving force in academe and industry around the world [1e3]. According to the classical thermodynamic volcano plot, a catalyst that follows the scaling relationship DGOH (*OH binding free energy) z 0.1 eV would show the optimum ORR kinetics [4,5]. Pt-based catalysts lie close to the top of the thermodynamic volcano, making them excellent ORR catalysts, and these have been widely used in PEMFCs [5]. However, the cost of Pt-based ORR catalysts has risen sharply because of the scarcity of Pt, making these catalysts prohibitively expensive for mainstream PEM fuel cells. To reduce the cost, various strategies have been used to increase the ORR efficiency and reduce the Pt loading, such as the use of low Pt loadings,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Guo), [email protected] (Y. Gao).

Pt alloys, and Pt- and metal-free catalysts [6,7]. For instance, the rhombic dodecahedral PtCu nanoframes as Pt alloy catalyst exhibited excellent catalytic activity for ORR with positive halfwave potential at 0.95 V [8]. In addition, several novel electrocatalysts with special structure also showed excellent ORR activity, such as Pt75Co25 nanodendritic assemblies [9], hierarchical PdAu nanodentrites [10], Co@G/N-GCNs [11] etc. From the perspective of long-term use, research into Pt-free and metal-free catalysts has become a research hotspot because these materials effectively reduce the catalyst cost [6,12,13]. Nevertheless, to date, low-Pt loading catalysts are considered to be the most promising selection for commercial fuel cell applications because the problem of sluggish ORR kinetics for Pt-free and metal-free catalysts has not been resolved [14]. Currently, most Pt-based catalysts for PEMFC applications use carbon as the support for Pt nanoparticles, so-called Pt/C-based catalysts [15]. The development of low-Pt loading catalysts is based on material and structural improvements for Pt/C catalysts. The technical goal for low-Pt loading is the content for Pt group metal (PGM, i.e., Pt, Ir, Os, Ru, Rh, and Pd) is less than 0.125 mgPt cm
2 (150 kPa, 94  C, rated power density >1 W cm
2), which is the United States Department of Energy (DOE) target for 2020 PEMFC’s catalyst loading [16]. However, the current Pt loading of fuel cell vehicles is usually 0.3e0.4 mgPt cm
2. For example, the Toyota

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Mirai, commercially available fuel-cell-based automobile, uses a PtCo/C catalyst with Pt loadings of 0.315 and 0.05 mgPt cm
2 for the cathode and anode, respectively, which are far from the technical targets of the DOE [17]. Recent research has suggested that the performance of low-Pt loading catalysts is limited by the local O2 resistance due to the small Pt specific surface area, especially when the cathode Pt loading is below 0.2 mgPt cm
2 [7]. This issue can be solved through catalyst redesign, for example, the use of a high and stable Pt dispersion (such as by lattice strain engineering [18], alloying [19], core-shell structures [20,21]), the application of substrate materials with high O2 permeability (such as accessible porous carbon [22] or nanoscale ordered arrays [23]), and the utilization of ionomers with high O2 transmission rates for interaction with Pt in a way that does not constrain ORR rates. In these approaches, the material must have a specific structure and size to achieve a high O2 permeability substrate. For instance, Yarlagadda et al. proposed that the available carbon pore size should be in the range of 4e7 nm to ensure O2 transport and limit ionomer penetration, but it is difficult to achieve due to the severe in homogeneous size of the carbon black particles [22]. Ono et al. have demonstrated that lower EW ionomer in the cathodes increased the apparent local resistance, while increasing the EW value of the ionomer was also accompanied by the reduction of proton conductivity [24]. Customized ionomer structures should still be investigated while meeting high O2 transport rates and high proton conductivity. Therefore, the synthesis of a new high-activity, highspecific-surface-area Pt-based catalyst is a simplest and most direct way to solve the above problems. Currently, the highest rated power density with an ultra-low-Pt cathode catalyst has been recorded by Kongkanand from General Motors [16]. It reported a PtCo/C alloy catalyst (PtCo/HSC-f, where HSC is high surface carbon) with a rated power density of 1.32 W cm
2 @ 2 A (Cathode Pt loading 0.063 mgPt cm
2). Kongkanand used a carbon support with a high surface area and large number of micropores and coated the PteCo alloy nanoparticles in the carbon support by using a chemical synthesis method. The PtCo/HSC-f exhibited substantially enhanced ORR activity compared with the PtCo/MSC (Solid carbon) and showed a 31% increase in the rated power density. However, the preparation process for this PtCo/HSC-f catalyst was not reported due to commercial reasons, particularly the details of the preparation of HSC-f. Kongkanand also reported a Pt-monolayer core-shell fuel cell cathode catalyst (PtML/Pd/C), which had a rated power density of 1.14 W cm
2 @ 2 A (Cathode Pt loading 0.05 mgPt cm
2). It is far better than those of PtCo/C (0.84 W cm
2 @ 2 A, cathode Pt loading 0.05 mgPt cm
2) and Pt/C (0.84 W cm
2 @ 2 A, cathode Pt loading 0.05 mgPt cm
2) [20]. Kongkanand proposed that the PtML/Pd/C catalyst showed superior performance because of the greater Pt surface area. More remarkably, a solid carbon (NEH carbon black) substrate material was used to prepare a PtML/Pd/C catalyst. This yielded similar performance to the power density of low-Pt catalysts, while avoided the performance degradation caused by O2 mass loss. For the preparation of more effective Pt-based/C ORR catalysts, various synthetic strategies have been proposed, such as the impregnation method [25,26], colloidal method [27], microemulsion method [28,29], electrochemical deposition method [30,31], ion-exchange method [32,33] and supercritical fluid method [34]. These chemical synthesis methods are complicated and waste of Pt during the production process [35]. For example, the preparation of the PtML/Pd/C core-shell catalyst requires two or more metal precursor chemical reduction processes [36]. Despite its excellent performance, the Pt waste in this process and the high cost of production greatly limit the practical application of coreshell synthesis. Therefore, the development of new preparation process for Pt-based/C catalyst has significant scientific and

practical implications. The magnetron sputtering deposition (MSD) method is a physical approach and has drawn increasing attention because of the simple and flexible preparation, controllable catalyst size, high productivity, low cost and environmental friendliness [37]. The use of the MSD method for the preparation of carbon-supported-Pt catalysts will allow tedious chemical approaches to be replaced, as well as the enhancement of the Pt specific surface area and the creation of new effective nanostructured electrocatalysts [35]. Since Hirano first proposed the application of the MSD for fuel cell catalyst preparation in 1997 [38], extensive research into the sputtering of Pt onto GDL [39,40], Nafion membrane [41,42], NSTF [43,44] and CoeOHeCO3 [45] has been undertaken. However, all preparation methods are based on 2D planar materials or 3D ordered materials as a substrate material. It limits specific surface area of Pt and reduces the effective ion and water transmission channels which make the active area for Pt sputtering low and ion transmission difficult, resulting in low catalytic performance. Here, we propose to use the MSD method for the preparation of an ultra-low-Pt and high specific surface area Pt/C catalyst. This is the first time that carbon-sphere-supported Pt catalysts have been prepared by the MSD method. In our study, we first prepared a monodisperse carbon-layer using water to disperse the surfactantsurface-treated carbon spheres. Subsequently, the MSD was used to deposit Pt nanoparticles on the monodisperse carbon layer to obtain an ultra-low-Pt and high-specific-surface-area Pt/C cayalyst (Fig. 1). In this paper, the performance of this sputtered-Pt/C catalyst is compared with that of the commercial-Pt/C catalyst, and the advantages of this material and potential further developments are also discussed. 2. Experiment section 2.1. Carbon supported Pt catalyst First, carbon powder (Vulcan XC-72, Cabot Corporation, USA) was added to a solution of 30 wt% H2O2 and 98 wt% H2SO4 (3:1 ratio) and treated in an ultrasonic bath for 30 min. Carbon powder, isopropanol (AR, Sinopharm Chemical Reagent Co., Ltd, China), deionized water, and zirconia balls were mixed in a ball mill tank in a certain mass ratio. After ball milling for 1.5 h, the carbon ink was slowly dropped on the surface of the deionized water in the culture dish; then, the carbon particles were spread evenly on the water surface to form an ultra-thin carbon layer. After drying, the ultrathin carbon layer was adsorbed on the bottom of the culture dish. The Pt was sputtered on the surface of the carbon film to prepare the carbon-supported Pt catalyst using a fully automatic magnetron sputtering system (TRP-450, Sky Technology Development Co., Ltd., CAS). The sputtering parameters were as follows: targetesubstrate distance: 90 mm, vacuum: 3  10
4 Pa, working gas pressure: 6 Pa, radio frequency power: 100 W, and sputtering time: 8 s. Fig. 1 shows a schematic diagram of the preparation of the carbonsupported Pt catalyst. 2.2. Preparation of MEA Gore membrane (15 mm) was selected as the proton exchange membrane, and the MEA active area was 5  5 cm2. Commercial Pt/ C (Pt 30 wt%, Johnson Matthey Co. Ltd. USA) was used as the catalyst on the anode side, and the Pt loading was 0.05 mg cm
2. The sputtered Pt/C catalyst was used on the cathode side, and the Pt loading was 0.08 mg cm
2. The catalyst layer was transferred to both sides of the membrane by hot pressing at 150  C for 150 s under a pressure of 1.5 MPa. SGL (Sigracet 25 BC, SGL Group, Germany) and used as the anode gas diffusion layer, where as 10%

Please cite this article as: K. Fu et al., Magnetron sputtering a high-performance catalyst for ultra-low-Pt loading PEMFCs, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152374

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Fig. 1. Schematic diagram of prepared method of sputtered-Pt/C.

hydrophobic GDL (WUT, China) was selected as the cathode layer. In addition, for comparison, an MEA with anode and cathode catalyst layers prepared from commercial Pt/C catalysts was prepared with Pt loadings of 0.05 and 0.08 mg cm
2, respectively.

automated HEPHAS (HTS-125, Hephas Energy Co., Ltd.) fuel cell test station. The single cells with active area of 25 cm2 were operated at 80  C, 100% relative humidity, and a backpressure of 150 kPa. Either pure H2/O2 or H2/air was used as reactant gases in the anode/ cathode.

2.3. Structure characterization Scanning TEM and EDS images were taken to understand the high-resolution morphology and element distribution of the sputtered Pt/C. The phases in the sputtered and commercial-Pt/C catalysts were characterized by XRD. ICP-OES was used to determine the Pt content of the sputtered Pt/C catalyst and the Pt loading of the cathode catalyst layer. XPS was used to determine the valence state and chemical environment of the sputtered Pt. 2.4. Electrochemical characterization The electrochemical characterization was performed in 0.1 M HClO4 solution at room temperature using an electrochemical workstation (CHI 660A, Pine Co. Ltd., USA) and a rotating disk electrode (RDE). CV, ORR, EIS and long-term stability measurements were performed on the sputtered-Pt/C and commercial-Pt/C with the same Pt loading of 0.02 mg cm
2. The measurement setup was a standard three-electrode system, consisting of a working electrode (catalyst ink dropping on glassy carbon with a geometric surface area of 0.196 cm2), a Pt black counter electrode and a RHE. For the CV measurements, the working electrodes were scanned in N2-saturated HClO4 solution between 0 V and 1.2 Vvs. RHE with 100 mV s
1 for 50 cycles to clean the electrode surface electrochemically. The working electrodes were then scanned in N2saturated HClO4 solution between 0.04 V and 1.2 V at 50 mV s
1 for 6 cycles to obtain steady-state CVs. For the ORR measurements, the working electrodes were scanned in O2-saturated HClO4 solution between 0.2 V and 1.1 V at 20 mV s
1 at rotation rates of 1600 rpm. 2.5. Fuel cell operation The single cell performance tests were performed on an

3. Results and discussion Transmission electron microscopy (TEM) micrographs are shown in Fig. 2(aed) and Fig. S1. As show in Fig. 2(a), the 3 nm sized Pt nanoparticles are uniformly distributed on the surface of the carbon spheres (50 nm), forming a Pt coating layer. Similar results are also shown in Fig. S1 (low-magnification (10 nm) and high-magnification (100 nm) images of the sputtered-Pt/C). In contrast with the commercial-Pt/C catalyst (Fig. 2(c)), the sputtered-Pt/C is distributed homogeneously and densely. Particle size analysis indicates that the sputtered-Pt/C possesses a narrower particle size distribution (2.5e3.5 nm, see Fig. 2(b)) than the commercial-Pt/C (1.5e4.5 nm, see Fig. 2(d)). The more uniform distribution and greater number of Pt nanoparticles should increase the Pt-specific surface area of the sputtered-Pt/C, which should improve the catalyst performance [46]. The thickness of the Pt nanoparticles of the sputtered-Pt/C can be calculated from the sputtering time. On the basis of the thickness of Pt-layer on the glass substrate with different sputtering times (Fig. S2), the Pt-layer thickness and sputtering time are linearly related when using the same sputtering parameters (target-substrate distance: 90 mm, vacuum degree: 3  10
4 Pa, argon pressure: 6 Pa, radio frequency power: 100 W): 15.2 nm @ 1 min, 45.6 nm @ 3 min, 136.9 nm @ 9 min. Therefore, the calculated thickness of the sputtered-Pt/C layer is about 2 nm with sputtering time 8 s. Elemental mapping of the original TEM image (Fig. 2(e)), C þ Pt (Fig. 2(f)), C (Fig. 2(g)) and Pt (Fig. 2(h)) maps after sputtering the Pt/C catalyst are shown in Fig. 2(eeh). The results clearly show that the Pt atoms are homogenously distributed over the surface of the carbon spheres without obvious agglomeration and vacancies. Moreover, energy dispersive X-ray spectroscopy (EDS) of the sputtered-Pt/C material indicates only Pt, C and O are presented in the catalyst (Fig. 2(i)).

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Fig. 2. TEM images and particle size analysis of sputtered-Pt/C (aeb) and commercial-Pt/C (ced); elemental mapping of original TEM image (e) and C þ Pt (f), C (g) and Pt (h) elements of sputtered-Pt/C; (i) EDS of sputtered-Pt/C.

Fig. 3. (a) XRD patterns of sputtered-Pt/C and commercial-Pt/C; (bed) TEM images of sputtered-Pt/C.

The X-ray diffraction (XRD) pattern of the sputtered-Pt/C (Fig. 3(a), black) indicates that the Pt nanoparticles fabricated by MSD method are polycrystalline, and the crystal structure is similar to that of the commercial-Pt/C materials (Fig. 3(a), red). Furthermore, as shown in Fig. 3(b, c), a typical polycrystalline structure for Pt nanoparticles was observed, showing lattice fringe spacings of 0.27 and 0.20 nm, which agrees well with spacing values for the Pt(111) and Pt(200) planes of the Pt nanoparticles, respectively. These results are consistent with the XRD analysis presented in Fig. 3(a). More interestingly, the Pt nanoparticles prepared by the MSD method show oriented growth of the Pt(111) plane, which is the highest intensity peak in the XRD data (Fig. 3(a)). This is because the (111) plane has the lowest surface energy and is expected to grow more rapidly than other faces. Fortunately, according to previous research, the Pt(111) crystal plane shows the optimal ORR activity [5]. Therefore, the Pt nanoparticles made by MSD have both enhanced Pt specific surface area and preferred Pt(111) orientation, which improve the overall performance of the sputtered-Pt/C electrodes. The Pt content in the sputtered-Pt/C catalyst was determined by inductively coupled plasma optical emission spectrometry (ICPOES). The integrated area of the ICP-OES curves reveal the Pt concentrations of the sputtered-Pt/C and commercial-Pt/C catalysts: 68.06 mg L
1 (9.5 wt%) and 261.36 mg L
1 (29.7 wt%), respectively (Table 1). Although the Pt content of sputtered-Pt/C is only onethird that of commercial-Pt/C, it exhibits a larger Pt-specific surface area (Fig. 2(a)). Because the Pt specific surface area directly determines the ORR efficiency of the catalyst, the sputtered-Pt/C

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Table 1 ICP-OES of sputtered-Pt/C and commercial-Pt/C. ICP-OES

Sample quality/g

Chemical element

Constant volume/ml

mg/L

wt.%

Sputtered-Pt/C Commercial-Pt/C

0.018 0.022

Pt Pt

25 25

68.06 261.36

9.5 29.7

has a higher Pt utilization rate than commercial-Pt/C proved by its low Pt loading and high specific surface area. X-ray photoelectron spectroscopy (XPS) analysis was employed to determine the chemical composition and bonding states of the elements in the sputtered-Pt/C catalysts (Fig. 4(a)) by using the commercial-Pt/C catalyst for comparison (Fig. 4(b)). In the fullrange XPS survey spectrum, the peak shapes of the sputtered-Pt/ C and commercial-Pt/C samples were found to be essentially the same, having a Pt 4d and 4f peak at ca. 320 and 75 eV, respectively, and a C 1speak at ca. 285 eV. An O 1s peak at ca. 532 eV was observed in the spectra of both the sputtered-Pt/C and commercialPt/C samples, probably because of platinum oxides in the sample. The high-resolution Pt 4f spectra of the sputtered-Pt/C and commercial-Pt/C catalysts are shown in Fig. 4(c and d). As shown in Fig. 4(c), Pt0 4f7/2 and Pt2þ 4f7/2 peaks were observed at 71.5 and 72.5 eV, respectively, and these are associated with two satellite peaks at 74.8 and 75.8 eV [11,47e49]. Clearly, both the sputtered-Pt/ C and commercial-Pt/C catalysts contain platinum oxide, and the Pt0/Pt2þ ratios are 3:1 and 2:1, respectively. The formation of platinum oxide in the catalyst is mainly due to catalyst oxidation in

the air [50]. The cyclic voltammetry (CV) curves of the sputtered-Pt/C and commercial-Pt/C with the same Pt loading (0.02 mg cm
2) are shown in Fig. 5(a). The hydrogen adsorption/desorption peak is at about 0.2 V vs. reversible hydrogen electrode (RHE), which is consistent with literature reports [51]. According to the hydrogen absorption/desorption peak (potential: 0.05e0.4 V vs. RHE) of the CV curves, the electrochemically active surface area (ECSA) of catalyst electrodes can be calculated [51]. The results show that, for the same Pt loading, the ECSA of the sputtered-Pt/C reached 79.6 m2 g
1 Pt , an increase of 34% compared to that of the commercial-Pt/C catalyst (59.2 m2 g
1 Pt ). As discussed in the previous section, the sputtered-Pt/C has a larger specific surface area at the same Pt loading, and the electrochemically active area is related to the Pt specific surface area; thus, the ECSA of the sputtered-Pt/C is greater than that of the commercial-Pt/C electrode. At the same time, the limiting current of sputtered-Pt/C and commercial-Pt/C samples in the linear scanvoltammetry (LSV) curves is between 0.2 V and 1.1 V (Fig. 5(a)). However, the half-wave potential of the sputtered-Pt/C (0.9 V) catalyst is larger than that of the commercial-

Fig. 4. Full-range XPS spectra of sputtered-Pt/C (a) and commercial-Pt/C (b); high-resolution spectra of Pt 4f of sputtered-Pt/C (c) and commercial-Pt/C (d).

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Fig. 5. CV (a) and LSV (b) curves of sputtered-Pt/C and commercial-Pt/C (Pt loading 0.02 mg cm
2, 0.1 M HClO4, N2-saturated for CV, O2-saturated for LSV, scan rate 50 mVs
1 for CV, scan rate 20 mVs
1 for LSV); (c) EIS patterns of ORR at 0.9 V on sputtered-Pt/C and commercial-Pt/C recorded in O2-saturated 0.1 M HClO4 at 1600 rpm in a frequency range from 106 to 10
1; (d) Chronoamperometric responses of sputtered-Pt/C and commercial-Pt/C for ORR in O2-saturated 0.1 M HClO4 at 1600 rpm.

Pt/C catalyst (0.88 V). A possible reason for this is that the surface of the sputter Pt/C catalyst has more exposed Pt(111) planes, which resulted in a higher catalytic activity than that of the commercialPt/C. According to the calculation, the specific activity of sputtered-Pt/C is 346.1 mA/cm2, while commercial Pt/C is 258.2 mA/ cm2. Fig. 5(c) shows the electrochemical impedance spectroscopy of sputtered-Pt/C and commercial-Pt/C at 0.9 V in O2-saturated 0.1 M HClO4 solution. It is observed that the semi-circle and its diameter of sputtered-Pt/C is smaller than that of commercial-Pt/C. This is due to the fact that the polarization resistance of the ORR process at 0.9 V on sputtered-Pt/C is found to be very low, indicating faster kinetics, as compared to that commercial Pt/C [52,53]. The long-term stability of sputtered-Pt/C and commercial-Pt/C for ORR was evaluated using chronoamperometric measurement at 0.6 V (vs. RHE) in an O2-saturated 0.1 M HclO4 solution, as shown in Fig. 5(d). The normalized current of sputtered-Pt/C decreases 26.5% after 50000 s chronoamperometric response, while commercial-Pt/ C fades by 78.6% after 50000 s stability test, confirming much better stability of sputtered-Pt/C as compared to commercial-Pt/C [54]. Such a higher stability of sputtered-Pt/C may be attributed to the narrower particle size distribution. To elucidate difference in the degradation between sputtered-Pt/C and commercial Pt/C, both catalysts before and after 50000 s stability tests were further investigated by TEM. As show in Figs. S3(a and c), Pt nanoparticles of both sputtered-Pt/C and commercial-Pt/C are evenly dispersed on the carbon support in the initial state. The particle size of Pt nanoparticles on both catalysts is increased after 50000 s stability tests (Figs. S3(b and d)), indicating some degree of agglomeration and sintering of the Pt nanoparticles. A comparison of the particle size implies that the particles on sputtered-Pt/C are smaller than the commercial-Pt/C after 50000 s stability tests. The average Pt particles size of sputtered-Pt/C increases to 3.51 nm, while

commercial-Pt/C increases to 3.89 nm. These results suggest that the higher ORR activity of sputtered-Pt/C after 50000 s stability test is attributed to the smaller particle size. The polarization curves of membrane electrode assembly (MEA) samples obtained using the sputtered-Pt/C and commercial-Pt/C cathode catalyst in H2/O2 and H2/air are shown in Fig. 6(a) and (b), respectively. The Pt loading of the sputtered-Pt/C and commercial-Pt/C are both 0.08 mg cm
2 in the MEA cathode (the anode catalyst is commercial-Pt/C, and the Pt loading is 0.05 mg cm
2). As shown in Fig. 6(a), the performance of the sputtered-Pt/C-MEA catalyst under H2/O2 test conditions reached 1.24 W cm
2 @ 2A, far greater than that of the commercial-Pt/CMEA catalysts (1.04 W cm
2 @ 2A). The maximum power density of the sputtered-Pt/C-MEA catalyst exceeded 1.87 W cm
2 @ 3.6 A. Under H2/air test conditions (Fig. 6(b)), the performances of the sputtered-Pt/C-MEA and commercial-Pt/C-MEA were 0.90 W cm
2 @ 2 A and 0.71 W cm
2 @ 2 A, respectively. Nevertheless, the power density at 2 A cm
2 of the sputtered-Pt/C-MEA was 26.7% higher than the commercial-Pt/C-MEA, which is similar to the highest level of performance improvement reported in the literature (PtML/ Pd/C, 1.14 W cm
2 @ 2 A @ 0.05 mg cm
2, 35.7% higher than that of the commercial-Pt/C) [20]. In contrast, our sputtered-Pt/C catalyst has the advantages of being easy to prepare, having high Pt utilization, and being non-polluting. In addition, the performances of sputtered-Pt/C-MEA is far beyond that of sputtering Pt on gas diffusion layer (the maximum power density less than 0.4 W cm
2) [55]. More interestingly, the power density of the sputtered-Pt/CMEA showed a significant drop at high current density (>2 A cm
2), which is caused by O2 mass loss. We believe that the use of a high-O2-permeability carbon (such as HSC-f) instead of Vulcan XC-72 will greatly improve performance of our sputteredPt/C catalyst, and we will investigate this in the future. Moreover,

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Fig. 6. The polarization curves of sputtered-Pt/C-MEA and commercial-Pt/C-MEA under H2/O2 (a) and H2/air (b) test conditions; (c) The internal resistance of sputtered-Pt/C-MEA and commercial-Pt/C-MEA.

the internal resistance of the sputtered-Pt/C-MEA is comparable to the commercial-Pt/C-MEA (~2.3 mU), indicating that the quality of these two MEAs are similar (Fig. 6(c)).

https://doi.org/10.1016/j.jallcom.2019.152374.

4. Conclusion

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A novel ultra-low-Pt Pt/C catalyst with high specific surface area was prepared by MSD method. Compared to the catalyst prepared by conventional chemical methods, the Pt/C catalyst made by MSD has advantages with easier operation, higher Pt utilization, and less pollution. With the same Pt loading, the ECSA of the sputtered-Pt/C showed an enhancement of 34% compared to that of the commercial-Pt/C. In addition, the fuel cell testing revealed that the rated power density of the sputtered-Pt/C is 26.7% higher than the commercial-Pt/C under H2/air test conditions. At present, this novel catalyst now is still limited by equipment capacity of MSD, resulting in it is difficult to achieve continuous production. Nevertheless, it is very promising for large-scale production when the equipment of MSD has been improved in the future. Magnetron sputtering technology has gradually achieved full automation and entered the stage of industrial production, and widely used in the manufacturing of various industries. Thus, we firmly believe that the ultra-low-Pt and high specific surface area Pt/C catalyst fabricated by MSD will have broad applications in commercial fuel cells. Acknowledgement This work was supported by a grant from the National Natural Science Foundation of China (No. 21706200), the Ph.D. Programs Foundation of Ministry of Education of China (No. 2018T110813) and the Fundamental Research Funds for the Central Universities (WUT: 2019IVA079). Appendix A. Supplementary data Supplementary data to this article can be found online at

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Please cite this article as: K. Fu et al., Magnetron sputtering a high-performance catalyst for ultra-low-Pt loading PEMFCs, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152374