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Performance evaluation of platinum-molybdenum carbide nanocatalysts with ultralow platinum loading on anode and cathode catalyst layers of proton exchange membrane fuel cells
Journal of Power Sources 378 (2018) 742–749
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Performance evaluation of platinum-molybdenum carbide nanocatalysts with ultralow platinum loading on anode and cathode catalyst layers of proton exchange membrane fuel cells
T
Shibely Saha, José Andrés Cabrera Rodas, Shuai Tan, Dongmei Li∗ Department of Chemical Engineering, University of Wyoming, Laramie, 80271, United States
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
nanoparticles (< 3nm) were de• Ptposited on β-Mo C nanotubes via ro2
• • •
tary ALD. Catalysts were applied to anode and cathode of MEAs without carbon black support. Enhanced durability was demonstrated using a rigorous ADT protocol. MEA performance was evaluated for two fabrication methods and GDL structures.
A R T I C L E I N F O
A B S T R A C T
Keywords: Transition metal carbide Phase-pure support Ultra-low platinum Cathodic performance Membrane electrode assembly Electrocatalyst durability
An alternative catalyst platform, consisting of a phase-pure transition carbide (TMC) support and Pt nanoparticles (NPs) in the range of subnanometer to < 2.7 nm, is established that can be used in both anode and cathode catalyst layers. While some TMCs with low Pt loadings have demonstrated similar activity as commercial Pt catalyst in idealized disk electrode screening tests, few to none have been applied in a realistic fuel cell membrane electrode assembly (MEA). We recently reported that β−Mo2C hollow nanotubes modified with Pt NPs via atomic layer deposition (ALD) possess better activity and durability than 20% Pt/C. This paper presents systematic evaluation of the Pt/Mo2C catalysts in a MEA, investigating effects of different MEA preparation techniques, gas diffusion layers (GDL) and various Pt loadings in the ultralow range (< 0.04 mg/cm2) on MEA performance. Most importantly, we demonstrate, for the first time, that Pt/Mo2C catalyst on both anode and cathode, with a loading of 0.02 mg (Pt) cm−2, generated peak power density of 414 mW cm−2 that corresponds to 10.35 kWgPt−1 using hydrogen (H2) and oxygen (O2). Accelerated degradation tests (ADT) on Pt/ Mo2C catalysts show 111% higher power density than commercial 20% Pt/C after the vigorous ADT.
1. Introduction Development and sustainable commercialization of low-temperature (LT) and high-temperature (HT) proton exchange membrane fuel cells (PEMFC) or electrolyzers needs further reduction of platinum (Pt) loading without compromising durability. Currently, the cost for
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PEMFC has decreased to $55/kW, of which 50% is for Pt catalyst. However, to achieve the cost target of $40/kW by 2020, Pt use need to be further optimized [1]. In addition, Department of Energy (DOE) Hydrogen and Fuel Cell Program Plan 2011 set the target of Pt utilization for fuel cell automobiles as 8 kW gPt−1 for 2017–2020 [1]. These goals may be met by improving effective surface area of Pt catalyst via
Corresponding author. E-mail address: [email protected] (D. Li).
https://doi.org/10.1016/j.jpowsour.2017.12.062 Received 19 October 2017; Received in revised form 18 December 2017; Accepted 21 December 2017 Available online 02 February 2018 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
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was reported as Pt support for ORR electrocatalysis due to its similar dband structure to Pt [14–16]. For example, Pt nanocrystal (20 wt%) deposited on Mo2C-C nanocrystal showed better ORR activity compared to 20% Pt/C [14]. However, Mo2C has also been reported to show strong oxidation characteristics [17]. Yan et al. demonstrated that 40% Pt supported on Mo2C, which used carbon black as a secondary support, demonstrated higher ORR activity compared to commercial 40% Pt/CTKK catalyst with sufficient stability [18]. The authors speculated that the improved stability of Pt/C-Mo2C towards oxidation might have resulted from the synergistic and binding effect of Pt and Mo2C. Xin et al. indicated that as the bonding strength between Pt and its support increases, resultant catalysts will be more durable in ORR condition [19]. Recent results from our group also demonstrated that after modification of Mo2C surface by depositing Pt NPs via ALD onto Mo2C nanotubes the stability of the catalyst significantly increased due to strong bonding between Pt and Mo2C [11]. These studies indicate that Pt supported on pristine β-Mo2C catalyst may provide higher ORR activity, due to increased bonding between Pt and phase-pure Mo2C support, than commercial Pt/C catalyst. Following these studies we hypothesize that the noticeable activity towards ORR along with the strong bonding between Pt and Mo2C as observed from XPS would allow this catalyst to be successfully applied as PEMFC cathode. In this paper, the 4.4%Pt/Mo2C catalyst was applied in both anode and cathode, with systematic investigation on effects of GDL, catalyst loading and MEA fabrication techniques. Activity and stability of the catalyst was rigorously tested in cathodic conditions using a rotating ring disk electrode (RRDE) and MEA via an accelerated degradation test (ADT) protocol specified by DOE [20]. To the best of our knowledge this is the first report in applying Pt/Mo2C catalyst in PEMFC cathode, without the need of using a secondary catalyst support.
rational catalyst design [2], since the triple phase boundary (TPB) among the catalyst, electrolyte and the reactants in the catalyst layer (CL) plays a crucial role in PEMFC performance. In 2006, Matsumoto et al. reported that 16% molybdenum carbide supported on carbon nanotube was used as an PEMFC anode and reached 50% of the open circuit potential commercial 29% Pt/C achieved [3]. Recent study on different morphologies of Mo2C showed further enhancement in electrocatalytic performance towards HER and HOR. However, activity of bare Mo2C still cannot compare to commercial 10% Pt/C. In addition, none of the Mo2C catalysts were used in a single cell. There are some TMCs that were used in the single cell, but very poor performance was observed compared to commercially available 10–20% Pt/C. For example maximum power density from WC and WNi/C was only 5.7 and 7.3%, respectively, of that from 20 wt% Pt/C [4]. Izhar et al. showed maximum power density of cobalt-tungsten carbide and molybdenum-tungsten carbide supported on Ketjen carbon was only 14 and 11%, respectively, of that from 20% Pt/C [5]. However, research in Pt modified TMCs (with ≥10% Pt loading) primarily focuses on anodic applications in PEMFC, in addition to the lack of performance evaluation in MEAs due to the poor stability of carbides at higher potential. For example, 20% Pt NPs supported on Mo2C were shown to have less anodic activity than 20% Pt/C in PEMFC with 0.4 mg/cm2 Pt loading [6]. Another study on 10% Pt/WC showed less anodic activity than 10% Pt/C [7]. Therefore, a systematic investigation is still needed to investigate effect of different preparation parameters on PGM supported on TMCs, especially exploring their applications on cathodes of PEMFC, based on the first principle of density functional theory (DFT) predications that Mo and W-based carbides can be effective supports for Pt single atoms [8]. Experimentally, Hunt et al. synthesized self-assembled transition metal carbides coated with Pt or Pt alloy monolayer, which were highly resistant to sintering and CO poisoning, in addition to enhanced activity and durability for methanol electrooxidation process [9,10]. Combining theoretical and experimental observations, we hypothesize that Pt particles (< 3 nm) supported on pristine TMC surfaces with low Pt loading (< 5%) may be used on the cathode size of PEMFC (or anode size of an electrolyzer) [11,12]. Moreover, it will be highly desirable to evaluate catalyst performance of such catalysts against DOE target in an MEA. From our earlier study it was observed that 100 cycles of Pt deposition via ALD resulted in 4.4% loading by weight (referred to as 4.4%Pt/ Mo2C hereafter), demonstrated higher mass activity of Pt than 20% Pt/ C [12]. Therefore, 4.4%Pt/Mo2C was chosen for the systematic MEA performance evaluation of the Pt/Mo2C catalyst platform. Since both microstructure of GDL and MEA preparation techniques play an important role in TPB interaction, we will investigate of PEMFC performance of 4.4%Pt/Mo2C using two different types of GDLs and fabrication techniques. Different microporous structures in GDL determine the difference in in-plane vs. through-plane conductivity. For example, SGL 10BC was reported to show best performance with 20% Pt/C than E-TEK GDL 1200W and Freudenberg H2315C2, resulting from its lower bulk density and porosity with higher permeability [13]. In this paper, we compare SGL 10 BC to Torray carbon paper by either brush painting or spray coating 4.4%Pt/Mo2C catalyst onto GDLs. The coated catalyst layer (CL) together with GDL will be referred to as gas diffusion electrode (GDE). It is worth mentioning that GDLs also help prevent the CL from water flooding. Catalyst loading is another important parameter that affects the performance of a PEMFC. Generally, increasing Pt loading in the electrode result in enhanced performance. However, increase in total catalyst loading also increases the CL thickness, introducing higher mass transfer resistance to the reactants and the products. In addition, increased total catalyst loading worsens particle agglomeration. As a result, it is important to optimize catalyst loading. In order to minimize mass transfer resistance, it is desirable to further reduce Pt loading for both anode and cathode. Therefore, the feasibility to use Pt/Mo2C catalyst in the cathode was investigated in this paper. Previously, Mo2C
2. Experimental 2.1. Synthesis of Mo2C and Pt/Mo2C Both β-Mo2C and Pt ALD modified β-Mo2C (referred to as Mo2C for simplicity hereafter) nanotube synthesis methods were discussed in detail previously [11,12]. Briefly, Mo2C nanotubes were synthesized via a salt flux reaction using sodium fluoride (NaF), sodium chloride (NaCl), multiwalled carbon nanotube (MWCNT) and molybdenum powder (all from Sigma-Aldrich and used as received). The synthesized Mo2C nanotubes were then pretreated in 15% CH4/H2 at 590 °C (3 °C/ min) over 4 h to remove any possible oxides, which was followed by 100 cycles of ALD to deposit Pt onto Mo2C nanotubes in a rotating reactor maintained at 200 °C, resulting in 4.4% Pt in mass loading. Platinum (IV) Trimethyl Methylcyclopentadienyl (MeCpPtMe3) and oxygen were used as ALD precursors. 2.2. Characterizations Transmission electron microscopy (TEM) images and Brunauer–Emmett–Teller (BET) surface area results for 4.4% Pt/Mo2C catalyst can be found in our recent publications [11,12]. In order to characterize morphologies of the catalyst layer via brush painting and ultrasonic spray, scanning electron microscope images (SEM) (FEI Quanta FEG 450 FESEM) were collected for both top surface and crosssection areas. Also, cross-section images of membrane electrode assemblies (MEAs) containing 4.4% Pt/Mo2C catalysts were obtained before and after ADT to investigate the structure change of MEAs during ADT. 2.3. MEA fabrication, conditioning and performance evaluation It should be mentioned that the catalysts were pretreated in 15% CH4/H2 at 590 °C (3 °C/min) over 4 h to be used in any electrochemical measurement, in order to eliminate any effects from possible oxides on 743
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Fig. 1. Polarization and Power density curve of brush painted (a) 20% Pt/C in both anode and cathode with the loading of 0.4 mg Pt/cm2 and (b) 4.4% Pt/Mo2C on anode onto different carbon papers. 4.4%Pt/Mo2C on anode with Pt loading of 0.05 mg Pt/cm2 for SGL 10BC and 0.06 mg Pt/cm2 for Torray Carbon Paper collected at 1 atm pressure.
(glassy carbon disk and a platinum ring) and a CompactStat e10800 potentiostat (Ivium Technology). Data were collected under N2 atmosphere (0.2 L per min) using 0.1 M HClO4 electrolyte at a rotating rate of 1600 rpm. A Ag/AgCl electrode saturated in 4 M KCl was used as the reference electrode, with Pt wire being the counter electrode. 4.4% Pt/ Mo2C catalyst ink was deposited (10 μL) onto the glassy carbon surface (5 mm diameter) to serve as the working electrode. We like to point out that the ink concentration is by no means optimized for the 4.4% Pt/ Mo2C. The ink was prepared by adding 2.66 mg of 4.4% Pt/Mo2C catalyst to 52.2 mL of Nafion® solution (5 wt% Nafion® ionomers in ethanol, Sigma-Aldrich), followed by sonicating the solution for 15 min. After the ink deposition, vacuum drying was allowed for 15 min to enable proper adhesion of the ink-catalyst to the glassy carbon surface. Prior to collecting data, the working electrode was conditioned by cycling for 30 cycles between 0 and 1.2 V (vs. RHE) at 50 mV s−1. To collect cyclic voltammetry curves for ECSA calculation, a lower scan rate of 40 mV s−1 was used. CV curves were collected after conditioning cycles, with ECSA being calculated by integrating the hydrogen adsorption/desorption area (0.06–0.4 V vs. RHE) [22,23].
Mo2C [6]. Membrane electrode assemblies (MEAs) in this study were made using Nafion® 212 (DuPoint), which was treated using the same process adopted from Herring et al. [21] Briefly, Nafion® 212 membrane was first boiled in 3% H2O2 for one hour, followed by one hour of boiling in deionized (DI) water, then one hour of boiling in 1 M H2SO4, and finally one hour of boiling in DI water. Treated membranes were stored in DI water in a dark location before use. SGL 10 BC carbon paper (Ion Power) was used as Gas Diffusion Layer (GDL). The gas diffusion electrode was prepared by either painting or spraying (using ultrasonic accumist spray) the GDL. Catalyst inks were prepared by combining desired catalyst, methanol (for 20% Pt/C) or ethanol (for Pt/ Mo2C), and Nafion® solution. 5% Nafion® ionomer solution (DuPont) was added such that the Nafion® solids were 30% of the total mass of catalysts and Nafion® solids in the ink. Methanol or Ethanol was added in an amount that was ten times the mass of catalyst in the ink for brush paint technique and at a volume required to maintain 0.8 mg of catalyst/ml of ink concentration for SonoTek Ultrasonic Accumist sprayer. The ink was then sonicated for 20 min. The catalyst loading on the GDL was determined by measuring the weight of an empty GDL and the weight after depositing catalyst on the GDL using a microbalance (Sartorius). The Nafion® membrane with gas diffusion electrode on both sides was pressed using digital combo multi-purpose heat press (DC14, GEO Knight & Co. Inc.). The pressing condition was maintained at 135 °C at 80 psig for 5 min. The MEA was tested using a fuel cell testing station (Scribner Associates Inc.) at 80 °C. H2 and O2 flow rates were maintained at 0.2 L/min with relative humidity of 100% for both gas streams. Before starting each polarization experiment MEAs were conditioned by humidifying them at 80 °C for one hour, followed by holding the potential at 0.6 V for one hour. Sequentially, potential was altered between 0.7 and 0.5 V (holding at each voltage for twenty minutes) for the total duration of twelve hours then holding at current of 1 ampere (A) for seven hours. To systematic evaluate anode performance, polarization experiments were performed with 4.4%Pt/Mo2C and 20% Pt/C on anode side, while keeping 20% Pt/C on the cathode with Pt loading of 0.4 mg Pt/cm2. To evaluate MEA performance on both anode and cathode, 4.4%Pt/Mo2C and 20% Pt/C with the same Pt loading (0.02 mg/cm2) were applied in the catalyst layers, respectively.
2.4.2. Accelerated degradation test (ADT) Durability of MEA with 4.4% Pt/Mo2C catalyst on both anode and cathode sides was investigated by the accelerated degradation test (ADT) procedure specified by DOE, as detailed by Han et al. [24] Based on this procedure, the anode of the fuel cell was used as reference electrode and cathode was used as the working electrode. On the anode side H2 flowrate was maintained at 0.2 L/minute (lpm), with cathode side flowing N2 at 0.05 lpm. The potential was cycled between 0.6 V to 1 V at a scan rate of 500 mV s−1 for 30,000 cycles [20]. 3. Results and discussion 3.1. Performance comparison for different gas diffusion layers (GDL) To investigate how Pt/Mo2C catalyst performs in MEAs fabricated with different GDLs, Torray carbon paper (Fuel Cell store) and Sigracet SGL 10BC (Ion Power) were selected since they are both widely used. Current density and peak power density were normalized by Pt loading on the anodes for GDL effect and are referred to as mass current density and mass peak power density, respectively. Fig. 1 demonstrates the effect of different gas diffusion layers (GDLs) on fuel cell performance for 4.4%Pt/Mo2C. Consistent with what Millington et al. reported, Sigracet SGL 10BC outperformed Torray carbon paper dramatically for both 20% Pt/C (Figs. 1a) and 4.4%Pt/Mo2C (Fig. 1b) [13].
2.4. Durability study 2.4.1. Potential cycling in RRDE Durability of the 4.4% Pt/Mo2C catalyst was tested using both rotating ring disk electrode (RRDE) and ADT. For RRDE, 10,000 potential cycles were carried out at the scan rate of 200 mVs−1 using Pine MSR 744
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4.4%Pt/Mo2C (0.015 mg/cm2) resulted in 114% increase in mass current density and 67.4% increase in peak mass power density, respectively, for ultrasonic spray technique which is consistent with the percent increase reported for 20% Pt/C [13]. To understand the effect of catalyst particle size distribution for 4.4%Pt/Mo2C with different fabrication techniques, electrodes were peeled off from the MEAs fabricated. Fig. 4 presents the SEM images of the cross section of the MEAs and the Nafion® membrane surfaces. Comparing the cross section of MEAs fabricated by two techniques (Fig. 4a and b) revealed that the MEA fabricated by brush paint has a bigger gap between membrane and the GDL possibly resulting from the fact that bigger particle size was observed for brush paint technique (Fig. 4c).
Impressively, 77.4% and 76.2% increase in current and peak power density per area, respectively, was observed with SGL 10BC for 4.4%Pt/ Mo2C compared to torray carbon paper. We postulate that the 3-D nonwoven, microporous structure of Sigracet SGL 10BC not only facilitates higher air permeability, due to its inherent open pore feature, it also significantly provides better dispersion of the 4.4%Pt/Mo2C which in turn increases the active sites of the catalyst. Consequently, the microporous layer of the SGL 10BC increased the through-plane conductivity, by significantly enhancing mass transfer, relative to 2-D fibrous structure of the Torray carbon paper [25]. Since the SGL 10BC outperformed Torray carbon paper, all our MEA were prepared using SGL 10BC as the GDL. 3.2. Effect of fabrication techniques on the performance of Pt modified Mo2C
3.3. Effect of catalyst loading in the anode fabricated by ultrasonic spray technique
With ultralow Pt loading (< 0.02 mg/cm2), 4.4%Pt/Mo2C catalyst outperformed commercial 20% Pt/C when the same amount of Pt loading of both catalysts was applied to anode, while keeping 0.4 mg/ cm2 Pt loading on cathode, as reported by our group previously [12]. To further optimize fuel cell performance in the ultralow Pt loading regime, ultrasonic spray technique (Acumist® from SonoTek Inc.) was applied to fabricate anodes. It has been reported in open literature that ultrasonic spray techniques resulted in further reduced Pt loading without compromising performance [13]. Specifically, for our Pt/Mo2C catalyst platform we hypothesize that a sophisticated spray technique, such as the Ultrasonic Accumist spray system by SonoTek, enables better catalyst dispersion by minimizing catalyst aggregation. The combination of ultrahigh surface areas, combined with optimized distribution of the catalyst, may allow further reduction of Pt loading. To establish a baseline for the fabrications techniques, 20% Pt/C catalyst with known performance was first applied in both anode and cathode by both brush-painting and ultrasonic spray (Fig. 2a and b). PEMFC performance data of the 20% Pt/C showed that MEAs fabricated by the spray technique resulted in 76.5% and 79% increase of mass current density at 0.6 V and peak mass power density, respectively. The performance enhancement by ultrasonic spray technique is consistent with what was reported by other groups. The improved performance was attributed to smaller particle size of the catalyst, thanks to the atomization of catalyst ink during ultrasonic spray processes. The MEA performance comparison of our 4.4%Pt/Mo2C catalyst from the two preparation approaches is presented in Fig. 3. Despite of the slightly lower Pt loading (0.015 mg/cm2), the MEA prepared by Ultrasonic Spray resulted in 58.3% higher current density per area at 0.6 V and 25.8% higher peak power density per area (Fig. 3a) compared to Brush Paint technique (Pt loading: 0.02 mg/cm2). As for the mass current density and peak mass power density (Fig. 3b), ultralow loading of
To probe how the catalyst loading affects the MEA performance in ultra-low loading regime, MEAs were fabricated by ultrasonic spray technique with Pt loadings of 0.015, 0.022 and 0.029 mg Pt/cm2 in the anode (Fig. 5). Interestingly, MEA with lower Pt loadings outperformed the ones with higher Pt loadings, which is consistent with what was observed previously for brush painted MEAs with 4.4% Pt/Mo2C on the anode side [12]. We postulate that Pt/Mo2C tends to agglomerate and better activity may be achieved by minimizing Pt agglomeration by lower loadings on the GDE. We do want to point that catalyst particles settled down in the tube of the sprayer during spraying the catalyst ink, possibly resulting in lower catalyst concentration in the ink at the end of spraying than the initial concentration. With Nafion® being more dispersed in the solution the weight used for calculation of the Pt loading might end up with larger contribution from Nafion® ionomer. The challenge of catalyst precipitating may also explain the compromised MEA performance as Pt loading increased to 0.029 mg/cm2. Since longer spraying time required to achieve this loading may have caused more catalyst particles settling out of the ink dispersion. To resolve this issue more investigation is required to pick a suitable dispersion solvent for Pt/Mo2C catalyst ink formation, in addition to customizing the SonoTek spray system by shortening the catalyst ink delivery line. 3.4. Performance of Pt/Mo2C in cathode Based on the CV data from a three-electrode static cell in our previous publication [12], the Pt/Mo2C catalysts demonstrated significant activity towards ORR and oxidation stability. Due to the technical challenge with our SonoTek spray system as discussed above, the cathode was prepared by brush painting to achieve a loading of
Fig. 2. (a) Polarization and Power density curves and (b) Polarization and Power density curves per Pt mass in anode. Commercial 20% Pt/C in both anode and cathode for both techniques with the loading of 0.3 mg Pt/cm2 in the cathode, 0.39 mg Pt/cm2 for ultrasonic spray and 0.38 mg Pt/cm2 for Brush Painting in the anode. GDL: Torray Carbon Paper.
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Fig. 3. (a) Area and (b) specific polarization and power density curves with 4.4%Pt/Mo2C on SGL 10BC as GDL (solid line: ultrasonic spray; dashed line: brush painting) 4.4%Pt/Mo2C on anode with Pt loading of 0.015 mg Pt/cm2 for ultrasonic spray and 0.02 mg Pt/cm2 for brush paint respectively.
Fig. 4. SEM images of cross sectional MEAs fabricated by (a) brush paint and (b) ultrasonic spray technique with 4.4%Pt/ Mo2C catalyst; top view of Nafion® membranes collected from MEA fabricated by (c) brush paint and (d) ultrasonic spray technique.
Fig. 5. Effect of 4.4%Pt/Mo2C catalyst loading on MEA performance with SGL 10BC as GDL via Ultrasonic Spray: (a) Polarization and Power density curves, and (b) Polarization and Power density curves per Pt loading in anode. Anode Pt loading: 0.015 mg/cm2; 0.022 mg/cm2 and 0.029 mg/cm2. Pt loading for 20% Pt/C was maintained constant on the cathode (0.4 mg Pt/cm2).
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Fig. 6. (a) Area and (b) specific polarization and power density curves of MEAs fabricated via brush painting with 4.4 wt % Pt/Mo2C and 20 wt% Pt/C (Vulcan). Solid line: 4.4% Pt/Mo2C catalyst; dashed line: commercial 20% Pt/C. Pt loading: 0.02 mg Pt/cm2 on both anode and cathode for both catalysts; Cell Area: 5 cm2; operating conditions: H2 flowrate in anode and O2 flowrate in cathode is 0.2 L/min; Cell Temperature: 80 °C; Back pressure: 2 bar.
0.02 mg/cm2. Anode Pt loading was maintained at 0.02 mg/cm2. A MEA with the same Pt loading on anode and cathode sides was fabricated via brush painting with commercial 20% Pt/C to serve as a bench mark. Shown in Fig. 6, Pt/Mo2C catalyst achieved 27.5% higher current density (at 0.6 V) and 32.2% higher maximum power density than 20% Pt/C. We note that MEA reported here has comparable or better performance, compared to MEAs made of ultralow Pt loading [13] The maximum power per mass of Pt was found to be 10.35 kWgPt−1 (Fig. 6b). As a reference, US DOE target for the period 2017–2020 is 8 kWgPt−1 for H2 and air in transportation applications [1]. The polarization curve data for 4.4%Pt/Mo2C reported here is collected after 6 days of continuous voltage scan in addition to the 21 h of conditioning and the performance kept increasing. This indicates this catalyst is durable for practical operating conditions. To determine the durability of our catalyst against Pt dissolution, the electrochemically active surface area (ESCA) before and after 10,000 cycles of potential cycling were calculated from the CV profile generated using a rotating ring-desk electrode. As shown in Fig. 7a, one irreversible peak appeared at 0.47 V vs. RHE, which could result from high concentration of Nafion® ionomer used in the catalyst ink [26,27] or oxidized molybdenum species [6]. Subbaraman et al. reported that the specific adsorption of sulfonate anions of Nafion® on Pt(111) could contribute to the so-called “mini-butterfly” CV irreversible peak at around 0.5 V vs. RHE [26,27]. This group also reported that if the peak near 0.47 V is caused by Nafion® ionomer, it will disappear after switching the electrolyte from HClO4 to H2SO4. Fig. 7b shows that this peak remains even with 0.5 M H2SO4 as the electrolyte. Therefore, the peak near 0.47 V is most likely from oxidized molybdenum species. Ongoing work in our group aims to further understand the surface chemistry of the oxidized Mo species via XPS and x-ray absorption near edge structure (XANES). In Fig. 7a, the degradation of 4.4% Pt/Mo2C catalyst was presented by the reduced integrated area in the hydrogen adsorption/desorption region (0.06–0.4 V vs. RHE). The ECSA decreased from 31.87 m2/gPt to 20.34 m2/gPt after the cycling. Consistent with the RRDE results, Fig. 7b and Table .1 summarize the ADT data, showing that the degradation from ADT was more severe for 20% Pt/C (87% decrease in maximum power density) than 4.4%Pt/Mo2C (78% decrease in maximum power density). However, it is worth pointing out that the reduced ECSA is still higher than the commercial 20% Pt supported on Vulcan (Alfa Aesar), which has an ECSA value of 13.54 before cycling under the same potential cycling conditions. The apparently lower ECSA value for 20%Pt/C may be attributed to the suboptimal ink composition.
The polarization curve of 20% Pt/C collected after ADT demonstrated wiggly performance at high current density indicating unreliable activity of 20% Pt/C. Abruptness in performance was also observed for 4.4%Pt/Mo2C after ADT. Nonetheless, the compromised performance of 4.4%Pt/Mo2C in a MEA (applied to both anode and cathode sides) was further characterized by SEM images of the MEA in crosssectional areas after the ADT, which showed the noticeable microporous voids that may have resulted in detrimental effects on the TPB, consequentially MEA performance (Fig. 7c). However, after this vigorous ADT 4.4%Pt/Mo2C still maintained 212.5% higher current density and 111% higher maximum power density than 20% Pt/C indicating 4.4%Pt/Mo2C to be more resistant to Pt dissolution than 20% Pt/C. We speculated that the synergetic interaction between Mo2C support and Pt particles (< 3 nm) may prohibit sintering and migration of Pt particles [11], responsible for better MEA performance after 30,000 potential cycling than Pt particles supported on inert carbon black. 4. Conclusion Systematic study on fabrication techniques, GDL and Pt loadings demonstrated that Pt/Mo2C catalysts perform very reliably in low loading regime (< 0.03 mg/cm2), while achieving better durability than commercial 20% Pt/C. Catalysts with ultralow Pt loading and desired durability are well sought after because they enable further reduction in cost and stack size of PEMFC automobiles. We note, as a reference, that the MEA peak power density using H2 and O2 with Pt/ Mo2C in both anode and cathode is higher than the target set by DOE for 2017–2020 for H2 and air [1]. Given that our MEAs were mostly fabricated by brush painting, our ongoing effort focuses on further optimization of MEA performance by improving catalyst dispersion and our ultrasonic spray system. Additionally, after the catalyst delivery system of our SonoTek system is optimized, MEAs will be only fabricated by the spray technique and tested under H2 and air conditions. To summarize, the 4.4%Pt/Mo2C catalyst showed significantly higher durability with 111% higher end of life activity than commercially 20% Pt/C after the rigorous ADT. Acknowledgements We would like to thank Professor Andrew Herrings for generously sharing their knowledge with MEA fabrication via brush painting and testing protocols. Shibely Saha acknowledges financial support for her study through the University of Wyoming Academic Affairs Energy 747
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Fig. 7. (a) CV profile of 4.4% Pt/Mo2C catalyst before and after 10,000 cycling in a rotating ring-desk electrode in 0.1 M HClO4 electrolyte; (b) CV profile of 4.4% Pt/Mo2C catalyst in 0.5 M H2SO4 electrolyte; (c) Polarization and power density curves of MEAs fabricated with 4.4%Pt/Mo2C and 20% Pt/C (solid line: 4.4%Pt/Mo2C catalyst; dashed line: 20% Pt/C) with Pt loading of 0.02 mg Pt cm−2 on both anode and cathode showing the performance after 30,000 cycles of potential cycling from 0.6 V to 1.0 V; (d) Cross section of MEA containing 4.4% Pt/Mo2C catalyst after 30,000 potential cycling. Dash cycle: microporous voids.
electrospray method, J. Power. Sources 229 (2013) 179–184. [3] T. Matsumoto, Y. Nagashima, T. Yamazaki, J. Nakamura, Fuel cell anode composed of Mo2C catalyst and carbon nanotube electrodes, Electrochem. Solid State Lett. 9 (2006) A160–A162. [4] M. Nagai, A.M. Zahidul, K. Matsuda, Nano-structured nickel-molybdenum carbide catalyst for low-temperature water-gas shift reaction, Appl. Catal. Gen. 313 (2006) 137–145. [5] S. Izhar, M. Yoshida, M. Nagai, Characterization and performances of cobalttungsten and molybdenum-tungsten carbides as anode catalyst for PEFC, Electrochim. Acta 54 (2009) 1255–1262. [6] A. Hassan, V.A. Paganin, A. Carreras, E.A. Ticianelli, Molybdenum carbide-based electrocatalysts for CO tolerance in proton exchange membrane fuel cell anodes, Electrochim. Acta 142 (2014) 307–316. [7] V.M. Nikolic, D.L. Zugic, I.M. Perovic, A.B. Saponjic, B.M. Babic, I.A. Pasti, M.P. Marceta Kaninski, Investigation of tungsten carbide supported Pd or Pt as anode catalysts for PEM fuel cells, Int. J. Hydrogen Energy 38 (2013) 11340–11345. [8] C.K. Poh, S.H. Lim, J. Lin, Y.P. Feng, Tungsten carbide supports for single-atom platinum-based fuel-cell catalysts: first-principles study on the metal-support interactions and O2 dissociation on WxC low-index surfaces, J. Phys. Chem. C 118 (2014) 13525–13538. [9] S.T. Hunt, M. Milina, A.C. Alba-Rubio, C.H. Hendon, J.A. Dumesic, Y. Románleshkov, Self-assembly of noble metal monolayers on transition metal carbide nanoparticle catalysts, Science 352 (2016) 974–978. [10] S.T. Hunt, M. Milina, Z. Wang, Y. Román-Leshkov, Activating earth-abundant electrocatalysts for efficient, low-cost hydrogen evolution/oxidation: sub-monolayer platinum coatings on titanium tungsten carbide nanoparticles, Energy Environ. Sci. 9 (2016) 3290–3301. [11] S. Tan, L. Wang, S. Saha, R.R. Fushimi, D. Li, Active site and electronic structure elucidation of Pt nanoparticles supported on phase-pure molybdenum carbide nanotubes, ACS Appl. Mater. Interfac. 9 (2017) 9815–9822. [12] S. Saha, B. Martin, B. Leonard, D. Li, Probing synergetic effects between platinum nanoparticles deposited via atomic layer deposition and a molybdenum carbide
Table 1 Different parameters before and after 30,000 potential cycling collected from the MEA performance of different catalysts. Catalyst
20% Pt/C 4.4%Pt/ Mo2C
Open Circuit Voltage/V
Current Density at 0.6 V/mA cm−2
Maximum Power Density/ W cm−2
Initial
After 30000 cycles
Initial
After 30000 cycles
Initial
After 30000 cycles
0.922 0.924
0.679 0.773
272 385
20 62.5
0.313 0.414
0.0418 0.0882
Graduate Assistantship program. Dongmei Li is grateful for the continuous support from School of Energy Resources (SER) at University of Wyoming for funding, in addition to Wyoming NASA Space Grant Consortium (NNX13AB13A) for the competitive Faculty Fellowship Award. References [1] U.S Department of Energy, The Department of Energy Hydrogen and Fuel Cells Program Plan, (2011), pp. 1–92. [2] S. Martin, B. Martinez-Vazquez, P.L. Garcia-Ybarra, J.L. Castillo, Peak utilization of catalyst with ultra-low Pt loaded PEM fuel cell electrodes prepared by the
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[21] J.R. Ferrell, M.C. Kuo, A.M. Herring, Direct dimethyl-ether proton exchange membrane fuel cells and the use of heteropolyacids in the anode catalyst layer for enhanced dimethyl ether oxidation, J. Power. Sources 195 (2010) 39–45. [22] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Highly crystalline multimetallic nanoframes with threedimensional electrocatalytic surfaces, Science 343 (2014) 1339–1343. [23] J.M. Doña Rodríguez, J.A. Herrera Melián, J. Pérez Peña, Determination of the real surface area of Pt electrodes by hydrogen adsorption using cyclic voltammetry, J. Chem. Educ. 77 (2000) 1195. [24] B. Han, C.E. Carlton, A. Kongkanand, R.S. Kukreja, B.R. Theobald, L. Gan, R. O'Malley, P. Strasser, F.T. Wagner, Y. Shao-Horn, Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells, Energy Environ. Sci. 8 (2015) 258–266. [25] M.S. Ismail, T. Damjanovic, D.B. Ingham, M. Pourkashanian, A. Westwood, Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells, J. Power Sources 195 (2010) 2700–2708. [26] R. Subbaraman, D. Strmcnik, A.P. Paulikas, V.R. Stamenkovic, N.M. Markovic, Oxygen reduction reaction at three-phase interfaces, Chem. Phys. Chem 11 (2010) 2825–2833. [27] R. Subbaraman, D. Strmcnik, V. Stamenkovic, N.M. Markovic, Three phase interfaces at electrified metal - solid electrolyte systems 1. Study of the Pt (hkl) - nafion interface, J. Phys. Chem. C 114 (2010) 8414–8422.
nanotube support, J. Mater. Chem. A Mater. Energy Sustain 4 (2016) 9253–9265. [13] B. Millington, V. Whipple, B.G. Pollet, A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique, J. Power. Sources 196 (2011) 8500–8508. [14] L. Elbaz, C.R. Kreller, N.J. Henson, E.L. Brosha, Electrocatalysis of oxygen reduction with platinum supported on molybdenum carbide-carbon composite, J. Electroanal. Chem. 720–721 (2014) 34–40. [15] M. Pang, C. Li, L. Ding, J. Zhang, D. Su, W. Li, C. Liang, Microwave-assisted preparation of Mo2C/CNTs nanocomposites as efficient electrocatalyst supports for oxygen reduction reaction, Ind. Eng. Chem. Res. 49 (2010) 4169–4174. [16] R.B. Levy, M. Boudart, Platinum-like behavior of tungsten carbide in surface catalysis, Science 181 (1973) 547–549. [17] E.C. Weigert, D.V. Esposito, J.G. Chen, Cyclic voltammetry and X-ray photoelectron spectroscopy studies of electrochemical stability of clean and Pt-modified tungsten and molybdenum carbide (WC and Mo2C) electrocatalysts, J. Power Sources 193 (2009) 501–506. [18] Z. Yan, M. Zhang, J. Xie, J. Zhu, P.K. Shen, A bimetallic carbide Fe2MoC promoted Pd electrocatalyst with performance superior to Pt/C towards the oxygen reduction reaction in acidic media, Appl. Catal. B Environ. 165 (2015) 636–641. [19] L. Xin, F. Yang, S. Rasouli, Y. Qiu, Z.F. Li, A. Uzunoglu, C.J. Sun, Y. Liu, P. Ferreira, W. Li, Y. Ren, L.A. Stanciu, J. Xie, Understanding Pt nanoparticle anchoring on graphene supports through surface functionalization, ACS Catal. 6 (2016) 2642–2653. [20] J. Spendelow, D. Papageorgopoulos, J. Garbak, Fuel Cell Technologies Program Record, (2012).
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Performance evaluation of platinum-molybdenum carbide nanocatalysts with ultralow platinum loading on anode and cathode catalyst layers of proton exchange membrane fuel cells
T
Shibely Saha, José Andrés Cabrera Rodas, Shuai Tan, Dongmei Li∗ Department of Chemical Engineering, University of Wyoming, Laramie, 80271, United States
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
nanoparticles (< 3nm) were de• Ptposited on β-Mo C nanotubes via ro2
• • •
tary ALD. Catalysts were applied to anode and cathode of MEAs without carbon black support. Enhanced durability was demonstrated using a rigorous ADT protocol. MEA performance was evaluated for two fabrication methods and GDL structures.
A R T I C L E I N F O
A B S T R A C T
Keywords: Transition metal carbide Phase-pure support Ultra-low platinum Cathodic performance Membrane electrode assembly Electrocatalyst durability
An alternative catalyst platform, consisting of a phase-pure transition carbide (TMC) support and Pt nanoparticles (NPs) in the range of subnanometer to < 2.7 nm, is established that can be used in both anode and cathode catalyst layers. While some TMCs with low Pt loadings have demonstrated similar activity as commercial Pt catalyst in idealized disk electrode screening tests, few to none have been applied in a realistic fuel cell membrane electrode assembly (MEA). We recently reported that β−Mo2C hollow nanotubes modified with Pt NPs via atomic layer deposition (ALD) possess better activity and durability than 20% Pt/C. This paper presents systematic evaluation of the Pt/Mo2C catalysts in a MEA, investigating effects of different MEA preparation techniques, gas diffusion layers (GDL) and various Pt loadings in the ultralow range (< 0.04 mg/cm2) on MEA performance. Most importantly, we demonstrate, for the first time, that Pt/Mo2C catalyst on both anode and cathode, with a loading of 0.02 mg (Pt) cm−2, generated peak power density of 414 mW cm−2 that corresponds to 10.35 kWgPt−1 using hydrogen (H2) and oxygen (O2). Accelerated degradation tests (ADT) on Pt/ Mo2C catalysts show 111% higher power density than commercial 20% Pt/C after the vigorous ADT.
1. Introduction Development and sustainable commercialization of low-temperature (LT) and high-temperature (HT) proton exchange membrane fuel cells (PEMFC) or electrolyzers needs further reduction of platinum (Pt) loading without compromising durability. Currently, the cost for
∗
PEMFC has decreased to $55/kW, of which 50% is for Pt catalyst. However, to achieve the cost target of $40/kW by 2020, Pt use need to be further optimized [1]. In addition, Department of Energy (DOE) Hydrogen and Fuel Cell Program Plan 2011 set the target of Pt utilization for fuel cell automobiles as 8 kW gPt−1 for 2017–2020 [1]. These goals may be met by improving effective surface area of Pt catalyst via
Corresponding author. E-mail address: [email protected] (D. Li).
https://doi.org/10.1016/j.jpowsour.2017.12.062 Received 19 October 2017; Received in revised form 18 December 2017; Accepted 21 December 2017 Available online 02 February 2018 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
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was reported as Pt support for ORR electrocatalysis due to its similar dband structure to Pt [14–16]. For example, Pt nanocrystal (20 wt%) deposited on Mo2C-C nanocrystal showed better ORR activity compared to 20% Pt/C [14]. However, Mo2C has also been reported to show strong oxidation characteristics [17]. Yan et al. demonstrated that 40% Pt supported on Mo2C, which used carbon black as a secondary support, demonstrated higher ORR activity compared to commercial 40% Pt/CTKK catalyst with sufficient stability [18]. The authors speculated that the improved stability of Pt/C-Mo2C towards oxidation might have resulted from the synergistic and binding effect of Pt and Mo2C. Xin et al. indicated that as the bonding strength between Pt and its support increases, resultant catalysts will be more durable in ORR condition [19]. Recent results from our group also demonstrated that after modification of Mo2C surface by depositing Pt NPs via ALD onto Mo2C nanotubes the stability of the catalyst significantly increased due to strong bonding between Pt and Mo2C [11]. These studies indicate that Pt supported on pristine β-Mo2C catalyst may provide higher ORR activity, due to increased bonding between Pt and phase-pure Mo2C support, than commercial Pt/C catalyst. Following these studies we hypothesize that the noticeable activity towards ORR along with the strong bonding between Pt and Mo2C as observed from XPS would allow this catalyst to be successfully applied as PEMFC cathode. In this paper, the 4.4%Pt/Mo2C catalyst was applied in both anode and cathode, with systematic investigation on effects of GDL, catalyst loading and MEA fabrication techniques. Activity and stability of the catalyst was rigorously tested in cathodic conditions using a rotating ring disk electrode (RRDE) and MEA via an accelerated degradation test (ADT) protocol specified by DOE [20]. To the best of our knowledge this is the first report in applying Pt/Mo2C catalyst in PEMFC cathode, without the need of using a secondary catalyst support.
rational catalyst design [2], since the triple phase boundary (TPB) among the catalyst, electrolyte and the reactants in the catalyst layer (CL) plays a crucial role in PEMFC performance. In 2006, Matsumoto et al. reported that 16% molybdenum carbide supported on carbon nanotube was used as an PEMFC anode and reached 50% of the open circuit potential commercial 29% Pt/C achieved [3]. Recent study on different morphologies of Mo2C showed further enhancement in electrocatalytic performance towards HER and HOR. However, activity of bare Mo2C still cannot compare to commercial 10% Pt/C. In addition, none of the Mo2C catalysts were used in a single cell. There are some TMCs that were used in the single cell, but very poor performance was observed compared to commercially available 10–20% Pt/C. For example maximum power density from WC and WNi/C was only 5.7 and 7.3%, respectively, of that from 20 wt% Pt/C [4]. Izhar et al. showed maximum power density of cobalt-tungsten carbide and molybdenum-tungsten carbide supported on Ketjen carbon was only 14 and 11%, respectively, of that from 20% Pt/C [5]. However, research in Pt modified TMCs (with ≥10% Pt loading) primarily focuses on anodic applications in PEMFC, in addition to the lack of performance evaluation in MEAs due to the poor stability of carbides at higher potential. For example, 20% Pt NPs supported on Mo2C were shown to have less anodic activity than 20% Pt/C in PEMFC with 0.4 mg/cm2 Pt loading [6]. Another study on 10% Pt/WC showed less anodic activity than 10% Pt/C [7]. Therefore, a systematic investigation is still needed to investigate effect of different preparation parameters on PGM supported on TMCs, especially exploring their applications on cathodes of PEMFC, based on the first principle of density functional theory (DFT) predications that Mo and W-based carbides can be effective supports for Pt single atoms [8]. Experimentally, Hunt et al. synthesized self-assembled transition metal carbides coated with Pt or Pt alloy monolayer, which were highly resistant to sintering and CO poisoning, in addition to enhanced activity and durability for methanol electrooxidation process [9,10]. Combining theoretical and experimental observations, we hypothesize that Pt particles (< 3 nm) supported on pristine TMC surfaces with low Pt loading (< 5%) may be used on the cathode size of PEMFC (or anode size of an electrolyzer) [11,12]. Moreover, it will be highly desirable to evaluate catalyst performance of such catalysts against DOE target in an MEA. From our earlier study it was observed that 100 cycles of Pt deposition via ALD resulted in 4.4% loading by weight (referred to as 4.4%Pt/ Mo2C hereafter), demonstrated higher mass activity of Pt than 20% Pt/ C [12]. Therefore, 4.4%Pt/Mo2C was chosen for the systematic MEA performance evaluation of the Pt/Mo2C catalyst platform. Since both microstructure of GDL and MEA preparation techniques play an important role in TPB interaction, we will investigate of PEMFC performance of 4.4%Pt/Mo2C using two different types of GDLs and fabrication techniques. Different microporous structures in GDL determine the difference in in-plane vs. through-plane conductivity. For example, SGL 10BC was reported to show best performance with 20% Pt/C than E-TEK GDL 1200W and Freudenberg H2315C2, resulting from its lower bulk density and porosity with higher permeability [13]. In this paper, we compare SGL 10 BC to Torray carbon paper by either brush painting or spray coating 4.4%Pt/Mo2C catalyst onto GDLs. The coated catalyst layer (CL) together with GDL will be referred to as gas diffusion electrode (GDE). It is worth mentioning that GDLs also help prevent the CL from water flooding. Catalyst loading is another important parameter that affects the performance of a PEMFC. Generally, increasing Pt loading in the electrode result in enhanced performance. However, increase in total catalyst loading also increases the CL thickness, introducing higher mass transfer resistance to the reactants and the products. In addition, increased total catalyst loading worsens particle agglomeration. As a result, it is important to optimize catalyst loading. In order to minimize mass transfer resistance, it is desirable to further reduce Pt loading for both anode and cathode. Therefore, the feasibility to use Pt/Mo2C catalyst in the cathode was investigated in this paper. Previously, Mo2C
2. Experimental 2.1. Synthesis of Mo2C and Pt/Mo2C Both β-Mo2C and Pt ALD modified β-Mo2C (referred to as Mo2C for simplicity hereafter) nanotube synthesis methods were discussed in detail previously [11,12]. Briefly, Mo2C nanotubes were synthesized via a salt flux reaction using sodium fluoride (NaF), sodium chloride (NaCl), multiwalled carbon nanotube (MWCNT) and molybdenum powder (all from Sigma-Aldrich and used as received). The synthesized Mo2C nanotubes were then pretreated in 15% CH4/H2 at 590 °C (3 °C/ min) over 4 h to remove any possible oxides, which was followed by 100 cycles of ALD to deposit Pt onto Mo2C nanotubes in a rotating reactor maintained at 200 °C, resulting in 4.4% Pt in mass loading. Platinum (IV) Trimethyl Methylcyclopentadienyl (MeCpPtMe3) and oxygen were used as ALD precursors. 2.2. Characterizations Transmission electron microscopy (TEM) images and Brunauer–Emmett–Teller (BET) surface area results for 4.4% Pt/Mo2C catalyst can be found in our recent publications [11,12]. In order to characterize morphologies of the catalyst layer via brush painting and ultrasonic spray, scanning electron microscope images (SEM) (FEI Quanta FEG 450 FESEM) were collected for both top surface and crosssection areas. Also, cross-section images of membrane electrode assemblies (MEAs) containing 4.4% Pt/Mo2C catalysts were obtained before and after ADT to investigate the structure change of MEAs during ADT. 2.3. MEA fabrication, conditioning and performance evaluation It should be mentioned that the catalysts were pretreated in 15% CH4/H2 at 590 °C (3 °C/min) over 4 h to be used in any electrochemical measurement, in order to eliminate any effects from possible oxides on 743
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Fig. 1. Polarization and Power density curve of brush painted (a) 20% Pt/C in both anode and cathode with the loading of 0.4 mg Pt/cm2 and (b) 4.4% Pt/Mo2C on anode onto different carbon papers. 4.4%Pt/Mo2C on anode with Pt loading of 0.05 mg Pt/cm2 for SGL 10BC and 0.06 mg Pt/cm2 for Torray Carbon Paper collected at 1 atm pressure.
(glassy carbon disk and a platinum ring) and a CompactStat e10800 potentiostat (Ivium Technology). Data were collected under N2 atmosphere (0.2 L per min) using 0.1 M HClO4 electrolyte at a rotating rate of 1600 rpm. A Ag/AgCl electrode saturated in 4 M KCl was used as the reference electrode, with Pt wire being the counter electrode. 4.4% Pt/ Mo2C catalyst ink was deposited (10 μL) onto the glassy carbon surface (5 mm diameter) to serve as the working electrode. We like to point out that the ink concentration is by no means optimized for the 4.4% Pt/ Mo2C. The ink was prepared by adding 2.66 mg of 4.4% Pt/Mo2C catalyst to 52.2 mL of Nafion® solution (5 wt% Nafion® ionomers in ethanol, Sigma-Aldrich), followed by sonicating the solution for 15 min. After the ink deposition, vacuum drying was allowed for 15 min to enable proper adhesion of the ink-catalyst to the glassy carbon surface. Prior to collecting data, the working electrode was conditioned by cycling for 30 cycles between 0 and 1.2 V (vs. RHE) at 50 mV s−1. To collect cyclic voltammetry curves for ECSA calculation, a lower scan rate of 40 mV s−1 was used. CV curves were collected after conditioning cycles, with ECSA being calculated by integrating the hydrogen adsorption/desorption area (0.06–0.4 V vs. RHE) [22,23].
Mo2C [6]. Membrane electrode assemblies (MEAs) in this study were made using Nafion® 212 (DuPoint), which was treated using the same process adopted from Herring et al. [21] Briefly, Nafion® 212 membrane was first boiled in 3% H2O2 for one hour, followed by one hour of boiling in deionized (DI) water, then one hour of boiling in 1 M H2SO4, and finally one hour of boiling in DI water. Treated membranes were stored in DI water in a dark location before use. SGL 10 BC carbon paper (Ion Power) was used as Gas Diffusion Layer (GDL). The gas diffusion electrode was prepared by either painting or spraying (using ultrasonic accumist spray) the GDL. Catalyst inks were prepared by combining desired catalyst, methanol (for 20% Pt/C) or ethanol (for Pt/ Mo2C), and Nafion® solution. 5% Nafion® ionomer solution (DuPont) was added such that the Nafion® solids were 30% of the total mass of catalysts and Nafion® solids in the ink. Methanol or Ethanol was added in an amount that was ten times the mass of catalyst in the ink for brush paint technique and at a volume required to maintain 0.8 mg of catalyst/ml of ink concentration for SonoTek Ultrasonic Accumist sprayer. The ink was then sonicated for 20 min. The catalyst loading on the GDL was determined by measuring the weight of an empty GDL and the weight after depositing catalyst on the GDL using a microbalance (Sartorius). The Nafion® membrane with gas diffusion electrode on both sides was pressed using digital combo multi-purpose heat press (DC14, GEO Knight & Co. Inc.). The pressing condition was maintained at 135 °C at 80 psig for 5 min. The MEA was tested using a fuel cell testing station (Scribner Associates Inc.) at 80 °C. H2 and O2 flow rates were maintained at 0.2 L/min with relative humidity of 100% for both gas streams. Before starting each polarization experiment MEAs were conditioned by humidifying them at 80 °C for one hour, followed by holding the potential at 0.6 V for one hour. Sequentially, potential was altered between 0.7 and 0.5 V (holding at each voltage for twenty minutes) for the total duration of twelve hours then holding at current of 1 ampere (A) for seven hours. To systematic evaluate anode performance, polarization experiments were performed with 4.4%Pt/Mo2C and 20% Pt/C on anode side, while keeping 20% Pt/C on the cathode with Pt loading of 0.4 mg Pt/cm2. To evaluate MEA performance on both anode and cathode, 4.4%Pt/Mo2C and 20% Pt/C with the same Pt loading (0.02 mg/cm2) were applied in the catalyst layers, respectively.
2.4.2. Accelerated degradation test (ADT) Durability of MEA with 4.4% Pt/Mo2C catalyst on both anode and cathode sides was investigated by the accelerated degradation test (ADT) procedure specified by DOE, as detailed by Han et al. [24] Based on this procedure, the anode of the fuel cell was used as reference electrode and cathode was used as the working electrode. On the anode side H2 flowrate was maintained at 0.2 L/minute (lpm), with cathode side flowing N2 at 0.05 lpm. The potential was cycled between 0.6 V to 1 V at a scan rate of 500 mV s−1 for 30,000 cycles [20]. 3. Results and discussion 3.1. Performance comparison for different gas diffusion layers (GDL) To investigate how Pt/Mo2C catalyst performs in MEAs fabricated with different GDLs, Torray carbon paper (Fuel Cell store) and Sigracet SGL 10BC (Ion Power) were selected since they are both widely used. Current density and peak power density were normalized by Pt loading on the anodes for GDL effect and are referred to as mass current density and mass peak power density, respectively. Fig. 1 demonstrates the effect of different gas diffusion layers (GDLs) on fuel cell performance for 4.4%Pt/Mo2C. Consistent with what Millington et al. reported, Sigracet SGL 10BC outperformed Torray carbon paper dramatically for both 20% Pt/C (Figs. 1a) and 4.4%Pt/Mo2C (Fig. 1b) [13].
2.4. Durability study 2.4.1. Potential cycling in RRDE Durability of the 4.4% Pt/Mo2C catalyst was tested using both rotating ring disk electrode (RRDE) and ADT. For RRDE, 10,000 potential cycles were carried out at the scan rate of 200 mVs−1 using Pine MSR 744
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4.4%Pt/Mo2C (0.015 mg/cm2) resulted in 114% increase in mass current density and 67.4% increase in peak mass power density, respectively, for ultrasonic spray technique which is consistent with the percent increase reported for 20% Pt/C [13]. To understand the effect of catalyst particle size distribution for 4.4%Pt/Mo2C with different fabrication techniques, electrodes were peeled off from the MEAs fabricated. Fig. 4 presents the SEM images of the cross section of the MEAs and the Nafion® membrane surfaces. Comparing the cross section of MEAs fabricated by two techniques (Fig. 4a and b) revealed that the MEA fabricated by brush paint has a bigger gap between membrane and the GDL possibly resulting from the fact that bigger particle size was observed for brush paint technique (Fig. 4c).
Impressively, 77.4% and 76.2% increase in current and peak power density per area, respectively, was observed with SGL 10BC for 4.4%Pt/ Mo2C compared to torray carbon paper. We postulate that the 3-D nonwoven, microporous structure of Sigracet SGL 10BC not only facilitates higher air permeability, due to its inherent open pore feature, it also significantly provides better dispersion of the 4.4%Pt/Mo2C which in turn increases the active sites of the catalyst. Consequently, the microporous layer of the SGL 10BC increased the through-plane conductivity, by significantly enhancing mass transfer, relative to 2-D fibrous structure of the Torray carbon paper [25]. Since the SGL 10BC outperformed Torray carbon paper, all our MEA were prepared using SGL 10BC as the GDL. 3.2. Effect of fabrication techniques on the performance of Pt modified Mo2C
3.3. Effect of catalyst loading in the anode fabricated by ultrasonic spray technique
With ultralow Pt loading (< 0.02 mg/cm2), 4.4%Pt/Mo2C catalyst outperformed commercial 20% Pt/C when the same amount of Pt loading of both catalysts was applied to anode, while keeping 0.4 mg/ cm2 Pt loading on cathode, as reported by our group previously [12]. To further optimize fuel cell performance in the ultralow Pt loading regime, ultrasonic spray technique (Acumist® from SonoTek Inc.) was applied to fabricate anodes. It has been reported in open literature that ultrasonic spray techniques resulted in further reduced Pt loading without compromising performance [13]. Specifically, for our Pt/Mo2C catalyst platform we hypothesize that a sophisticated spray technique, such as the Ultrasonic Accumist spray system by SonoTek, enables better catalyst dispersion by minimizing catalyst aggregation. The combination of ultrahigh surface areas, combined with optimized distribution of the catalyst, may allow further reduction of Pt loading. To establish a baseline for the fabrications techniques, 20% Pt/C catalyst with known performance was first applied in both anode and cathode by both brush-painting and ultrasonic spray (Fig. 2a and b). PEMFC performance data of the 20% Pt/C showed that MEAs fabricated by the spray technique resulted in 76.5% and 79% increase of mass current density at 0.6 V and peak mass power density, respectively. The performance enhancement by ultrasonic spray technique is consistent with what was reported by other groups. The improved performance was attributed to smaller particle size of the catalyst, thanks to the atomization of catalyst ink during ultrasonic spray processes. The MEA performance comparison of our 4.4%Pt/Mo2C catalyst from the two preparation approaches is presented in Fig. 3. Despite of the slightly lower Pt loading (0.015 mg/cm2), the MEA prepared by Ultrasonic Spray resulted in 58.3% higher current density per area at 0.6 V and 25.8% higher peak power density per area (Fig. 3a) compared to Brush Paint technique (Pt loading: 0.02 mg/cm2). As for the mass current density and peak mass power density (Fig. 3b), ultralow loading of
To probe how the catalyst loading affects the MEA performance in ultra-low loading regime, MEAs were fabricated by ultrasonic spray technique with Pt loadings of 0.015, 0.022 and 0.029 mg Pt/cm2 in the anode (Fig. 5). Interestingly, MEA with lower Pt loadings outperformed the ones with higher Pt loadings, which is consistent with what was observed previously for brush painted MEAs with 4.4% Pt/Mo2C on the anode side [12]. We postulate that Pt/Mo2C tends to agglomerate and better activity may be achieved by minimizing Pt agglomeration by lower loadings on the GDE. We do want to point that catalyst particles settled down in the tube of the sprayer during spraying the catalyst ink, possibly resulting in lower catalyst concentration in the ink at the end of spraying than the initial concentration. With Nafion® being more dispersed in the solution the weight used for calculation of the Pt loading might end up with larger contribution from Nafion® ionomer. The challenge of catalyst precipitating may also explain the compromised MEA performance as Pt loading increased to 0.029 mg/cm2. Since longer spraying time required to achieve this loading may have caused more catalyst particles settling out of the ink dispersion. To resolve this issue more investigation is required to pick a suitable dispersion solvent for Pt/Mo2C catalyst ink formation, in addition to customizing the SonoTek spray system by shortening the catalyst ink delivery line. 3.4. Performance of Pt/Mo2C in cathode Based on the CV data from a three-electrode static cell in our previous publication [12], the Pt/Mo2C catalysts demonstrated significant activity towards ORR and oxidation stability. Due to the technical challenge with our SonoTek spray system as discussed above, the cathode was prepared by brush painting to achieve a loading of
Fig. 2. (a) Polarization and Power density curves and (b) Polarization and Power density curves per Pt mass in anode. Commercial 20% Pt/C in both anode and cathode for both techniques with the loading of 0.3 mg Pt/cm2 in the cathode, 0.39 mg Pt/cm2 for ultrasonic spray and 0.38 mg Pt/cm2 for Brush Painting in the anode. GDL: Torray Carbon Paper.
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Fig. 3. (a) Area and (b) specific polarization and power density curves with 4.4%Pt/Mo2C on SGL 10BC as GDL (solid line: ultrasonic spray; dashed line: brush painting) 4.4%Pt/Mo2C on anode with Pt loading of 0.015 mg Pt/cm2 for ultrasonic spray and 0.02 mg Pt/cm2 for brush paint respectively.
Fig. 4. SEM images of cross sectional MEAs fabricated by (a) brush paint and (b) ultrasonic spray technique with 4.4%Pt/ Mo2C catalyst; top view of Nafion® membranes collected from MEA fabricated by (c) brush paint and (d) ultrasonic spray technique.
Fig. 5. Effect of 4.4%Pt/Mo2C catalyst loading on MEA performance with SGL 10BC as GDL via Ultrasonic Spray: (a) Polarization and Power density curves, and (b) Polarization and Power density curves per Pt loading in anode. Anode Pt loading: 0.015 mg/cm2; 0.022 mg/cm2 and 0.029 mg/cm2. Pt loading for 20% Pt/C was maintained constant on the cathode (0.4 mg Pt/cm2).
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Fig. 6. (a) Area and (b) specific polarization and power density curves of MEAs fabricated via brush painting with 4.4 wt % Pt/Mo2C and 20 wt% Pt/C (Vulcan). Solid line: 4.4% Pt/Mo2C catalyst; dashed line: commercial 20% Pt/C. Pt loading: 0.02 mg Pt/cm2 on both anode and cathode for both catalysts; Cell Area: 5 cm2; operating conditions: H2 flowrate in anode and O2 flowrate in cathode is 0.2 L/min; Cell Temperature: 80 °C; Back pressure: 2 bar.
0.02 mg/cm2. Anode Pt loading was maintained at 0.02 mg/cm2. A MEA with the same Pt loading on anode and cathode sides was fabricated via brush painting with commercial 20% Pt/C to serve as a bench mark. Shown in Fig. 6, Pt/Mo2C catalyst achieved 27.5% higher current density (at 0.6 V) and 32.2% higher maximum power density than 20% Pt/C. We note that MEA reported here has comparable or better performance, compared to MEAs made of ultralow Pt loading [13] The maximum power per mass of Pt was found to be 10.35 kWgPt−1 (Fig. 6b). As a reference, US DOE target for the period 2017–2020 is 8 kWgPt−1 for H2 and air in transportation applications [1]. The polarization curve data for 4.4%Pt/Mo2C reported here is collected after 6 days of continuous voltage scan in addition to the 21 h of conditioning and the performance kept increasing. This indicates this catalyst is durable for practical operating conditions. To determine the durability of our catalyst against Pt dissolution, the electrochemically active surface area (ESCA) before and after 10,000 cycles of potential cycling were calculated from the CV profile generated using a rotating ring-desk electrode. As shown in Fig. 7a, one irreversible peak appeared at 0.47 V vs. RHE, which could result from high concentration of Nafion® ionomer used in the catalyst ink [26,27] or oxidized molybdenum species [6]. Subbaraman et al. reported that the specific adsorption of sulfonate anions of Nafion® on Pt(111) could contribute to the so-called “mini-butterfly” CV irreversible peak at around 0.5 V vs. RHE [26,27]. This group also reported that if the peak near 0.47 V is caused by Nafion® ionomer, it will disappear after switching the electrolyte from HClO4 to H2SO4. Fig. 7b shows that this peak remains even with 0.5 M H2SO4 as the electrolyte. Therefore, the peak near 0.47 V is most likely from oxidized molybdenum species. Ongoing work in our group aims to further understand the surface chemistry of the oxidized Mo species via XPS and x-ray absorption near edge structure (XANES). In Fig. 7a, the degradation of 4.4% Pt/Mo2C catalyst was presented by the reduced integrated area in the hydrogen adsorption/desorption region (0.06–0.4 V vs. RHE). The ECSA decreased from 31.87 m2/gPt to 20.34 m2/gPt after the cycling. Consistent with the RRDE results, Fig. 7b and Table .1 summarize the ADT data, showing that the degradation from ADT was more severe for 20% Pt/C (87% decrease in maximum power density) than 4.4%Pt/Mo2C (78% decrease in maximum power density). However, it is worth pointing out that the reduced ECSA is still higher than the commercial 20% Pt supported on Vulcan (Alfa Aesar), which has an ECSA value of 13.54 before cycling under the same potential cycling conditions. The apparently lower ECSA value for 20%Pt/C may be attributed to the suboptimal ink composition.
The polarization curve of 20% Pt/C collected after ADT demonstrated wiggly performance at high current density indicating unreliable activity of 20% Pt/C. Abruptness in performance was also observed for 4.4%Pt/Mo2C after ADT. Nonetheless, the compromised performance of 4.4%Pt/Mo2C in a MEA (applied to both anode and cathode sides) was further characterized by SEM images of the MEA in crosssectional areas after the ADT, which showed the noticeable microporous voids that may have resulted in detrimental effects on the TPB, consequentially MEA performance (Fig. 7c). However, after this vigorous ADT 4.4%Pt/Mo2C still maintained 212.5% higher current density and 111% higher maximum power density than 20% Pt/C indicating 4.4%Pt/Mo2C to be more resistant to Pt dissolution than 20% Pt/C. We speculated that the synergetic interaction between Mo2C support and Pt particles (< 3 nm) may prohibit sintering and migration of Pt particles [11], responsible for better MEA performance after 30,000 potential cycling than Pt particles supported on inert carbon black. 4. Conclusion Systematic study on fabrication techniques, GDL and Pt loadings demonstrated that Pt/Mo2C catalysts perform very reliably in low loading regime (< 0.03 mg/cm2), while achieving better durability than commercial 20% Pt/C. Catalysts with ultralow Pt loading and desired durability are well sought after because they enable further reduction in cost and stack size of PEMFC automobiles. We note, as a reference, that the MEA peak power density using H2 and O2 with Pt/ Mo2C in both anode and cathode is higher than the target set by DOE for 2017–2020 for H2 and air [1]. Given that our MEAs were mostly fabricated by brush painting, our ongoing effort focuses on further optimization of MEA performance by improving catalyst dispersion and our ultrasonic spray system. Additionally, after the catalyst delivery system of our SonoTek system is optimized, MEAs will be only fabricated by the spray technique and tested under H2 and air conditions. To summarize, the 4.4%Pt/Mo2C catalyst showed significantly higher durability with 111% higher end of life activity than commercially 20% Pt/C after the rigorous ADT. Acknowledgements We would like to thank Professor Andrew Herrings for generously sharing their knowledge with MEA fabrication via brush painting and testing protocols. Shibely Saha acknowledges financial support for her study through the University of Wyoming Academic Affairs Energy 747
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Fig. 7. (a) CV profile of 4.4% Pt/Mo2C catalyst before and after 10,000 cycling in a rotating ring-desk electrode in 0.1 M HClO4 electrolyte; (b) CV profile of 4.4% Pt/Mo2C catalyst in 0.5 M H2SO4 electrolyte; (c) Polarization and power density curves of MEAs fabricated with 4.4%Pt/Mo2C and 20% Pt/C (solid line: 4.4%Pt/Mo2C catalyst; dashed line: 20% Pt/C) with Pt loading of 0.02 mg Pt cm−2 on both anode and cathode showing the performance after 30,000 cycles of potential cycling from 0.6 V to 1.0 V; (d) Cross section of MEA containing 4.4% Pt/Mo2C catalyst after 30,000 potential cycling. Dash cycle: microporous voids.
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Table 1 Different parameters before and after 30,000 potential cycling collected from the MEA performance of different catalysts. Catalyst
20% Pt/C 4.4%Pt/ Mo2C
Open Circuit Voltage/V
Current Density at 0.6 V/mA cm−2
Maximum Power Density/ W cm−2
Initial
After 30000 cycles
Initial
After 30000 cycles
Initial
After 30000 cycles
0.922 0.924
0.679 0.773
272 385
20 62.5
0.313 0.414
0.0418 0.0882
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