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The effect of Nafion ionomer loading coated on gas diffusion electrodes with in-situ grown Pt nanowires and their durability in proton exchange membrane fuel cells
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The effect of Nafion ionomer loading coated on gas diffusion electrodes with in-situ grown Pt nanowires and their durability in proton exchange membrane fuel cells Shangfeng Du a,*, Benjamin Millington a,b, Bruno G. Pollet a,b a b
School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK PEM Fuel Cell Research Group, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
article info
abstract
Article history:
The effect of varying Nafion ionomer loadings coated on the surface of gas diffusion
Received 9 November 2010
electrodes (GDEs) with in-situ grown single crystal Pt nanowires and their durability in
Received in revised form
Proton Exchange Membrane Fuel Cells (PEMFCs) were investigated. GDEs were fabricated by
27 December 2010
growing Pt nanowires directly onto the Gas Diffusion Layer (GDL) surface with a simple one-
Accepted 5 January 2011
step wet chemical approach at room temperature, as reported in our previous studies. The samples were then coated with various Nafion ionomer loadings and tested as cathodes in a 25 cm2 PEMFC hardware with Hydrogen/Air. The data were compared to commercial GDEs
Keywords:
(E-TEK ELAT GDE LT 120E-W). Performance results showed that the as-prepared GDEs with
PEM fuel cell
Pt nanowires required higher Nafion ionomer loading coating compared to the commer-
Pt nanowire
cial ones. Accelerated ageing tests (500 cycles of voltage scan) were performed in views of
Catalyst
evaluating the as-prepared GDE durability. The experimental data showed that the as-
Ionomer
prepared GDEs exhibited much larger current densities at 0.7 V but higher degradation rates
Durability
compared to commercial GDEs, indicating that the as-prepared GDEs gave poor durability.
Gas diffusion electrode (GDE)
This was due to the difference in GDE surface nanostructures influenced by the electrolyte ionomer loading coating. This effect is further discussed in this paper. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
To obtain high PEMFC performances, smaller Pt nanoparticles of nanometer range (1e10 nm) exhibiting high specific surface areas are required. The electrocatalytic behaviour is usually altered by the “particle size effect”, especially for extremely small Pt nanoparticles [1], which have also the tendency to aggregate in turns reducing their surface areas and leading to much lower mass activities (current per unit mass) as well as efficiencies [2]. However, Pt nanowires, known as a type of one-dimensional (1-D) nanomaterials, can overcome these
issues due to their high aspect ratios compared to small spherical nanoparticles, especially at high catalyst loadings. Therefore, the development of Pt-based nanowires possessing large surface areas in views of achieving high catalytic performances and utilisation efficiencies, has attracted a lot of attention in the past few years [3,4]. In recent years, many methods have been proposed for the preparation of Pt-based nanowires [3,5], with a few of them being currently used in practical PEM fuel cells, including template approaches [6e8], electro-deposition [9], electrospinning [10,11] and wet chemical methods [12,13]. These
* Corresponding author. Tel.: þ44 121 414 5081; fax: þ44 121 414 5377. E-mail address: [email protected] (S. Du). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.014
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methods produce polycrystalline and single crystal Pt or Ptbased nanowires, exhibiting excellent catalytic properties when evaluated by Cyclic Voltammetry (CV) and Membrane Electrode Assembly (MEA) experiments. The results showed that, even at low catalyst loadings, similar or better performances were obtained for both the Oxygen Reduction Reaction (ORR) and the Hydrogen Oxidation Reaction (HOR). However, as one of the most crucial steps in MEA fabrication, the content of the electrolyte ionomer has little been investigated for the electrode layer with catalyst nanowires, compared to many other studies on supported or unsupported spherical shaped catalysts in PEMFCs [14e17]. An optimum electrolyte content coated on the catalyst layer is required in order to obtain good MEA performances as it maximises the Triple Phase Boundary (TPB) as well as retaining moisture and preventing membrane dehydration [18]. This is important for any types of electrocatalysts in GDEs i.e. both spherical nanoparticles and 1-D nanowires. Therefore, there is a need to carry out this investigation on the GDEs with Pt nanowires to move this promising technique forward into practical PEMFC applications. Furthermore, the durability of the GDEs with Pt nanowires, which is directly related to the overall cost of the PEM fuel cell and its widespread commercialisation, has rarely been reported before. This is due to (i) complex electrode fabrication, (ii) high cost and (iii) issues on process scalability. These factors lead to challenges in producing durable catalyst layers with Pt-based nanowire catalysts especially when using conventional PEMFC processes. In our previous studies, we reported a facile one-step procedure for the development of GDEs with in-situ grown single crystal Pt nanowires in PEMFCs [19]. This was performed by directly reducing the Pt precursor with formic acid in aqueous solution at room temperature, without using any organic solvents, templates or growing catalysts. Better performances were obtained with this novel GDE leading to much lower manufacturing costs. Based on our preliminary results, we report here for the first time, the effect of Nafion ionomer loading coating on the performance of this new type of GDEs, and then evaluate their durability towards ORR in PEMFCs. The data were compared to commercial GDEs (E-TEK ELAT GDE LT 120E-W) with conventional spherical shaped catalysts. The experiments were performed by polarization and Electrochemical Impedance Spectroscopy (EIS) measurements in a 25 cm2 Hydrogen/ Air PEMFC hardware. The electrocatalyst durability was tested by applying a severe cycling regime (hundreds of voltage scans), as a method to accelerate the ageing and degradation process. From our data, the influence mechanism of GDE surface structures on the electrolyte ionomer loading coatings and GDE durability is proposed.
(H2PtCl6$6H2O, 99.95%) and formic acid (CH2O2, 98%) were purchased from SigmaeAldrich. GDL materials: E-TEK ELAT GDL LT 1200-W and E-TEK ELAT GDE LT 120E-W (Pt loading 0.5 mg cm2), Nafion NRE-212 membrane and Nafion (DE 1021, 10 wt%) solution were purchased and used as received from Fuel Cell Store. All aqueous solutions were prepared using ultrapure water (18.2 MU cm) from a Millipore water system.
2.2.
Experimental section
2.1.
Materials
All chemicals were used as received without any further purification. Sulphuric acid (H2SO4), hydrogen peroxide (H2O2), tetrahydrofuran (THF), ethanol (C2H6O), polyvinyl pyrrolidone (PVP, M.W. ¼ 55,000), hydrogen hexachloroplatinic acid
GDE preparation
E-TEK ELAT GDL LT 1200-W carbon cloth was used as the GDL support to grow Pt nanowires. The detailed procedure is described in our previous studies [19]. Typically, to grow Pt nanowires on GDL surfaces, pieces of carbon cloth of 5 5 cm2 were used. Carbon cloth samples were first treated by sonication for 3 min in ethanol aqueous solutions (5 vol%), and then immersed in an aqueous solution of H2PtCl6 and formic acid at room temperature. Normally, for the growth of 0.5 mg.cm2 Pt nanowires on GDLs, 33.2 mg H2PtCl6$6H2O (12.5 mg Pt, 0.5 mg.cm2 on GDL), 1.0 ml formic acid and 5 mg PVP were added to 25 ml of water. The samples were stored at room temperature for 72 h. After completion of the Pt reduction reaction and the growth of nanowires onto the substrates, the samples were rinsed with D.I. water 3 times, Ethanol 3 times and D.I. water 3 times, followed by drying at 65 C for 24 h. The loading of 0.5 mg.cm2 Pt nanowires on the carbon cloth was controlled by monitoring the Pt precursor weight versus the supporting GDL area. The carbon cloth samples with in-situ grown Pt nanowires were used directly as integrated GDEs.
2.3.
MEA fabrication
The Nafion NRE-212 membrane was pretreated by boiling for 1 h in 3% H2O2, D.I. Water, 1 M H2SO4 and D.I. water, respectively. Then the as-prepared GDEs were assembled as cathodes with Nafion NRE-212 membranes to produce MEAs, while anodes were commercial GDEs of E-TEK ELAT GDE LT 120EW, onto which a thin Nafion ionomer layer was painted using a THF solution of Nafion DE 1021 (volume ratio of Nafion solution DE 1021 to THF ¼ 1:2) followed by drying for 24 h at 90 C. The GDEs were then hot-pressed against the Nafion NRE-212 membrane at 140 C for 1 min under a constant pressure of 50 kg cm2. For comparison purposes, MEAs were simultaneously fabricated with Nafion NRE-212 membrane and commercial GDEs (E-TEK ELAT GDE LT 120EW) used for both electrodes. The hard Teflon film with a thickness of 254 mm was used as gasket material in the PEM fuel cell hardware.
2.4.
2.
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Fuel cell measurement
The MEAs with the as-prepared and commercial GDEs were tested as cathodes in a 25 cm2 PEMFC single cell hardware at a temperature of 65 C, using pure H2 and air gases at 100% relative humidity (RH) and gas flows in the range of 120 and 300 ml min1 with stoichiometries of 1.5(H2)/2.0(Air) respectively. The gas pressure of the water-saturated H2 or air (identical at the anode and cathode sides) was 2.5 bar absolute (1.5 bar backpressure). Measurements were controlled and
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recorded by a Bio-logic FCT-50S PEMFC test station (PaxiTech). The MEA was conditioned by break-in at 0.6 V for 12 h, and thereafter the polarization curves were recorded at a scan rate of 1 mV s1. The system was then cooled down to room temperature. Prior to durability experiments, the single cell hardware was reheated to 75 C and the backpressure was increased to 2 bar. Durability experiments were performed by cycling voltage scan (500 cycles) from Open Circuit Voltage (OCV) to 0.25 V with a scan rate of 30 mV s1. EIS measurements were performed in the frequency range of 10 kHz to 0.1 Hz with an amplitude of 100 mV prior and after durability experiments.
2.5.
MEA characterization
Scanning Electron Microscopy (SEM) analysis of the asprepared and the commercial GDEs, and cross sections of the MEAs before and after durability test were performed by field emission scanning electron microscope at 20 kV (Jeol JSM7000F FE-SEM, Japan). The MEA cross sections were coated with Au at 10 mA for 100 s before observation.
Fig. 1 e Polarization curves for MEAs with various Nafion loading (unit: mg cmL2) coated on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires. Catalyst loading for all electrodes: 0.5 mg Pt cmL2.
Fig. 2 e Plots of the current density at 0.7 V versus Nafion loading on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the asprepared cathode GDEs with in-situ grown Pt nanowires.
Fig. 3 e Electrochemical impedance spectra measured at 0.8 V and 0.6 V for MEAs with various Nafion loading (unit: mg cmL2) on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires.
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3.
Results and discussion
In GDE layers, optimising the electrolyte content is paramount for enhancing the TPB and catalyst utilization. Low ionomer loading cannot provide enough ‘connection’ between the catalyst, the carbon support and the electrolyte, but too high ionomer loading leads to a complete coverage of the entire catalyst surface active sites in turn blocking the electrode pores [18]. Compared to spherical Pt nanoparticles, Pt nanowires in-situ grown on the GDL offer much higher aspect ratios (>20), exhibiting lengths of 100 nme200 nm with diameters of only a couple of nanometers [19]. For the Pt nanowires in the as-prepared GDEs, the unconventional shape means that a different optimum electrolyte ionomer loading is required on the GDE surface. Fig. 1(a) and 1(b) show a series of polarization curves for various Nafion ionomer loadings coated on the cathode surface for the commercial and as-prepared GDEs respectively. It was found that an optimum Nafion ionomer loading of 0.9 mg cm2 gave the highest performance for the commercial cathode (E-TEK ELAT GDE LT 120E-W). With a loading below this optimum point, the performance was improved when increasing the Nafion ionomer loading. The maximum power density on the polarization curve rose from 0.590 W cm2 at a ionomer loading of 0.4 mg cm2 to 0.639 W cm2 at 0.9 mg cm2. For ionomer loadings above this optimum point, the performance decreased rapidly with increasing Nafion ionomer loadings. The maximum power density dropped to 0.567 W cm2 at 1.5 mg cm2. A very similar trend was observed
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for the as-prepared Pt-nanowire GDEs, where a maximum power density of 0.655 W cm2 was obtained at a Nafion ionomer loading of 1.2 mg cm2 [Fig. 1(b)]. However, at higher ionomer loadings after this optimum point, the performance decreased much faster than that of the commercial GDE, especially in the high current density region. Furthermore, this optimum amount of 1.2 mg cm2 found for the as-prepared GDE was higher than 0.9 mg cm2 for the commercial GDE. In order to elucidate the effect of the Nafion ionomer loading on the performance of GDEs used in this study, Fig. 2 shows comparison plots of the current density at 0.7 V on the polarization curve of different Nafion ionomer loadings for both types of MEAs, which is the common voltage operated for PEMFCs in practical applications [20]. The plots exhibited the same variation trend as that shown by the peak power density point in Fig. 1. The maximum current densities, with the value of 0.635 A cm2 and 0.494 A cm2 at 0.7 V, were obtained at the Nafion ionomer loadings of 1.2 mg cm2 and 0.9 mg cm2 for the as-prepared and commercial GDEs respectively. The much larger current density of the as-prepared GDE compared to the commercial one is due to the excellent catalytic ability of the in-situ grown single crystal Pt nanowires, as previously observed [19]. In order to understand the origins of this loading effect, EIS experiments were performed for both types of MEAs with the commercial and the as-prepared cathode GDEs at various Nafion ionomer loadings (Fig. 3). The MEAs with the commercial cathode GDEs (E-TEK ELAT GDE LT 120E-W) exhibited generally larger impedance than those with the asprepared GDEs, which is in very good agreement with our
Fig. 4 e SEM images and schematic illustration of the Nafion ionomer chain coated on the surface of the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W with commercial spherical catalyst nanoparticles ((a) and (c)) and the as-prepared cathode GDEs with in-situ grown Pt nanowires ((b) and (d)).
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previous findings [19]. Both types of GDEs showed a similar trend on the impedance at various ionomer loadings, which is in good agreement with our early data (see Figs. 1 and 2). At the optimum Nafion ionomer loading, both types of MEAs exhibited low impedance as well as low charge transfer and mass transfer resistances. The impedance of GDEs with a low loading showed a large charge transfer resistance, especially the ones measured at a high voltage of 0.8 V, indicating a poor triple phase boundary. Although increasing the loadings improved the overall performance, excessive loadings also led to high mass transfer resistances, possibly due to blocked pores caused by Nafion ionomer in the porous electrode [17]. To understand the mechanism of this effect of electrolyte ionomer loadings on the GDE performance, a schematic illustration of the Nafion ionomer coating on the GDE surface, based on the MEA measurement and SEM results is presented in Fig. 4. It is evident that for the commercial electrodes, the small spherical catalyst nanoparticles do not show any major impacts on the coated electrolyte ionomer, which formed a very good contact with the catalyst particle surface as well as the carbon support. Whereas, the as-prepared electrodes possess a different surface structure to the commercial one. A rough surface was formed by very small nanowire ends, as shown in Fig. 4(b) and 4(d), indicating that a higher electrolyte ionomer loading is required to maximise the triple phase boundary. From Fig. 4, it can also be observed that the electrolyte ionomer chain is mainly in contact with the Pt-nanowire end parts, and the interface contact area between them is not as stable as that for the spherical catalyst nanoparticles in the commercial electrodes, where the electrolyte ionomer chain possesses a large contact area with the carbon support around the catalyst nanoparticles. The only weak contact between the electrolyte ionomer and the Pt nanowire ends may result in poor durability for the as-prepared GDEs compared to the commercial ones. Fig. 5 shows degradation plots for both types of MEAs on the current densities at 0.7 V from 500 cycles of voltage scan, where the Nafion ionomer loading coating for the commercial and the as-prepared GDEs are 0.9 mg cm2 and
Fig. 5 e Degradation plots shown by the current densities at 0.7 V in each voltage scan cycle versus cycle numbers for both types MEAs with (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W and (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires.
1.2 mg cm2 respectively. It can be seen that both types of GDEs exhibited similar decay behaviours. During the early stage, GDE performances were improved due to the catalyst and membrane activation, but this was very short for the asprepared GDE i.e. only about 20 cycles compared with ca. 300 cycles for the commercial one. The current density reached a maximum value after the activation process had ended, and then a slow degradation commenced and worsened over increasing cycles. After 500 cycles, both GDEs showed an obvious decline in their performances. However, the asprepared GDE displayed a much larger degradation ratio of ca. 6.5% from the top current density compared with only ca. 1.7% for the commercial GDE. Of course, the commercial GDE (E-TEK ELAT GDE LT 120E-W) is well known for its durability and is currently used in commercial PEMFC applications [21,22]. The excellent durability shown in our experiments here confirmed this point again, although the ‘real’ degradation process only occurred through 217 cycles from the maximum current density point at 283rd cycle in the ageing process, compared with 480 cycles ageing for the as-prepared GDE. To investigate these degradation phenomena in more detail, polarization curves were performed and are shown in Fig. 6 for MEAs with two types of cathode GDEs at the 1st cycle, the cycle with the best performance, and the 500th cycle. Both GDEs showed a very similar trend at these typical cycles. The
Fig. 6 e Polarization curves for two types of MEAs at the selected cycles in durability test: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the asprepared cathode GDEs with in-situ grown Pt nanowires.
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pronounced degradation occurred at high current densities, i.e. the mass transport region, especially for the as-prepared GDE. In views of clarifying our observations on the degradation mechanisms, EIS were carried out on the samples as shown in Fig. 7 for both type of MEAs. The measurements were performed at three voltages of 0.8 V, 0.6 V and 0.4 V, corresponding to low, mid-range and very high current density region, respectively, as that shown by the polarization curves in Fig. 6. It can be observed that EIS data are in very good agreement with our previous results shown in Figs. 5 and 6. The commercial GDE exhibited a slight decrease in both the charge transfer and mass transfer resistances at the three voltages after the durability test. A better performance was obtained at the 500th cycle than the 1st one, indicating there was a pronounced improvement on the performance due to possibly good activation and low degradation rates in the durability test, as shown in Figs. 5 and 6(a). However, a different degradation occurred for the as-prepared GDE, as shown by Fig. 7(c) and 7(d). The EIS measured at 0.8 V showed an increase in impedance, corresponding to a larger kinetic resistance after cycling, indicating either a possible degradation process occurring on the catalyst itself or a poor contact between the electrolyte and the catalysts or both. However, a decrease in the diameter of the second semi-circle on the EIS measured at 0.6 V and 0.4 V, indicates an improvement on the mass transfer resistance, in other words, blocked pores were probably opened after the durability experiments. If
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this degradation was mainly caused by catalyst decay, the activation resistance would significantly increase, but in our case, there was no growth on the charge transfer resistance of the EIS measured at 0.6 V and 0.4 V, indicating, that in our conditions, the catalyst decay is not the main reason for this degradation. Furthermore, the good stability of single crystal Pt nanowires has been previously confirmed by other techniques, such as TEM and CV measurements [23,24]. Therefore, other mechanisms may occur, leading to this observable degradation. Due to the high aspect ratio of the Pt nanowires, the electrolyte ionomer chain was mainly in poor contact with the Pt nanowires end parts in the as-prepared GDEs, as discussed previously. This poor contact may worsen during the severe cycling regime in turn improving the blocked effect due to the coated electrolyte ionomer and reducing the mass transfer resistance. Although the impact from this poor contact is not so obvious at larger current densities with low voltages e.g. 0.6 V or 0.4 V, the activation resistance become much larger at relatively smaller current densities at high voltages, e.g. 0.8 V. This observation is in good agreement with our results shown in Fig. 6(b), 7(c) and 7(d). Thus, it can be deduced that it is the contact decline between the catalyst nanowires and electrolyte ionomer which leads to this pronounced degradation of the as-prepared GDEs. Because the electrolyte ionomer coated onto the GDE surface also works as the ‘glue’ during the hot-pressing process in MEA assembling, a poor contact between this ‘glue’
Fig. 7 e Electrochemical impedance spectra (EIS) measurement before and after the durability cycling for both types MEAs with the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W ((a) and (b)) and the as-prepared cathode GDEs with insitu grown Pt nanowires ((c) and (d)), respectively.
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Fig. 8 e Cross section SEM image of the MEA with the asprepared Pt-nanowire GDE (a), Nafion 212 membrane (b) and the commercial GDE of E-TEK ELAT GDE LT 120E-W (c).
and the Pt-nanowire GDE surface will also result in a bad link between the GDE and the electrolyte membrane. The SEM image of the MEA cross section before the test is shown in Fig. 8. This MEA has the as-prepared Pt-nanowire GDE at one side (a) and the commercial GDE at another side (b). It can be seen that the commercial GDE side has a much better contact with the electrolyte membrane than the as-prepared Pt-nanowire GDE, confirming the poor contact between the coated electrolyte ionomer and the Pt-nanowire GDE surface. Fig. 9 shows the SEM images of the MEA cross sections before and after durability test at cathode side, with the
commercial GDE [(a) and (b)] and the as-prepared Pt-nanowire GDE [(c) and (c)], respectively. The images show that the good contact between the commercial GDE and the electrolyte membrane did not change too much, whereas this situation became much worse for the as-prepared GDE, which may be one of the factors leading to the large degradation rate of the MEA as shown in Fig. 5. Some place was even detached from the membrane surface, demonstrating the bad ‘glue’ performance, resulting from the poor contact between the electrolyte ionomer chain and Pt nanowires grown on the GDL surface. In order to improve the durability of the as-prepared GDEs, a good contact between the Pt nanowires and electrolyte ionomer is required. This can be achieved by controlling the growth density and length of the Pt nanowires. Low growth density with short nanowire lengths, e.g. 20e40 nm enables larger contact areas between the catalyst nanowires and the electrolyte ionomer, and possibly even between the carbon support and the electrolyte ionomer. In this case, a stable contact between the catalyst and the electrolyte can be obtained similarly to that of the commercial GDE surface, and thus, an improved durability should be expected. Another way is to modify the coating process by changing the electrolyte ionomer with either a shorter chain or coating, using a low viscosity electrolyte ionomer solution. The contact interface area may also be ‘increased’ in this way but at the risk of increasing the blocked pores in the porous electrodes. In general, for the as-prepared GDEs with in-situ grown Pt nanowires made by a very simple approach, the degradation rates shown in this study are still very low when compared to other values reported in the literature. Although the durability
Fig. 9 e Cross section SEM images before and after the durability cycling for both types MEAs with the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W ((a) and (b)) and the as-prepared Pt-nanowire cathode GDEs ((c) and (d)), respectively.
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is not as good as the commercial E-TEK ELAT GDE, a much higher current density at 0.7 V was successfully obtained even after severe durability testing. Furthermore, it should be noted that we are showing preliminary data of this novel approach, bearing in mind that commercial GDEs have undertaken process optimization for several decades. The authors believe that with this very simple process leading to high performances of the novel GDEs, this one-step facile approach for preparing electrodes with in-situ grown single crystal Pt nanowires, will help PEMFCs towards successful commercial applications.
4.
Conclusions
The effect of Nafion ionomer loading coated onto the surface of the GDEs with in-situ grown Pt nanowires and their durabilities were investigated as cathodes by the polarization and EIS measurements in a 25 cm2 PEMFC single cell hardware with H2/Air. The results were compared to commercial GDEs (E-TEK ELAT GDE LT 120E-W). It was found that the electrolyte ionomer loading was an important parameter for optimizing performance. An optimum Nafion ionomer loading of 1.2 mg cm2 was found for the as-prepared GDE, which was higher than 0.9 mg cm2 for the commercial one, due to the special GDE surface formed by the in-situ grown Pt nanowires. However, because of this unconventional structure of the catalyst nanowires, by the current coating process, the contact between the electrolyte ionomer and catalyst nanowires was not as good as that for the commercial spherical catalyst particles. The durability results, obtained from the accelerated ageing measurement of 500-cycle voltage scan, showed a degradation of 6.5% on the current density at 0.7 V for the as-prepared GDE, compared to 1.7% for the commercial one. This issue could be solved by controlling the catalyst growth or improving the electrolyte ionomer coating process prior to moving this simple approach into practical applications for fabricating PEMFC electrodes.
Acknowledgements This work was supported by the Advantage West Midlands (AWM) Science City Interdisciplinary Research Alliance (SCIRA) Research Fellowships and the Higher Education Funding Council for England (HEFCE) awarded to Dr Du.
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[4] Cademartiri L, Ozin GA. Ultrathin nanowires e a materials chemistry perspective. Adv Mater 2009;21:1013e20. [5] Yousfi-Steinera N, Moc¸ote´guya PH, Candussoc D, Hissel D. A review on polymer electrolyte membrane fuel cell catalyst degradation and starvation issues: causes, consequences and diagnostic for mitigation. J Power Sources 2009;194:130e45. [6] Choi WC, Woo SI. Bimetallic PteRu nanowire network for anode material in a direct-methanol fuel cell. J Power Sources 2003;124:420e5. [7] Choi SM, Kim JH, Jung JY, Yoon EY, Kim WB. Pt nanowires prepared via a polymer template method: its promise toward high Pt-loaded electrocatalysts for methanol oxidation. Electrochim Acta 2008;53:5804e11. [8] Liang ZX, Zhao TS. New DMFC anode structure consisting of platinum nanowires deposited into a Nafion membrane. J Phys Chem C 2007;111:8128e34. [9] Zhao GY, Xu CL, Guo DJ, Li H, Li HL. Template preparation of PteRu and Pt nanowire array electrodes on a Ti/Si substrate for methanol electro-oxidation. J Power Sources 2006;162:492e6. [10] Kim YS, Nam SH, Shim HS, Ahn HJ, Anan M, Kim WB. Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation for methanol electrooxidation. Electrochem Commun 2008;10:1016e9. [11] Kim HJ, Kim YS, Seo MH, Choi SM, Kim WB. Pt and PtRh nanowire electrocatalysts for cyclohexane-fueled polymer electrolyte membrane fuel cell. Electrochem Commun 2009;11:446e9. [12] Lee EP, Peng ZM, Cate DM, Yang H, Campbell CT, Xia YN. Growing Pt nanowires as a densely packed array on metal gauze. J Am Chem Soc 2007;129:10634e5. [13] Sun SH, Zhang GX, Zhong Y, Liu H, Li RY, Zhou XR, et al. Ultrathin single crystal Pt nanowires grown on N-doped carbon nanotubes. Chem Commun 2009;45:7048e50. [14] Lee SJ, Mukerjee S, McBreen J, Rho YW, Kho YT, Lee TH. Effects of Nafion impregnation on performances of PEMFC electrodes. Electrochim Acta 1998;43:3693e701. [15] Chu YH, Shul YG, Choi WC, Woo SI, Han HS. Evaluation of the Nafion effect on the activity of PteRu electrocatalysts for the electro-oxidation of methanol. J Power Sources 2003;118: 334e41. [16] Krishnamurthy B, Deepalochani S, Dhathathreyan KS. Effect of ionomer content in anode and cathode catalyst layers on direct methanol fuel cell performance. Fuel Cells 2008;8:404e9. [17] Lee D, Hwang S. Effect of loading and distributions of Nafion ionomer in the catalyst layer for PEMFCs. Int J Hydrogen Energy 2008;33:2790e4. [18] Sasikumar G, Ihm JW, Ryu H. Optimum Nafion content in PEM fuel cell electrodes. Electrochim Acta 2004;50:601e5. [19] Du SF. A facile route for polymer electrolyte membrane fuel cell electrodes with in situ grown Pt nanowires. J Power Sources 2010;195:289e92. [20] Barbir F. PEM fuel cells: theory and practice. London: Elsevier Academic Press; 2005. [21] Ramaswamy N, Arruda TM, Wen W, Hakim N, Saha M, Gulla A, et al. Enhanced activity and interfacial durability study of ultra low Pt based electrocatalysts prepared by ion beam assisted deposition (IBAD) method. Electrochim Acta 2009;54:6756e66. [22] Agro S. Development of higher temperature membrane and electrode assembly for proton exchange membrane fuel cell device based on speck blends. DOE project report: DE-FG3603GO13172; 2005. [23] Lee EP, Peng Z, Chen W, Chen S, Yang H, Xia YN. Electrocatalytic properties of Pt nanowires supported on Pt and W gauzes. ACS Nano 2008;2:2167e73. [24] Sun SH, Jaouen F, Dodelet JP. Controlled growth of Pt nanowires on carbon nanospheres and their enhanced performance as electrocatalysts in PEM fuel cells. Adv Mater 2008;20:3900e4.
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The effect of Nafion ionomer loading coated on gas diffusion electrodes with in-situ grown Pt nanowires and their durability in proton exchange membrane fuel cells Shangfeng Du a,*, Benjamin Millington a,b, Bruno G. Pollet a,b a b
School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK PEM Fuel Cell Research Group, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
article info
abstract
Article history:
The effect of varying Nafion ionomer loadings coated on the surface of gas diffusion
Received 9 November 2010
electrodes (GDEs) with in-situ grown single crystal Pt nanowires and their durability in
Received in revised form
Proton Exchange Membrane Fuel Cells (PEMFCs) were investigated. GDEs were fabricated by
27 December 2010
growing Pt nanowires directly onto the Gas Diffusion Layer (GDL) surface with a simple one-
Accepted 5 January 2011
step wet chemical approach at room temperature, as reported in our previous studies. The samples were then coated with various Nafion ionomer loadings and tested as cathodes in a 25 cm2 PEMFC hardware with Hydrogen/Air. The data were compared to commercial GDEs
Keywords:
(E-TEK ELAT GDE LT 120E-W). Performance results showed that the as-prepared GDEs with
PEM fuel cell
Pt nanowires required higher Nafion ionomer loading coating compared to the commer-
Pt nanowire
cial ones. Accelerated ageing tests (500 cycles of voltage scan) were performed in views of
Catalyst
evaluating the as-prepared GDE durability. The experimental data showed that the as-
Ionomer
prepared GDEs exhibited much larger current densities at 0.7 V but higher degradation rates
Durability
compared to commercial GDEs, indicating that the as-prepared GDEs gave poor durability.
Gas diffusion electrode (GDE)
This was due to the difference in GDE surface nanostructures influenced by the electrolyte ionomer loading coating. This effect is further discussed in this paper. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
To obtain high PEMFC performances, smaller Pt nanoparticles of nanometer range (1e10 nm) exhibiting high specific surface areas are required. The electrocatalytic behaviour is usually altered by the “particle size effect”, especially for extremely small Pt nanoparticles [1], which have also the tendency to aggregate in turns reducing their surface areas and leading to much lower mass activities (current per unit mass) as well as efficiencies [2]. However, Pt nanowires, known as a type of one-dimensional (1-D) nanomaterials, can overcome these
issues due to their high aspect ratios compared to small spherical nanoparticles, especially at high catalyst loadings. Therefore, the development of Pt-based nanowires possessing large surface areas in views of achieving high catalytic performances and utilisation efficiencies, has attracted a lot of attention in the past few years [3,4]. In recent years, many methods have been proposed for the preparation of Pt-based nanowires [3,5], with a few of them being currently used in practical PEM fuel cells, including template approaches [6e8], electro-deposition [9], electrospinning [10,11] and wet chemical methods [12,13]. These
* Corresponding author. Tel.: þ44 121 414 5081; fax: þ44 121 414 5377. E-mail address: [email protected] (S. Du). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.014
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methods produce polycrystalline and single crystal Pt or Ptbased nanowires, exhibiting excellent catalytic properties when evaluated by Cyclic Voltammetry (CV) and Membrane Electrode Assembly (MEA) experiments. The results showed that, even at low catalyst loadings, similar or better performances were obtained for both the Oxygen Reduction Reaction (ORR) and the Hydrogen Oxidation Reaction (HOR). However, as one of the most crucial steps in MEA fabrication, the content of the electrolyte ionomer has little been investigated for the electrode layer with catalyst nanowires, compared to many other studies on supported or unsupported spherical shaped catalysts in PEMFCs [14e17]. An optimum electrolyte content coated on the catalyst layer is required in order to obtain good MEA performances as it maximises the Triple Phase Boundary (TPB) as well as retaining moisture and preventing membrane dehydration [18]. This is important for any types of electrocatalysts in GDEs i.e. both spherical nanoparticles and 1-D nanowires. Therefore, there is a need to carry out this investigation on the GDEs with Pt nanowires to move this promising technique forward into practical PEMFC applications. Furthermore, the durability of the GDEs with Pt nanowires, which is directly related to the overall cost of the PEM fuel cell and its widespread commercialisation, has rarely been reported before. This is due to (i) complex electrode fabrication, (ii) high cost and (iii) issues on process scalability. These factors lead to challenges in producing durable catalyst layers with Pt-based nanowire catalysts especially when using conventional PEMFC processes. In our previous studies, we reported a facile one-step procedure for the development of GDEs with in-situ grown single crystal Pt nanowires in PEMFCs [19]. This was performed by directly reducing the Pt precursor with formic acid in aqueous solution at room temperature, without using any organic solvents, templates or growing catalysts. Better performances were obtained with this novel GDE leading to much lower manufacturing costs. Based on our preliminary results, we report here for the first time, the effect of Nafion ionomer loading coating on the performance of this new type of GDEs, and then evaluate their durability towards ORR in PEMFCs. The data were compared to commercial GDEs (E-TEK ELAT GDE LT 120E-W) with conventional spherical shaped catalysts. The experiments were performed by polarization and Electrochemical Impedance Spectroscopy (EIS) measurements in a 25 cm2 Hydrogen/ Air PEMFC hardware. The electrocatalyst durability was tested by applying a severe cycling regime (hundreds of voltage scans), as a method to accelerate the ageing and degradation process. From our data, the influence mechanism of GDE surface structures on the electrolyte ionomer loading coatings and GDE durability is proposed.
(H2PtCl6$6H2O, 99.95%) and formic acid (CH2O2, 98%) were purchased from SigmaeAldrich. GDL materials: E-TEK ELAT GDL LT 1200-W and E-TEK ELAT GDE LT 120E-W (Pt loading 0.5 mg cm2), Nafion NRE-212 membrane and Nafion (DE 1021, 10 wt%) solution were purchased and used as received from Fuel Cell Store. All aqueous solutions were prepared using ultrapure water (18.2 MU cm) from a Millipore water system.
2.2.
Experimental section
2.1.
Materials
All chemicals were used as received without any further purification. Sulphuric acid (H2SO4), hydrogen peroxide (H2O2), tetrahydrofuran (THF), ethanol (C2H6O), polyvinyl pyrrolidone (PVP, M.W. ¼ 55,000), hydrogen hexachloroplatinic acid
GDE preparation
E-TEK ELAT GDL LT 1200-W carbon cloth was used as the GDL support to grow Pt nanowires. The detailed procedure is described in our previous studies [19]. Typically, to grow Pt nanowires on GDL surfaces, pieces of carbon cloth of 5 5 cm2 were used. Carbon cloth samples were first treated by sonication for 3 min in ethanol aqueous solutions (5 vol%), and then immersed in an aqueous solution of H2PtCl6 and formic acid at room temperature. Normally, for the growth of 0.5 mg.cm2 Pt nanowires on GDLs, 33.2 mg H2PtCl6$6H2O (12.5 mg Pt, 0.5 mg.cm2 on GDL), 1.0 ml formic acid and 5 mg PVP were added to 25 ml of water. The samples were stored at room temperature for 72 h. After completion of the Pt reduction reaction and the growth of nanowires onto the substrates, the samples were rinsed with D.I. water 3 times, Ethanol 3 times and D.I. water 3 times, followed by drying at 65 C for 24 h. The loading of 0.5 mg.cm2 Pt nanowires on the carbon cloth was controlled by monitoring the Pt precursor weight versus the supporting GDL area. The carbon cloth samples with in-situ grown Pt nanowires were used directly as integrated GDEs.
2.3.
MEA fabrication
The Nafion NRE-212 membrane was pretreated by boiling for 1 h in 3% H2O2, D.I. Water, 1 M H2SO4 and D.I. water, respectively. Then the as-prepared GDEs were assembled as cathodes with Nafion NRE-212 membranes to produce MEAs, while anodes were commercial GDEs of E-TEK ELAT GDE LT 120EW, onto which a thin Nafion ionomer layer was painted using a THF solution of Nafion DE 1021 (volume ratio of Nafion solution DE 1021 to THF ¼ 1:2) followed by drying for 24 h at 90 C. The GDEs were then hot-pressed against the Nafion NRE-212 membrane at 140 C for 1 min under a constant pressure of 50 kg cm2. For comparison purposes, MEAs were simultaneously fabricated with Nafion NRE-212 membrane and commercial GDEs (E-TEK ELAT GDE LT 120EW) used for both electrodes. The hard Teflon film with a thickness of 254 mm was used as gasket material in the PEM fuel cell hardware.
2.4.
2.
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Fuel cell measurement
The MEAs with the as-prepared and commercial GDEs were tested as cathodes in a 25 cm2 PEMFC single cell hardware at a temperature of 65 C, using pure H2 and air gases at 100% relative humidity (RH) and gas flows in the range of 120 and 300 ml min1 with stoichiometries of 1.5(H2)/2.0(Air) respectively. The gas pressure of the water-saturated H2 or air (identical at the anode and cathode sides) was 2.5 bar absolute (1.5 bar backpressure). Measurements were controlled and
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recorded by a Bio-logic FCT-50S PEMFC test station (PaxiTech). The MEA was conditioned by break-in at 0.6 V for 12 h, and thereafter the polarization curves were recorded at a scan rate of 1 mV s1. The system was then cooled down to room temperature. Prior to durability experiments, the single cell hardware was reheated to 75 C and the backpressure was increased to 2 bar. Durability experiments were performed by cycling voltage scan (500 cycles) from Open Circuit Voltage (OCV) to 0.25 V with a scan rate of 30 mV s1. EIS measurements were performed in the frequency range of 10 kHz to 0.1 Hz with an amplitude of 100 mV prior and after durability experiments.
2.5.
MEA characterization
Scanning Electron Microscopy (SEM) analysis of the asprepared and the commercial GDEs, and cross sections of the MEAs before and after durability test were performed by field emission scanning electron microscope at 20 kV (Jeol JSM7000F FE-SEM, Japan). The MEA cross sections were coated with Au at 10 mA for 100 s before observation.
Fig. 1 e Polarization curves for MEAs with various Nafion loading (unit: mg cmL2) coated on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires. Catalyst loading for all electrodes: 0.5 mg Pt cmL2.
Fig. 2 e Plots of the current density at 0.7 V versus Nafion loading on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the asprepared cathode GDEs with in-situ grown Pt nanowires.
Fig. 3 e Electrochemical impedance spectra measured at 0.8 V and 0.6 V for MEAs with various Nafion loading (unit: mg cmL2) on the cathode GDE surface: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires.
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3.
Results and discussion
In GDE layers, optimising the electrolyte content is paramount for enhancing the TPB and catalyst utilization. Low ionomer loading cannot provide enough ‘connection’ between the catalyst, the carbon support and the electrolyte, but too high ionomer loading leads to a complete coverage of the entire catalyst surface active sites in turn blocking the electrode pores [18]. Compared to spherical Pt nanoparticles, Pt nanowires in-situ grown on the GDL offer much higher aspect ratios (>20), exhibiting lengths of 100 nme200 nm with diameters of only a couple of nanometers [19]. For the Pt nanowires in the as-prepared GDEs, the unconventional shape means that a different optimum electrolyte ionomer loading is required on the GDE surface. Fig. 1(a) and 1(b) show a series of polarization curves for various Nafion ionomer loadings coated on the cathode surface for the commercial and as-prepared GDEs respectively. It was found that an optimum Nafion ionomer loading of 0.9 mg cm2 gave the highest performance for the commercial cathode (E-TEK ELAT GDE LT 120E-W). With a loading below this optimum point, the performance was improved when increasing the Nafion ionomer loading. The maximum power density on the polarization curve rose from 0.590 W cm2 at a ionomer loading of 0.4 mg cm2 to 0.639 W cm2 at 0.9 mg cm2. For ionomer loadings above this optimum point, the performance decreased rapidly with increasing Nafion ionomer loadings. The maximum power density dropped to 0.567 W cm2 at 1.5 mg cm2. A very similar trend was observed
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for the as-prepared Pt-nanowire GDEs, where a maximum power density of 0.655 W cm2 was obtained at a Nafion ionomer loading of 1.2 mg cm2 [Fig. 1(b)]. However, at higher ionomer loadings after this optimum point, the performance decreased much faster than that of the commercial GDE, especially in the high current density region. Furthermore, this optimum amount of 1.2 mg cm2 found for the as-prepared GDE was higher than 0.9 mg cm2 for the commercial GDE. In order to elucidate the effect of the Nafion ionomer loading on the performance of GDEs used in this study, Fig. 2 shows comparison plots of the current density at 0.7 V on the polarization curve of different Nafion ionomer loadings for both types of MEAs, which is the common voltage operated for PEMFCs in practical applications [20]. The plots exhibited the same variation trend as that shown by the peak power density point in Fig. 1. The maximum current densities, with the value of 0.635 A cm2 and 0.494 A cm2 at 0.7 V, were obtained at the Nafion ionomer loadings of 1.2 mg cm2 and 0.9 mg cm2 for the as-prepared and commercial GDEs respectively. The much larger current density of the as-prepared GDE compared to the commercial one is due to the excellent catalytic ability of the in-situ grown single crystal Pt nanowires, as previously observed [19]. In order to understand the origins of this loading effect, EIS experiments were performed for both types of MEAs with the commercial and the as-prepared cathode GDEs at various Nafion ionomer loadings (Fig. 3). The MEAs with the commercial cathode GDEs (E-TEK ELAT GDE LT 120E-W) exhibited generally larger impedance than those with the asprepared GDEs, which is in very good agreement with our
Fig. 4 e SEM images and schematic illustration of the Nafion ionomer chain coated on the surface of the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W with commercial spherical catalyst nanoparticles ((a) and (c)) and the as-prepared cathode GDEs with in-situ grown Pt nanowires ((b) and (d)).
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previous findings [19]. Both types of GDEs showed a similar trend on the impedance at various ionomer loadings, which is in good agreement with our early data (see Figs. 1 and 2). At the optimum Nafion ionomer loading, both types of MEAs exhibited low impedance as well as low charge transfer and mass transfer resistances. The impedance of GDEs with a low loading showed a large charge transfer resistance, especially the ones measured at a high voltage of 0.8 V, indicating a poor triple phase boundary. Although increasing the loadings improved the overall performance, excessive loadings also led to high mass transfer resistances, possibly due to blocked pores caused by Nafion ionomer in the porous electrode [17]. To understand the mechanism of this effect of electrolyte ionomer loadings on the GDE performance, a schematic illustration of the Nafion ionomer coating on the GDE surface, based on the MEA measurement and SEM results is presented in Fig. 4. It is evident that for the commercial electrodes, the small spherical catalyst nanoparticles do not show any major impacts on the coated electrolyte ionomer, which formed a very good contact with the catalyst particle surface as well as the carbon support. Whereas, the as-prepared electrodes possess a different surface structure to the commercial one. A rough surface was formed by very small nanowire ends, as shown in Fig. 4(b) and 4(d), indicating that a higher electrolyte ionomer loading is required to maximise the triple phase boundary. From Fig. 4, it can also be observed that the electrolyte ionomer chain is mainly in contact with the Pt-nanowire end parts, and the interface contact area between them is not as stable as that for the spherical catalyst nanoparticles in the commercial electrodes, where the electrolyte ionomer chain possesses a large contact area with the carbon support around the catalyst nanoparticles. The only weak contact between the electrolyte ionomer and the Pt nanowire ends may result in poor durability for the as-prepared GDEs compared to the commercial ones. Fig. 5 shows degradation plots for both types of MEAs on the current densities at 0.7 V from 500 cycles of voltage scan, where the Nafion ionomer loading coating for the commercial and the as-prepared GDEs are 0.9 mg cm2 and
Fig. 5 e Degradation plots shown by the current densities at 0.7 V in each voltage scan cycle versus cycle numbers for both types MEAs with (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W and (b) the as-prepared cathode GDEs with in-situ grown Pt nanowires.
1.2 mg cm2 respectively. It can be seen that both types of GDEs exhibited similar decay behaviours. During the early stage, GDE performances were improved due to the catalyst and membrane activation, but this was very short for the asprepared GDE i.e. only about 20 cycles compared with ca. 300 cycles for the commercial one. The current density reached a maximum value after the activation process had ended, and then a slow degradation commenced and worsened over increasing cycles. After 500 cycles, both GDEs showed an obvious decline in their performances. However, the asprepared GDE displayed a much larger degradation ratio of ca. 6.5% from the top current density compared with only ca. 1.7% for the commercial GDE. Of course, the commercial GDE (E-TEK ELAT GDE LT 120E-W) is well known for its durability and is currently used in commercial PEMFC applications [21,22]. The excellent durability shown in our experiments here confirmed this point again, although the ‘real’ degradation process only occurred through 217 cycles from the maximum current density point at 283rd cycle in the ageing process, compared with 480 cycles ageing for the as-prepared GDE. To investigate these degradation phenomena in more detail, polarization curves were performed and are shown in Fig. 6 for MEAs with two types of cathode GDEs at the 1st cycle, the cycle with the best performance, and the 500th cycle. Both GDEs showed a very similar trend at these typical cycles. The
Fig. 6 e Polarization curves for two types of MEAs at the selected cycles in durability test: (a) the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W, (b) the asprepared cathode GDEs with in-situ grown Pt nanowires.
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pronounced degradation occurred at high current densities, i.e. the mass transport region, especially for the as-prepared GDE. In views of clarifying our observations on the degradation mechanisms, EIS were carried out on the samples as shown in Fig. 7 for both type of MEAs. The measurements were performed at three voltages of 0.8 V, 0.6 V and 0.4 V, corresponding to low, mid-range and very high current density region, respectively, as that shown by the polarization curves in Fig. 6. It can be observed that EIS data are in very good agreement with our previous results shown in Figs. 5 and 6. The commercial GDE exhibited a slight decrease in both the charge transfer and mass transfer resistances at the three voltages after the durability test. A better performance was obtained at the 500th cycle than the 1st one, indicating there was a pronounced improvement on the performance due to possibly good activation and low degradation rates in the durability test, as shown in Figs. 5 and 6(a). However, a different degradation occurred for the as-prepared GDE, as shown by Fig. 7(c) and 7(d). The EIS measured at 0.8 V showed an increase in impedance, corresponding to a larger kinetic resistance after cycling, indicating either a possible degradation process occurring on the catalyst itself or a poor contact between the electrolyte and the catalysts or both. However, a decrease in the diameter of the second semi-circle on the EIS measured at 0.6 V and 0.4 V, indicates an improvement on the mass transfer resistance, in other words, blocked pores were probably opened after the durability experiments. If
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this degradation was mainly caused by catalyst decay, the activation resistance would significantly increase, but in our case, there was no growth on the charge transfer resistance of the EIS measured at 0.6 V and 0.4 V, indicating, that in our conditions, the catalyst decay is not the main reason for this degradation. Furthermore, the good stability of single crystal Pt nanowires has been previously confirmed by other techniques, such as TEM and CV measurements [23,24]. Therefore, other mechanisms may occur, leading to this observable degradation. Due to the high aspect ratio of the Pt nanowires, the electrolyte ionomer chain was mainly in poor contact with the Pt nanowires end parts in the as-prepared GDEs, as discussed previously. This poor contact may worsen during the severe cycling regime in turn improving the blocked effect due to the coated electrolyte ionomer and reducing the mass transfer resistance. Although the impact from this poor contact is not so obvious at larger current densities with low voltages e.g. 0.6 V or 0.4 V, the activation resistance become much larger at relatively smaller current densities at high voltages, e.g. 0.8 V. This observation is in good agreement with our results shown in Fig. 6(b), 7(c) and 7(d). Thus, it can be deduced that it is the contact decline between the catalyst nanowires and electrolyte ionomer which leads to this pronounced degradation of the as-prepared GDEs. Because the electrolyte ionomer coated onto the GDE surface also works as the ‘glue’ during the hot-pressing process in MEA assembling, a poor contact between this ‘glue’
Fig. 7 e Electrochemical impedance spectra (EIS) measurement before and after the durability cycling for both types MEAs with the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W ((a) and (b)) and the as-prepared cathode GDEs with insitu grown Pt nanowires ((c) and (d)), respectively.
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Fig. 8 e Cross section SEM image of the MEA with the asprepared Pt-nanowire GDE (a), Nafion 212 membrane (b) and the commercial GDE of E-TEK ELAT GDE LT 120E-W (c).
and the Pt-nanowire GDE surface will also result in a bad link between the GDE and the electrolyte membrane. The SEM image of the MEA cross section before the test is shown in Fig. 8. This MEA has the as-prepared Pt-nanowire GDE at one side (a) and the commercial GDE at another side (b). It can be seen that the commercial GDE side has a much better contact with the electrolyte membrane than the as-prepared Pt-nanowire GDE, confirming the poor contact between the coated electrolyte ionomer and the Pt-nanowire GDE surface. Fig. 9 shows the SEM images of the MEA cross sections before and after durability test at cathode side, with the
commercial GDE [(a) and (b)] and the as-prepared Pt-nanowire GDE [(c) and (c)], respectively. The images show that the good contact between the commercial GDE and the electrolyte membrane did not change too much, whereas this situation became much worse for the as-prepared GDE, which may be one of the factors leading to the large degradation rate of the MEA as shown in Fig. 5. Some place was even detached from the membrane surface, demonstrating the bad ‘glue’ performance, resulting from the poor contact between the electrolyte ionomer chain and Pt nanowires grown on the GDL surface. In order to improve the durability of the as-prepared GDEs, a good contact between the Pt nanowires and electrolyte ionomer is required. This can be achieved by controlling the growth density and length of the Pt nanowires. Low growth density with short nanowire lengths, e.g. 20e40 nm enables larger contact areas between the catalyst nanowires and the electrolyte ionomer, and possibly even between the carbon support and the electrolyte ionomer. In this case, a stable contact between the catalyst and the electrolyte can be obtained similarly to that of the commercial GDE surface, and thus, an improved durability should be expected. Another way is to modify the coating process by changing the electrolyte ionomer with either a shorter chain or coating, using a low viscosity electrolyte ionomer solution. The contact interface area may also be ‘increased’ in this way but at the risk of increasing the blocked pores in the porous electrodes. In general, for the as-prepared GDEs with in-situ grown Pt nanowires made by a very simple approach, the degradation rates shown in this study are still very low when compared to other values reported in the literature. Although the durability
Fig. 9 e Cross section SEM images before and after the durability cycling for both types MEAs with the commercial cathode GDEs of E-TEK ELAT GDE LT 120E-W ((a) and (b)) and the as-prepared Pt-nanowire cathode GDEs ((c) and (d)), respectively.
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is not as good as the commercial E-TEK ELAT GDE, a much higher current density at 0.7 V was successfully obtained even after severe durability testing. Furthermore, it should be noted that we are showing preliminary data of this novel approach, bearing in mind that commercial GDEs have undertaken process optimization for several decades. The authors believe that with this very simple process leading to high performances of the novel GDEs, this one-step facile approach for preparing electrodes with in-situ grown single crystal Pt nanowires, will help PEMFCs towards successful commercial applications.
4.
Conclusions
The effect of Nafion ionomer loading coated onto the surface of the GDEs with in-situ grown Pt nanowires and their durabilities were investigated as cathodes by the polarization and EIS measurements in a 25 cm2 PEMFC single cell hardware with H2/Air. The results were compared to commercial GDEs (E-TEK ELAT GDE LT 120E-W). It was found that the electrolyte ionomer loading was an important parameter for optimizing performance. An optimum Nafion ionomer loading of 1.2 mg cm2 was found for the as-prepared GDE, which was higher than 0.9 mg cm2 for the commercial one, due to the special GDE surface formed by the in-situ grown Pt nanowires. However, because of this unconventional structure of the catalyst nanowires, by the current coating process, the contact between the electrolyte ionomer and catalyst nanowires was not as good as that for the commercial spherical catalyst particles. The durability results, obtained from the accelerated ageing measurement of 500-cycle voltage scan, showed a degradation of 6.5% on the current density at 0.7 V for the as-prepared GDE, compared to 1.7% for the commercial one. This issue could be solved by controlling the catalyst growth or improving the electrolyte ionomer coating process prior to moving this simple approach into practical applications for fabricating PEMFC electrodes.
Acknowledgements This work was supported by the Advantage West Midlands (AWM) Science City Interdisciplinary Research Alliance (SCIRA) Research Fellowships and the Higher Education Funding Council for England (HEFCE) awarded to Dr Du.
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