Fuel cell testing of Pt–Ru catalysts supported on differently prepared and pretreated carbon nanotubes

Electrochimica Acta 98 (2013) 94–103

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Fuel cell testing of Pt–Ru catalysts supported on differently prepared and pretreated carbon nanotubes Wojciech Tokarz a , Grzegorz Lota b,c,1 , Elzbieta Frackowiak b,1 , a,d,1 ´ Andrzej Czerwinski , Piotr Piela a,∗ a

Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, Piotrowo 3, 60-965 Poznan, Poland c Institute of Non-Ferrous Metals, Branch in Poznan, Central Laboratory of Batteries and Cells, Forteczna 12, 61-362 Poznan, Poland d University of Warsaw, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland b

a r t i c l e

i n f o

Article history: Received 16 November 2012 Received in revised form 2 March 2013 Accepted 9 March 2013 Available online 16 March 2013 Keywords: Carbon nanotubes Pt–Ru catalyst Fuel cell Methanol Hydrogen

a b s t r a c t Proton-exchange membrane fuel cell (PEMFC) testing of Pt–Ru catalysts supported on differently prepared multiwall carbon nanotube (MCNT) supports was performed to elucidate the influence of the different supports on the operating characteristics of the catalysts under real direct methanol fuel cell (DMFC) anode and H2 -PEMFC anode conditions. The MCNTs were either thin, entangled or thick, disentangled. Pretreatment of the MCNTs was also done and it was either high-temperature KOH etching or annealing (graphitization). The performance of the catalysts was compared against the performance of a commercial Pt–Ru catalyst supported on a high-surface-area carbon black. Among the different MCNT supports, the graphitized, entangled support offered the best performance in all tests, which was equal to the performance of the commercial catalyst, despite the MCNT catalyst layer was ca. 2.2 times thicker than the carbon black catalyst layer. Even for an MCNT catalyst layer, which was almost 7 times thicker than the carbon black catalyst layer, the transport limitations were not prohibitive. This confirmed the expected potential of nanotube supports for providing superior reactant transport properties of the PEMFC catalyst layers. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The acidic-electrolyte, low-temperature fuel cell catalysis at its present stage of development relies entirely on the use of noble metals. Pt–Ru is a particularly useful noble metal combination, which is active enough to allow for building practical direct methanol fuel cell (DMFC) anodes [1]. It is also the alloy of choice for proton-exchange membrane fuel cell (PEMFC) anodes operating on reformate gas because of the increased CO tolerance [2]. One of the most important issues is the long-term stability of the Pt–Ru catalyst [3]. When this and other active noble metal catalysts are supported on inert conductive supports (usually carbon) in order to increase the noble metal utilization, the long-term stability issue no longer concerns only the metal part of the catalyst but extends to the support [4]. In the search for more stable catalyst supports, carbon nanotubes (CNTs) [5] are a viable material because they are carbon structures of low energy.

∗ Corresponding author. Tel.: +48 22 568 2908; fax: +48 22 568 2390. E-mail address: [email protected] (P. Piela). 1 ISE members. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.047

Another problem faced by the fuel cell technology is the nonoptimal transport of reactants and products to/from the catalytic active sites. In this respect, CNTs can provide for a more open catalyst layer structure and could even enable designing an optimal internal geometry of the catalyst layer [6]. The issue of poor mass transport in the catalyst layer is even more severe in the area of nonnoble fuel cell catalysts. Because of the intrinsically lower active site spatial density and, presumably, active site turnover rate of present-day non-noble catalysts, there is need for thick catalyst layers (high catalyst loadings) to obtain enough power from the unit area of the fuel cell [7]. Hence, carbon nanostructures, such as CNTs, can be of use in non-precious-metal catalysis, too [8]. Indeed, CNTs have been recognized as promising materials for fuel cells [9], because they can provide a larger three-phase boundary (electronic conductor/ionic conductor/reactant plenum) for the fuel cell reactions than it is possible with carbon-black-type materials [10]. It has been found that CNTs are very stable under highly oxidizing conditions present in fuel cell cathodes [11] and some authors postulated that they even exhibit a catalytic effect in fuel cells [12]. Elementary analysis of CNTs and carbon blacks also reveals the former are purer materials, which are less likely to poison the metallic catalytic sites [13]. For all these reasons, in the

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recent years, the interest in the use of CNTs as a supporting material for fuel cell catalysts significantly increased [14–23]. As it is known from the literature, carbon treated with KOH under harsh conditions increases its porosity and enlarges its external surface area [24–27]. On the other hand, high-temperature annealing of carbon increases its degree of crystallinity because large areas of low-energy graphitic layers tend to form [28]. In the case of CNTs, the process leads to a closing of the nanotube tips. In this contribution we present DMFC- and H2 -PEMFC studies of Pt–Ru/CNT catalysts, in which two types of CNTs strongly differing in their morphology as well as the above carbon pretreatment methods were used to elucidate a potential effect. These catalysts were screened against a comparable commercial Pt–Ru/carbon black catalyst subjected to the same fuel cell tests to find out, whether there is reason to use any such prepared CNTs instead of the carbon black. 2. Experimental 2.1. Catalysts In this work we used either thick, disentangled (later referred to as “NT templ”) or thin, entangled CNTs (later called “NT”). In the case of the latter ones, two further treated versions were used, one subjected to graphitization by annealing at 2500 ◦ C (later called “NT graph”) and a second one subjected to KOH treatment at 850 ◦ C (later called “NT act”). To obtain the catalysts, Pt–Ru (1:1 atomic) nanoparticles with diameters in the range 1–3 nm were loaded onto the above CNT supports by the impregnation method to obtain a loading of 10 wt.% of metals in the final material in each case. More detailed information about the preparation and morphology of the catalysts is found in a previous paper [29]. In summary, the BET surface area of the supports was determined to be 15, 290, 159, and 402 m2 g−1 for NT templ, NT, NT graph, and NT act, respectively. From TEM images, it was established that: (i) the NT templ is composed of straight, thick (few hundred nanometers) tubes while the NT-based supports had much thinner (10–30 nm) and buckled tubes, and (ii) all metalized supports were characterized by a relatively even distribution of the Pt–Ru particles with diameters in the range given above. In the present work, additional TEM characterization of the catalysts was performed, which consisted of acquiring and comparing images of the pristine catalysts and of the catalysts removed from the anodes after fuel cell tests. The images were collected using a Zeiss Libra 120 TEM. For reference, a (former) commercial 20 wt.% Pt–Ru (1:1 atomic) on Cabot Co.’s Vulcan XC72R carbon black (E-TEK, Inc., De Nora, USA) was also used. These catalysts were employed to prepare the anode catalytic layers. The cathode catalyst used in every fuel cell was a (former) commercial fuel-cell-grade unsupported Pt black (E-TEK, Inc.). 2.2. Fuel cell preparation In order to amplify any differences in the anode catalysts’ behavior due to the use of different supports it was necessary to prepare fuel cells capable of performing in the DMFC mode. In this mode the anodic reaction – methanol oxidation – is much more demanding than the fast anodic reaction of the H2 -PEMFC. It is known that the DMFC requires high metal loadings at both the anode and the cathode to achieve good performance [30]. On the other hand, the anode catalysts used possessed a low wt.% of metals, so that making anodes with a high metal loading would have required unusually thick catalytic layers. Making such thick layers is not beneficial for reactant transport and it is also technically very difficult to obtain very thick layers of good quality. This led us to a compromise: the Pt–Ru loading of the anodes prepared was 0.56 mg cm−2 .

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With such loading, the CNT-based catalyst layer thickness was estimated to be 73 ␮m and the Vulcan-based catalyst layer thickness was estimated to be 33 ␮m. (Catalyst layer thickness calculation: From the fuel cell preparation recipe – see below – and from the literature [31] it can be assumed the volume of the catalytic layer is one third catalyst, one third recast NafionTM , and one third void. This leads to: catalyst layer thickness = 3 · catalyst layer metals loading/weight fraction of metals in catalyst/bulk density of catalyst. The bulk density of carbon-supported metal catalysts is well estimated using the volume fractions of the catalyst components and the components’ bulk densities. Such calculations, obviously leading to underestimations of the catalyst layer thickness for hollow supports such as the CNTs, were verified by SEM measurements – see below.) For every fuel cell, a Pt loading of 6 mg cm−2 was used on the cathode, which leads to an estimate of the cathode catalytic layer thickness of 8.5 ␮m. With such over-catalyzed cathodes the probability was less to observe prevailing cathode limitations in the fuel cell tests. The anode and the cathode catalyst layers were directly formed on the opposite sides of pieces of NafionTM 117 membrane (DuPont, USA) using the painting technique [32,33]. Anode and cathode catalyst inks were prepared by ultrasonically mixing the catalyst with double-distilled water and a NafionTM dispersion (5 wt.% in alcohols, DuPont). The NafionTM dispersion was added in an amount suitable to achieve a 1:1 volumetric ratio between the bulk catalyst (internal void of the CNTs not counted in the catalyst volume) and the NafionTM in the dried ink, and water was added at a level suitable to obtain a well paintable ink. The obtained membrane-electrode assemblies (MEAs) were mounted in self-made single-cell fuel cell hardware using single-sided and double-sided cloth-type diffusion backings (E-TEK, Inc.) for the anode and the cathode sides, respectively. Thus, fuel cells were obtained, which could perform relatively well in both the DMFC and the H2 -PEMFC modes. The active geometric area of the MEAs was 1.50 cm2 . Two identical fuel cells were built and tested for every anode catalyst. 2.3. MEAs structural characterization MEAs prepared and tested in this work were removed from fuel cell hardware, cut with a scalpel perpendicularly to the constituent layers, and the MEAs cross sections were examined using a Zeiss Merlin field-emission SEM. EDS linear analysis (built in the SEM) was applied on the cross sections to identify the boundaries of the various MEA layers so as to reliably determine the layers’ thicknesses. 2.4. Fuel cell testing For testing of the anodes as direct methanol anodes, a 0.3 M aqueous methanol solution was supplied to the anode at a flow rate of 1.3 cm3 min−1 . Methanol was p.a. grade (POCh, Poland) and double-distilled water was used. All gases used (H2 , N2 , synthetic air) were of 99.999% purity. A self-made fuel cell testing station was used for control of the fuel cells’ operating parameters and for data collection. For steady-state dc polarization tests, a series connection of an Agilent 6051 A dc load with a Hewlett-Packard 6031 A power supply was used. High-frequency resistance of the fuel cell was measured during the dc polarizations with a Solartron SI1260 impedance/gain-phase analyzer connected in parallel to the fuel cell. When the anodes worked as direct methanol anodes (DMFC and methanol anode polarization experiments), the cell was also polarized dynamically with a Solartron SI1287 potentiostat/galvanostat for better current resolution. The SI1287 was also

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used in cyclic voltammetry and stripping experiments performed on the fuel cell anodes. The sequence of electrochemical tests performed identically on all assembled fuel cells was the following: (i) Cyclic voltammetry of the Pt–Ru anode (ambient conditions; cathode supplied with humidified H2 , anode supplied with humidified N2 ); (ii) Steady-state H2 -PEMFC polarization curve (ambient conditions; cathode supplied with humidified air, anode supplied with humidified H2 ); (iii) Steady-state H2 -PEMFC polarization curve (increased temperature and pressure; gases as above); (iv) Steady-state and dynamic DMFC polarization curves (increased temperature, ambient pressure; cathode supplied with dry air, anode supplied with aqueous methanol); (v) Dynamic methanol anode polarization curve (increased temperature, ambient pressure; cathode supplied with humidified H2 , anode supplied with aqueous methanol); (vi) Methanol stripping of the Pt–Ru anode (ambient conditions; cathode supplied with humidified H2 , anode supplied with aqueous methanol during adsorption step and with humidified N2 during methanol flushing and anode stripping). More detailed operating conditions are given in figure captions. 3. Results and discussion The microstructure of selected catalysts in their pristine state and after work in the fuel cell is presented in Fig. 1. From the TEM images of the unused catalysts (parts a, c, and e of the figure) it can be concluded that the catalysts prepared were not entirely homogeneous on the tens-of-nanometers scale. They were prevailingly composed of regions with a relatively even distribution of small, 1-to-3-nm Pt–Ru particles on the CNT support (white arrows), however, there were also regions with metal particles’ agglomerates of varying size (black arrows). Scarcely, one could find agglomerates as large as a few hundreds of nanometers, which were obviously remnants of the Pt–Ru black used in the preparation of the catalysts by the impregnation method. With regard to the NT act support (Fig. 1e), one could notice a large number of defects on the surface of the nanotubes, which were the effect of the KOH etching of the NT material. Average metal particle size for the regions of even distribution of the small particles on the supports was around 2.1 nm for all supports (see Table 1). TEM images of the NT templ and the NT graph catalysts removed from MEAs after fuel cell tests (Figs. 1b and d, respectively) reveal a metal particle agglomeration phenomenon in the regions of even distribution of the small Pt–Ru particles, which took place upon operation of the fuel cells. From measurements of the particles’ size it followed that, despite the large difference between the structures of the two supports, the extent of the agglomeration was the same for both (see Table 1). A feature apparent from the images of the used catalysts is that the small Pt–Ru particles sometimes detached from the supports with the recast Nafion (for example, see Fig. 1d between the two upper white arrows). Also of note, in the catalysts having worked in the fuel cell we could not find the large metallic agglomerates present in the pristine materials (the black arrow in Fig. 1d indicates a large carbon black particle assigned as coming from the microporous layer of the gas diffusion layer). It can be concluded from the TEM analysis that the impregnation method used to prepare the CNT-supported catalysts resulted in materials, which were perhaps not very stable compared with those obtained by the more common precipitation method, however, materials that possessed a similar spatial distribution of Pt–Ru

particles with the same size distribution. As such, the prepared series of catalysts was suitable to pinpoint the influence of the different CNT supports on, at least the initial, fuel cell performance. Fig. 2 is a collection of SEM images of tested MEAs cross sections. Insets show the structure of the anode catalyst layers in more detail. One thing noticed immediately is that the CNT catalyst layers (parts a, b, c, and d of the figure) contained plenty of void space as micro- as well as meso-porosity when compared with the Vulcan-based catalyst layer (Fig. 2e). As a result, the effective thicknesses of the CNT layers were very large (see Table 1) and much larger than the thickness estimated in the Experimental section. The effective thickness is obviously mostly a function of the ability of a particular support to pack itself into a layer within the realms of the applied catalyst layer fabrication method. In the case of the coarse NT templ material it was interesting to note a large portion of the recast Nafion in the catalyst layer existed aside the CNTs in the form of large particles of the polymer (inset to Fig. 2b). The rest of the polymer coated the nanotubes in the accessible areas, as can be seen in the inset to Fig. 1b (black arrows). This non-optimal morphology had its impact on the metal utilization in the NT templ catalyst layer, the internal cell resistance, and the reactant transport (see below). It must be borne in mind the anode catalyst layers preparations were not optimized in this work. Such an optimization would be necessary to reveal the full potential of the particular CNT supports and the present study should be treated as a basic screening of the different CNTs influence on the fuel cell performance. Fig. 3 presents cyclic voltammograms (CVs) of the fuel cells’ anodes. The voltammetry behavior was very stable from cycle to cycle for all the cells, so the third cycle is presented, only. The currents were also corrected for the hydrogen crossover (uncorrected curves were shifted positive on the current scale by a constant because of oxidation of a small hydrogen stream crossing the MEA from the H2 -fed side to the N2 -fed side). The shapes of all the CVs, including the reference Pt–Ru catalyst, are similar and typical for the Pt–Ru alloy with a significant Ru content, i.e., with no resolved peaks in the hydrogen adsorption/desorption region (below 0.3 V). There are differences, though. In the pseudodouble-layer region of the curves (around 0.3 V) one can notice two catalysts displaying a higher current density. These are the NT and the NT act, and particularly the latter. Because the double layer currents are dominated by the charging currents of the carbon support material, it can be concluded that: (i) the pristine, entangled CNTs (NT) provide for a higher specific area of the support in the fuel cell catalyst layer than the pristine, disentangled CNTs (NT templ), (ii) the KOH etching of the NT support leads to a much increased specific area of the support (NT act), which also follows from the respective TEM images, and (iii) the graphitization of the NT support leads to a decrease of the specific area of the support (NT graph). This is qualitatively consistent with the BET results for the different supports. When looking at the region of the onset of hydrogen evolution (below 0.1 V), we see that the specific area of the support is not related to the ability of the Pt–Ru/CNT catalyst to evolve H2 because the material standing out in this respect is the NT graph. It is on par with the reference Pt–Ru/Vulcan catalyst. Perhaps, the conductivity of the support is more important than its specific area. Methanol stripping of fuel cell electrodes sheds more light on the activity of the catalyst and on its electrochemically active surface area (EASA). Fig. 4 presents such data for the fuel cell anodes of this work. Every stripping CV contains the first anodic stripping scan and the full potential cycle immediately following the stripping scan. The curves were corrected for the hydrogen crossover current. Parameters to look at when assessing the catalytic activity of the Pt–Ru catalysts toward methanol oxidation are the positions of the onset of the stripping peak and of the peak

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Fig. 1. TEM images of pristine Pt–Ru/CNT catalysts (a, c, e) and Pt–Ru/CNT catalysts having worked in fuel cell (b, d). Type of CNT support indicated in images. Insets show lower magnification.

itself on the potential scale [34]. For all the catalysts studied, as well as for the reference catalyst, the onset of the methanol adsorption product oxidation falls in the same potential range (between 0.3 and 0.4 V). On the other hand, the stripping peak potential for the CNT-supported catalysts is somewhat less than for the reference

catalyst (except for the NT templ, for which a peak did not form). This might be explained as an effect of a more optimal composition of the Pt–Ru alloy particles used for the CNT-based catalysts as compared with the reference catalyst, or as a catalytic effect of the CNT supports. A suggestion that the CNT support might play

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Fig. 2. SEM images of MEA cross sections. Type of anode support indicated in images. Insets are close-ups for inspection of anode catalyst layer structure.

a certain catalytic role in these catalysts is found in the largest negative shift of the peak potential for NT act. NT act is expected to have a significantly different surface chemistry than the other supports due to the KOH etching. The EASA, which is particularly relevant for the functioning of the anodes as methanol anodes, can be best inferred from the limiting stripping charge, i.e., the stripping charge corresponding to

catalyst surface saturation with the product of methanol adsorption under the given adsorption conditions [34]. Such charges can only be converted to the EASA knowing the average electron-per-site (eps) parameter, which is different for different metals and different adsorption conditions [35]. Nevertheless, not knowing the average eps, as in the case of Pt–Ru and methanol, the limiting stripping charge can be used directly for comparisons of the dif-

Table 1 Parameters of Pt–Ru anode catalyst layers from methanol stripping, TEM, and SEM.

Average metal particle diametera (nm) Exposed metal areab (cm2 cm−2 geom. ) Qdes c (mC cm−2 geom. ) Limiting coverage by adsorbatec (%) Qdes /exposed metal area (mC cm−2 ) Average anode catalyst layer thicknessd (␮m) a b c d e

NT

NT templ

NT act

NT graph

Ref. Pt–Ru/Vulcan

2.11 468 55 66 0.118 150

2.10 (2.87e ) 470 (344e ) 30 54 0.064 (0.087e ) 275

2.13 464 71 92 0.153 310

2.10 (2.82e ) 470 (350e ) 48 88 0.102 (0.137e ) 100

2.00 494 48 82 0.097 45

From TEM. From average metal particle diameter and Pt–Ru loading, assuming half of supported (spherical) particle surface is exposed to reactant. From methanol stripping (see text). From SEM. After having worked in fuel cell.

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Fig. 3. In-fuel-cell cyclic voltammograms of Pt–Ru anodes. Cell temperature: 20 ◦ C; cathode feed: H2 , 40 std cm3 min−1 , humidifier temperature 20 ◦ C, no backpressure; anode feed: N2 , 100 std cm3 min−1 , humidifier temperature 20 ◦ C, no backpressure. Potential scan rate: 10 mV s−1 . Lines’ designations given in graph.

ferent Pt–Ru electrodes because it is proportional to the amount of catalytic sites that are capable of adsorbing methanol, i.e., are accessible for it, and that are capable of oxidizing the methanol adsorption product. Both these steps are needed during methanol anode operation. From Fig. 4 it is clear that not all the sites that are electrochemically active fulfill this double requirement. A sometimes significant residual charge from hydrogen desorption is seen at the beginning of the anodic stripping scans. Using this residual charge and the full hydrogen desorption charge from the second

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anodic scan, the limiting coverage of the catalyst by the methanol adsorption products could be estimated (see Table 1). The limiting stripping charges, Qdes , have been determined from the methanol stripping voltammograms using a calculation procedure described in ref. [34] and their values are given in Table 1. For the different supports, Qdes values followed this relation: NT act  NT ≈ NT graph ≈ Vulcan  NT templ. A parameter that even more precisely reflects the Pt–Ru utilization in anodes when operated as methanol anodes is the ratio of Qdes and the exposed metal area from TEM. The ratio ordered the different catalysts the same as Qdes because the exposed metal areas were very close for all of them (see Table 1). It is evident that despite the same loadings of Pt–Ru in all the anode catalyst layers, the layers displayed different quantities and to some extent also qualities of the active catalytic sites. To summarize the results of the CV and the methanol stripping experiments, from the basic electrocatalysis point of view NT act appeared to be the most promising catalyst layer for the DMFC, while NT templ – the worst. Fig. 5 presents methanol anode polarization curves for the fuel cells recorded at a very slow polarization rate (0.833 mV s−1 ). In this experiment, the cathode of the fuel cell is under pure, humidified hydrogen making it a pseudo-reference electrode, therefore the recorded voltage can be treated as the potential of the anode versus that reference electrode. The potentials were further corrected for the iR (ohmic) drop as described below in connection with DMFC results. The polarization curves are not stationary (the forward potential scan differs from the back scan), however an analysis of the charge-transfer and mass-transport features of the anodes under real DMFC operating conditions is possible. Firstly, one can notice all the anodes start to oxidize methanol in the same potential range (0.25–0.30 V), which is in accordance with the methanol stripping data, except that the onset of oxidation is now ca. 100 mV

Fig. 4. In-fuel-cell methanol stripping voltammograms of Pt–Ru anodes. Cell temperature: 20 ◦ C; cathode feed: H2 , 100 std cm3 min−1 , humidifier temperature 20 ◦ C, no backpressure; anode feeds: during methanol adsorption: 0.3 M methanol, 1.3 cm3 min−1 , 20 ◦ C, no backpressure; during methanol flushing and stripping: N2 , 200 std cm3 min−1 , humidifier temperature 20 ◦ C, no backpressure. Methanol adsorption for 30 min at 0.2 V, methanol flushing for 60 min, potential scan rate: 10 mV s−1 . Lines’ designations are given in graph.

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Fig. 5. Dynamic methanol anode polarization curves. Cell temperature: 80 ◦ C; cathode feed: H2 , 100 std cm3 min−1 , humidifier temperature 95 ◦ C, no backpressure; anode feed: 0.3 M methanol, 1.3 cm3 min−1 , 20 ◦ C, no backpressure. Potential scan rate: 0.833 mV s−1 .

more negative because of the higher temperature. For the CNTsupported anodes, contrary to the Vulcan-supported anode, some transport limitations set on very early after the onset of oxidation. This is attested by the difference in the forward and the backward scans at the very beginning of the curves (0.3–0.4 V). Disregarding that, an order of catalytic activity under the real DMFC conditions, as judged from the low-current parts of the curves, is now the following: NT graph > Vulcan ≈ NT ≈ NT act  NT templ. Generally, the order from stripping is confirmed, except for NT act and NT graph. The worse apparent kinetic behavior of the NT act and the better kinetic behavior of the NT graph than expected form the stripping data might be due to the fact that it is not a real kinetic behavior but a transport-influenced behavior, as mentioned above (large transport limitations for NT act and small for NT graph). In the transport-dominated region of the methanol anode polarization curves (above 0.45 V) all the curves, except for the NT graph, exhibit a similar behavior. A peak is formed in the forward scan. In the back scan, a smaller anodic peak is formed under the peak from the forward scan. For NT act, only an inflection point is noticeable in the back scan. Such behavior must originate from two things: (i) the consumption of methanol at the catalytic sites being much faster than the transport of methanol across the anode catalyst layer and the anode diffusion backing, which creates a large methanol concentration gradient at the catalytic sites (non-stationary conditions) – mostly responsible for the forward scan being better and for the forward peak, and (ii) a passivation of the Pt–Ru catalyst above a certain potential that can be reversed by lowering the potential – partly responsible for the forward peak and entirely responsible for the backward peak. Considering this, the current values on these non-stationary curves that are the closest to the stationary anode limiting currents are those of the backward peak/inflection point. At this point of the non-stationary polarization experiment, all excess methanol in the catalyst layer is already consumed (the methanol concentration distribution in the catalyst layer changes no more) and the Pt–Ru passivation is mostly reversed. Accordingly, an order of the limiting currents for the various electrodes can be given: NT graph > NT ≈ Vulcan > NT act  NT templ. The NT act catalyst layer, indeed, exhibited a much higher mass transport resistance than the NT graph layer, which may explain their relative behavior in the low-current parts of the methanol anode polarization curves. The reasons for the quite severe mass transport limitations under methanol for NT act and, particularly, NT templ become clear after looking at the corresponding catalyst layer thicknesses in Table 1

Fig. 6. (A) Dynamic DMFC polarization and power density curves and (B) internal cell resistances measured during steady-state DMFC polarizations. Cell temperature: 80 ◦ C; cathode feed: air, 100 std cm3 min−1 , no humidification, no backpressure; anode feed: 0.3 M methanol, 1.3 cm3 min−1 , 20 ◦ C, no backpressure. Dynamic polarization potential scan rate: 0.833 mV s−1 ; cell impedances measured in constant dc current mode with current amplitude 20 mA and frequency 2 kHz.

and at the NT templ catalyst layer structure in Fig. 2b (inset). These two catalyst layers were particularly thick, whereas the access of methanol to the bunched tubes of NT templ was certainly additionally limited. From the above analysis of the Fig. 5 data and the stripping data it can be concluded that the NT graph’s good performance as a methanol anode originates from an optimum balance of catalytic activity (good EASA) and transport properties of the catalyst layer. Overall, NT graph performed as the reference (Vulcan) catalyst in this test, which is particularly interesting vis-à-vis the much thicker anode catalyst layer with NT graph. In fact, the even thicker NT-based catalyst layer performed well. Nanotubes clearly give a mass transport advantage. The methanol anode test results obtained here are somewhat different from the results of a previous study of methanol oxidation using these catalysts performed in aqueous H2 SO4 [29], in which NT act did not perform well, while NT templ did. Clearly, there is no absolute correspondence between such tests performed in an aqueous versus in a solid polymer electrolyte. One of the reasons might be a different access of electrolyte and methanol to the catalytic sites (different EASAs). DMFC polarization curves of the fuel cells are presented in Fig. 6A. The curves have been recorded at the same slow voltage scan rate as the methanol anode polarization curves (Fig. 5). Two voltage scans were done for each curve (from high to low cell voltage, then back to high cell voltage) and the scans were averaged to obtain single lines. The average lines were further corrected for

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the iR drop, each using a corresponding average value of the cell’s internal resistance. Resistance values used for obtaining the average values are shown in Fig. 6B and were collected during steady-state DMFC polarizations (not shown due to a low current resolution) under the same DMFC operating conditions as for the dynamic DMFC polarizations of Fig. 6A. The iR-corrected polarization curves reflect the catalytic activity (charge-transfer kinetics) and the mass-transport properties of the cells, only. To show the performance of the cells with the ohmic polarization included, uncorrected power density curves are also presented in Fig. 6A. The DMFC performance was anode-limited in the whole range of cell voltage because the order of the DMFC curves for the different catalysts corresponded with the order found in the methanol anode polarizations, both in the kinetics- (high cell voltage) and the transport-dominated (low cell voltage) regions. It is evident that the DMFC performance of fuel cells with CNT-based anodes can depend strongly on the morphology of the CNTs used (NT templ vs. NT) and also on the pretreatment of the CNT support. In this test, we find that the graphitization of thin, entangled CNTs leads to an improvement of the anode’s properties (NT graph vs. NT), while the KOH treatment has little influence (NT act vs. NT). Although the NT graph cell only matched the Vulcan cell, it has to be underlined again that the latter cell had the advantage of the ca. 2.2 times thinner anode catalyst layer. One should also note that with such thick anode catalyst layers, often nearly as thick as the anode backing, the DMFC performance would benefit from a methanol concentration higher than the one used in this work (anodes were somewhat starved in this study). That is because the thick anode catalyst layer creates an additional diffusion barrier for methanol and lowers the methanol crossover rate. The use of 0.3 M methanol in this work allowed to strongly distinguish the different anode catalyst layers with respect to their transport properties. The cells’ internal resistances in the DMFC mode, shown in Fig. 6B, were independent of the current density, which was a consequence of the good hydration of the cells in this mode (contact with 0.3 M methanol). However, a quite large variation in the resistances for the different cells could be noticed. Also, the CNT-based cells all had higher internal resistances than the reference cell with the Vulcan-based anode. Surprisingly, the highest resistances were found for the NT templ and the NT graph. Particularly the latter was expected to have a low resistance because the graphitization lowers the resistance of carbon. Therefore, a structural factor must have decided about the effective resistance of the CNT-based catalyst layer (cf. Fig. 2). It may be envisioned that the higher the number of contacts between the individual nanotubes in the catalyst layer, the lower will be the effective resistance. The distribution of the solid ionomer in the layer, which is a function of the shape of the catalyst support nanostructures, is also important. Such reasoning helps understand the low resistance found for the entangled, thin CNTs (NT, inset to Fig. 2a) and the high resistance with the thick, disentangled CNTs (NT templ, inset to Fig. 2b) because the former may have allowed for many more contact points between the individual tubes than the latter ones. Unfortunately, it was impossible to scrutinize, why the resistance of NT graph was so much higher than the resistance of NT given the apparent high degree of similarity between both catalyst layers’ structures as evidenced by SEM (insets to Fig. 2a and d). It can only be hypothesized that the difference must have had to do with the recast Nafion distribution in the two layers, and, perhaps, a little excessive an amount of Nafion in the lower-specific-area (by BET) NT graph layer. Despite the sometimes large internal resistance of the CNT-based DMFCs, thanks to the low DMFC current densities the maximum iR drop noted was ca. 30 mV, so it did not preclude the CNTs from practical use, particularly in the segment of low-power DMFC applications.

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Fig. 7. (A) Steady-state H2 -PEMFC polarization and power density curves and (B) internal cell resistances measured concurrently. Cell temperature: 80 ◦ C; cathode feed: air, 200 std cm3 min−1 , humidifier temperature 95 ◦ C, 1.4 bar backpressure; anode feed: H2 , 100 std cm3 min−1 , humidifier temperature 105 ◦ C, 1.4 bar backpressure. Cell impedances measured in constant dc current mode with current amplitude 20 mA and frequency 2 kHz.

Fig. 7A shows the results of steady-state H2 -PEMFC polarizations of the fuel cells at the increased temperature and pressure. Two voltage scans were done for each polarization experiment (from high to low cell voltage, then back to high cell voltage) and the scans were averaged yielding the lines shown in the figure. Before averaging, the scans were corrected for the iR drop using the values of the cells’ internal resistances from Fig. 7B, which were simultaneously acquired during the scans. It is noticed in Fig. 7A that even for the fast anodic reaction of the H2 -PEMFC mode and, particularly, at the relatively high loadings of the Pt–Ru catalyst, the different anode catalyst layers strongly influenced the performance. Among the CNTs, the best support material for hydrogen operation is NT graph, which is on par with the reference catalyst, i.e., just as in the DMFC mode. On the other hand, the outstandingly worst DMFC performer, the NT templ, joins the rest of the materials in the hydrogen mode. The worst curve is now for the entangled, untreated support (NT). Because there was more than enough catalyst on the anodes to oxidize hydrogen and ample hydrogen was supplied to the anodes, these differences between the supports in the H2 PEMFC mode and the relative differences in the performance of the supports on methanol- vs. hydrogen operation (different rankings of the materials’ performance) point to different hydrogen transport limitations in the catalyst layers. More specifically, there may be different characteristics of the anode catalyst layers with respect to the management of the water supplied from the anode humidifier. Another factor playing a role may be too long a path of the H2 to the Pt–Ru particles because of strong confinement of some of them, if they are deeply embedded in recast polymer electrolyte. The

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factors are certainly dependent on the catalyst layer thickness and structure, the latter being perhaps more important (cf. the worst performance of the moderately thick NT layer). On the graph with the cells’ internal resistance plots (Fig. 7B) we interestingly see that the CNT cells’ resistances were generally lower than in the DMFC mode and that the spread between the different cells was much smaller. Moreover, the CNT-based cells that had the highest resistance in the DMFC mode (NT templ and NT graph) had the lowest resistance in the H2 -PEMFC mode. We may attribute these differences to a swelling phenomenon of the anode catalyst layers under methanol operation, as opposed to the lack of such swelling in the hydrogen mode. The swelling may lead to a worsening of the mutual electrical contact between the nanotubes. There was almost no difference between the resistance of the Vulcan cell in both modes suggesting a lack of such swelling for the carbon black material or a lesser influence of the swelling on the effective resistance. With no swelling, the bulk resistivity of the different nanotubes should have come into play and, indeed, NT graph and NT templ (with the thick CNTs) exhibited the lowest resistances, while NT act (NT degraded by KOH etching) exhibited the highest resistance. Also, remembering the tests on methanol were done in this work after the tests on H2 (see Section 2), a potential MEA durability problem strongly affecting the NT graph and the NT templ resistances on going from H2 to methanol cannot be completely ruled out. This could be, e.g., some anode catalyst layer delamination from the membrane. Yet, such a durability problem was not very likely because the NT and the NT act cells did not undergo any large resistance transformations upon switching to methanol. Fluctuations of the cells’ resistances observed during the polarization experiments under hydrogen (Fig. 7B) are typical for cells designed to operate on methanol, although the fluctuations are usually not as pronounced as in this work. DMFC cells have a more hydrophilic anode backing, which causes unsteady anode- and cell humidification conditions under humidified-gas operation because there is an increased tendency for liquid water formation in the anode backing. In the case of the cells prepared for this study, which had unusually thick anode catalytic layers, the humidification instability was probably additionally aggravated because the thick catalytic layer constituted another important hydrophilic element on the anode side. On the other hand, a slight tendency for a higher cell resistance at high current densities seen in Fig. 7B is also typical for the H2 -PEMFC technology and is caused by the polymer membrane drying at the anode side. The drying is because of a high electro-osmotic drag of water by the protons in the membrane, which can no longer be balanced by the water supply with the humidified hydrogen. This last phenomenon might as well be amplified by the thick anode catalyst layers acting as additional barriers for the external, anode-side humidity on its way to the electrolyte membrane. Both types of influence of the thick anode catalytic layers on the water management in the cells resulted in this rather peculiar flooding–drying behavior shown in Fig. 7B. 4. Conclusions CNTs with different morphology and subjected to different pretreatment, which are used as the conductive support for catalytic metal particles in PEMFCs, have a marked influence on the performance of the PEMFCs. In the case of the Pt–Ru/CNT anode catalysts of this work, the best performance, both in the DMFC and the H2 PEMFC modes, was obtained using thin, entangled CNTs subjected to high-temperature graphitization. This is because this support yielded a catalyst giving the optimum combination of metal utilization (EASA) and transport properties of the resulting catalyst layer. The improved conductivity of the graphitized CNTs also played a role, however the significance of this property was secondary and regarded only the H2 -PEMFC mode. The fuel cell with

this best CNT-based catalyst matched the cell prepared using the reference Vulcan-supported catalyst in performance despite the much thicker anode catalyst layer of the former relative to the latter. This demonstrated the usefulness of the CNTs for the preparation of fuel cell catalyst layers with improved transport properties. It is purposeful to revisit the problem of optimization of DMFC anodes, where the Pt–Ru/CNT catalysts might give way to good energy conversion efficiency at metal loadings lower than with the state-of-the-art use of Pt–Ru blacks. Acknowledgements This work was supported by the National Centre for Research and Development of Poland through grant no. R05 0010 04 awarded for the years 2008–2011. The SEM and TEM images were obtained using equipment purchased within CePT project no. POIG.02.02.0014-024/08-00. References [1] M.P. Hogarth, T.R. Ralph, Catalysis for low temperature fuel cells. Part III: Challenges for the direct methanol fuel cell, Platinum Metals Review 46 (2002) 146. [2] T.R. Ralph, M.P. Hogarth, Catalysis for low temperature fuel cells. Part II: The anode challenges, Platinum Metals Review 46 (2002) 117. [3] P. Piela, C. Eickes, E. Brosha, F. Garzon, P. Zelenay, Ruthenium crossover in direct methanol fuel cell with Pt–Ru black anode, Journal of the Electrochemical Society 151 (2004) A2053. [4] Y. Shao, G. Yin, Y. Gao, Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cell, Journal of Power Sources 171 (2007) 558. [5] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Applied Catalysis A: General 253 (2003) 337. [6] P.S. Ruvinskiy, A. Bonnefont, M. Houllé, C. Pham-Huu, E.R. Savinova, Preparation, testing and modeling of three-dimensionally ordered catalytic layers for electrocatalysis of fuel cell reactions, Electrochimica Acta 55 (2010) 3245. [7] H. Tributsch, U.I. Koslowski, I. Dorbandt, Experimental and theoretical modeling of Fe-Co-, Cu-, Mn-based electrocatalysts for oxygen reduction, Electrochimica Acta 53 (2008) 2198. [8] A.L. Mohana Reddy, N. Rajalakshmi, S. Ramaprabhu, Cobalt-polypyrrolemultiwalled carbon nanotube catalysts for hydrogen and alcohol fuel cells, Carbon 46 (2008) 2. [9] G. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Carbon nanotubule membranes for electrochemical energy storage and production, Nature 393 (1998) 346. [10] Z. He, J. Chen, D. Liu, H. Zhou, Y. Kuang, Electrodeposition of Pt–Ru nanoparticles on carbon nanotubes and their electrocatalytic properties for methanol electrooxidation, Diamond and Related Materials 13 (2004) 1764. [11] J.F. Lin, C.W. Mason, A. Adame, X. Liu, X.H. Peng, A.M. Kannan, Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells, Electrochimica Acta 55 (2010) 6496. [12] G. Centi, M. Gangeri, M. Fiorello, S. Perathoner, J. Amadou, D. Begin, M.J. Ledoux, C. Pham-Huu, M.E. Schuster, D.S. Su, J.-P. Tessonnier, R. Schlogl, The role of mechanically induced defects in carbon nanotubes to modify the properties of electrodes for PEM fuel cell, Catalysis Today 147 (2009) 287. [13] C.A. Bessel, K. Laubernds, N.M. Rodriguez, R.T.K. Baker, Graphite nanofibers as an electrode for fuel cell applications, Journal of Physical Chemistry B 105 (2001) 1115. [14] K. Kardimi, T. Tsoufis, A. Tomou, B.J. Kooi, M.I. Prodromidis, D. Gournis, Synthesis and characterization of carbon nanotubes decorated with Pt and PtRu nanoparticles and assessment of their electrocatalytic performance, International Journal of Hydrogen Energy 37 (2012) 1243. [15] Z.H. Zhou, W.S. Li, P.P. Guo, H.Y. Yang, X.D. Xiang, L.Z. Zeng, Z. Fu, Catalytic activity and stability toward methanol oxidation of PtRu/CNTs prepared by adsorption in aqueous solution and reduction in ethylene glycol, Journal of Solid State Electrochemistry 14 (2010) 191. [16] C.-H. Wang, H.-Y. Du, Y.-T. Tsai, C.-P. Chen, C.-J. Huang, L.C. Chen, K.H. Chen, H.-C. Shih, High performance of low electrocatalysts loading on CNT directly grown on carbon cloth for DMFC, Journal of Power Sources 171 (2007) 55. [17] K.-T. Jeng, C.-C. Chien, N.-Y. Hsu, W.-M. Huang, S.-D. Chiou, S.-H. Lin, Fabrication and impedance studies of DMFC anode incorporated with CNT-supported highmetal-content electrocatalyst, Journal of Power Sources 164 (2007) 33. [18] A.L. Ocampo, M. Miranda-Hernandez, J. Morgado, J.A. Montoya, P.J. Sebastian, Characterization and evaluation of Pt–Ru catalyst supported on multi-walled carbon nanotubes by electrochemical impedance, Journal of Power Sources 160 (2006) 915. [19] Z. Cui, C. Liu, J. Liao, W. Xing, Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electrooxidation, Electrochimica Acta 53 (2008) 7807.

W. Tokarz et al. / Electrochimica Acta 98 (2013) 94–103 [20] K. Han, J. Lee, H. Kim, Preparation and characterization of high metal content Pt–Ru alloy catalysts on various carbon blacks for DMFCs, Electrochimica Acta 52 (2006) 1697. [21] S. Garbarino, A. Ponrouch, S. Pronovost, D. Guay, Enhanced stability and activity of PtRu nanotubes for methanol electrooxidation, Electrochemistry Communications 11 (2009) 1449. [22] S.P. Somani, P.R. Somani, A. Sato, M. Umeno, Platinum and ruthenium nanoparticles decorated multi walled carbon nanotubes as electrodes for polymer electrolyte membrane fuel cells, Diamond and Related Materials 18 (2009) 497. [23] M.-C. Tsai, T.-K. Yeh, C.-Y. Chen, C.-H. Tsai, A catalytic gas diffusion layer for improving the efficiency of a direct methanol fuel cell, Electrochemistry Communications 9 (2007) 2299. [24] M.J.B. Evans, E. Halliop, J.A.F. MacDonald, The production of chemicallyactivated carbon, Carbon 37 (1999) 269. [25] A. Ahmadpour, D.D. Do, The preparation of active carbons from coal by chemical and physical activation, Carbon 34 (1996) 471. [26] E. Frackowiak, S. Delpeux, K. Jurewicz, K. Szostak, D. Cazorla-Amoros, F. Beguin, Enhanced capacitance of carbon nanotubes through chemical activation, Chemical Physics Letters 361 (2002) 35. [27] E. Raymundo-Pinero, D. Cazorla-Amoros, A. Linares-Solano, S. Delpeux, E. Frackowiak, K. Szostak, F. Beguin, High surface area carbon nanotubes prepared by chemical activation, Carbon 40 (2002) 1614.

103

[28] S. Delpeux-Ouldriane, K. Szostak, E. Frackowiak, F. Beguin, Annealing of template nanotubes to well-graphitized multi-walled carbon nanotubes, Carbon 44 (2006) 814. [29] E. Frackowiak, G. Lota, T. Cacciaguerra, F. Beguin, Carbon nanotubes with Pt–Ru catalyst for methanol fuel cell, Electrochemistry Communications 8 (2006) 129. [30] S.C. Thomas, X. Ren, S. Gottesfeld, P. Zelenay, Direct methanol fuel cells: progress in cell performance and cathode research, Electrochimica Acta 47 (2002) 3741. [31] S.C. Thomas, X. Ren, S. Gottesfeld, Influence of ionomer content in catalyst layers on direct methanol fuel cell performance, Journal of the Electrochemical Society 146 (1999) 4354. [32] M.S. Wilson, S. Gottesfeld, High performance catalyzed membranes of ultralow Pt loadings for polymer electrolyte fuel cells, Journal of the Electrochemical Society 139 (1992) L28. [33] R. Bashyam, P. Zelenay, A class of non-precious metal composite catalysts for fuel cells, Nature 443 (2006) 63. ´ [34] W. Tokarz, H. Siwek, P. Piela, A. Czerwinski, Electro-oxidation of methanol on Pt–Rh alloys, Electrochimica Acta 52 (2007) 5565. ´ [35] H. Siwek, W. Tokarz, P. Piela, A. Czerwinski, Electrochemical behavior of CO, CO2 and methanol adsorption products formed on Pt–Rh alloys of various surface compositions, Journal of Power Sources 181 (2008) 24.