High platinum utilization in ultra-low Pt loaded PEM fuel cell cathodes prepared by electrospraying

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Technical Communication

High platinum utilization in ultra-low Pt loaded PEM fuel cell cathodes prepared by electrospraying S. Martin, P.L. Garcia-Ybarra*, J.L. Castillo Dept. Fisica Matematica y de Fluidos, Facultad de Ciencias, UNED, Senda del Rey 9, 28040 Madrid, Spain

article info

abstract

Article history:

Cathode electrodes for proton exchange membrane fuel cells (PEMFCs) with ultra-low

Received 11 May 2010

platinum loadings as low as 0.012 mgPtcm
2 have been prepared by the electrospray

Received in revised form

method. The electrosprayed layers have nanostructured fractal morphologies with

12 July 2010

dendrites formed by clusters (about 100 nm diameter) of a few single catalyst particles

Accepted 13 July 2010

rendering a large exposure surface of the catalyst. Optimization of the control parameters

Available online 13 August 2010

affecting this morphology has allowed us to overcome the state of the art for efficient electrodes prepared by electrospraying. Thus, using these cathodes in membrane electrode

Keywords:

assemblies (MEAs), a high platinum utilization in the range 8e10 kW g
1 was obtained for

Ultra-low platinum loading

the fuel cell operating at 40  C and atmospheric pressure. Moreover, a platinum utilization

Electrostatic spray deposition

of 20 kW g
1 was attained under more suitable operating conditions (70  C and 3.4 bar over-

Electrospray

pressure). These results substantially improve the performances achieved previously with

Electrohydrodynamic atomization

other low platinum loading electrodes prepared by electrospraying. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

PEMFC

1.

Introduction

A major barrier that PEM fuel cells has to overcome for becoming a real option in power generation is the reduction of the catalyst content in their electrodes. The most widely used catalyst in PEM fuel cells is platinum so recent research efforts focus on reducing the platinum loading and increasing the utilization of platinum [1]. Several methods such as ion-beam assisted deposition [2,3], electrodeposition [4] and those based on sputtering [5e10] have been used to reduce the platinum loading on the electrodes below 0.05 mgPtcm
2. At present, the highest utilization of platinum at ultra-low loadings has been achieved with electrodes prepared by sputtering methods [10]. But these methods require a strict atmosphere control and vacuum conditions that make them relatively expensive and

not easily adaptable to mass production. Electrohydrodynamic atomization or electrospraying [11,12] is another promising method to generate nanostructured materials [13] which can be used as efficient electrodes for PEM fuel cells. This method involves the atomization of an ink (suspension of catalyst particles in a solvent) under the influence of an electric field. The benefits of this technique are the simplicity of the experimental set up (only a pump-needle system and a high voltage power supply), the high platinum utilization due to the small electrosprayed particle size and the easy scale-up suitable for industrial production. Several authors have applied this technique to generate electrodes for PEMFC [14e17]. Thus, Baturina and Wnek [14] lowered the amount of platinum in their electrosprayed electrodes down to 0.09 mgPtcm
2. Umeda et al. [15] used the same technique

* Corresponding author. Tel.: þ34 913986743; fax: þ34 913987628. E-mail addresses: [email protected] (S. Martin), [email protected] (P.L. Garcia-Ybarra), [email protected] (J.L. Castillo). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.069

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Fig. 1 e SEM micrographs of electrosprayed catalyst layers corresponding to a platinum loading of 0.012 mgPtcmL2 and Nafion contents of (a) 20 wt.% (b) 40 wt.% and (c) 60 wt.%. The basic constituents of the Pt/C powder and the electrosprayed deposit are showed in (d) and (e), respectively. The micron bar in figures (a), (b) and (c) corresponds to 100 mm and 5 mm length for the main and the enlarged inset respectively, whereas in (d) and (e) the micron bar is 500 nm in length.

reaching a platinum loading of 0.1 mgPtcm
2. And, Chaparro et al. [16] achieved a Pt loading of 0.024 mgPtcm
2. Moreover, in a previous work [17], we also used the electrospray method to spread catalyst particles on cathode electrodes reducing the platinum loading from 0.1 mgPtcm
2 down to 0.012 mgPtcm
2. A relatively low value of platinum utilization (3.3 kW g
1) was obtained for the ultra-low platinum loading of 0.012 mgPtcm
2. The present study is focussed in the optimization of the preparation method to improve the performance of the electrodes with this 0.012 mgPtcm
2 ultra-low Pt loading. Thus, to optimize the electrode performance three independent parameters were varied: Nafion content in the suspensions, flow rate imposed to electrospray the catalyst ink, and pressure applied during the hot pressing of the MEAs. Moreover, a long-term operation of an optimized MEA was carried out.

2.

Experimental

Catalyst inks were prepared by mixing Pt/C powder (Pt 10 wt.% on Vulcan XC-72R), ethanol as solvent and Nafion (Aldrich, 5 wt.% in lower aliphatic alcohols and water) as ionomer. Different percentages of Nafion ranging from 20% to 60% (weight percentage in deposited solid fraction) were added to the suspension. A well dispersed suspension was obtained by immersing the catalyst inks in an ultrasonic bath during at least 2 h.

Cathode electrodes with a Pt loading of 0.012 mgPtcm
2 were prepared by electrospraying the catalyst ink according to the experimental set up described in [17]. The substrate consisted of a 5 cm2 square size untreated carbon paper (Toray TGP-H-060 not hydrophobized). The control parameters in the electrospray deposition were: the flow rate of the suspension, the voltage drop and the distance between the needle and the substrate [12]. A flow rate of 0.20 ml h
1 was selected to get a fractal deposit composed of small clusters, each cluster formed by aggregation of a few catalyst particles [13,18]. Furthermore, the voltage drop was set to 9 kV to ensure a stable long-term operation of the electrospray in the conejet mode and a distance between the needle and the substrate of 7 cm was chosen to allow for the complete evaporation of the ethanol during the flight of the droplets. Observation of the electrosprayed catalyst layers was made by means of a scanning electron microscope (Hitachi S-3000N). The MEAs consisted of a Nafion 212 membrane (Electrochem Inc) sandwiched between the anode and the cathode electrodes. To evaluate the cathode performance, home-made anodes were prepared by the impregnation technique with a high Pt loading of 1 mgPtcm
2 and a fixed Nafion content of 30%. The electrodes and the Nafion membrane were bonded by hot pressing at a temperature of 120  C and a pressure of 5 MPa applied for 2 min. The electrochemical performance of these MEAs was evaluated in a commercial fuel cell hardware (FC05-01SP Electrochem, Inc.) connected to an external electronic load (Hocher & Hackl PL306). Except where indicated otherwise, all measurements were carried out at a cell

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a

1 0.9 Nafion: 20% Nafion: 30% Nafion: 40% Nafion: 50% Nafion: 60%

0.8

Voltage / V

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

200

400

600

800

Current density / mA cm-2

10

100

8

80

6

-1

120

60 4 40 2

20 0

0 0

200

400

600

Current density / mA cm

c

Power per Pt loading / kW g

Power density / mW cm

-2

b

800

-2

1

300

Voltage / V

0.7

200

0.6 0.5

150

0.4 100

0.3 0.2

50

Power density / mW cm

250

0.8

-2

0.9

0.1 0 0

200

400

600

800

Current density / mA cm

1000

0 1200

-2

Fig. 2 e (a): Currentevoltage characteristics. (b): power density (left axis) and specific power curves (right axis). In both cases for MEAs with a Pt loading of 0.012 mgPtcmL2, different Nafion concentrations, and data collected at a temperature of 40  C, ambient pressure and dry H2/O2. (c): Currentevoltage characteristics (left axis) and power density curves (right axis) for a MEA with a Pt loading of 0.012 mgPtcmL2, Nafion content of 30 wt.% and two different operating conditions: void symbols correspond to the same conditions as in (a) and (b) whereas solid symbols stand for a temperature of 70  C, back-pressure of 3.4 bar and dry H2/O2. temperature of 40  C and ambient pressure. Feeding gases were dry oxygen and dry hydrogen supplied by mass flow controllers (Bronkhorst Hi-Tec).

3.

Results and discussion

Fig. 1 shows the morphology of the electrosprayed deposits corresponding to a platinum loading of 0.012 mgPtcm
2 and

different Nafion contents. The SEM pictures reveal a deposit with a fractal structure. The building blocks of this fractal structure are clusters (100 nm diameter, approx.) consisting of a few catalyst particles. The single particle size corresponds to the original nanoparticles in the Pt/C powder (see Fig. 1d). The result is a porous deposit with a high dispersion of the catalyst. Thus, the material is suitable for use as an electrode allowing a large exposure of platinum to the reactant gas due to the small size of the catalyst clusters and to the fractal structure of the deposit. As it was remarked in a previous work [17], after hot pressing, the fractality of the catalyst layer is diminished but the average grain size is maintained. Moreover, as the electrosprayed catalyst deposits mostly on the outer fibres of the carbon paper substrate, the catalyst losses are minimal. It should be mentioned that numerical simulations of depositing particles have shown that the fractal characteristic of the deposit increases with the particle diffusion [19]. In electrospray deposition the pseudo-diffusion of the charged particles is induced by the local fluctuations of the electric field. Experiments show an increase in the fractality by raising the ionomer concentration in the catalytic ink (Fig. 1, aec) which entails an enlargement of the size range of macropores in the catalytic layer: from the lower cut-off dictated by the inter-cluster microporosity up to some upper cut-off which increases with the ionomer concentration. The average electrode inter-fibre distance imposes a natural bound to this upper scale. Thus, in Fig. 1(a) where the minimum Nafion loading of 20% is applied, large voids between the fibres of the substrate can be observed. This voids become smaller when a catalyst layer with higher Nafion loading is deposited (see Fig. 1b). When the upper Nafion concentration of 60% is applied (Fig. 1c), the catalytic material (Pt/C and ionomer) is enough to fill the voids between fibres and completely cover them forming a pseudo-continuous porous layer displaying the whole allowed range of macroporous scales. In Fig. 2, the current-voltage characteristic power density and power specific curves are shown for different Nafion concentrations (in the cathode) and different operating conditions. In Fig. 2 (a), a sharp drop in performance is observed for the two extreme values of Nafion content, 20% and 60%, while the electrodes with a Nafion content of 30%, 40% and 50% exhibit roughly the same performance. These features comply well with the usual drawbacks related to a defect or to an excess of ionomer in the catalytic ink, reported previously by several authors [20]. However, when the electrospray technique is used, there is an additional coupling with the variation of the catalytic layer macroporosity which, as remarked above, is seen to depend on the ionomer concentration in the ink. For the lowest Nafion concentration the cause of the performance drop can be mainly attributed to the poor morphological characteristics of the deposit. In Fig. 1a, it becomes clear that the lack of Nafion renders the deposit more compact by leaving large regions devoid of catalyst between the electrode fibres, reducing the possibility of formation of triple phase boundaries and leading to an increased ionic resistance. The value of the internal resistance obtained by current interruption for this Nafion concentration was 0.61 Ohm cm2 while, in the range of 30e50% of Nafion content, values slightly below 0.4 Ohm cm2 were measured. In the other extreme case, at sufficiently high

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Table 1 e Summary of cathodic specific power values reported in the literature for MEAs with ultra-low Pt loading electrodes prepared by sputtering or by electrospraying methods. Pt loading, mg cm
2

Cathodic specific power, kW g
1

Sputter Sputter Sputter Sputter Electrospray Electrospray Electrospray Electrospray Electrospray

0.01 0.01 0.01 0.011 0.09 0.1 0.024 0.0125 0.012

40 18 14 14 12 0.7 13 3 20

Fuel cell operating conditions

Ref.

Temperature ( C)

Pressure, cathode e anode (bar)

Feeding

80 80 80 21 80 80 80 40 70

3.8e3.1 3e3 3e3 ambient 3e3 ambient 1e1 ambient 3.4e3.4

humid dry dry humid humid humid humid dry dry

Nafion concentrations, the catalyst is well dispersed throughout the surface (see Fig. 1c) but the excessive increase of resistances to mass transport and electronic conduction may explain the drop in performance. On the one hand, oxygen diffusion may be limited by an increased thickness of the ionomer covering the catalyst particles. Moreover, even in the gas phase, mass transport resistances can appear due to an increased thickness of the catalyst layer. On the other hand, ohmic resistances increase since the electron paths through the catalyst layer become longer. Also, some of the carbon support particles can undergo complete ionomer coverage being electrically isolated with a significant loss of active catalyst. Therefore, the intermediate range of Nafion contents is free from the difficulties arising in the two extreme cases and shows a better performance with a slight dependence on Nafion concentration. In Fig. 2(b), the power density (left axis) and the power per unit mass of platinum (specific power, right axis) are depicted for different Nafion contents and the fuel cell working at a temperature of 40  C and ambient pressure conditions. Even under these operating conditions a high utilization of platinum is achieved for these MEAs reaching a utilization peak of 8e10 kW g
1. These results represent a substantial improvement over our previous work [17] where the cathode utilization of platinum was 3.3 kW g
1. The improvement responds to an optimization of the parameters in the processing of the electrode and the MEA. On the one hand, taking in to account that the lower the flow rate the smaller the size of the electrosprayed particle [12], the flow rate was decreased (respect to the applied in [17]) until the particle size was almost the same as the Pt/C powder (see Fig. 1dee). Thus, big clusters of catalyst particles produced by higher flow rates were avoided in order to increase the Pt utilization. On the other hand, during the hot pressing of MEAs it was used an external force about 50% lower than in [17] and as a consequence less damage was caused to the labile electrosprayed deposit. To compare with platinum utilization values previously reported for low platinum loading electrodes prepared by electrospray (see Table 1), a new set of measurements were performed for temperature and pressure conditions similar to those used by other authors. Thus, the MEA with 30 wt.% Nafion content was operated at a temperature of 70  C and a back-pressure of 3.4 bar. The resulting current-voltage

[10] [8] [9] [5] [14] [15] [16] [17] Present work

characteristic (left axis) and the power density curves (right axis) are shown in Fig. 2 (c) where the curves for the same MEA working at 40  C and ambient pressure are also plotted for comparison. A maximum power density of 243 mW cm
2 and a cathodic specific power of 20 kW g
1 were attained for this MEA at the new experimental conditions. For comparison, Table 1 also provides a summary of the maximum platinum utilization achieved by different research groups using electrodes with similar ultra-low Pt loadings prepared either by sputtering or by electrospraying (other works dealing with a different range of Pt loadings or with electrodes prepared by some other techniques are not included here, to avoid extending the comparison to operating conditions beyond the scope of the present work). The highest value was obtained for electrodes with 0.01 mgPtcm
2 prepared by sputtering. As mentioned before, this technique requires a strict atmosphere control which does not facilitate the required scale-up for mass production. Besides, the specific power data obtained by the electrospray method with different platinum loadings are given in the table. To the best of our knowledge, the specific power obtained in the present work is the highest obtained for ultra-low Pt loading electrodes prepared by the electrospray method. Therefore, the electrospray method may reach platinum efficiencies which are comparable to those achieved by sputtering methods although with the added benefits of an inexpensive equipment, a simple set up and feasible scale-up.

0.9 0.8 0.7

Voltage / V

Electrode preparation Method

0.6 0.5 0.4 0.3 0.2 0.1 0 0

2000

4000

6000

8000

10000

12000

Time / min

Fig. 3 e Time evolution of the cell voltage for fuel cell operation under constant current demand (200 mA cmL2).

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In addition, a long-term operation experiment was conducted for a MEA with a Nafion content of 40 wt.%. Fig. 3 depicts the voltage-time curve for more than 200 h of uninterrupted running in galvanostatic mode with a current density demand of 200 mA cm
2, constant temperature of 50  C and ambient pressure. The cell was operated in the self e humidification mode [21,22] with dry hydrogen and dry oxygen. The voltage provided by the cell shows small oscillations which are probably due to slight unbalances in the water management. During the last 80 h of operation a voltage loss of 76 mV h
1 was observed which lies within the degradation rate expected for fuel cell operating in self-humidifying mode [23,24]. A SEM analysis of the electrodes after the test does not provide any insights of corrosion or chemical damage.

[3]

[4]

[5]

[6]

[7]

4.

Conclusions [8]

The electrospray technique has been used to deposit catalyst layers on electrodes with a Pt loading of 0.012 mgPtcm
2 and Nafion concentrations in the range of 20e60 wt.%. SEM images of these catalytic layers show a high dispersion of the catalyst powders forming fractal deposits made by small clusters of the primary catalyst Pt/C nanoparticles, with the clusters arranging in a dendritic growth. These electrodes were used as cathodes in a single PEM fuel cell. The fuel cell performance was almost independent of the Nafion content in the range of 30e50 wt.%, but a sharp performance drop was noticed for both lower or higher Nafion concentrations. Even for the simplest working conditions used in the experiments (low temperature, ambient pressure and no humidification of inlet gases), a high utilization of platinum, in the range of 8e10 kW g
1, was reached by the MEAs with optimal Nafion content. Moreover, when the fuel cell was operated at a higher temperature (70  C) and backpressure (3.4 bar) the utilization of platinum increased up to 20 kW g
1. The electrospray technique can be easily implemented in the laboratory and the scale-up is quite straightforward, becoming a competitive option for mass production of industrial electrodes. Further analysis on electrochemical characterizations, fuel cell performance under extreme operating conditions and cyclic operation regimes are currently under way.

[9]

[10]

[11] [12]

[13]

[14]

[15]

[16]

Acknowledgements Work made with financial support from the Spanish Ministerio de Ciencia e Innovacion (MICINN) under project ENE2008-06683-C03-01.

[17]

[18]

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