SiO2 Platinum Thin Film Electrode

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Journal of The Electrochemical Society, 162 (7) F634-F638 (2015)

Fabrication and Cell Analysis of a Pt/SiO2 Platinum Thin Film Electrode Masanori Inaba,z Takahisa Suzuki, Tatsuya Hatanaka,∗ and Yu Morimoto∗ Toyota Central R&D Laboratories, Inc., Nagakuge, Aichi 480-1192, Japan A platinum thin film electrode was fabricated on the surface of SiO2 nanofibers by atomic layer deposition (ALD) and its cell performances were analyzed with an MEA using it as the cathode to reveal general features of platinum thin film electrodes. Although the Pt/SiO2 cell showed comparable performance to the conventional Pt/C cell under suitable condition in spite of the much smaller electrochemical surface area (ECSA), it showed poor performance under overly dry or wet conditions as is the case with the 3M’s nanostructured thin film (NSTF) electrode. An analysis applying a transmission line model of the electrode to the Pt/SiO2 nanofiber electrode indicated that the performance loss of the Pt/SiO2 electrode under dry condition is mainly due to the increase in the reaction resistance rather than that in the ohmic resistance. The increment in the reaction resistance was suppressed by addition of ionomer to the Pt/SiO2 electrode. Furthermore, performance loss under wet condition due to cathode flooding was drastically alleviated by addition of a hydrophobic polymer to the Pt/SiO2 electrode. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0201507jes] All rights reserved. Manuscript submitted February 4, 2015; revised manuscript received March 19, 2015. Published March 27, 2015. This was Paper 1207 presented at the Cancun, Mexico, Meeting of the Society, October 5–9, 2014.

A polymer electrolyte fuel cell (PEFC) is a promising power source for automotive application. For the wider commercialization of the fuel cell vehicles (FCVs), however, there still remains challenge to reduce the amount of precious Pt used as the electrocatalyst. For that purpose, it is essential to develop a novel electrode which enables high oxygen reduction reaction (ORR) activity, high durability, and high power density.1,2 Conventional PEFCs generally use platinum particles supported on high surface area carbon particles as electrode catalysts. Great efforts have been made in improving performance of those particulate catalysts by reducing particle size,3 alloying,4–9 or applying core-shell structure.10–12 However, the chemical and electrochemical stabilities of the carbon supports,13,14 as well as the dissolution of platinum particles,15–17 raise concern about the durability of the catalyst. Furthermore, the oxygen reduction reaction (ORR) activity per surface area of platinum (the specific activity) decreases with reducing the particle size.3,17–22 These problems limit further improvement of the dispersed catalysts. Recently, platinum thin film electrodes, such as 3M’s nanostructured thin film (NSTF), have attracted much attention as alternative catalysts for PEFCs.23 The structure of the NSTF electrode is totally different from that of the conventional Pt/C electrode in following three points: low platinum surface area (10 – 25 cm2 Pt /cm2 planar ), support material without electrical conductivity, and containing no ionomer. Although NSTF electrodes showed higher efficiency and power density than conventional Pt/C electrodes under suitable operating conditions,24,25 it has been reported that they tend to show larger performance loss under dry or wet conditions.26–28 The aim of this research is to examine all those properties of NSTF on other thin film electrodes and reveal their general features. We adopted atomic layer deposition (ALD) as a fabrication process of platinum thin films. ALD is a gas-phase process that allows for a conformal growth even on high-aspect-ratio nanostructures with a precise deposit-thickness control.29–33 Therefore, various support materials with different morphologies are adaptable unlike vacuum spattering process, which is used in the fabrication of the NSTF electrode. In this paper, a platinum thin film electrode was fabricated on a surface of SiO2 nanofibers (NF). Performances of the cell with the electrode were studied and its various properties were analyzed. Experimental Fabrication of Pt/SiO2 NF electrode.— Platinum was deposited on an electrospun SiO2 NF web34,35 (Japan Vilene Company, 200 nm ∗ z

Electrochemical Society Active Member. E-mail: [email protected]

in fiber diameter and 6 μm in web thickness) by atomic layer deposition (ALD). The ALD processes were carried out by a homemade apparatus using (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3 , Tri Chemical Laboratories), argon (99.999%), and hydrogen (99.99999%) as the platinum precursor, carrier gas, and reactant gas, respectively.31,32 The ALD reactor was kept at 150◦ C throughout the deposition process. The following steps were followed to deposit platinum onto the SiO2 NFs. 1. 2. 3. 4.

Ar saturated with precursor is introduced into the reactor at 0.05 L/min for 15 min. Ar saturated with precursor is replaced with Ar at 0.2 L/min for 5 min. Ar is replaced with H2 at 0.1 L/min for 5 min. H2 is replaced with Ar at 0.2 L/min for 5 min.

Steps 1–4 were carried out for 100 times. The cross-section of the obtained Pt/SiO2 NF web was observed by scanning electron microscopy (SEM). The back-scattered electron (BSE) image is shown in Figure 1. Apparently, the surface of the SiO2 NFs are fully covered with Pt thin films. The thickness of the Pt thin film was estimated to be 4 nm from increment of weight of the web after Pt deposition and the surface area of the SiO2 NF web (9 m2 /m2 geometric ). Addition of ionomer to the Pt/SiO2 NF electrode.— Nafion solution (DE2020, DuPont) was diluted to concentration of 0.5 wt% by addition of isopropyl alcohol (Wako Pure Chemical). A Pt/SiO2 NF web was immersed in the diluted Nafion solution for 24 hours and then thoroughly drained and dried in atmosphere at room temperature.36 The amount of added Nafion was estimated to be 16 wt% from the weight increase. Addition of a hydrophobic polymer to the Pt/SiO2 NF electrode.— Teflon AF (AF 2400, DuPont-Mitsui Fluorochemicals) solution was diluted to concentration of 0.11–0.22 wt% by addition of hydrofluoroether (Novec 7200, 3M Japan). A Pt/SiO2 NF web was dipped in the diluted Teflon AF solution and then drained and dried at 100◦ C for 3 hours. The amount of added Teflon AF was estimated to be 6–19 wt% from the weight increase. Fabrication of MEAs and cell measurement.— Nafion membrane (50 μm thick) was sandwiched between the Pt/SiO2 NF electrode as a cathode and a conventional Pt/C electrode (0.2 mgPt /cm2 ) as an anode to form a membrane electrode assembly (MEA).37 The size of the electrode was 1 cm2 . The cross-sectional SEM images (secondary electron (SE) image) of the MEA are shown in Figure 2. Thickness of the cathode is ca. 3 μm.

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Current density / mA cm-2

5 2.5 0 -2.5 Pt/SiO2 NF Pt/SiO2 NF, Nafion (16wt%) Pt/SiO2 NF, Teflon AF (13wt%) Pt/C (TEC10E50E)

-5 -7.5 0

Figure 1. Cross-sectional SEM image of the Pt/SiO2 nanofiber web (BSE image). Pt, a heavier element, appears brighter than SiO2 .

Cell performance measurements were conducted using a 1 cm2 single-cell hardware and the electrochemical measurement unit (SI 1280 B, Solartron). Gas diffusion layers (GDL) with micro porous layer (MPL) and porous flow fields were applied for both cathode and anode side. The electrochemical surface area (ECSA), proton conductivity and oxygen transport resistance were analyzed by CO stripping voltammetry, electrochemical impedance spectroscopy, and

0.2

0.6 0.4 0.8 Potenal / V vs. RHE

1

Figure 3. Cyclic voltammograms of Pt/SiO2 NF, Nafion-added Pt/SiO2 NF, Teflon AF-added Pt/SiO2 NF, and Pt/C as cathode in 80◦ C, 100% RH, H2 /N2 , at 50 mV/s.

diffusion-limited current measurement using diluted oxygen, respectively as described below. CO stripping voltammetry.— 100 ppm CO/N2 was supplied while the potential was held at 0.05 V (vs the anode with H2 at 1 atm as the reference) for 30 min at 80◦ C, 100%RH. After purging with N2 , the potential was swept to 1.0 V to strip the electrochemically oxidizable CO adsorbed on the Pt. This potential sweep was carried out twice. The CO stripping charge was estimated from the difference between the currents during the first and second sweep, and then converted to ECSA value using the conversion coefficient of 420 μC/cm2 Pt .38,39 Electrochemical impedance spectroscopy.— The AC impedance spectra were acquired while the potential was held at 0.2 V and N2 was supplied to the cathode. The voltage perturbation of 5 mV (amplitude) was used at frequencies ranging from 20,000 to 1 Hz. The AC impedance spectra were analyzed by fitting the through-plane proton transfer resistance, capacitance, and high frequency resistance (HFR) using a transmission-line model.40 Diffusion-limited current measurement.— The diffusion-limited currents of polarization curves were measured at 80◦ C, 70%RH under 5 pressure conditions (101, 121, 141, 161, and 181 kPa) while O2 partial pressure was constant (0.95 kPa). Other transport resistance, Rother , was estimated by subtracting the molecular diffusion resistance from total gas transport resistance, RTotal , obtained from the measured limiting current.41 Results and Discussion

Figure 2. Cross-sectional SEM image of the MEA using the Pt/SiO2 nanofiber as cathode (a) and amplification of the Pt/SiO2 nanofiber electrode (b).

Cell performance and electrochemical properties under 80◦ C, 100%RH condition.— Cyclic voltammograms and polarization curves of the Pt/SiO2 NF electrode are shown in Fig. 3 and Fig. 4, respectively, in comparison with those of a conventional Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo) electrode with the equivalent platinum loading. The Pt/SiO2 NF electrode shows comparable performance to the Pt/C electrode at 80◦ C, 100% RH condition in spite of the much smaller ECSA without ionomer. Analyzed results of various cell properties are shown in Table I. Roughness factor (RF) of the Pt/SiO2 NF electrode (15), which is only 21% of that of the Pt/C electrode (72), is within a reasonable range considering that the roughness factor of the SiO2 NF web is estimated to be 9 and the Pt surface seems mostly smooth. ORR mass activity (MA) at 0.9 V of the Pt/SiO2 NF and the Pt/C are equivalent. Consequently, ORR specific activity (SA) of the Pt/SiO2 NF is five times as high as that of the Pt/C. This high specific activity indicates that extended platinum thin film of the Pt/SiO2 NF electrode have a bulk-like property. Proton conductivity of the Pt/SiO2 NF electrode without ionomer is 7% of


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Journal of The Electrochemical Society, 162 (7) F634-F638 (2015) 1

0.8

1 Pt/SiO2 NF

80%RH 60%RH 40%RH

0.8 Cell voltage / V

Cell voltage / V

(a)

Pt/SiO2 NF Pt/SiO2 NF, Nafion (16wt%) Pt/SiO2 NF, Teflon AF (13wt%) Pt/C (TEC10E50E)

0.6

0.4

0.6

0.4

0.2 0

0.5

1

2

1.5

0.2

Current density / A cm-2 Figure 4. Fuel cell polarization curves of Pt/SiO2 NF, Nafion-added Pt/SiO2 NF, Teflon AF-added Pt/SiO2 NF, and Pt/C as cathode in 80◦ C, 100% RH, H2 /Air with total pressure of 147 kPa, at 20 mV/s (anodic scan).

where RH+ is the areal proton transfer resistance per unit length in cm, Rreac is the areal reaction resistance on unit length in cm3 , and l is thickness of the catalyst layer in cm.44,45 Using Eq. 1, cathode resistance RC can be divided into resistance derived from proton transfer through the electrode (RH+ ) and resistance derived from ORR (Rreac ). Validity of the assumption that ORR has a linear resistance is discussed in supplemental information. RC was obtained by subtracting HFR (resistance of the membrane and the diffusion layer) from the slope of the polarization curve in Fig. 5a at 0.7 V. RH+ was obtained from the impedance spectroscopy measurement (see supplemental information). Rreac was obtained by numerically solving Eq. 1 using those values of RC , RH+ , and l (3 μm). The results are shown in Fig. 7. The reaction resistance significantly increases with decreasing RH, while the proton transfer resistance increase is limited. This indicates that the performance loss of the Pt/SiO2 nanofiber electrode under dry condition is mainly due to the increase in the reaction resistance. Fur-

0.4 0.8 Current density / A cm-2

1.2

(b)

1 Pt/SiO2 NF, Nafion 0.8 Cell voltage / V

that of the Pt/C; this result confirms proton conduction on platinum under a humidified condition without ionomer. Although proton conduction mechanism on electrolyte-free Pt surface has been discussed in literature,42,43 the exact mechanism remains unclear. Humidity dependence of the cell performance.— Although cell performance of the Pt/SiO2 NF electrode is comparable to that of the Pt/C electrode under 80◦ C, 100%RH condition, it drastically deteriorates under dry conditions as shown in Fig. 5a. As the performance loss under dry condition is also reported on the NSTF electrode,27,28 this phenomenon appears common to ionomer-free electrodes. To analyze the performance loss under dry conditions, a transmission line model of the electrode shown in Fig. 6 was applied to the Pt/SiO2 NF electrode. Assuming the ORR in a cathode catalyst layer has a linear resistance for simplicity, the areal cathode resistance, RC [ cm2 ], is given by [1] Rc = R H + Rr eac coth l R H + /Rr eac

0

80%RH 60%RH 40%RH

0.6

0.4

0.2 0


1.2

Figure 5. Fuel cell polarization curves under various humidity conditions of (a) pristine and (b) Nafion-added Pt/SiO2 NF electrode in 80◦ C, H2 /Air with O2 partial pressure of 20 kPa, at 20 mV/s (anodic scan).

ther analysis on ORR under dry conditions on ionomer-free platinum surface were not carried out in the scope of this paper. Influence of ionomer addition to the electrode.— An obvious possible strategy for mitigating the large performance loss under dry condition is to coat the Pt/SiO2 NF electrode with an ionomer. Cyclic voltammogram and polarization curves of the Nafion-added Pt/SiO2 NF electrode are shown in Fig. 3, Fig. 4, and Fig. 5b. The cyclic voltammogram and the polarization curve of Nafion-added Pt/SiO2 NF under 80◦ C, 100%RH condition are exactly similar to those of pristine Pt/SiO2 NF. In contrast, performance loss under dry conditions is alleviated by the ionomer addition as shown in Fig. 5. The analysis using the transmission line model was also applied to the Nafion-added electrode. The results are shown in Fig. 7. Proton transfer resistivity of the Nafion-added electrode shows similar humidity dependence to that of the ionomer-free electrode and are ca. 60% of that of the ionomer-free electrode (Fig. 7b). In contrast, the strong humidity dependence of reaction resistance of the ionomer-free electrode is mostly diminished for the Nafion-added electrode; the increase in

Table I. Analyzed cell properties of Pt/SiO2 NF, Nafion-added Pt/SiO2 NF, Teflon AF-added Pt/SiO2 NF, and conventional Pt/C (TEC10E50E) under 80◦ C, 100%RH condition: Pt loading, roughness factor (RF), electrochemical surface area (ECSA), ORR mass activity (MA) and specific activity (SA) (current at 0.9 V in H2 /O2 , 147 kPa, not IR free), and electrode proton conductivity. The amount of the added Teflon AF is 13 wt%.

Pt/SiO2 NF Pt/SiO2 NF, Nafion Pt/SiO2 NF, Teflon AF Pt/C (TEC10E50E)

Pt loading [mgPt /cm2 ]

RF [-]

ECSA [m2 /gPt ]

MA0.9 V [A/gPt ]

SA0.9 V [μA/cm2 Pt ]

H+ conductivity [S/cm]

0.08 0.10 0.09 0.09

15 18 16 72

19 18 18 80

2.1×102 1.8×102 9.4×10 1.7×102

1.1×103 1.0×103 5.2×102 2.1×102

1.2×10−3 4.7×10−3 1.2×10−3 9.1×10−3



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membrane

cathode catalyst layer

Cathode resistance, RC / Ω cm2

(a) diffusion layer

H+

Pt thin layer

e-

l RH+

5 Pt/SiO2 NF 4

Pt/SiO2 NF Nafion

3 2 1 0

20

Rreac

40

60

80

100

Humidity / % RH Proton transfer resisvity, RH+ / kΩ cm

(b) Figure 6. Equivalent circuit of the transmission-line model for a O2 working electrode, showing the proton transfer resistance (RH+ ), oxygen reduction reaction resistance (Rreac ), and thickness of the electrode (l). Electric resistance of Pt thin layer is negligible.

the reaction resistance under lower RH was significantly suppressed by the ionomer addition. These results indicate that ionomer not only enhances proton conduction in the electrode but also significantly facilitates ORR reaction.

Summary A platinum thin film electrode was fabricated on the surface of SiO2 NF by atomic layer deposition (ALD) to reveal general features of the platinum thin film electrodes. Although the Pt/SiO2 cell showed comparable performance to the conventional Pt/C cell under 80◦ C, 100%RH condition in spite of the much smaller electrochemical surface area (ECSA), it showed poor performance under overly dry or

Pt/SiO2 NF

5

Pt/SiO2 NF Nafion

4 3 2 1 0

20

40

60

80

100

Humidity / % RH (c)

Reacon resistance per unit length, Rreac / mΩ cm3

Alleviation of electrode flooding.— In addition to the performance loss under lower RH, drastic decline in cell performance under overly wet condition was also observed as shown in Fig. 8. Limiting current density of the unmodified Pt/SiO2 NF electrode under 40◦ C, 100% RH is much lower than that under 80◦ C, 100%RH condition (Fig. 4). This phenomenon is attributed to cathode flooding with product water since the Pt/SiO2 NF electrode, which is fully covered with Pt, is highly hydrophilic unlike conventional Pt/C electrodes. In order to enhance hydrophobicity of the Pt/SiO2 NF electrode, the effect of adding hydrophobic materials to the Pt/SiO2 NF electrode was examined. In the case of adding Nafion, which has a hydrophobic backbone in its molecular structure, the limiting current density was improved only slightly. In contrast, performance of the Pt/SiO2 NF electrode under 40◦ C, 100% RH condition was improved significantly when Teflon AF, which is a commercially available fluoropolymer that is soluble in fluorine solvents, was added (Fig. 8). To examine the effect of added amount of Teflon AF, both limiting current density of polarization curve at 40◦ C, 100% RH and oxygen transport resistance at 80◦ C, 70% RH of Pt/SiO2 NF with various Teflon AF amount were evaluated. Oxygen transport resistance in the catalyst layer, Rother , including Knudsen diffusion resistance and transport resistance through Teflon AF and liquid water, is estimated by subtracting the molecular diffusion resistance from total gas transport resistance.41 As shown in Fig. 9, the limiting current density under 40◦ C, 100% RH condition was considerably improved while the oxygen transport resistance under 80◦ C, 70% RH condition remains low when added amount of Teflon AF is around 10 wt%. This indicates that electrode flooding of Pt thin film electrodes can be alleviated by adding a proper amount of fluoropolymers.

6

1.2 Pt/SiO2 NF

1

Pt/SiO2 NF Nafion

0.8 0.6 0.4 0.2 0 20

40

60

80

100

Humidity / % RH Figure 7. Analyzed results applying the transmission-line model under various humidity conditions of the pristine Pt/SiO2 NF electrode and the ionomeradded Pt/SiO2 NF electrode: (a)cathode resistance RC , (b)proton transfer resistivity RH+ , and (c)reaction resistance per unit length Rreac .

wet conditions as is the case with the 3M’s NSTF. From an analysis applying transmission line model of the electrode to the Pt/SiO2 NF electrode, it became clear that the performance loss of the Pt/SiO2 NF electrode under dry condition is mainly due to the increase in the reaction resistance rather than that in the ohmic resistance. The increment in the reaction resistance was suppressed by addition of an ionomer to the Pt/SiO2 NF electrode. Furthermore, cathode flooding, the cause of the performance loss under wet condition, was considerably inhibited by addition of hydrophobic materials to the Pt/SiO2 NF electrode.


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Journal of The Electrochemical Society, 162 (7) F634-F638 (2015) 1 Pt/SiO2 NF Pt/SiO2 NF, Nafion (16wt%) Pt/SiO2 NF, Teflon AF (13wt%)

Cell voltage / V

0.8

0.6

0.4

0.2 0


1.2

60

Liming current density at 40°C, 100%RH/ A cm-2

2

50 1.5 40 30

1

20 0.5 10 0

0

5

10

15

0 20

O2 transport resistance, Rother at 80°C, 70%RH / s m-1

Figure 8. Fuel cell polarization curves of pristine, Nafion-added, and Teflon AF-added Pt/SiO2 NF electrode in 40◦ C, 100% RH, at 20 mV/s (anodic scan). The amount of added Teflon AF is 13 wt%.

Added amount of Teflon AF / wt% Figure 9. Dependence of limiting current density of polarization curves in 40◦ C, 100% RH, H2 /Air with total pressure of 108 kPa, at 20 mV/s (anodic scan) and O2 transport resistance in the electrode, Rother, under 80◦ C, 70% RH on added amount of Teflon AF to the Pt/SiO2 NF electrode.

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