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On the role of the silica-containing catalyst layer for proton exchange membrane fuel cells
Energy 68 (2014) 794e800
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
On the role of the silica-containing catalyst layer for proton exchange membrane fuel cells Chi-Young Jung a, Jae-You Yi a, Sung-Chul Yi a, b, * a b
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Hydrogen and Fuel Cell Technology, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 16 February 2014 Accepted 1 March 2014 Available online 26 March 2014
The performance of a PEMFC (proton exchange membrane fuel cell) severely decreases as the relative humidity decreases. Herein, we present size-controlled SiO2 (silica) nanoparticles in the CLs (catalyst layers) to provide sufficient water to the Nafion ionomer. It is found that the microstructure of the agglomerated CL is notably improved using the SiO2 particles with smaller diameter. In addition, as the SiO2 particle diameter decreases, both the electrochemical surface area and ohmic performance are improved, as well as the wettability, for the PEMFC application. The highest performance is achieved for the CL with the 8 nm SiO2 particle, which results in 2.93 times increased current density at 0.5 V relative to the 80 nm SiO2-containing CL, when SiO2-to-carbon ratio was fixed to 0.20. Consequently, it is more effective to improve the electrode morphology of the SiO2 CL than simply increase the SiO2 content, in order to enhance the fuel-cell performance under low relative humidity. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PEMFC Relative humidity Silica Catalyst layer Wettability
1. Introduction A PEMFC (proton exchange membrane fuel cell) is highly efficient and green energy-conversion device, regarded as one of the promising next-generation power suppliers in the 21st century. In order to use atmospheric air as an oxidizer in the fuel-cell system, a stable performance must be achieved in a wide range of RH (relative humidity) [1]. However, typical perfluorosulfonic acid membranes require external humidification, because their proton transport mechanism is strongly dependent on the membrane WU (water uptake) [2]. For successful commercialization, the US DOE (Department of Energy) focuses on producing a MEA (membrane electrode assembly) that can yield at least 0.6 V at 1.5e2.0 A cm2 with a total platinum (Pt) loading of 0.125 mg cm2 under 0e100% RH conditions [3]. To present, water management is still regarded as an important issue in operation of the PEMFC [4,5]. Since Watanabe et al. [6,7] have invented a new concept of the self-humidifying PEM (proton exchange membrane) that contained Pt and metal-oxide particles, which initiated the chemical oxidation of H2 (hydrogen) and O2 (oxygen) and retention of moisture, respectively, a number of researchers [8e10] have investigated the self-humidifying membranes to obtain superior low-humid * Corresponding author. Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Tel.: þ82 2 2220 0481; fax: þ82 2 2298 5147. E-mail address: [email protected] (S.-C. Yi). http://dx.doi.org/10.1016/j.energy.2014.03.009 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
performance by improving the membrane WU. However, increasing the WU in the anode CL (catalyst layer) may be more required than in the membrane and cathode CL, because the anode dehydration can be severed by the electro-osmotic drag [11]. Therefore, several designs of the self-humidifying CLs [12e15], mostly with hygroscopic additives, have been constructed. Han et al. [12] fabricated a thin SiO2 (silica)eNafion functional layer between the PEM and CL, by airbrushing deposition, to increase the WU in the CL under dehumidified condition. However, the fuel-cell performance of the MEA under sufficiently humidified condition should be decreased relative to the conventional one because of the increased proton-transfer resistance. Recently, Inoue et al. [13,14] have provided a facile procedure on SiO2-containing CL, which conducts both hydrolysis/condensation of SiO2 sol and catalyst ink preparation in a single vial. These authors reported that the ESA (electrochemical surface area) is increased as the SiO2 content increases, although others [15,16] argued that the ESA mostly decreased as the SiO2 blocks the electrocatalytic sites of the Pt. Besides Han et al. [12] and Inoue et al. [14] have argued that SiO2containing CL has reduced performance fluctuation in not only low RH but also high RH condition. Despite these efforts, the electrode morphology of the SiO2 CL has not been clarified yet. In addition, the influences of hygroscopic material in the CL on the chemical and electrochemical performance were either not clarified or controversial.
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To best of our knowledge, the Pt/CeNafion suspension forms an agglomerated structure covered with thin Nafion film with a thickness below 50 nm (Fig. 1) [17]. In order to improve the CL WU, it is important to make a close contact between the SiO2 particle and Nafion ionomer in the central part of the agglomerate, which is mainly formed by the micropores. Therefore, the electrode morphology may vary with different SiO2 particle diameters, hence dominating the chemical and electrochemical performance of the SiO2 CL. Under dehumidified condition, an intimate contact between SiO2 and Nafion ionomer can be of great importance to improve the ohmic polarization. Herein, the SiO2 particles with three different diameters of 8 nm, 30 nm and 80 nm were employed as a hygroscopic additive in the CLs with different SiCRs (SiO2-tocarbon ratios) of 0.01, 0.04 and 0.20. By using sufficiently smallsized SiO2 particles, an improved fuel-cell performance under low RH was obtained with increased ESA. In addition, the wettability of the CL was successfully controlled by varying the specific surface area of SiO2, which may affect the concentration polarization of the PEMFCs.
Table 1 Synthetic variables used in the modified Stöber method to prepare nano-sized SiO2 colloids with three different particle diameters.
2. Experimental
In a 50 mL vial equipped with a magnetic stirrer, the 2.24 g of 5 wt% Nafion dispersion was added into 3.1 g of a propane-1,2,3triol, followed by an addition of the prepared 10 wt% colloidal SiO2 solution to adjust the SiO2-to-carbon mass ratio to be 0.01, 0.04 and 0.20. After that, the 0.2 g of 20 wt% carbon-supported Pt catalyst was added into the mixture and homogenized at 50 C. Then, the 0.1 g of 40 wt% TBAOH was applied, followed by 24-h stirring. The 0.3 T PTFE (polytetrafluoroethylene) film was thoroughly cleansed and repeatedly painted with the SiO2-containing catalyst ink and annealed in the convection oven at 130 C until the Pt loading reached 0.2 mg cm2. After the CL is formed, a sufficient amount of 1,5-pentanediol was applied onto the CL surface in order to obtain a high transfer rate of over 99%. Prior to the decal transfer, the Nafion membrane was converted into the sodium form by lightly boiling in 1 wt% sodium hydroxide aqueous solution, followed by boiling in deionized water. After that, the CL-coated decal was assembled by sandwiching the sodium-form Nafion membrane and hotpressing at 140 C [20,21] for 10 min. After the PTFE film was peeled off, the MEA was boiled in deionized water at 100 C, in order to remove any residual 1,5-pentanediol, and 0.5 M sulfuric acid solution at 70 C, followed by cleansing in a lightly boiling water. After the cleansing, the MEA was wiped and dried in atmospheric condition less than 30 min in order to promote the pre-activation process of the cell performance [22].
2.1. Materials The 20 wt% Pt/C (Pt on carbon black) and 40 wt% TBAOH (tetrabutylammonium hydroxide) were purchased from Alfa Aesar. The 99.5 wt% TEOS (tetraethyl orthosilicate), ammonia, methanol, propane-1,2,3-triol and 1,5-pentanediol were purchased from Sigma Aldrich. The Nafion 212 membrane was purchased from Ion Power and solely used as a PEM. The 5 cm2, three-serpentine cell fixture was purchased from CNL Co. to measure the fuel-cell performance. 2.2. Preparation of the spherical SiO2 particle Preparation of the SiO2 particles was performed by the hydrolysis/condensation method [18], which is originally proposed by Stöber et al. [19]. The 1.2 g of TEOS was added into a 3.0 M ammonia solution in a 1-propanol/methanol mixture. Solution was physically stirred in room temperature to form spherical colloidal SiO2 by the solegel reaction. Then, the mixture was centrifuged and washed three times and dried in the vacuum oven at 50 C over 24 h to separate the SiO2 particles. Then, the 10 wt% aqueous mixture of SiO2 dispersion with different particle diameter of 8 nm, 30 nm and 80 nm was obtained as a hygroscopic agent. Molar concentration of the TEOS and ammonia was controlled to obtain different diameters of the SiO2 particles (Table 1).
Fig. 1. Conceptual illustration of the Pt/CeSiO2eNafion agglomerate.
Sample
Molar concentration of TEOS (M)
Molar concentration of ammonia (M)
80 nm SiO2 30 nm SiO2 8 nm SiO2
0.35 0.20 0.10
2.0 1.0 0.5
As reported in the previous literature [17], diameter of the Pt/Ce Nafion agglomerate varies in between 100 and 500 nm (Fig. 1) while the thickness of Nafion ionomer covering layer is only a few 10 nm. In order to reveal true behavior of the size effect, the SiO2 particle of three different diameters that represent for smaller (8 nm), similar (30 nm) and larger (80 nm) than the Nafion covering layer thickness is needed for the CL preparation. 2.3. Fabrication of SiO2-containing membrane electrode assembly
2.4. Chemical characterization Size of the prepared SiO2 nanoparticle was examined by TEM (transmission electron microscopy) (JEM-2000EXII) at an accelerated voltage of 200 kV. Microstructures of the SiO2 particles and SiO2-containing CL were characterized by SEM (scanning electron microscopy) (Jeol JSM-6330F) equipped with an energy dispersive spectrometer. SEM images of the electrode morphology were verified with 100,000 magnification. The thickness and contact between membrane and electrode were verified by cross-section analysis. Contact angle of the SiO2-containing CL was measured by the static sessile drop method (KRUSS DSA100 Drop Shape Analysis System). The contact angle image was taken by a digital camera (Phillips SPC900NC) at 5 s after the water drop was put on the CL surface. In order to obtain a completely flat surface, the CL was coated on the PTFE sheet, which is tightly attached to the slide glass using an acid-resistant adhesive. For accuracy, an averaged value
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was achieved by measuring the contact angles at three different points. 2.5. Electrochemical characterization Cell polarization was measured with a 5 cm2 single-serpentine cell fixture. The 99.999% H2 and O2 gas feed were supplied into the cell at 70 C and ambient pressure, while the humidifying temperatures were fixed to satisfy the target RHs. For cyclic voltammetry, the 99.999% H2 and nitrogen were supplied into the cell at 25 C and 20 sc cm, while humidification temperature was maintained at 30 C [20]. The potential was cycled from 0.05 V to 1.20 V by 50 mV s1.
was also observed in the cathode CL-coated membranes by Lee et al. [23]. In a viewpoint of the agglomerate size, the ESA is expected to be further improved when smaller SiO2 particles were applied. For larger SiO2 CLs, the ohmic resistance may be severed by decrease of the charge-transfer paths due to the weak connectivity among the agglomerates. For thickness and compactness of the SiO2 CLs, Fig. 4 presents the cross-sectional SEM images of the CLs. As observed in the figure, the CLs have become slightly thinner and denser as the SiO2 particle diameter decreases. This is well matched with Fig. 3def, which exhibit more shrinked and densely-packed agglomerates as the SiO2 particle diameter decreases. One possible explanation is due to the attractive interaction between the Pt/CeSiO2 aggregate and Nafion ionomer [24]. On this perspective, it is reasonable to have denser CL structure as the specific surface area of the SiO2 particle increases.
3. Results and discussions 3.1. Microstructure of SiO2 particle and SiO2-containing electrode
3.2. Wettability of the SiO2-containing electrode
In order to obtain the diameter and aggregation behavior, collected SiO2 products with different particle size were investigated by TEM analysis. Fig. 2aec shows the TEM images of the 80 nm, 30 nm and 8 nm spherical SiO2 particles, respectively. The spherical SiO2 particles with different diameters are successfully prepared by the modified Stöber method. As already reported by Stöber et al. [19], the aggregation of the 8 nm SiO2 particle is more developed compare to the 30 nm and 80 nm SiO2 particles. It is expected that 8 nm SiO2 particle may suffer severe aggregation when the SiO2 content is increased. However, the aggregation of the Pt/C, mainly caused by carbon black (supporting material), is more critical to fuel-cell performance due to higher concentration and surface charge. Fig. 3 shows the SEM images of the 80 nm, 30 nm and 8 nm spherical SiO2 particles and surface of the SiO2-containing CLs. The diameter of the prepared SiO2 particle is confirmed in addition to TEM images (Fig. 2). In order to use the SiO2 particles in the catalyst ink for PEMFC application, the SiO2 nanoparticles were prepared without using any surfactant. Hence, as observed in Fig. 3aec, the surface of the prepared SiO2 was getting rougher and more aggregated as the particle diameter decreases. To confirm the microstructure of the SiO2 CL, SEM images on the surface of the CLs are shown in Fig. 3def. For 80 nm SiO2 CL, the SiO2 particles are frequently observed as separated from the Pt/Ce Nafion agglomerates. More specifically, the CL showed imparted Pt/ CeNafion agglomerate structure with break-in SiO2 particles. Distribution of the Pt/CeNafion agglomerate was poor in prospects of both agglomerate size and density. However, as the SiO2 particle diameter decreases, the SiO2 CL showed well-connected morphology with superior distribution of Pt/CeSiO2eNafion agglomerate relative to larger SiO2-containing CLs. Recently, for larger SiO2-containing CLs, less-connected Pt/CeNafion structure
In typical, the hygroscopic materials are known to not only directly block the pore but also facilitate liquidewater retention due to their hydrophilic behavior [15]. This affects the masstransport limitation of the PEMFC, especially when high current density is applied. It is already mentioned that the PEMFC performance is greatly decreased as the RH decreases [17] because of the lowered WU in the CLs. Therefore, it is important to provide higher content of the hygroscopic materials, such as SiO2 particles, without increasing the hydrophilicity of the CL. Fig. 5 shows the contact angle image of the SiO2-containing CLs with different diameters of SiO2 particles. Fig. 5a stands for the conventional CL, which was fabricated from the catalyst ink without SiO2 additive. Fig. 5bed exhibits contact angle of the CLs with SiO2 particle diameter of 80 nm, 30 nm and 8 nm, respectively. Interestingly, the 8 nm SiO2 CL presented the most hydrophobic surface with sufficient amount of SiO2 particles, e.g., the SiCR of 0.20, which is designed to act as a hygroscopic material. In addition, there is distinct difference in contact angle with respect to the SiO2 particle diameter. This can be explained by considering the hydrogen bonding between the hydroxyl group (SiO2) and sulfonate group (Nafion). According to the recent findings of Paul et al. [24], the hydrophobic Nafion film surface can be only obtained, when sufficiently small (<0.1 wt%) or large amount (>3 wt%) of the Nafion ionomer concentration was applied. However, for the catalyst ink, the amount of Nafion ionomer content is not simply adjustable because the role of the Nafion is solidly fixed as an electrolyte and binding material. Therefore, for the 8 nm SiO2 CL, the increased specific surface of the SiO2 can facilitate the sulfonate groups attach to the hydrophilic surface due to the hydrogen bonding, which is similar to the thin Nafion film from dilute solution [24]. As tabulated in Table 2, the dependence of the wettability
Fig. 2. TEM images of the prepared SiO2 colloidal particles. a: 80 nm SiO2, b: 30 nm SiO2 and c: 8 nm SiO2.
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Fig. 3. SEM images of the SiO2 particles and SiO2-containing CL surfaces. a: 80 nm SiO2, b: 30 nm SiO2, c: 8 nm SiO2, d: 80 nm SiO2 CL surface, e: 30 nm SiO2 CL surface and f: 8 nm SiO2 CL surface. The SiCR was fixed as 0.20 in order to compare the electrode morphology.
Fig. 4. Cross-sectional SEM images of the SiO2-containing CLs. a: 80 nm SiO2 CL MEA, b: 30 nm SiO2 CL MEA and c: 8 nm SiO2 CL MEA.
on the SiO2 particle diameter was consistent in a wide range of the SiCR. As a consequence, both low-humid performance and masstransport limitation can be further improved by using smaller SiO2 particles in the CLs with adequate amount of the SiCR. 3.3. Electrochemical surface area of the SiO2 catalyst layer Because of the SiO2 particles penetrated into the Pt/CeNafion agglomerate or remained outside the agglomerate, the ESA can be varied due to different degree of agglomerate dispersion, catalytic surface poisoning, etc. Controversial ESA results on the SiO2 contents have been reported by the previous literature [13e16]. To clarify the influence of both the SiO2 content and particle size, the ESA of the 8 nm, 30 nm and 80 nm SiO2 MEAs was measured by the CV (cyclic voltammetry) analysis.
As observed in Fig. 6, the ESA of the 80 nm SiO2 MEA has decreased from 53 m2 g1 to 32 m2 g1 as the SiCR increases except for the SiCR of 0.01. As observed from the electrode morphology (SEM image in Fig. 3a), the degree of dispersion and size of the agglomerates were severely affected as large SiO2 particles approach among the agglomerates. For the SiCR of 0.01, the ESA slightly increased because of the suppression of sulfonate anion adsorption on the Pt surface [16], although the SiO2 particles are not sufficiently penetrated or made contact with Nafion electrolyte. However, as the SiCR increases, the 30 nm and 8 nm SiO2 MEA shows a notable increase in the ESA up to 57 m2 g1 and 62 m2 g1, respectively. In summary, the Pt/CeNafion contact is improved in the small-sized SiO2 CL as evidenced in the SEM (Fig. 3) and ESA (Fig. 6) evaluation, although aggregation of SiO2 particle is somewhat developed. Interestingly, our results correspond to
Fig. 5. Contact angle of the SiO2-containing catalyst layer with different SiO2 particle diameter. a: SiCR of 0, b: 80 nm SiO2 with the SiCR of 0.20, c: 30 nm SiO2 with the SiCR of 0.20 and d: 8 nm SiO2 with the SiCR of 0.20.
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Table 2 Contact angle data with different SiO2 particle diameters and SiCRs. SiO2-to-carbon ratio
0.01
0.04
0.20
80 nm SiO2 CL 30 nm SiO2 CL 8 nm SiO2 CL
126.7 138.6 144.5
115.1 129.6 137.7
109.2 127.0 134.1
actual contribution to the WU of Nafion electrolyte inside the agglomerate. From our experiments, the cell performance was increased as the SiCR increases, when the SiO2 particle diameter was sufficiently smaller than the Nafion covering layer. Consequently, it is of great importance to enhance the electrode morphology, by efficiently penetrate the SiO2 nanoparticles inside the Pt/CeNafion agglomerate.
experimental observations of Inoue et al. [14] and Su et al. [15]. Concretely, the ESA was rapidly increased at first but gradually increased or even decreased as the SiCR increases with different diameters.
3.4. Fuel-cell performance of the SiO2-containing electrode To investigate the effect of SiO2 particles in the CLs on the fuelcell performance, we have characterized the cell polarization with different SiO2 particle diameters and SiO2 contents. All MEA samples were operated with H2/O2 feed gas at the anode/cathode flow channel under 100% RH and 20% RH conditions. As observed in Fig. 7, for 100% RH operation, the cell performances of 80 nm, 30 nm and 8 nm SiO2 CL MEAs were similar with different SiO2 contents. Precisely, the cell performance was slightly decreased as the SiCR increases because of the increase in the ohmic resistance by the SiO2 particles [25]. In addition, the cell performance of the 8 nm SiO2 CL MEA was increased from 1.99 A cm2 to 2.11 A cm2 at 0.5 V compare to the 80 nm SiO2 CL MEA due to the increase of the ESA and ohmic performance. In contrast, Fig. 8 shows the cell polarization with different SiO2 particle diameters and contents in the 20% RH condition. For the 20% RH operation, a typical type of cell voltage decrease in low-current region [26] was obtained for all MEA samples. Overall, the cell performance of the 8 nm SiO2 CL MEA has drastically improved by exhibiting a maximum of 2.93fold current density at 0.5 V relative to the 80 nm SiO2 CL MEA. As seen in Fig. 8c, for the SiCR of 0.20 of the 8 nm SiO2 CL, approximately 1.61 A cm2 of current density was achieved, at a cell voltage of 0.5 V, under 20% RH condition. It can be concluded that the SiO2 particles in the agglomerate have a great contact with Nafion electrolyte, as confirmed in SEM image (Fig. 3), hence providing low ohmic resistance under low RH condition, as the SiO2 particle diameter decreases. In addition, the cell performance of 80 nm SiO2 CL has remained identical although the SiCR varies, which implies that the increase of the 80 nm SiO2 content, that is larger than the thin Nafion layer covering the agglomerate, has no
Fig. 6. Electrochemical surface area of SiO2-containing CL with different SiO2-to-carbon ratio and SiO2 particle diameter.
Fig. 7. Cell polarization of the SiO2-containing CL MEAs with different SiO2 diameters under 100% RH condition. a: SiCR of 0.01, b: SiCR of 0.04 and c: SiCR of 0.20.
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4. Conclusions Experimental exploration of the SiO2-containing CL was conducted with different size of SiO2 particle and different amount of the SiO2 content. The SiO2 CLs with different SiO2 particle diameters exhibited that the ESA and ohmic performance is highly improved
799
as the SiO2 particles diameter decreases. Especially, from the CV result, smaller SiO2 nanoparticles have improved ESA of the CL by successfully forming Pt/CeSiO2eNafion agglomerate structure despite the SiO2 aggregation evidenced by TEM image. In addition, the SiO2 CL became more hydrophobic as the SiO2 particle diameter decreases, because the amount of hydrogen bonding sites between the hydroxyl group of SiO2 and sulfonate group of Nafion ionomer is increased with the specific surface area of the SiO2 particles. It can be concluded that the SiO2 CL shows an enhanced PEMFC performance under low RH condition with smaller SiO2 particles. For 8 nm SiO2 CL, the cell polarization under 20% RH has improved with approximately 2.93-fold current density at 0.5 V relative to the conventional CL. Furthermore, it is anticipated to increase the specific surface area of the SiO2 nanoparticles, e.g., by decreasing the particle diameter or providing high aspect ratio, in order to improve the overall cell performance under low RH. Acknowledgments This work was supported by the Manpower Development Program for Energy supported by the Ministry of Knowledge and Economy (MKE). It was also partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2010-0024794). The authors are grateful for their financial support. References
Fig. 8. Cell polarization of the SiO2-containing CL MEAs with different SiO2 diameters under 20% RH condition. a: SiCR of 0.01, b: SiCR of 0.04 and c: SiCR of 0.20. Bars indicate standard deviation due to insufficient RH.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
On the role of the silica-containing catalyst layer for proton exchange membrane fuel cells Chi-Young Jung a, Jae-You Yi a, Sung-Chul Yi a, b, * a b
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Hydrogen and Fuel Cell Technology, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 16 February 2014 Accepted 1 March 2014 Available online 26 March 2014
The performance of a PEMFC (proton exchange membrane fuel cell) severely decreases as the relative humidity decreases. Herein, we present size-controlled SiO2 (silica) nanoparticles in the CLs (catalyst layers) to provide sufficient water to the Nafion ionomer. It is found that the microstructure of the agglomerated CL is notably improved using the SiO2 particles with smaller diameter. In addition, as the SiO2 particle diameter decreases, both the electrochemical surface area and ohmic performance are improved, as well as the wettability, for the PEMFC application. The highest performance is achieved for the CL with the 8 nm SiO2 particle, which results in 2.93 times increased current density at 0.5 V relative to the 80 nm SiO2-containing CL, when SiO2-to-carbon ratio was fixed to 0.20. Consequently, it is more effective to improve the electrode morphology of the SiO2 CL than simply increase the SiO2 content, in order to enhance the fuel-cell performance under low relative humidity. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PEMFC Relative humidity Silica Catalyst layer Wettability
1. Introduction A PEMFC (proton exchange membrane fuel cell) is highly efficient and green energy-conversion device, regarded as one of the promising next-generation power suppliers in the 21st century. In order to use atmospheric air as an oxidizer in the fuel-cell system, a stable performance must be achieved in a wide range of RH (relative humidity) [1]. However, typical perfluorosulfonic acid membranes require external humidification, because their proton transport mechanism is strongly dependent on the membrane WU (water uptake) [2]. For successful commercialization, the US DOE (Department of Energy) focuses on producing a MEA (membrane electrode assembly) that can yield at least 0.6 V at 1.5e2.0 A cm2 with a total platinum (Pt) loading of 0.125 mg cm2 under 0e100% RH conditions [3]. To present, water management is still regarded as an important issue in operation of the PEMFC [4,5]. Since Watanabe et al. [6,7] have invented a new concept of the self-humidifying PEM (proton exchange membrane) that contained Pt and metal-oxide particles, which initiated the chemical oxidation of H2 (hydrogen) and O2 (oxygen) and retention of moisture, respectively, a number of researchers [8e10] have investigated the self-humidifying membranes to obtain superior low-humid * Corresponding author. Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Tel.: þ82 2 2220 0481; fax: þ82 2 2298 5147. E-mail address: [email protected] (S.-C. Yi). http://dx.doi.org/10.1016/j.energy.2014.03.009 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
performance by improving the membrane WU. However, increasing the WU in the anode CL (catalyst layer) may be more required than in the membrane and cathode CL, because the anode dehydration can be severed by the electro-osmotic drag [11]. Therefore, several designs of the self-humidifying CLs [12e15], mostly with hygroscopic additives, have been constructed. Han et al. [12] fabricated a thin SiO2 (silica)eNafion functional layer between the PEM and CL, by airbrushing deposition, to increase the WU in the CL under dehumidified condition. However, the fuel-cell performance of the MEA under sufficiently humidified condition should be decreased relative to the conventional one because of the increased proton-transfer resistance. Recently, Inoue et al. [13,14] have provided a facile procedure on SiO2-containing CL, which conducts both hydrolysis/condensation of SiO2 sol and catalyst ink preparation in a single vial. These authors reported that the ESA (electrochemical surface area) is increased as the SiO2 content increases, although others [15,16] argued that the ESA mostly decreased as the SiO2 blocks the electrocatalytic sites of the Pt. Besides Han et al. [12] and Inoue et al. [14] have argued that SiO2containing CL has reduced performance fluctuation in not only low RH but also high RH condition. Despite these efforts, the electrode morphology of the SiO2 CL has not been clarified yet. In addition, the influences of hygroscopic material in the CL on the chemical and electrochemical performance were either not clarified or controversial.
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To best of our knowledge, the Pt/CeNafion suspension forms an agglomerated structure covered with thin Nafion film with a thickness below 50 nm (Fig. 1) [17]. In order to improve the CL WU, it is important to make a close contact between the SiO2 particle and Nafion ionomer in the central part of the agglomerate, which is mainly formed by the micropores. Therefore, the electrode morphology may vary with different SiO2 particle diameters, hence dominating the chemical and electrochemical performance of the SiO2 CL. Under dehumidified condition, an intimate contact between SiO2 and Nafion ionomer can be of great importance to improve the ohmic polarization. Herein, the SiO2 particles with three different diameters of 8 nm, 30 nm and 80 nm were employed as a hygroscopic additive in the CLs with different SiCRs (SiO2-tocarbon ratios) of 0.01, 0.04 and 0.20. By using sufficiently smallsized SiO2 particles, an improved fuel-cell performance under low RH was obtained with increased ESA. In addition, the wettability of the CL was successfully controlled by varying the specific surface area of SiO2, which may affect the concentration polarization of the PEMFCs.
Table 1 Synthetic variables used in the modified Stöber method to prepare nano-sized SiO2 colloids with three different particle diameters.
2. Experimental
In a 50 mL vial equipped with a magnetic stirrer, the 2.24 g of 5 wt% Nafion dispersion was added into 3.1 g of a propane-1,2,3triol, followed by an addition of the prepared 10 wt% colloidal SiO2 solution to adjust the SiO2-to-carbon mass ratio to be 0.01, 0.04 and 0.20. After that, the 0.2 g of 20 wt% carbon-supported Pt catalyst was added into the mixture and homogenized at 50 C. Then, the 0.1 g of 40 wt% TBAOH was applied, followed by 24-h stirring. The 0.3 T PTFE (polytetrafluoroethylene) film was thoroughly cleansed and repeatedly painted with the SiO2-containing catalyst ink and annealed in the convection oven at 130 C until the Pt loading reached 0.2 mg cm2. After the CL is formed, a sufficient amount of 1,5-pentanediol was applied onto the CL surface in order to obtain a high transfer rate of over 99%. Prior to the decal transfer, the Nafion membrane was converted into the sodium form by lightly boiling in 1 wt% sodium hydroxide aqueous solution, followed by boiling in deionized water. After that, the CL-coated decal was assembled by sandwiching the sodium-form Nafion membrane and hotpressing at 140 C [20,21] for 10 min. After the PTFE film was peeled off, the MEA was boiled in deionized water at 100 C, in order to remove any residual 1,5-pentanediol, and 0.5 M sulfuric acid solution at 70 C, followed by cleansing in a lightly boiling water. After the cleansing, the MEA was wiped and dried in atmospheric condition less than 30 min in order to promote the pre-activation process of the cell performance [22].
2.1. Materials The 20 wt% Pt/C (Pt on carbon black) and 40 wt% TBAOH (tetrabutylammonium hydroxide) were purchased from Alfa Aesar. The 99.5 wt% TEOS (tetraethyl orthosilicate), ammonia, methanol, propane-1,2,3-triol and 1,5-pentanediol were purchased from Sigma Aldrich. The Nafion 212 membrane was purchased from Ion Power and solely used as a PEM. The 5 cm2, three-serpentine cell fixture was purchased from CNL Co. to measure the fuel-cell performance. 2.2. Preparation of the spherical SiO2 particle Preparation of the SiO2 particles was performed by the hydrolysis/condensation method [18], which is originally proposed by Stöber et al. [19]. The 1.2 g of TEOS was added into a 3.0 M ammonia solution in a 1-propanol/methanol mixture. Solution was physically stirred in room temperature to form spherical colloidal SiO2 by the solegel reaction. Then, the mixture was centrifuged and washed three times and dried in the vacuum oven at 50 C over 24 h to separate the SiO2 particles. Then, the 10 wt% aqueous mixture of SiO2 dispersion with different particle diameter of 8 nm, 30 nm and 80 nm was obtained as a hygroscopic agent. Molar concentration of the TEOS and ammonia was controlled to obtain different diameters of the SiO2 particles (Table 1).
Fig. 1. Conceptual illustration of the Pt/CeSiO2eNafion agglomerate.
Sample
Molar concentration of TEOS (M)
Molar concentration of ammonia (M)
80 nm SiO2 30 nm SiO2 8 nm SiO2
0.35 0.20 0.10
2.0 1.0 0.5
As reported in the previous literature [17], diameter of the Pt/Ce Nafion agglomerate varies in between 100 and 500 nm (Fig. 1) while the thickness of Nafion ionomer covering layer is only a few 10 nm. In order to reveal true behavior of the size effect, the SiO2 particle of three different diameters that represent for smaller (8 nm), similar (30 nm) and larger (80 nm) than the Nafion covering layer thickness is needed for the CL preparation. 2.3. Fabrication of SiO2-containing membrane electrode assembly
2.4. Chemical characterization Size of the prepared SiO2 nanoparticle was examined by TEM (transmission electron microscopy) (JEM-2000EXII) at an accelerated voltage of 200 kV. Microstructures of the SiO2 particles and SiO2-containing CL were characterized by SEM (scanning electron microscopy) (Jeol JSM-6330F) equipped with an energy dispersive spectrometer. SEM images of the electrode morphology were verified with 100,000 magnification. The thickness and contact between membrane and electrode were verified by cross-section analysis. Contact angle of the SiO2-containing CL was measured by the static sessile drop method (KRUSS DSA100 Drop Shape Analysis System). The contact angle image was taken by a digital camera (Phillips SPC900NC) at 5 s after the water drop was put on the CL surface. In order to obtain a completely flat surface, the CL was coated on the PTFE sheet, which is tightly attached to the slide glass using an acid-resistant adhesive. For accuracy, an averaged value
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was achieved by measuring the contact angles at three different points. 2.5. Electrochemical characterization Cell polarization was measured with a 5 cm2 single-serpentine cell fixture. The 99.999% H2 and O2 gas feed were supplied into the cell at 70 C and ambient pressure, while the humidifying temperatures were fixed to satisfy the target RHs. For cyclic voltammetry, the 99.999% H2 and nitrogen were supplied into the cell at 25 C and 20 sc cm, while humidification temperature was maintained at 30 C [20]. The potential was cycled from 0.05 V to 1.20 V by 50 mV s1.
was also observed in the cathode CL-coated membranes by Lee et al. [23]. In a viewpoint of the agglomerate size, the ESA is expected to be further improved when smaller SiO2 particles were applied. For larger SiO2 CLs, the ohmic resistance may be severed by decrease of the charge-transfer paths due to the weak connectivity among the agglomerates. For thickness and compactness of the SiO2 CLs, Fig. 4 presents the cross-sectional SEM images of the CLs. As observed in the figure, the CLs have become slightly thinner and denser as the SiO2 particle diameter decreases. This is well matched with Fig. 3def, which exhibit more shrinked and densely-packed agglomerates as the SiO2 particle diameter decreases. One possible explanation is due to the attractive interaction between the Pt/CeSiO2 aggregate and Nafion ionomer [24]. On this perspective, it is reasonable to have denser CL structure as the specific surface area of the SiO2 particle increases.
3. Results and discussions 3.1. Microstructure of SiO2 particle and SiO2-containing electrode
3.2. Wettability of the SiO2-containing electrode
In order to obtain the diameter and aggregation behavior, collected SiO2 products with different particle size were investigated by TEM analysis. Fig. 2aec shows the TEM images of the 80 nm, 30 nm and 8 nm spherical SiO2 particles, respectively. The spherical SiO2 particles with different diameters are successfully prepared by the modified Stöber method. As already reported by Stöber et al. [19], the aggregation of the 8 nm SiO2 particle is more developed compare to the 30 nm and 80 nm SiO2 particles. It is expected that 8 nm SiO2 particle may suffer severe aggregation when the SiO2 content is increased. However, the aggregation of the Pt/C, mainly caused by carbon black (supporting material), is more critical to fuel-cell performance due to higher concentration and surface charge. Fig. 3 shows the SEM images of the 80 nm, 30 nm and 8 nm spherical SiO2 particles and surface of the SiO2-containing CLs. The diameter of the prepared SiO2 particle is confirmed in addition to TEM images (Fig. 2). In order to use the SiO2 particles in the catalyst ink for PEMFC application, the SiO2 nanoparticles were prepared without using any surfactant. Hence, as observed in Fig. 3aec, the surface of the prepared SiO2 was getting rougher and more aggregated as the particle diameter decreases. To confirm the microstructure of the SiO2 CL, SEM images on the surface of the CLs are shown in Fig. 3def. For 80 nm SiO2 CL, the SiO2 particles are frequently observed as separated from the Pt/Ce Nafion agglomerates. More specifically, the CL showed imparted Pt/ CeNafion agglomerate structure with break-in SiO2 particles. Distribution of the Pt/CeNafion agglomerate was poor in prospects of both agglomerate size and density. However, as the SiO2 particle diameter decreases, the SiO2 CL showed well-connected morphology with superior distribution of Pt/CeSiO2eNafion agglomerate relative to larger SiO2-containing CLs. Recently, for larger SiO2-containing CLs, less-connected Pt/CeNafion structure
In typical, the hygroscopic materials are known to not only directly block the pore but also facilitate liquidewater retention due to their hydrophilic behavior [15]. This affects the masstransport limitation of the PEMFC, especially when high current density is applied. It is already mentioned that the PEMFC performance is greatly decreased as the RH decreases [17] because of the lowered WU in the CLs. Therefore, it is important to provide higher content of the hygroscopic materials, such as SiO2 particles, without increasing the hydrophilicity of the CL. Fig. 5 shows the contact angle image of the SiO2-containing CLs with different diameters of SiO2 particles. Fig. 5a stands for the conventional CL, which was fabricated from the catalyst ink without SiO2 additive. Fig. 5bed exhibits contact angle of the CLs with SiO2 particle diameter of 80 nm, 30 nm and 8 nm, respectively. Interestingly, the 8 nm SiO2 CL presented the most hydrophobic surface with sufficient amount of SiO2 particles, e.g., the SiCR of 0.20, which is designed to act as a hygroscopic material. In addition, there is distinct difference in contact angle with respect to the SiO2 particle diameter. This can be explained by considering the hydrogen bonding between the hydroxyl group (SiO2) and sulfonate group (Nafion). According to the recent findings of Paul et al. [24], the hydrophobic Nafion film surface can be only obtained, when sufficiently small (<0.1 wt%) or large amount (>3 wt%) of the Nafion ionomer concentration was applied. However, for the catalyst ink, the amount of Nafion ionomer content is not simply adjustable because the role of the Nafion is solidly fixed as an electrolyte and binding material. Therefore, for the 8 nm SiO2 CL, the increased specific surface of the SiO2 can facilitate the sulfonate groups attach to the hydrophilic surface due to the hydrogen bonding, which is similar to the thin Nafion film from dilute solution [24]. As tabulated in Table 2, the dependence of the wettability
Fig. 2. TEM images of the prepared SiO2 colloidal particles. a: 80 nm SiO2, b: 30 nm SiO2 and c: 8 nm SiO2.
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Fig. 3. SEM images of the SiO2 particles and SiO2-containing CL surfaces. a: 80 nm SiO2, b: 30 nm SiO2, c: 8 nm SiO2, d: 80 nm SiO2 CL surface, e: 30 nm SiO2 CL surface and f: 8 nm SiO2 CL surface. The SiCR was fixed as 0.20 in order to compare the electrode morphology.
Fig. 4. Cross-sectional SEM images of the SiO2-containing CLs. a: 80 nm SiO2 CL MEA, b: 30 nm SiO2 CL MEA and c: 8 nm SiO2 CL MEA.
on the SiO2 particle diameter was consistent in a wide range of the SiCR. As a consequence, both low-humid performance and masstransport limitation can be further improved by using smaller SiO2 particles in the CLs with adequate amount of the SiCR. 3.3. Electrochemical surface area of the SiO2 catalyst layer Because of the SiO2 particles penetrated into the Pt/CeNafion agglomerate or remained outside the agglomerate, the ESA can be varied due to different degree of agglomerate dispersion, catalytic surface poisoning, etc. Controversial ESA results on the SiO2 contents have been reported by the previous literature [13e16]. To clarify the influence of both the SiO2 content and particle size, the ESA of the 8 nm, 30 nm and 80 nm SiO2 MEAs was measured by the CV (cyclic voltammetry) analysis.
As observed in Fig. 6, the ESA of the 80 nm SiO2 MEA has decreased from 53 m2 g1 to 32 m2 g1 as the SiCR increases except for the SiCR of 0.01. As observed from the electrode morphology (SEM image in Fig. 3a), the degree of dispersion and size of the agglomerates were severely affected as large SiO2 particles approach among the agglomerates. For the SiCR of 0.01, the ESA slightly increased because of the suppression of sulfonate anion adsorption on the Pt surface [16], although the SiO2 particles are not sufficiently penetrated or made contact with Nafion electrolyte. However, as the SiCR increases, the 30 nm and 8 nm SiO2 MEA shows a notable increase in the ESA up to 57 m2 g1 and 62 m2 g1, respectively. In summary, the Pt/CeNafion contact is improved in the small-sized SiO2 CL as evidenced in the SEM (Fig. 3) and ESA (Fig. 6) evaluation, although aggregation of SiO2 particle is somewhat developed. Interestingly, our results correspond to
Fig. 5. Contact angle of the SiO2-containing catalyst layer with different SiO2 particle diameter. a: SiCR of 0, b: 80 nm SiO2 with the SiCR of 0.20, c: 30 nm SiO2 with the SiCR of 0.20 and d: 8 nm SiO2 with the SiCR of 0.20.
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Table 2 Contact angle data with different SiO2 particle diameters and SiCRs. SiO2-to-carbon ratio
0.01
0.04
0.20
80 nm SiO2 CL 30 nm SiO2 CL 8 nm SiO2 CL
126.7 138.6 144.5
115.1 129.6 137.7
109.2 127.0 134.1
actual contribution to the WU of Nafion electrolyte inside the agglomerate. From our experiments, the cell performance was increased as the SiCR increases, when the SiO2 particle diameter was sufficiently smaller than the Nafion covering layer. Consequently, it is of great importance to enhance the electrode morphology, by efficiently penetrate the SiO2 nanoparticles inside the Pt/CeNafion agglomerate.
experimental observations of Inoue et al. [14] and Su et al. [15]. Concretely, the ESA was rapidly increased at first but gradually increased or even decreased as the SiCR increases with different diameters.
3.4. Fuel-cell performance of the SiO2-containing electrode To investigate the effect of SiO2 particles in the CLs on the fuelcell performance, we have characterized the cell polarization with different SiO2 particle diameters and SiO2 contents. All MEA samples were operated with H2/O2 feed gas at the anode/cathode flow channel under 100% RH and 20% RH conditions. As observed in Fig. 7, for 100% RH operation, the cell performances of 80 nm, 30 nm and 8 nm SiO2 CL MEAs were similar with different SiO2 contents. Precisely, the cell performance was slightly decreased as the SiCR increases because of the increase in the ohmic resistance by the SiO2 particles [25]. In addition, the cell performance of the 8 nm SiO2 CL MEA was increased from 1.99 A cm2 to 2.11 A cm2 at 0.5 V compare to the 80 nm SiO2 CL MEA due to the increase of the ESA and ohmic performance. In contrast, Fig. 8 shows the cell polarization with different SiO2 particle diameters and contents in the 20% RH condition. For the 20% RH operation, a typical type of cell voltage decrease in low-current region [26] was obtained for all MEA samples. Overall, the cell performance of the 8 nm SiO2 CL MEA has drastically improved by exhibiting a maximum of 2.93fold current density at 0.5 V relative to the 80 nm SiO2 CL MEA. As seen in Fig. 8c, for the SiCR of 0.20 of the 8 nm SiO2 CL, approximately 1.61 A cm2 of current density was achieved, at a cell voltage of 0.5 V, under 20% RH condition. It can be concluded that the SiO2 particles in the agglomerate have a great contact with Nafion electrolyte, as confirmed in SEM image (Fig. 3), hence providing low ohmic resistance under low RH condition, as the SiO2 particle diameter decreases. In addition, the cell performance of 80 nm SiO2 CL has remained identical although the SiCR varies, which implies that the increase of the 80 nm SiO2 content, that is larger than the thin Nafion layer covering the agglomerate, has no
Fig. 6. Electrochemical surface area of SiO2-containing CL with different SiO2-to-carbon ratio and SiO2 particle diameter.
Fig. 7. Cell polarization of the SiO2-containing CL MEAs with different SiO2 diameters under 100% RH condition. a: SiCR of 0.01, b: SiCR of 0.04 and c: SiCR of 0.20.
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4. Conclusions Experimental exploration of the SiO2-containing CL was conducted with different size of SiO2 particle and different amount of the SiO2 content. The SiO2 CLs with different SiO2 particle diameters exhibited that the ESA and ohmic performance is highly improved
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as the SiO2 particles diameter decreases. Especially, from the CV result, smaller SiO2 nanoparticles have improved ESA of the CL by successfully forming Pt/CeSiO2eNafion agglomerate structure despite the SiO2 aggregation evidenced by TEM image. In addition, the SiO2 CL became more hydrophobic as the SiO2 particle diameter decreases, because the amount of hydrogen bonding sites between the hydroxyl group of SiO2 and sulfonate group of Nafion ionomer is increased with the specific surface area of the SiO2 particles. It can be concluded that the SiO2 CL shows an enhanced PEMFC performance under low RH condition with smaller SiO2 particles. For 8 nm SiO2 CL, the cell polarization under 20% RH has improved with approximately 2.93-fold current density at 0.5 V relative to the conventional CL. Furthermore, it is anticipated to increase the specific surface area of the SiO2 nanoparticles, e.g., by decreasing the particle diameter or providing high aspect ratio, in order to improve the overall cell performance under low RH. Acknowledgments This work was supported by the Manpower Development Program for Energy supported by the Ministry of Knowledge and Economy (MKE). It was also partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2010-0024794). The authors are grateful for their financial support. References
Fig. 8. Cell polarization of the SiO2-containing CL MEAs with different SiO2 diameters under 20% RH condition. a: SiCR of 0.01, b: SiCR of 0.04 and c: SiCR of 0.20. Bars indicate standard deviation due to insufficient RH.
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