- Email: [email protected]
Nafion effect on dual-bonded structure cathode of PEMFC
Electrochemistry Communications 8 (2006) 1615–1620 www.elsevier.com/locate/elecom
Nafion effect on dual-bonded structure cathode of PEMFC Xuewei Zhang, Pengfei Shi
*
Department of Applied Chemistry, Harbin Institute of Technology, Harbin, Box 411, 150001, PR China Received 15 June 2006; received in revised form 19 July 2006; accepted 19 July 2006 Available online 28 August 2006
Abstract Hydrophobic and hydrophilic electrodes have inherent problems in proton/gas transport caused by properties of binder material (PTFE or Nafion) in catalyst layer (CL). A novel cathode with dual-bonded CL (hydrophilic/hydrophobic) has been designed and evaluated through testing cell performance. Two groups (hydrophilic/constant hydrophobic CL and constant hydrophilic/hydrophobic CL) of cathodes with the same Pt loading of 0.2 mg cm2 were fabricated to investigate Nafion content/loading effect on cell performance. For hydrophilic/constant hydrophobic electrodes, cell performance increased from 0.56 W cm2 to 0.59 W cm2 when the Nafion content in hydrophilic layer increased from 35 wt% to 55 wt%. Reducing Nafion loading in hydrophobic layer from 0.3 mg cm2 to 0 mg cm2, peak power density of constant hydrophilic/hydrophobic electrodes increased from 0.64 W cm2 to 0.77 W cm2 at ambient pressure. Ó 2006 Elsevier B.V. All rights reserved. Keywords: PEMFC; Dual-bonded cathode; Nafion content; Nafion loading; Recast film
1. Introduction The structure of membrane electrode assembly (MEA) affects performance of proton exchange membrane fuel cell (PEMFC) to a large extent. Over the past two decades, many efforts have been done to decrease Pt loading in catalyst layer (CL) of electrode, especially in cathode owing to its sluggish kinetics of oxygen reduction reaction (ORR). Pt loadings were reduced from 4 mg cm2 to as little as 0.2 mg cm2 for oxygen operation [1–3]. The decrease of Pt loading was due to the alteration of the structure of CL. Supported platinum was widely adopted to substitute for platinum black as electrocatalyst and the same level of performance could be achieved with only one tenth of Pt loading [5]. From then on, the preparation methods can be mainly classified by different binder materials in CL. The structure of the CL in early stage was adapted from phosphoric acid fuel cell and the entire catalyst structure *
Corresponding author. Tel.: +86 451 86413721; fax: +86 451 86413720. E-mail address: [email protected] (P. Shi). 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.029
was bonded together with 25–50 wt% PTFE [1]. Then PTFE-bonded Pt/C layer was impregnated with solubilized Nafion by brushing, spraying or floating/dipping way [5– 8]. Another method was developed as thin-film hydrophilic CL for using Nafion ionomer as binder material, in which supported platinum and Nafion solution were mixed with presence of additional organic solvents. The ink of hydrophilic mixture can be applied on gas diffusion layer (GDL)/ membrane directly [1–4]. Both two types of CLs have inherent problems in proton/ gas transport of cathodic reaction. Hydrophobic CL uses PTFE as a binder and imparts hydrophobicity to the gas diffusion region of the electrode, especially when operating in reactant gases with high humidity, which decrease gas transport resistance. Although impregnation of solubilized Nafion provided an extension of three-dimensional reaction zone in the porous CL, variations in the impregnation depth of the solubilized Nafion resulted in its low catalyst utilization of only about 10–20% [1,4,5]. On the other hand, hydrophilic CL used Nafion ionomer as a binder and enhanced protonic conductivity of whole CL, thus increased the utilization of catalyst with a penalty of water flooding in CL [9].
1616
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
Flooding is always concerned for the dimensions of a hydrophilic ionomer swelling by excess water that will clog reactant gases and result in a decrease in cell performance. In this paper, dual-bonded structure CL containing hydrophilic and hydrophobic layers was designed to investigate Nafion effect on proton transfer/gas transport of cathodic reaction. Two groups of electrodes were made of the different Nafion content/loading in CL. One group focused on effect of Nafion content in hydrophilic layer on protonic conductivity and the other group concerned effect of Nafion loading on hydrophobic layers on gas transport. 2. Experiments 2.1. Structure of dual-bonded cathode All cathodes were fabricated with dual-bonded CL structure and the sketch of dual-bonded CL cathode is shown in Fig. 1. This dual-bonded CL cathode has two different property layers from GDL to polymer electrolyte membrane [10]. The hydrophobic layer, used PTFE as binder material, was fabricated on surface of GDL directly, and then impregnated solubilized Nafion on the surface. A recast film formed front surface of hydrophobic layer [11]. Hydrophilic layer adopted Nafion as binder material and was fabricated on the front surface of hydrophobic layer. 2.2. Preparation of electrodes This research aims at investigating the effect of Nafion ionomer on the proton/gas transport in cathode, so the electrodes of each group used here were prepared with the same anode, GDL.
All the slurry prepared in this paper was mixed ultrasonically in organic solution. Vulcan XC-72 carbon black and PTFE were firstly mixed for 1 h in ethanol and then applied on single side of water proofed carbon paper (TGP-H-060, Toray) as micro-porous layer (MPL). After being sintered at 350 °C, GDL was obtained. Pt/C (40%Pt/C, Johnson Matthey) containing PTFE was ultrasonically dispersed in ethanol for 1 h. The resulting mixture was applied on the surface of MPL as the hydrophobic layer, and then impregnated solubilized Nafion on the surface by spraying. Nafion ionomer and Pt/C catalyst were mixed for 1 h and applied above the hydrophobic layer. Each layer of cathode had the same Pt loading of 0.1 mg cm2 and total Pt loading of the whole CL was 0.2 mg cm2. Anode of the cell was hydrophilic electrode and Pt loading was about 0.2 mg cm2. Nafion 212 membrane was chosen as solid polymer electrolyte. After dual-bonded CL treatment, they were assembled and hot-pressed on the both side of membrane at 150 °C, 80 atm for 2 min. Two groups of cathodes were fabricated and designated as group A and group B. Structure of CL in group A is of hydrophilic/constant hydrophobic layer with the same amount Nafion impregnated on the constant hydrophobic layer. Different Nafion content of hydrophilic layer was 55 wt% (A1), 45 wt% (A2) and 35 wt% (A3), respectively. Group B used the constant hydrophilic/hydrophobic layer with various amount of Nafion impregnated on the hydrophobic layer. The impregnated Nafion loading was 0.3 mg cm2, 0.15 mg cm2 and 0 mg cm2 and then designated as B1, B2 and B3, respectively. The cell was operated with the fuel cell test station (ElectroChem, Inc., FCT 890B) at ambient pressure. The MEAs were tested at 60 °C with humidified hydrogen and oxygen at 65 °C using a commercialized cell fixture with 5 cm2 active area. 3. Results and discussion 3.1. Comparison of dual-bonded cathode with hydrophobic/ hydrophilic electrode
Fig. 1. Sketch of dual-bonded CL structure in cathode for PEMFC: (A) Nafion membrane (B) hydrophilic layer (ionomer-bonded Pt/C layer) (C) recast film (D) hydrophobic layer (PTFE-bonded Pt/C layer) and (E) gas diffusion layer.
In Wilson’s point, a catalyst site must satisfy three criteria for it to contribute to the electrochemical reaction in a fuel cell. The criteria are proton access, gas access and electronic path continuity [1]. Such a catalyst site is called effective catalyst site. Yu et al. [12] pointed out that the electron transport was fast enough in electrochemical reaction. It indicates the main problems existed in the CL are proton/gas transport resistances. Conventional electrode usually has hydrophobic or hydrophilic CL for ORR. As hydrophobic CL, PTFEbonded Pt/C is distributed uniformly in the whole CL and is more favorable for gas transport. Nafion ionomer concentration is higher in front surface of PTFE-bonded layer than that in the layer close to GDL. Effective reaction sites are mainly distributed on the front surface of PTFE-bonded layer, where Nafion ionomer is abundant and proton
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
3.2. Nafion effect on dual-bonded cathode performance Figs. 2 and 3 show the cell potential-current density characteristics of PEM fuel cell with diminished Nafion content/loading. The initial drop of the polarization curve is due to electrochemical activation process, which is caused by the sluggish kinetics of ORR at the cathode. The following linear decrease of polarization curve with the increasing current density is due to ohmic overpotential, which is attributed to the proton flow and gas transport through the three-dimensional reaction zone. At the end of the curve, there is a departure from linearity which is considered to be concentration overpotential. In Fig. 2, the polarization curves of group A with varying Nafion content in hydrophilic layer have similar cell performance in ohmic control region besides apparent differences occurred in concentration overpotential region. Three electrodes maintain their linear region in the range of current density up to 1.2 A cm2, 1.1 A cm2,
A1 (55 wt.%) A2 (45 wt.%) A3 (35 wt.%)
1.0 0.9
Voltage (V)
0.8
-2
B1 (0.3 mg cm ) -2 B2 (0.15mg cm ) -2 B3 (0.0 mg cm )
1.0 0.9 0.8
Cell voltage (V)
transfer is swift. As hydrophilic CL, it has high protonic conductivity because its binder material is Nafion ionomer. When ORR occurs, protons carry molecules of water from anode to the cathode and the inner CL of cathode is occupied by much more water than that of the outer layer. Too much water is a barrier for oxygen to permeate into the effective sites close to membrane. So effective reaction sites are mainly distributed along the CL close to substrate material where the fresh oxygen supplied and consumed. The dual-bonded CL is different because it possesses characters of both hydrophobic and hydrophilic layers simultaneously. Compared with hydrophobic CL, substitute the relatively rigid ionomer-bonded Pt/C matrix for solubilized Nafion ensures the proton path continuity deeper into hydrophobic layer. From another point of view, in presence of PTFE, more oxygen could permeate into inner part of hydrophilic layer. It was expected that ORR in cathode would be enhanced by both proton transfer and gas transport, thereby increasing the cell performance.
1617
0.7 0.6 0.5 0.4 0.3 0.2 0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Current density (A cm-2)
Fig. 3. Performance of dual-bonded cathodes with diminished Nafion loading in group B.
1.0 A cm2 from high Nafion content to low Nafion content. When current densities of three cathodes are higher than the points mentioned above, the polarization curves depart from linearity. It has been reported that lower Nafion content is favorable to oxygen transport and electronic conduction, whereas higher Nafion content is advantageous in proton conduction [13]. It should be pointed out that the different slope at the end of three curves is mainly caused by varied Nafion content in hydrophilic layer. Inner hydrophilic layer acts as proton transfer path and increase of Nafion content improves proton transfer ability. Nafion content in hydrophilic layer results in a slight effect on ORR of dual-bonded CL in ohmic region. Fig. 3 shows polarization curves of group B. The recast film layer is reduced gradually and cell performance increases gradually. Electrode B1 and B2 have the similar shape because they all impregnated with solubilized Nafion. Curve of B2 gives a better performance than B1 owing to the thinner recast film formed on front surface of hydrophobic layer. Though recast film is indispensable for conventional hydrophobic CL, this film is a barrier for gas transport in dual-bonded CL. Electrode B3 without recast film demonstrates a linear curve even in concentration control region. 3.3. Kinetic parameter analyses
0.6
Cell voltage–current density data can be fitted by using the following semi-empirical equation until the end of the linear region in the single cell [4,11,14,15]:
0.5
E ¼ E0 b log i Ri
0.4
where
0.3
E0 ¼ Er þ b log i0
0.7
0.2 0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
Current density (A cm-2)
Fig. 2. Performance of dual-bonded cathodes with diminished Nafion content in group A.
ð1Þ ð2Þ
In the equation, E0 is the cell potential at the current density 1 mA cm2, i0 is exchange current density, where i is expressed in mA cm2, b is the Tafel slope for oxygen reduction, Er is the reversible potential for the oxygen electrode reaction and R is predominantly the ohmic resistance in
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
O2 þ Hþ ! intermediate species
ð3Þ
Two aspects affect rate-determining step (oxygen reduction): oxygen permeation and protonic conductivity in CL. Fig. 5 shows that Tafel slope for group A increases with the decrease of Nafion content in hydrophilic layer. Three
60
group A group B
0.3
0.2
50
45 0.1 40 0.0
Nafion loading (mg cm-2)
55
Nafion content (%)
35
30
-0.1 1.03
1.04
1.05
1.06
1.07
E0 (V)
Fig. 4. E0 as a function of Nafion content/loading.
60
group A group B
0.3
55
50
0.2
45 0.1 40 0.0
Nafion loading (mg cm-2)
the electrode, electrolyte and the gas transport resistance, responsible for the linear variation of the potential vs. current density plot. The above equation is fitted to the experimental data by the non-linear least-squares method and the derived electrode kinetic parameters E0, b and R are listed in Table 1, together with peak power density, the Nafion content of group A and Nafion loading of group B. Note that Nafion content represents weight of Nafion in hydrophilic layer whilst Nafion loading represents impregnated Nafion on hydrophobic layer. E0, b, R and peak power density as function of Nafion content/loading are plotted in Figs. 4–7, respectively. Fig. 4 shows E0 of group A and B with a similar tendency, which drops a little with the decrease of Nafion content/loading. E0 is the cell potential at the current density 1 mA cm2, so the value of E0 can approximately presents open circuit potential (OCP). Crossover of oxygen and hydrogen through the Nafion membrane was proven [8,14]. Inevitable crossover of reactant gases, via the polymer electrolyte membrane to opposite electrode, causes potential of anode (cathode) up (down) [16]. It has been reported that the recast film is more soluble and permeable than commercial membrane and the recast film affects the proton transfer ability more or less [17]. Groups A and B have the same trend that E0 decreases with the increase of Nafion content/loading in dual-bonded CL. The reason is that amount of reactant gases crossover to the opposite electrode is down with the increase of Nafion content/loading, thus potential difference between two electrodes is increased. A Tafel slope of 60 mV dec1 has been explained by a mechanism of oxygen reduction where the reaction involves an initial fast charge-transfer step followed by a chemical step, which is the rate-determining step under Langmuir condition [15]. The ORR may be summarized as follows: After a fast chemical oxygen-adsorption step conducted at equilibrium and involving a very low partial coverage, there is a first electrochemical step. The latter corresponds to the protonation of oxygen molecule, which is rate-determining step [11],
Nafion content (%)
1618
35
30 60
61
62
-0.1 63
Tafel slope (mV dec -1) Fig. 5. Tafel slope as a function of Nafion content/loading.
same recast films in group A generate identical resistance for oxygen transport, so the protonic conductivity in hydrophilic layer is a decisive factor for Tafel slope. High Nafion content in hydrophilic layer is favorable for proton transfer to effective catalyst sites, which is advantageous to ORR. Tafel slope of B2 in group B deviates from the linearity of other two electrodes. Nafion loading of B1 is the highest among the three electrodes and more protons could transfer to effective catalyst sites than that of B2, which leads to a low value of Tafel slope. Though B3 has a similar Tafel
Table 1 Electrode kinetic parameter with different Nafion content/loading of two groups Electrode
Nafion content/loading (%/mg cm2)
E0 (V)
b (mV dec1)
R (mX cm2)
Peak power density (W cm2)
A1 A2 A3 B1 B2 B3
55% 45% 35% 0.3 0.15 0
1.0389 1.0378 1.0285 1.0678 1.0461 1.0297
60.7 61 62.6 61.1 62.6 60.6
30 30 29 30 23 22
0.59 0.58 0.56 0.64 0.72 0.77
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620 60
group A group B
0.3
0.2
50
45 0.1 40 0.0
Nafion loading (mg cm-2)
Nafion content (%)
55
35
30 21
22
23
24
25
26
27
28
29
30
-0.1 31
R (mΩ cm2) Fig. 6. Cell resistance as a function of Nafion content/loading.
60
group A group B
0.3
50
0.2
45 0.1 40 0.0
Nafion loading (mg cm-2)
Nafion content (%)
55
35
30 0.55
0.60
0.65
0.70
0.75
-0.1 0.80
Power density (W cm-2)
Fig. 7. Peak power density as a function of Nafion content/loading.
slope to that of B1, the reason may be different. Absence of recast film is favorable for oxygen accessing in CL and contributes to low Tafel slope. B2 with a thinner recast membrane has less proton transfer path than that of B1 and thinner recast film is a handicap for oxygen access, which leads to a slight higher Tafel slop than those of the other two electrodes. The cell resistance as a function of Nafion content/loading is plotted in Fig. 6. A different system resistance is found in the two groups of electrodes. The difference in system resistance is due to different recast films formed on these two groups. Although there is an obvious change in Nafion content of hydrophilic layer, cell resistances of group A have approximately same values. Considering the same amount solubilized Nafion applied on the hydrophobic layer, which formed the identical recast film, cell resistance is up to the recast film. Focus on B1, B2 and B3, the cell resistance also has direct relation to the recast film. The cell performance in ohmic region increased largely with the decrease of recast film thickness. Although B3 is slightly lower than that of B2 in ohmic region, no
1619
concentration overpotential is observed in curve of B3 because no recast film formed on hydrophobic layer. Although the Nafion content and Nafion loading were changed in cathode of those two groups, the cell resistance varied with the Nafion loading only. Peak power density vs. Nafion content/loading is also plotted in Fig. 7. Comparing Fig. 7 with Figs. 4–6, it indicates peak power density of two groups have a direct relation with cell resistance. The peak power density of group A varied from 0.56 W cm2 to 0.59 W cm2 in a small scope according to corresponding cell resistance. Opposite to this phenomenon, peak power density of group B varied from 0.64 W cm2 to 0.77 W cm2 with decrease of cell resistance. It must be pointed out that: although current density of B2 is slightly higher than B3 at range of 800– 600 mV, peak power density of B3 is higher than B2 because it maintains linearity even at concentration overpotential region. It shows B3 possesses more gas passages and water drainage capacity in CL. Considering its applied hydrophilic layer directly on the unimpregnated hydrophobic layer, dual-bonded CL without recast films is more favorable for gas transport. 4. Conclusions Electrodes with dual-bonded structure (hydrophilic/ hydrophobic) were evaluated and analyzed. Though equal Pt loadings were applied on both hydrophilic/constant hydrophobic and constant hydrophilic/hydrophobic cathodes, Nafion distribution in CL shows an apparent effect to the cell performance. For hydrophilic/constant hydrophobic electrodes, no obvious improvement in cell performance was obtained in ohmic region while increase of Nafion content in the hydrophilic layer. For constant hydrophilic/hydrophobic electrodes, decrease of the amount of Nafion loading on the hydrophobic layer enhanced gas transport and then resulted in a significant improvement in cell performance. Acknowledgements This work was partially supported by Fuel Cell R&D centre, Coslight Storage Battery Co. Ltd., and by Harbin Institute of Technology. The author wishes to thank Miss Jiao Jian for her kindly help. References [1] M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 22 (1992) 1. [2] H.A. Gasteiger, J.E. Panels, S.G. Yan, J. Power Source 127 (2004) 162. [3] Sergei Gamburzev, A. John Appleby, J. Power Source 115 (2002) 5. [4] Mahlon S. Wilson, Jundith A. Valerio, Shimshon Gottjbfeld, Elecrrochim. Acta 40 (1995) 355. [5] S. Srinivasan, E.A. Ticianelli, C.R. Derouin, A. Redondo, J. Power Source 22 (1988) 359. [6] E.A. Ticianelli, C.R. Derouin, S. Srinivasan, J. Electroanal. Chem. Interf. Electrochem. 251 (1988) 275.
1620
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
[7] E.A. Ticianelli, C.R. Derouin, A. Redondo, S. Srinivasan, J. Electrochem. Soc. 135 (1988) 2209. [8] Jingrong Yu, z Md Nazrul Islam, Toyoaki Matsuura, Masayuki Tamano, Yoshihiko Hayashi, Michio Hori, Electrochem. Solid-State Lett. 8 (6) (2005) A320. [9] Jinhua Chen, Toyoaki Matsuura, Michio Hori, J. Power Sources 131 (2004) 155. [10] Xuewei Zhang, Pengfei Shi, Electrochem. Commun., in press, doi:10.1016/j.elecom.2006.05.026. [11] Xuezhong Du, Jingrong Yu, Baolian Yi, Ming Han, Kewan Bi, Phys. Chem. Chem. Phys. 3 (2001) 3175.
[12] Jingrong Yu, Yusuke Yoshikawa, Toyoaki Matsuura, Md Nazrul Islam, Michio Hori, Electrochem. Solid-State Lett. 8 (2005) A152. [13] Y.-G. Yoon, T.-H. Yang, G.-G. Park, W.-Y. Lee, C.-S. Kim, J. Power Sources 118 (2003) 189. [14] Masanobu Wakizoe, Omourtag A. Velev, Supramaniam Arinivasan, Electrochim. Acta 40 (1995) 335. [15] E. Antolini, L. Giorgi, A. Pozio, E. Passalacqua, J. Power Sources 77 (1999) 136. [16] K. Broka, P. Ekdunge, J. Appl. Electrochem. 27 (1997) 117. [17] F. Moussy, S. Jakeway, D.J. Harrison, R.V. Rajotte, Anal. Chem. 66 (1994) 3882.
Nafion effect on dual-bonded structure cathode of PEMFC Xuewei Zhang, Pengfei Shi
*
Department of Applied Chemistry, Harbin Institute of Technology, Harbin, Box 411, 150001, PR China Received 15 June 2006; received in revised form 19 July 2006; accepted 19 July 2006 Available online 28 August 2006
Abstract Hydrophobic and hydrophilic electrodes have inherent problems in proton/gas transport caused by properties of binder material (PTFE or Nafion) in catalyst layer (CL). A novel cathode with dual-bonded CL (hydrophilic/hydrophobic) has been designed and evaluated through testing cell performance. Two groups (hydrophilic/constant hydrophobic CL and constant hydrophilic/hydrophobic CL) of cathodes with the same Pt loading of 0.2 mg cm2 were fabricated to investigate Nafion content/loading effect on cell performance. For hydrophilic/constant hydrophobic electrodes, cell performance increased from 0.56 W cm2 to 0.59 W cm2 when the Nafion content in hydrophilic layer increased from 35 wt% to 55 wt%. Reducing Nafion loading in hydrophobic layer from 0.3 mg cm2 to 0 mg cm2, peak power density of constant hydrophilic/hydrophobic electrodes increased from 0.64 W cm2 to 0.77 W cm2 at ambient pressure. Ó 2006 Elsevier B.V. All rights reserved. Keywords: PEMFC; Dual-bonded cathode; Nafion content; Nafion loading; Recast film
1. Introduction The structure of membrane electrode assembly (MEA) affects performance of proton exchange membrane fuel cell (PEMFC) to a large extent. Over the past two decades, many efforts have been done to decrease Pt loading in catalyst layer (CL) of electrode, especially in cathode owing to its sluggish kinetics of oxygen reduction reaction (ORR). Pt loadings were reduced from 4 mg cm2 to as little as 0.2 mg cm2 for oxygen operation [1–3]. The decrease of Pt loading was due to the alteration of the structure of CL. Supported platinum was widely adopted to substitute for platinum black as electrocatalyst and the same level of performance could be achieved with only one tenth of Pt loading [5]. From then on, the preparation methods can be mainly classified by different binder materials in CL. The structure of the CL in early stage was adapted from phosphoric acid fuel cell and the entire catalyst structure *
Corresponding author. Tel.: +86 451 86413721; fax: +86 451 86413720. E-mail address: [email protected] (P. Shi). 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.029
was bonded together with 25–50 wt% PTFE [1]. Then PTFE-bonded Pt/C layer was impregnated with solubilized Nafion by brushing, spraying or floating/dipping way [5– 8]. Another method was developed as thin-film hydrophilic CL for using Nafion ionomer as binder material, in which supported platinum and Nafion solution were mixed with presence of additional organic solvents. The ink of hydrophilic mixture can be applied on gas diffusion layer (GDL)/ membrane directly [1–4]. Both two types of CLs have inherent problems in proton/ gas transport of cathodic reaction. Hydrophobic CL uses PTFE as a binder and imparts hydrophobicity to the gas diffusion region of the electrode, especially when operating in reactant gases with high humidity, which decrease gas transport resistance. Although impregnation of solubilized Nafion provided an extension of three-dimensional reaction zone in the porous CL, variations in the impregnation depth of the solubilized Nafion resulted in its low catalyst utilization of only about 10–20% [1,4,5]. On the other hand, hydrophilic CL used Nafion ionomer as a binder and enhanced protonic conductivity of whole CL, thus increased the utilization of catalyst with a penalty of water flooding in CL [9].
1616
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
Flooding is always concerned for the dimensions of a hydrophilic ionomer swelling by excess water that will clog reactant gases and result in a decrease in cell performance. In this paper, dual-bonded structure CL containing hydrophilic and hydrophobic layers was designed to investigate Nafion effect on proton transfer/gas transport of cathodic reaction. Two groups of electrodes were made of the different Nafion content/loading in CL. One group focused on effect of Nafion content in hydrophilic layer on protonic conductivity and the other group concerned effect of Nafion loading on hydrophobic layers on gas transport. 2. Experiments 2.1. Structure of dual-bonded cathode All cathodes were fabricated with dual-bonded CL structure and the sketch of dual-bonded CL cathode is shown in Fig. 1. This dual-bonded CL cathode has two different property layers from GDL to polymer electrolyte membrane [10]. The hydrophobic layer, used PTFE as binder material, was fabricated on surface of GDL directly, and then impregnated solubilized Nafion on the surface. A recast film formed front surface of hydrophobic layer [11]. Hydrophilic layer adopted Nafion as binder material and was fabricated on the front surface of hydrophobic layer. 2.2. Preparation of electrodes This research aims at investigating the effect of Nafion ionomer on the proton/gas transport in cathode, so the electrodes of each group used here were prepared with the same anode, GDL.
All the slurry prepared in this paper was mixed ultrasonically in organic solution. Vulcan XC-72 carbon black and PTFE were firstly mixed for 1 h in ethanol and then applied on single side of water proofed carbon paper (TGP-H-060, Toray) as micro-porous layer (MPL). After being sintered at 350 °C, GDL was obtained. Pt/C (40%Pt/C, Johnson Matthey) containing PTFE was ultrasonically dispersed in ethanol for 1 h. The resulting mixture was applied on the surface of MPL as the hydrophobic layer, and then impregnated solubilized Nafion on the surface by spraying. Nafion ionomer and Pt/C catalyst were mixed for 1 h and applied above the hydrophobic layer. Each layer of cathode had the same Pt loading of 0.1 mg cm2 and total Pt loading of the whole CL was 0.2 mg cm2. Anode of the cell was hydrophilic electrode and Pt loading was about 0.2 mg cm2. Nafion 212 membrane was chosen as solid polymer electrolyte. After dual-bonded CL treatment, they were assembled and hot-pressed on the both side of membrane at 150 °C, 80 atm for 2 min. Two groups of cathodes were fabricated and designated as group A and group B. Structure of CL in group A is of hydrophilic/constant hydrophobic layer with the same amount Nafion impregnated on the constant hydrophobic layer. Different Nafion content of hydrophilic layer was 55 wt% (A1), 45 wt% (A2) and 35 wt% (A3), respectively. Group B used the constant hydrophilic/hydrophobic layer with various amount of Nafion impregnated on the hydrophobic layer. The impregnated Nafion loading was 0.3 mg cm2, 0.15 mg cm2 and 0 mg cm2 and then designated as B1, B2 and B3, respectively. The cell was operated with the fuel cell test station (ElectroChem, Inc., FCT 890B) at ambient pressure. The MEAs were tested at 60 °C with humidified hydrogen and oxygen at 65 °C using a commercialized cell fixture with 5 cm2 active area. 3. Results and discussion 3.1. Comparison of dual-bonded cathode with hydrophobic/ hydrophilic electrode
Fig. 1. Sketch of dual-bonded CL structure in cathode for PEMFC: (A) Nafion membrane (B) hydrophilic layer (ionomer-bonded Pt/C layer) (C) recast film (D) hydrophobic layer (PTFE-bonded Pt/C layer) and (E) gas diffusion layer.
In Wilson’s point, a catalyst site must satisfy three criteria for it to contribute to the electrochemical reaction in a fuel cell. The criteria are proton access, gas access and electronic path continuity [1]. Such a catalyst site is called effective catalyst site. Yu et al. [12] pointed out that the electron transport was fast enough in electrochemical reaction. It indicates the main problems existed in the CL are proton/gas transport resistances. Conventional electrode usually has hydrophobic or hydrophilic CL for ORR. As hydrophobic CL, PTFEbonded Pt/C is distributed uniformly in the whole CL and is more favorable for gas transport. Nafion ionomer concentration is higher in front surface of PTFE-bonded layer than that in the layer close to GDL. Effective reaction sites are mainly distributed on the front surface of PTFE-bonded layer, where Nafion ionomer is abundant and proton
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
3.2. Nafion effect on dual-bonded cathode performance Figs. 2 and 3 show the cell potential-current density characteristics of PEM fuel cell with diminished Nafion content/loading. The initial drop of the polarization curve is due to electrochemical activation process, which is caused by the sluggish kinetics of ORR at the cathode. The following linear decrease of polarization curve with the increasing current density is due to ohmic overpotential, which is attributed to the proton flow and gas transport through the three-dimensional reaction zone. At the end of the curve, there is a departure from linearity which is considered to be concentration overpotential. In Fig. 2, the polarization curves of group A with varying Nafion content in hydrophilic layer have similar cell performance in ohmic control region besides apparent differences occurred in concentration overpotential region. Three electrodes maintain their linear region in the range of current density up to 1.2 A cm2, 1.1 A cm2,
A1 (55 wt.%) A2 (45 wt.%) A3 (35 wt.%)
1.0 0.9
Voltage (V)
0.8
-2
B1 (0.3 mg cm ) -2 B2 (0.15mg cm ) -2 B3 (0.0 mg cm )
1.0 0.9 0.8
Cell voltage (V)
transfer is swift. As hydrophilic CL, it has high protonic conductivity because its binder material is Nafion ionomer. When ORR occurs, protons carry molecules of water from anode to the cathode and the inner CL of cathode is occupied by much more water than that of the outer layer. Too much water is a barrier for oxygen to permeate into the effective sites close to membrane. So effective reaction sites are mainly distributed along the CL close to substrate material where the fresh oxygen supplied and consumed. The dual-bonded CL is different because it possesses characters of both hydrophobic and hydrophilic layers simultaneously. Compared with hydrophobic CL, substitute the relatively rigid ionomer-bonded Pt/C matrix for solubilized Nafion ensures the proton path continuity deeper into hydrophobic layer. From another point of view, in presence of PTFE, more oxygen could permeate into inner part of hydrophilic layer. It was expected that ORR in cathode would be enhanced by both proton transfer and gas transport, thereby increasing the cell performance.
1617
0.7 0.6 0.5 0.4 0.3 0.2 0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Current density (A cm-2)
Fig. 3. Performance of dual-bonded cathodes with diminished Nafion loading in group B.
1.0 A cm2 from high Nafion content to low Nafion content. When current densities of three cathodes are higher than the points mentioned above, the polarization curves depart from linearity. It has been reported that lower Nafion content is favorable to oxygen transport and electronic conduction, whereas higher Nafion content is advantageous in proton conduction [13]. It should be pointed out that the different slope at the end of three curves is mainly caused by varied Nafion content in hydrophilic layer. Inner hydrophilic layer acts as proton transfer path and increase of Nafion content improves proton transfer ability. Nafion content in hydrophilic layer results in a slight effect on ORR of dual-bonded CL in ohmic region. Fig. 3 shows polarization curves of group B. The recast film layer is reduced gradually and cell performance increases gradually. Electrode B1 and B2 have the similar shape because they all impregnated with solubilized Nafion. Curve of B2 gives a better performance than B1 owing to the thinner recast film formed on front surface of hydrophobic layer. Though recast film is indispensable for conventional hydrophobic CL, this film is a barrier for gas transport in dual-bonded CL. Electrode B3 without recast film demonstrates a linear curve even in concentration control region. 3.3. Kinetic parameter analyses
0.6
Cell voltage–current density data can be fitted by using the following semi-empirical equation until the end of the linear region in the single cell [4,11,14,15]:
0.5
E ¼ E0 b log i Ri
0.4
where
0.3
E0 ¼ Er þ b log i0
0.7
0.2 0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
Current density (A cm-2)
Fig. 2. Performance of dual-bonded cathodes with diminished Nafion content in group A.
ð1Þ ð2Þ
In the equation, E0 is the cell potential at the current density 1 mA cm2, i0 is exchange current density, where i is expressed in mA cm2, b is the Tafel slope for oxygen reduction, Er is the reversible potential for the oxygen electrode reaction and R is predominantly the ohmic resistance in
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
O2 þ Hþ ! intermediate species
ð3Þ
Two aspects affect rate-determining step (oxygen reduction): oxygen permeation and protonic conductivity in CL. Fig. 5 shows that Tafel slope for group A increases with the decrease of Nafion content in hydrophilic layer. Three
60
group A group B
0.3
0.2
50
45 0.1 40 0.0
Nafion loading (mg cm-2)
55
Nafion content (%)
35
30
-0.1 1.03
1.04
1.05
1.06
1.07
E0 (V)
Fig. 4. E0 as a function of Nafion content/loading.
60
group A group B
0.3
55
50
0.2
45 0.1 40 0.0
Nafion loading (mg cm-2)
the electrode, electrolyte and the gas transport resistance, responsible for the linear variation of the potential vs. current density plot. The above equation is fitted to the experimental data by the non-linear least-squares method and the derived electrode kinetic parameters E0, b and R are listed in Table 1, together with peak power density, the Nafion content of group A and Nafion loading of group B. Note that Nafion content represents weight of Nafion in hydrophilic layer whilst Nafion loading represents impregnated Nafion on hydrophobic layer. E0, b, R and peak power density as function of Nafion content/loading are plotted in Figs. 4–7, respectively. Fig. 4 shows E0 of group A and B with a similar tendency, which drops a little with the decrease of Nafion content/loading. E0 is the cell potential at the current density 1 mA cm2, so the value of E0 can approximately presents open circuit potential (OCP). Crossover of oxygen and hydrogen through the Nafion membrane was proven [8,14]. Inevitable crossover of reactant gases, via the polymer electrolyte membrane to opposite electrode, causes potential of anode (cathode) up (down) [16]. It has been reported that the recast film is more soluble and permeable than commercial membrane and the recast film affects the proton transfer ability more or less [17]. Groups A and B have the same trend that E0 decreases with the increase of Nafion content/loading in dual-bonded CL. The reason is that amount of reactant gases crossover to the opposite electrode is down with the increase of Nafion content/loading, thus potential difference between two electrodes is increased. A Tafel slope of 60 mV dec1 has been explained by a mechanism of oxygen reduction where the reaction involves an initial fast charge-transfer step followed by a chemical step, which is the rate-determining step under Langmuir condition [15]. The ORR may be summarized as follows: After a fast chemical oxygen-adsorption step conducted at equilibrium and involving a very low partial coverage, there is a first electrochemical step. The latter corresponds to the protonation of oxygen molecule, which is rate-determining step [11],
Nafion content (%)
1618
35
30 60
61
62
-0.1 63
Tafel slope (mV dec -1) Fig. 5. Tafel slope as a function of Nafion content/loading.
same recast films in group A generate identical resistance for oxygen transport, so the protonic conductivity in hydrophilic layer is a decisive factor for Tafel slope. High Nafion content in hydrophilic layer is favorable for proton transfer to effective catalyst sites, which is advantageous to ORR. Tafel slope of B2 in group B deviates from the linearity of other two electrodes. Nafion loading of B1 is the highest among the three electrodes and more protons could transfer to effective catalyst sites than that of B2, which leads to a low value of Tafel slope. Though B3 has a similar Tafel
Table 1 Electrode kinetic parameter with different Nafion content/loading of two groups Electrode
Nafion content/loading (%/mg cm2)
E0 (V)
b (mV dec1)
R (mX cm2)
Peak power density (W cm2)
A1 A2 A3 B1 B2 B3
55% 45% 35% 0.3 0.15 0
1.0389 1.0378 1.0285 1.0678 1.0461 1.0297
60.7 61 62.6 61.1 62.6 60.6
30 30 29 30 23 22
0.59 0.58 0.56 0.64 0.72 0.77
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620 60
group A group B
0.3
0.2
50
45 0.1 40 0.0
Nafion loading (mg cm-2)
Nafion content (%)
55
35
30 21
22
23
24
25
26
27
28
29
30
-0.1 31
R (mΩ cm2) Fig. 6. Cell resistance as a function of Nafion content/loading.
60
group A group B
0.3
50
0.2
45 0.1 40 0.0
Nafion loading (mg cm-2)
Nafion content (%)
55
35
30 0.55
0.60
0.65
0.70
0.75
-0.1 0.80
Power density (W cm-2)
Fig. 7. Peak power density as a function of Nafion content/loading.
slope to that of B1, the reason may be different. Absence of recast film is favorable for oxygen accessing in CL and contributes to low Tafel slope. B2 with a thinner recast membrane has less proton transfer path than that of B1 and thinner recast film is a handicap for oxygen access, which leads to a slight higher Tafel slop than those of the other two electrodes. The cell resistance as a function of Nafion content/loading is plotted in Fig. 6. A different system resistance is found in the two groups of electrodes. The difference in system resistance is due to different recast films formed on these two groups. Although there is an obvious change in Nafion content of hydrophilic layer, cell resistances of group A have approximately same values. Considering the same amount solubilized Nafion applied on the hydrophobic layer, which formed the identical recast film, cell resistance is up to the recast film. Focus on B1, B2 and B3, the cell resistance also has direct relation to the recast film. The cell performance in ohmic region increased largely with the decrease of recast film thickness. Although B3 is slightly lower than that of B2 in ohmic region, no
1619
concentration overpotential is observed in curve of B3 because no recast film formed on hydrophobic layer. Although the Nafion content and Nafion loading were changed in cathode of those two groups, the cell resistance varied with the Nafion loading only. Peak power density vs. Nafion content/loading is also plotted in Fig. 7. Comparing Fig. 7 with Figs. 4–6, it indicates peak power density of two groups have a direct relation with cell resistance. The peak power density of group A varied from 0.56 W cm2 to 0.59 W cm2 in a small scope according to corresponding cell resistance. Opposite to this phenomenon, peak power density of group B varied from 0.64 W cm2 to 0.77 W cm2 with decrease of cell resistance. It must be pointed out that: although current density of B2 is slightly higher than B3 at range of 800– 600 mV, peak power density of B3 is higher than B2 because it maintains linearity even at concentration overpotential region. It shows B3 possesses more gas passages and water drainage capacity in CL. Considering its applied hydrophilic layer directly on the unimpregnated hydrophobic layer, dual-bonded CL without recast films is more favorable for gas transport. 4. Conclusions Electrodes with dual-bonded structure (hydrophilic/ hydrophobic) were evaluated and analyzed. Though equal Pt loadings were applied on both hydrophilic/constant hydrophobic and constant hydrophilic/hydrophobic cathodes, Nafion distribution in CL shows an apparent effect to the cell performance. For hydrophilic/constant hydrophobic electrodes, no obvious improvement in cell performance was obtained in ohmic region while increase of Nafion content in the hydrophilic layer. For constant hydrophilic/hydrophobic electrodes, decrease of the amount of Nafion loading on the hydrophobic layer enhanced gas transport and then resulted in a significant improvement in cell performance. Acknowledgements This work was partially supported by Fuel Cell R&D centre, Coslight Storage Battery Co. Ltd., and by Harbin Institute of Technology. The author wishes to thank Miss Jiao Jian for her kindly help. References [1] M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 22 (1992) 1. [2] H.A. Gasteiger, J.E. Panels, S.G. Yan, J. Power Source 127 (2004) 162. [3] Sergei Gamburzev, A. John Appleby, J. Power Source 115 (2002) 5. [4] Mahlon S. Wilson, Jundith A. Valerio, Shimshon Gottjbfeld, Elecrrochim. Acta 40 (1995) 355. [5] S. Srinivasan, E.A. Ticianelli, C.R. Derouin, A. Redondo, J. Power Source 22 (1988) 359. [6] E.A. Ticianelli, C.R. Derouin, S. Srinivasan, J. Electroanal. Chem. Interf. Electrochem. 251 (1988) 275.
1620
X. Zhang, P. Shi / Electrochemistry Communications 8 (2006) 1615–1620
[7] E.A. Ticianelli, C.R. Derouin, A. Redondo, S. Srinivasan, J. Electrochem. Soc. 135 (1988) 2209. [8] Jingrong Yu, z Md Nazrul Islam, Toyoaki Matsuura, Masayuki Tamano, Yoshihiko Hayashi, Michio Hori, Electrochem. Solid-State Lett. 8 (6) (2005) A320. [9] Jinhua Chen, Toyoaki Matsuura, Michio Hori, J. Power Sources 131 (2004) 155. [10] Xuewei Zhang, Pengfei Shi, Electrochem. Commun., in press, doi:10.1016/j.elecom.2006.05.026. [11] Xuezhong Du, Jingrong Yu, Baolian Yi, Ming Han, Kewan Bi, Phys. Chem. Chem. Phys. 3 (2001) 3175.
[12] Jingrong Yu, Yusuke Yoshikawa, Toyoaki Matsuura, Md Nazrul Islam, Michio Hori, Electrochem. Solid-State Lett. 8 (2005) A152. [13] Y.-G. Yoon, T.-H. Yang, G.-G. Park, W.-Y. Lee, C.-S. Kim, J. Power Sources 118 (2003) 189. [14] Masanobu Wakizoe, Omourtag A. Velev, Supramaniam Arinivasan, Electrochim. Acta 40 (1995) 335. [15] E. Antolini, L. Giorgi, A. Pozio, E. Passalacqua, J. Power Sources 77 (1999) 136. [16] K. Broka, P. Ekdunge, J. Appl. Electrochem. 27 (1997) 117. [17] F. Moussy, S. Jakeway, D.J. Harrison, R.V. Rajotte, Anal. Chem. 66 (1994) 3882.