Design of Pore Structure in Gas Diffusion Layers for Oxygen

Mar 5, 2014 - A series of oxygen depolarized cathodes (ODC) with different gas diffusion layers (GDLs), determined by different carbon black fillers, ...
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Design of Pore Structure in Gas Diffusion Layers for Oxygen Depolarized Cathode and Their Effect on Activity for Oxygen Reduction Reaction Jingjun Liu, Chao Yang, Chenguang Liu, Feng Wang,* and Ye Song State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: A series of oxygen depolarized cathodes (ODC) with different gas diffusion layers (GDLs), determined by different carbon black fillers, PTFE contents, and pore-forming agents, were fabricated. The structure-sensitive activity for the oxygen reduction reaction (ORR) on these cathodes has been studied in alkaline solutions, with the aim of revealing the effect of pore structures in GDLs on the ORR activity. A linear-type gas permeability (K) dependence on the ORR activity is observed, indicating that K is one of the key parameters representative of the ability of gas diffusion through GDLs. The added PTFE as binder favors the formation of the secondary pores instead of the primary pores in GDLs. By introducing (NH4)2C2O4 as poreforming agents in GDL, the best performance of the ODC was obtained. The resulting GDL possessed the average secondary pore diameter of 144 nm and the secondary pore volume of 0.3032 mL/g.

1. INTRODUCTION Chlorine and caustic soda have been regarded as fundamental chemicals in a variety of industries, which are essentially produced in large scale through sodium chloride or brine electrolysis around the world by now. However, we have to recognize the fact that chlor-alkali electrolysis is an energyintensive process that consumes an estimated ∼2% of electricity in the United States1 and ∼1.5% of electricity in China.2 It is very urgent, therefore, to reduce the electrical power consumption in the chlor-alkali industry, taking into account the energy supply is becoming more and more tight and the consequent depletion of global energy. However, a substantial improvement in the energy savings for either sodium chloride or brine electrolysis is not likely to be obtained with the current chlor-alkali approach, because it involves the generation of hydrogen gas that is an energy-intensive process.3 Fortunately, significant power savings may be easily achieved by introducing an oxygen depolarized cathode (ODC) into the conventional ion-exchange membrane electrolyzer, as a substitute for the energy-intensive hydrogen evolving cathodes.4 It is believed that the application of oxygen depolarized cathode (ODC) in the chlor-alkali electrolysis can save ∼30% energy in comparison with that of the hydrogen evolving cathode by reducing the cell voltage of ∼1.23 V (Figure.1), even though at current density of 3 kA/m.2,5 Besides the large-scale energy saving, such oxygen depolarized cathode approach can also eliminate the possibilities of explosion and dichromate pollution in the chlor-alkali industry. It is well-known that the oxygen depolarized cathode used for chlor-alkali electrolysis is a typical porous gas-diffusion electrode, which is composed of a gas diffusion layer (GDL) that serves as a passageway for oxygen diffusion and a catalyst layer that serves as three-phase reaction sites, where oxygen gas, electrolyte, and the electro-catalytic catalyst meet.3 In fact, the efficient oxygen depolarized cathodes depend on not only the efficient electro-catalysts with low overpotentils for ORR but © 2014 American Chemical Society

Figure 1. Comparison of chlor-alkali electrolysis with different active cathodes (a) hydrogen evolving cathode (b) and oxygen depolarized cathode.

also the ideal porous structures in the gas diffusion layer that facilitate oxygen diffusion to the catalyst layer under very high applied current density conditions. Therefore, for a given electro-catalyst for ORR, such as the well-known carbon supported platinum and platinum-based alloys, the design of the satisfactory porous structure of GDL in supplying oxygen gas to the three phase boundary is one of the most urgent works to enhance the ODC performance. To date, lots of efforts have been made to investigate the correlation of the structural parameters of the porous GDLs and their ORR activity.6−11 Watanabe6 proposed in early studies that there is a bimodal pore size distribution in the gas diffusion layer, the primary and secondary pores, which are attributed to the spaces formed within carbon black Received: Revised: Accepted: Published: 5866

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the corresponding activity is not clear by now. Moreover, the interaction between the filled carbon black and PTFE binder on the electrode activity still remains a fuzzy. Therefore, the exploring of the synergistic effects between carbon black supports and PTFE binds on the electrochemical performances of ODC is still a challenge by now. Therefore, in this work, a series of oxygen depolarized cathodes with different porous structures in GDL, adjusted by different carbon blacks, contents of PTFE binder, and the added pore-forming agents, were fabricated. By varying the filled carbon blacks and PTFE contents, the structure-sensitive activity for ORR on these oxygen depolarized cathodes has been studied systematically in alkaline solutions. The goal is to highlight the effect of the porous structures of GDL on the perfect electrode activity for ORR. On this basis, by introducing various pore-forming agents (PEG-200, (NH 4 ) 2 C 2 O 4 , NH4HCO3, and NaCl) we show the optimal performance of the electrode was obtained though tuning the pore structure of as-fabricated GDL, which will helps us to develop the highperformance oxygen depolarized cathodes for ORR.

agglomerates and the spaces formed among agglomerates, ́ é 7 defined the pores with respectively. Furthermore, B́elek diameter 0.05 μm as secondary pores (Vsec). Figure 2 gives a

Figure 2. Schematic diagram of the typical boundary size of the different pores including micropore, primary and secondary pore, and macropore existing in as-synthesized GDL.

schematic diagram for the typical boundary size of the different pores in GDLs. Hence, Morimoto8 investigated the effects of the average pore diameter in GDL on cathode performances and put forward that the ORR overpotential will become small, when the average pore diameter becomes large within a certain size range. However, the overpotential of the ORR will remain unchanged, when the average pore diameter becomes greater than 0.15 μm. This conclusion is confirmed by the fact that GDLs with greater pores in a certain size range maybe promote gas supply and further decrease mass transport resistance for oxygen diffusion, thus resulting in the perfect performance of the electrode.9 Besides the pore diameter, Passalacqua and coworkers10 further studied the impacts of pore volumes in the gas diffusion layers on the performances; they further claimed that pore volume is another key factor to determine the electrode performance. However, deeply exploring the relationship between structural parameters of GDL and the activity of electrodes is very difficult due to the inadequate understanding we have for the complex porous structure of the GDL in nanoscale. Recently, Osaka and co-workers11 used the electrochemical impedance spectroscopy (EIS) to deeply study the effect of flooding in the microstructured cathode catalyst layer of the polymer electrolyte fuel cell and they further determined the correlation of the performances and pore structure for the cathode on the basis of the resistance of the catalytic reaction in the primary and secondary pores in the catalyst layer. According to their results, the EIS method may be a powerful tool to further investigate the correlation between gas diffusion processes in the GDL and the structure-sensitive performances of electrodes. Moreover, Passalacqua10 prepared GDLs by using different types of carbon blacks in order to identify the effect of the porous structure derived from carbon blacks on electrode performances. The resulting findings showed that the GDL prepared with acetylene black (Shawinigan) led to a better performance because of its higher porosity for oxygen diffusion, which is beneficial for the oxygen diffusion trough gas diffusion layer to reach the catalyst layer. In addition, Park and Popov12 investigated the impacts of PTFE contents on electrode activity and discovered a volcano-type PTFE contents dependence is observed in the activity and excessive PTFE content would result in the poor electrode performance in return. But, how the PTFE contents affect the formation of the various porous structures of GDL including primary and secondary pores and

2. EXPERIMENTAL SECTION The as-fabricated oxygen depolarized cathodes (ODC) consisted of a reaction layer (RL), a gas diffusion layer (GDL), and a current collector. The GDL in the ODC was made up of a hydrophobic carbon black as filler and polytetrafluoroethylene (PTFE) as binder that is highly hydrophobic in order to enable high diffusivity of oxygen and barrier water, whereas the reaction layer (RL) was prepared by mixingan amount of the homemade silver nanoparticle (size: 20−50 nm), hydrophilic carbon black (acidified Vulcan XC-72) support, hydrophobic carbon black (graphitizated carbon black of Vulcan XC-72), and polytetrafluoroethylene. Here, the acidified carbon black was obtained by the acidification of the original Vulcan XC-72 in concentrated nitric acid (14.6 M) at 120 °C for 10 h of reflow, while graphitizated carbon black was obtained by the graphitization of the Vulcan XC-72 in a homemade graphitization furnace at 2600 °C for 1 h under argon atmosphere. A nickel foam with 1 mm mesh and 1 mm thick was integrated in the gas diffusion layer and used as the current collector. To prepare the RL and GDL, aqueous suspensions of isopropyl alcohol (Sinopharm Chemical Reagent Co., Ltd.) containing silver catalysts, a PTFE suspension, a nonionic surfactant (Triton X-100, Sinopharm Chemical Reagent Co., Ltd.), and a hydrophilic/hydrophobic carbon black were prepared for RL, while a PTFE suspension, a nonionic surfactant (Triton X-100), and a hydrophobic carbon black were prepared for GDL. Then, the suspensions were treated with a homogenizer (Ultra-Turrax T18 with dispersing tool S25N-25F, IKA) for 30 min to obtain the slurries with high dispersion and uniformity. The slurries of GDL and RL were then painted on the nickel foam, respectively, and then dried in an oven at 80 °C for 10 h. The obtained ODC painted GDL and RL on the nickel foam was pressed at room temperature. After the ODC was sintered at 290 °C in air atmosphere, the ODC was pressed at 360 °C under the pressure of 4.9 MPa for 1 min. In this work, four types of carbon blacks, oil-furnace carbon black (CB; Vulcan XC-72, Cabot Co. Ltd.), graphitizated Vulcan XC-72 carbon black (GCB), Black Pearls 2000 (BP-2000; Cabot Co., Ltd.), and acetylene black (AB; Beijing Baishun Chemical Technology Co., Ltd.), were used in GDLs as filler for the study of their effects on the pore structure characteristics and electrochemical properties. The particle size 5867

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Figure 3. Surface morphology of the gas diffusion layers (GDLs) prepared by different carbon blacks with the same PTFE content of 20 wt %.

morphologies for the as-fabricated GDLs using different carbon black fillers (30 mg cm−2) with the same PTFE content of 20 wt %, which revealed the distribution of the filled carbon black nanoparticles and profiles of the surface morphology of the GDLs. It can be seen that the as-fabricated GDL containing the GCB show uniform carbon distribution, which facilitates the reactant gas to diffuse to the active catalyst sites easily. Similar to GCB, the GDL built up by AB also show the uniform surface morphology. On the contrary, the surface morphology of GDL constituted by the oil-furnace CB and Black Pearls (BP-2000) respectively, exhibit a strongly compactness of the surface structure. This difference in morphology may be originated from the different particle sizes and aggregate sizes of these carbon blacks as shown in Table 1. In fact, the GCB and AB

of carbon black was measured by a laser particle size instruments (Malvern Instruments Ltd.). The PTFE contents in the GDL are 10, 20, 30, and 40 wt %, respectively. The effect of mass ratios of C/PTFE on the pore structure characteristics and electrochemical properties was also investigated. Regarding the RL, the content of PTFE in the RL are 20 wt %, while the content of silver catalyst is 133 wt % relative to the carbon filler. Finally, the pore-forming agents (PEG-200, (NH4)2C2O4, NH4HCO3, and NaCl) of the same added amount was employed to modify the porous structure of as-fabricated GDLs. The pore-forming agents such as PEG-200, (NH4)2C2O4, and NH4HCO3 were removed by thermal decomposition, while NaCl was removed by water boiling. The invariable PTFE content in these GDLs is 30 wt.%, as the pore-forming agents are employed to produce such GDLs. Surface morphology of the as-fabricated and pore-forming GDLs was examined by using a scanning electron microscope. The hydrophobic nature of these electrodes was characterized by surface contact angle measurement using a contact angle standard goniometer (Dataphysics OCA20). The pore size distributions in the GDL were measured with MP (mercury porosimetry) by using an automatic porosimeter device (AutoPoreIV9520, Micromeritics Instrument Co.). The measurements in oxygen reduction activity were carried out in a 1 cm2 ODC with platinum patch as counter electrode and saturated calomel electrode (SCE) as reference electrode in 30 wt % NaOH using PARSTAT2273 electrochemical workstation. The temperature was maintained at constant operating temperature (80 °C) as regulated by a thermostatted water bath, the flow of reactant oxygen to the rear side of the electrode was kept at 40 mL/min, and current−potential readings were obtained for the assessment of the various types of electrodes. All of the potentials in this paper refer to the Hg/ Hg2Cl2/Cl− reference electrode.

Table 1. Particle Sizes and Aggregate Sizes for Various Carbon Blacks carbon black

particle size (nm)

aggregate size (nm)

Vulcan XC-72 (CB) GCB BP-2000 AB

40−60 40−70 15−30 50−70

325.5 334.2 268.2 602.9

have the largest particle sizes and aggregate sizes, while BP2000 possesses the smallest particle size among the used carbon black materials. For the aggregate size of these carbon blacks estimated from the inset pictures in Figure 3, follow the order AB > GCB > CB > BP-2000, with aggregate size values of 602.9, 334.2, 325.5, and 268.2 nm, respectively. The reason for the larger aggregate size of GCB can be explained by the graphitization treatment with respect to that of the original CB. As evidenced by the results from HR-TEM images for two carbon particles, the CB spheres are comprised of discontinuous graphite sheets, indicating the low degree of graphitization. This disordered structure lead to not only bad conductivity but also bad durability. On the contrary, after high temperature graphitization treatment, the resulting GCB

3. RESULTS AND DISCUSSION 3.1. Surface Morphologies of GDLs Filled by Different Carbon Materials. Figure.3 shows the typical surface 5868

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Figure 4. Effect of different carbon blacks on pore structures in the as-fabricated GDLs. (a) The pore-size distribution, (b) the cumulative pore volume for mercury intrusion, (c) the local porosity/%, (d) average pore diameter of primary and secondary pore of GDLs with various carbon supports, and (e) the theoretical gas permeability calculated from the K−C equation included primary and secondary pore of GDLs only.

and 0.91 × 10−6 darcy (μm2), respectively. These findings indicate that the AB-filled GDL have the largest K−C equation gas permeability, owing to the largest porosity and average pore diameter (Figure 4c,d). Similar to AB filler, GCB filled the GDL also exhibits the larger gas permeability among these fabicated GDLs, which may promote the accessibility of oxygen gas to the three phase boundary of the catalyst layer and reduce mass transport resistance compared to other carbon materials. 3.3. Relationship between Gas Permeability of GDL and ORR Activity. To investigate the relationship between the pore structures in GDLs on the activity toward ORR, a series of ODC electrodes were prepared by using different carbon-filler GDLs coated with a same catalytic layer (homemade Ag/C catalyst). The cathodic polarization curves for these ODC in large sweeping potential ranges in concentrated alkaline medium (30 wt.% NaOH) are illustrated in Figure 5a. For

particles are surrounded by continuous graphitic layers and some particles seem to be connected to each other. This continuous structure observed in GCB particles reveals the increase in structural integrity and crystallinity of carbon. 3.2. Effect of Different Carbon Blacks on Pore Structures in GDL. To better understand the effect of the filled carbon materials on the pore structures of GDL in nanoscale, mercury porosimetry was employed to investigate the unique porous structures of as-fabricated GDLs built up by these different carbon blacks, as shown in Figure 4. The resulting cumulative and differential pore volume as a function of the pore diameter for GDLs are observed for these different carbons, respectively. In fact, the curves of cumulative pore volumes appear to be quite dissimilar with different carbon blacks, as shown in Figure 4a. Two different types of micropores can be seen from the pore-size distribution curves for these carbon black materials in Figure 4b. Definitely, the first peak of the pore volume distribution is located at a pore size of approximately ∼35 nm, while the second peak with a broad distribution which is not obvious has an average pore size of about ∼120 nm (Figure 4b). Clearly, the resulting first and second peaks are ascribed to the primary pores and secondary pores that corresponded to a space within the carbon black agglomerates (or aggregate) and a space among the agglomerates, respectively. However, another huge pore with diameter larger than 1 μm is classified into macropore, which originated from the microcracks of the electrodes and will not contribute to the property of ODC. Furthermore, to determine the gas permeability derived from the carbon black fillers, which depended on both the local porosity (Figure 4c) and average pore diameter (Figure 4d), we employed the well-known Kozeny−Carman equation to calculate the gas permeability for these GDLs without the reaction layer, and the K−C equation is given by9 K=

Figure 5. Effect of gas permeability of GDL prepared by using different carbon blacks on ORR activity for electrodes with the GDL coated with the same catalytic layer (homemade Ag/C catalyst) in 30 wt % NaOH at 80 °C, (a) the cathodic polarization curves for ORR and (b) relationship between the cathodic current density and the gas permeability at specific potentials.

the given catalyst, it can be seen that the cathodic current densities at specific potentials increase sharply as the K−C equation gas permeability of GDLs increase in Figure 5b, although it increases slowly after the value of the gas permeability exceeds 3 × 10−6 darcy (μm2). On the contrary, Figure 5b shows that the supply of the reactant gas is not sufficient to the catalyst layer when the gas permeability is less than 3 × 10−6 darcy (μm2) because of poor penetration of oxygen in GDLs. So, the significant phenomenon demonstrates a clear dependence of cathode performance on the gas permeability of GDL for these tested electrodes.

φn + 1 , C = 180/λmean 2 C(1 − φ)n

where ϕ is the porosity, λmean is the mean pore diameter; the value of the exponential n is 2, and constant C is called the Kozeny−Carman constant. The resulting K−C equation of gas permeability shown in Figure 4e follows the order AB > GCB > CB > BP-2000, with gas permeability values of 15.41, 3.02, 2.20, 5869

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carbons. The results show that the gas permeability for the resulting GDL derived from the acidified CB is almost similar to that of the GDL derived from the original CB. The similar result was observed for the acidified BP-2000 sample. It further confirms the weak effect of the hydrophilicity of the carbon fillers on the gas permeability of their GDLs. Therefore, it is evident that the perfect gas permeability of the GDLs mainly result from the pore structure characteristics in nanoscale, which is attributed to the particle sizes and aggregate sizes of these carbon blacks as shown in Table 1. Considering the excessive hydrophilic nature of the gas diffusion layer could result in water or electrolyte flooding for GDLs and decrease performance of ODC,13 we employed the GCB as a desired carbon filler and systematically studied the effects of a polymer binder such as PTFE in GDL on the electrode performance for ORR. 3.4. Effect of PTFE Contents on Activity toward ORR. We employed the GCB as filler and the various contents of PTFE binder to build up a series of GDLs to study the effects of PTFE contents on the pore structures and performance of the electrodes. The PTFE contents in these GDLs are 10 wt.%, 20 wt.%, 30 wt.% and 40 wt.%, respectively. Figure 7a,b presents the cumulative pore volume and the pore-size distribution for mercury intrusion of GDLs with various PTFE contents. Similar to the different carbon fillers in Figure 4, the contents of PTFE exhibit obvious effect on the porous structures including cumulative pore volume and the pore-size distribution in GDLs and there are bimodal pore size distribution observed, as shown in Figure 7a,b. Definitely, the percentage of the local porosity and average pore diameter of GDLs with various PTFE loading are illustrated in Figure 7c and Figure 7d. Although the little change in the total local porosity with the increasing contents of PTFE, the average pore diameter grow first, and then drop when the PTFE content reach up to 40 wt.%. A volcano-type PTFE contents in GDL dependence on the average pore diameter was observed. The as-synthesized GDL with 30 wt.% PTFE exhibits the largest average pore diameter of 61 nm in Figure 7d and the highest K−C equation gas permeability shown in Figure 7e, suggesting that dependence of PTFE contents on the electrochemical

In addition, the relationship between the surface hydrophobicity and the gas permeability for these as-fabricated GDLs was assessed by using contact angle measurements, as shown in Figure 6. The resulting data suggest the contact angles of these

Figure 6. Surface contact angles of the gas diffusion layers (GDLs) prepared by different carbon black fillers with the same PTFE content of 20 wt % (a) CB, (b) GCB, (c) BP-2000, and (d) AB.

GDLs derived from CB, GCB, BP-2000, and AB fillers were 134.1°, 139.9°, 128.7°, and 72.4°, respectively, indicating that the GDL built up by GCB is highly hydrophobic due to it is high temperature graphitization process among the employed carbon blacks. Unfortunately, the GDL containing AB was highly hydrophilic with a contact angle of just only about 72.4°, although the carbon material exhibits the largest gas permeability (Figure 4e). To further clarify the correlation between the hydrophilicity of these carbon materials and the gas permeability of GDLs, we performed the acidification treatments for the carbon materials (CB and BP-2000) in concentrated nitric acid (14.6 M) at 120 °C for 10 h of reflow respectively, aimed at improving the hydrophilicity of these carbon materials. Subsequently, the resulting acidified samples were used as fillers to fabricate the GDLs according to similar procedures for fabricating GDLs containing the original

Figure 7. Effect of various PTFE contents on the pore structures in GDL and activity toward ORR. (a) The pore-size distribution, (b) the cumulative pore volume for mercury intrusion, (c) the local porosity %, (d) average pore diameter of primary and secondary pore of GDLs, (e) theoretical gas permeability calculated from the K−C equation of GDLs included primary and secondary pore of GDLs only, and (f) the cathodic polarization curves of GDLs coated by the same catalytic layer for ORR in 30 wt % NaOH at 80 °C. 5870

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diffusion layers, we introduced the different pore-forming agents to modify the pore structures of GDLs, in order to verify the relationships between the pore characteristics of the GDL and the electrochemical properties of electrodes. The poreforming agents are utilized to effectively impart porosity to the gas diffusion layer structure so as to enhance contact between oxygen gas and catholyte. Of course, when these pore-forming agents are employed to produce such gas diffusion layers, they must be removed fully after formation of the GDLs. It was believed that these thermally decomposable pore-forming agents, such as PEG-200, (NH4)2C2O4, and NH4HCO3 are easily removed completely after formation of the GDLs by thermal decomposition, while the water-soluble pore-forming agent (NaCl) can be removed completely by water boiling. Moreover, the removing processes of these pore-forming agent have no damage to the structural integrity of the electrodes. Figure 9 present the changes in the surface morphologies of the modified GDLs prepared by adding the same amount of the different pore-forming agents (PEG-200, (NH 4 ) 2 C 2 O 4 , NH4HCO3, and NaCl) in comparison with that of the blank electrode (without any pore-forming agent), respectively. Figure 10a gives the changes in the average pore diameters for these pore-forming GDLs, it can be seen that the primary average pore diameter and secondary average pore diameter in the pore-forming GDLs clearly increase after adding these poreforming agents, compared to the blank sample. Definitely, the calculated secondary average pore diameter in GDLs by mercury porosimetry testing are compared in the order, (NH4)2C2O4 > NaCl > NH4HCO3 > blank > PEG-200, indicating that these pore-forming agents all have the perfect pore-forming ability in GDLs (Figure.10a). In addition, among these pore-forming agents, (NH4)2C2O4 additive exhibits the highest supply of reactant gas to the catalyst sites because it has to the largest secondary average pore diameter about 144 nm shown in Figure.10a. Besides pore diameter, since the pore volume is another key parameter to determine the oxygen delivery in GDLs, the calculated pore volumes for these pore-forming GDLs are shown in Figure 10b. As a result, (NH4)2C2O4 exhibits the

activity of the electrodes. This outcome is verified that the electrode with 30 wt.% PTFE in the GDL has the highest maximum current density for ORR in the sweeping potential ranges, as shown in Figure 7f. To deeply understand the enhanced activity toward ORR, the dependences of the PTFE contents on the formation of the primary pores and secondary pores in gas diffusion layers were investigated. Figure 8a-b present the changes in the average

Figure 8. Changes in the average pore diameter and volume for both primary pores (dprim) and secondary pores in the as-fabricated GDLs by varying the PTFE content, (a) average pore diameter of the primary pores, dprim, the secondary pores, dsec, and that of combination primary and secondary pores, dprim+sec, and (b) pore volume of the primary pores, Vprim, the secondary pores, Vsec, and that of combination primary and secondary pores, Vprim+sec.

pore diameter and volume for both primary pores (dprim) and secondary pores (dsec), respectively. It is visible that the obtained average pore diameters and pore volumes in GDLs substantially depend on the PTFE contents, as shown in Figure 8a,b. The synthesized GDL with 30 wt % PTFE exhibits the largest secondary pore volume (0.2928 mL/g) when PTFE contents increased from 10 to 40 wt %, indicating that the PTFE binder plays a crucial role in the increase in the secondary pore volume by adding to the large number of secondary pore. However, as PTFE content exceeds 30 wt %, the secondary pore volume will decline, thus decreasing the performance of GDE for ORR. 3.5. Effect of Pore-Forming Agents on Activity for ORR. To better understanding of oxygen delivery in the gas

Figure 9. Surface morphologies of the GDLs introduced by different pore-forming agents (PEG-200, (NH4)2C2O4, NH4HCO3, and NaCl) and the blank GDL (without pore-forming agent). 5871

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to the three phase boundary of the catalyst layers. Furthermore, compared to the blank electrode, the modified GDL by (NH4)2C2O4 as pore-forming agent shows perfect ORR activity due to the largest secondary pore diameter and the largest secondary pore volume existed the GDL, which verify the understanding of why the electrode activity is improved.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Funds of China (Grant Nos. 51272018 and 51125007) and the National Key Technology R&D Program (2009BAE87B00) from the Ministry of Science and Technology of the People’s Republic of China.

Figure 10. Effect of the different pore-forming agents on the pore structures of the modified GDLs and the activity for ORR on these GDLs coated by the same catalytic layer for ORR in 30 wt % NaOH at 80 °C, (a) calculated average pore diameter of the GDLs, (b) calculated pore volume on the GDLs, (c) the cathodic polarization curves, and (d) the chronopotentiometry curves. Blank represents the GDL without adding any pore-forming agent.

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REFERENCES

(1) Liang, Y.; Wang, H.; Diao, P.; Chang, W.; Hong, G.; Li, Y.; Dai, H. Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes. J. Am. Chem. Soc. 2012, 134, 15849−15857. (2) Liu, Z. Basic importance of chlor-alkali industry development is energy conservation and decrease of consumption. China Chlor-Alkali 2002, 1, 1−3. (3) Bidault, F.; Brett, D.; Middleton, P.; Brandon, N. Review of gas diffusion cathodes for alkaline fuel cells. J. Power Sources 2009, 187, 39−48. (4) Moussallem, I.; Pinnow, S.; Wagner, N.; Turek, T. Development of high-performance silver-based gas-diffusion electrodes for chloralkali electrolysis with oxygen depolarized cathodes. Chem. Eng. Process.: Process Intens. 2012, 52, 125−131. (5) Baolian, Y.; Wenhua, Z. Current efficiency in the chlorate cell process with an oxygen cathode (III): a gas analysis method for chlorate current efficiency. J. Appl. Electrochem. 1994, 24, 503−508. (6) Watanabe, M.; Tomikawa, M.; Motoo, S. Experimental analysis of the reaction layer structure in a gas diffusion electrode. J. Electroanal. Chem. 1985, 195, 81−93. ́ e, ́ A. B.; Miyatake, K.; Uchida, H.; Watanabe, M. Gas (7) B́ elek diffusion electrodes containing sulfonated polyether ionomers for PEFCs. Electrochim. Acta 2007, 53, 1972−1978. (8) Morimoto, T.; Suzuki, K.; Matsubara, T.; Yoshida, N. Electrochim. Acta 2000, 45, 4257−4262. (9) Tseng, C.-J.; Lo, S.-K. Effects of microstructure characteristics of gas diffusion layer and microporous layer on the performance of PEMFC. Energ. Convers. Manage. 2010, 51, 677−684. (10) Passalacqua, E.; Squadrito, G.; Lufrano, F.; Patti, A.; Giorgi, L. Effects of the diffusion layer characteristics on the performance of polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 2001, 31, 449−454. (11) Nara, H.; Momma, T.; Osaka, T. Impedance analysis of the effect of flooding in the cathode catalyst layer of the polymer electrolyte fuel cell. Electrochim. Acta 2013, 113, 720−729. (12) Park, S.; Popov, B. N. Effect of cathode GDL characteristics on mass transport in PEM fuel cells. Fuel 2009, 88, 2068−2073. (13) Jiao, K.; Zhou, B. Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode. J. Power Sources 2008, 175, 106−119.

largest secondary pore volume of 0.3032 mL/g among these pore-forming agents in use, although the primary average pore diameter of (NH4)2C2O4 electrode is relatively small (Figure 10a). Considering both the largest secondary pore diameter and the largest secondary pore volume in GDL produced by adding the (NH4)2C2O4, the pore-forming GDL coated with a catalytic layer will have the perfect activity for ORR. This inference is verified further by the cathodic polarization curves and the chronopotentiometry curves for the as-fabricated electrodes in 30 wt % NaOH at 80 °C, as shown in Figure 10 c,d. As expected, the pore-forming electrode by (NH4)2C2O4 have the largest cathodic reduction current density and the most positive electrode potential due to the largest secondary pore diameter and secondary pore volume it owned. This outcome indicates that the enhanced activity of the porous electrode strongly depends on the parameters of the secondary pore diameter and volume existed in GDLs, which speed up the delivery of the reacted oxygen, thus the electrochemical activity of oxygen diffusion cathodes will enhance accordingly.

4. CONCLUSIONS In this study, the effects of the physical pore characteristics, including gas permeability, pore size distribution, the pore volume of both primary and secondary pores in GDLs determined by different carbon black fillers, PTFE contents, and additions of pore-forming agents (PEG-200, (NH4)2C2O4, NH4HCO3, and NaCl), on the electrochemical catalytic performance of GDE for ORR are systematically investigated. By comparing the pore characteristics of different carbon blacks in GDLs, the ODC activity for ORR significantly increases as the K−C equation gas permeability of GDLs increase and the enhanced activity of the porous electrode strongly depends on the parameters of the secondary average pore diameter and secondary pore volume of GDL in nanoscale. When GCB was utilized as the optimum carbon black in gas diffusion layers there exists an optimum amount of PTFE loading of 30 wt % in GDLs, which resulting in the enhanced activity for ORR by enlarging of the supply the reactant gas from gas diffusion layers 5872

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