Design of a catalytic layer with hierarchical proton transport structure

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Design of a catalytic layer with hierarchical proton transport structure: the role of Nafion nanofiber Yiyan Sun, Lirui Cui, Jian Gong, Jin Zhang, Yan Xiang, and Shanfu Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03910 • Publication Date (Web): 05 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Sustainable Chemistry & Engineering

Design of a catalytic layer with hierarchical proton transport structure: the role of Nafion nanofiber Yiyan Sun,‡a Lirui Cui,‡a Jian Gong,b Jin Zhang,*a Yan Xianga and Shanfu Lu*a a Beijing

Key Laboratory of Bio-inspired Materials and Devices & School of Space

and Environment, Beihang University, Beijing, 100191, China b National

Key Laboratory of Science and Technology on Aero-Engine Aerothermo-

dynamics, Collaborative Innovation Center of Advanced Aero-Engine, School of Energy and Power Engineering, Beihang University, Beijing, 100191, China Corresponding authors: Shanfu Lu, [email protected] Jin Zhang, [email protected] KEYWORDS: Nafion nanofiber, Catalytic layer, Proton transportation, Pt utilization, Proton exchange membrane fuel cell

ABSTRACT: The chemical composition and architecture of catalytic layers significantly affect the performance of membrane electrode assemblies in proton exchange membrane fuel cells. In this work, a novel catalytic layer with hierarchical proton transport pathways has been designed by simultaneously employing Nafion nanofibers and Nafion ionomer. A H2/O2 fuel cell based on the hierarchical catalytic layer shows an increase of 32.3% on power output in comparison with the conventional fuel cell at 70 oC and 100% relative humidity. That is attributed to unique roles of Nafion nanofibers in the hierarchical catalytic layer. First, the addition of Nafion nanofibers significantly increases proton conductivity of the catalytic layer up to 8.3 × 10-1 S cm-1 at 70 oC and 100% relative humidity, which is 15.7 times higher than that of the catalytic layer where 1

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the Nafion nanofibers are replaced by the same content of Nafion ionomers. That is confirmed by the accumulated proton transportation of a single Nafion nanofiber in the Nafion-nanofiber-based catalytic layer by a simulated model. Second, the porosity of the catalytic layer is increased due to the introduction of Nafion nanofiber, leading to enhancement of mass transfer. Third, the Pt/C nanoparticles are homogeneously anchored on the surface of the Nafion nanofibers, which improves the electrochemical surface area and the utilization of the Pt catalyst.

INTRODUCTION Proton exchange membrane fuel cell (PEMFC) has been considered as one of the most promising and attractive technologies for future power generation due to many advantages including high energy efficiency and fast start-up/shut down cycles1-2. However, the application of PEMFCs still confronts great challenges including the performance improvement of membrane electrode assemblies (MEAs)3-4. Generally, the performance of MEAs is greatly dominated by their components and architecture of catalytic layers5-6. A conventional catalytic layer comprises a mixture of carbon-supported Pt catalyst and proton conducting ionomer7. Nevertheless, the random structure of the conventional catalytic layer results in low Pt utilization and low MEAs performance8. To enhance the Pt utilization and the MEAs performance, many advanced catalytic layers have been developed9. Debe et al. fabricated an ordered Pt 2


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electrode with high performance based on an organic nanostructured thin film (NSTF)10. Inspired by the Pt/NSTF electrode, vertically aligned carbon nanotubes11 and arrays of conducting carbon coated metal dioxide12 and conducting polymers13 are employed as ordered catalyst supports to fabricate three-dimensional (3D) electrodes. These 3D electrode structures significantly increase the power density of a single cell to 1.02 W cm-2 at 80 oC with a Pt loading of 0.065 mg cm-2.14 That is due to decrease of the mass transfer resistance of the catalytic layer and increase of the electrochemical surface area (ECSA) of Pt. Moreover, it has been revealed that the orientation of Nafion ionomer improves the Pt utilization and the MEAs performance15. When Nafion ionomer and Pt/C catalyst were fabricated to long chain-like strings by an electrospun method, the power density of the cell based on the composite catalyst reached up to 0.48 W cm-2 at 80 oC with a Pt loading of 0.02 mg cm-2.16-17 The one-dimensional Nafion nanofiber has unique properties including outstanding proton conductivity and aligned structure18. The proton conductivity of a single Nafion nanofiber reaches to 8.5  106 S cm-1 with the diameter of 500 nm at 25 oC and 100% relative humidity (RH) due to connective proton transport pathways along the direction of the nanofiber19. The ordered proton diffusion pathways can also be easily formed by the pack of one-dimensional nanofibers20. Elabd et al. fabricated a Nafion-nanofiber-based cathodic catalytic layer with high

performance

via

a

simultaneous

electrospinning/electrospraying

technique21. However, not only the architecture of the catalytic layer but also the 3



content of Pt catalyst and Nafion nanofiber are hardly controlled by the complex fabrication process. Moreover, the effect of Nafion nanofibers in the advanced catalytic layer on the performance of PEMFCs is still unknown. Herein, we designed an advanced catalytic layer by introducing Nafion nanofibers to the conventional catalytic layer that contains Nafion ionomer (Figure 1). The combination of Nafion nanofibers and Nafion ionomer in the advanced catalytic layer tends to form hierarchical proton transport pathways for fast proton diffusion. The properties of a single Nafion nanofiber and the cell performance of PEMFC with Nafion-nanofiber-based catalytic layer were comprehensively investigated. Moreover, the effect of Nafion nanofiber on the catalytic layer was revealed in terms of proton conductivity, porosity and electrochemical surface area (ECSA) of Pt. Overall, the peak power density of a H2/O2 fuel cell based on the hierarchical catalytic layer reached up to 1.39 W cm-2 at 70 oC and 100% RH, which is 32.3% higher than that of the fuel cell based on the conventional catalytic layer in the same condition.

Figure 1. The scheme of a catalytic layer with hierarchical proton transport channels.

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EXPERIMENTAL SECTION Materials. D2020 Nafion dispersion (EW=1000, 20 wt %) and Nafion 211 membrane were obtained from Dupont, USA. Polyvinylpyrrolidone (PVP K90, Mw=1,300,000 g·mol-1), H2O2 (28 wt %), H2SO4 (98 wt %), N, Ndimethylformamide (DMF) and isopropanol were purchased from Beijing Chemical Factory, China. Pt supported on XC-72 active carbon (40 wt % Pt) was purchased from Johnson Matthey, UK. Carbon paper with the gas diffusion layer (GDS-3260) was purchased from Sunlaite company, China. The Nafion 211 membrane was treated by H2O2 and H2SO4 solutions sequentially according to the report22. Preparation of Nafion nanofibers. The Nafion nanofibers were fabricated by an electrospinning method with the assistance of a high molecular weight carrier polymer, PVP K90. Briefly, 1.0 g D2020 Nafion dispersion and 0.01 g PVP K90 were mixed in 0.5 mL DMF and stirred at 50 oC for 2 hours to form a transparent solution. After that, the solution was cooled down to room temperature. Then, it was fed to a 1 mL syringe fitted with a metallic needle with the inner diameter of 0.7 mm. For the setup of the electrospinning apparatus, one electrode that connected to high voltage power was clamped to the metal needle tip and the other electrode was clamped to the aluminium collector. The distance from the tip to the collector was kept at 15 cm. In addition, the feed rate for the Nafion solution was 0.5 mL h-1 and the applied voltage was 15 kV.

5



Preparation of Nafion-nanofiber-based catalytic layers. The Nafionnanofiber-based catalyst ink was prepared by mixing Nafion nanofibers, Pt/C (40 wt % Pt, JM) and D2020 Nafion dispersion. Then the catalyst ink was sprayed onto the surface of Nafion 211 membrane. The Nafion-nanofiber-based catalytic layers contained 20 wt % Nafion ionomer and various loadings of Nafion nanofibers. The content of Nafion nanofibers varied from 0 to 40 wt %. The catalytic layers were denoted as NNF-X, where X (X=0, 20, 30, 40) is the percentage of Nafion nanofibers in the catalytic layer. Preparation of conventional catalytic layers. Conventional catalyst ink was prepared by mixing Pt/C (40 wt % Pt, JM) and D2020 Nafion dispersion. Then the catalyst ink was sprayed onto the surface of Nafion 211 membrane. The conventional catalytic layers contained 20 wt % Nafion ionomer and additional Nafion ionomer. The content of additional Nafion ionomer varied from 0 to 40 wt %. The catalytic layers were denoted as NNI-X, where X (X=0, 20, 30, 40) is the percentage of additional Nafion ionomer in the catalytic layer. Fabrication of catalyst coated membranes (CCMs). Four CCMs contained Nafion nanofibers were fabricated where the NNF-X catalytic layers were employed as their cathodic catalytic layers. Another four conventional CCMs were fabricated where the conventional NNI-X catalytic layers were employed as their cathodic catalytic layers. The Pt loading in all the cathodic catalytic layers was 0.4 mg cm-2. In addition, the anodic catalytic layers for all the CCMs were the same and contained only Nafion ionomer and Pt/C catalyst. The loading 6


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for the Pt and Nafion ionomer in the anodic catalytic layer is 0.1 mg cm-2 and 20 wt %, respectively. In addition, the content of Pt/C, Nafion ionomer, and Nafion nanofiber in NNF-X and NNF-I electrodes were summarized in Table S1. The active area for all the CCMs was 4 cm2 (2 cm × 2 cm). Characterizations. The morphology of the catalytic layer in CCMs was observed by scanning electron microscopy (SEM, ZEISS Sigma500, Germany) with an accelerating voltage of 2 kV. Moreover, the morphology of the Pt/C catalyst and the Nafion nanofiber in the Nafion-nanofiber-based catalytic layer was determined by transmission electron microscopy (TEM, JEM-1200EX, Japan) with an accelerating voltage of 200 kV. Proton conductivity of a single Nafion nanofiber was tested by electrochemical impedance spectroscopy (EIS) via a two-electrode method in a frequency range of 100 kHz to 100 Hz with an amplitude voltage of 10 mV23 by a potentiostat/galvanostat of Princeton VersaSTAT4. The single nanofiber was located between two parallel carbon electrodes on a glass substrate. The distance between the two electrodes varied from 2.5 mm to 8.0 mm. The apparatus was stored in an environmental chamber with different temperature and humidity for 2.0 h before the proton conductivity measurement. Proton conductivity of a catalytic layer was also measured by EIS via a fourelectrode method24. Nafion nanofiber, Nafion ionomer and silica nanoparticles (30 nm) with the same size of Pt/C catalyst were mixed and sprayed on carbon paper. Four Pt electrode with an equal distance were placed on the surface of the 7



carbon paper. The operating temperature and humidity of the sample were control by an environmental chamber. The frequency range was 100 kHz to 0.1 Hz with an amplitude voltage of 10 mV. The electronic resistance of the catalytic layer was tested via a four-electrode method. In the test, active carbon, Nafion ionomer and Nafion nanofiber were mixed and sprayed on the Nafion 211 membrane. Then four Pt wire electrodes were placed with an equal distance on the catalytic layer. The operating temperature and humidity of the sample were control by an environmental chamber. A direct current was applied between the two reference electrodes while the voltage between the working electrode and counter electrode was measured. Eventually, the electron conductivity of the catalytic layer was calculated by Ohm’s laws. A membrane electrode assembly was assembled by sandwiching a CCM with two pieces of carbon paper with gas diffusion layer (GCS-3260). Then the MEA was assembled to a single cell. The performance of the single cell was tested at 70 oC and different RH without back pressure by a fuel cell test station (G20, Greenlight Innovation, Canada). The cell was fed by humidified H2 and O2 with a flow rate of 0.1 L min-1 and 0.2 L min-1, respectively. For the H2/air test, the feed rate for humidified H2 and air was 0.1 L min-1 and 1.0 L min-1, respectively. In addition, the electrochemical active surface area of Pt in the cathodic catalytic layer was measured by cyclic voltammetry at 25 oC when the cell was fed with

8


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humidified H2 (0.05 L min-1) and N2 (0.1 L min-1) in the anode and the cathode, respectively. The scan rate was 50 mV s-1. Simulation model for the Nafion-nanofiber-based catalytic layer. The proton conductivity of a catalytic layer with Nafion nanofibers was simulated by finite element method. A model assumed that the conventional catalytic layer comprised Pt/C agglomerates that was covered by a thin layer of Nafion ionomer. The size of the Pt/C agglomerate was 200 nm and the thickness of the Nafion ionomer layer was 10 nm. The Pt/C agglomerates were randomly distributed in a 1 m × 1 m square. The proton conductivity of the catalytic layer was obtained by calculating the average conductivity of 5 random cases with the same number of agglomerates. The gas flow and reactions inside the catalytic layer was neglected in this model to alleviate the computational complexity under reasonable level. For the simulation of the Nafion-nanofiber-based catalytic layer, a nanofiber with diameter of 200 nm was added to the model of the conventional catalytic layer. In this model, multiple Pt/C agglomerates were generated at random x/y positions in the two-dimensional computational domain. Moreover, the Pt/C agglomerates were connected with each other during the simulation processes to transfer protons. The boundary condition for the voltage was always 1.0 V, while the simulated proton conductivity of the catalytic layers was calculated from the Ohm's law (1) and (2).

i  0

(1)

i  

(2)

Where i, σand  are electrolyte current density, electrolyte conductivity of Nafion polymer or nanofiber and electrolyte potential, respectively. 9



RESULTS AND DISCUSSION Proton conductivity of a single Nafion nanofiber. A Nafion/PVP solution with a weight ratio of 12 wt %: 2 wt % (Nafion:PVP) was employed to fabricate Nafion nanofibers via the electrospinning method. A SEM image in Figure 2a shows the successful fabrication of homogeneous Nafion nanofibers with a solid structure and an average diameter of 300 nm. The diameter of Nafion nanofiber is significantly affected by the Nafion: PVP weight ratio. When the ratio increases from 12 wt %: 2 wt % to 5 wt %: 7.5 wt %, the average diameter of the Nafion nanofiber increases from 300 nm to 1.2 µm (Figure S1).

Figure 2. (a) The SEM image of Nafion nanofibers. Inset is the diameter distribution of the Nafion nanofibers. (b) Proton conductivity of Nafion nanofibers with different diameter under 70 oC and 100% RH. (c)Proton conductivity of a single Nafion nanofiber with diameter of 320 nm at temperatures from 30 to 80 oC and 100% RH. (d) Proton conductivity of the single Nafion nanofiber with diameter of 320 nm at relative humidity from 40% to 100% and 70 oC. 10


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The proton conductivity of a single Nafion nanofiber was measured by EIS reported in the literature23. The effect of distance and water condensation between two tested electrodes on the proton conductivity of the single Nafion nanofiber is negligible (Figure S2). The proton conductivity of Nafion nanofibers at 70 oC and 100% RH substantially drops from 11.8 S cm-1 with diameter of 205 nm to 1.8 S cm-1 with diameter of 800 nm (Figure 2b). With further increasing the diameter of Nafion nanofibers, the value slightly decreases to 1.2 S cm-1 at 3.0 μm and then to 1.0 × 10-1 S cm-1 at 12 μm. That shows the same trend as reported data19. In addition, the proton conductivity of a single Nafion nanofiber with diameter of 320 nm significantly increases from 5.6 S cm-1 at 30 oC to 10.3 S cm-1 at 70 oC under 100% RH (Figure 2c). On the contrary, the proton conductivity of Nafion 211 membrane is only 1.0 × 10-1 S cm-1 at 70 oC and 100% RH (Figure 1b). The results indicate that the proton conductivity of Nafion nanofiber is much higher than that of the Nafion membrane at the same condition. That might due to the preferential orientation of the rod-like Nafion aggregate in the longitude direction of the one-dimensional Nafion nanofiber.19 Noteworthy, the value of Nafion nanofiber in our case is between the Nafion/PVP nanofibers (8.5 × 106 S cm-1 at 25 oC and 100% RH)19 and the high purity Nafion nanofiber (1.5 S cm-1 at 25 oC and 100% RH)23. The huge difference between the conductivity of Nafion nanofibers in different research might depend on the content of polymer carrier and the diameter of Nafion nanofibers. Besides, the proton conductivity of the single Nafion nanofiber is also affected by the relative 11



humidity. The Nafion nanofiber with diameter of 320 nm slightly increases from 4.6 S cm-1 at 40% RH to 7.2 S cm-1 at 80% RH. Nevertheless, the proton conductivity jumps to 9.6 S cm-1 at 90% RH (Figure 2d), which is due to water dependence of Nafion nanofiber for proton diffusion25.

Figure 3. (a, b) SEM images for surface section of a Nafion-nanofiber-based catalytic layer (NNF-30) at different magnifications; (c) A SEM image, (d) F element mapping and (f) the overlay image of c and d for the cross section of a Nafion-nanofiber-based catalytic layer (NNF-30). Novel catalytic layer with Nafion nanofiber. The morphology of a Nafionnanofiber-based catalytic layer was characterized by the scanning electron microscopy. Take the NNF-30 catalytic layer as an example, individual Nafion nanofibers are uniformly distributed in the catalytic layer (Figure 3a), while Nafion ionomer covers on the surface of the Pt/C aggregates, as shown in Figure 3b. In other words, the combination of Nafion nanofibers and Nafion ionomer forms hierarchical proton conducting pathways in the catalytic layer (Figure 3c). The interconnected F element mapping in Figure 3d and e compliments Figure 3c by further confirming the hierarchical proton transportation networks in the 12


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catalytic layer. Compared to the conventional catalytic layer (Figure S3), the Nafion-nanofiber-based catalytic layer shows higher porosity, larger pore size and smaller catalyst particle aggregates. The hierarchical proton conducting pathways is expected to provide an array for fast proton diffusion26, while the high porosity is favour for accelerating the mass transport in the catalytic layer27.

Figure 4. (a) Polarization and power density curves for H2/O2 fuel cells of NNFX at 70 oC and 100% RH. (b)I-V curves of H2/O2 fuel cell of NNF-30 and NNI30 CCMs at 70 oC and 100% RH. (c) EIS of the NNF-X cells at 0.6 V under 70 oC and 100% RH. (c) Durability of a H /O fuel cell based on NNF-30 at a 2 2 o constant voltage of 0.6 V at 70 C and 100% RH. The effect of Nafion nanofibers on cell performance. When NNF-X CCMs were assembled into fuel cells, their H2/O2 cell performance was evaluated at 70 oC

and 100% RH, as shown in Figure 4a. The addition of Nafion nanofibers to

the cathodic catalytic layer significantly increases the cell performance of NNF0. When Nafion nanofibers were added into the cathode with a loading of 20%, the peak power density of the cell reaches to 1.27 W cm-2, which is 20.1% higher than that of the cell based on NNF-0 (1.05 W cm-2). Besides, the cell performance 13



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shows a volcano shape against the content of Nafion nanofibers in the cathodic catalytic layer. The fuel cell based on NNF-30 CCM shows the highest peak power density of 1.39 W cm-2, which is 8.1 times higher than that of NNI-30 fuel cell at the same test condition (Figure 4b). In addition, the peak power density of NNF-30 cell is 0.59 W cm-2 under H2/air at 70 oC and 100% RH without backpressure (Figure S4). The value is 43.9% higher than that of the cell performance of NNF-0 at the same test condition. More specifically, during the activation polarization region, the activation polarization loss gradually decreases from NNF-0 to NNF-30. It indicates that the addition of Nafion nanofibers increases the kinetics of the electrochemical reactions over the Pt catalyst. It is likely due to the fact that the addition of Nafion nanofibers enhance the contact of Pt nanoparticles with the Nafion polyelectrolyte, leading to increase the area of triple phase boundary (TPB)28. However, excess content of Nafion nanofibers tends to isolate the Pt catalyst clusters and then decreases the area of TPB, resulting in the increase of the activation polarization loss from NNF-30 to NNF-40

29.

The addition of Nafion

nanofibers also significantly decreases the charge transfer resistance from 0.29 Ω cm2 for NNF-0 to 0.16 Ω cm2 for NNF-30 (Figure 4c). Nevertheless, excess content of Nafion nanofibers results in the increase of the charge transfer resistance to 0.25 Ω cm2 for NNF-40. Meanwhile, the polarization curve of NNF40 fuel cell shows obvious mass transfer loss in the concentration polarization region. The results indicate that the excess content of Nafion nanofiber may result 14


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in flooding of water in the cathodic catalytic layer30, which decreases the cell performance from 1.39 W cm-2 for NNF-30 fuel cell to 1.13 W cm-2 for NNF-40 fuel cell at 70 oC and 100% RH. Besides, durability is another critical issue for PEMFCs. PEMFC based on conventional catalytic layer shows outstanding durability during the fuel cell operating condition

31-32.

However, the stability of fuel cells based on advanced

catalytic layers is still a great challenge because of corrosion of the Pt/C catalyst, collapse of the aligned structure during wetting/drying cycles and membrane degradation by the attack of H2O2 in the cathode

33.

The current density of fuel

cell based on NNF-30 is stable at 1.25 A cm-2 for 50 h with a constant voltage of 0.6 V at 70 oC and 100% RH, as shown in Figure 4d. The results illustrate that the addition of Nafion nanofibers to the catalytic layer shows neglected effect on the durability of the conventional catalytic layer. Nevertheless, further investigation in the future is required to confirm the intact hierarchical proton diffusion structure after the fuel cell operating conditions. Figure 5 shows the cell performance of NNF-30 and NNF-0 under different relative humidity and 70 oC. At relative humidity of 50%, the peak power density of NNF-30 fuel cell is 0.56 W cm-2 (Figure 5a), which is 143.5% higher than that of the NNF-0 fuel cell at the same test condition (Figure 5b). With increase of the relative humidity, the cell performance of NNF-30 fuel cell jumps to 1.23 W cm-2 at 70% RH, whilst the peak power density of the cell slightly increases to 1.39 W cm-2 when the relative humidity further increases to 100% (Figure 5c). 15



On the contrary, the peak power density of NNF-0 almost linearly increases from 0.23 W cm-2 at 50% RH to 1.02 W cm-2 at 100% RH. The results indicate that the PEMFC with Nafion nanofibers catalytic layer has less humidity dependence than the one with conventional catalytic layer.

Figure 5. The polarization curves of single cell based on (a) NNF-30 and (b) NNF-0 CCMs at 70 oC and different relative humidity. (c) Peak power density of NNF-30 cell and NNF-0 cell at 70 oC and different relative humidity.

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The role of Nafion nanofibers in the catalytic layers. The aforementioned results demonstrate that the addition of Nafion nanofibers to the cathodic catalytic layer significantly affects the fuel cell performance. Moreover, an optimum loading of Nafion nanofibers in the cathodic catalytic layer is 30% when the cell was operated at 70 oC and 100% RH. Generally, the performance of a catalytic layer is mainly affected by its conductivity, ECSA of Pt and porosity of the catalytic layer 34. Thus, the three parameters have been carefully evaluated for the cathodic catalytic layers of four NNF-X CCMs. In addition, the cathodic layers of the NNI-X were employed as the control group.

Figure 6. (a) The electron conductivity and (b) the ionic conductivity of the cathodic catalytic layers of NNF-X and NNI-X at 70 oC and 100% RH.

Electronic conductivity and proton conductivity of the cathodic catalytic layers were measured for both NNF-X and NNI-X CCMs 35, as shown in Figure 6. The electronic conductivity of NNF-0 is 2.28 S cm-1 at 70 oC and 100% RH (Figure 6a). With increasing the content of either Nafion nanofibers or Nafion ionomer, the electronic conductivity of the catalytic layer gradually decreases. That is attributed to the fact that the Nafion polyelectrolyte acts as an insulator and hinders the electron transfer among the Pt catalyst clusters 17


36.

Nevertheless, the


electronic conductivity of the cathodic layer of NNF-40 is 1.15 S cm-1, which is 8.3 times higher than that of the cathodic layer of NNI-40. The results demonstrate that the addition of Nafion nanofibers in catalytic layers shows less negative effect on the electronic conductivity of the catalytic layers than the addition of Nafion ionomer. That is likely due to the fact that the Nafion nanofiber alleviates the coverage of polyelectrolyte on Pt/C clusters. Obviously, the change of the electron conductivity of the cathodes is inconsistent with the trend of the power density of the fuel cell, indicating that the electronic conductivity of the hierarchical catalytic layer is not the dominated parameter for the cell performance. Moreover, Figure 6b shows the proton conductivity of cathodic catalytic layers of NNF-X and NNI-X measured at 70 oC and 100% RH. It shows that the proton conductivity of the catalytic layers gradually improves with increasing the content of Nafion polyelectrolytes. More specifically, when 30% Nafion nanofibers was added, the proton conductivity of the catalytic layer sharply increases to 8.3 × 10-1 S cm-1. That value is 15.7 times higher than that of the catalytic layer of NNI-30 (5.0 × 10-2 S cm-1) tested in the same condition. In other word, the addition of Nafion nanofibers is more efficient to increase the proton conductivity of the catalytic layer than Nafion ionomer with the same weight content in the catalytic layer. However, the proton conductivity of the catalytic layer slightly decreases from 8.3 × 10-1 S cm-1 to 8.2 × 10-1 S cm-1 in the same test condition when the content of the Nafion nanofibers increases from 30 wt % 18


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to 40 wt %. Park et al. also found that the proton conductivity of the Nafion fibre/polyvinylidene fluoride composite membrane reached a plateau when the content of Nafion nanofiber was higher than 60%

37.

In our case, the Nafion-

nanofiber-based catalytic layer forms a hierarchical architecture with high porosity and high proton conductivity with the addition of optimum content of Nafion nanofiber. However, the further addition of excess Nafion nanofibers only increases the thickness of the catalytic layer. That compromises the positive effect of the addition of Nafion nanofiber in the catalytic layer, leading to decrease of the proton conductivity. Overall, the proton conductivity of the hierarchical catalytic layer is constant with the cell performance, indicating that the proton conductivity is one of the dominated parameters for the cell performance.

Figure 7. (a) A computational domain of the simulated catalytic layer with randomly distribution of Pt/C agglomerates in a 1 m × 1 m square; (b) simulation results of ionic conductivity of the cathodic layers of NNF-X and NNI-X; (c) potential distribution and (d) current density distribution in Nafion ionomer and the Nafion nanofiber. 19



To further evaluate the effect of Nafion nanofibers on proton conductivity of a catalytic layer, a two-dimensional mathematical model was developed. Figure 7a shows the model of Nafion-nanofiber-based catalytic layer (NNF-X). It only covers ionic conductivity in catalyst layer with a Nafion nanofiber and random Pt/C agglomerates distributed across the computational domain. When the Nafion nanofiber is absent in the model, it is denoted as NNI-X. The model is mainly based on ohm’s law and built with finite element method. Figure 7b depicts that the simulated proton conductivity of a catalytic layer increases with increasing the content of Nafion polyelectrolytes in the catalytic layer. Nevertheless, the proton conductivity of the catalytic layer with the addition of Nafion nanofibers is much higher than that of the catalytic layer with the addition of the same content of Nafion ionomer. The simulated results are consistent with the trend of the measured proton conductivity in Figure 6b. In addition, during the simulated operation of the Nafion-nanofiber-based catalytic layer, only small potential drop exists in the nanofiber due to its high ionic conductivity (Figure 7c). Moreover, majority of the protons passes through the nanofiber since the proton conductivity of a nanofiber is much higher than that of Nafion ionomer (Figure 7d). Consequently, the results demonstrate that the Nafion nanofiber is the “highway” for proton transportation in a simulated Nafion-nanofiber-based catalytic layer.

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Figure 8. (a) Proton conductivity and (b) electronic conductivity of the catalytic layer of NNF-30 at 70 oC and various relative humidity.

Figure 8a shows that proton conductivity of NNF-30 is 1.9  10-1 S cm-1 at 70 oC

and 40% RH. Nevertheless, the value significantly increases to 1.6 S cm-1 at

70% RH and then slightly increases to 2.0 S cm-1 at 100% RH. On the contrary, the electronic conductivity of the catalytic layer remains constant during the relative humidity range from 40% to 100% (Figure 8b). Moreover, the trend is consistent with the cell performance of NNF-30 fuel cell against the relative humidity in Figure 5. The results demonstrate that the high performance of NNFX cell at low relative humidity is due to the high proton conductivity in the same condition. In a conventional catalytic layer, active carbon nanoparticles are adhered by Nafion ionomer to form agglomerate exhibiting primary (2-20 nm) and secondary pores (>20nm). The pores create a sufficiently connected percolation ionomer network for proton conduction. To further evaluate the effect of Nafion nanofibers on microstructure of a catalytic layer, N2 adsorption/desorption isotherms of NNF-0 and NNF-30 catalytic layer were measured, as shown in Figure 9. When 30 wt % Nafion nanofibers were added to the catalytic layer, 21



type IV curve with H1 hysteresis indicates the mesoporous structure of the catalytic layer (Figure 9a). The BET surface area of the catalytic layer increases to 97.1 m2 g-1 (Table S2), which is 2.2 times higher than that of the NNF-0 catalytic layer. Furthermore, the pore size of NNF-30 is larger than that of NNF-0 and the pore volume of NNF-30 from 1 to 140 nm (0.23 m3 g-1) is 2.3 times higher than that of the NNF-0 catalytic layer (Figure 9b). In addition, the proton conductivity of a single Nafion nanofiber shows high dependence on the relative humidity (Figure 2d). Consequently, the high proton conductivity and cell performance of NNF-X is due to the improved porosity with the capillary effect of mesoporous structure38 after addition of the Nafion nanofiber.

Figure 9. (a) N2 adsorption/desorption isotherms and (b) pore size distribution of NNF-0 and NNF-30 catalytic layer. Figure 10 shows the electrochemical performance of Pt catalysts in the catalytic layers of NNF-X. As shown in Figure 10a, the addition of Nafion nanofibers significantly improves the catalytic activity of the conventional Pt catalytic layer. Take NNF-20 as an example, the ECSA of Pt in the cathodic layer of NNF-20 is 48.3 m2 g-1, which is 90% higher than that of the cathodic layer of NNF-0 (Figure 10b). When Nafion nanofiber was introduced to the conventional 22


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catalytic layer, the catalytic layer possesses larger pore volumes in the primary and secondary pore size than the NNF-0 electrode (Figure 9). Thereby, the addition of Nafion nanofiber tend to increase the TPB and ECSA for Pt nanoparticle in the catalytic layer39, which is consistent with the results of SEM and BET measurement. In addition, Figure 10c reveals that the Pt/C nanoparticles are homogenous anchored on the surface of the Nafion nanofibers, which increases the contact area of Pt nanoparticles with the proton conductors in the catalytic layer. In other words, more Pt nanoparticles are exposed to Nafion polyelectrolytes and gases. In addition, the ECSA of Pt shows volcano shape against the content of Nafion nanofibers in the catalytic layer, while the highest value of 55.9 m2 g-1 is achieved for the cathodic layer of NNF-30. When the content of Nafion nanofiber increases to 40 wt %, the ECSA of Pt decreases to 39.3 m2 g-1. That is due to the fact that the excess Nafion nanofibers dilutes the contact of the Pt catalyst clusters with the Nafion polyelectrolytes and then decreases the TPB area in the catalytic layer 7. Consequently, the ECSA of Pt in the Nafion-nanofiber-based catalytic layer is consistent with the performance of the fuel cells, indicating that the ECSA is also a dominated parameter affecting the cell performance. Overall, the addition of Nafion nanofibers to the catalytic layer increases the proton conductivity, the accessibility by oxygen, water retention at low relative humidity and the Pt utilization of the catalytic layer, leading to the high performance of the fuel cells with Nafion-nanofiber-based catalytic layers. 23



Figure 10. (a) Cyclic voltammetry curves of the cathodic layers of NNF-X with a scan rate 20 mV s-1 at room temperature and 100% RH; (b) the electrochemical surface area of Pt catalysts of the cathodic layers of NNF-X; (c) The TEM image of the cathodic layer of NNF-30.

CONCLUSIONS In this work, hierarchical catalytic layers have been fabricated by simultaneously employing both Nafion nanofibers and Nafion ionomer as polyelectrolyte binders in catalytic layers. The addition of Nafion nanofibers significantly enhances the proton diffusion rate of the catalytic layer because of the outstanding proton conductivity of Nafion nanofibers. Moreover, the one24


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dimensional structure of nanofibers increases the volume of mesoporous in the catalytic layer, resulting in improved gases accessibility. Finally, the Pt nanoparticles are homogeneously anchored on the surface of the onedimensional Nafion nanofibers, which mitigates the coverage of Nafion polyelectrolytes on Pt nanoparticles and increases the Pt utilization. The optimum loading of the Nafion nanofibers in the catalytic layer is 30 wt % with a peak power density of 1.39 W cm-2 at 70 oC and 100% RH, which is 32.3% higher than the fuel cell with the conventional catalytic layer. The hierarchical polyelectrolyte nanofiber-based catalytic layers are easy to fabricate with accurate control of the components and have excellent cell performance. It shows promising application in polyelectrolyte membrane fuel cells including proton exchange membrane fuel cells, anion exchange membrane fuel cell and bipolar membrane fuel cells. ASSOCIATE CONTENT Supporting Information Components of the cathodes with Nafion nanofiber and their BET results, SEM images of the nanofibers and catalytic layers, proton conductivity of nanofibers and single cell performance. AUTHOR INFORMATION Corresponding author *Tel.: +86 10 61716639. Fax: +86 10 61716639. E-mail: [email protected] (Prof. Shanfu Lu); [email protected] (Dr. Jin Zhang); 25



Author Contributions ‡ These authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors thank the financial support by grants from Key Research and Development Program of Beijing (Z171100000917011), the National Natural Science Foundation of China (No.21722601, No.21576007), Beijing Municipal Natural Science Foundation (No.2194076), and the Fundamental Research Funds for the Central Universities. REFERENCES (1) Lu, S.; Xu, X.; Zhang, J.; Peng, S.; Liang, D.; Wang, H.; Xiang, Y. A SelfAnchored Phosphotungstic Acid Hybrid Proton Exchange Membrane Achieved via One-Step Synthesis. Adv. Energy Mater. 2014, 4 (17), 1400842, DOI: 10.1002/aenm.201400842. (2) Yang, H.; Zhang, J.; Li, J.; Jiang, S. P.; Forsyth, M.; Zhu, H. Proton Transport in Hierarchical-Structured Nafion Membranes: A NMR Study. J. Phys. Chem. Lett. 2017, 8 (15), 3624-3629, DOI: 10.1021/acs.jpclett.7b01557. (3) Breitwieser, M.; Bayer, T.; Büchler, A.; Zengerle, R.; Lyth, S. M.; Thiele, S. A fully spray-coated fuel cell membrane electrode assembly using Aquivion ionomer with a graphene oxide/cerium oxide interlayer. J. Power Sources 2017, 351, 145-150, DOI: 10.1016/j.jpowsour.2017.03.085. (4) Wang, J.; Wang, H.; Fan, Y. Techno-Economic Challenges of Fuel Cell Commercialization. Engineering 2018, 4 (3), 352-360, DOI: 10.1016/j.eng.2018.05.007. (5) Wu, J.; Melo, L. G. A.; Zhu, X.; West, M. M.; Berejnov, V.; Susac, D.; Stumper, J.; Hitchcock, A. P. 4D imaging of polymer electrolyte membrane fuel cell catalyst layers by soft X-ray spectro-tomography. J. Power Sources 2018, 381, 72-83, DOI: 10.1016/j.jpowsour.2018.01.074. (6) Fritz, K. E.; Beaucage, P. A.; Matsuoka, F.; Wiesner, U.; Suntivich, J. 26


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For Table of Contents Use Only

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The introduction of Nafion nanofibers in catalytic layer increases its porosity, proton conductivity, Pt ultilization and improves PEMFCs performance.

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Design of a catalytic layer with hierarchical proton transport structure

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