Improving Oxygen Reduction Performance by Using Protic Poly(Ionic

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Improving Oxygen Reduction Performance by Using Protic Poly(ionic liquid) as Proton Conductor Xiaocong Yan, Fangfang Zhang, Haining Zhang, Haolin Tang, Mu Pan, and Pengfei Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20587 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Improving Oxygen Reduction Performance by Using Protic Poly(ionic liquid) as Proton Conductor Xiaocong Yan,†, Fangfang Zhang,‡, Haining Zhang,*,† Haolin Tang,† Mu Pan,† Pengfei Fang*,‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Nr. 122 Luoshi Rd., Wuhan, China 430070 ‡

College of Physics and Science Technology, Wuhan University, Luojiashan Rd.,

Wuhan, China 430072 * Corresponding

authors: [email protected] (H Zhang) [email protected] (P Fang)

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Abstract: Improving catalytic performance of oxygen reduction reaction (ORR) of Pt/C catalysts is essential for reducing Pt-loading and the according cost of proton exchange membrane fuel cells (PEMFCs). Herein, we report a new conceptual design of catalyst layer to improve the ORR performance of Pt/C catalysts by replacing perfluorosulfonated ionomers with protic poly(ionic liquid) as proton conductor. The specific activity of designed catalyst at 0.9 V under acidic conditions is over three times higher than that of catalyst using Nafion as proton conductor. Furthermore, the durability test reveals that the introduction of protic poly(ionic liquid) ionomer can protect Pt nanoparticles against aggregation during potential cycles but it is less durable than Nafion due to the hydrocarbon natures. Nevertheless, we believe that replacing perfluorosulfonated ionomers with protic poly(ionic liquid) as proton conductor could be a promising strategy to design efficient cathode for low Pt-loading PEMFCs.

Keywords: electrochemistry; fuel cell; platinum; oxygen reduction; poly(ionic liquid)

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1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention as electrochemical energy conversion devices with zero emission of pollutants and high energy conversion efficiency.1-4 However, cost and durability are still the remaining barriers hindered the widespread applications of PEMFCs. The sluggish kinetics of oxygen reduction reaction (ORR) at air electrode induced high platinum (Pt) loading and the corrosion of air electrode are important contributors to the cost and durability of PEMFCs. Since development of non-platinum catalysts for ORR is still in their infancy stage, optimization of the structure and composition of catalyst layer in air electrode is an important approach for improvement in Pt utilization which can lead to enhanced fuel cell performance and reduced cost of PEMFC.5-8 In a typical air electrode of PEMFC, perfluronated sulfonated acid (PFSA) ionomers as binder and proton conductor are usually covered on the surface of Pt/C catalysts due to electrostatic interactions.9-13 During ORR process in air electrode, oxygen molecules are required to pass through the covered ionomer layer to reach the active sites of catalyst. Thus, the solubility and transport rate of oxygen in ionomer layers are critical to the kinetics of ORR. As an example, Kongkanand et al. reported that the ionomers layers on the surface of Pt could constrain the ORR kinetics due to the resistance of oxygen transport on or near the Pt surface.14 Moreover, the strong complexation of sulfonate groups on ionomer with Pt and the associated water guaranteeing proton transport in electrode can result in the decreased active sites, which in turn leads to the decreased cell performance particularly for low Pt loading 3



PEMFC.15,16 Inspired by this understanding, hydrophobic ionic liquids (ILs) have been realized as promising additives to catalyst layer of air electrode due to their unique characteristics including high solubility and transport rate for oxygen, great ionic conductivity, low volatility, and excellent electrochemical stability.17,18 With the introduction of specifically synthesized ILs to catalyst layer of air electrode, both electrocatalytic activity towards ORR and tolerance to cell reversal of PEMFC can be improved.19-22 However, the unavoidable leaching of IL molecules and the electrostatic interaction of IL molecules with sulfonate groups on ionomers may limit their functions in air electrode. This has led out interest to poly(ionic liquid)s (PILs) that has been widely applied as electrolyte membrane materials as described in recent topical reviews.23,24 Herein, we report the electrocatalytic ORR performance of Pt/C catalysts using copolymer containing protic ionic liquid segments as binder and proton conductor to replace PFSA ionomers. It is expected that the protic ionic liquid moieties on the polymer offer proton transport media and the weak interaction between ionic groups and Pt can increase the active sites on the surface of Pt/C catalysts, thus improving the catalytic activity of Pt/C catalysts towards ORR. This conceptual design would provide a new idea for fabrication of efficient catalyst layers in PEMFC, particularly for low Pt loading PEMFCs. 2. Experimental Section

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Materials and reagents. Styrene (St, > 98%, Sinopharm Chemical Reagent, China) was distilled under vacuum prior to use and N,N-Dimethylvinylbenzylamine (DMVBA, mixture of isomers 97%, Aldrich) was used without further purification. Azobisisbutyronitrile (AIBN, 99%, recrystallized from ethanol, Aladdin), 1, 4-dioxane (anhydrous, 99.5%, Aladdin), trifluoromethanesulfonic acid (HTfO, 98+%, Alfa Aesar), and dimethylformamide (DMF, 98%, Sinopharm Chemical reagent, Chain) were used as received. Pt/C catalyst (20 wt% platinum on active carbon) was supplied by Hesen Electric Co. (Shanghai, China). Water was purified through a Millipore system with 18.2 MΩcm-1. All the other chemical reagents were purchased from Sinopharm Chemical Reagent and used as received. Synthesis

of

poly(DMVBAn-co-Stm).

Copolymers

of

poly[(N,N-

Dimethylvinylbenzylamine)n-co-(styrene)m], denoted as poly(DMVBAn-co-Stm) (n:m refers to the molar ratio of initially added monomers DMVBA:St), were synthesized by conventional free-radical copolymerization. The concentration of mixed monomers in 1, 4-dioxane was 50% in volume and the amount of AIBN was 0.8% with respect to the weight of mixed monomer. In general, to the solution of mixed monomer solution in 1, 4-dioxane, desired amount initiator of AIBN was added under stirring. The obtained solution was degassed through three freezing-thaw cycles to remove the dissolved oxygen and placed in a thermostat pre-set to 60  1 oC. After 20 h polymerization, the reaction solution was added dropwise to cold ethanol/water mixture (1:4 in volume). The precipitated copolymer was collected by filtration and washed extensively with

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cold ethanol/water mixture. The final copolymer product was obtained after drying at 80 oC under vacuum. Protonation of poly(DMVBAn-co-Stm). Typically, 0.5 g of copolymers was completely dissolved in 50 mL solvent (absolute ethanol for poly(DMVBA5-co-St5) and poly(DMVBA7-co-St3), DMF for poly(DMVBA3-co-St7)) under magnetic stirring in an ice bath. After addition of 1.5 g of HTfO in 20 mL solvent slowly, white precipitation was formed. The mixture was continuously stirred for 12 h to ensure complete protonation of amine groups. For poly(DMVBA3-co-St7), the products was precipitated from cold deionized water. The white solid products were collected by filtration, followed by extensively washing with cold ethanol. The products were finally dried at 80 oC under vacuum. The protonated samples were denoted as poly(DMVBAnTfO-co-Stm). The average molecular weight of the synthesized copolymer is around 104 gmol-1 and the detailed parameters for different designed copolymers were listed in Table 1. Table 1. Average number molecular weight and polydispersity index (PDI) of poly(DMVBAn-TfO-co-Stm) determined by GPC analysis. n:m

Mn (103 gmol-1)

PDI

3:7

10.48

1.54

5:5

14.18

1.51

7:3

8.15

1.53

Preparation of catalyst inks. The catalyst inks for evaluation of catalytic activity towards oxygen reduction reactions (ORR) were prepared by dispersing 4.0 mg Pt/C 6


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(20 wt%) catalyst in a mixture of 900 µL isopropanol, 100 µL deionized water, and 20 µL ionomer dispersion (5 wt% of Nafion or 3 wt% of poly(DMVBAn-TfO-co-Stm in isopropanol/water mixture with volume ratio of 9:1 unless otherwise stated), followed by ultrasonic treatment to obtain an uniform suspension. General characterization. 1H NMR spectra of poly(DMVBAn-Tfo-co-Stm) were recorded on a Bruker Nuclear Magnetic Spectrometer (400 MHz) using deuterated dimethyl sulfoxide (DMSO) as solvent and tetramethyl silane (TMS) as internal standard to investigate the chemical environment of according H-atoms and to calculate the final segment ratio of DMVBA and St. Elemental analysis was carried out on a Vario EL Cube analyzer (Elementar, Germany) to determine the chemical structure and the degree of protonation of copolymers. Average molecular weight and polydispersity of poly(DMVBAn-TfO-co-Stm) were determined by gel permeation chromatography (GPC) equipped with a Waters 600E GPC system, where DMF was used as eluent and the narrow-polydispersity polystyrene was used as calibration standard. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analyses were carried out using STA449F3, Netzsch with a ramp rate of 10 oCmin-1 under nitrogen atmosphere to determine thermal properties of poly(DMVBAn-TfO-co-Stm). To determine the hydrophobicity of the formed catalysts, water contact angle measurements were carried out a WCA instrument (Theta Attension, Biolin Scientific, Finland) at room temperature, and recorded by OneAttension software. The water droplet volume for contact angle measurement was 4 µL. On each sample, four water droplets were deposited on different areas to acquire the average WCA and at least two samples were 7



tested for each experiment. Transmission electron microscopy (TEM) was carried out on a H-600 STEM/JEM2100F (Hitachi) to investigate the morphology of platinum nanoparticles in catalysts before and after degradation tests. Electrochemical measurements. Electrochemical measurements were carried out on electrochemical workstation (CHI 660 E) with a rotating system (Pine Research Instruments, USA). Potentials reported in this paper were calibrated against reversible hydrogen electrode (RHE) unless otherwise specified. To obtain the protonic resistance of catalyst layers, electrochemical impedance spectroscopy (EIS) was conducted at 0.45 V in N2-saturated HClO4 with a rotation speed of 2500 rpm at room temperature.15, 26 Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were employed to evaluate the ORR performance of catalysts in 0.1 M saturated HClO4 solution at room temperature in a typical three-electrode system using saturated calomel electrode (SCE) as reference electrode, 1 cm2 platinum foil as counter electrode, and glassy carbon rotating disk electrode (GC-RDE, 0.196 cm2) as working electrode. Before electro-chemical tests, the prepared catalyst ink (10 µL) was applied to the surface of GC-RDE and dried in air. The electrochemical cleaning of working electrode was conducted through rapid voltage scanning from -0.3 V vs. SCE to 0.9 V vs. SCE with a scan rate of 100 mVs-1 for 50 cycles in N2 saturated 0.1 M HClO4 solution until the CV curve became stable. Afterwards, CV data were recorded from -0.3 V vs. SCE to 0.9 V vs. SCE with a scan rate of 20 mVs-1 for 5 cycles in N2 saturated 0.1 M HClO4 solution to determine the electrochemically surface area (ECSA). LSV curves were recorded in the O2 saturated 0.1 M HClO4 solution from -0.3 V vs. SCE to 0.9V vs. 8


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SCE with a scan rate of 20 mVs-1 and rotation speeds 1600 rpm to investigate the ORR activity. The durability of catalysts was analyzed by recording the CV data from 0.1 V vs. SCE to 0.9 V vs. SCE with a scan rate of 50 mVs-1 for 3000 cycles in O2 saturated 0.1 M HClO4 solution at room temperature. 3. Results and Discussion The model copolymers were synthesized by a simple free radical polymerization using mixed monomer solutions of N, N-dimethylvinylbenzylamine (DMVBA) and styrene (St) in 1, 4-dioxane, followed by protonation using trifluoromethanesulfonic acid (HTfO), as shown in Scheme 1. The accordingly synthesized copolymers were denoted as poly(DMVBAn-TfO-co-Stm), where n:m refers to the initial molar ratio of the added monomers of DMVBA and St for polymerization. The successful synthesis of copolymers was confirmed by 1H NMR spectra since the characteristic resonance peaks of polystyrene and poly(N, N-dimethylvinylbenzylamine) are clearly observed as indicated in the figure (Figure 1). Particularly, the observed resonance signal at 9.38 ppm assigned to the transferred proton from sulfonic acid group of TfO to tertiary amine of copolymers indicates the successful quaternization process. Elemental analysis was applied to determine the chemical composition and the degree of quaternization of copolymers. The nitrogen contents from elemental analysis are very close to the calculated values from the ratio of initially added monomer mixture (Table 2), indicating that the ratio of monomeric segments on copolymer chains is very similar to the initial molar ratio of monomers for polymerization. The molar ratios of S-atoms and N-atoms (S/N) which reflects the degree of quaternization of amine are calculated based 9



on elemental analysis to be 1.00, 0.99, and 0.89 for poly(DMVBA7-TfO-co-St3), poly(DMVBA5-TfO-co-St5), and poly(DMVBA3-TfO-co-St7), respectively. Moreover, the final segments ratio of DMVBA and St calculated from the integration of characteristic resonance peaks agrees well with the elemental results, as displayed in Table 2. This can be understood that the stretched polymer chains induced by electrostatic repulsion can facilitate the migration of TfO for copolymers with relatively high fraction of DMVBA whereas the shrinkage of polymer chains and the possible coverage of amine groups by polystyrene segments can lead to the low degree of quaternization for copolymers with low fraction of DMVBA.

Scheme 1

Figure 1

1H

Synthetic route of poly(DMVBAn-TfO-co-Stm) ionomers.

NMR spectra and assignment of resonance peaks for poly(DMVBA5-

TfO-co-St5) (black line) and poly(DMVBA3-TfO-St7) (grey line). 10


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Table 2. Elemental analysis data, calculated value of N-content, and density of ionic groups on poly(DMVBAn-TfO-co-Stm). n:m

Elemental analysis (wt%)

initial

from NMR

C

H

N

S

Molar ratio of S:N

3:7

3:7.58

66.36

6.41

2.66

5.42

0.89

2.53

1.69

5:5

5:5.28

54.11

6.48

3.38

7.68

0.99

3.37

2.40

7:3

7:3.22

50.07

5.67

3.71

8.47

1.00

3.94

2.65

Calculated N (wt%)

Density of ionic groups (mmolg-1)

Thermal properties of the synthesized copolymers were investigated since they significantly affect the formation process of membrane electrode assembly in practical integration of PEMFCs. Differential scanning calorimetry (DSC) curves in Figure 2a revealed that the glass transition temperature (Tg) of all the three tested samples of poly(DMVBAn-TfO-co-Stm) is about 125 oC, similar to that of commercial Nafion ionomers.25 This would suggest that the formation of catalyst layers in electrode using the synthesized PILs as proton conductor can follow similar process for catalyst layers using PFSA ionomer as proton conductor. Moreover, the thoroughly protonated samples of poly(DMVBA7-TfO-co-St3) and poly(DMVBA5-TfO-co-St5) are thermally stable up to 350

oC

whereas the non-protonated tertiary amine groups in

poly(DMVBA3-TfO-co-St7) starts to decompose at about 250 oC as observed in thermalgravimetric analysis curves (Figure 2b).

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Figure 2

DSC (a) and TGA (b) curves of poly(DMVBAn-TfO-co-Stm) as indicated in the figure.

Reasonable proton conductivity inside catalyst layer of Pt/C is a prerequisite for ORR. It has been recently reported that the proton diffusion resistance in catalyst layer can be calculated by the intercept of the linear portion on the x-axis in electrochemical impedance spectrum.15 Figure 3a displays Nyquist plots of electrochemical impedance spectra normalized to area of electrode for different catalyst layers under nitrogensaturated 0.1 M HClO4 aqueous solutions and the calculated overall proton diffusion resistance (RH+) normalized to the area of catalyst layers were plotted in Figure 3b. The calculated RH+ value for Nafion-Pt/C catalyst layer is about 1.61 Ωcm2 that is very similar to the reported data.15,

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After replacing Nafion with the synthesized

copolymers, RH+ values significantly decreased for all the three tested samples, indicating the enhanced proton conductivity in catalyst layers. It is also apparent that the proton diffusion resistance decreased with the increase in the ionic liquid segments in copolymers due to the increased density of ionic groups (also listed in Table 2).

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Figure 3

(a) Nyquist plots of impedance spectra normalized to electrode area

measured under N2-saturated 0.1M HClO4 for different catalyst layers. (b) Proton resistance RH+ of different catalyst layers calculated from (a). It has been proposed that for sulfonated hydrocarbon ionomers applied in electrode, ionomers with low ion exchange capacity value do not possess sufficient proton conductivity and ionomers with high ion exchange capacity value can lead to the decreased active area and constrained oxygen permeation, thus affecting electrocatalytic activity.13 To elucidate the effect of density of ionic groups of the synthesized PIL ionomers on the electrocatalytic activity towards ORR, CV curves of commercial Pt/C catalysts using the synthesized PIL ionomers or Nafion as proton conductor and binder were recorded from nitrogen-saturated 0.1 M HClO4 solutions at a scanning rate of 20 mVs-1, as shown in Figure 4a. Apart from the observed H adsorption/desorption peaks at the potential region from 0.05 to 0.4 V, the peak corresponding to the formation of oxygenated species at the surface of Pt was also observed at potential beyond 0.6 V and the coverage of oxygenated species (O-ad) was calculated through dividing the oxygenated species adsorption area (0.6 – 0.9 V) by the overall active surface area according to literatures.15,27-30 Figure 4b shows the accordingly calculated electrochemical surface area (ECSA) and the coverage of 13



oxygenated species (O-ad) from CV curves. It is apparent that the ECSA increased with the decrease in density of ionic groups whereas the O-ad significantly decreased with the decrease in density of ionic groups. Specifically, ECSA of poly(DMVBA3-TfO-coSt7)-Pt/C is about 82.4 m2g-1, slightly smaller than that of Nafion-Pt/C (87.7 m2g-1). However, the coverage of oxygenated species at the surface of Pt/C significantly decreased from 32.5% for Nafion-Pt/C to 22.8% for poly(DMVBA3-TfO-co-St7)-Pt/C. This result implies that PIL ionomers can form a protection layer on the surface of Pt/C catalysts probably through the electrostatic interaction between ionic moieties on ionomer and Pt. This was further confirmed by the variation of water contact angle results (Figure 4c). It can be seen that Pt/C catalyst is hydrophilic with water contact angle of about 15o. After incorporation with poly(DMVBA3-TfO-co-St7) which has water contact angle of about 94o, the water contact angle of the according catalyst changes to 126o, indicating the introduction of poly(DMVBA3-TfO-co-St7) does cause the significant change in the hydrophobicity of Pt/C. Since the adsorption of oxygenated species at the surface of Pt/C can severely suppress the ORR kinetics,27 poly(DMVBA3TfO-co-St7)-Pt/C may exhibit an enhanced ORR activity compared to Nafion-Pt/C catalysts.

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Figure 4

(a) CV curves of poly(DMVBAn-TfO-co-Stm)-Pt/C and Nafion-Pt/C in

N2-saturated 0.1M HClO4 solution at a scanning rate of 20 mVs−1. (b) The derived ECSA and coverage of oxygenated species (θO-ad) on the surface of Pt for different catalysts. (c) Water contact angles on Pt/C, poly(DMVBA3-TfO-co-St7), and poly(DMVBA3-TfO-co-St7)-Pt/C layers as indicated in the figure. The electrocatalytic activity of poly(DMVBAn-Tfo-co-Stm)-Pt/C and Nafion-Pt/C towards ORR was evaluated in oxygen-saturated 0.1 M HClO4 aqueous solutions using linear sweep voltammetry (LSV) at room temperature with a scanning rate of 20 mVs1

under rotating rate of 1600 rpm. Pt loading for all the samples is about 40 μgcm-2 and

ionomer loads are about 43.62 μgcm-2 for Nafion and 24.95 μgcm-2 for the synthesized ionomers. The according polarization curves were shown in Figure 5a. It is apparent that the ORR performance improved with the decrease in the fraction of ionic segments in the synthesized copolymers, as evidenced by the positive shift of half-wave potentials (E1/2).29 Moreover, the sample of poly(DMVBA3-TfO-co-St7)-Pt/C exhibits higher electrocatalytic activity towards ORR compared with Nafion-Pt/C catalysts whereas the other two samples have lower ORR activity compared to Nafion-Pt/C catalysts. The results demonstrate that ionic segments in the applied ionomers induced hydrophilicity of micro-environment is responsible for ORR activity while proton conductivity is sufficient in catalyst layers. With the increase in ionic segments of ionomers and the according hydrophilicity, the increased occupation of active sites of 15



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the Pt surface by nonreactive oxygenated species and the increased difficulty in oxygen permeation lead to the decrease in ORR activity. For Nafion-Pt/C catalysts, it has been reported that the loading of Nafion ionomers in catalysts layer has significant influence on the catalytic activity toward ORR and the according fuel cell performance.31-35 Therefore, the effect of PIL loadings on the ORR performance of PIL-Pt/C catalysts was further investigated using poly(DMVBA3-TfO-co-St7) as example proton conductor, as shown in Figure 5b. The loading of PIL ionomer was controlled by varying the concentration of ionomer solutions to keep the constant volume of catalyst ink. It can be seen that catalyst with ionomer loading of 24.95 μgcm-2 (corresponding to ionomer concentration of 3 wt.%) exhibited the best ORR performance among the tested samples. This can be qualitatively understood that the low ionomer loading provides insufficient proton conductivity and the high ionomer loading may affect the transport of oxygen through catalyst layer, as suggested for Nafion-Pt/C system.31,

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Thus, optimized ionomer

loading including Nafion according to literature and PILs were selected in the following study. Figure 5c displays the comparison of LSV curves for poly(DMVBA3-TfO-co-St7)Pt/C and Nafion-Pt/C catalysts. The observed positive shift of half-wave potential by 24 mV for poly(DMVBA3-TfO-co-St7)-Pt/C compared to Nafion-Pt/C suggests that poly(DMVBA3-TfO-co-St7)-Pt/C exhibits a decreased overpotential and an improved electrode reaction kinetics for ORR. The according specific activity (SA) and mass specific activity (MSA) were calculated by considering the current kinetics at 0.9V 16


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derived from LSV curves and ECSA of the applied catalysts to elucidate the difference in catalytic activity towards ORR,15, 36 as shown in Figure 5d. The SA and MSA values are 0.06 mAcmPt-2 and 0.052 AmgPt-1 for poly(DMVBA3-TfO-co-St7)-Pt/C, which are about 3.16 and 3.06 times higher than those for Nafion-Pt/C, respectively. Despite the same Pt loading, very similar proton conductivity, and ECSA, the ORR performance is quite different for poly(DMVBA3-TfO-co-St7)-Pt/C and Nafion-Pt/C. In addition to the suppressed occupation of non-reactive oxygenated species at surface of Pt, the inherent rigid structure induced by aromatic rings on ionomers is also favorable for oxygen transport to the surface of Pt, leading to improved ORR activity.37

Figure 5

LSV curves of different catalysts (a) and of poly(DMVBA3-TfO-co-St7)-

Pt/C with different ionomer loadings (b) recorded in O2-saturated 0.1 M HClO4 solution at a scanning rate of 20 mVs−1 under rotating speed of 1600 rpm. (c) LSV curves and (d) the calculated specific activity at 0.9 V for poly(DMVBA3-TfO-co-St7)17



Pt/C and Nafion-Pt/C recorded in O2-saturated 0.1 M HClO4 solution at a scanning rate of 20 mVs−1 under rotating speed of 1600 rpm. The stability of the synthesized poly(DMVBA3-TfO-co-St7)-Pt/C and Nafion-Pt/C catalysts was also evaluated by potential cycles in oxygen-saturated 0.1 M HClO4 solution at room temperature with a scan rate of 50 mVs-1. Figure 6a shows LSV curves of poly(DMVBA3-TfO-co-St7)-Pt/C and Nafion-Pt/C catalysts after 3000 potential cycles. It is apparent that the ORR performance of poly(DMVBA3-TfO-co-St7)-Pt/C is still higher than that of Nafion-Pt/C. However, the difference in half-wave potential significantly decreased from initial 24 mV to 8 mV, indicating the faster decay in ORR performance for poly(DMVBA3-TfO-co-St7)-Pt/C compared with Nafion-Pt/C. To understand such degradation behavior, ECSA values of both catalysts before and after 3000 CV cycles derived from CV curves (Figure 6b) were calculated and plotted in Figure 6c. It was found that Nafion-Pt/C loses about 67.4% of its initial ECSA whereas a loss of about 58.1% in initial ECSA for poly(DMVBA3-TfO-co-St7)-Pt/C was observed. However, the coverage of oxygenated species (Figure 6d) for poly(DMVBA3-TfO-co-St7)-Pt/C increased dramatically from initial 22.8% to 37.3% after 3000 CV cycles compared with Nafion-Pt/C, indicating the significant change of micro-environment for poly(DMVBA3-TfO-co-St7)-Pt/C. It should be noted that, although the coverage of oxygenated species for poly(DMVBA3-TfO-co-St7)-Pt/C is higher than that of Nafion-Pt/C catalysts, the high retention of ECSA of poly(DMVBA3-TfO-co-St7)-Pt/C makes the ORR performance still slightly higher than that of Nafion-Pt/C catalysts. The good ECSA retention rate for poly(DMVBA3-TfO18


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co-St7)-Pt/C was further confirmed by aggregation of Pt nanoparticles. The initial average size of Pt nanoparticles for both poly(DMVBA3-TfO-co-St7)-Pt/C and Nafion-Pt/C is about 2.30 nm (Figure 6e and 6f). After 3000 CV cycles, the average size of Pt nanoparticles increased to 5.01 nm for Nafion-Pt/C (Figure 6g) whereas it only increased to 3.58 nm for poly(DMVBA3-TfO-co-St7)-Pt/C (Figure 6h), indicating that aggregation of Pt nanoparticles during potential cycling is not the major factor for the decay of ORR performance of poly(DMVBA3-TfO-co-St7)-Pt/C. Since the model PIL ionomer poly(DMVBA3-TfO-co-St7)-Pt/C is a hydrocarbon polymer, the potential cycles induced degradation of ionomer which in turn affects the micro-environment around Pt/C catalysts should be responsible for the decay of ORR performance. Moreover, the proton diffusion resistance in the catalyst layer after CV cycles is very similar to the initial value, indicating that the degradation of the synthesized PILs started from the main chain during the tested CV cycles. The accordingly remained ionic liquid moieties can protect the Pt nanoparticles against aggregation and provide sufficient proton conductivity. Thus, we can propose that the stability issue might be solved with special design of the chemical structure of PIL ionomers, for example using perfluoronated PIL ionomers.

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Figure 6

LSV curves (a), CV curves (b), ECSA values (c), and coverage of

oxygenated species (d) of PIL-Pt/C and Nafion-Pt/C before and after 3000 CV cycles in oxygen-saturated 0.1 M HClO4 solution at a scanning rate of 50 mVs−1. Initial TEM images of Nafion-Pt/C (e) and PIL-Pt/C (f). TEM images of Nafion-Pt/C (g) and PILPt/C (h) after 3000 CV cycles. The insets in TEM images represents the size distribution of Pt nanoparticles in the image. 4. Conclusion We reported that the electrocatalytic activity of Pt/C catalysts towards ORR can be improved through replacement of Nafion ionomers with protic poly(ionic liquid) 20


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ionomers as proton conductor and binder in electrode. It was found that the increase in hydrophobicity of poly(ionic liquid) ionomers can lead to the suppressed adsorption of non-reactive oxygenated species on the surface of Pt and the enhanced oxygen transport through ionomers if the applied ionomers provide sufficient proton conductivity. In addition, the aromatic rings induced rigid structure of the synthesized poly(ionic liquid) ionomer also facilitate oxygen transport to the surface of Pt, leading to enhanced catalytic activity towards ORR. Although the applied poly(ionic liquid) ionomers can protect Pt nanoparticles against aggregation during potential cycles, the decomposition of hydrocarbon ionomers makes the according catalysts less durable compared to Nafion-Pt/C catalysts. Nevertheless, these findings may provide a new conceptual idea for design of the catalyst layer in cathode for low Pt-loading fuel cells.

Author Information Corresponding authors *E-mail:

[email protected] (H. Zhang) [email protected] (P. Fang)

ORCID Haining Zhang: 0000-0002-5546-2347 Pengfei Fang: 0000-0001-6836-0318 Haolin Tang:

0000-0003-1041-5376

Author Contribution 

X. Yan and F. Zhang contributed equally to this work. 21



Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the Natural Science Foundation of China under grant numbers of 21878239 and 21576216, and International Cooperation Project of Hubei Province (2017AHB057).

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Improving Oxygen Reduction Performance by Using Protic Poly(Ionic

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