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Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
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Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy Emmanuel O. Balogun a,*, Nabeel Hussain b, Jessica Chamier b, Paul Barendse a a b
Department of Electrical Engineering, University of Cape Town, Rondebosch, South Africa HySA Catalysis, Department of Chemical Engineering, University of Cape Town, Rondebosch, South Africa
highlights Aquivion® based catalyst-binder showed lower ohmic resistance compared to Nafion®. Aquivion® ionomer-based catalyst-binder showed higher ECSA compared to Nafion®. MEA degradation is more pronounced in the Nafion® ionomer compared to Aquivion®. Catalyst utilization was higher for Aquivion® compared to Nafion® ionomer.
article info
abstract
Article history:
Perfluorosulfonic acids (PFSA) are the most widely used ionomers in Polymer electrolyte
Received 27 August 2019
membrane fuel cells as both membrane and catalyst layer support. This study presents an
Received in revised form
electrochemical based analysis of using long-side-chain Nafion® and short-side-chain
8 October 2019
Aquivion® PFSA ionomers as binders in the catalyst-layers of PEMFC. Membrane elec-
Accepted 13 October 2019
trode assemblies were designed with consistent components, varying only the catalyst
Available online xxx
ink's ionomer type from 28 wt% Aquivion® to 28 wt% Nafion®. The durability and performance profile of the resulting catalyst-coated-membranes made from using these PFSA
Keywords:
ionomers as binders in the catalyst layers was investigated. Semi-empirical modeling
PFSA ionomers
shows that the Nafion® ionomer based binders give a catalyst-coated-membrane with a
Electrochemical impedance
46.6% higher ohmic resistance compared to using Aquivion® ionomers as binders. The
spectroscopy
power density analysis also showed that catalyst-coated-membranes made with Aqui-
Accelerated stress testing
vion® ionomer based binder gives an output power that is 22.33% higher than catalyst-
Catalyst coated membranes
coated-membranes made with Nafion® ionomers as binders in the catalyst layers. The
Polarization
Aquivion® ionomer binder also shows higher catalyst utilization compared to the Nafion®
Power curve
ionomer binders. The Electrochemical Impedance Spectroscopye Equivalent Circuit Model analysis showed that the membrane electrode assembly degradation is more pronounced in the Nafion® ionomer based binder compared to its Aquivion® counterpart. It is thus
* Corresponding author. E-mail address: [email protected] (E.O. Balogun). https://doi.org/10.1016/j.ijhydene.2019.10.079 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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shown that Aquivion® ionomers are better catalyst binders compared to the Nafion® ionomers as they result in catalyst-coated-membranes that are better performing and more durable. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increased attention given to fuel cell research and technology is driven by the need for a decarbonized, sustainable and renewable source of energy to meet 21st century energy demands. With over 80% of the fuel cell market, Polymer Electrolyte Membrane Fuel Cells (PEMFC) has been at the forefront of these investigations and the Membrane Electrode Assembly (MEA) the main focus of attention [1e3]. Per-Fluoro-Sulphonic-Acid (PFSA) ionomers have been singled out as the preferable ionomers for use in PEMFC applications to provide support, bind catalyst (Platinum) in the catalyst layer (CL) and provide proton conductivity. PFSA ionomers have extensive intrinsic chemical stability and super sulfonic acid strength which is core to the PEMFC proton conductivity. Until recently, Nafion®, which is a long-side chain (LSC) ionomer developed by Dupont in 1960, has been the most reliable ionomer for use in the PEMFC membranes as well as catalyst support [4]. In recent times, there have been several breakthroughs in designing other substitute ionomers, one of which is the short-side chain (SSC) ionomer made by Solvay Solexis, popularly called Aquivion® [6]. By shortening the length of the PFSA side chain it was possible to design an ionomer with higher mechanical stability [9,10]. The PFSA ionomers are added to the catalyst surface area in order to enhance proton conduction. PEMFC performance is dependent on the extent of its proton and electron conductivity and the proton conductivity is also dependent on degree of hydration. Thus, higher proton conductivity is achieved by a higher level of the humidity [4]. Ionomers used in the CL must allow for efficient water and gas transport. In the right quantity, ionomers enhance the PEMFC performance and in excess proportion they stand the possibility of inhibiting the PEMFC's performance by blocking the Pt sites. In this work, we will be comparing both the widely used long-side chain (LSC) Nafion® and the short side chain (SSC) Aquivion® PFSA ionomer MEAs under exposure to same degradation techniques and conditions. There have been some studies on the comparison of Nafion® and Aquivion® PFSA membranes but little information in literature that compares their performance when used as binders in the catalyst layers. Jeon et al. [12] investigated using advanced durability protocols, the MEAs composed of SSC (Aquivion®) and LSC (Nafion®) conducting polymer at the high temperature of 120 C and low humidity of 40%RH and they observed that Nafion®-based MEAs led to a greater increase in membrane resistance than Aquivion®-based MEAs. Skulimowska et al. [14] showed that a SSC Aquivion® PEM
based water electrolysers unlike its LSC Nafion® counterpart allows for high temperature operation up to 140 C, with lower H2 crossover and higher performance. Postnov et al. [5] described the behaviour of nanocomposites upon doping with Nafion® and Aquivion® ionomers. They found that doping with Aquivion® results in an increase in proton conductivity of hybrid membranes with low relative humidity. Gebert et al. [7] and Arico et al. [8] had earlier investigated the performance of the Nafion® membrane in contrast to the Aquivion® membrane at a stack level under high temperature application above 100 C. Following up on earlier works on high temperature fuel cells (HTFC 90e140 C), Peron et al. [13] showed in their work that Aquivion® membranes performed better than the Nafion® membranes in HTFC and at low RH (dry-20%) where there is no problem of flooding due to RH and water management as observed in low-temperature PEMFC. These studies suggest that SSC Aquivion® membranes during low RH operation, enhances water mobility, proton conductivity, and oxygen reduction reaction kinetics through selfhumidification in HTFCs [13,15]. Li et al. [16] in their work asserted that the improved chemical and mechanical stability observed in the SSC PFSA membrane is due to its shorter sidependant chain and absence of the ether group and tertiary carbon readily found in the LSC PFSA membrane. Furthermore, earlier reports by Pica et al. [17], Stassi et al. [18] and Xiao et al. [19], elucidated that the SSC Aquivion® PFSA membrane performs better under varying experimental conditions compared to the LSC Nafion® PFSA membrane. Lee et al. [20] reported that the improved performance observed in the SSC Aquivion® PFSA is not just based on its superior chemical and mechanical stability, but also largely a resultant effect of the optimized motion of water within the polymer. Thus, it was shown that the SSC PFSA membranes possess superior water sorption properties compared to the LSC Nafion® membranes. In this study, we investigate the influence of using LSC Nafion® and SSC Aquivion® based PFSA ionomers as binders in the CL on low-temperature PEMFC performance and durability. Catalyst coated membranes (CCMs) were made with either Nafion® or Aquivion® ionomer in the catalyst layer (CL). A comparative analysis of the performance and durability of LSC Nafion® and SSC Aquivion® ionomer based CCMs was made using a modified DOE accelerated stress test (AST) protocol, semi-empirical modeling, polarization and power curves, Electrochemical Impedance Spectroscopy (EIS) and Equivalent circuit modelling (ECM) analysis. The EIS is a powerful tool that allows a deep in-situ kinetic analysis of catalytic phenomena as well as the separation of different processes contributing to overpotential and the polarization curves. These methods determine the trend of
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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cell potential and the output power density as a function of the current density.
Experimental MEA preparation For this study, two sets of CCMs were prepared under the same conditions using equal weight percentage (%wt) for both the Aquivion® and Nafion® ionomers in the CL, referred to hereafter as Aquivion® CCM and Nafion® CCM respectively . A Gore® M735.8 membrane was used as the proton exchange membrane. The CCMs were prepared using commercial catalyst (HyPlat) with 40% Pt loading and ionomer content of 28 wt%, the type of ionomer used was however varied. The ionomers used in this study were Nafion D2021 and Aquivion D72-25BS. The resulting CCMs were sandwiched between two AvCarb MB30 GDLs with an uncompressed thickness of 0.200 mm. The loading of the CCMs were 0.1 mg/cm2 Pt on the anode and 0.4 mg/cm2 on the cathode. The MEAs were mounted on a 25 cm2 Baltic PEMFC single cell test fixtures. The test fixture consists of gold plated copper current collector plates at the anode and cathode side. The cell fixture offered 5 multi-channel serpentine gas flow field (1 mm 1 mm) with robust aluminium end-plates and titanium monopolar plates. The cell was operated at 80 C and the bubbler humidifier temperature was set at 80 C and 75 C at the anode and cathode respectively. The operating parameters were as follows: Anode Fuel: Hydrogen (99.999% purity), 1.5 Stoichiometry, 100% RH Cathode Oxidant: Air, 2.0 Stoichiometry, 81% RH Temperature (oC): 80 Pressures (bar): 1 bar
Physical characterization of the MEAs The prepared CCM's were characterized using scanning electron microscopy (SEM) to view the consistency of the electrode surface and to determine the layer thickness (Nova NanoSEM). X-ray photospectroscopy (XPS) was used to measure the CCM's surface composition major elements, fluoride, carbon, and platinum, were investigated before and after AST testing, using a PHI 5000 Scanning ESCA Microprobe. A 100 mm diameter monochromatic Al Ka x-ray beam (hn ¼ 1486.6 eV) generated by a 25 W, 15 kV electron beam was used to analyze the different binding energy peaks.
Semi-empirical modeling To further analyze these PFSA binders, basic electrochemistry was used to introduce a simple equation that fits the experimental data over the entire range of current density with optimum accuracy. The resulting equation as highlighted below was tested and proved to be accurate under several experimental conditions. Using experimental data derived
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with a 25 cm2 H2/air single cell using both Aquivion® and Nafion® based CCMs, we were able to analyze, quantify and contrast the degree of activation polarization, ohmic losses, and mass transfer losses in both CCMs. According to S. D. Fraser et al. [25] and from the investigation of the various empirical fitting models [21e24], the Yutaro's model [22] gave the best representation of the fuel cell's polarization curve and thereby used in this study. For fitting the measured data and the equations, Matlab Curve Fitting tool was used to perform regression analysis, using the non-linear least-squares method and Levenberg-Marquardt algorithm with a bisquare robust method to fit a line through a set of data.
EIS analysis of prepared membranes An in-built FuelWork software on the FuelCon Evaluator-C test station was used for the cell analysis. EIS analysis was performed using a TrueData-EIS which is a high current AC impedance meter with a maximum DC of 1000A. The FuelWork software on the Evaluator test station automatically applied control of the fuel cell's operating conditions. The current and the voltage range depends on the impedance of the test item. To an impedance of 800 mU, the maximum modulation voltage of the test item is 20 mV (2% of offload voltage of a single cell). The impedance data were taken using TrueData-EIS software which preformed galvanostatic impedance measurements over a frequency range of 0.1 Hze30 kHz. During analysis, the system draws a small direct current (DC) while superimposing a small magnitude of alternating current (AC) e about 10% of the DC value. The EIS measurement was repeated for current density values of 100 mA/cm2, 300 mA/ cm2, 800 mA/cm2 and 1000 mA/cm2 for both Aquivion® and Nafion® CCMs. These setpoints were chosen to represent different sections of the polarization curves. The data were then fitted to the equivalent circuit model (ECM) described in Fig. 2 with the help of the EC-Lab software. The EC-lab software uses a combination of Randomize and Simplex leastsquare nonlinear fitting algorithm to determine the path of best fits for different data points.
Accelerated stress test To determine the durability of both the Nafion® and Aquivion® CCMs, AST protocol was applied as shown in Table 1. The procedure was modified from the DOE protocol [26]. This modified DOE protocol involves a series of relative humility (R.H) cycling at both wet and dry conditions simultaneously with load cycling of the MEA under very low and high current loadings. The cell was operated at 80 C and the bubbler humidifier temperature was cycled between wet condition at 80 C at the cathode inlet and 75 C at the anode inlet, while the dry conditions relative humidity was at 25 C. The cell was cycled between alternating loads of 1.2 and 0.02 A/cm2 each for 5 cycles and between 5 cycles of 0.1 and 0.02 A/cm2 for the dry relative humidity cycle. Each relative humidity cycle between wet and dry conditions lasted for 10 min and the process was repeated over 24 h. A 2 min transition time was allowed while operating the cell at 0.6 A/cm2 current density
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Table 1 e Modified Drive-Cycle Durability Protocol [26]. Test Point #
Anode Cathode Test Point Run Current Time density (A/ Inlet Temp Inlet Temp (oC) (oC) (Minutes) cm2)
Wet with Voltage Cycling RH1 0.02 RH2 1.2 RH3 0.02 RH4 1.2 RH5 0.02 RH6 1.2 RH7 0.02 RH8 1.2 RH9 0.02 RH10 1.2 Trans 0.6 1 Dry With Load Cycling RH11 0.1 RH12 0.02 RH13 0.1 RH14 0.02 RH15 0.1 RH16 0.02 RH17 0.1 RH18 0.02 RH19 0.1 RH20 0.02
75 75 75 75 75 75 75 75 75 75 75
80 80 80 80 80 80 80 80 80 80 80
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2.0
25 25 25 25 25 25 25 25 25 25
25 25 25 25 25 25 25 25 25 25
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 5.0
to allow for some transition between the wet and dry RH cycle. The 5 min hold in step RH20 as observed in Table 1 was intended to represent a system idle point to represent the end of a complete cycle and set the pace for a new cycle. The DOE drive-cycle testing reflects only degradation losses associated with wet and dry cyclic operation. However, in the modified AST, an additional stress was placed on the MEA because of load cycling. The load cycling employed in this protocol may have a minor effect on the catalyst [31] however, in this study, the focus is to provide a holistic analysis of the MEA degradation.
The ECM in Fig. 1 is an extension of the Randles cell for a circuit with mixed kinetic and charge transfer control. Like the Randles circuit, RU corresponds to the cell's ohmic resistance connected in series with two parallel combinations of CPEdl, A, - Rct,A, and CPEdl,C - Rct, C, representing the distributed doublelayer capacitive effects and the charge transfer resistance at the anode and cathode respectively. An inductive element L was connected in parallel with the Warburg element (W) to model the low-frequency inductive effect and mass action respectively.
Results and discussions SEM images of the CCMs indicates that the anode and cathode layers of the Aquivion® and Nafion® ionomer based catalyst layers (CLs) were of similar thickness. The average thickness of the cathode was at 11.5 mm and that of the anode at 4.2 mm. This similar thickness allows for comparison of the Ohmic resistances due to similar electronic pathlength in the CL. Thicker layers generally imply higher Ohmic resistance. The morphology of the electrode layers for the respective CCMs was smooth, with no visible cracking of defects. The quality of the MEAs allowed for further comparative electrochemical evaluations. Both Nafion® and Aquivion® CCMs require adequate humidification to maintain proton conductivity through the MEA's operation life-cycle [23,27]. The conditioning of the MEAs occur at relatively high humidity (80e100%), and studies have shown that Aquivion® demonstrates a higher charge carrier concentration and increased proton conductivity compared to Nafion® at these operating conditions [15]. Nafion® CCMs are likely to normalize to their peak performance quicker compared to Aquivion® CCMs. The longer side chain ionomer is prone to swelling, which will result in the quicker hydration of the catalyst layer, thus, resulting in accelerated maximization and stabilization of the CCM's performance.
Polarization and power curve analysis Equivalent circuit model (ECM) An equivalent circuit can be used to represent an electrode/ electrolyte interface when it is analyzed by EIS. For PEMFC analysis, the Randles Cell [11] is the most common type of electronic circuit used for EIS analysis. In this work, an ECM based on the simplified Randles Cell Schematic was adapted.
The performance of the Aquivion® CCM was compared to the Nafion® CCM using polarization curves determined at the specified operating conditions. Fig. 3 presents the beginning of life (BoL) results of the polarization curves for the two CCMs. The OCV values of the Nafion® CCM was 0.968 V which is higher than the Aquivion® CCM's OCV value of 0.948 V.
Fig. 1 e Equivalent circuit model used for EIS fitting [11]. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 2 e SEM images illustrating the surface morphology and the layer thickness of the anode and cathode layers on the (A) Aquivion® and (B) Nafion® based CL CCM.
However, upon the application of load, as observed in Fig. 3(a), the Aquivion® CCM shows superior performance to its Nafion® counterpart at 0.65V. Similarly, the power curve analysis from Fig. 3(b) shows that the Aquivion® CCM has a maximum power output of 587 mW/cm2 (14.68 W) which is quite higher compared to the Nafion® CCM maximum power output of 480 mW/cm2 (12 W). To better understand the difference in performance, semi-empirical modelling was performed as discussed in the section below.
Semi-empirical modeling Fig. 4 shows the Aquivion® and Nafion® ionomer based CL CCM's polarization curves fitted to Yutaro's model for a cell operated at 80 C. The obtained regression parameters fitted for these models are listed in Table 2. The Yutaro’ semiempirical model was a good fit for the data, and the correlation coefficients Rcorr were greater than 0.99. The ohmic resistance, Rohmic, formed the core basis of the comparison. The Rohmic of the Nafion® CCM was greater than that of
Aquivion® CCM, indicating higher ohmic losses, and therefore lower performance, for the Nafion® CCM. This result suggests that the Aquivion® ionomer based catalyst binder offers enhanced proton conductivity due to its shorter chain, resulting in lower protonic resistance in the catalyst layers compared to the LSC Nafion® based catalyst support MEA [7]. The activation loss value a in the Yutaro's model for Nafion® CCM was higher compared to the Aquivion® CCM, indicating that the activation overpotential loss is higher for the Nafion® compared to the Aquivion® CCM. As activation is generally related to the catalyst, this would imply that the latter has better surface interface with the catalyst, allowing for the exposure of higher catalyst surface area. Similarly, from Table 2, the concentration loss parameters m and n showed that the Nafion® CCM suffered higher mass transport losses compared to the Aquivion® CCM. This means that the reactants will be delivered to the catalyst sites with less inhibition using an Aquivion® ionomer compared to using a Nafion® ionomer in the CL.
Fig. 3 e (a) Polarization Curve (b) Power curve Analysis of Nafion® and Aquivion® ionomer based CL e CCMs. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 4 e Polarization curve model fitting for the (a) Aquivion ® ionomer based CL (b) Nafion® ionomer based CL using Yutaro’s model.
EIS-ECM modeling The Nyquist plots for both Aquivion® and Nafion® CCMs are shown in Fig. 5. Extracted data from the Nyquist plot presented in Table 3 shows that membrane resistance (Rm) decreases with increasing load from 2.5A (100 mA/cm2) to 25A (1000 mA/cm2) across both CCMs. This is most likely attributed to the resulting increase in charge transport due to an increase in cell's thermodynamics at higher current density values. The back diffusion of water taking place at high current density also increases the water content in the membrane, hence leading to an increase in membrane humidity. Since the membrane resistance is the main contributor to the ohmic resistance, a well-hydrated CCM will result in an overall reduction in electronic and protonic resistance of the cell. The Aquivion® CCM has a lower ohmic resistance compared to the Nafion® based CCM, indicating that the Aquivion® CCM has an overall higher conductivity. The anodic CPEdl,A and the Rct,A values are gotten at higher frequencies compared to their cathodic counterpart. This is because the electrochemical rate at the cathode is slow compared to that of the anode, as a result, the RC time constant of the oxygen reduction reaction at cathode has a higher Rct,C. The reaction kinetics of the charge transfer at the anode and cathode cannot be differentiated by just looking at the EIS impedance arc, as the cathode and anode arcs overlap with the cathode arc dominating the whole process [28]. From the extracted parameter values summarized in Table 3, it was observed that Rct,A is small compared to Rct,C for the entire load current range. From the merged cathode and anode impedance arcs, the effective fuel cell charge transfer resistance (Rct) is the sum of both, i.e.
Rct ¼ Rct;A þ Rct;C
(6)
®
The Nafion CCM has a larger charge transfer resistance compared to the Aquivion® CCM, indicating that the Aquivion® CCM boasts a comparatively faster reaction kinetics. The Nafion® CCMs shows a higher mass transport impedance compared to the Aquivion® CCMs, thus, indicating longer timeframes for reactants to move and penetrate further through the GDLs [28]. Overall, EIS-ECM analysis shows that using Aquivion® ionomer binders in the CL makes better CCMs that perform better in comparison to using Nafion® ionomer binders in the CL. The Aquivion ionomer based CL CCMs have lower values for the ohmic losses, charge transfers losses and mass transfer losses.
Durability studies From previous studies [29], R.H cycling has many complex effects on the PEMFC membrane durability since both flooding and drying contribute to PEMFC performance loss. Cycling the cell at low humidification is detrimental to the membrane, causing higher chemical degradation of the membrane and the lack of water makes the membrane brittle and fragile. Excessive humidification, however, under the low current density condition can lead to excess water percolation during the fuel cell operation causing flooding of the membrane and catalyst layers and blockage of active reactant diffusion sites. Fig. 6 shows the potential-plot for the wet load cycle. Each R.H cycle has 5 cyclic load loops between 0.02 A/cm2 and 1.2 A/cm2, with the potential recorded at the lower current density of each cycle. Fig. 7 shows the potential - time plot for
Table 2 e Yutaro’s model fitting for MEAs made for the PFSA ionomers.
Aquivion Nafion®
®
Rcorr
Rohmic [Ucm2]
a [V]
b [V]
m [V]
n [cm2/A]
0.9986 0.9988
0.1792 0.2258
4.816e-05 3.213e-05
0.001021 0.0008955
0.01566 0.0169
1.76 2.153
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 5 e EIS Nyquist plots determined for Nafion® and Aquivion® ionomer based CL CCMs at (a) 100 mA/cm2, (a) 300 mA/cm2, (a) 800 mA/cm2 and (a) 1000 mA/cm2 (fit data were obtained using the ECM in Fig. 1).
the dry load cycle. During the dry load cycle, the first load cycle is still somewhat humid as the membrane and catalyst layers may have retained some water carried over from the wet load cycle operation. With continual operation in the dry condition, the membrane and catalyst layers loose humidity resulting in decreased performance. From Figs. 6 and 7, it is observed that after the AST, the Aquivion® CCM outperformed the Nafion® CCM both in the wet and dry load cycle conditions.
The combined effect of the R.H and load cycling significantly degraded the CCMs in 24 h over a 40 cyclic period of wet and dry R.H conditions. The polarization curve plot in Fig. 8(b) shows that the Aquivion® CCM is also more durable than the Nafion® CCM after being exposed to AST conditions. Fig. 8(a) shows an overview of percentage degradation per voltage point of both CCMs before and after the AST protocol. The degree of degradation is very likely related to the stability and the water-retaining capacity of the ionomers. As
Table 3 e Fitted values for the EIS e ECM data, (Rm (U) is gotten from the high frequency (HF) intercept with the x axis, Rct, A(U) þ Rct, C (U) is the circumference of the semicircle. Cdl, A (mF) þ Cdl, C (mF) is gotten from the fit values, but can be calculated by finding inverse of the product of maximum frequency and Rct. L(H) is fitted parameter indicating losses as a result of wiring inductances, often observed when the Nyquist plot extends beyond 0 and Zw is the warburg inductance derived from fitting ECM circuit.). Parameter Aquivion Nafion Aquivion Nafion Aquivion Nafion Aquivion Nafion
Current density (mA/cm2)
Rm (U)
Rct, A(U)
Rct, C (U)
100 100 300 300 800 800 1000 1000
2.844 3.101 2.816 3.021 2.160 2.350 2.167 2.272
2.298 12.000 3.058 3.389 3.449 4.763 3.297 3.601
16.470 18.410 10.320 11.77 19.540 23.060 34.21 47.49
Cdl,
A
(mF)
0.161 1.148 3.003e3 0.831e3 2.289e3 3.651e3 0.718 1.063
Cdl,
C
(mF)
1.213 3.599 0.943 3.599 1.479 1.033 1.625 1.045
L(H)
Zw
0.081 0.059 0.157 0.277 0.218 0.443 0.238 0.388
1.340 5.162 8.072 8.763 4.322 7.199 7.337 9.420
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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0.9 0.89
Voltage (V)
0.88 0.87 0.86 0.85 0.84 0.83 14:24:00
19:12:00
00:00:00
04:48:00
09:36:00
14:24:00
19:12:00
Time (t) Nafion Aquivion Fig. 6 e Wet Load Cycling Voltage-Time Response on the Aquivion® and Nafion® ionomer based CL CCMs.
0.86 0.84
Voltage (V)
0.82 0.8 0.78 0.76 0.74 14:24:00
19:12:00
00:00:00
04:48:00
09:36:00
14:24:00
19:12:00
Time (t) Aquivion
Nafion
Fig. 7 e Dry Load Cycling Voltage-Time Response on the Aquivion® and Nafion® ionomer based CL CCMs.
Fig. 8 e (a) Percentage voltage loss after AST at each current density point. (b) Polarization curve analysis of Aquivion® and Nafion® ionomer based CL CCMs before and after the AST test completed after 24 h.
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 9 e EIS Nyquist plot for Aquivion® and Nafion® at (a) 100 mA/cm2 and (b) 300 mA/cm2 (fit data were obtained using the ECM in Fig. 1).
Table 4 e Fitted values for EIS e ECM data (Rm (U) is gotten from the high frequency (HF) intercept with the x axis, Rct, A(U) þ Rct, C (U) is the circumference of the semicircle. Cdl, A (mF) þ Cdl, C (mF) is gotten from the fit values, but can be calculated by finding inverse of the product of maximum frequency and Rct. L(H) is fitted parameter indicating losses as a result of wiring inductances, often observed when the Nyquist plot extends beyond 0 and Zw is the warburg inductance derived from fitting ECM circuit.). Parameter Aquivion Nafion Aquivion Nafion
Current density (mA/cm2)
Rm(U)
Rct, A (U)
Rct, C (U)
100 100 300 300
8.254 9.454 2.652 7.355
11.06 15.25 13.51 8.437
32.44 47.4 14.27 25.18
pointed out by Park et al. [30], The Aquivion® ionomer has higher water retention capacity, higher crystallinity and a higher glass transition temperature (Tg) compared to the Nafion® ionomer. Attack on the per-fluorocarbon backbone side chain of the ionomer is the core of the membrane and ionomer degradation. This loss of sulfonic acid sites leads to a reduction in membrane and catalyst layer proton conductivity. The easier
Cdl,
A
(mF)
21.55e3 26.58e3 11.81e6 8.602e6
Cdl,
C
(mF)
0.76 1.033 0.106 0.362
L(H)
Zw
0.314 0.483 0.318 0.325
3.657 12.43 3.101 24.12
this side chain is broken, the more severe and faster the rate of degradation. From the above comparative analysis, it can be inferred that the short-side chain Aquivion® based ionomer catalyst support has a stronger bond with its per-fluorocarbon backbone as compared with the long-side chain Nafion® based catalyst support MEA, thus, the reason for an extended lifecycle after undergoing the R.H-load cycling AST.
EIS e ECM modelling for ionomer catalyst support characterization after AST Fig. 9(a) and (b) shows the Nyquist plots of the Nafion® and Aquivion® ionomer based CL CCMs taken after the AST. Table 4 shows the fitted parameters from the ECM modeling, and it is observed that ohmic resistance (RU) decreases with increasing load from 4A (100 mA/cm2 Current density) to 12A (300 mA/cm2 Current density) across both MEAs. After exposure to AST through R.H and load cycling of the cell, the Aquivion® ionomer based CL CCM shows a lower ohmic resistance compared to the Nafion® CL MEA, hence indicating higher conductivity of the Aquivion® based CL MEA.
Table 5 e CV scan data showing ECSA of Nafion® and Aquivion® ionomer based CL CCMs. Ionomer Type Fig. 10 e CV Scan analysis of Aquivion® and Nafion® ionomer based CL CCMs.
®
Nafion Aquivion®
ECSA (m2/g Pt) 64.30 87.70
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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From the extracted parameter values summarized in Table 4, it was observed that Rct,A is smaller compared to Rct, C. From the merged cathode and anode impedance arcs, the effective fuel cell charge transfer resistance (Rct) is the sum of both, i.e. Rct¼ Rct,Aþ Rct,C. After undergoing degradation, the Nafion® ionomer based CL CCM has a larger charge transfer resistance compared to the Aquivion® ionomer, thus indicating that the Aquivion® ionomer based CL CCM is more durable, possessing longer lifecycle under continuous operation. The corresponding decrease in the Capacitive layers (Cdl) for both sets of MEAs strongly suggest a decrease in both electronic and proton conductivity, likely related to the water content, in the CL. Aquivion® CL MEAs presented a better electro-catalytic activity due to lower Zw values, which is on average 3 times higher after AST testing. The lower resistance to mass transfer, can infer that there is a higher rate of catalyst utilization in the Aquivion® ionomer based CL CCM compared to the Nafion® ionomer based CCM. The most significant difference however, is observed in the Rm after AST for both MEAs at 300 mA/cm2. The Aquivion® ionomer based CL CCM also displays a better electrocatalytic activity due to lower Zw values. Fig. 10 and Table 5 shows the cyclic voltammetry scan (CV) of both PFSA Ionomer CCMs. The electrochemical
active surface area (ECSA) of the CL made from using Aquivion® ionomer as a binder is observed to be higher compared to CL made from using Nafion® ionomer. The shorter side chains of the Aquivion® ionomer facilitates rapid gas access to the reaction sites in comparison to the longer side chain Nafion® ionomer. The longer the side chain of the PFSA ionomer, the more the pore spaces in the catalyst layer are filled, proton conductivity increases, but catalyst sites become blocked to gas. This is the reason for the lower ECSA observed for the LSC Nafion® ionomer compared to the SSC Aquivion® ionomer binder in CL. This result is in agreement with earlier results presented by Garsany et al. [36]. Where they showed that the mesopore volume of the SSC ionomers based CL is almost 4 times higher than that of the LSC ionomers based CL. As confirmed in earlier works by Wong and Eikerling [32,33], the larger primary pores of the SSC ionomers based CL is an indication of more hydrophobicity, which translates to improved reactant transport and water removal. Thus, improved ECSA observed is an indication of higher catalytic utilization which leads to increase in electrochemical activity of the cell and also higher Pt/C in contact with the ionomer when Aquivion® ionomers are used as catalyst binders. After undergoing AST, the Aquvion® CCM
Fig. 11 e High-resolution XPS spectra for Pt 4f, C 1s, and F 1s signals before and after AST for the Aquivion® (a,c,e) and Nafion® (b, d,f) in the MEA catalyst layer. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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experienced an 18% reduction in its ECSA (71.3 m2/g Pt) while the Nafion® CCM experienced a 20% reduction in its ECSA (52.51m2/g Pt). Thus, the modified DOE AST protocol impacted on both the CL and the ionomers. The conductivity of PFSA membranes is found to depend on hydration [34], and the length of the side chain of the PFSA has been shown in earlier studies to impact the level of hydration of the CL. Thus, the above result is in agreement with previous report that stated that higher micropore volume for the CCMs with the SSC based CL indicates less filling of the pores in CL, thus giving room for faster gas and water transport [36]. The XPS measurements were carried out to investigate the electrode surface composition. The XPS wide scans showed the presence of the outermost layers of platinum, carbon, and fluorine. Pt 4f doublet peaks at 71.2 and 74.6 eV are attributable to Pt 4f7/2 and Pt 4f5/2, respectively. These peaks can be assigned to Pt0, i.e. Pt in the metallic state. It is evident that the AST test didn't have a significant impact on the oxidation state of the catalyst on either of the coated CCMs. The C 1s peak corresponding to CeC binding (284.3 eV) on the respective MEAs (Fig. 11(c) and d) was also consistent, increasing slightly on the Nafion® ionomer based CL CC. This slight increase is likely due to minor variation in the coating of the CCMs. The consistent Pt4f and CeC binding peaks for the CCMs strongly suggest that the carbon support materials remained relatively stable during testing. The CeF2 peak, however, disappeared after testing. The F1s signal in the XPS-spectra decreased after AST significantly from 688.3 eV to 687.5 V confirming extensive oxidation of the CF2 functional groups to CHFO and CFO functionalities for both MEAs. The Pt 4f signal shifted to slightly higher binding energies likely indicating the metallic Pt being oxidized to the PtO and PtO2 species [35]. As the catalyst support remained stable with a marginal increase in oxidation of the catalyst, it is suggested that the major component which underwent degradation in the CL was the ionomer, in the case of both Nafion® as well as Aquivion®.
Conclusion The empirical modelling analysis of the PEMFC shows that ohmic resistance for the Nafion® ionomer based CL CCM is greater than the Aquivion® ionomer based CL CCM's ohmic resistance. The Nafion® ionomer based catalyst support exhibits lower proton conductivity and thus a reduced performance compared to the Aquivion® ionomer based CL CCM. Both the polarization curve and EIS analysis of the CCMs showed that the Aquivion® ionomer catalyst support performs better and is also more durable than the Nafion® ionomer CL CCM. The CV analysis of both PFSA ionomers shows that the Aquivion® ionomer binders result in a CCM with a larger ECSA compared to the Nafion® ionomer based binder. Also, The Nafion® based catalyst support CCM has a larger charge transfer resistance compared to the Aquivion® based catalyst support, indicating that the Aquivion® ionomer boasts lower charge transfer losses and faster reaction kinetics. The effect of degradation was more pronounced on the
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Nafion® ionomer based binders compared to the Aquivion® ionomer based binders. Hence it can be concluded from observed results that the Aquivion® ionomer is a better and more durable option as a binder in the catalyst layers of lowtemperature PEMFCs.
Aknowlegement This project was funded by the Mandela Rhodes Foundation (MRF PG Scholarship) and Hydrogen South Africa (HySA catalysis).
references
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Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy Emmanuel O. Balogun a,*, Nabeel Hussain b, Jessica Chamier b, Paul Barendse a a b
Department of Electrical Engineering, University of Cape Town, Rondebosch, South Africa HySA Catalysis, Department of Chemical Engineering, University of Cape Town, Rondebosch, South Africa
highlights Aquivion® based catalyst-binder showed lower ohmic resistance compared to Nafion®. Aquivion® ionomer-based catalyst-binder showed higher ECSA compared to Nafion®. MEA degradation is more pronounced in the Nafion® ionomer compared to Aquivion®. Catalyst utilization was higher for Aquivion® compared to Nafion® ionomer.
article info
abstract
Article history:
Perfluorosulfonic acids (PFSA) are the most widely used ionomers in Polymer electrolyte
Received 27 August 2019
membrane fuel cells as both membrane and catalyst layer support. This study presents an
Received in revised form
electrochemical based analysis of using long-side-chain Nafion® and short-side-chain
8 October 2019
Aquivion® PFSA ionomers as binders in the catalyst-layers of PEMFC. Membrane elec-
Accepted 13 October 2019
trode assemblies were designed with consistent components, varying only the catalyst
Available online xxx
ink's ionomer type from 28 wt% Aquivion® to 28 wt% Nafion®. The durability and performance profile of the resulting catalyst-coated-membranes made from using these PFSA
Keywords:
ionomers as binders in the catalyst layers was investigated. Semi-empirical modeling
PFSA ionomers
shows that the Nafion® ionomer based binders give a catalyst-coated-membrane with a
Electrochemical impedance
46.6% higher ohmic resistance compared to using Aquivion® ionomers as binders. The
spectroscopy
power density analysis also showed that catalyst-coated-membranes made with Aqui-
Accelerated stress testing
vion® ionomer based binder gives an output power that is 22.33% higher than catalyst-
Catalyst coated membranes
coated-membranes made with Nafion® ionomers as binders in the catalyst layers. The
Polarization
Aquivion® ionomer binder also shows higher catalyst utilization compared to the Nafion®
Power curve
ionomer binders. The Electrochemical Impedance Spectroscopye Equivalent Circuit Model analysis showed that the membrane electrode assembly degradation is more pronounced in the Nafion® ionomer based binder compared to its Aquivion® counterpart. It is thus
* Corresponding author. E-mail address: [email protected] (E.O. Balogun). https://doi.org/10.1016/j.ijhydene.2019.10.079 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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shown that Aquivion® ionomers are better catalyst binders compared to the Nafion® ionomers as they result in catalyst-coated-membranes that are better performing and more durable. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increased attention given to fuel cell research and technology is driven by the need for a decarbonized, sustainable and renewable source of energy to meet 21st century energy demands. With over 80% of the fuel cell market, Polymer Electrolyte Membrane Fuel Cells (PEMFC) has been at the forefront of these investigations and the Membrane Electrode Assembly (MEA) the main focus of attention [1e3]. Per-Fluoro-Sulphonic-Acid (PFSA) ionomers have been singled out as the preferable ionomers for use in PEMFC applications to provide support, bind catalyst (Platinum) in the catalyst layer (CL) and provide proton conductivity. PFSA ionomers have extensive intrinsic chemical stability and super sulfonic acid strength which is core to the PEMFC proton conductivity. Until recently, Nafion®, which is a long-side chain (LSC) ionomer developed by Dupont in 1960, has been the most reliable ionomer for use in the PEMFC membranes as well as catalyst support [4]. In recent times, there have been several breakthroughs in designing other substitute ionomers, one of which is the short-side chain (SSC) ionomer made by Solvay Solexis, popularly called Aquivion® [6]. By shortening the length of the PFSA side chain it was possible to design an ionomer with higher mechanical stability [9,10]. The PFSA ionomers are added to the catalyst surface area in order to enhance proton conduction. PEMFC performance is dependent on the extent of its proton and electron conductivity and the proton conductivity is also dependent on degree of hydration. Thus, higher proton conductivity is achieved by a higher level of the humidity [4]. Ionomers used in the CL must allow for efficient water and gas transport. In the right quantity, ionomers enhance the PEMFC performance and in excess proportion they stand the possibility of inhibiting the PEMFC's performance by blocking the Pt sites. In this work, we will be comparing both the widely used long-side chain (LSC) Nafion® and the short side chain (SSC) Aquivion® PFSA ionomer MEAs under exposure to same degradation techniques and conditions. There have been some studies on the comparison of Nafion® and Aquivion® PFSA membranes but little information in literature that compares their performance when used as binders in the catalyst layers. Jeon et al. [12] investigated using advanced durability protocols, the MEAs composed of SSC (Aquivion®) and LSC (Nafion®) conducting polymer at the high temperature of 120 C and low humidity of 40%RH and they observed that Nafion®-based MEAs led to a greater increase in membrane resistance than Aquivion®-based MEAs. Skulimowska et al. [14] showed that a SSC Aquivion® PEM
based water electrolysers unlike its LSC Nafion® counterpart allows for high temperature operation up to 140 C, with lower H2 crossover and higher performance. Postnov et al. [5] described the behaviour of nanocomposites upon doping with Nafion® and Aquivion® ionomers. They found that doping with Aquivion® results in an increase in proton conductivity of hybrid membranes with low relative humidity. Gebert et al. [7] and Arico et al. [8] had earlier investigated the performance of the Nafion® membrane in contrast to the Aquivion® membrane at a stack level under high temperature application above 100 C. Following up on earlier works on high temperature fuel cells (HTFC 90e140 C), Peron et al. [13] showed in their work that Aquivion® membranes performed better than the Nafion® membranes in HTFC and at low RH (dry-20%) where there is no problem of flooding due to RH and water management as observed in low-temperature PEMFC. These studies suggest that SSC Aquivion® membranes during low RH operation, enhances water mobility, proton conductivity, and oxygen reduction reaction kinetics through selfhumidification in HTFCs [13,15]. Li et al. [16] in their work asserted that the improved chemical and mechanical stability observed in the SSC PFSA membrane is due to its shorter sidependant chain and absence of the ether group and tertiary carbon readily found in the LSC PFSA membrane. Furthermore, earlier reports by Pica et al. [17], Stassi et al. [18] and Xiao et al. [19], elucidated that the SSC Aquivion® PFSA membrane performs better under varying experimental conditions compared to the LSC Nafion® PFSA membrane. Lee et al. [20] reported that the improved performance observed in the SSC Aquivion® PFSA is not just based on its superior chemical and mechanical stability, but also largely a resultant effect of the optimized motion of water within the polymer. Thus, it was shown that the SSC PFSA membranes possess superior water sorption properties compared to the LSC Nafion® membranes. In this study, we investigate the influence of using LSC Nafion® and SSC Aquivion® based PFSA ionomers as binders in the CL on low-temperature PEMFC performance and durability. Catalyst coated membranes (CCMs) were made with either Nafion® or Aquivion® ionomer in the catalyst layer (CL). A comparative analysis of the performance and durability of LSC Nafion® and SSC Aquivion® ionomer based CCMs was made using a modified DOE accelerated stress test (AST) protocol, semi-empirical modeling, polarization and power curves, Electrochemical Impedance Spectroscopy (EIS) and Equivalent circuit modelling (ECM) analysis. The EIS is a powerful tool that allows a deep in-situ kinetic analysis of catalytic phenomena as well as the separation of different processes contributing to overpotential and the polarization curves. These methods determine the trend of
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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cell potential and the output power density as a function of the current density.
Experimental MEA preparation For this study, two sets of CCMs were prepared under the same conditions using equal weight percentage (%wt) for both the Aquivion® and Nafion® ionomers in the CL, referred to hereafter as Aquivion® CCM and Nafion® CCM respectively . A Gore® M735.8 membrane was used as the proton exchange membrane. The CCMs were prepared using commercial catalyst (HyPlat) with 40% Pt loading and ionomer content of 28 wt%, the type of ionomer used was however varied. The ionomers used in this study were Nafion D2021 and Aquivion D72-25BS. The resulting CCMs were sandwiched between two AvCarb MB30 GDLs with an uncompressed thickness of 0.200 mm. The loading of the CCMs were 0.1 mg/cm2 Pt on the anode and 0.4 mg/cm2 on the cathode. The MEAs were mounted on a 25 cm2 Baltic PEMFC single cell test fixtures. The test fixture consists of gold plated copper current collector plates at the anode and cathode side. The cell fixture offered 5 multi-channel serpentine gas flow field (1 mm 1 mm) with robust aluminium end-plates and titanium monopolar plates. The cell was operated at 80 C and the bubbler humidifier temperature was set at 80 C and 75 C at the anode and cathode respectively. The operating parameters were as follows: Anode Fuel: Hydrogen (99.999% purity), 1.5 Stoichiometry, 100% RH Cathode Oxidant: Air, 2.0 Stoichiometry, 81% RH Temperature (oC): 80 Pressures (bar): 1 bar
Physical characterization of the MEAs The prepared CCM's were characterized using scanning electron microscopy (SEM) to view the consistency of the electrode surface and to determine the layer thickness (Nova NanoSEM). X-ray photospectroscopy (XPS) was used to measure the CCM's surface composition major elements, fluoride, carbon, and platinum, were investigated before and after AST testing, using a PHI 5000 Scanning ESCA Microprobe. A 100 mm diameter monochromatic Al Ka x-ray beam (hn ¼ 1486.6 eV) generated by a 25 W, 15 kV electron beam was used to analyze the different binding energy peaks.
Semi-empirical modeling To further analyze these PFSA binders, basic electrochemistry was used to introduce a simple equation that fits the experimental data over the entire range of current density with optimum accuracy. The resulting equation as highlighted below was tested and proved to be accurate under several experimental conditions. Using experimental data derived
3
with a 25 cm2 H2/air single cell using both Aquivion® and Nafion® based CCMs, we were able to analyze, quantify and contrast the degree of activation polarization, ohmic losses, and mass transfer losses in both CCMs. According to S. D. Fraser et al. [25] and from the investigation of the various empirical fitting models [21e24], the Yutaro's model [22] gave the best representation of the fuel cell's polarization curve and thereby used in this study. For fitting the measured data and the equations, Matlab Curve Fitting tool was used to perform regression analysis, using the non-linear least-squares method and Levenberg-Marquardt algorithm with a bisquare robust method to fit a line through a set of data.
EIS analysis of prepared membranes An in-built FuelWork software on the FuelCon Evaluator-C test station was used for the cell analysis. EIS analysis was performed using a TrueData-EIS which is a high current AC impedance meter with a maximum DC of 1000A. The FuelWork software on the Evaluator test station automatically applied control of the fuel cell's operating conditions. The current and the voltage range depends on the impedance of the test item. To an impedance of 800 mU, the maximum modulation voltage of the test item is 20 mV (2% of offload voltage of a single cell). The impedance data were taken using TrueData-EIS software which preformed galvanostatic impedance measurements over a frequency range of 0.1 Hze30 kHz. During analysis, the system draws a small direct current (DC) while superimposing a small magnitude of alternating current (AC) e about 10% of the DC value. The EIS measurement was repeated for current density values of 100 mA/cm2, 300 mA/ cm2, 800 mA/cm2 and 1000 mA/cm2 for both Aquivion® and Nafion® CCMs. These setpoints were chosen to represent different sections of the polarization curves. The data were then fitted to the equivalent circuit model (ECM) described in Fig. 2 with the help of the EC-Lab software. The EC-lab software uses a combination of Randomize and Simplex leastsquare nonlinear fitting algorithm to determine the path of best fits for different data points.
Accelerated stress test To determine the durability of both the Nafion® and Aquivion® CCMs, AST protocol was applied as shown in Table 1. The procedure was modified from the DOE protocol [26]. This modified DOE protocol involves a series of relative humility (R.H) cycling at both wet and dry conditions simultaneously with load cycling of the MEA under very low and high current loadings. The cell was operated at 80 C and the bubbler humidifier temperature was cycled between wet condition at 80 C at the cathode inlet and 75 C at the anode inlet, while the dry conditions relative humidity was at 25 C. The cell was cycled between alternating loads of 1.2 and 0.02 A/cm2 each for 5 cycles and between 5 cycles of 0.1 and 0.02 A/cm2 for the dry relative humidity cycle. Each relative humidity cycle between wet and dry conditions lasted for 10 min and the process was repeated over 24 h. A 2 min transition time was allowed while operating the cell at 0.6 A/cm2 current density
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Table 1 e Modified Drive-Cycle Durability Protocol [26]. Test Point #
Anode Cathode Test Point Run Current Time density (A/ Inlet Temp Inlet Temp (oC) (oC) (Minutes) cm2)
Wet with Voltage Cycling RH1 0.02 RH2 1.2 RH3 0.02 RH4 1.2 RH5 0.02 RH6 1.2 RH7 0.02 RH8 1.2 RH9 0.02 RH10 1.2 Trans 0.6 1 Dry With Load Cycling RH11 0.1 RH12 0.02 RH13 0.1 RH14 0.02 RH15 0.1 RH16 0.02 RH17 0.1 RH18 0.02 RH19 0.1 RH20 0.02
75 75 75 75 75 75 75 75 75 75 75
80 80 80 80 80 80 80 80 80 80 80
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2.0
25 25 25 25 25 25 25 25 25 25
25 25 25 25 25 25 25 25 25 25
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 5.0
to allow for some transition between the wet and dry RH cycle. The 5 min hold in step RH20 as observed in Table 1 was intended to represent a system idle point to represent the end of a complete cycle and set the pace for a new cycle. The DOE drive-cycle testing reflects only degradation losses associated with wet and dry cyclic operation. However, in the modified AST, an additional stress was placed on the MEA because of load cycling. The load cycling employed in this protocol may have a minor effect on the catalyst [31] however, in this study, the focus is to provide a holistic analysis of the MEA degradation.
The ECM in Fig. 1 is an extension of the Randles cell for a circuit with mixed kinetic and charge transfer control. Like the Randles circuit, RU corresponds to the cell's ohmic resistance connected in series with two parallel combinations of CPEdl, A, - Rct,A, and CPEdl,C - Rct, C, representing the distributed doublelayer capacitive effects and the charge transfer resistance at the anode and cathode respectively. An inductive element L was connected in parallel with the Warburg element (W) to model the low-frequency inductive effect and mass action respectively.
Results and discussions SEM images of the CCMs indicates that the anode and cathode layers of the Aquivion® and Nafion® ionomer based catalyst layers (CLs) were of similar thickness. The average thickness of the cathode was at 11.5 mm and that of the anode at 4.2 mm. This similar thickness allows for comparison of the Ohmic resistances due to similar electronic pathlength in the CL. Thicker layers generally imply higher Ohmic resistance. The morphology of the electrode layers for the respective CCMs was smooth, with no visible cracking of defects. The quality of the MEAs allowed for further comparative electrochemical evaluations. Both Nafion® and Aquivion® CCMs require adequate humidification to maintain proton conductivity through the MEA's operation life-cycle [23,27]. The conditioning of the MEAs occur at relatively high humidity (80e100%), and studies have shown that Aquivion® demonstrates a higher charge carrier concentration and increased proton conductivity compared to Nafion® at these operating conditions [15]. Nafion® CCMs are likely to normalize to their peak performance quicker compared to Aquivion® CCMs. The longer side chain ionomer is prone to swelling, which will result in the quicker hydration of the catalyst layer, thus, resulting in accelerated maximization and stabilization of the CCM's performance.
Polarization and power curve analysis Equivalent circuit model (ECM) An equivalent circuit can be used to represent an electrode/ electrolyte interface when it is analyzed by EIS. For PEMFC analysis, the Randles Cell [11] is the most common type of electronic circuit used for EIS analysis. In this work, an ECM based on the simplified Randles Cell Schematic was adapted.
The performance of the Aquivion® CCM was compared to the Nafion® CCM using polarization curves determined at the specified operating conditions. Fig. 3 presents the beginning of life (BoL) results of the polarization curves for the two CCMs. The OCV values of the Nafion® CCM was 0.968 V which is higher than the Aquivion® CCM's OCV value of 0.948 V.
Fig. 1 e Equivalent circuit model used for EIS fitting [11]. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 2 e SEM images illustrating the surface morphology and the layer thickness of the anode and cathode layers on the (A) Aquivion® and (B) Nafion® based CL CCM.
However, upon the application of load, as observed in Fig. 3(a), the Aquivion® CCM shows superior performance to its Nafion® counterpart at 0.65V. Similarly, the power curve analysis from Fig. 3(b) shows that the Aquivion® CCM has a maximum power output of 587 mW/cm2 (14.68 W) which is quite higher compared to the Nafion® CCM maximum power output of 480 mW/cm2 (12 W). To better understand the difference in performance, semi-empirical modelling was performed as discussed in the section below.
Semi-empirical modeling Fig. 4 shows the Aquivion® and Nafion® ionomer based CL CCM's polarization curves fitted to Yutaro's model for a cell operated at 80 C. The obtained regression parameters fitted for these models are listed in Table 2. The Yutaro’ semiempirical model was a good fit for the data, and the correlation coefficients Rcorr were greater than 0.99. The ohmic resistance, Rohmic, formed the core basis of the comparison. The Rohmic of the Nafion® CCM was greater than that of
Aquivion® CCM, indicating higher ohmic losses, and therefore lower performance, for the Nafion® CCM. This result suggests that the Aquivion® ionomer based catalyst binder offers enhanced proton conductivity due to its shorter chain, resulting in lower protonic resistance in the catalyst layers compared to the LSC Nafion® based catalyst support MEA [7]. The activation loss value a in the Yutaro's model for Nafion® CCM was higher compared to the Aquivion® CCM, indicating that the activation overpotential loss is higher for the Nafion® compared to the Aquivion® CCM. As activation is generally related to the catalyst, this would imply that the latter has better surface interface with the catalyst, allowing for the exposure of higher catalyst surface area. Similarly, from Table 2, the concentration loss parameters m and n showed that the Nafion® CCM suffered higher mass transport losses compared to the Aquivion® CCM. This means that the reactants will be delivered to the catalyst sites with less inhibition using an Aquivion® ionomer compared to using a Nafion® ionomer in the CL.
Fig. 3 e (a) Polarization Curve (b) Power curve Analysis of Nafion® and Aquivion® ionomer based CL e CCMs. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 4 e Polarization curve model fitting for the (a) Aquivion ® ionomer based CL (b) Nafion® ionomer based CL using Yutaro’s model.
EIS-ECM modeling The Nyquist plots for both Aquivion® and Nafion® CCMs are shown in Fig. 5. Extracted data from the Nyquist plot presented in Table 3 shows that membrane resistance (Rm) decreases with increasing load from 2.5A (100 mA/cm2) to 25A (1000 mA/cm2) across both CCMs. This is most likely attributed to the resulting increase in charge transport due to an increase in cell's thermodynamics at higher current density values. The back diffusion of water taking place at high current density also increases the water content in the membrane, hence leading to an increase in membrane humidity. Since the membrane resistance is the main contributor to the ohmic resistance, a well-hydrated CCM will result in an overall reduction in electronic and protonic resistance of the cell. The Aquivion® CCM has a lower ohmic resistance compared to the Nafion® based CCM, indicating that the Aquivion® CCM has an overall higher conductivity. The anodic CPEdl,A and the Rct,A values are gotten at higher frequencies compared to their cathodic counterpart. This is because the electrochemical rate at the cathode is slow compared to that of the anode, as a result, the RC time constant of the oxygen reduction reaction at cathode has a higher Rct,C. The reaction kinetics of the charge transfer at the anode and cathode cannot be differentiated by just looking at the EIS impedance arc, as the cathode and anode arcs overlap with the cathode arc dominating the whole process [28]. From the extracted parameter values summarized in Table 3, it was observed that Rct,A is small compared to Rct,C for the entire load current range. From the merged cathode and anode impedance arcs, the effective fuel cell charge transfer resistance (Rct) is the sum of both, i.e.
Rct ¼ Rct;A þ Rct;C
(6)
®
The Nafion CCM has a larger charge transfer resistance compared to the Aquivion® CCM, indicating that the Aquivion® CCM boasts a comparatively faster reaction kinetics. The Nafion® CCMs shows a higher mass transport impedance compared to the Aquivion® CCMs, thus, indicating longer timeframes for reactants to move and penetrate further through the GDLs [28]. Overall, EIS-ECM analysis shows that using Aquivion® ionomer binders in the CL makes better CCMs that perform better in comparison to using Nafion® ionomer binders in the CL. The Aquivion ionomer based CL CCMs have lower values for the ohmic losses, charge transfers losses and mass transfer losses.
Durability studies From previous studies [29], R.H cycling has many complex effects on the PEMFC membrane durability since both flooding and drying contribute to PEMFC performance loss. Cycling the cell at low humidification is detrimental to the membrane, causing higher chemical degradation of the membrane and the lack of water makes the membrane brittle and fragile. Excessive humidification, however, under the low current density condition can lead to excess water percolation during the fuel cell operation causing flooding of the membrane and catalyst layers and blockage of active reactant diffusion sites. Fig. 6 shows the potential-plot for the wet load cycle. Each R.H cycle has 5 cyclic load loops between 0.02 A/cm2 and 1.2 A/cm2, with the potential recorded at the lower current density of each cycle. Fig. 7 shows the potential - time plot for
Table 2 e Yutaro’s model fitting for MEAs made for the PFSA ionomers.
Aquivion Nafion®
®
Rcorr
Rohmic [Ucm2]
a [V]
b [V]
m [V]
n [cm2/A]
0.9986 0.9988
0.1792 0.2258
4.816e-05 3.213e-05
0.001021 0.0008955
0.01566 0.0169
1.76 2.153
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 5 e EIS Nyquist plots determined for Nafion® and Aquivion® ionomer based CL CCMs at (a) 100 mA/cm2, (a) 300 mA/cm2, (a) 800 mA/cm2 and (a) 1000 mA/cm2 (fit data were obtained using the ECM in Fig. 1).
the dry load cycle. During the dry load cycle, the first load cycle is still somewhat humid as the membrane and catalyst layers may have retained some water carried over from the wet load cycle operation. With continual operation in the dry condition, the membrane and catalyst layers loose humidity resulting in decreased performance. From Figs. 6 and 7, it is observed that after the AST, the Aquivion® CCM outperformed the Nafion® CCM both in the wet and dry load cycle conditions.
The combined effect of the R.H and load cycling significantly degraded the CCMs in 24 h over a 40 cyclic period of wet and dry R.H conditions. The polarization curve plot in Fig. 8(b) shows that the Aquivion® CCM is also more durable than the Nafion® CCM after being exposed to AST conditions. Fig. 8(a) shows an overview of percentage degradation per voltage point of both CCMs before and after the AST protocol. The degree of degradation is very likely related to the stability and the water-retaining capacity of the ionomers. As
Table 3 e Fitted values for the EIS e ECM data, (Rm (U) is gotten from the high frequency (HF) intercept with the x axis, Rct, A(U) þ Rct, C (U) is the circumference of the semicircle. Cdl, A (mF) þ Cdl, C (mF) is gotten from the fit values, but can be calculated by finding inverse of the product of maximum frequency and Rct. L(H) is fitted parameter indicating losses as a result of wiring inductances, often observed when the Nyquist plot extends beyond 0 and Zw is the warburg inductance derived from fitting ECM circuit.). Parameter Aquivion Nafion Aquivion Nafion Aquivion Nafion Aquivion Nafion
Current density (mA/cm2)
Rm (U)
Rct, A(U)
Rct, C (U)
100 100 300 300 800 800 1000 1000
2.844 3.101 2.816 3.021 2.160 2.350 2.167 2.272
2.298 12.000 3.058 3.389 3.449 4.763 3.297 3.601
16.470 18.410 10.320 11.77 19.540 23.060 34.21 47.49
Cdl,
A
(mF)
0.161 1.148 3.003e3 0.831e3 2.289e3 3.651e3 0.718 1.063
Cdl,
C
(mF)
1.213 3.599 0.943 3.599 1.479 1.033 1.625 1.045
L(H)
Zw
0.081 0.059 0.157 0.277 0.218 0.443 0.238 0.388
1.340 5.162 8.072 8.763 4.322 7.199 7.337 9.420
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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0.9 0.89
Voltage (V)
0.88 0.87 0.86 0.85 0.84 0.83 14:24:00
19:12:00
00:00:00
04:48:00
09:36:00
14:24:00
19:12:00
Time (t) Nafion Aquivion Fig. 6 e Wet Load Cycling Voltage-Time Response on the Aquivion® and Nafion® ionomer based CL CCMs.
0.86 0.84
Voltage (V)
0.82 0.8 0.78 0.76 0.74 14:24:00
19:12:00
00:00:00
04:48:00
09:36:00
14:24:00
19:12:00
Time (t) Aquivion
Nafion
Fig. 7 e Dry Load Cycling Voltage-Time Response on the Aquivion® and Nafion® ionomer based CL CCMs.
Fig. 8 e (a) Percentage voltage loss after AST at each current density point. (b) Polarization curve analysis of Aquivion® and Nafion® ionomer based CL CCMs before and after the AST test completed after 24 h.
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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Fig. 9 e EIS Nyquist plot for Aquivion® and Nafion® at (a) 100 mA/cm2 and (b) 300 mA/cm2 (fit data were obtained using the ECM in Fig. 1).
Table 4 e Fitted values for EIS e ECM data (Rm (U) is gotten from the high frequency (HF) intercept with the x axis, Rct, A(U) þ Rct, C (U) is the circumference of the semicircle. Cdl, A (mF) þ Cdl, C (mF) is gotten from the fit values, but can be calculated by finding inverse of the product of maximum frequency and Rct. L(H) is fitted parameter indicating losses as a result of wiring inductances, often observed when the Nyquist plot extends beyond 0 and Zw is the warburg inductance derived from fitting ECM circuit.). Parameter Aquivion Nafion Aquivion Nafion
Current density (mA/cm2)
Rm(U)
Rct, A (U)
Rct, C (U)
100 100 300 300
8.254 9.454 2.652 7.355
11.06 15.25 13.51 8.437
32.44 47.4 14.27 25.18
pointed out by Park et al. [30], The Aquivion® ionomer has higher water retention capacity, higher crystallinity and a higher glass transition temperature (Tg) compared to the Nafion® ionomer. Attack on the per-fluorocarbon backbone side chain of the ionomer is the core of the membrane and ionomer degradation. This loss of sulfonic acid sites leads to a reduction in membrane and catalyst layer proton conductivity. The easier
Cdl,
A
(mF)
21.55e3 26.58e3 11.81e6 8.602e6
Cdl,
C
(mF)
0.76 1.033 0.106 0.362
L(H)
Zw
0.314 0.483 0.318 0.325
3.657 12.43 3.101 24.12
this side chain is broken, the more severe and faster the rate of degradation. From the above comparative analysis, it can be inferred that the short-side chain Aquivion® based ionomer catalyst support has a stronger bond with its per-fluorocarbon backbone as compared with the long-side chain Nafion® based catalyst support MEA, thus, the reason for an extended lifecycle after undergoing the R.H-load cycling AST.
EIS e ECM modelling for ionomer catalyst support characterization after AST Fig. 9(a) and (b) shows the Nyquist plots of the Nafion® and Aquivion® ionomer based CL CCMs taken after the AST. Table 4 shows the fitted parameters from the ECM modeling, and it is observed that ohmic resistance (RU) decreases with increasing load from 4A (100 mA/cm2 Current density) to 12A (300 mA/cm2 Current density) across both MEAs. After exposure to AST through R.H and load cycling of the cell, the Aquivion® ionomer based CL CCM shows a lower ohmic resistance compared to the Nafion® CL MEA, hence indicating higher conductivity of the Aquivion® based CL MEA.
Table 5 e CV scan data showing ECSA of Nafion® and Aquivion® ionomer based CL CCMs. Ionomer Type Fig. 10 e CV Scan analysis of Aquivion® and Nafion® ionomer based CL CCMs.
®
Nafion Aquivion®
ECSA (m2/g Pt) 64.30 87.70
Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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From the extracted parameter values summarized in Table 4, it was observed that Rct,A is smaller compared to Rct, C. From the merged cathode and anode impedance arcs, the effective fuel cell charge transfer resistance (Rct) is the sum of both, i.e. Rct¼ Rct,Aþ Rct,C. After undergoing degradation, the Nafion® ionomer based CL CCM has a larger charge transfer resistance compared to the Aquivion® ionomer, thus indicating that the Aquivion® ionomer based CL CCM is more durable, possessing longer lifecycle under continuous operation. The corresponding decrease in the Capacitive layers (Cdl) for both sets of MEAs strongly suggest a decrease in both electronic and proton conductivity, likely related to the water content, in the CL. Aquivion® CL MEAs presented a better electro-catalytic activity due to lower Zw values, which is on average 3 times higher after AST testing. The lower resistance to mass transfer, can infer that there is a higher rate of catalyst utilization in the Aquivion® ionomer based CL CCM compared to the Nafion® ionomer based CCM. The most significant difference however, is observed in the Rm after AST for both MEAs at 300 mA/cm2. The Aquivion® ionomer based CL CCM also displays a better electrocatalytic activity due to lower Zw values. Fig. 10 and Table 5 shows the cyclic voltammetry scan (CV) of both PFSA Ionomer CCMs. The electrochemical
active surface area (ECSA) of the CL made from using Aquivion® ionomer as a binder is observed to be higher compared to CL made from using Nafion® ionomer. The shorter side chains of the Aquivion® ionomer facilitates rapid gas access to the reaction sites in comparison to the longer side chain Nafion® ionomer. The longer the side chain of the PFSA ionomer, the more the pore spaces in the catalyst layer are filled, proton conductivity increases, but catalyst sites become blocked to gas. This is the reason for the lower ECSA observed for the LSC Nafion® ionomer compared to the SSC Aquivion® ionomer binder in CL. This result is in agreement with earlier results presented by Garsany et al. [36]. Where they showed that the mesopore volume of the SSC ionomers based CL is almost 4 times higher than that of the LSC ionomers based CL. As confirmed in earlier works by Wong and Eikerling [32,33], the larger primary pores of the SSC ionomers based CL is an indication of more hydrophobicity, which translates to improved reactant transport and water removal. Thus, improved ECSA observed is an indication of higher catalytic utilization which leads to increase in electrochemical activity of the cell and also higher Pt/C in contact with the ionomer when Aquivion® ionomers are used as catalyst binders. After undergoing AST, the Aquvion® CCM
Fig. 11 e High-resolution XPS spectra for Pt 4f, C 1s, and F 1s signals before and after AST for the Aquivion® (a,c,e) and Nafion® (b, d,f) in the MEA catalyst layer. Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079
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experienced an 18% reduction in its ECSA (71.3 m2/g Pt) while the Nafion® CCM experienced a 20% reduction in its ECSA (52.51m2/g Pt). Thus, the modified DOE AST protocol impacted on both the CL and the ionomers. The conductivity of PFSA membranes is found to depend on hydration [34], and the length of the side chain of the PFSA has been shown in earlier studies to impact the level of hydration of the CL. Thus, the above result is in agreement with previous report that stated that higher micropore volume for the CCMs with the SSC based CL indicates less filling of the pores in CL, thus giving room for faster gas and water transport [36]. The XPS measurements were carried out to investigate the electrode surface composition. The XPS wide scans showed the presence of the outermost layers of platinum, carbon, and fluorine. Pt 4f doublet peaks at 71.2 and 74.6 eV are attributable to Pt 4f7/2 and Pt 4f5/2, respectively. These peaks can be assigned to Pt0, i.e. Pt in the metallic state. It is evident that the AST test didn't have a significant impact on the oxidation state of the catalyst on either of the coated CCMs. The C 1s peak corresponding to CeC binding (284.3 eV) on the respective MEAs (Fig. 11(c) and d) was also consistent, increasing slightly on the Nafion® ionomer based CL CC. This slight increase is likely due to minor variation in the coating of the CCMs. The consistent Pt4f and CeC binding peaks for the CCMs strongly suggest that the carbon support materials remained relatively stable during testing. The CeF2 peak, however, disappeared after testing. The F1s signal in the XPS-spectra decreased after AST significantly from 688.3 eV to 687.5 V confirming extensive oxidation of the CF2 functional groups to CHFO and CFO functionalities for both MEAs. The Pt 4f signal shifted to slightly higher binding energies likely indicating the metallic Pt being oxidized to the PtO and PtO2 species [35]. As the catalyst support remained stable with a marginal increase in oxidation of the catalyst, it is suggested that the major component which underwent degradation in the CL was the ionomer, in the case of both Nafion® as well as Aquivion®.
Conclusion The empirical modelling analysis of the PEMFC shows that ohmic resistance for the Nafion® ionomer based CL CCM is greater than the Aquivion® ionomer based CL CCM's ohmic resistance. The Nafion® ionomer based catalyst support exhibits lower proton conductivity and thus a reduced performance compared to the Aquivion® ionomer based CL CCM. Both the polarization curve and EIS analysis of the CCMs showed that the Aquivion® ionomer catalyst support performs better and is also more durable than the Nafion® ionomer CL CCM. The CV analysis of both PFSA ionomers shows that the Aquivion® ionomer binders result in a CCM with a larger ECSA compared to the Nafion® ionomer based binder. Also, The Nafion® based catalyst support CCM has a larger charge transfer resistance compared to the Aquivion® based catalyst support, indicating that the Aquivion® ionomer boasts lower charge transfer losses and faster reaction kinetics. The effect of degradation was more pronounced on the
11
Nafion® ionomer based binders compared to the Aquivion® ionomer based binders. Hence it can be concluded from observed results that the Aquivion® ionomer is a better and more durable option as a binder in the catalyst layers of lowtemperature PEMFCs.
Aknowlegement This project was funded by the Mandela Rhodes Foundation (MRF PG Scholarship) and Hydrogen South Africa (HySA catalysis).
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Please cite this article as: Balogun EO et al., Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.079