Determining electrochemically active surface area in PEM fuel cell electrodes with electrochemical impedance spectroscopy and its application to catalyst durability

Electrochimica Acta 114 (2013) 278–284

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Determining electrochemically active surface area in PEM fuel cell electrodes with electrochemical impedance spectroscopy and its application to catalyst durability O’Rian Reid, Farhana S. Saleh, E. Bradley Easton ∗ Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, Canada L1H 7K4

a r t i c l e

i n f o

Article history: Received 6 August 2013 Accepted 10 October 2013 Available online 23 October 2013 Keywords: PEM fuel cell Electrochemical impedance spectroscopy Accelerated degradation testing protocol Degradation mechanisms Electrochemically active surface area Pseudo-capacitance

a b s t r a c t Here we have derived a simple expression to relate faradaic pseudo-capacitance, CF , determined by electrochemical impedance spectroscopy to the electrochemically active surface area (ECSA) of Pt electrocatalysts. To test this expression, two commercially available Pt/C catalysts were subjected to accelerated degradation testing protocol (ADTP) during which catalyst layer health was assessed using cyclic voltammetry (CV) and EIS to monitor the degradation process. CF was determined by acquiring the EIS response at two different DC bias potentials: the first at a bias potential where the faradaic process was present, and the second at a bias potential where only double layer capacitance was present which enables accurate tracking of changes in CF throughout the ADTP. A near-identical decay profile for both ECSA (determined by CV) and CF (determined by EIS) was observed, providing an excellent fit to the derived expression. Using the EIS model, similar potential-dependant proportionality constants were determined for hydrogen adsorption/desorption on each catalyst indicating that they are universally applicable across Pt catalysts. These constants can therefore be used to effectively determine ECSA values without performing CV measurements. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction In the world today fossil fuels are responsible for approximately 80% of the world’s energy, and with the decrease in reserves the development of an alternative power source has gained interest [1]. Finding a source of sustainable energy is a topic of interest and recently worldwide research has gone into other energy sources that would be able to replace fossil fuels. At present research on fuel cells has increased since it is one of few technologies capable of producing energy by using simple fuels. Consequently, this technology can also be deployed in numerous applications, including automotive, and stationary and portable power. Fuel cells are ecofriendly devices that produce electricity and water as by-products with the input of hydrogen and oxygen gas. One of the major cost drivers in polymer electrolyte membrane fuel cells (PEMFCs) is the use of platinum-based electrodes to catalyze both the anodic oxidation of hydrogen (anode reaction) and the cathodic reduction of oxygen (cathode reaction). Consequently, much of the past research has been focused on determining whether automotive platinum loading targets (0.2 gPt/kW), driven by cost and platinum supply considerations, could be met with currently known catalyst

∗ Corresponding author. Tel.: +1 905721 8668x2936; fax: +1 905 721 3304. E-mail address: [email protected] (E.B. Easton). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.050

technology. Recent studies have reached an important milestone toward automotive PEMFC applications by envisioning a feasible technical roadmap toward meeting the stringent automotive target [2,3], which consist of membrane electrode assembly (MEA) and diffusion media (DM) optimization in parallel with engineering of Pt-alloy cathode catalysts. Catalyst durability during PEMFC operation remains a key challenge to developing PEMFCs with acceptable lifetime in transportation and stationary sectors. Significant research and development efforts are focused on elucidating and quantifying catalyst degradation mechanisms in order to determine whether currently known carbon-supported platinum (Pt/C) and platinum-alloy (Pt-alloy/C) catalysts will meet long-term performance degradation requirements (automotive applications  10 ␮ V/h). In order for polymer electrolyte fuel cells (PEFCs) to find practical application as automotive power sources and as stationary residential generators, it is essential to reduce the performance degradation caused by the operating conditions or environmental conditions. Thus, finding a way to track the degradation of platinum over time can give an idea on the important processes that occur within the fuel cell, and with this knowledge the utilization of platinum in the fuel cell can be increased. Catalyst degradation in PEMFCs is witnessed as an apparent loss of platinum surface area over time [4–6], associated with different degradation processes described elsewhere [7–13]. Any sintering/agglomeration of the platinum (or Pt-alloy) particles leads to

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Pt/C-Nafion Catalyst Layer

Table 1 DC bias potentials used for EIS analysis.

Relectronic

GDL

279

PEM

Rionic Fig. 1. Diagram of circuit describing the impedance of a PEM fuel cell electrode.

platinum surface area loss in the electrode and results in a loss of fuel cell performance. Due to the nature of PEMFC catalysts and their complexity usually involving precious metals or alloys, it is hard to evaluate the degradation method of the electro-catalyst and differentiate between carbon corrosion and other factors contributing to activity loss (e.g., platinum sintering or migration). It is therefore necessary in most cases to employ additional methodologies besides electrochemical measurements to determine the relative activity loss due to carbon corrosion. However, common techniques for characterizing carbon corrosion, including potential cycling [14–16], potential holding [17,18] and open circuit operation [19], are time consuming. By developing another method which reduces the time of testing and can be used to evaluate the entire catalyst layer degradation would allow a better understanding of the degradation of platinum in a functioning fuel cell. To shorten the length of time needed to determine catalyst stability, an accelerated degradation testing protocol (ADTP) can be used. An ADTP normally consists of an accelerated aging process coupled with periodic assessment of performance or a performance indicator like SPt by CV. We have recently reported that when electrochemical impedance spectroscopy (EIS) is added to ADTP additional catalyst layer degradation modes can be elucidated. Specific changes is the EIS profiles occur upon degradation that are characteristic of specific catalyst layer degradation processes, including the degradation of the carbon support and the ionomer [20]. Here, we have extended that methodology by varying the DC bias potential at which the impedance spectra are recorded. By doing so, changes in faradaic pseudo-capacitance associated with hydrogen adsorption determined by EIS are monitored overtime and can be directly related to changes in ECSA determined by CV. In the following section, our EIS model is described in more detail and subsequently applied to the accelerated testing of two commercially available Pt catalysts, E-TEK 20% Pt on Vulcan XC72 carbon and a Johnson Matthey 20% Pt on carbon black. 2. The EIS model The basis of our EIS data treatment lies in the finite transmissionline model developed by the Pickup group for use with PEM fuel cell electrode structures [21,22]. Pickup’s model is essentially a modification of the traditional porous electrode model, describing ionic resistance (Rionic ) and electronic resistance (Relectronic ) using two parallel resistive rails. These two rails are connected by capacitors that represent the double layer capacitance at the Pt/C-electrolyte interface, which is assumed to be uniformly distributed throughout the catalyst layer. The equivalent circuit representation of this model is shown in Fig. 1. Normally, one can assume that Relectronic is much smaller than Rionic , thus the impedance of the catalyst layer would be dominated by Rionic . As such, this model has been widely

Region

Potential (vs. RHE) (V)

Region description

A B C D E

0.085 0.215 0.380 0.608 0.885

H-adsorption/desorption Peak 1 H-adsorption/desorption Peak 2 Double layer Quinone/Hydroquinone growth Pt oxide growth/stripping

used to study ion transport in electrodes that contain Nafion, as well as numerous alternative ion conductors [21–30]. One key figure of merit that can be derived from the EIS data acquired with this model is the limiting capacitance, which is the limiting capacitance value at low frequency. The value can be easily extracted from a “capacitance plot”, where the series capacitance, −1/(ωZ ), is plotted against the real impedance, Z . It has been demonstrated that this limiting capacitance value is directly related to the electrochemically active surface area (ECSA), and can even yield more accurate measurements than CV in some cases [22]. Thus capacitance plots enable the easy visualization of active area and catalyst layer resistance in a single plot. It is from this point in that we are extending the model. The assumption that the limiting capacitance is representative of active electrode area is truly only valid in the absence of faradaic pseudo-capacitance–that is an active redox process like adsorption/desorption. However, pseudo-capacitance can in fact be present in Pt/C layers, the magnitude of which is both composition and potential (E) dependent. Thus, we will now define the total capacitance measured as CT , which is the sum of the double layer capacitance, Cdl , and the pseudo-capacitance due to a fast faradaic process on the surface, CF , to yield the following general equation: CT (E) = Cdl (E) + CF (E)

(1)

This universal equation essentially describes these two processes as being capacitive elements connected in series, and would be valid only at low frequencies (where limiting capacitance is determined) so that transport limited capacitance can be accounted for. For the application to catalyst layers, it is useful to refer to a typical CV such as that shown in Fig. 3, where four main features can be observed: peaks associated with hydrogen adsorption/desorption on Pt (ca. 0–0.3 V), a double layer region (ca. 0.3–0.45 V), possible peaks associated with surface quinone/hydroquinone groups on carbon (ca. 0.5–0.65 V), and growth/stripping of Pt-oxide (ca. 0.7–1.2 V). Three of these main features involve surface redox process. Thus, the DC bias potential at which the EIS spectra are recorded can alter the limiting capacitance measured [22]. We shall consider the EIS response at 5 different potentials, denoted as regions A–E, as defined in Table 1. Normally, the Pickup model is used at a DC bias potential in the double layer region, region C. The capacitance at the double layer region (ca. 0.35–0.45 V vs NHE) can then be written as: CT,C = Cdl,C + CF,C

(2)

Since in region C there are no faradaic process occurring, we can safely assume that CT,C = Cdl,C . Furthermore, we will also now assume that the double layer capacitance does not vary significantly with potential, and can therefore state that Cdl (E) = Cdl,C = CT,C . This assumption is reasonable in that true double layer capacitors are symmetrical except when faradaic processes are present [31]. For PEM fuel cell catalysts, the primary contributor to Cdl will be the carbon support. Most stable carbon supports employed in fuel cell catalysts tend to be highly graphitic with minimal contributions from redox active surface groups that could also contribute to total

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Fig. 2. TEM images of (a) E-20 catalyst and (b) JM-20 catalysts at 180,000× magnification.

capacitance at different potentials, so this assumption would be reasonable for most catalyst systems below 1 V vs. RHE [32,33]. If we now consider the capacitance measured in region A where hydrogen adsorption/desorption occurs, the total capacitance can be expressed as follows: CT,A = Cdl,A + CF,A

(3)

Using the previous assumption, we know that Cdl,A = CT,C , thus: CF,A = CT,A − CT,C

(4)

Thus the difference between the limiting capacitances measured at these two potentials will yield a value for faradaic capacitance. Furthermore, since pseudo-capacitance is a surface phenomenon, CF,A must be proportional to the electrochemically active surface area of Pt: CF,A = kA (ECSA)

(5)

where kA is the proportionality constant in region A. Combing Eqs. (4) and (5) gives the following expression: CF,A = (CT,A − CT,C ) = k,A (ECSA)

(6)

Using Eqs. (5) and (6), a plot of CF,A vs. ECSA should yield straight line with a slope equal to kA . The above derivation takes on similar form when EIS is collected in regions B and E where Pt surface electrochemistry takes place. Likewise, this equation is expected to be universally applicable to other surface electrochemical processes, with a proportionality between CF and the number of electrochemically active moieties. 3. Experimental To verify the validity of the model, we will apply this method to an accelerated durability study of two commercially available Pt/C catalysts that employ highly corrosion stable carbon supports. E-TEK 20% Pt on Vulcan XC72 carbon black (BASF) and Johnson Matthey HiSpec 3000 20% Pt on carbon black (Alfa Aesar) were used as received and are hereafter referred to as E-20 and JM-20, respectively. E-20 and JM-20 were chosen as the model catalysts since they have been previously demonstrated to only degrade by Pt dissolution/agglomeration under these ADT conditions [20,23]. As such, additional complications due to carbon corrosion or ionomer degradation are not expected to be present.

3.1. Materials characterization Transmission electron microscopy (TEM) images of the E-20 and JM-20 catalyst materials were acquired using a Philips CM 10 instrument equipped with an AMT digital camera system. Samples for TEM analysis were dispersed in ultrapure water and applied to nickel 400 mesh formvar coated carbon reinforced grids and allowed to dry under air before they were introduced in the chamber. 3.2. Electrochemical measurements Catalyst inks were prepared by mixing approximately 10 mg of catalyst with 400 ␮L of deionized water and isopropyl alcohol (IPA) in a 50:50 ratio, and 100 ␮L Nafion (Aldrich, 5 wt% in aliphatic alcohols and water). The mixture was then placed in a sonication bath for 30 min to produce a uniform colloidal mixture. A microsyringe was used to deposit 2 ␮L of the mixture on the surface of a clean, polished glassy carbon (GC) electrode (diam = 3 mm, CH instruments). The newly formed catalyst layer was allowed to dry at room temperature for at least 30 min prior to use. A Pt loading of 0.12 ± 0.02 mg/cm2 was used in all experiments. Electrochemical experiments were performed in a single compartment cell using the newly inked GC electrode as the working electrode, an Hg/Hg2 SO4 reference electrode, and a platinum wire counter electrode. The Hg/Hg2 SO4 electrode was used to eliminate the presence of chloride ions which are known to increase platinum dissolution [7,34,35]. All potentials reported here have been corrected to the reversible hydrogen electrode (RHE) scale. Measurements were made using 0.5 M H2 SO4 (aq) (Aldrich) electrolyte that was purged with nitrogen for 10 min to remove oxygen from the cell. All electrochemical measurements were performed at 25 ◦ C using a Solatron 1470E multichannel potentiostat and a Solartron 1260 frequency response analyzer controlled using Multstat software (Scribner). 3.3. 3.3 ADT Protocol Accelerated aging was accomplished using a standard CV ADT protocol. Potential was cycled between 0.056 V and 1.32 V vs. RHE at a sweep rate of 50 mV/s. The catalyst layers were aged for 4,000 cycles in N2 -purged 0.5 M H2 SO4 solution, with periodic assessment of catalyst layer health after 200, 400, 1000, 2000, 3000, and 4000 cycles. Catalyst layer health was assessed by both CV (to determined ECSA) and EIS. Those CVs were collected at a sweep rate of 20 mV/s for three cycles to resolve all peaks. All EIS spectra were

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Fig. 3. Cyclic voltammograms of (a) E-20 catalyst and (b) JM-20 catalyst in 0.5 M H2 SO4 (half-cell). (c) Plot displaying relationship between ECSA and cycle number for E-20 and JM-20 catalysts.

collected over a frequency range of 100 kHz to 0.1 Hz. Five EIS spectra were collected at each assessment, 1 at each of the 5 different DC bias potentials listed in Table 1. For each catalyst, a minimum of two catalyst layer replicates were prepared and subjected to ADT, enabling the estimation of measurement error. 4. Results 4.1. Materials characterization TEM images were collected for both Pt/C catalysts (E-20 and JM-20) and are shown in Fig. 2. It is expected that both catalysts look similar since they both have the same loading of platinum dispersed on the carbon black surface. Both catalysts have comparable particle sizes which show similar surface area of platinum. 4.2. Electrochemical characterization Fig. 3 shows the change in the platinum CV characteristics over 4000 cycles. In both catalysts the peaks associated with hydrogen adsorption/desorption and oxide growth/stripping decreased over the 4000 cycles. Both catalysts show a similar degradation profile, with a slightly faster initial rate of decay in ECSA for JM-20 catalysts as shown in Fig. 3(c).

Fig. 4. EIS data showing complex (a) Nyquist plot, (b) capacitance plot, and (c) normalized capacitance plot at Region C for E-20 catalyst in the half cell.

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Fig. 5. EIS data showing complex (a) Nyquist plot, (b) capacitance plot, and (c) normalized capacitance plot at Region A for E-20 catalyst in the half cell.

Fig. 6. EIS data showing complex (a) Nyquist plot, (b) capacitance plot, and (c) normalized capacitance plot at Region C for JM-20 catalyst in the half cell.

EIS was collected at different DC bias potentials to probe the different faradaic processes that occur on the electrode surface. The data collected can be displayed in three ways, a Nyquist plot, capacitance plot, and a normalized capacitance plot. Fig. 4 shows the EIS response obtained for the E-20 catalyst layer in potential region C at different stages of ADT. A typical 45◦ Warburg–like response is present in the high frequency region of the Nyquist plot. The projection of the Warburg length onto the real axis corresponds to a value of R˙ /3, where R˙ corresponds to the sum of ionic and electronic resistance in the catalyst layer [21]. In this case, R˙ is dominated by the ionic contribution. The initial decrease in resistance is due to improved hydration of the catalyst layer, which has been observed previously [20,23]. Over the courses of 4000 cycles there is no significant change in the Warburg length which indicates no change in either ionic or electronic resistance upon cycling. The capacitance plots show a small decrease in capacitance over 4000 cycles. This decrease in capacitance is due to the reduction in Pt surface area and its contribution to the double layer capacitance. Fig. 5 shows the EIS response obtained for the E-20 catalyst layer in potential region A at different stages of ADT. A clear change in the lower frequency region of the Nyquist plot is observed upon cycling. However, the Warburg length remains stable as observed in Region C. This shows that the change in impedance has to be due to the change in surface area of platinum in the catalyst layer. This is better seen in the capacitance plots shown in Fig. 5(b) and (c). The capacitance plot shows a gradual decrease in limiting capacitance which is due to the decrease of platinum surface area over 4000 cycles. A similar trend was observed in region B, which is also in the Hupd region (not shown). The limiting capacitance at the end of life (EOL) is also quite close to the EOL value measured in region C, which would be expected from our model since CF,A would be quite small at the EOL. The normalized capacitance plots also show that the change in the impedance is due to the change in platinum surface area since there is no change in resistance from the carbon support or the ionomer. The variation in EIS response obtained for the JM-20 catalyst layer in potential region C at different stages of ADT is shown in Fig. 6. The Nyquist plots show an initial decrease in resistance due to hydration, slightly larger than that observed for E-20. The high frequency region shows a Warburg region that does not change with cycle number after becoming fully hydrated. The capacitance plot shows an initial increase in limiting capacitance again due to

improved hydration, after which it remains fairly steady. The normalized capacitance plot shows no change in the resistance of the carbon support or the ionomer after hydration of the catalyst layer. Fig. 7 shows the EIS response obtained for the JM-20 catalyst layer in potential region A at different stages of ADT. The high frequency region shows similar inflections in the Warburg region which indicate no change in carbon support over 4000 cycles. The capacitance plot shows the same trend as E-20 with a decreasing limiting capacitance with cycle number. The normalized capacitance plot shows no changes in resistance which confirms again that there is no change to the carbon support or ionomer. 5. Discussion The data for each catalyst was analyzed in the context of the EIS model. The double layer capacitance, CT,C , was subtracted from the total limiting capacitance in all other regions to yield the pseudocapacitance. This step is critical since Cdl , which our model assumes

Fig. 7. EIS data showing complex (a) Nyquist plot, (b) capacitance plot, and (c) normalized capacitance plot at Region A for JM-20 catalyst in the half cell.

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283

Table 2 Summary of electrochemical data obtained for E-20 and JM-20 catalysts.

Initial ECSA from CV (m2 /gPt ) Final ECSA from CV (m2 /gPt ) kA (F/cmPt 2 ) kB (F/cmPt 2 )

Fig. 8. Variation in Cdl with cycle number for E-20 and JM-20 catalysts.

to be equal to CT,C , can vary greatly depending upon the catalyst layer composition. For example, Fig. 8 compares the variation in Cdl with cycle number obtained for the E-20 and JM-20 catalysts. Both catalysts show an initial decay in Cdl which is due to the loss of Pt surface area. This highlights the need to make a Cdl measurement along with each and every measurement of CT in order to account for variations in the capacitive background overtime. Furthermore, this figure also highlights the fact that these two catalysts have substantially different Cdl values throughout the measurement despite having similar Pt and Nafion loadings. This difference arises from differences in the surface properties of the carbon support used in each catalyst. In this particular case the carbon supports have similar BET surface areas (200–250 m2 /g) yet still produced a significant variation in Cdl . An even more dramatic difference in Cdl would be expected when vastly different carbon supports are employed (e.g. Black Pearls 2000, carbon nanotube), and also when different wt% Pt are used [20,23]. Fig. 9 shows the relationship between CF and cycle number for each catalyst. Regions A and B show a decay in capacitance

E-20

JM-20

Average

55.7 ± 3.5 6.3 ± 2.3 12.1 ± 0.7 7.1 ± 1.1

53.2 ± 0.31 9.3 ± 0.2 13.8 ± 0.7 6.4 ± 1.3

– – 12.95 ± 1.13 6.75 ± 1.08

with cycle number that is very similar to how ECSA decays upon cycling. This similar behavior was expected since both are due to the decrease of platinum surface area [36]. The pseudo-capacitance at region D shows an increase in capacitance which shows that there is a small growth of quinone/hydroquinone surface groups on the carbon surface. This growth is region D is larger for E-20 and is consistent with the CV results. Fig. 10 shows plots of CF vs. ECSA for each catalyst in each of the four regions where pseudo-capacitance is present. There is an excellent linear correlation in regions of Pt surface electrochemistry, as predicted by Eq. 6. The strongest correlation occurs in regions A and B and will therefore be used for the development of ECSA relationships. The linear fits to Eq. 6 in regions A and B for both catalysts are listed in Table 2, along with the standard deviations from the replicate tests. The proportionality constants between ECSA and CF were higher in Region A than in Region B, which may be related to a higher concentration of Pt (1 1 1) and Pt (1 1 0) crystal planes in the catalysts. Furthermore, the values of kA and kB measured for the two different catalysts were very similar, confirming that the proportionality constants is dependent upon the energetics at the Pt surface. The overall average values of kA and kB were determined to be 12.95 and 6.74 F/cmPt 2 , respectively. This means that by making capacitive EIS measurement and employing these constants, accurate measurements of any Pt surface area can be made that will be equally valid as those obtained from CV. It is important to note here that in order to perform these measurements accurately and rapidly, one must subsequently determine both CT and Cdl , and determine their difference in order to account

Fig. 9. Plot showing the relationship between pseudo-capacitance (CF ) and cycle number for the E-20 and JM-20 catalysts.

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Fig. 10. Plot showing the relationship between capacitance (CF ) and ECSA for the E-20 and JM-20 catalysts.

for variations in Cdl arising from differences in catalyst layer composition and/or changes in Cdl overtime. Upon doing so, these values of kA and kB could be used in place of CV measurements to determine ECSA, which may be beneficial in case where CV data is unreliable. 6. Conclusion In this study, we have derived an expression relating pseudocapacitance and the electrochemical surface area of Pt. The expression was tested by examining the accelerated degradation of two commercially available Pt-based catalysts that employ stable carbon supports. A linear relationship between the pseudo capacitance and ECSA was established, yielding potentials dependent proportionality constants that do not vary between catalysts and are universally applicable across Pt catalysts. These constants can be used to effectively determine ECSA values without running cyclic voltammetry. Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the Discovery Grants program, and UOIT. The authors also acknowledge equipment support from the Canada Foundation for Innovation. References [1] S. Shafiee, E. Topal, When will fossil fuel reserves be diminished? Energy Policy 37 (2009) 181–189, http://dx.doi.org/10.1016/j.enpol.2008.08.016. [2] H.A. Gasteiger, J.E. Panels, S.G. Yan, J. Power Sources 127 (2004) 162. [3] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal., B 56 (2005) 9. [4] J. Xie, D.L. Wood, D.M. Wayne, T.A. Zawodzinski, P. Atanassov, R.L. Borup, J. Electrochem. Soc. 152 (2005) A104. [5] M.S. Wilson, F.H. Garzon, K.E. Sickafus, S. Gottesfeld, J. Electrochem. Soc. 140 (1993) 2872.

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