Evaluation of Alkylamine Modified Pt Nanoparticles as ORR

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Evaluation of Alkylamine Modified Pt Nanoparticles as ORR Electrocatalyst for Fuel Cells via Electrochemical Impedance Spectroscopy Prerna Joshi, Toshihiko Okada, Keiko Miyabayashi, and Mikio Miyake Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00247 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Analytical Chemistry

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Evaluation of Alkylamine Modified Pt Nanoparticles as ORR Electrocatalyst for Fuel Cells via Electrochemical Impedance Spectroscopy Prerna Joshi§, Toshihiko Okada§, Keiko Miyabayashi§*, Mikio Miyakeǂ §

Department of Engineering, Graduate School of Integrated Science and Technology, Shizuoka University, Japan Department of Environmental Engineering and Green Technology, Malaysia – Japan International Institute of Technology University Technology Malaysia, Malaysia ǂ

ABSTRACT: Organically (octyl amine, OA) surface modified electrocatalyst (OA-Pt/CB) was studied for its oxygen reduction reaction (ORR) activity via d.c. methods and its charge and mass transfer properties were studied via Electrochemical Impedance spectroscopy (EIS). Comparison with a commercial catalyst (TEC10V30E) with similar Pt content was also carried out. In EIS, both the catalysts showed a single time-constant with an emerging high frequency semi-circle of very small diameter which was fitted using suitable equivalent circuits. The organically modified catalyst showed lower charge-transfer resistance and hence, low polarization resistance in high potential region as compared to the commercial catalyst. The dominance of kinetic processes was observed at 0.925-1.000V whereas domination of diffusion based processes was observed at lower potential region for the organic catalyst. No effect due to the presence of carbon was observed in the EIS spectra. Using the hydrodynamic method, higher current penetration depth was obtained for the organically modified catalyst at 1600 rpm. Exchange current density and Tafel slopes for both the electrocatalysts were calculated from the polarization resistance obtained from EIS which was in correlation with the results obtained from d.c. methods.

Keywords: Fuel Cell, Electrocatalysts, Organic surface modification, Oxygen reduction reaction, Electrochemical Impedance Spectroscopy Oxygen Reduction Reaction (ORR) is one of the most important reaction playing a leading role in various electrochemical devices including sensors and fuel cells1,2. Also, it is one of the well-investigated electrochemical processes due to its sluggish kinetics and thus, requires a cost effective and highly durable electrocatalyst3-5. Various efforts have been made to overcome the two major obstacles of ORR kinetics investigation and catalyst durability6,7. Currently, Pt nanoparticles (nps) dispersed on carbon are used as practical catalysts and hence, have been widely studied for electrocatalytic purposes with further modifications by tailoring the nps and/or the carbon support8-11. Shape/size control12 via studying the bimetallic catalysts13 and crystalline nanoframes14, alloy formation15,16, tagging ionic liquids (IL)17-19, use of nanoceramics20 and catalysts on carbon nitride as a support21 are some of the approaches for developing an efficient electrocatalyst. Further, use of organic compounds such as calixarenes22, chlorophenyl group23, electronegative ligands24, cyanide25, and amines6,26 have also emerged as

another effective approach for developing nps with structured controlled morphology. In our previous research work6, we studied the synthesis and effect of organic modification (octyl amine, OA) on Pt nps using rotating disk electrode (RDE) technique. The synthesized nps, OA-Pt/CB showed significant improvement in ORR kinetics and improved mass (jkm: 326 A gPt-1) and area specific activities (jksp: 426 μA cmPt-2) due to the change in the adsorption kinetics /oxygen concentration at Pt nps surface owing to the surface modification using OA as the protective agent. These OA-stabilized/modified electrocatalyst have shown superior ORR activity than the commercial catalyst, 30% Pt/CB, (jkm: 208 A gPt-1, jksp: 332 μA cmPt-2) with similar Pt content. In this research work, we extend our previous study to characterize the ORR mechanism and estimate the kinetic parameters for Pt nps by using both d.c. and a.c. techniques as the latter enables a comprehensive study of various kinetic and diffusion processes.

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The ORR activity of electrocatalysts have been traditionally evaluated using conventional d.c. methods such as CV, LSV and so forth27,28. Electrochemical impedance spectroscopy (EIS), however, emerges as a sensitive a.c. method which determines the chargetransfer and mass-transfer processes occurring in system in a resolved manner29. This is a simple yet complex technique which involves the application of a small sinusoidal perturbation to the potential difference between the working electrode (WE) and a reference electrode (RE) and the current associated is analyzed in the frequency domain in terms of impedance. Various equivalent circuit models are used to fit the spectra and the obtained parameter values define the cell processes. EI spectroscopy is a well-known technique among researchers as it is highly being applied in studying various electrochemical process due to its high sensitivity to i) minute capacitive and resistive change and ii) its flexibility and compatibility with several systems. For fuel cells, several research groups have focused on this technique with different electrodes with first been used by Springer et. al. for studying the impedance spectra of ORR on a gas diffusion electrode (GDE)30. After which, several reports31-33 were published showing EIS studied based on different electrochemical systems. However, depending on the operand and the operating conditions, a wide variety of spectra was reported which were described based up on the experimental and constitutional conditions34-36. EIS studies have also been carried out in a half-membrane electrode assembly (MEA) configuration. For the simplification of spectra obtained, the a.c. and d.c. analysis were also conducted in a thin-film RDE configuration31,37,38, correlating the parameters obtained from both the methods. However, these studies eliminated the complication arising from anode and other components of the cell, and hence, were carried out in a single electrode system. Further, Singh et. al.39 reported the EIS characteristics of a thin-film catalyst in a three-electrode configuration comparing the data obtained for an ionomer-free and an ionomer containing catalyst layer with and without carbon supported platinum. The impedance spectra obtained were explained using a series of equivalent circuits in the kinetic and diffusion controlled region and gave an insight on the nature of the EI spectra depending on the ionomer content in the catalyst layer and the rotation rate as well. Further, impedance studies of ORR have also been carried out via EIS on bulk platinum in alkaline media using gas diffusion electrode (GDE)40. In this work, both 4-electron and 2-electron pathways were considered for simulation of the EIS spectra. In the current research work, we have used the OAmodified catalyst (OA-Pt/CB) with 31.2 wt % Pt for ORR kinetic studies and compared its electrochemical characteristics with the commercially available TEC10V30E (29 wt. % Pt) using electrochemical impedance spectroscopy. Since, ORR occurs at the

interface of the electrode (electrocatalyst) and the electrolyte, studying it via EIS acts as an efficient tool in studying the interface which leads to detailed study of ORR. Further, electrochemical variability of EIS also helps in the determination of the electrode surface processes and the undergoing reaction mechanism. Spectral fitting using various equivalent circuits gave details about the nature of the catalyst in a specific range of potential. Conventional characterization methods such as CV, LSV and RDE were used along with EIS to validate the results for OA-Pt/CB (31.2 wt % Pt). ORR kinetic parameters (Tafel slope and kinetic current density) were estimated from the polarization resistance (Rp) obtained from EIS circuit fitting and confirmed with the already-established CV/RDE results. MATERIALS AND METHODS: Although described previously6, a brief detail of the experimental conditions is provided here to maintain the continuity. Hydrogen hexachloroplatinate hexahydrate (≥98.5%, H2PtCl6·6H2O), tetraoctylammonium bromide (≥97.0%, TOAB), acetone (≥99.5%), dichloromethane (≥99.5%), toluene (≥99.5%), methanol (≥99.8%), and 2propanol (≥99.9%) were obtained from Wako Pure Chemicals Co., Ltd. Octylamine (≥98%, OA) was purchased from Tokyo Chemical Industry Co., Ltd. Hydrogen perchlorate (62.2%, Ultrapure) was obtained from Kanto Chemical Co., Inc. Nafion (5 wt %) was purchased from Aldrich. All chemicals were used as purchased without further purification. Ultrapure water (18.8 MΩ cm, Komatsudenshi K. K.) was used for preparation of Pt and NaBH4 aqueous solutions and electrochemical measurements. Synthesis of Pt Nanoparticles Modified with Octylamine (OA-Pt NPs): Platinum NPs were prepared via a modified Brust method.24 Further, OA modification to synthesize OA-Pt was done according to the literature resulting into 31.2 wt. % Pt in OA-Pt. Also, carbonsupported (carbon black, CB) catalysts OA-Pt/CB were prepared according to our previously reported work6. Preparation of Electrode: A glassy carbon (GC) electrode was prepared with a little modification in the already reported procedure6. GC (diameter, 5 mm; area, 0.196 cm2) was polished with 0.05 μm alumina and washed with ultrapure water thrice under sonication. GC electrode with Pt catalyst was prepared as follows: OAPt/CB (1 mg) was dispersed in 2-propanol (0.34 mL), ultrapure water (1.08 mL) and Nafion solution (6.98 μL, 5 wt %) by ultrasonic treatment for minimum 45 mins at 0 °C. The aliquot of the catalyst solution was casted on a GC electrode placed on a rotating shaft and the electrode was rotated at 700 rpm till dry. Further, the electrode was heated at 60 oC for 30 minutes to obtain a completely dry electrode. The coated electrode was then used for electrochemical studies. Each electrode was prepared to contain ~9 μg cm−2 Pt. The Pt content of the catalyst solutions was measured by inductively coupled plasma


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atomic emission spectroscopy (ICP-AES) before casting on the GC. Characterization: ICP-AES (Shimadzu Co., ICPS7100) was used to determine the Pt content of OA-Pt/CB by dissolving OA-Pt/CB in hot aqua regia (1 mL) followed by optimum dilution. All the electrochemical measurements were carried out in similar procedures as reported in the literature.41 ORR activity for OA-Pt/CB casted on GC was studied by cyclic voltammograms (CVs) using electrochemical analyzer ALS-CH, 760C at room temperature in 0.1 M HClO4 solution as the electrolyte. OA-Pt/CB casted on GC, Pt wire, and reversible hydrogen electrode (RHE) were used as working (WE), counter (CE), and reference electrodes(RE), respectively. All the potentials are reported with respect to RHE. Pretreatment was carried out on GC by applying cycling sweeps between 0.02 and 1.2 V with a scan rate of 200 mV s−1 under Ar (>99.9999%) atmosphere until reproducible CVs were obtained (ca. 100 cycles). CV was carried out at the same potential range at a scan rate of 50 mVs-1. RDE technique was used for evaluating ORR after constant purging with oxygen (>99.99995%) for 35 mins maintaining a gentle oxygen flow during the measurements. ORR measurement was conducted at six different rotation speeds (100, 400, 900, 1600, 2000, and 2500 rpm) by applying a potential of 0.2−1.0 V vs. RHE in the positive direction at a rate of 10 mV s−1. The data were presented after iR-drop correction. TEC10V30E (29 wt% Pt) (Tanaka Kikinzoku Kogyo K. K., TEC10V30E)19 was used as a reference catalyst for comparison and was electrochemically evaluated following the above described procedures. The electrochemical active surface area (ECSA) values for OA-Pt/CB and TEC10V30E were estimated from the adsorbed hydrogen peak area between 0.06 and 0.45 V observed on the CV with a scan rate of 50 mV s−1 after correction of the electrical double layer and by adopting a capacitance value of 210 μC cm−2 for the adsorption of hydrogen as a monolayer. EIS measurements were obtained in a three-electrode configuration using the same electrochemical system as used for CV/ORR measurements. The impedance spectra were recorded under the frequency range from 0.02 Hz to 1 MHz using an a.c. amplitude of 5 mV at room temperature in O2 atmosphere. All the data were obtained in a single sine mode. Further, the spectra were fitted using ZSimpWin software from Princeton Applied Research. Comparison was carried out using comercial catalyst TEC10V30E using the same procedure. OA-Pt/CB is an organically modified electrocatalyst which used surface modification agent, octylamine tagged onto Pt supported on carbon black. OA is known to enhance the catalytic activity as, i) it prevents the agglomeration of Pt nps, thus, allowing better dispersion, ii) acts as a protective agent by preventing oxidation of the Pt surface and further, iii) does not interfere with

electrode surface reactions i.e. H+ adsorption or hydrogen desorption etc. Depending on the content and extent of modification, ORR activity of OA-Pt/CB by d.c. methods has already been studied in our previous research work6,26. Investigation of ORR characteristics by a.c. technique for this catalyst have not been studied to date. In this study, we emphasize upon the a.c. study of OA-Pt/CB via EIS, with a brief description of the d.c. techniques. The CV of OA-Pt/CB casted on GC electrode was measured in 0.1 M HClO4 and was compared with that of commercial catalyst TEC10V30E (30% Pt) on a GC electrode (Figure S1). OA-Pt/CB showed high ECSA (66.9−70.7 m2g Pt−1) as compared to TEC10V30E (63.7-73.00 m2g Pt−1). High ECSA of OA-Pt/CB was achieved due to the smooth dispersion of Pt nps on CB. Further, in case of OA-Pt/CB, ORR appears at a slightly higher positive potential (0.78 V) as compared to TEC10V30E (0.76 V) indicating that higher activity of the modified catalyst. Figure S2 and S3 show the ORR polarization curves obtained for OAPt/CB and TEC10V30E respectively measured in O2saturated 0.1 M HClO4 solutions using rotating disk electrodes at six different rotation speeds (100, 400, 900, 1600, 2000, and 2500 rpm). These ORR current values were analyzed by Koutecky-Levich Plots (KL plots) (Figure S4 and S5) and from the slope of KL plots, the number of electrons (n) involved in the reaction were calculated. Value of n was observed to be 4, which coincided with the theoretical value. High mass and area specific activities were obtained for OA-Pt/CB as compared to TEC10V30E. Stability of the synthesized catalysts was also studied during 10,000 potential cycle tests and the change in ECSA values were calculated. The evidence for high catalytic activity and reasons behind are already reported in the literature by our research group6. The impedance analysis of OA-Pt/CB and TEC10V30E was carried out to investigate the effect of surface modification on electrocatalysts between 0.8 to 1.0 V. Figure 1a shows the impedance spectra for OAPt/CB and TEC10V30E at 0 and 1600 rpm between 0.81.0V in O2-saturated 0.1M HClO4 solution. For both the catalysts, the spectra appear to be a single semi-circle of decreasing diameter with decreasing potential. An expanded view of the intercept of the first high frequency (HF) loop on the real axis shows an emerging HF semicircle of small diameter and the diameter of this HF semicircle does not depend on the potential applied (Figure S7). To clarify components of large semicircle, EI spectrum was acquired at different potential. We can clearly say from Figure 1 that the process occurring here is dominantly a charge-transfer process, as Rct decreases with a decrease in the electrode potential. An equivalent distributed resistance (EDR) recognized by a high frequency straight line with a phase angle of 45o was missing which corresponds to the electrochemical pure capacitive behavior with porous electrodes42 in the case of both the catalysts, most probably due to a significantly



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lower amount of carbon loading. Absence of EDR in the presence of carbon (porous) can be interpreted to be due to presence of active layer with uniform thickness and absence of surface roughness of the catalyst. Mass transfer effect on EI spectra was evaluated by hydrodynamic technique. Measurement was carried out at two different conditions, a) stationary state, at no rotation or no convection and b) at 1600 rpm. The decrease in the diameter of semi-circle with increasing the rotation rate below 0.95 V reveals the presence of mass transfer effect even at kinetic controlled region as also referred in an article by Singh et.al.39 A low frequency small inductive loop had also been observed for 1600 rpm at 0.8 V. An inductive loop in such spectra has been reported to occur due to relaxation of adsorbed intermediates on Pt surface involved in different steps of ORR (Pt-OHads, Pt-OOHads and PtOads (Figure S1)43,44. The inductive behavior is characterized by positive phase shift. When the surface coverage due to adsorbed intermediates result into a change in kinetic current but shows a sluggish response to the change in electrode potential. This leads to a time lag (positive phase shift, Figure S8) between a.c. potential perturbation and kinetic

current stabilization indicated by appearance of an inductive loop45. At low potential and at low frequency, Cole-cole plot and Bode plot became noisy owing to limited supply of O2 (data not shown). The phase disturbance observed in Bode plot is improved by increasing the rotation speed of electrode. Equivalent circuit fitting is difficult in the presence of these noises and a large contribution of mass transfer to polarization resistance may increase the error for estimation of charge transfer resistance and therefore, the impedance spectra were fitted with equivalent circuit only in the kinetic and the mixed kinetic diffusion controlled region (above 0.8 V). The effect of Nafion content was not studied in the current research work as low Nafion amount does not bring any significant changes in the impedance spectra whereas high Nafion loading only separates the timeconstants of the charge and mass transfer processes and adds extra mass transfer resistance to the overall polarization resistance. No new semi-circle appears due to the change in amount of Nafion. Equivalent circuit fitting was conducted to evaluate the charge and mass transfer resistances. Electrical circuit models equivalent to the electrochemical process were

Figure 1. Impedance spectra for OA-Pt/CB (31.2 wt.% Pt) at a) 0 rpm and b) at 1600 rpm and TEC10V30E at c) 0 rpm and d) at 1600 rpm between 0.8-1.0V vs. RHE taken in O2-saturated 0.1M HClO4 solution


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adapted for arriving at the exact parameters. These electrical circuits are designed depending on the composition of the catalyst and use combination of elements i.e. solution resistance (Rs), polarization resistance, denoted as Rp and constant phase element (CPE) corresponding to the double layer capacitance. Polarization resistance (Rp) can be stated to be the sum of charge –transfer resistance (Rct) and diffusion (masstransfer) resistance (Rd)46,47. Spectral fitting was carried out using a combination of Voigt type analog48. For, OAPt/CB (Figure 1 a, b), a 4-element circuit was designed which used a combination of solution resistance, Rs modified using a CPE element, Q1, coupled with three QR elements coupled in series, (Q1Rs) (Q2Rorg) (Q3Rd) (Q4Rct). Depending on the composition of the catalyst and the electrochemical process, the circuit was chosen, and the polarization resistance was calculated. In the case of TEC10V30E (Figure 1 c, d), the circuit used for fitting involved a three element circuit, three resistances coupled with CPE element, Q in series, (Q1Rs) (Q2Rd) (Q3Rct). Use of an extra QR element in the case of OA-Pt/CB for electrical circuit fitting is attributed to the presence of an additional organic layer required for surface modification. Hence, the 4 different resistances can be denoted as solution resistance, resistance due to the organic layer, diffusion resistance and charge transfer resistance. In the case of TEC10V30E, the organic layer is not present, hence, a three-element circuit suffices. Q1, Q2, Q3 and Q4 are the CPE elements which are used in place of a capacitor. Use of CPE is preferred as it takes various factors in consideration such as surface inhomogeneity, roughness, reactivity and potential distributions associated with the geometry of the electrode. Hence, fitting a spectrum with a CPE element is safer as compared to a capacitive element (C)49. Further, for diffusion, we have used a resistor (Rd) coupled with another CPE element, (Q3) instead of using a Warburg element (W). During ORR, considering only the first layer of oxygen present next to the active site gets

Figure 2. Plot of Rp vs Potential for OA-Pt/CB (31.2 wt.% Pt) and TEC10V30E (30 w.% Pt) at 1600 rpm taken in O2 saturated 0.1 M HClO4

Figure 3. Plot of Rct vs Potential for OA-Pt/CB (31.2 wt.% Pt and TEC10V30E (30 wt.% Pt) at 1600 rpm taken in O2 saturated 0.1 M HClO4

reduced, using W would indicate an infinite diffusion condition, whereas in the case of fuel cells, an openboundary, finite length diffusion is observed50. And at low frequency, finite length diffusion acts like a resistor. Therefore, a resistor element owing to finite length diffusion coupled with a CPE indicating double layer capacitance is used here. R1, in both the cases, refers to the solution resistance (Rs). The charge transfer resistance of the catalysts is plotted against potential to estimate the kinetic current density. The polarization resistance, Rp with respect to potential was plotted in Figure 2 at 1600 rpm in O2 saturated 0.1 M HClO4. It was observed that with decreasing potential, an exponential decrease was observed in the Rp which suggests the domination by charge-transfer process in that potential region. On reducing the potential further from 0.9V to 0.8V, the decrease was gradual indicating the contribution of mass-transfer in the overall resistance in the low potential region. Further, OA-Pt/CB initially showed comparable Rp values with that of TEC10V30E in the potential region less than 0.95V vs RHE, however, as can be seen from the graph, Rp for OA-Pt/CB was lesser than TEC10V30E in pure kinetic region from 0.95V-1.0V vs RHE. This was also supported by the plot of Rct as a function of potential at 1600rpm (Figure 3). For OA-Pt/CB, in the kinetic region, low value of chargetransfer resistance was observed which lowered the overall polarization resistance value as well. The obtained result is consistent with the high ORR activity of OA-Pt/CB in d.c. method (linear sweep voltammogram). All these results suggest that organic surface modification not only improves the ORR characteristics but also enhances the reaction kinetics by facilitating the charge-transfer process in the high potential region or the region closer to the thermodynamic electrode potential for ORR.



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Calculation of exchange current density For ORR, 1.229 V is considered as the thermodynamic electrode potential and it is expected to obtain ORR at potentials as close as possible to this reversible electrode potential28. Extrapolation of Rct to the reversible ORR thermodynamic potential (~1.229V) gives the value of resistance required to calculate the exchange current density, given by 𝑅𝑇

jo=𝑛𝐹𝑅𝑐𝑡 At this potential, the process is purely kinetic, and no mass-transport occurs. Since, estimation of pure chargetransfer resistance is difficult, hence, at 1.229V, Rp can also be termed as Rct and can be used directly in the above equation51. Kinetic current density was estimated by a graph between extrapolated Rp vs potential on a logarithmic scale (Figure. S9). As well known, exchange current density is a kinetic parameter that depends entirely on the reaction and on the electrode surface on which the reaction occurs28. Its magnitude determines how easily a reaction occur on an electrode surface. The exchange current density at 1600 rpm, thus calculated is 1.261 x 10-8 A cm-2 for OA-Pt/CB and 0.912 x 10-8 A cm-2 for TEC10V30E considering n=4, R= 8.314 kg m2 s-2 K-1 mol-1, T=298K and F=96500 C mol-1. These values are comparable to the exchange current density obtained for pure Pt catalyst measured at Pt/Nafion interface at 30 oC28. Exchange current density, jo for OA-Pt/CB is higher than TEC10V30E indicating that OA-Pt/CB requires a smaller overpotential to produce high current flow as compared to the commercial catalyst. The charge transfer resistance obtained for both the electrocatalysts showed a difference even though both the nanocatalysts used pure Pt with similar Pt content. One possible explanation can be the change in Pt electronic states in OA-Pt/CB due to surface modification26,52. Organic functionalization of Pt nanoparticles using amine based modification agents leads to a change in the electronic states of Pt resulting into effective charge transfer53. Calculation of current penetration depth The current penetration depth in catalyst layer and double layer were evaluated to figure the mechanism of improved electrocatalytic activity by the surface modification. To determine the effect of mass-transfer on the current penetration depth, the CPE exponent obtained from the electrochemical circuit fitting was plotted w.r.t

Figure 4. The CPE exponent (ϕ) as a function of potential for OA-Pt/CB (31.2 wt.% Pt) and TEC10V30E (30 w.% Pt) at a) 0 rpm and b) 1600 rpm

potential39. Mathematically, impedance of a CPE is given by, ZCPE =1/(𝑄(𝑗𝜔)ɸ ) where Q is the CPE coefficient, and ɸ is the CPE exponent. When ɸ =1, the equation becomes the same as that for an impedance of a capacitor, with Q=C, however, phase independent. For ɸ = 0, the equation describes impedance response of an ideal resistor, Q = 1/R 29. Figure 4 shows CPE exponent (ɸ) as a function of potential. ɸ values ranging within 0.8-0.9 indicated a low current penetration depth at 0 rpm in diffusion controlled region. Also, value of ɸ decreased with decreasing potential which indicated the extent of mass transfer of reactants. For TEC10V30E, the values of ɸ decreased from 1.00 to 0.81 with decrease in potential. However,

Table 1. Tafel slopes obtained for OA-Pt/CB and TEC10V30E Catalyst

Pt (wt. %)

OA-Pt/CB TEC10V30E

From a.c. method: EIS at 1600 rpm

From d.c. method

At high potential region (mV/dec)

At low potential region (mV/dec)

Apparent Tafel slope At 0.95 V (mV/dec)

Apparent Tafel slope At 0.825 V (mV/dec)

31.2

71.30±0.0011

126.18±0.014

57.95±0.04

139.22±0.03

29.0

63.25±0.0018

118.01±0.0051

61.19±0.05

116.97±0.03


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Figure 5. a) log (Rp-1) vs. IR-corrected cathode potential for OA-Pt/CB and TEC10V30E for calculating the Tafel slope at high and low potentials. Tafel plots obtained via CV along with the Slope values at 0.95 V and 0.85V vs RHE for b) OA-Pt/CB and c) TEC10V30E

the decrease in OA-Pt/CB was gradual and stayed in the range of 0.93 to 0.86. This gradual decrease can be attributed to the high solubility of oxygen in octylamine leading to better diffusivity of oxygen to Pt surface. At 1600 rpm, for OA-Pt/CB, a true capacitive nature with ɸ ≈ 0.99 was observed in the potential range of 0.80.925V. ɸ values reduced later at 0.95 V but increased again at 0.975V. Whereas for TEC10V30E, at 1600 rpm also, the current penetration depth was low at lower potentials but increased at higher potentials yet was lesser than that of OA-Pt/CB at high potentials (0.925-1.0V). This also correlates to the absence of EDR straight line in high frequency region in Figure 1. It can now be confirmed to be related to a uniform, even surface which also results in high current penetration depth, especially for OA-Pt/CB which includes an organic coating on Pt and increased amount of carbon loading as compared to the commercial catalyst. Estimation of Tafel slope Tafel slope is the most common parameter that distinguishes ORR on a Pt interface. Usually, for ORR on bulk platinum, Tafel slope at low current densities is close to 60 mVdec-1 and 120 mVdec-1 at high current densities (based on the electrode materials and potential range)40. By means of Tafel slope estimation, we have tried to display the similarity between d.c. and a.c. methods of ORR evaluation. Rp value obtained from the electric circuit fitting can be used efficiently to obtain the Tafel slope. From the EIS data, Tafel slope can be obtained by plotting the log (Rp-1) as a function of potential51, as shown in Figure 5. From the plot, two different regions of potential can be obtained. In this work, the Tafel slopes obtained via EIS were in accordance with the Tafel slopes obtained via d.c. methods. Table 1 shows the Tafel slopes obtained via a.c.

and d.c. methods for both the catalysts. The difference in Tafel slopes is generally accredited to the change in the adsorption conditions associated with the change in surface state of the electrode at different potentials i.e Pt surface and Pt-PtO surface6,26. Shown by a sudden dip in Figure 5, for OA-Pt/CB, the change in surface state occurs at the kinetic region, i.e. 0.925 V unlike TEC10V30E, for which the Tafel slope changes after 0.9 V. In this research work, we have tried to study the EIS characteristics of an organically modified Pt electrocatalyst and compared its behavior with a commercial catalyst, TEC10V30E. A study of the EI spectra of these catalysts were investigated in a threeelectrode system. Depending on the studied potential range and on the constituent of the electrocatalyst, the ORR impedance spectra were obtained and analyzed for a range of potential i.e. 0.8-1.0V vs RHE. A single semicircle appearing in the impedance spectra for both the catalysts indicated the polarization resistance which kept decreasing with decreasing potential. This suggested that the process is primarily kinetic. Similar effect was observed at a rotation rate of 1600 rpm. Based on the constituents of the electrocatalyst, the electric circuits were used for spectral fitting. Kinetic parameters such as exchange current density, current penetration depth and Tafel slope were calculated from EIS and were compared to the values obtained for commercial catalyst. The validity of EIS for ORR was shown by the means of Tafel slope estimation. Use of combined d.c. technique to evaluate ORR have been developed over several decades via applying several assumptions and corrections. The focus on development of EIS for ORR evaluation requires more attention. Since, EIS is based on resonance of frequency with individual



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processes, this single technique allows more distinguished and precise characterization of all the processes occurring in the electrochemical system such as electron transfer w.r.t overpotential, surface adsorption, diffusion in electrolyte etc. This cannot be achieved by d.c. methods distinctively as they are based on the net cell polarizations which cannot be broken to study the effect of individual polarizations. EIS being an extremely sensitive technique can be effective in designing and analyzing new catalyst formulations, providing detailed information of the electrochemical interfacial system and obtaining the mechanism of the reaction.

ASSOCIATED CONTENT Supporting Information The Supporting information (SI) includes the results obtained via d.c. methods (Cyclic voltammograms, linear sweep voltammograms and KL plots) and a.c. methods (supporting Nyquist/Bode plots and the tabular data of resistance values obtained for the respective spectrum of the discussed catalyst). (Supporting information for publication, PDF)

AUTHOR INFORMATION Corresponding author *Dr. Keiko Miyabayashi Phone & Fax: +81-53-478-1151 Email: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

CONFLICT OF INTEREST: The authors declare no competing financial interest

ACKNOWLEDGMENTS This research was carried out under the framework of project financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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Evaluation of Alkylamine Modified Pt Nanoparticles as ORR

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