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Nov 8, 2016 - Comparisons of cyclic voltammograms for a polycrystalline Pt electrode with various amount of Nafion: Nafion free (black solid line), on...
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Impacts of Perchloric Acid, Nafion, and Alkali Metal Ions on Oxygen Reduction Reaction Kinetics in Acidic and Alkaline Solutions Shangqian Zhu, Xiaomeng Hu, Lulu Zhang, and Minhua Shao* Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of ClO4−, Nafion ionomer, and cations on Pt/C, Pd/C, and transition metal, N codoped carbon-based (Me− N−C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO4− had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand, showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however, is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The noncovalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This noncovalent interaction, however, was not observed on Pd/C, which showed the same ORR activity in all three electrolytes (LiOH, NaOH, and KOH). The ORR activity of Me−N−C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C. The mechanisms for the material-dependent activity trend in different cation solutions were discussed. et al., the adsorption of ClO4− anions on Pt could even influence the adsorption of methanol molecules and their further oxidations.20 All these results imply that ClO4− may impact the ORR kinetics on noble metal surfaces. Nafion ionomer is another commonly used chemical in ORR measurements in liquid cells, where it serves as the binder of the catalyst film on the RDE tip, unlike in fuel cells where it is also a conductor medium for protons in electrodes. In liquid cell, the adsorption of side-chain sulfonate groups of Nafion on bulk Pt surfaces was observed in spectroscopic studies21,22 and CO displacement experiments.17 The partially confined sulfonate groups in Nafion ionomer have similar interfacial behavior as compared to free (bi)sulfate anions in sulfuric acid solutions, resulting in a considerable loss of ORR activity due to their occupation on active sites,23 although their adsorption is weaker than (bi)sulfate anions.24 Solid-state cell test by Kodama et al. even found that the adsorption of sulfonate groups on Pt surfaces was enhanced under dry conditions.25 Thus, Nafion ionomers in catalyst films on both RDE tips and fuel cell electrodes are expected to have a big impact on ORR kinetics. Since it is rather difficult to prepare a perfect catalyst thin film on a glassy carbon electrode without adding Nafion in the catalyst ink,26 it is unclear whether Nafion ionomer significantly lowers the ORR activity of evaluated samples.

1. INTRODUCTION The reaction kinetics of oxygen reduction reaction (ORR) is rather sluggish even on noble-metal-based catalysts (Pt, Pd, etc.) in low-temperature fuel cells.1 Besides the types of catalytic materials,2−4 the electrolytes that are used in evaluating the ORR activity also significantly affect the performance of the evaluated material due to specific adsorption of ions. Fundamental understandings on the behaviors of those ions on different surfaces are of importance to the development of more active catalysts.5 Specific adsorptions of some strongly adsorbed ions (e.g., SO42−, Cl−) and their impacts on ORR kinetics on Pt surfaces have been well studied by various techniques.6−12 However, the impacts of some weakly or nonadsorbed ions on ORR are still under debate. For instance, ClO4− has been long-term regarded as a nonspecifically adsorbed anion, which rendered the most frequent usage of HClO4 as the electrolyte for ORR activity evaluation based on rotating disk electrode (RDE) and addition of perchlorate salts to enhance the conductivity in liquid cells.13−18 Later on, Swatari et al. found the first spectroscopic evidence of ClO4− adsorption on Pt surface in an in situ infrared spectroscopy (IR) study.19 The potential-dependent adsorption band at ∼1200 cm−1 indicated that ClO4− was adsorbed on the Pt surface. Electrochemical quartz crystal microbalance (EQCM) study by Santos et al. revealed that the adsorption of one ClO4− anion on Pt was accompanied by two water molecules.10 According to the in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study by Kunimastsu © XXXX American Chemical Society

Received: September 27, 2016 Revised: November 7, 2016 Published: November 8, 2016 A

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a precleaned glassy carbon RDE tip (5 mm in diameter, Pine Instruments) and allowed to dry in air. A leak-free Ag/AgCl electrode (with saturated KCl solution) calibrated by a reversible hydrogen electrode (RHE) was used as reference. A Pt wire was used as the counter electrode. A 0.1 M HClO4 + 0.1 M NaClO4 solution was prepared from ultrapure NaOH (VWR Chemicals) and HClO4 (GFS Chemicals, Veritas double distilled, Cl− ≤ 0.1 ppm, SO42− ≤ 1 ppm) to control the impurity content below 50 ppb. For comparison, one more 0.1 M HClO4 + 0.1 M NaClO4 solution was prepared from NaClO4 with a lower purity (Aldrich, ACS Reagent, 98%) and HClO4 (GFS Chemicals, Veritas double distilled) to allow a slightly higher impurity level (maximum 400 ppb). The electrode was cycled between 0.02 and 1.2 V vs RHE for 10 cycles in an argon-saturated 0.1 M HClO4 solution at 100 mV s−1. Then a stable cyclic voltammetry (CV) curve was recorded at 50 mV s−1. The polarization curves for ORR were recorded in oxygen-saturated solutions following the order of 0.1 M HClO4, 0.1 M HClO4 + 0.1 M NaClO4, and 0.2 M HClO4 using the same electrode at a scanning rate of 10 mV s−1 at 1600 rpm. CV curves were recorded in Ar-saturated solutions following the same order. 2.3. Nafion Impact. The Nafion effect on ORR activity was evaluated on both a flat polycrystalline Pt RDE tip (5 mm in diameter, Pine Instrument) and 46% Pt/C deposited on a glassy carbon electrode. The Pt electrode was polished using the 50 nm Al2O3 powder, followed by potential cycling between 0.05 and 1.4 V in an Ar-saturated 0.1 M HClO4 solution for 40 cycles at a scanning rate of 100 mV s−1. Then a stabilized CV curve was recorded between 0.05 and 1.2 V at 50 mV s−1. The polarization curves for ORR were recorded in an O2-saturated 0.1 M HClO4 solution. Then the polycrystalline Pt electrode was washed with ultrapure water for several times, and 5 μL of diluted Nafion solution (80 μL of 5% Nafion in 4 mL of H2O and 1 mL of isopropanol) was dropped on the electrode and dried in air. The estimated thickness of the Nafion layer was 113 nm according to the density of solid Nafion (1.5 g cm−3). The Nafion-covered Pt electrode was cycled in an Ar-saturated 0.1 M HClO4 solution until a stable CV curve was obtained. The ORR polarization curve was recorded in an O2-saturated 0.1 M HClO4 solution. The process was repeated to obtain a thicker layer of Nafion (∼226 nm) by dropping the same volume of Nafion solution. Nafion effect was also evaluated in 0.1 M NaOH solutions following similar procedures. A special protocol was developed in order to prepare a smooth Nafion-free thin Pt/C catalyst film on the glassy carbon electrode. Approximately 10 mg of Pt/C was ultrasonically dispersed in a mixing solvent consisting of 2 mL of water and 8 mL of isopropanol for 10 min. 10 μL of the suspension was deposited on the precleaned glassy carbon RDE tip and allowed to dry in a vacuum oven with a low pressure (0.2 bar) for 3 min. The CV and ORR polarization curves were recorded in 0.1 M HClO4 solutions following the procedures described in section 2.2. Then the electrode was carefully washed with water to remove residual electrolyte. 5 μL of diluted Nafion solution was dropped on the electrode and allowed to dry in air. The estimated ionomer to carbon (I/C) ratio was 0.67. Then the ORR polarization curve was recorded after cycling the electrode in an Ar-saturated 0.1 M HClO4 solution for five cycles. The electrode with a higher I/C ratio (1.33) was prepared by dropping another 5 μL of diluted Nafion solution on the same electrode following the previous procedure.

More suitable methods of preparing Nafion-free catalyst thin film should be developed in order to assess the impacts of Nafion on ORR. Cation effects on ORR are of great importance to the choice of electrolyte for alkaline fuel cells. CO2 tolerance (carbonate precipitation), O2 solubility, and conductivity are main concerns in practical applications, which lead to a more frequent usage of KOH as the electrolyte as compared to NaOH.27,28 The impact of cations themselves on the ORR kinetics receives less attention. Early studies by Kazarinov et al. revealed that various cations could adsorb on Pt surfaces.29,30 Later on, Adzic et al. found that the adsorption of cations originated from their interaction with the surface oxide species on Pt.31 Similar cation adsorption on Ru surfaces and its effect on ORR kinetics was studied by Anastasijevic et al.32 As proposed by Strmcnik et al. recently, hydrated cations (e.g., Na+, K+) would noncovalently adsorb on OH groups preadsorbed on bulk Pt (111) surface.33 Active sites for ORR were partially blocked by the formed M+(H2O)x clusters, which led to a decreased activity. The presence of immobilized cations at ∼4 Å away from the surface was confirmed by soft X-ray spectroscopy, indicating that cations were not directly adsorbed on the electrode surface.34 Thus, the impacts of these ions on ORR activity on carbon-supported nanocatalysts (e.g., Pt/C, Pd/C, and transition metal doped carbon-based catalysts) are of great interest from both fundamental and practical viewpoints. In this work, the ClO4− and Nafion impacts on ORR activity of noble metals and the cation (Li+, Na+, and K+) effects on both noble metals and carbon-based non-precious-metal catalysts were carefully studied.

2. EXPERIMENTAL SECTION 2.1. Preparation of Transition Metal and N Codoped Carbon-Based (Me−N−C) Electrocatalysts. Fe−N−C composites were synthesized following a modified protocol reported previously.35 All the chemicals used in the synthesis were purchased from Aldrich (ACS Reagent) without further purification. In a typical synthesis, 0.75 mL of aniline was added into 125 mL of 1.0 M HCl solution under magnetic stirring. Then 3.15 g of the metal precursor FeCl3·6H2O was added into the solution. After the dissolution of metal precursor, 1.25 g of (NH4)2S2O8 was added into the solution to polymerize aniline followed by vigorously stirring for 4 h at room temperature. 0.1 g of carbon (Ketjen Black EC-300J) was pretreated at 80 °C in a 70% nitric acid solution for 8 h, then it was ultrasonically dispersed in 10 mL of 1.0 M HCl solution, and finally mixed with the polymerized aniline solution. The solution was stirred for 48 h and then dried at 90 °C with magnetic stirring. The dried powder was ground and then heat-treated at 900 °C under an Ar atmosphere for 1 h. The obtained powder was subsequently leached in 150 mL of 0.5 M H2SO4 solution at 80−90 °C for 8 h in order to remove any iron and iron oxide nanoparticles and washed with sufficient deionized water for several times. The powder was then dried at 100 °C and heattreated again at 900 °C for 1 h to obtain the final product. Co− N−C was synthesized following the same procedure except that Co(NO3)2·6H2O was used as the metal precursor. 2.2. ClO4− Impact. Approximately 10 mg of Pt/C (46%, TEC10E50E, TKK) was ultrasonically dispersed in a mixing solvent consisting of 8 mL of water (Milli-Q UV-plus), 2 mL of isopropanol (VWR Chemicals), and 40 μL of 5% Nafion (Aldrich) for 10 min. 10 μL of the suspension was deposited on B

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Figure 1. Comparisons of cyclic voltammograms for Pt/C supported on a glassy carbon electrode in (a) Ar-saturated 0.1 M HClO4 (black solid line), 0.1 M HClO4 + 0.1 M NaClO4 (red dashed line), and 0.2 M HClO4 (blue dotted line) solutions at a scan rate of 50 mV s−1 and (b) O2saturated 0.1 M HClO4 (black solid line), 0.1 M HClO4 + 0.1 M NaClO4 (red dashed line), and 0.2 M HClO4 (blue dotted line) solutions at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

Figure 2. Comparisons of cyclic voltammograms for a polycrystalline Pt electrode with various amount of Nafion: Nafion free (black solid line), one Nafion layer (red dashed line), and two Nafion layer (blue dotted line) capped on the surface in (a) an Ar-saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1 and (b) an O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

2.4. Cation Impact. The Pt/C and Pd/C (35%, TKK) electrode were prepared according to the procedures described in section 2.2. The electrode was cycled between 0.02 V (0.07 for Pd/C) and 1.2 V vs RHE for 10 cycles in an Ar-saturated 0.1 M NaOH solution at a scan rate of 100 mV s−1. Then a stable CV curve was recorded at a scan rate of 50 mV s−1. The polarization curves for ORR were recorded in oxygen-saturated solutions following the order of 0.1 M KOH (VWR Chemicals), NaOH (VWR Chemicals), and LiOH (Fluka) solutions at a scanning rate of 10 mV s−1 at 1600 rpm. A different recipe was used to prepare the catalyst suspension for Me−N−C catalysts. Approximately 60 mg of Me−N−C catalyst was ultrasonically dispersed in a mixing solvent consisting of 8 mL of water, 2 mL of isopropanol, and 100 μL of 5% Nafion for 30 min. 10 μL of the suspension was deposited on a precleaned glassy carbon RDE tip and allowed to dry in air. The electrode was cycled between 0.05 and 1.0 V vs RHE for 10 cycles in an Ar-saturated 0.1 M KOH solution at a scan rate of 100 mV s−1. Then a stable CV curve was recorded at a scan rate of 50 mV s−1. Steady-state ORR polarization curves were recorded in O2-staturated solutions using potential step of 25 mV (between 0.4 and 0.95 V vs RHE) and step duration of 30 s at 1600 rpm. ORR activity was measured in O2-saturated solutions following the order of 0.1 M KOH, NaOH, and LiOH to investigate the impact of cations. Then ORR performance was also evaluated in O2-saturated 0.1 M NaOH, 0.2 M NaOH, and 0.1 M NaOH + 0.1 M NaCl (Alfa Aesar, 99.99%) solutions for the purpose of evaluating the cation concentration impacts. In all electrochemical measurements mentioned, a CHI Electrochemical Workstation (Model

760E, CH Instruments) was used, and IR correction was conducted to exclude the interference from electrolyte conductivity.

3. RESULTS AND DISCUSSION 3.1. ClO4− Impact on the ORR Kinetics of Pt/C. We first assessed the concentration effects of HClO4 on the CV and ORR polarization curves. Figure 1a shows the CVs of the same sample (Pt/C-coated RDE tip) in Ar-saturated 0.1 M (black solid line) and 0.2 M HClO4 (blue dotted line) solutions. A higher concentration of ClO4− does not affect the shape of the CV of Pt/C, implying that ClO4− might not be adsorbed on Pt surface in the potential window between 0.05 and 1.2 V. The overlapping of ORR polarization curves in Figure 1b also supports this argument. A similar experiment conducted in strong adsorbate-contained ((bi)sulfate anions) solutions shows different result (Figure S1). Changes on both CV and ORR polarizations are clearly observed after increasing the concentration of electrolyte. Specifically, the H adsorption/ desorption region between 0.05 and 0.35 V are narrowed in CV, along with a 10 mV negative shift on ORR half-wave potential. This result further supports the argument that ClO4− does not or only weakly adsorb on Pt surfaces. To exclude possible effect from pH change, similar experiments were conducted in 0.1 M HClO4 + 0.1 M NaClO4 solutions (red dashed line in Figure 1) with impurity level below 50 ppb. Clearly, the CV of Pt/C does not change in Figure 1a, while the limiting current is slightly lower due to a slower O2 diffusion rate in 0.1 M HClO4 + 0.1 M NaClO4 than that in pure HClO4 solutions. The poisoning of ClO4− anions on the kinetics of C

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echoes previous findings on severe poisoning effect of Nafion ionomer on Pt surfaces.23 A thicker Nafion layer does not cause further reduction of activity possibly due to the full coverage of the first Nafion layer, which is in consistency with the CVs in Figure 2a. In order to exclude the experimental error, a control experiment was conducted by dropping 5 μL of Nafion-free solution on the Pt surface and following the same experimental procedure. The negligible change in both CV and ORR polarization (Figure S4a,c) curves excludes the possibility of activity loss caused by other factors (exposure to air, extra electrochemical cycling, etc.) rather than Nafion ionomer itself. The Nafion effect was also evaluated in the alkaline electrolytes. As shown in Figure 3a, new faradaic peaks from the specific adsorption of Nafion on Pt surfaces were not observed in the whole potential window studied. The effect of Nafion layer on the ORR activity is much smaller in alkaline as compared to that in acidic electrolyte with only a 7% decrease on specific activity (at 0.8 V vs RHE) with one layer of Nafion on the surface (Figure 3b). There are two possible reasons for the observed discrepancy between acidic and alkaline electrolytes: (i) the more negative potential window (vs SHE) in alkaline than in acidic electrolyte may impose a stronger electrostatic repulsion to the sulfonate anion groups, which partially hinders the approach and specific adsorption of these groups to the surface of Pt; (ii) the cation exchange of H+ by Na+ may decrease the adsorption ability of sulfonate groups on Pt, as the latter binds to the sulfonate group more strongly. To validate this hypothesis, CV curves were recorded in Arsaturated 0.1 M HClO4 and 0.1 M HClO4 + 0.1 M NaClO4 solutions. As shown in Figure S5, the onset potential of H adsorption in the cathodic scanning direction is positively shifted, and the sulfonate adsorption faradaic peak at around 0.17 V is attenuated in the Na+ containing solution. These phenomena clearly demonstrate that the competitive adsorption is weakened upon replacement of proton with Na+ in sulfonate groups. In membrane electrode assembly (MEA), Nafion ionomers are mixed with Pt/C to conduct protons to the reaction sites as well as to serve as the binder of electrodes. Since carbon black contributes most of the volume and surface areas in the electrode, the dispersion of Nafion on Pt nanoparticles and its subsequent impact on ORR probably are different from those on a flat Pt surface. In order to evaluate the effect of Nafion ionomers on ORR kinetics in fuel cell electrodes, we conducted the thin-film RDE study using Pt/C. The fabrication of Nafionfree catalyst layer with a uniform coverage on the glassy carbon is very challenging. As reported by Shinozaki et al.,36 drying the Nafion-free catalyst suspension under an isopropanol atmosphere for several hours could produce a uniform catalyst layer, although a trace amount of nonionic surfactant was needed to improve the dispersion of catalysts. In our study, a low pressure (∼0.2 bar) drying technique was developed to prepare a good catalyst film within a very short period (∼3 min). A careful study of adjusting the isopropanol to water ratio (from 1:5 to 5:1) in the catalyst suspension suggested that the 4:1 ratio produced the most uniform film. A certain amount of diluted Nafion solution was added on the dried catalyst film to make the I/C ratio reach 0.67 and 1.33, which are close to the value that is commonly used to fabricate fuel cell electrodes. It should be noted that Nafion ionomers are expected to penetrate into the porous catalyst film instead of an overlayer capping on the top of the electrode. As shown in Figure 4a, the CV curves are nearly identical for the Pt/C samples with and without Nafion,

ORR was also negligible in the mixed kinetics and mass transport controlled region between 0.8 and 1.0 V. It should be noted that an extremely low impurity level of electrolyte is critical to obtain reliable results. In a controlled experiment, the same measurement was conducted in a 0.1 M HClO4 + 0.1 M NaClO4 solution containing ∼400 ppb SO42− and ∼300 ppb Cl− at maximum prepared from NaClO4 with a lower purity level. As shown in Figure S2, besides the narrowed H adsorption/desorption region and attenuated OH formation on CV, a considerable 12 mV negative shift of the half-wave potential on ORR curves was observed compared to the one obtained in a 0.1 M HClO4 solution. Despite strong evidence of specific adsorption of ClO4− on Pt surfaces,10,19,20 our result demonstrates that its impact on the ORR activity of Pt/C catalyst is negligible. It is possible that the interaction of ClO4− with Pt surfaces is too weak to compete with the adsorption of O2 and other oxygen containing species and consequently has a negligible impact on ORR kinetics. 3.2. Nafion Impact on a Bulk Pt Electrode and Pt/C. As mentioned in the Introduction, the possible impact of Nafion ionomer on the ORR activity of Pt/C has been studied by several groups.26,36,37 In the current study, the impact of Nafion on ORR activity was first evaluated on a polycrystalline Pt electrode. The CVs of a flat Pt electrode without (black solid line) and with a Nafion layer (∼226 nm in thickness, red dashed line) are compared in Figure 2a. The CV curve of a Nafion-capped Pt electrode shows distinct features as compared to the bare Pt surface. Two pairs of sharp faradaic peaks are observed between 0.05 and 0.4 V along with the narrowed H adsorption/desorption regions. The similar features between CVs of a Nafion-capped Pt in a HClO4 solution and a bare Pt electrode in a H2SO4 solution (Figure S3) indicate that the changes in CV are caused by competitive adsorption of sulfonate groups from Nafion ionomer in the H adsorption/ desorption region. This observation is also in consistency with the previous report by Subbaraman et al.17 If the thickness of the Nafion layer is doubled (blue dotted line), the CV shows no difference from the one with one layer. The overlapping of CV curves with different thickness of Nafion indicates that the first ionomer layer may have already covered the whole electrode surface. The impacts of Nafion capping on ORR activity are significant as shown in Figure 2b. A considerable 24 mV negative shift of ORR half-wave potential and the reduction of limit current density by ∼0.35 mA cm−2 are observed with one layer of Nafion on the Pt electrode. Doubling the amount of Nafion does not lead to a further shift of the half-wave potential, although a smaller limiting current density is observed due to the larger resistance on O2 diffusion in a thicker Nafion layer. The electrochemical surface areas (ECSAs) and specific activities of the Pt electrode were calculated and are summarized in Table 1. The specific activity is significantly reduced by ∼55% with one Nafion layer, which Table 1. Ehalf‑wave, ECSA, and Activity of Pt Electrode with Various Amounts of Nafion Capping

Ehalf‑wave (V) ECSA (cm2) specific activity (mA cm−2Pt) at 0.9 V

Nafion free

one Nafion layer

two Nafion layers

0.879 0.22 2.49

0.855 0.23 1.11

0.856 0.23 1.11

D

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Figure 3. Comparisons of cyclic voltammograms for a polycrystalline Pt electrode with various amount of Nafion: Nafion free (black solid line), one Nafion layer (red dashed line), and two Nafion layer (blue dotted line) capped on the surface in (a) an Ar-saturated 0.1 M NaOH solution at a scan rate of 50 mV s−1 and (b) an O2-saturated 0.1 M NaOH solution at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

Figure 4. Comparisons of cyclic voltammograms for Pt/C with various I/C ratio: 0 (black solid line), 0.67 (red dashed line), and 1.33 (blue dotted line) supported on a glassy carbon electrode in (a) an Ar-saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1 and (b) an O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

black, and only a small portion of Pt surface is covered. As a result, the poisoning effect from the sulfonate groups is significantly attenuated on carbon-loaded nanocatalysts. 3.3. Cation Impacts on Pt/C, Pd/C, and Me−N−C. NaOH and KOH are commonly used electrolytes in alkaline fuel cells and also in ORR measurements in alkaline media. In this study, we first assessed the cation impacts on the ORR activity of Pt/C using 0.1 M NaOH and KOH as electrolytes. As shown in Figure 5a, the CVs of the same Pt/C sample are almost identical in these two different electrolytes except that the current in the OH formation region between 0.7 and 0.8 V is slightly higher in the KOH solution. As a result, a higher coverage of OH on Pt surfaces is expected in this solution. The ORR polarization curve in the KOH solution, however, is 13 mV more positive than that in the NaOH solution, as shown in Figure 5b. When LiOH was used as the electrolyte, the differences in CVs and ORR polarization curves are even more obvious. The faradaic current in the OH formation region is totally attenuated (Figure 5a), and the ORR activity of Pt/C is 40 mV lower compared with that in a KOH solution (Figure 5b). According to Stremcnik et al.,33 hydrated cations could bond to surface OHads groups on several noble metal surfaces and serve as site block species by preventing the direct adsorption of O2 molecules on the electrode surface. Cations with a higher hydration energy had a stronger interact with surface OH groups and led to a lower ORR activity. Their finding on ORR activity trend of Pt (111) surface in different alkaline solutions is in consistency with our observation on Pt/ C. This result indicates that this noncovalent interaction also occurs on Pt nanocatalysts (∼2.5 nm). In order to gain more insight into cation effect, ORR polarization curves were compared between 0.1 and 0.2 M NaOH/KOH solutions. As

which is totally different from that of polycrystalline Pt electrode (Figure 2a). As expected, the limiting current density in the mass transport control region slightly decreased upon adding Nafion due to larger resistance for O2 transport (Figure 4b). However, the ORR polarization curves in the kinetic and mass transport mixed controlled region (0.8−1.0 V) showed no noticeable difference even with an I/C as high as 1.33. As shown in Table 2, the half-wave potentials only shifted by 2 and Table 2. ORR Activity of Pt/C with Different Nafion Content I/C: 0 Ehalf‑wave (V) Pt mass activity at 0.9 V vs RHE (A mg−1Pt)

0.903 0.27

I/C: 0.67 I/C: 1.33 0.901 0.26

0.900 0.25

3 mV compared with the Nafion free sample for I/C ratios of 0.67 and 1.33, respectively. The corresponding mass activity dropped by 0.01 and 0.02 A mg−1Pt from 0.27 A mg−1Pt, respectively. The mass activity is similar to that obtained in a typical ORR measurement with Nafion ionomer being mixed in the catalyst ink (I/C ratio is around 0.34).4 These results suggest that Nafion as a binder in typical ORR measurements does not affect the activity noticeably. The impact of Nafion on ORR activity measurement of Pt/C catalysts was also reported by Shinozaki et al. with a somewhat larger difference between Nafion free and contained samples.37 On the basis of our CV and ORR curves of Pt/C in Figure 4, we may conclude that the coverage of Nafion ionomers on surfaces of Pt nanoparticles is low. One of the possible reasons is due to the high specific area of carbon black (∼800 m2 g−1) as compared to Pt nanoparticles (∼80 m2 g−1). Most of Nafion ionomers may coat on carbon E

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Figure 5. Comparisons of cyclic voltammograms for Pt/C supported on a glassy carbon electrode in (a) Ar-saturated 0.1 M LiOH (black solid line), NaOH (red dashed line), and KOH (blue dotted line) solutions at a scan rate of 50 mV s−1, (b) O2-saturated 0.1 M LiOH (black solid line), NaOH (red dashed line), and KOH (blue dotted line) solutions at a scan rate of 10 mV s−1, (c) O2-saturated 0.1 (black solid line) and 0.2 M (red dashed line) NaOH solutions at a scan rate of 10 mV s−1, and (d) O2-saturated 0.1 M (black solid line) and 0.2 M (red dashed line) KOH solutions at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

Figure 6. Comparisons of cyclic voltammograms for Pd/C supported on a glassy carbon electrode in (a) Ar-saturated 0.1 M LiOH (black solid line), NaOH (red dashed line), and KOH (blue dotted line) solutions at a scan rate of 50 mV s−1 and (b) O2-saturated 0.1 M LiOH (black solid line), NaOH (red dashed line), and KOH (blue dotted line) solutions at a scan rate of 10 mV s−1; rotation speed = 1600 rpm.

bulk surface. This may alter the dipole moment of surface OH groups and thus change the hydrogen bond strength between OHads and water molecules around cations. It is likely that the interaction between OHads and hydrated K+ is not strong enough on Pt nanoparticles, which renders the negligible site blocking effect. It is also worth noting that the noncovalent bonding between hydrated cations and surface OHads may block some active sites for further formation of OHads. The weaker noncovalent bonding in KOH solutions can explain the larger OH coverage than that in NaOH and LiOH solutions. Then cation effects were also evaluated on Pd/C, which shows much higher activity in alkaline solutions than in acidic ones.39 As shown in Figure 6a, the CV curves recorded in NaOH and KOH are overlapped. Besides that the ORR polarization curves are almost identical (Figure 6b). This observation is totally different from that of Pt/C. In order to further confirm this observation, the CV and ORR curves were also recorded in 0.1 M LiOH solutions. Li+ cation has a higher

shown in Figure 5c, the half-wave potential of ORR is negatively shifted by 5 mV from 0.875 to 0.870 V after doubling the concentration of NaOH. On the other hand, there is no such a shift in KOH solutions as shown in Figure 5d. The decrease of limiting current in a higher concentration electrolyte is due to the lower O2 solubility, diffusion coefficient, and higher viscosity.38 Since the changes of pH are identical in these two electrolytes, cation concentration is the main reason for the observed ORR difference. Because of the equilibrium between bulk and adsorbed cations, more activity sites are likely blocked in the solution with a higher cation concentration, which can reasonably explain the lower ORR activity when doubling the NaOH concentration. In consideration of the obvious site block effect of K+ cation on Pt (111) surface,33 the negligible impact from the KOH (K+ cation) concentration in Figure 5d is intriguing. It should be noted that surface atoms of the Pt nanoparticle (∼2.5 nm) have a smaller average coordination number as compared with the F

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Figure 7. Comparisons of cyclic voltammograms for Fe−N−C supported on a glassy carbon electrode in (a) Ar-saturated 0.1 M LiOH (black line), NaOH (red line), and KOH (blue line) solutions at a scan rate of 50 mV s−1 and (b) O2-saturated 0.1 M LiOH (black line), NaOH (red line), and KOH (blue line) solutions; rotation speed = 1600 rpm.

Figure 8. Comparisons of steady-state ORR polarization curves for Fe−N−C supported on a glassy carbon electrode in (a) O2-saturated 0.1 M (black line) and 0.2 M (red line) NaOH solutions and (b) O2-saturated 0.1 M NaOH (black line) and 0.1 M NaOH + 0.1 M NaCl (red line) solutions at 1600 rpm.

hydration energy than that of Na+ and K+; thus, a lower ORR activity is expected if the noncovalent interaction also occurs on Pd surfaces. Interestingly, no difference is observed in either CV or ORR (blue dotted line in Figure 6a,b). Our results indicate that the nature of catalyst surfaces plays an essential role in determining the adsorption behavior of ions. It should be noted that the OHads formation on Pd/C occurs between 0.48 and 0.78 V, above which the formation of Pd oxide gradually starts.40,41 The kinetics predominant region of ORR lies at a more positive potential window between 0.85 and 1.0 V, which is overlapped with the potential window where Pd− OH becomes Pd−O. It is likely that hydrated cations are noncovalently adsorbed on Pd surfaces but are removed from the surface due to the formation of Pd oxides, or the Pd−OH gradually loses the ability to bond hydrated cations due to the change of dipole moment at higher potentials. As a result, the site block effects of cations are not observed on Pd surfaces. Future work needs to be conducted in order to validate this hypothesis. Transition metal and N codoped carbon-based materials show comparable activities to Pt/C in alkaline solutions. The cation effect in ORR activity, however, has not been studied. In this work, we use Fe−N−C synthesized by thermal pyrolysis as a model catalyst illustrating possible cation effect on its ORR activity. The CV and ORR curves of the same sample were recorded in 0.1 M LiOH, NaOH, and KOH solutions. Featureless CVs shown in Figure 7a give little information apart from the charge and discharge behavior of the electrochemical double layer, which is a common phenomenon observed on transition metal doped carbon-based materials. However, the half-wave potentials of steady-state ORR polarization curves change following the order of LiOH >

NaOH > KOH. The difference in half-wave potential between LiOH and KOH is 10 mV (Figure 7b). The similar trend was also found in Co−N−C (Figure S6), indicating that the alkali metal cation induced effect is independent of the type of doped metals. It is worth noting that the activity trend observed on Me−N−C catalyst in alkaline solutions is totally reversed compared with that on Pt (111) and Pt/C and has not been reported previously. Similarly, the impact of cation concentration on ORR activity was investigated. Figure 8a shows the comparison of ORR polarization curves in O2-saturated 0.1 and 0.2 M NaOH solutions. The half-wave potential in the 0.2 M NaOH is 9 mV higher than that in a 0.1 M one. A similar experiment was conducted in an O2-saturated 0.1 M NaOH + 0.1 M NaCl solution to exclude the possible impact of pH change. As shown in Figure 8b, a similar positive shift on half-wave potential is observed in the mixed solution, indicating that a higher Na+ concentration is the reason for the improved ORR kinetics. The higher ORR activity in the solutions with a higher Na+ concentration suggests the absence of site blocking effect caused by the interaction between surface OHads species and hydrated cations; otherwise, a negative shift in ORR curve is expected. An ORR mechanism on Me−N−C proposed by Ramaswamy et al.38,42,43 suggested that the 4e− reduction involving the direct adsorption of O2 on the surface was the predominant reaction pathway, and the bonding strength of OH groups on the catalyst surface was rather weak that could be facilely removed by the competitive adsorption of O2 molecules. This argument implies that hydrated cations cannot be adsorbed on catalyst surfaces through the noncovalent interaction with OHads and is consistent with our findings. It is worth noting G

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alkaline solutions. Thus, protonation of reaction intermediates is not expected to be the rate-determining step. In addition, the potential window at the SHE scale of ORR in acidic electrolytes is much higher than that in alkaline ones. The strong electrostatic repulsion between the catalyst surface and hydrated cations may drive the cations out of the OHP, thus rendering the negligible impact of cations in acidic media. Based on the results and discussion above, the interaction models of hydrated cations with Pt, Pd, and Me−N−C surfaces are illustrated in parts a, b, and c of Figure 10, respectively. A

that anchored hydrated cations can also facilitate certain electrochemical reactions on noble metal surfaces. For instance, Lopes et al.44 found that the steric effects from the adsorbed hydrated cations can benefit the adsorption of ethanol molecules. In addition, the EtOads intermediate can be stabilized by consequently formed OHads−Me+(H2O)x−EtOads network, resulting in a faster reaction kinetics at the low overpotential region. However, these effects from neighboring adsorbed cations are unlikely to occur on Me−N−C catalysts due to the isolation of activity sites (Me−N sites) from each other. Thus, the adsorption of cations can only lead to the unavailability of active sites, and a lowered ORR activity is expected if alkali metal cation has a higher hydration energy, which is apparently not the case observed in the current study. The analysis above redirected our attention to other aspects of the double-layer structure in order to explain the cation-induced enhancement on ORR activity. During the ORR in alkaline electrolytes, the inner-Helmholtz plane (IHP) is occupied by adsorbed O2 molecules, OH groups, and water, and the hydrated cations are expected to occupy the outer-Helmholtz plane (OHP). The water shell in the hydrated cation clusters is one of the ready proton sources for the reactions due to their short distance to reaction intermediates. It should be noted that in alkaline solutions the rate of proton transfer from water to intermediates is much slower as compared with the acidic counterpart.38 As reported by Subbaraman et al.,45 cations with a higher hydration energy can destabilize the H−OH bond of the surrounding water molecules more significantly, accelerating the reaction rate of HER in alkaline electrolytes. Herein, we propose that this effect also occurs on ORR. By using the supporting electrolyte with alkali metal cations of a higher hydration energy (Li+ > Na+ > K+), the protonation rate of reaction intermediates in high pH electrolytes can be improved. The improved ORR activity of the Me−N−C catalysts in the electrolyte with a higher cation concentration is likely caused by a faster compensation of “activated” water molecules to the electrode surface upon consumption in the ORR. To further support our argument, ORR polarization curves in O2-saturated 0.1 M HClO4 and 0.1 M HClO4 + 0.1 M NaClO4 solutions were also recorded. As shown in Figure 9, the addition of Na+ cations into the acidic electrolyte shows no improvement on the ORR activity. It should be noted that hydronium cations (H3O+) is the source for intermediate protonation process in acidic electrolytes, which has a much higher activity than the destabilized water molecules in hydrated cation clusters in

Figure 10. Interaction models of hydrated cations with (a) Pt, (b) Pd, and (c) Me−N−C surfaces.

noncovalent bonding between the hydrated cations with surface OH groups was proposed on Pt surfaces, while unlikely on Pd surfaces. On the other hand, the proton transfer from water molecules in the hydrated cation clusters to the adsorbed oxygen molecules was proposed on the Me−N−C surfaces.

4. CONCLUSION In this study, the specific adsorption of ClO4−, Nafion ionomer, and cations on Pt/C, Pd/C, and Me−N−C catalysts and their effects on the ORR kinetics were studied. The main findings are summarized below. The specific adsorption of ClO4− was revisited on Pt/C, and the results showed that its adsorption strength might be too weak to have a discernible impact on ORR. In terms of Nafion, a severe loss on ORR activity of a bulk Pt disk electrode in an acidic electrolyte was observed when it was covered with a Nafion thin film. This was mainly due to the significant poisoning effect from the sulfonate group adsorbed on Pt surfaces. This poisoning effect was much weaker in alkaline electrolytes due to the exchange of H+ by Na+ in the ionomer and stronger electrostatic repulsion that prevented the approach of sulfonate groups to Pt surfaces. On the other hand, a careful study revealed a negligible effect of Nafion ionomer on ORR kinetics of Pt/C with an I/C ratio below 1.33. This result demonstrated that adding Nafion ionomer in catalyst ink in RDE experiment would not change the activity of Pt/C. In addition, it implied that the Nafion ionomer in the electrode of MEA will not negatively impact the ORR kinetics of Pt/C but on oxygen transport in wet conditions. Then cation impacts on Pt/C, Pd/C, and Me−N−C catalysts were systematically studied for the first time. A lower ORR

Figure 9. Comparisons of steady-state ORR polarization curves for Fe−N−C supported on a glassy carbon electrode in O2-saturated 0.1 M HClO4 (black line) and 0.1 M HClO4 + 0.1 M NaClO4 (red line) solutions at 1600 rpm. H

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Supported on a Pt (111) Electrode. J. Phys. Chem. B 2003, 107, 9813− 9819. (8) Climent, V.; Markovic, N. M.; Ross, P. N. Kinetics of Oxygen Reduction on an Epitaxial Film of Palladium on Pt (111). J. Phys. Chem. B 2000, 104, 3116−3120. (9) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Sulfuric Acid Solution: Rotating Ring-Pt (hkl) Disk Studies. J. Phys. Chem. 1995, 99, 3411−3415. (10) Santos, M. C.; Miwa, D. W.; Machado, S. A. S. Study of Anion Adsorption on Polycrystalline Pt by Electrochemical Quartz Crystal Microbalance. Electrochem. Commun. 2000, 2, 692−696. (11) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J. The Oxygen Reduction Reaction on a Pt/Carbon Fuel Cell Catalyst in the Presence of Chloride Anions. J. Electroanal. Chem. 2001, 508, 41−47. (12) Stamenkovic, V.; Markovic, N. M.; Ross, P. N. StructureRelationships in Electrocatalysis: Oxygen Reduction and Hydrogen Oxidation Reactions on Pt (111) and Pt (100) in Solutions Containing Chloride Ions. J. Electroanal. Chem. 2001, 500, 44−51. (13) Lazarescu, V.; Clavilier, J. pH Effects on the Potentiodynamic Behavior of the Pt (111) Electrode in Acidified NaClO4 Solutions. Electrochim. Acta 1998, 44, 931−941. (14) Li, M. F.; Liao, L. W.; Yuan, D. F.; Mei, D.; Chen, Y. X. pH Effect on Oxygen Reduction Reaction at Pt (111) Electrode. Electrochim. Acta 2013, 110, 780−789. (15) Macia, M. D.; Campina, J. M.; Herrero, E.; Feliu, J. M. On the Kinetics of Oxygen Reduction on Platinum Stepped Surfaces in Acidic Media. J. Electroanal. Chem. 2004, 564, 141−150. (16) Markovic, N. M.; Gasteiger, H.; Ross, P. N. Kinetics of Oxygen Reduction on Pt (hkl) Electrodes: Implications for the Crystallite Size Effect with Supported Pt Electrocatalysts. J. Electrochem. Soc. 1997, 144, 1591−1597. (17) Subbaraman, R.; Strmcnik, D.; Stamenkovic, V.; Markovic, N. M. Three Phase Interfaces at Electrified Metal-Solid Electrolyte Systems 1. Study of the Pt (hkl)-Nafion Interface. J. Phys. Chem. C 2010, 114, 8414−8422. (18) Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E. SiteSpecific Vs Specific Adsorption of Anions on Pt and Pt-Based Alloys. J. Phys. Chem. C 2007, 111, 9267−9274. (19) Sawatari, Y.; Inukai, J.; Ito, M. The Structure of Bisulfate and Perchlorate on a Pt (111) Electrode Surface Studied by Infrared Spectroscopy and Ab-Initio Molecular Orbital Calculation. J. Electron Spectrosc. Relat. Phenom. 1993, 64, 515−522. (20) Kunimatsu, K.; Hanawa, H.; Uchida, H.; Watanabe, M. Role of Adsorbed Species in Methanol Oxidation on Pt Studied by ATRFTIRAS Combined with Linear Potential Sweep Voltammetry. J. Electroanal. Chem. 2009, 632, 109−119. (21) Ayato, Y.; Kunimatsu, K.; Osawa, M.; Okada, T. Study of Pt Electrode/Nafion Ionomer Interface in HClO4 by in Situ SurfaceEnhanced FTIR Spectroscopy. J. Electrochem. Soc. 2006, 153, A203− A209. (22) Gómez-Marín, A. M.; Berná, A.; Feliu, J. M. Spectroelectrochemical Studies of the Pt (111)/Nafion Interface Cast Electrode. J. Phys. Chem. C 2010, 114, 20130−20140. (23) Subbaraman, R.; Strmcnik, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 2010, 11, 2825−2833. (24) Kodama, K.; Shinohara, A.; Hasegawa, N.; Shinozaki, K.; Jinnouchi, R.; Suzuki, T.; Hatanaka, T.; Morimoto, Y. Catalyst Poisoning Property of Sulfonimide Acid Ionomer on Pt (111) Surface. J. Electrochem. Soc. 2014, 161, F649−F652. (25) Kodama, K.; Jinnouchi, R.; Suzuki, T.; Murata, H.; Hatanaka, T.; Morimoto, Y. Increase in Adsorptivity of Sulfonate Anions on Pt (111) Surface with Drying of Ionomer. Electrochem. Commun. 2013, 36, 26− 28. (26) Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E.; Kocha, S. S. Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction. Anal. Chem. 2010, 82, 6321−6328.

activity in an alkaline solution containing the cation with a higher hydration energy was observed on Pt/C. The result indicated the noncovalent interaction between surface OHads groups and hydrated cations also occurred on Pt/C, and the active sites were blocked by the anchored clusters on the surface. However, this effect was totally absent on Pd/C, indicating that the nature of catalyst surfaces also played an important role in the interfacial behavior of ions. The ORR activity of Me−N−C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C; i.e., the highest activity was observed in the solution containing the cation with the highest hydration energy. The cationinduced enhancement on the ORR activity of Me−N−C was reported for the first time. The destabilized water molecules in the hydration cation clusters due to the interaction with alkali metal cations were proposed to be the proton sources for intermediates protonation. The stronger the hydration energy of the cation, the easier the proton transfer and hence the higher ORR activity. The findings in this paper emphasize the importance of specific adsorption of ions and double-layer interfacial effects on the ORR activity measurement in aqueous solutions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09769. Additional electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.). ORCID

Minhua Shao: 0000-0003-4496-0057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from Research Grant Council of the Hong Kong Special Administrative Region (IGN13EG05 and 26206115) and a startup fund from the Hong Kong University of Science and Technology.

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REFERENCES

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