Charting the Outer Helmholtz Plane and the Role of Nitrogen Doping

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Charting the Outer Helmholtz Plane and the Role of Nitrogen Doping in the Oxygen Reduction Reaction Conducted in Alkaline Media Using Non-Precious Metal Catalysts Stanfield Youngwon Lee, Dong Young Chung, Myeong Jae Lee, Yun Sik Kang, Heejong Shin, Mi-Ju Kim, Christopher W. Bielawski, and Yung-Eun Sung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04771 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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The Journal of Physical Chemistry

Charting the Outer Helmholtz Plane and the Role of Nitrogen Doping in the Oxygen Reduction Reaction Conducted in Alkaline Media Using Non-Precious Metal Catalysts Stanfield Youngwon Lee,1,2,3,4,† Dong Young Chung,3,4 Myeong Jae Lee,3,4 Yun Sik Kang,3,4 Heejong Shin,3,4 Mi-Ju Kim 3,4 Christopher W. Bielawski,12,*and Yung-Eun Sung3,4,* 1

Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919,

Republic of Korea

2

Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of

Science and Technology (UNIST), Ulsan 44919, Republic of Korea

3

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826 Republic of

Korea 4

School of Chemical and Biological Engineering, Seoul National University, Seoul 08826

Republic of Korea

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ABSTRACT

This study was focused on elucidating the origin of the catalytic activity displayed by non-precious metal-based oxygen reduction reaction (ORR) catalysts before and after heat treatment. Electrochemical measurements were recorded using a series of metal phthalocyanines calculated to exhibit varying oxygen adsorption energies before and after heat treatment at a temperature sufficiently high to facilitate degradation. Collectively, the results indicate that while the oxygen adsorption is germane to the catalytic activity before heat treatment, the ORR appears to proceed through a different pathway that is not dependent on adsorption energy after heat treatment. These conclusions help to explain the high catalytic activities exhibited by carbon- or nitrogen-based materials containing metal ions after heat treatment and may lead to the realization of substitutes for ORR catalysts that utilize precious transition metals.

INTRODUCTION

Benefits such as a high theoretical output voltage and environmentally friendly byproducts are some of the reasons why the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) play integral roles in a broad range of energy conversion1-2 and energy storage devices.3 Unfortunately, the large scale commercialization of such systems has been hampered by the lack of an abundant, viable, and cost-efficient catalyst.1 While platinum has been the most widely used catalyst for the ORR, it is relatively scarce, expensive,4 and susceptible to carbon monoxide poisoning which, in turn, requires the use of ultra-pure reactants.5-6 As a result, catalysts that show activities similar to or greater than that of platinum but are less expensive and less susceptible to poisoning are necessary if alternative energy devices based on the aforementioned reactions are to be widely implemented. 2 ACS Paragon Plus Environment

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Numerous reports have shown that carbon-based materials containing nitrogen or nitrogen coordinated metal ions catalyze the ORR with rates comparable to that of platinum.7-10 In addition, when heat treated, these catalysts often exhibit even higher activities which relegate the importance of choosing specific metal ions.11-14 Such characteristics, when coupled with significantly lower material costs, provide hope that an effective non-precious metal catalyst to replace platinum can be developed.10 Regardless, a more in depth understanding of how the ORR proceeds using such non-precious metal-based catalysts is warranted. Using theoretical15-16 as well as experimental17-18 methods, the mechanism through which the ORR proceeds on platinum surfaces has been studied in detail. As a result, various structures, including core shell nanoparticles19-20 and nanoparticles with platinum enriched surfaces3,21-22 have been designed to not only increase catalytic activity but also to minimize the amount of platinum required. While non-precious metal-based catalysts have also been shown to exhibit high ORR catalytic activities,7-9 their underlying mechanisms have yet to be fully elucidated. Indeed, although numerous pathways involving the nitrogen sites,23-24 the carbons adjacent to the nitrogen sites,25 and/or the coordinated metal ions8, 26 have all been proposed as the active sites, an overall pathway has not yet been universally accepted. To gain a deeper understanding of the factors that influence the oxygen reduction reaction in alkaline media, copper as well as iron phthalocyanine complexes were independently loaded onto high surface area carbon supports and electrochemically characterized in the presence of various electrolytes. Additional electrochemical measurements were taken after heat treatment at a temperature that was sufficiently high to cause degradation of the phthalocyanine component. Assessment of the corresponding catalytic activities should not only help to



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determine if metal coordinated species are the active sites but also clarify how the inner and outer Helmholtz planes influence the ORR.

RESULTS Electrolyte Influence on the Catalytic Activities Displayed by Platinum and Gold A summary of the activities measured for a 40% Pt/C based catalyst in the presence of alkaline or acidic electrolytes is shown in Figure S1. The ORR polarization curves obtained in the presence of the former with different cations showed several distinct characteristics. For example, at potentials higher than 0.7 V, the catalytic performances began to deviate and followed in the general order of: LiOH < NaOH < KOH ≤ CsOH < HClO4. For ORRs monitored at potentials lower than approximately 0.7 V, similar performances were observed with the exception of the measurement taken in the presence of CsOH. Markovic and co-workers theorized that the anomaly may be due to the differences caused by a lower working electrode potential in alkaline media which resulted in the outer Helmholtz plane, and consequently the metal cations, to be closer in proximity to the catalyst surface.27 Furthermore, the 40% Pt/Cbased catalyst exhibited the highest activity in the presence of acidic electrolytes likely because the higher working potential would repel the outer Helmholtz plane such that oxygen diffusion would be unhindered.27 Although this groundbreaking study was performed on platinum single crystals, Katsounaros and Mayrhofer reported that the trend may also be observed with polycrystalline platinum surfaces.28 Opposite to that observed with platinum catalysts, an as-synthesized gold on Vulcan XC72 (Au/C) based catalyst exhibited significantly higher ORR activities when acidic electrolytes


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were replaced with basic analogues (see Figure S2). Although rationales for this enhancement have been reported,29 a detailed understanding of the underlying processes has not been experimentally determined to the best of our knowledge. Regardless, since no change in the ORR activities was observed in the presence of different electrolytes, we concluded that the dominant pathway in alkaline media is not dependent on oxygen diffusion to the inner Helmholtz plane and thus may not rely on direct adsorption of oxygen to the catalyst surface. The Mukerjee group reported that electron tunneling from the electrode to the outer Helmholtz plane may lead to the formation superoxide anions in alkaline media.30 Indeed, Blizanac et al. also calculated that such superoxide anions, though unstable in acidic media, are relatively stable in alkaline media.31 Nonetheless, the use of electrolytes containing different cations can be a useful and effective technique to help elucidate favored reaction pathways under different conditions.

Electrolyte Influence on the Catalytic Activities Displayed by Copper Phthalocyanine (CuPc/C) and Iron Phthalocyanine (FePc/C) Copper and iron phthalocyanines (i.e., CuPc and FePc) were chosen as model substrates because oxygen binding to the corresponding metal ions should occur in a similar fashion and also because the varying oxygen adsorption strengths may confirm that the ORR occurs through in the inner Helmholtz plane.32-33 Density functional theory calculations have indicated that CuPc should not bind to oxygen whereas FePc should bind and facilitate its four electron reduction.3233

As shown in Figure 1, the UV-Vis spectra recorded for CuPc and FePc before and after

loading onto a carbon support were similar, consistent with retention of the metal coordination structures. In contrast, the UV-Vis absorbance spectra recorded after heat treatment at 8000C revealed a significant reduction in the aforementioned absorbance signals. As such, we



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concluded that most of the metal coordination sites decomposed during heat treatment. Heat treatment at 6000C as well as 9000C were also performed to gain further insight into the decomposition of the phthalocyanine structures (see Figure S3).

Figure 1. UV-Vis absorbance spectra recorded for (a) CuPc, (b) CuPc/C, (c) FePc, and (d) FePc/C dispersed in DMF before (blue) and after heat treatment at 8000C (green). After the metal phthalocyanines were loaded onto carbon supports, a series of electrochemical measurements were performed. Although the underlying mechanism is not well understood, carbon materials have been shown to catalyze the ORR. Differing theories involving the stability of the superoxide generated during the course of the reaction,34 the presence of unsaturated carbons at crystallite edges35 and/or the presence of oxygen containing surface groups36 have been offered to explain how carbon-based materials may catalyze the ORR. Collectively, these theories indicate that carbon-based catalysts proceed mostly through a two electron reduction pathway, which results in oxygen reduction to peroxide. In addition, 6 ACS Paragon Plus Environment

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differences in ORR activity have been observed when different electrolytes were used. For example, as summarized in Figure 2, the ORR activities measured for Vulcan XC-72 in the presence of various electrolytes increased in the following order: LiOH ≤ NaOH < CsOH < KOH (see ESI for the corresponding kinetic current plots). Moreover, the ORR polarization curves measured for CuPc/C were found to increase in the following order: LiOH ≤ NaOH < KOH ≤ CsOH. However, when comparing these activities to that of the carbon support (see Figure S4), we concluded that the activity was mostly from the latter. In addition, a decrease in catalytic activity was observed as the initial weight loading of CuPc was increased (see figures

S5-S6). Consistent with previously reported density functional calculations,32-33 we surmise that the CuPc decreases the carbon surface exposed to the electrolyte and reactant, which results in lower catalytic activities.



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Figure 2. (a, b) Cyclic voltammetry data and RDE polarization curves as recorded in a 0.1 M aqueous solution of LiOH (red), NaOH (green), KOH (blue), or CsOH (black) for Vulcan XC72. (c, d) Cyclic voltammetry data and RDE polarization curves as recorded in a 0.1 M aqueous solution of LiOH (red), NaOH (green), KOH (blue), or CsOH (black) for CuPc/C. Two distinct redox processes corresponding to the Fe1+/2+ and Fe2+/3+ redox couples were recorded at approximately 0.25 V and 0.8 V for the FePc/C-based catalyst using cyclic voltammetry (see Figure 3a).37 Of the two processes, the Fe2+/3+ couple appeared to play an integral role in the ORR kinetics. As shown in Figure 3b, the ORR activity curve was found to sharply rise from the limiting current value to a nearly zero current value at a potential close to where the Fe2+/Fe3+ redox couple was measured. The sharp incline is in contrast to that reported for Pt/C catalysts as signals corresponding to redox couples were not observed and the respective ORR activity was presumably deactivated due to the blocking of active sites by hydroxide ion adsorption and/or the formation of platinum oxide over time.27 The gradual change was not observed in the presence of FePc because the ORR may proceed only when the Fe ions are in the Fe2+ state and instantaneous deactivation of all the active sites would occur when the iron ions are relatively oxidized (i.e., in the Fe3+ state). As a result, a nearly instantaneous decrease in the reduction activity may be expected to be observed near the redox peak potential, which is consistent with the data shown in Figure 3c.

Density functional theory calculations and

experimental data obtained in situ were also in agreement with this hypothesis.38-41 No significant differences were observed when the ORR was examined in the presence of other electrolytes. While slight differences were recorded at potentials exceeding 0.8 V, which may be attributed to changes that occur after the Fe ions undergo oxidation (see above), the coordinated


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metal species appear to exhibit the appropriate oxygen binding energy to adsorb and release oxygen, which leads to a relatively high catalytic activity. To probe any roles that the Fe+2-N-C coordinated sites play in the ORR, KCN ([KCN]0 = 10 mM) was added to the electrolyte and a series of electrochemical measurements were performed. Under these conditions, the ORR activity was measured to be relatively low and the underlying redox processes were not observed via cyclic voltammetry (see Figure 3d). Since cyanide ions (CN-) are known to ligate to iron species and can block the active sites found on catalytically-active surfaces,7, 11, 42 the aforementioned results were consistent with an impeded

ORR upon Fe coordination and indicated that the ORR proceeded through an inner Helmholtz plane on the surface of the catalyst. Figure 3. (a) Cyclic voltammetry data as recorded in a 0.1 M aqueous solution of LiOH (red), NaOH (green), KOH (blue), or CsOH (black) for FePc/C. (b) Comparison of the RDE polarization curves as recorded in a 0.1 M aqueous solution of KOH for FePc/C (blue) or 40% Pt/C (orange). (c) RDE polarization curves as recorded in a 0.1 M aqueous solution of LiOH 9 ACS Paragon Plus Environment


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(red), NaOH (green), KOH (blue), or CsOH (black) for FePc/C. (d) Comparison of the RDE polarization curves for FePc/C as recorded in a 0.1 M aqueous solution of KOH (blue) or 0.1 M KOH + 0.010 M KCN (green). Heat Treatment Effects When comparing the electrochemical data recorded before and after heat treatment, a loss in ORR activity was observed after the Vulcan XC-72 catalyst was heat treated (see Figures 4a and 4b). As mentioned earlier, theories explaining the reduction of oxygen to peroxide on carbon-based catalysts have been proposed.34-36 One of these theories indicated that the oxygen containing functional groups present on the surface may alter the surface properties of the carbon that facilitates the ORR.36, 43 Heat treatment usually leads to a loss of surface functional groups35 and is also frequently used to reduce graphene oxide.44-45 Thus, heating the catalyst appeared to result in a loss of oxygen containing functional groups, which may explain the aforementioned loss in ORR activity. The relative activity displayed by the Vulcan XC-72 catalyst in the presence of different electrolytes was also found to be unchanged (i.e., LiOH ≤ NaOH

Charting the Outer Helmholtz Plane and the Role of Nitrogen Doping

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