Article pubs.acs.org/JPCC
In Situ XAFS and HAXPES Analysis and Theoretical Study of Cobalt Polypyrrole Incorporated on Carbon (CoPPyC) Oxygen Reduction Reaction Catalysts for Anion-Exchange Membrane Fuel Cells Koichiro Asazawa,*,†,‡ Hirofumi Kishi,†,‡ Hirohisa Tanaka,† Daiju Matsumura,§ Kazuhisa Tamura,§ Yasuo Nishihata,§ Adhitya Gandaryus Saputro,# Hiroshi Nakanishi,# Hideaki Kasai,# Kateryna Artyushkova,⊥ and Plamen Atanassov⊥ †
Frontier Technology R&D Division, Daihatsu Motor Company Ltd., Shiga 520-2593, Japan Japan Science and Technology Agency, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Quantum Beam Science Center, Japan Atomic Energy Agency, Sayo-gun, Hyogo 679-5148, Japan # Department of Applied Physics, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ⊥ Chemical and Nuclear Engineering Department, Center for Emerging Energy Technologies, University of New Mexico, Albuquerque, New Mexico 87131, United States ‡
S Supporting Information *
ABSTRACT: Non-noble metal electrocatalysts not only are a solution to limited resources but also achieve higher efficiency for fuel cells, especially in alkaline media such as alkaline membrane fuel cells. Co-polypyrrole-based electrocatalysts provide high oxygen reduction reaction (ORR) reactivity, but the active sites and reaction mechanism have yet to be elucidated fully. In this study, ex situ and in situ synchrotron characterization and theoretical study have been combined to evaluate the ORR mechanism on two possible active sites consisting of Co coordinated with pyrrolic nitrogen and Co coordinated with pyridinic nitrogen.
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INTRODUCTION
remarkably better in direct hydrazine fuel cells in comparison with polymer electrolyte fuel cells with a power density of 500 mW cm−2.1,11 The CoPPyC-based catalyst materials evaluated in this study can be generally classified as metal−N−C type catalyst materials. This class of non-Pt catalyst was initially inspired by biomimetic origins. Jasinski was the first to show the potential of utilizing transition metal−macrocycle compounds as ORR catalyst precursors in alkaline media.12 Due to the heterogeneous nature of the materials, a basic understanding of the interplay between the chemistry and activity and the mechanism of ORR is still missing. To rationally design active and durable ORR electrocatalysts, the mechanism of oxygen reduction and the active site or sites where the reaction happens need to be clearly identified. Discussion of the possible active sites and the role of a transition metal (Me) in the activity of non-noble metal electrocatalysts derived from nitrogen-doped carbon has been contested in the past few years. Studies of ORR on N-doped carbon nanotubes (CNTs) showed that without metal, only the
Platinum is a limited resource on earth, leading to substantial efforts toward the replacement of platinum by non-noble metal electrocatalysts.1−3 Under alkaline conditions, the kinetics of the oxygen reduction reaction (ORR) on the cathode are enhanced, leading to improved fuel cell energetic efficiency. Alkaline fuel cells (AFC) with an alkaline liquid electrolyte such as KOH(aq) are the best performing of all known conventional hydrogen oxygen fuels. The application of alkaline conditions at the electrodes opens the potential use of a range of low-cost non-precious-metal catalysts. In an attempt to potentially enable liquid electrolyte-free AFCs, a number of groups have begun research efforts devoted to the fabrication and engineering of anion-exchange membranes and ionomer solutions.4−10 In addition, there are incentives to develop new and novel materials for AFC systems that have the potential to alleviate or eliminate the technical issues associated with liquid electrolyte systems. This may include the use of a much more diverse selection of potential fuels that are thermodynamically favorable in alkaline media. Cobalt polypyrrole incorporated on carbon (CoPPyC) electrocatalysts have been investigated for anion-exchange membrane fuel cells.11 The performance of these catalysts is © 2014 American Chemical Society
Received: July 27, 2014 Revised: October 3, 2014 Published: October 8, 2014 25480
dx.doi.org/10.1021/jp507534f | J. Phys. Chem. C 2014, 118, 25480−25486
The Journal of Physical Chemistry C
Article
first step of the oxygen reduction reaction O2 → H2O2 occurs.13 Transition metal appears to be imperative for complete reduction of O2 to H2O, yet there is no clear evidence of the mechanism (2 × 2e− or 4e−). There are three possible mechanisms for ORR representing (I) direct 4e− in which full conversion of oxygen to water occurs and (II) indirect reactions (2 × 2e−) in which during the first step oxygen is reduced to hydrogen peroxide, which is further reduced to water during the second step. Two types of 2 × 2e− indirect reactions are possible, that is, (IIa) a single-site consequential 2 × 2e− mechanism and (IIb) a dual-site consequential 2 × 2e− bifunctional mechanism. Recently, the dual-site mechanism of ORR on CoPPyC electrocatalysts in alkaline media was proposed,14 and similar investigations on the nature of the active sites of CoPPyC in acid by Zhang et al. were conducted.15 Through structure-to-property analysis between XPS and activity, the dual-site consequential type of mechanism was reported in which a 2e− reduction of O2 on a Co−Nx type site into HO2− is reported as a first step. The HO2− species further reacts via either electrochemical reduction to form OH− or chemical disprotonation to form OH− and O2 on the decorating CoxOy/Co nanoparticle.14 In a theoretical study, four types of active-site structures and a possible reaction mechanism were evaluated.15 There are many possible active ORR structures that are possible with Co(III) and Co(II) being an active center. The redox potential and oxygen binding energy was reported to be affected by the polypyrrole chain size. The structure of the cobalt polypyrrole electrocatalyst is very complex, and further studies are required to get a complete understanding of this system. In this study, CoPPyC-based ORR electrocatalysts, chemically modified in different ways, were analyzed with synchrotron X-ray radiation, and the correlations between the electrochemical properties and the structure of electrocatalysts, which are cobalt monoclusters associated with different types of nitrogen, are discussed. Cobalt associated with pyrrolic nitrogen showed a clear change of chemical environment and coordination number upon ORR. Cobalt associated with pyridinic nitrogen showed changes of coordination number. Cobalt may be not the only active site, but it also clearly shows adsorption of different species such as oxygen and OH−. In parallel to experimental studies, we performed theoretical studies using density functional theory (DFT) calculations and investigated the ORR mechanism in a CoPPyC-based catalyst. DFT calculations of the cobalt−pyrrole structure were obtained previously, where a model of active sites on Co−N and the role of pyrrolic and pyridinic nitrogen were proposed through experimental and theoretical studies.16 In this study, we investigated the ORR mechanism based on optimized Co−N structure from DFT calculations.
pyrolyzed in N2 gas at 700 °C and treated in 1 M sulfuric acid is CoPPyC-PA (3). This sample has 0.12 wt % of Co. Co on carbon (CoC (4)) was synthesized with carbon black (Vulcan XC-72, Cabot Corp.) and Co(NO3)2·6H2O reduced in NaBH4 aqueous solution. X-ray Adsorption Fine Structure (XAFS) Data Collection and Analysis. XAFS measurements were carried out at the BL14B2 line of SPring-8. XAFS measurements were conducted at the Co K-edge. CoPPyC as synthesized was measured in transmission, and CoPPyC-A and CoPPyC-PA were measured in fluorescence using 19ch SSD. In situ XAFS has been analyzed using an electrochemical cell consisting of a Hg/HgO reference electrode and a Pt wire counter electrode. The working electrode is a carbon paper coated with an ORR catalyst layer. KOH (1 M) air-saturated electrolyte was circulated through a cell. Before the in situ XAFS analysis, working electrodes were treated in cyclic voltammetry until the voltammogram was stable. During in situ XAFS analysis, the potential was set at 0.1, −0.2, and −0.4 V versus Hg/HgO (1.024, 0.724, and 0.524 V vs RHE). XAFS data processing was done using EXAFS analysis software (Ifeffit). The coordination number was calculated using curve fitting in the same software. Hard X-ray Photo Electron Spectroscopy (HAXPES) Analysis. HAXPES measurements were carried out at BL46XU and BL47XU of SPring-8. The source X-ray energy was 7940 eV. C 1s, N 1s, O 1s, and Co 2p spectra were acquired. Spectra were charge calibrated to the binding energy for the Au standard plate of 84 eV (Au 4f). Rotating Ring-Disk Electrode (RRDE) Preparation and Testing. The catalyst layer was applied to the disk electrode as follows: 10 mg/mL solutions of the catalysts were prepared by mixing 10 mg of the catalyst, 0.8 mL of water, and 0.2 mL of isopropyl alcohol. Next, 1 mg/mL solutions were prepared by mixing 0.1 mL of the concentrated 10 mg/mL solution, 0.75 mL of water, 0.1 mL of IPA, and 0.05 mL of 10× diluted 5 wt % Nafion solution. Then 10 μL of the 1 mg/mL solutions was deposited on a disk electrode. The RRDE experimental setup utilized a BAS potentiostat model 1030. A Pine RRDE assembly consisting of a glassy carbon disk (geometric area = 0.25 cm2) and Pt ring working electrodes was utilized. The electrochemical cell consisted of a Hg/HgO reference electrode and a Pt wire counter electrode. All data were collected in 1.0 M KOH electrolyte and at room temperature. The presented data were collected with a 10 mV/s potential scan rate and a rotation rate of 1600 rpm. The Pt ring potential was held constant at 0.6 V versus Hg/HgO (1.524 V vs RHE). The % HO2− generated was calculated using eq 1, where N is the collection efficiency of the ring electrode for the intermediate reaction product, HO2−:
EXPERIMENTAL SECTION Catalyst Preparation. CoPPyC (10 wt % of Co) was synthesized with a commercially available 20 wt % polypyrrole supported on a carbon black (Vulcan XC-72, Cabot Corp.) composite material, polypyrrole on carbon (PPyC; Aldrich 577065) and Co(NO3)2·6H2O. CoPPyC (1) was treated in 1 M sulfuric acid to remove cobalt oxide species (CoPPyC-A) (2). In CoPPyC-A electrocatalysts, Co was not detected by standard X-ray photoelectron spectroscopy (XPS), because the concentration of Co as detected by inductively coupled plasma atomic emission spectroscopy is only 0.02 wt %. CoPPyC
Computational Study. All calculations were carried out using the Gaussian 09 program.17 We first use the B3LYP exchange-correlation functional and mixed basis sets to perform initial geometry optimization. The 6-31G** basis sets are used for light atoms (H, C, N, and O), and the double-ζ valence orbital (LANL2DZ) basis sets with an effective core potential are used for Co atoms. Next, the energy and the resulting spin ground-state species are refined and recalculated using the B3LYP functional and the 6-311+G(d,2p) basis sets.18−20 Co− Nx clusters are spin-polarized systems, but spin contamination is considered to be sufficiently small and does not affect the
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% HO−2 = 2 × Ir /(N × Id + Ir) × 100
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dx.doi.org/10.1021/jp507534f | J. Phys. Chem. C 2014, 118, 25480−25486
The Journal of Physical Chemistry C
Article
bond distances because all of the calculated ⟨S2⟩ values vary by
