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N-Co-O Triply-Doped Highly Crystalline Porous Carbon: An Acid-Proof Non-Precious Metal Oxygen Evolution Catalyst Shiliu Yang, Yi Zhan, Jingfa Li, and Jim Yang Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00437 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016
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ACS Applied Materials & Interfaces
N-Co-O Triply-Doped Highly Crystalline Porous Carbon: An Acid-Proof Non-Precious Metal Oxygen Evolution Catalyst Shiliu Yang, Yi Zhan, Jingfa Li and Jim Yang Lee*,† †
Department of Chemical and Biomolecular Engineering, National University of Singapore,
10 Kent Ridge Crescent, Singapore 119260, Singapore. KEYWORDS: Li-air battery, Acidic electrolyte, OER, N-Co-O doped carbon, Active sites.
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ABSTRACT: In comparison with non-aqueous Li-air batteries, aqueous Li-air batteries are kinetically more facile and there is more variety of non-noble metal catalysts available for oxygen electrocatalysis, especially in alkaline solution. The alkaline battery environment is however vulnerable to electrolyte carbonation by atmospheric CO2 resulting in capacity loss over time. The acid aqueous solution is immune to carbonation but is limited by the lack of effective non-noble metal catalysts for the oxygen evolution reaction (OER). This is contrary to the oxygen reduction reaction (ORR) in acid solution where a few good candidates exist. We report here the development of a N-Co-O triply-doped carbon catalyst with substantial OER activity in acid solution by the thermal co-decomposition of polyaniline, cobalt salt and cyanamide in nitrogen. Cyanamide and the type of cobalt precursor salt were found to determine the structure, crystallinity, surface area, extent of Co doping and consequently the OER activity of the final carbon catalyst in acid solution. We have also put forward some hypotheses about the active sites that may be useful for guiding further work.
1. INTRODUCTION Non-aqueous Li-air batteries with their exceptionally high theoretical energy density of 11680 Wh/kg are ideal for the large-scale storage of electrical energy.1-3 However, after two decades of research, there is still no satisfactory solution to tenacious problems such as the accumulation of insoluble and insulating discharge products (Li2O2 or Li2O) in the air electrode resulting in pore-plugging and reduced accessibility of catalytically active sites; and side reactions of electrolyte with moisture from the air. The overall result is low round-trip energy efficiency, unsatisfactory rate performance and insufficient cycle life in battery operation.3,4 For these reasons, aqueous Li-air batteries5 and dual-electrolyte hybrid Li-air batteries6 have been researched as alternatives. The main advantages of aqueous or hybrid electrolyte
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systems are more facile oxygen electrochemistry and the elimination of insoluble discharge products. Many of the current aqueous or dual-electrolyte hybrid Li-air batteries are operating at high pH conditions. Li2CO3 precipitation due to the carbonation of alkaline solution by atmospheric CO2 then becomes an issue. The common mitigation measures are periodic electrolyte refreshment7,8 or placing an O2 permselective membrane in the cathode compartment (which slows the O2 transport from air). The cost is inevitably higher due to the more complex battery operation or construction.3 A simpler alternative would be to operate Li-air batteries under acidic conditions.9,10 The advantages of acid Li-air batteries are not only the elimination of side reactions with atmospheric CO2, but also a higher theoretical cell voltage equal to (4.25-0.059×pH) V vs Li/Li+ from the reaction 2Li + ½O2 + 2H+ → 2Li+ +H2O.3 Regardless of the pH of the aqueous electrolyte, catalysts are still needed to improve the rates of the oxygen reactions to an acceptable level in actual practice. Many catalysts have been investigated including the platinum-group metals (PGMs, most notably Pt, Ru and Ir and their alloys), transition metal oxides and hydroxides, perovskites, nitrogen-doped and metal-doped carbons.11 The PGMs (e.g. Pt
12
and Ru,
Ir13) have shown the highest ORR and OER activities in acid solution to date. But the high cost of PGMs and the lack of long-term stability of carbon-supported PGMs in acid solution are strong disincentives. Among the small class of non-PGM catalysts investigated to date,14-20 nitrogen-doped14-16 and nitrogen-metal co-doped17-21 carbons have emerged as effective catalysts for ORR in acids. However, their OER performance is seldom explored or reported, or in the few reports where it was reported,22-24 rather dismal.
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In this study, a N-Co-O triply-doped carbon catalyst was prepared by the thermal co-decomposition of pre-synthesized polyaniline (PAn) nanofibers, cobalt chloride hexahydrate (CoCl2⋅6H2O), and cyanamide (CM). The catalyst prepared as such has a highly crystalline porous structure, a large surface area, uniform N-Co-O doping and is effective for OER in acid. Both CM and the cobalt precursor were found to affect the structure, crystallinity, extent of doping, and hence the OER activity of the final carbon catalyst. Some hypotheses about the OER active sites were made based on the results of catalyst characterizations by various microstructural analysis methods. 2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals were supplied by Sigma-Aldrich and used as received. PAn was prepared by a previously reported method.25 In a typical preparation, aniline (0.76 mL) and FeSO4·7H2O (0.36 g) were dissolved in 80 mL 0.1 M hydrogen chloride (HCl), and
stirred to form a clear solution. A freshly prepared solution of
ammonium persulphate (0.48 g) in 80 mL 0.1 M HCl was then added without stirring. The polymerization reaction was allowed to proceed for 6 h at room temperature without disturbance (to promote the formation of a fiber-like structure). The dark green precipitate was recovered by centrifugation, washed with water twice. The PAn prepared above was washed with 20 mL absolute ethanol twice. 25 mg of it (corresponding to 0.25 mmol of monomers) was dispersed in 7 mL absolute ethanol. 1.5 mL of absolute ethanol containing 0.3 mmol cobalt chloride hexahydrate (CoCl2⋅6H2O), and 1.5 mL of absolute ethanol containing 2 mmol CM were added dropwise to the PAn dispersion under gentle stirring. The solvent was slowly evaporated by heating at 60 oC without stirring. The dried solid was heated in 50 cm3/min of flowing N2, firstly at 2 oC/min to 550 oC (the optimal condition for graphitic carbon nitride (g-C3N4) formation), resting at this temperature for 1h, and
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then at 5 oC/min to 900 oC, and resting at this temperature for 2h. The heat-treated product was ground to a fine powder, and ultrasonically etched in 1.5 mL 0.5 M H2SO4 solution for 30 min twice to remove the Co residues. The etched product was washed with water and ethanol several times, centrifuged and dried at 80 oC for 3 h. In this paper, we refer the N-Co-O triply-doped samples prepared from CoCl2⋅6H2O, Co(NO3)2⋅6H2O and Co(CH3COO)2⋅4H2O as TDC-Cl, TDC-NO3 and TDC-Ac respectively (where TDC stands for triply-doped carbon). 2.2. Characterizations. The phase compositions of the carbon products were analyzed by X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å) and scanning the 10° to 80° 2θ range at 0.02° s-1. Field-emission transmission electron microscopy (TEM) images were taken from a JEOL JEM-2010F transmission electron microscope operating at 200 kV. N2 adsorption-desorption isotherms were measured by a Quadrasorb SI Surface Area and Pore Size Analyzer at liquid nitrogen temperature. The sample specific surface areas were calculated by the multipoint Brunauer-Emmett-Teller (BET) procedure. XPS analysis was carried out on a Kratos Axis Ultra DLD spectrometer, with binding energy correction by referencing the C1s peak of adventitious carbon to 284.5 eV. Thermogravimetric analysis (TGA) measurements in nitrogen or air were carried out on a Shimadzu DTG-60H spectrometer from room temperature to 900 oC using a heating rate of 10 oC/min and a gas flow rate of 50 cm3/min. The measurements only began after equilibrating the samples at 100 oC for 15 min to remove the absorbed moisture. 2.3. Electrochemical measurements. All electrochemical measurements were performed in a single compartment three-electrode glass cell at room temperature. A Pt foil and a Ag/AgCl (3M KCl) electrode were used as the counter electrode and
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reference electrode respectively. All potentials were referenced to the reversible hydrogen electrode (RHE) standard at pH = 0 (ERHE = EAg/AgCl + 0.21 + 0.059×pH V). The working electrode was a rotating glassy carbon disk (5 mm diameter, 0.196 cm2) polished with an aqueous alumina suspension on a felt polishing pad; followed by water cleansing and drying in flowing air. The catalyst ink was prepared by mixing x amount of catalyst (2< x
