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NanoCOT: Low-Cost Nanostructured Electrode Containing Carbon, Oxygen, Titanium for Efficient Oxygen Evolution Reaction Zhichao Shan, Panikar Sathyaseelan Archana, Gang Shen, Arunava Gupta, Martin Bakker, and Shanlin Pan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b05367 • Publication Date (Web): 04 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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Journal of the American Chemical Society

NanoCOT: Low-Cost Nanostructured Electrode Containing Carbon, Oxygen, Titanium for Efficient Oxygen Evolution Reaction Zhichao Shan,1,2 Panikar Sathyaseelan Archana,1,2 Gang Shen,4 Arunava Gupta,1, 2,3 Martin G. Bakker,1 and Shanlin Pan1,2* 1. Department of Chemistry, The University of Alabama, Tuscaloosa, AL, USA, 35487-0336 2.

Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL, USA, 35487-0209

3. Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, AL, USA, 35487-0203 4. Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL, USA, 35487-0200 *Corresponding Author: [email protected]

ACS Paragon Plus Environment


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Abstract Developing high efficiency, durable, and low-cost catalysts based on earth-abundant elements for oxygen evolution reaction (OER) is essential for renewable energy conversion and storage devices. In this study, we report a highly active nanostructured electrode NanoCOT, which contains carbon, oxygen and titanium, for efficient OER in alkaline solution. The NanoCOT electrode is synthesized from carbon transformation of TiO2 in an atmosphere of methane, hydrogen and nitrogen at a high temperature. The NanoCOT exhibits enhanced OER catalytic activity in alkaline solution, providing current density of 1.33 mA/cm2 at an overpotential of 0.42V. This OER current density of a NanoCOT electrode is about 4 times higher than an oxidized Ir electrode and 15 times higher than a Pt electrode because of its nanostructured high surface area and favorable OER kinetics. The enhanced catalytic activity of NanoCOT is attributed to the presence of a continuous energy band of the titanium oxide electrode with predominantly reduced defect states of Ti (e.g., Ti1+, Ti2+ and Ti3+) formed by chemical reduction with hydrogen and carbon. OER performance of NanoCOT can also be further enhanced by decreasing its overpotential 150 mV at a current density of 1.0 mA/cm2 after coating its surface electrophoretically with 2.0 nm IrOx nanoparticles (NPs).

1. Introduction Oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e− in acid; 4OH− → O2 + 2H2O + 4e− in alkaline solution) is one of the critical half-reactions of solar water splitting.1 Efficient OER holds the promise for addressing the challenge of solar water splitting for large-scale storage of intermittent energy using hydrogen from the sun, wind, and other renewable sources.210

However, one of the major obstacles to widespread adoption of water splitting is the low


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efficiency of the electrode materials and their high capital cost because of the use of noble metals. The OER involves sluggish kinetics at most electrode that has limited electrocatalysis being viable on an industrial scale.11-14 Catalysts such as Ru, Ir and Pt have shown desirable electrocatalytic activity, however their scarcity and prohibitive cost renders their use impractical for large-scale applications.15-20 Thus it is important to develop inexpensive and highly active OER catalysts in order to overcome these obstacles. There has been an enormous amount of research efforts in investigating new earth–abundant materials containing Co, Ni, Mn, Fe, Mo, or graphene-based materials.21-33 Among these materials, Fe or Co-based catalysts have seen extensive efforts. For example, catalyst containing Co2+ in neutral aqueous phosphate solution 21 and the optimal intrinsic catalyst Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF).22 Nanostructured metal oxides such as MnO2, 34 NiOx, 31 or Co3O4 on graphene

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and complex oxides such as NiCo2O4,

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CaMn4Ox, 13 MxFe3-xO4 (M = Ni, Co, Mn, Fe, or Zn) 36 and mesoporous NixCo3-xO4 nanowire,

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have been developed for enhancing OER efficiency. Improved understanding of the structurefunction relationship of these catalytic electrode materials for OER and their large scale applications under harsh conditions have yet to be addressed. Recently, our study showed that anodized Ti electrode can be doped with carbon to form highly active nanostructured electrode materials for charge storage and other relevant redox reactions (e.g., electrogenerated chemiluminescence).

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Here, we present the synthesis and

characterization of such NanoCOT electrode containing carbon, oxygen and titanium for efficient OER and complete water splitting. The NanoCOT electrode is a low-cost electrode material and has a nanostructured surface with excellent activity for OER in alkaline solutions. NanoCOT is synthesized by the facile carbon thermal transformation of nanostructured TiO2 film or TiO2 NPs in an atmosphere of methane, hydrogen and nitrogen. In comparison to semimetallic



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titanium oxycarbides synthesized by carbothermal reactions as reported previously in the literature, 38, 39 the NanoCOT possesses enhanced OER reactivity that is comparable to IrO2 or Pt in alkaline solution and can be engineered to serve as cathode and anode for complete water splitting. 2. Experimental

Figure 1. Schematic of preparation procedures for the NanoCOT electrode and a photograph of NanoCOT electrode (lower right inset) after carbon transformation of a nanostructured TiO2 film. 2.1 Materials. Titanium plate (Ti, 99%, 0.5 mm in thickness), titanium wire (Ti, 99.98%, 0.5 mm in diameter), sodium molybdate dihydrate (Na2MoO4·2H2O, 99%), tetrabasic sodium pyrophosphate

(Na4P2O7,

98%),

sodium

bicarbonate

(NaHCO3,

99%),

potassium

hexachloroiridate (IV) (K2IrCl6, Ir 39% min), iridium wire (99.8%, 0.5 mm in diameter), and platinum wire (Pt, 99.9%, 0.45 mm in diameter) were purchased from Alfa Aesar. Nickel (II) chloride hexahydrate (NiCl2·6H2O, 97%), anhydrous zinc chloride (ZnCl2, 99%), sodium sulfate (Na2SO4, 99%), sulfuric acid (H2SO4, ACS grade), hydrochloric acid (HCl, ACS grade), and methanol (CH3OH, ACS grade) were purchased from Fisher Scientific. TiO2 nanopowder


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(Aeroxide® P25), Iron (III) nitrate nonahydrate (Fe(NO3)3‧9H2O, 98%), poly(methyl methacrylate) (PMMA M.W. 15000) and hydrazine hydrate (N2H4, 64%) were purchased from Acros Organics. Hexaammineruthenium (III) chloride (Ru(NH3)6Cl3, 98%) and Nafion solution (5% in ethanol) were purchased from Sigma-Aldrich. Poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol), or Pluronic P123, was purchased from BASF. Sodium Hydroxide (KOH, ACS grade) was purchased from BDH. Gas mixture of 16 % CH4 and 20.51 % H2 balanced with N2, 99.5% N2 and 99.5% O2 were purchased from Airgas. All reagents were used as received without further purification. 2.2 Fabrication of NanoCOT electrode. As shown in Figure 1, NanoCOT electrode was fabricated by hydrothermal reaction of a Ti substrate to form Ti nanowires (NWs) followed by carbon transformation of the oxidized Ti NWs in a chemical vapor deposition (CVD) system.40 Briefly, Ti plates were cleaned with ethanol and water prior to being loaded into a 45 ml Teflonlined stainless steel autoclave containing 10 mL 0.6 M HCl solution. The autoclave was heated at 190 ℃ for 12 hours. After cooling to room temperature, the as-prepared Ti NW samples were annealed in air at 450 oC for 10 hours to form the desired surface oxidized Ti NW substrates. The thermally annealed Ti NW substrates were then treated with 0.5 M Fe(NO3)3 for 20 min and dried in air. The Fe catalyst coated substrates were horizontally placed into a quartz boat with a quartz plate cover on top to allow sufficient mixture gas retention time for the carbon transformation reaction. The boat was loaded in a tube furnace (×1100, MTI Corporation, Richmond, CA). The chamber was purged by several vacuum/N2 purge cycles to dispose of the oxygen in the furnace followed by a constant N2 gas flow. The furnace temperature was set to 900 °C with a ramp rate of 50 °C/min and a dwell time of 1 h. At approximately 600 °C the N2 gas was turned off and the CH4/H2/N2 gas mixture was turned on at a flow rate of around 1000



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sccm and reduced to around 100 sccm when the temperature reached 800 oC. After the carbon transformation reaction, the furnace was allowed to cool under CH4/H2/N2 flow until the temperature of the furnace reached approximately 600 oC. At this temperature the CH4/H2/N2 was turned off and the N2 flow was turned back on. The samples were allowed to cool to room temperature in a N2 stream before being removed from the furnace. 2.3 Fabrication of Planar COT. Planar COT electrode fabrication followed the same procedure as for the NanoCOT electrode described above, except using electropolished Ti substrates instead of nanostructured Ti NW substrates. Optically reflective Ti substrates were obtained by electropolishing Ti substrates in a methanol solution containing 3 volume % sulfuric and 3 volume % hydrochloric at a current density of 0.1 mA/cm2 at -40 oC.

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The electropolished Ti

substrates were then calcined at 550 oC for 10 hours in air to form a thin layer of TiO2. The TiO2 layer was then converted to COT electrode by following the same procedure outlined in section 2.2. 2.4 Synthesis of COT NPs. COT NPs were obtained by direct carbon transformation of TiO2 NPs using the same CVD setup described above for both NanoCOT and planar COT samples. 100 mg P25, 20 mg P123 and 20 mg PMMA were added to 30 mL of 3 mM Fe(NO3)3 acetone solution and stirred for 5 hours at room temperature in a capped beaker. The resultant precipitate of Fe and surfactant-modified P25 were then centrifuged and dried prior to being ground to powder in a mortar. The obtained powder was placed horizontally in a quartz boat and heated at 450 oC in air for 1 hour to remove the surfactant. N2 gas was then used to purge the tube furnace to remove residual O2 within the chamber prior to the carbon transformation process as described in section 2.2.


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2.5 Synthesis of NiMoZn/NanoCOT cathode. NiMoZn alloy was electrodeposited onto a NanoCOT electrode by following the method reported by Nocera and coworkers.42 Briefly, a solution of nickel (II) chloride hexahydrate (9.51 g L–1), sodium molybdate dihydrate (4.84 g L– 1

), anhydrous zinc chloride (0.0409 g L–1), tetrabasic sodium pyrophosphate (34.57 g L–1) and

sodium bicarbonate (74.77 g L–1) was used as electrodeposition solution. Hydrazine hydrate (1.21 mL L–1) was added immediately before plating. The NiMoZn alloy was deposited onto NanoCOT electrode at a potential of -1.5V vs Ag/AgCl for 20 min. The obtained electrodeposited film was then stored in 10 M KOH for 16 hours to obtain suitable stoichiometry for enhanced proton reduction. 2.6 Electrophoretic deposition of 2.0 nm IrOx NPs onto NanoCOT electrode. 2.0 nm IrOx particles were prepared and deposited onto NanoCOT by following the method reported by Murray and coworkers’.43 Briefly, 2.4 mM aqueous K2IrCl6 at pH 13 was heated at 90 °C for 20 min, and immediately cooled in an ice-bath, producing a blue 2.0 nm IrOx nanoparticle solution. The 2.0 nm IrOx particles were deposited by electrophoresis onto NanoCOT wire at 1.0 V vs. Ag/AgCl. 2.7 Characterization. Surface morphology and chemical composition of the prepared catalysts were characterized using a JEOL 7000 field emission scanning electron microscope (FE-SEM). High resolution transmission electron microscopy (HRTEM) images were recorded using a FEI Tecnai F-20 TEM. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos XIS 165 system. X-ray Diffraction patterns of samples were obtained using a Bruker D8 XRD system. A Nova 2200e surface area analyzer system was used to measure Brunauer–Emmett– Teller (BET) surface area.



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2.8 Electrochemical characterization. The OER characterization of the NanoCOT, planar COT or IrO2 coated NanoCOT samples were measured in oxygen-saturated 0.1 M KOH in a threeelectrode configuration using a CHI 760C potentiostat (CH Instruments, Austin, TX) at a scan rate of 5 mV/s. The electrochemical cell was comprised of a catalytic electrode as the working electrode, a graphite rod counter electrode, and a Ag/AgCl reference electrode filled with saturated KCl solution. Polarization curves for water oxidation of COT samples were compared with indium tin oxide (ITO,

NanoCOT: Low-Cost Nanostructured Electrode ... - ACS Publications

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