CoSe 2 Nanoparticles Grown on Carbon Fiber Paper: An efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

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1 SUPPORTING INFORMATION for CoSe 2 Nanoparticles Grown on Carbon Fiber Paper: An efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction Desheng Kong, Haotian Wang, Zhiyi Lu,,# and Yi Cui,,* Materials and Synthesis. CoSe 2 nanoparticles are synthesized through a two-step reaction. A precursor ink is prepared by dissolving 40 wt% cobalt (II) nitrate hexahydrate and 5 wt% Polyvinylpyrrolidone (PVP) in dimethylformamide (DMF) under vigorous stir at 80 o C for 2h. The ink is then applied to various electrodes by drop casting, and extra precursor ink is removed by lab wiper. Notice that carbon fiber paper is activated by oxygen plasma to be hydrophilic prior to dip coating. Afterwards, the electrode is pyrolyzed inside a horizontal tube furnace. For pyrolysis, the electrode is placed at the hot center of the tube furnace. The tube is pumped to a base pressure of 100 mtorr and flushed with Ar gas several times to remove residual oxygen. The heating center of the furnace is quickly elevated to 600 C in 20 min and maintained for 2h under the flow of Ar gas at 50 sccm. The process yields the formation of cobalt oxide nanoparticles. The second reaction is a selenization step performed inside a horizontal tube furnace to convert the cobalt oxide into cobalt diselenide. The substrate is placed at center of the tube furnace, and selenium powder (from Sigma Aldrich) is placed at the upstream side of the furnace at carefully adjusted locations to set the temperature. After flushed with Ar gas, the center of the furnace is quickly elevated to the reaction temperature of 450 C in 20 min, and the selenium precursor is kept at ~300 C. During the synthesis, Ar gas is kept flowing at a rate of 100 sccm to transport sulfur/selenium to the substrate. The furnace is held at the reaction temperature for ~20 to 30 min to fully convert the metal films, followed by natural cool-down. Notice that a cold trap (loaded with dry ice in isopropyl alcohol, from A&N Corporation) is installed on the exhaust gas line to prevent the release of toxic selenium vapor during the reaction. The mass loading of the CoSe 2 catalyst layer is in the range of 2.5 to 3.0 mg/cm 2, determined by using a high precision microbalance (Sartarious SE2, 0.1 μg resolution). CoSe 2 films are prepared on mirror-polished glassy carbon substrates from HTW Hochtemperatur-Werkstoffe GmbH. A 20 nm-thick Co film is coated on glassy carbon as the precursor by DC sputtering in a ATC Orion Sputtering System (AJA International, Inc.). The Co film is then converted S1

2 into CoSe 2 by the selenization reaction in tube furnace. Carbon fiber paper (from Fuel Cell Store, 2050-A) used in our study is ~200 μm think, consisting of 8 μm think carbon microfibers. To quantify the specific area, the carbon fiber paper is activated by oxygen plasma and then conformally coated with a 50 nm-thick Al 2 O 3 layer by atomic layer deposition (Cambridge NanoTech Savannah 100 Atomic Layer Deposition System). The coating is about 0.26 mg/cm 2 determined by the microbalance. It suggests the roughness factor of the carbon fiber paper is ~13. The synthesis is also applicable to other metal chalcogenides. For example, NiSe 2 nanoparticles are prepared on carbon fiber paper with similar procedures, by using a precursor ink consisting of 40 wt% nickel (II) nitrate hexahydrate and 5 wt% PVP dissolved in DMF. The loading of the catalyst is 2.2 mg/cm 2. Structural characterizations and electrochemical measurements of the catalyst are summarized in Figure S6 and Figure S7. Characterizations. Characterizations were carried out using SEM (FEI Nova NanoSEM 450), Raman spectroscopy (WITEC Raman spectrometer), X-ray Diffraction (PANalytical X Pert diffractometer using copper K-edge X-rays) and X-ray photoelectron spectroscopy (SSI SProbe XPS spectrometer with Al Kα source). The Raman spectra, which have a spectral cut-off at ~175 cm -1, are excited by a 531 nm laser with attenuated intensity to avoid local overheating. Electrochemical Measurements. CoSe 2 nanoparticles are grown on carbon fiber paper as a binder-free cathode for HER. Electrochemically inert polyimide tape is employed to define the 1 cm 2 electrode area. A metal alligator clip is used to connect the electrode with an external circuit. The measurements are performed in 0.5 M H 2 SO4 solution using a three electrode setup, with a saturated calomel reference electrode (SCE, from Accumet), a graphite rod (99.999%, from Sigma Aldrich) counter electrode, and the glassy carbon working electrode. The reference electrode is calibrated in H 2 saturated electrolyte with respect to an in situ reverse hydrogen electrode (RHE), by using two platinum wires as working and counter electrodes, which yields the relation E(RHE) = E(SCE) V. The saturation condition is confirmed by minimizing the potential difference between the working and counter electrodes to less than a few mv. Linear sweep voltammetry (scan rate of 5 mv/s) and AC impedance spectroscopy (at zero overpotential) are recorded using a Biologic VSP potentiostat. For a Tafel plot, the linear portion is fit to the Tafel equation. All data have been corrected for a small ohmic drop (<3 Ω) based on impedance spectroscopy. S2

3 Commercial platinum wire (Accumet Model ) is measured to establish a reference point of state-of-art HER catalyst. The experimental conditions are the same except that another platinum wire is used as counter electrode. The potential cycling stability test is examined by taking continuous cyclic voltammograms at an accelarated scanning rate of 100 mv/s. These cyclic voltammograms are carried out between 0.2 V and a low potential limit that drives a large current density of -200 ma/cm 2. The polarization curves before and after cycling are recorded under quasi-equilibrium conditions at a slower scan rate of 5 mv/s. The potentiostatic electrolysis test is performed by extended electrolysis at fixed potentials. The ohmic resistance of the cell is measured by AC impedance spectroscopy every 10 minutes. A small drift in potential due to the ohmic resistance fluctuation is compensated accordingly. Supplementary Figures Figure S1. A comparison of the prices of a few metals. The price comes from public available values from September, 2012 to October, 2012 accordingly to MetalPrices.com. It suggests cobalt dichalcogenides exhibit comparable material cost to molybdenum or tungsten dichalcogenides, which are much more affordable than noble metal like platinum. Figure S2. SEM images of carbon fiber paper revealing the highly textured surface of carbon microfibers. S3

4 Figure S3. (a) Overview SEM images of a CoSe 2 layer synthesized on a silicon wafer. (b) High-resolution SEM image revealing the structure of CoSe 2 layer formed by nanoparticles in the dimension of tens of nanometers. Figure S4. Polarization curves for eight individual CoSe 2 catalyst paper electrodes in 0.5 M H 2 SO 4. The corresponding electrochemical parameters are summarized in Table S1. S4

5 Figure S5. A typical Tafel plot of CoSe 2 nanoparticle grown on carbon fiber paper. The derivative of the linear region is used to determine the Tafel slope. The linear Tafel region extends to very high cathodic current density of ~ 80 ma/cm 2, beyond which the logarithmic current density deviates from the linear dependence on the overpotential, likely limited by mass transport. Figure S6. Characterizations on NiSe 2 nanoparticles grown on carbon fiber paper. (a) SEM image of NiSe 2 nanoparticles grown on carbon fiber paper. (b) High-resolution SEM image revealing the structure of NiSe 2 coating consisting of nanoparticles with the dimension ranging from tens of nanometers to about 200 nm. (c) Raman spectrum from as-grown nanoparticles on carbon fiber paper, revealing the active modes at 215 and 242 cm -1 corresponding to NiSe 2 with pyrite-type structure 49. S5

6 Figure S7. (a) Polarization curves of NiSe 2 nanoparticle (NP) /carbon fiber paper (CP) electrode. The polarization curve of pristine CP is also shown for comparison. (b) Correponding Tafel plot of NiSe 2 NP/CP electrode, revealing a small Tafel slope of 50.1 mv/dec. (c) Stability tests through potential cycling, in which the polarization curves before and after 5000 potential cycles are displayed. Negligible cathodic current density is lost through the potential cycling, confirming its exceptional durability for practical applications. Supplementary Tables Table S1. Electrochemical parameters of CoSe 2 catalyst paper electrodes Sample b (mv/dec.) j 0 (A/cm 2) μ@10ma/cm 2 μ@20ma/cm 2 μ@50ma/cm 2 μ@100ma/cm 2 S E S E S E S E S E S E S E S E Table S2. HER activities of some non-precious catalysts in 0.5 M H 2 SO 4 Material Loading (mg/cm 2 ) μ@10ma/cm 2 μ@50ma/cm 2 μ@100ma/cm 2 Reference CoSe 2 NP /CP this work* NiSe 2 NP /CP this work MoS 2 -graphene/gc N.A. Ref 6 MoS 2 /graphene/ Ni foam Ref 12 Ni 2 P / Ti ~ Ref 19 * The data of a representative sample is listed for comparison. S6

7 Supplementary Movies Movie S1. This movie shows CoSe 2 nanoparticle/carbon fiber paper catalyst operated at a large cathodic current density of 100 ma/cm 2 to drive HER. Generated hydrogen gas is efficiently released from the electrode in the form of tiny bubbles. The property suppresses the loss in available surface area blocked by the evolved hydrogen, likely contributing to the exceptional performance of the catalyst at higher overpotentials. Movie S2. This movie shows pristine carbon fiber paper operated at a large cathodic current density of 100 ma/cm 2 to drive HER. Apparently, molecular hydrogen exhibits stronger interaction with carbon fiber paper, so evolved hydrogen gas accumulates into millimeter-sized bubbles on the electrode. The hydrogen bubbles effectively blocks appreciable fraction of the surface area, which is detrimental to the practical operation of HER catalyst at high current density. Supplementary Reference (49) Heras, C. d. l.; Agulló-Rueda, F. Journal of Physics: Condensed Matter 2000, 12, S7