Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction

Abstract Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X‐ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well‐dispersed Ag2O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra‐small Ag2O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co–Ag oxides. The current density of mesostructured Co3O4 at 1.7 VRHE is increased from 102 to 211 mA cm−2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co3O4.

in a Teflon cell as electrolyte. Before the electrochemical measurement, argon was purged through the cell for 30 min to remove oxygen. During all measurements, argon was continuously purged to remove generated oxygen and the temperature of the cell was controlled at 25 o C using a water circulation system. Working electrodes were fabricated by depositing target materials on glass carbon (GC) electrodes (PINE, 5 mm diameter, 0.196 cm 2 area). Before use, all GC electrodes were thoroughly cleaned by polishing with Al2O3 suspension (5 and 0.25 μm, Allied High Tech Products, Inc.). For fabricating working electrode, 4.8 mg of powder sample was dispersed in a mixed solution containing 0.75 mL of H2O, 0.25 mL of 2-propanol (Aldrich, 99.5 %) and 50 μL of Nafion (5% in a mixture of water and alcohol). Afterwards, the mixture solution was immersed in a sonication bath for 30 min to form a homogeneous ink. After that, 5.25 μL of catalyst ink was dropped onto the GC electrode and dried under light irradiation for 10 min. The catalyst loading was calculated to be 0.12 mg/cm 2 in all cases for GC electrodes.
The linear sweep voltammetry (LSV) curves were collected by sweeping the potential from 0.7 V to 1.7 V vs RHE with a scan rate of 10 mV/s. Cyclic voltammetry (CV) measurements were carried out in a potential range between 0.7 and 1.6 vs RHE with a scan rate of 50 mV/s. In all measurements, a rotating disc electrode configuration was kept a rotation speed of 2000 rpm. The IR drop was compensated at 85 % automatically via the potentiostat software (EC-Lab V11.01).
The electrochemical impedance spectroscopy (EIS) was conducted in the same configuration with applying an anodic potential of 1.6 V vs RHE on the glassy carbon electrode. The spectra were collected from 10 5 Hz to 0.1 Hz with an amplitude of 5 mV.
The relative comparison of the resistance for the powder samples was carried out using a homemade cell. Two copper tapes with widths of 7 mm were utilized as an electrode. The gap between two copper tapes that are attached on the teflon holder was 1 mm. 5 mg of the sample was loaded onto the space between the gap of copper tapes and pressurized under 39.2 kPa during the measurement. The current-voltage curves were collected by sweeping the potential from 9 to 1 V using a power supply (2450 SourceMeter, KEITHLEY) for Co3O4 and CoxAg oxide (x = 16, 8, 4, 2, and 1) powder, leading to the calculation of the resistance. As reference materials, the resistance of commercially-available Ag nanoparticles (Aldrich) and Ag2O powder (Acros Organics) was also measured.
Characterization. Powder X-ray diffraction (XRD) patterns were collected at room temperature on a STOE theta/theta diffractometer in Bragg-Brentano geometry (Cu Kα1/2 radiation) with a secondary monochromator. N2-physisorption isotherms were measured using 3Flex Micrometrics at 77 K. Prior to the measurements, the samples were degassed at 150 °C for 10 h. Brunauer-Emmett-Teller (BET) surface areas were determined from the relative pressure range between 0.06 and 0.2. Transmission electron microscopy (TEM) images of samples were measured at 100 kV by an H-7100 electron microscope from Hitachi. High resolution TEM (HR-TEM) and scanning electron microscopy (SEM) images were taken on HF-2000 and Hitachi S-5500 microscopes, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a SPECS GmbH spectrometer with a hemispherical analyzer (PHOIBOS 150 1D-DLD). The monochromatized Al Kα X-ray source (E = 1486.6 eV) was operated at 100 W. An analyzer pass energy of 20 eV was applied for the narrow scans. The medium area mode was used as lens mode. The base pressure during the experiment in the analysis chamber was 5 x 10 -10 mbar. The binding energy scale was corrected for surface charging by use of the C 1s peak of contaminant carbon as reference at 284.5 eV.
Total scattering data for pair distribution function analysis (PDF) were collected on an in-house X-ray powder diffractometer (STOE STADI P) in transmission diffractometer using Mo radiation (0.7093 Å). The instrument is equipped with a primary Ge (111) monochromator (MoKa1) and a position sensitive Mythen1K detector. A generator setting of 50 kV and 40 mA was applied for the generation of X-rays. Data were collected in the range between 5 and 120°2 q with a step width of 0.015° 2q. For the measurements, the samples were filled into glass capillaries (Ø 0.5 mm). The program PDFgetX3 [3] was used for processing PDFs from the integrated scattering data and PDFgui [4] was used to visualize and simulate PDFs. For correction of background scattering from air and sample container, an empty glass capillary was measured. PDF curves were calculated for Qmax of 16 Å -1 . Crystal structure data for the simulation of PDFs of Ag, Ag2O and Co3O4 were taken from references. [5][6][7] Co K-edge XAS was used to measure nanocast Co3O4 and Co8Ag oxide samples. All samples were prepared as solutions in boron nitride to avoid self-absorption and sealed with 30 µm Kapton tape. Measurements were carried out at beamline 20 BM-B (Static XANES and EXAFS) at Advanced Photon Source (APS). The incident energy was selected by a Si (111) double crystal monochromator. Incident flux was 10 8 photons/seconds using a beam size of 5µm x 5µm and slits size of 3 µm (wide) x 1 µm (high). Samples were kept bellow 20 K in a He displex cryostat. Undamaged data was collected by detuning the incident flux by 15% and stability of the incident beam was monitored by collecting simultaneously Co foil. Incident energy was calibrated by assigning the first inflection point of Co foil to 7709.3 eV. Fluorescence spectra were recorded using a multielement Canberra Ge detector. A step size of 0.3 eV was used in the XANES region (1 sec integration time) and 0.05 Å −1 in the EXAFS region to k = 15 Å −1 (10-22 seconds integration time). Final spectra were processed and normalized using Athena program, included in the DEMETER package. [8] Figure S1. TEM images of nanocast Co3O4 using KIT-6 as hard template.               In a simplified Randles circuit, Rs and Rct represent the solution resistance and charge transfer resistance, respectively. Cdl element models the double-layer capacitance. The kinetics for Faradaic OER is determined by charge transfer resistance (Rct).       Figure S24. XPS survey of Co8Ag oxide@carbon fiber paper before and after OER stability test. C, S, and F were detected on the electrode, which were from Nafion. Figure S25. (a) SEM image with corresponding EDX analysis of Co8Ag oxide deposited on carbon fiber paper using Nafion binder, (b) SEM image and corresponding elemental mapping images of (c) carbon, (d) fluorine, (e) cobalt, (f) silver, and (g) oxygen. Note: EDX analysis excluded the content of carbon. S and F elements were due to Nafion, and a small amount of Al came from the sample holder. Figure S26. (a) SEM image with corresponding EDX analysis of Co8Ag oxide deposited on carbon fiber paper which was collected after the chronopotentiometry for 12 h, (b) SEM image and corresponding elemental mapping images of (c) carbon, (d) fluorine, (e) cobalt, (f) silver, and (g) oxygen. Note: EDX analysis excluded the content of carbon. A small amount of K was due to the residue of KOH on the electrode.  [17] Reduced Co 3 O 4 nanowires 1 M KOH 0.136 72 400 [18] Nanoparticulate Co 3 O 4 1 M NaOH 0.8 60.9 500 ± 10 [19]