Connected iridium nanoparticle catalysts coated onto silica with high density for oxygen evolution in polymer electrolyte water electrolysis

We propose connected Ir nanoparticle catalysts (Ir/SiO2) by coating 1.8 nm Ir particles with high density onto silica for the oxygen evolution reaction. Nanoparticles form electron-conducting networks, which can eliminate the need for an electron-conducting support. Ir/SiO2 showed a high electrochemical surface area, mass activity, and water electrolysis performance.

covered completely (Fig. S5), and with the higher precursor concentration of 10.3 M, excess nanoparticles were formed on silica (Fig. S6). For control of nanoparticles sizes, Ir/SiO 2 was treated by the silica coating, the supercritical treatment, and the alkaline treatment. As shown in Table S2, crystallite sizes of the catalysts after the supercritical treatment at 330 °C or 380 °C were calculated to be 2.5 nm and 2.8 nm, respectively.

Structural characterization of the catalysts
The as-synthesized Ir/SiO 2 was characterized using inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectrometry (EDX), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). For comparison, commercially available Ir black (Alfa Aesar (AA)) was used as a catalyst. The Ir loading for Ir/SiO 2 was measured by ICP-AES. Ir/SiO 2 (5 mg) was dissolved in aqua regia (10 mL) and hydrogen fluoride, which was diluted to 50 mL with water. Then, ICP-AES for the solution was measured using ICP Emission Spectroscopy ICPS-8100 (Shimadzu Corporation, Japan). XRD for the catalysts was conducted using a Multipurpose X-ray diffraction system Ultima IV (Rigaku Corporation, Japan) with a CuKα (λ = 0.154 nm) X-ray source. The peaks were ascribed with card data for fcc Ir (Inorganic Crystal Structure Database (ICSD) No. 64992). The powder X-ray analysis software PDXL 2.6 (Rigaku Corporation) was used for peak separation in the obtained XRD patterns, following which the Bragg angles and full width at half maximum (FWHM) were obtained. The crystallite sizes for the catalysts were calculated using the Scherrer equation 3 , which is expressed as D = Kλ/βcosθ, where the Bragg angle is θ, FWHM of the Ir (111) peak is β, wavelength of the X-ray is λ (0.154 nm) and the value of the shape factor is K (1.0747), assuming spherically shaped nanoparticles. 4 Field emission transmission electron microscope HF5000 (Hitachi High-Technologies Corporation, Japan) was used for STEM, EDX, and EELS analysis of Ir/SiO 2 .
Spherical aberration-corrected STEM/SEM HD-2700 (Hitachi High-Technologies) was used for parts of STEM of IrSiO 2 , and a transmission electron microscope H-8100 (Hitachi High-Technologies) was used for transmission electron microscopy (TEM) of Ir/SiO 2 and Ir black (AA). The average diameter of Ir/SiO 2 obtained by STEM was calculated from the diameter values of 32 particles. XPS analysis was measured using PHI Quantera II (ULVAC-PHI, Inc., Japan).

Electrochemical measurements
Electrochemical characterization of the catalysts was then performed on glassy carbon electrodes (GCE). The Ir/SiO 2 catalysts ink was prepared by mixing Ir/SiO 2 (4.4 mg), 25 wt% IPA aq. (6.3 mL) and 5 wt% Nafion ® solution (6.3 μL) with ultrasonication for 1 hour. Then, 10 μL of the catalyst ink was drop cast onto a GCE, which was dried at room temperature with rotation at a speed of 600 rpm. A GCE for Ir black (AA) was prepared using almost the same method. The Ir loading for the Ir/SiO 2 and Ir black (AA) electrodes was 9.3 μg/cm 2 Fig. S2(c). For comparison, the ECSA of Ir/SiO 2 was calculated using another method based on the charge of the hydrogen desorption before the pretreatment with a conversion factor of 179 μC cm Ir −2 . 8,9 The charge was calculated from the anodic current of the peak near 0.1 V after correcting for double layer charging by subtraction of the current at 0.3 V from the total current. CVs for OER were measured at a scan rate of 10 mV s −1 and rotation speed of 1600 rpm for 10 cycles. IR-free OER curves were obtained by averaging the current densities of the anodic and cathodic scans, and then removing the overpotential due to an ohmic resistance, which is dependent on the concentration of electrolyte, the distance between electrodes, and so forth. The overpotential was calculated by multiplying the measured current (I) and cell resistance (R) using the cyclic voltammetry and the solution resistance measurement, respectively. Subsequently, the IR-free curves were obtained by subtracting the overpotential from the measured potential, as shown in following literature. Mass activities for the OER were calculated using the IR-free currents at 1.48 V and the Ir loading on the electrode (9-10 μg cm −2 ). The OER performance was evaluated based on the highest performance in 2-10 cycles for each catalyst: 2nd cycle for Ir/SiO 2 , 5th cycle for Ir black (AA). The Tafel slope for the catalysts was calculated using Tafel plots for the IR-free OER curves in the low current density region below 1 mA cm −2 .

MEA measurements
A 3.5 cm square Nafion ® membrane N115 was treated in 1 M HNO 3 aq. at 90 °C for 2 hours; then, the membrane was cleaned in RO water (12.8 MΩ cm, supplied by Elix 3UV Essential, Merck Millipore) at 100 °C for 1 hour. The membrane electrode assembly (MEA) was made by using the treated Nafion ® N115 as electrolyte membrane, Ir/SiO 2 as anode catalyst, Pt/C as cathode catalyst and Nafion ® solution as ionomer. An ink of the anode catalyst was prepared by mixing Ir/SiO 2 (60 mg), 20 wt% Nafion ® solution (11.25 mg), IPA (6.6 mL) and RO water (8.4 mL) with ultrasonication for a few hours, followed by treatment in an autoclave at 200 °C for 20 hours to ensure the formation of a uniform ionomer coating on the catalyst. After ultrasonication for 1 hour, the obtained ink was coated onto the membrane with a catalyst layer size of 5 cm 2 with a spray method using a pulse spray system (Nordson Corporation, United States). An ink of the cathode catalyst was prepared by mixing Pt/C (2 g), RO water (8.5 g), 20 wt% Nafion ® solution (5.3 g), NPA (8.5 g) and IPA (8.5 g) with ball milling at a rotating rate of 200 rpm for 1 hour using zirconia balls with a diameter of 5 mm. A cathode catalyst layer was prepared using a decal method using the ink, which was then attached onto the opposite side of the membrane by pressing at 5.11 kN at 135 °C for 5 min to obtain an MEA. An electrolyzer cell (FC Development Co., Ltd., Japan) was set up with the MEA using carbon paper (SGL carbon, Germany) as a porous transfer layer. Electrochemical impedance spectroscopy (EIS) was measured using an electrochemical interface SI 1287 and impedance/gain-phase analyzer SI 9 100 kHz. Water electrolysis was measured using battery charge/discharge system HJ1010SD8 (HOKUTO DENKO CORPORATION) from 0.1 A cm −2 to 1.1 A cm −2 with each current density value held constant for a few minutes. The EIS and water electrolysis performance of the cell was measured at 80 °C with water circulating in the anode flow channel at a flow rate of 10 mL min −1 .