Flower‐Like Amorphous MoO3− x Stabilized Ru Single Atoms for Efficient Overall Water/Seawater Splitting

Abstract Benefitting from the maximum atom utilization efficiency, special size quantum effects and tailored active sites, single‐atom catalysts (SACs) have been promising candidates for bifunctional catalysts toward water splitting. Besides, due to the unique structure and properties, some amorphous materials have been found to possess better performance than their crystalline counterparts in electrocatalytic water splitting. Herein, by combining the advantages of ruthenium (Ru) single atoms and amorphous substrates, amorphous molybdenum‐based oxide stabilized single‐atomic‐site Ru (Ru SAs‐MoO3− x /NF) catalysts are conceived as a self‐supported electrode. By virtue of the large surface area, enhanced intrinsic activity and fast reaction kinetics, the as‐prepared Ru SAs‐MoO3− x /NF electrode effectively drives both oxygen evolution reaction (209 mV @ 10 mA cm−2) and hydrogen evolution reaction (36 mV @ 10 mA cm−2) in alkaline media. Impressively, the assembled electrolyzer merely requires an ultralow cell voltage of 1.487 V to deliver the current density of 10 mA cm−2. Furthermore, such an electrode also exhibits a great application potential in alkaline seawater electrolysis, achieving a current density of 100 mA cm−2 at a low cell voltage of 1.759 V. In addition, Ru SAs‐MoO3− x /NF only has very small current density decay in the long‐term constant current water splitting test.


XAFS analysis
Data reduction, data analysis, and EXAFS fitting were performed with the Athena and Artemis programs of the Demeter data analysis packages that utilizes the FEFF6 program to fit the EXAFS data. The energy calibration of the sample was conducted through standard Mo foil and Ru foil, which was simultaneously measured as a reference. A linear function was subtracted from the pre-edge region, then the edge jump was normalized using Athena software. The χ(k) data were isolated by subtracting a smooth, third-order polynomial approximating the absorption background of an isolated atom. The k3-weighted χ(k) data were Fourier transformed after applying a Kaiser-Bessel window function (Δk = 1.0). For EXAFS modeling, the global amplitude EXAFS (CN, R, σ 2 and ΔE 0 ) was obtained by using Artemis software to nonlinear fit the EXAFS equation with Fourier transform data in R space (least square refinement). Then, to determine the coordination numbers (CNs) in the Mo/Ru-O/Ru scattering path in sample, EXAFS of the Mo foil and Ru foil was fitted and the obtained amplitude reduction factor S 0 2 values (0.854 and 0.880) were set in the EXAFS analysis.

Electrochemical Measurements
All electrochemical measurements were performed in a conventional three-electrode system at room temperature using a CHI 660E electrochemical analyzer (CHI Instruments, Shanghai, China). A graphite rod and Hg/HgO were used as the counter electrode and the reference electrode, respectively. The as-prepared catalysts with dimensions of 0.5 cm×0.5 cm were directly employed as the working electrodes, and the loading of catalysts is about 3.5 mg cm -2 . As for powdery catalysts (RuO 2 and Pt/C), the working electrodes were prepared by sonicating the mixture containing 5 mg powder RuO 2 or Pt/C catalysts, 440 μL of isopropanol, 50 μL of water and 10 μL of 5 wt % Nafion for 30 min. The catalyst loading was 3.5 mg cm -2 , similar to that of Ru SAs-MoO 3-x /NF. The electrolytes were 1 M KOH freshwater solution and 1 M KOH seawater solution. To prepare the alkaline seawater media, the collected seawater was first filtered to remove the insoluble impurities. Then the potassium hydroxide was added into the seawater to obtain the 1 M KOH solution.
After stirring for 30 min, the solution was filtered again to remove the precipitated substances. The alkaline electrolyte mainly contains cationic K + , Na + , anionic OH -, Cl -, SO 4 2-, as well as other trace ions. The chloride concentration in the alkaline seawater media was measured to be 10960.07 mg/L by the Ion Chromatography test.
In HER and OER characterizations, all the polarization curves were recorded at a scan rate of 5 mV s -1 , and the polarization curves were iR-corrected using the equation: The generated H 2 and O 2 gases during overall water splitting were quantitatively collected by the water drainage method. The electrochemical active surface area (ECSA) was estimated by the obtained C dl according to the formula: ECSA = C dl /C s , where the C s means the specific capacitance of an ideal 1 cm 2 flat surface. Here we adopt the general value of 60 μF cm -2 for C s .
As for the two-electrode water splitting performance, the polarization curve without iR compensation was recorded with the as-synthesized bifunctional electrode material as both anode and cathode. The Pt/C||RuO 2 couple (3.5 mg cm -2 ) was also measured as benchmark. The stability was assessed through CV cycling test and chronoamperometry. The generated H 2 and O 2 were separated in a typical H-style cell with an anion-exchange membrane, and then collected and quantitatively evaluated by the drainage method.

Turnover frequency (TOF) calculations
The TOF (s -1 ) value was estimated by the following formula:

TOF = I/mnF
where I is current (A) during the linear sweep voltammetry (LSV) tests in 1 M KOH, n is the number of active sites (mol), F is the Faraday constant (96485, C mol -1 ), m is the factor (m for hydrogen evolution and oxygen evolution reactions are 2 and 4, respectively).

7
The number of active sites (mol) was calculated by the following formula: where Q is the voltammetric charge, F is the Faraday constant (C mol -1 ), I stands for the current (A), t is the time (s), V is the voltage (V) and v is the scanning rate (V s -1 ). Figure