MaterialsRuthenium acetylacetonate (Ru(acac)3, 98%) was obtained from Adamas-beta. Gallium acetylacetonate (Ga(acac)3, 99.9%) and 2-methylimidazole (C4H6N2, 98%) were purchased from Shanghai Aladdin Biochemical Co., Ltd. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Methanol (CH3OH, 99.8%) and ethanol (C2H5OH, 99.8%) were purchased from Beijing Tong Guang Fine Chemicals Company. AEM membrane (X37-50, 0.05 mm) was acquired from Dioxide Materials Sustainion. PEM membrane (Nafion 117, 0.18 mm) was obtained from DuPont Co. All reagents were used without further purification.Synthesis of GaSA/N–C and N–CThe GaSA/N–C was synthesized by a chemical and pyrolysis strategy. Typically, Zn(NO3)2·6H2O (3 g) and Ga(acac)3 (200 mg) were dissolved in 40 mL methanol as solution A. 2-methylimidazole (6.5 g) was dissolved in 80 mL methanol as solution B. Then, two solutions were rapidly mixed under magnetic stirring for 24 h at room temperature. The precipitate was centrifuged, washed with methanol for three times and dried in an oven at 60 °C, followed by annealing at 900 °C for 2 h in a 5% H2/Ar atmosphere. The N–C sample was prepared without the addition of Ga(acac)3.Synthesis of Ru–GaSA/N–C and Ru/N–CIn a typical procedure, GaSA/N–C (50 mg) was mixed with Ru(acac)3 (18 mg) in a 100 mL beaker containing 40 mL ethanol. After evaporation, the dried powder was then subjected to high-temperature H2-reduction treatment in 5% H2/Ar at 400 °C for 2 h. The synthetic process of Ru/N–C was similar to that of Ru–GaSA/N–C except using N–C instead of GaSA/N–C as support.CharacterizationTransmission electron microscope (TEM) was conducted on JEOL JEM-2010F. HAADF-STEM and energy dispersive X-ray spectroscopy (EDS) element mappings were taken on the JEOL 2200FS STEM/TEM microscope at 300 kV. XRD was performed on Bruker D8 Advance diffractometer at 40.0 kV and 120 mA with Cu–Kα radiation. XPS was carried out on a Thermo Scientific K-Alpha spectrometer. ICP-OES was applied to the Agilent 7800 instrument. XAFS spectra were applied at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) and BL1W1B in the Beijing Synchrotron Radiation Facility (BSRF). The Athena module of the IFEFFIT software package was conducted to analyze the XAFS raw data according to standard procedures, and the EXAFS data fitting was performed on the Artemis program.Electrochemical measurementsElectrochemical measurements were conducted in a three-electrode cell on an electrochemical workstation (CHI 760E) setup with a rotating disk electrode (RDE). Hg/HgO (1 M KOH) and graphite rod (Φ = 6 mm) were used as reference and counter electrodes, respectively. The catalyst-coated glassy carbon (GC) electrode (diameter: 5 mm) was conducted as a working electrode. 1 M KOH solution was used as an electrolyte and the electrolytic cell volume was about 100 mL. Before testing, the Hg/HgO electrode was calibrated by cyclic voltammetry using a purified Pt wire as the working and counter electrode in H2-saturated 1 M KOH electrolyte. The average potential at which the current crosses zero was regarded as the thermodynamic potential relative to Hg/HgO37. All recorded potentials (E) were converted to the reversible hydrogen electrode (RHE) based on the following equation: ERHE = EHg/HgO + 0.913. To prepare the working electrode, 5 mg as-prepared catalyst was firstly dispersed into 1 mL solution containing 975 μL of isopropanol and 25 μL of Nafion solution (D521), followed by ultrasonication for 30 min to form a homogenous ink. Afterward, 6 μL of catalyst ink was dropped onto the freshly polished GC electrode with a Ru loading of about 12 μg cm−2 and naturally dried in air. The noble metal loading of commercial PtRu/C and Pt/C catalysts was also controlled to be about 12 μg cm−2, respectively. To assess the performance of the PtRu/C catalyst with the same Ru loading as Ru–GaSA/N–C and Ru/N–C catalysts, we also prepared PtRu/C catalyst with higher loading on the GC electrode (Ru + Pt of 36 μg cm−2 and Ru of 12 μg cm−2). Before the HER test, the electrolyte was purged with N2 with a flow rate of 30 mL min−1 to form the N2-saturated 1 M KOH solution (pH is 13.8 ± 0.1). Linear sweep voltammetry polarization curves were tested three times independently at a scan rate of 5 mV s−1 with 95% iR-compensation under a rotating rate of 1600 rpm. Accelerated durability tests (ADT) were performed on RDE by cycling between 0 to −0.2 V vs RHE for 10,000 cycles at room temperature. The durability of the catalysts was also assessed by CP at a current density of 10 mA cm−2 for 100 h via coating the catalysts on carbon paper with a noble metal loading of about 24 μg cm−2. EIS measurements were recorded at an overpotential of 27 mV with a 5 mV amplitude and a frequency range from 100,000 to 0.1 Hz on RDE. To evaluate the HER performance of Ru–GaSA/N–C at ampere-level current density, we directly coated the catalysts on the carbon paper with a noble metal loading of about 40 μg cm−2.CO stripping testThe ECSA of the catalysts was evaluated via a CO stripping test. Briefly, CO adsorption was firstly conducted at 0.1 V vs RHE for 10 min in a CO-saturated 0.1 M HClO4 solution. Then, the electrode was transferred into an N2-saturated 0.1 M HClO4 solution. The CO stripping current was obtained by CV in the potential range of 0–1.0 V vs RHE at a scan rate of 50 mV s−1.The ECSA was calculated according to the following equation:$${ECSA}=\frac{{S}_{{{CO}}}}{V\times Q\times M}$$
(1)
where Q is the charge constant of CO stripping (420 μC cm−2), and SCO, V, and M represent the integration area of CO stripping, sweep speed, and noble metal loading, respectively.Calculation method of TOFThe TOF, s−1 was calculated according to the following equation:$${{TOF}}=\frac{I}{2\times F\times n}$$
(2)
where I, F, and n represent the measured current, Faraday constant (96,485 C mol−1), and molar number of active sites.n was calculated according to the following equation:$$n=\frac{{S}_{{{CO}}}}{2\times V\times F}$$
(3)
where SCO and V represent the integration area of CO desorption and sweep speed, respectively.Electrochemical measurements in AEMWE deviceThe AEMWE device was composed of the cathode (Ru–GaSA/N–C or Pt/C coated on carbon paper), anode (RuO2 coated on Ni foam), and commercial AEM membrane (X37-50, Dioxide Materials Sustainion). The membrane was immersed in 1 M KOH solution for at least 24 h prior to being employed as an electrolyte. The cathode and anode were prepared by air-spraying catalyst ink onto the carbon paper and Ni foam. The Ru and Pt loading was controlled to be about 0.08 mg cm−2 and 0.2 mg cm−2 for the cathode. The RuO2 loading was controlled to be about 1 mg cm−2 for the anode. The MEA was prepared by integrating the cathode, membrane, and anode between two Ti bipolar plates with a torque of 10 N m to complete an AEMWE device. The cell size is 5 × 5 × 4 cm3 with an active area of 1 cm2. 1 M KOH was circulated through the anodic side with a flow rate of 20 mL min−1 by a peristaltic pump (YZ1525, RONGBAI PUMP). The cell was activated at 100 mA cm−2 for 1 h prior to the test. The performance of AEMWE was evaluated by measuring polarization curves from 0.0 A cm−2 to 1.0 A cm−2 at 60 °C on a battery test system (CT-4008T-5V12A-S1-F, NEWARE). The stability of the AEMWE was evaluated by CP test at 1.0 A cm−2 for 170 h. All measurements in AEMWE were recorded without iR-correction.Electrochemical measurements of the PEMWE device using commercial Pt/C and IrO2 catalystsTo prepare the membrane electrode assembly (MEA), commercial Pt/C and IrO2 were used as cathode and anode catalysts, Nafion 117 membrane was employed as electrolyte, and Pt-coated Ti fiber was used as gas diffusion layers (GDL). Nafion 117 was firstly treated with H2O2 and 0.5 M H2SO4 at 80 °C for 1 h, respectively. The cathode and anode catalysts were directed air sprayed onto the Nafion 117 membrane with a Pt loading of about 0.4 mg cm−2 and IrO2 loading of about 1.5 mg cm−2. The MEA with an active area of 1 cm2 was prepared by hot-pressing the catalyst-loaded membrane and GDL at 130 °C for 2 min under a pressure of 3 MPa, which was sandwiched by two Ti bipolar plates with a torque of 10 N m to complete a PEMWE device. DI water was circulated through the anodic side with a flow rate of 20 mL min-1 by a peristaltic pump (YZ1525, RONGBAI PUMP). The cell was activated at 100 mA cm−2 for 1 h prior to the test. The performance of PEMWE was evaluated by measuring polarization curves from 0.0 to 1.0 A cm−2 at 60 °C on a battery test system (CT-4008T-5V12A-S1-F, NEWARE). All measurements in PEMWE were recorded without iR-correction.KIEs experimentsThe KIE experiments were conducted in 1.0 M KOH/H2O and 1.0 M KOD/D2O solution, respectively. The corresponding current densities at the same overpotential were donated as jH and jD, respectively. The KIE was calculated using the following equation:$${{KIE}}=\frac{{j}_{{{\rm{H}}}}}{{j}_{{{\rm{D}}}}}$$
(4)
Calculation detailsWe perform total energy and electronic properties calculations using the Vienna Ab initio Simulation Package based on DFT38,39,40. To describe electron-ion interaction, we employ the projector augmented wave method, while the exchange-correlation kernel is treated with the Perdew Burke Ernzerhof form of the generalized gradient approximation41. We also include the DFT + D3 method to effectively characterize weak interactions, which integrates empirical corrections following Grimme’s scheme. A cutoff energy of 500 eV is expanded for the plane wave basis set, ensuring a total energy convergence of less than 1 meV/atom.Our slab models are constructed with reference to experimental observations. The geometric structures are constructed using a supercell of graphene with dimensions of \(\left(6\times 6\times 1\right)\). Experimental observations suggest an even number of nitrogen atoms coordinated in the systems. In our models, we substituted carbon atoms with nitrogen, resulting in substrates containing four nitrogen atoms. To explore the targeting effect of Ga atoms, we also construct slabs by introducing Ga atoms into the nitrogen-based graphene slab. For simplicity, we label the slab configurations N4, while those containing Ga are denoted as GaN4. Afterward, the Ru13 nanoparticle is absorbed into these slabs. A vacuum slab of 16 Å is employed to separate periodic images along the c direction. We utilize a Monkhorst-Pack k-mesh42 initial set with dimensions (1 × 1 × 1), which is subsequently increased to (3 × 3 × 1) for DOS calculations. Geometry optimization is performed with full relaxation, maintaining the shape and volume constraints until the residual force on each atom is smaller than 0.01 eV/Å. In this study, the catalytic reaction is facilitated by the presence of the Ru13 nanoparticle. Therefore, the corresponding structure-properties relationship, electronic properties, and catalytic performance are calculated upon these configurations containing Ru13 nanoparticles. The properties considered include ELF, HER activity, d band center, and DOS.Following the optimization of structures, we evaluate catalytic properties by computing the hydrogen adsorption free energy, as determined by Eq. (5) for the Gibbs free energy of H* adsorption43:$${\Delta G}_{{H}^{*}}^{0}={\Delta E}_{H}+{\Delta E}_{{{ZPE}}}-{T\Delta S}_{H}$$
(5)
Here, \({\Delta E}_{{{ZPE}}}\) is the correction for zero-point energy, and \({\Delta S}_{H}\) denotes the entropy difference between H* adsorption and the H2 molecule. T represents the temperature at 298.15 K. \({\Delta E}_{H}\) refers to the total energy of H adsorption on the system, defined as:$${\Delta E}_{H}={E}_{{H}^{*}}-\frac{1}{2}{E}_{{H}_{2}}-{E}_{{{system}}}$$
(6)
where \({E}_{{H}^{*}}\), \({E}_{{H}_{2}}\), and \({E}_{system}\) represent the total energy of the system with H* adsorption, the H2 molecule, and systems without H*, respectively.The expression for \({\Delta E}_{{{\rm{ZPE}}}}\) is as follows:$${\Delta E}_{{{ZPE}}}={E}_{{{ZPE}}}^{H}-\frac{1}{2}{E}_{{{ZPE}}}^{{H}_{2}}$$
(7)
where \({E}_{{{ZPE}}}^{H}\) is calculated based on the vibration frequency of the systems with H adsorption, and \({E}_{{{ZPE}}}^{{H}_{2}}\) denotes the zero-point energy of the final states.Next, we can express \({\Delta S}_{H}\) conveniently as half of the entropy of the final states under ambient conditions:$${\Delta S}_{H}\cong -\frac{1}{2}{S}_{{H}_{2}}^{0}$$
(8)
\({\Delta E}_{{{ZPE}}}-{T\Delta S}_{H}\) equals −0.24 eV at a temperature of 298.15 K. Consequently, Eq. (5) can be simplified to:$${\Delta G}_{{H}^{*}}^{0}={\Delta E}_{H}+0.24$$
(9)
Considering the predominant electron donation originating from the d states of Ga atoms among these configurations, DOS projected onto these d states can be accurately characterized by the d band center. The expression is specified by Eq. (10):$${\varepsilon }_{d}=\frac{{\int }_{-\infty }^{\infty }{ED}\left(E\right){dE}}{{\int }_{-\infty }^{\infty }D\left(E\right){dE}}$$
(10)
where \(D\left(E\right)\) refers to the DOS corresponding to energy \(E\).
Oxophilic gallium single atoms bridged ruthenium clusters for practical anion-exchange membrane electrolyzer
