Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Abstract Single atomic catalysts have shown great potential in efficiently electro‐converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon‐based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP‐NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni‐N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high‐performance catalysts for practical H2O2 electrosynthesis.


Material synthesis
Preparation of NiO nanosheet.
1.0 g of Ni(NO3)2• 6H2O and 1.0 g of hexamine (HMT) were first dissolved in 20 mL of ultrapure water to form a clear solution, which was then transferred into a Teflonlined autoclave with volume of 50 mL and heated at 95 °C for 9 h.After carefully washing and drying, the obtained powder was placed in a porcelain boat and heated at 350 °C for 2 h in a muffle furnace to generate the NiO nanosheet.

Preparation of C3N4.
6 g of melamine was placed in a porcelain boat and then heated at 550 °C for 2 h in a muffle furnace to obtain C3N4 (a yellow block).

Preparation of NiSA/NiNP-NCNT.
The mixture powder of the as-synthesized NiO and C3N4 at a mass ratio of 1:100 was loaded into a tubular furnace.Subsequently, the mixture was heated from room temperature to 900 ℃ at a rate of 2 ℃ min -1 under a N2 atmosphere.Following this, the mixture was kept at 900 ℃ for 5 minutes before being cooled back to room temperature.The resulting material was identified as NiSA/NiNP-NCNT.Similarly, using the same procedure, control samples were prepared at different temperatures while keeping the other parameters constant, except for the temperature variation, which were denoted as NiSA/NiNP-NCNT-800 and NiSA/NiNP-NCNT-1000, respectively.The above NiSA/NiNP-NCNT was mixed with dibenzyl disulfide in a certain mass ratio (mass ratio 1:1) and placed in a tube furnace.Under the protection of N2 atmosphere, the mixture was heated from room temperature to 350 °C at a rate of 2°C min -1 , held at this temperature for 120 min, and then cooled to room temperature.The resulting black powder was then treated with 3 M HCl for 24 h to remove exposed metal aggregations.

Preparation of Ni-NSC
Ni-NSC catalysts were prepared according to the reported literature [1] .Solution A, comprising 491.0 mg of Zn(NO3)2• 6H2O and 276.0 mg of dibenzothiophene dissolved in 35.0 mL of MeOH, was prepared, while Solution B, involving 620.0 mg of 2methylimidazole dissolved in 10.0 mL of MeOH, was separately formulated.Upon creation of these solutions, Solution B was meticulously added dropwise to solution a under continuous stirring over a period of 1 hour.Subsequently, the resultant S-doped ZIF-8 was retrieved through centrifugation, subjected to thorough washing, and thereafter dried and set aside for further processing.The acquired S-doped ZIF-8 was subsequently subjected to a heating process in a tube furnace at 1000°C for 2 hours, with a heating rate of 5 °C min -1 , under a N2 atmosphere to yield NSC. Following this, 100.0 mg of NSC was immersed in a solution comprising 86.7 mL of isopropanol and reverse osmosis (RO) water at a 1:1 volume ratio.Moreover, a new solution, designated as Solution C, was prepared by blending 200.0 mL of aqueous isopropanol/RO mixture (with a 1:1 volume ratio) containing 124.4 mg of Ni(OAc)2• 4H2O and 290.0 mg of 1,10-phenanthroline.Subsequently, Solution C (13.35 mL) was introduced into the previously prepared dispersion, followed by stirring at room temperature for a period of 6 hours.The solvent was subsequently eliminated using a rotary evaporator and the residues were dried under vacuum at 60 °C.The resulting powder was then subjected to calcination under an Ar atmosphere at 800 °C for 2 hours, ramping up to temperature at a rate of 5 °C min -1 .The final product derived from this intricate process was termed Ni-NSC.

Characterization
Scanning electron microscope (SEM) images were obtained on a Zeiss Geminisem 500 microscope at 5 kV.Transmission electron microscopy (TEM), high resolution TEM (HR-TEM), high angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX) mapping images were obtained on the JEOL JEM-2100F 300kV.X-ray diffraction (XRD) patterns were recorded using Rigaku Ultima IV operating at 40 kV with Cu Kα radiation (λ = 0.15406 nm), a theta range of 5° to 90° and a scan rate of 10°/min.Raman spectra were collected on a Raman spectrometer system (Raman spectrometer model: HORIBA HR Evolution) using a laser with a wavelength of 532nm.Brunauer-Emmett-Teller (BET) surface area of the obtained material was measured using a relative pressure of P/P0 = 0.05-0.1.X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific K-Alpha instrument, which operates at 12 kV using Al-Ka radiation.The Ni-K-edge XANES spectra of the catalysts were measured at the optical emission end station of BSRF's beamline 4B9A in Beijing, China.

Electrochemical measurements
The electrochemical tests are all carried out using a three-electrode system via the Nafion 117 anion exchange membrane was used to separate the cathode and anode chambers.The H2O2 yield was determined using the colorimetric method:2Ce 4+ + H2O2 →2Ce 3+ + 2H + + O2 and cerium sulfate Ce(SO4)2 titration.A standard UV absorption curve was calibrated with a series of Ce(SO4)2 solutions of known concentration (0.1, 0.15, 0.2, 0.3 and 0.4mM).The yield of H2O2 can be calculated using the following equation: where C is the concentration of H2O2 in the electrolyte, V is the total volume of electrolyte in the cathode chamber, m is the catalyst loading, and h is the test time.At the end of the test, 0.5 mL of electrolyte was titrated into 5 mL of Ce(SO4)2 solution and 4.5 mL of sulfuric acid solution.In the experiment, V in the cathode half-cell was 9 65 mL, and t was set to 1h.The cathode half-cell was used in the experiment.C1 is the concentration of Ce 4+ before the reaction, V1 is the volume of Ce 4+ before the addition of the reaction; C2 is the concentration of Ce 4+ remaining after the reaction, V2 is the total volume of the electrolyte after the addition of the electrolyte.V3 is the volume of the electrolyte added.

Computational details
Density functional theory as implemented in the Vienna Ab-initio Simulation Package (VASP) was employed to optimize geometry structures [2] .The exchange-correlation interactions were described by the generalised gradient approximation (GGA) [3] in the form of the Perdew-Burke-Ernzerhof functional (PBE) [4] .A cut-off energy of 500 eV for plain-wave basis sets was adopted and the convergence threshold was 10 -5 eV, and 5×10 -3 eV/Å for energy and force, respectively.The weak interaction was described by DFT+D3 method using empirical correction in Grimme′s scheme. [5]The vacuum space was set to be more than 20 Å, which was enough to avoid the interaction between periodical images.
The reaction Gibbs free energy changes (∆G) for each elementary steps were based on the computational hydrogen electrode model, which can be calculated by the following equation;
CHI760E electrochemical workstation.The rotating ring disc electrode (RRDE) measurement consists of a rotating disc electrode (glassy carbon disc, 4 mm diameter), a rotating ring disc (glassy carbon disc, 5 mm inner diameter, 7 mm outer diameter, Pt ring), a platinum wire and an Ag/AgCl electrode (saturated KCl solution) as working electrode.The platinum wire was used as the counter electrode and the Ag/AgCl electrode as the reference electrode.The reference electrode Ag/AgCl (saturated KCl solution) was calibrated relative to the reversible hydrogen electrode (RHE), ERHE = EAg/AgCl +0.960.A catalyst ink was made by sonicating 5 mg of sample, 10 ul of Nafion(5 wt %), 700 ul of water and 290 ul of isopropanol for 1 h.Afterwards, 5 ul of the dispersion was dropped onto the disc electrode and then dried at room temperature.The mass loading of the catalyst at the disc electrode was 0.20 mg cm -2 .O2 was passed into 0.1 M KOH for 30 min before testing to ensure O2 saturation was achieved.Linear scanning voltammetry (LSV) tests were performed at 1600 rpm in an O2 saturated 0.1M KOH electrolyte.Repeat at a scan rate of 10mV s -1 over a potential range of 0.1V vs. RHE to 1.0V vs. RHE, The ring potential was also held at 1.2V vs.RHE.Normalise all current densities to the area of the disc electrode.In performing Ni single atom masking experiments to verify the active site test.Change the electrolyte from 0.1 M KOH to 0.5 ml 1 M KSCN + 0.1 M KOH.The rest of the procedure is the same.The hydrogen peroxide yield (H2O2 (%)) and the number of electron transfers (n) are calculated based on the following equations:n = 4 ×    +  /  2  2 (%) = 200 ×   /N   +  /Nwhere Id is the disk current, Ir is the ring current and N = 0.42 is the current collection efficiency of the platinum ring.Electrochemical test in the H-cell.Experiments on H2O2 production were carried out in an H-cell electrolyser using a three-electrode structure:NiSA/NiNP-NSCNT electrocatalyst-coated carbon paper, carbon rods, and Ag/AgCl electrodes as the working electrode, counting electrode, and reference electrode, respectively.The contact area of the carbon paper working electrode with the electrolyte was 1 cm 2 .

Figure S2 .
Figure S2.SEM image a) and XRD pattern b) of as-prepared NiO nanosheets.

Figure S3 .
Figure S3.TEM image of NiSA/NiNP-NSCNT, indicating that small nanoparticles are distributed randomly at the tips or inside within carbon nanotubes.

Figure S6 .
Figure S6.Morphological structure of NiSA/NiNP-NBCNT catalysts.a) SEM image of NiSA/NiNP-NBCNT, b) and c) are TEM and HRTEM image of NiSA/NiNP-NBCNT, d) HRTEM image and its e) corresponding IFFT images and f) lattice plane spacing (All due to enlargem ent in the red dotted box).

Figure S10 .
Figure S10.CV curve in 0.1 M KOH solutions under H2-saturated condition with a Ptfoil as the working electrode for RHE calibration.Based on the test, in 0.1 M KOH solutions, E(RHE) = E(Ag/AgCl) + 0.960 V.

Figure S14 .
Figure S14.XPS survey spectra of as-prepared four catalysts.

Figure
Figure S16.a) The S 2p XPS spectra of NiSA/NiNP-NSCNT samples prepared with different temperature (800-1000 °C) for the first-step thermal treatment.b) The P 2p XPS spectra of NiSA/NiNP-NPCNT samples prepared with different temperature (800-1000 °C) for the first-step thermal treatment.

Figure S20 .
Figure S20.Wavelet transforms of the EXAFS spectra of Ni foil and NiPc.

Figure S24 .
Figure S24.Bulk H2O2 production performance on NiSA/NiNP-NSCNT with a home-made Hcell in 0.1 M KOH electrolyte.a) UV-Vis spectrometer was used to determine different Ce 4+ concentrations and adsorption at 320 nm was recorded.b) LSV polarization curve.c) Ampere i-t curves at 0.66V vs. RHE potential.

Figure S25 .
Figure S25.Evaluation of ORR performance for carbon-supported Ni nanoparticles and Ni single atoms.a) Polarization curves of NiSA/NiNP-NSCNT after adding KSCN to poison the single atomic sites.b) The calculated H2O2 selectivity and transfer electron number (n).c) Polarization curves of Ni-NSC.d)The calculated H2O2 selectivity and transfer electron number (n).

Figure S26 .
Figure S26.The constructed model structures with only single Ni atomic sites, referred as NiN4, NiN3B, NiN3P and NiN4S.Brown, gray, yellow, green, blue and purple spheres represent C, Ni, S, B, N and P atoms, respectively.

Table S2 .
List of previously reported catalysts for hydrogen peroxide (H2O2) production.