Hollow-Sphere Tin Nanocatalysts for Converting CO2 into Formate

Abstract

Three-dimensional (3D) hollow nanosphere electrocatalysts that convert CO.sub.2 into formate with high current density and Faradaic efficiency (FE). The SnO.sub.2 nanospheres were constructed from small, interconnected SnO.sub.2 nanocrystals. The size of the constituent SnO.sub.2 nanocrystals was controlled between 2-10 nm by varying the calcination temperature and observed a clear correlation between nanocrystal size and formate production. In situ Raman and time-dependent X-ray diffraction measurements confirmed that SnO.sub.2 nanocrystals were reduced to metallic Sn and resisted microparticle agglomeration during CO.sub.2 reduction. The nanosphere catalysts outperformed comparably sized, non-structured SnO.sub.2 nanoparticles and commercially-available SnO.sub.2 with a heterogeneous size distribution.

Claims

1. A SnO.sub.2 powder, comprising at least 90 mass % hollow spheres in the (diameter) size range of 175 to 225 nm, preferably 180 to 220 nm, in some embodiments 190 to 210 nm; and wherein the spheres are comprised of SnO.sub.2 particles.

2. The SnO.sub.2 powder of claim 1 wherein at least 90 mass % hollow spheres are in the (diameter) size range of 180 to 220 nm or 190 to 210 nm

3. A SnO.sub.2 powder, comprising hollow spheres having a diameter of 100 nm or greater, wherein the spheres are comprised of SnO.sub.2 particles, and wherein at least 90 mass % of the spheres have diameters within a 10 nm range.

4. The SnO.sub.2 powder of claim 3 wherein at least 90 mass % of the spheres have diameters within a size range of from 200 to 220 nm.

5. The SnO.sub.2 powder of claim 1 wherein at least 90 mass % of the spheres have diameters within a 7 nm size range or a 5 nm size range or a 3 nm size range.

6. The SnO.sub.2 powder of claim 1 wherein the hollow spheres have a wall thickness in the range of 20 to 35 nm or 25 to 30 nm.

7. The SnO.sub.2 powder of any of claim 3 wherein the hollow spheres are comprised of nanocrystals having a mass average diameter in the range of 5 to 15 nm, or 5-10 nm, or 6 to 9 nm.

8. The SnO.sub.2 powder of claim 1 wherein, as measured by XRD, the hollow spheres are comprised of nanocrystals having an average crystallite size in the range of 5 to 10 nm, or 6 to 9 nm, or 6 to 8 nm.

9. The SnO.sub.2 powder of claim 3 characterizable by a durability of maintaining a j.sub.formate (mA cm.sup.−2) of at least 35 at 1.2 V vs. RHE for at least two days without regeneration.

10. (canceled)

11. The SnO.sub.2 powder of claim 1 characterizable by a j.sub.total/mA cm.sup.−2.sub.geo of at least 50 at 1.2 V vs. RHE.

12. (canceled)

13. The SnO.sub.2 powder of claim 3 characterizable by an ESCA of at least 35 cm.sup.2.

14. A method of making a SnO.sub.2 catalyst, comprising: providing a suspension of polymer particles, combining a tin salt with the suspension, removing the liquid from the suspension (preferably by evaporation) to form tin-coated polymer particles, drying the tin-coated polymer particles, and calcining the dried particles to burn out the polymer particles leaving hollow SnO.sub.2 spheres.

15. The method of claim 14 wherein the suspension is an aqueous suspension.

16. (canceled)

17. The method of claim 14 wherein the calcining is carried out at a temperature in the range of 300 to 600° C.

18. A catalyst ink, comprising the SnO.sub.2 particles powder of claim 1 dispersed in a liquid phase along with conductive particles and binder particles.

19-20. (canceled)

21. The catalyst ink of claim 18 wherein the conductive particles comprise carbon black, carbon fibers, carbon or graphene sheets, or carbon nanotubes.

22-24. (canceled)

25. The catalyst ink of claim 18 wherein the liquid phase comprises at least 50% of an alcohol or mixture of alcohols.

26. An electrode, comprising a conductive substrate coated with the SnO.sub.2 powder of claim 1.

27. The electrode of claim 26 wherein the conductive substrate comprises a porous carbon paper.

28. A method of making an electrode by impregnating, drop-casting or coating the ink of claim 18 into or on a conductive substate.

29. A system comprising the electrode of claim 26 disposed in a solution that is saturated with CO.sub.2

30. A system comprising the electrode of claim 26 comprising a tin catalyst disposed in a solution that is saturated with CO.sub.2, and further wherein the system or catalyst is characterizable by a durability of maintaining a j.sub.formate (mA cm.sup.2) of at least 35 or at least 40 or in the range of 40 to 55 at 1.2 V vs. RHE for at least one or at least two or at least three days or from one to five days.

31-37. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1. (A) Scheme illustrating the synthesis of 3D hollow SnO.sub.2 nanospheres by a combined sol-gel and templating method. (B) Representative FE-SEM and (C) HR-TEM images of SnO.sub.2 spheres calcined at 500° C. (D) XRD crystallite size as a function of calcination temperature.

[0020] FIG. 2. (A) Formate partial current density vs. cathodic potential of hollow SnO.sub.2 nanospheres calcined at different temperatures. (B) Representative Faradaic efficiency for formate, CO, and H.sub.2 for SnO.sub.2 nanospheres calcined at 500° C. (C) Geometric and (D) ECSA-normalized formate current densities for commercial-SnO.sub.2 nps, non-templated SnO.sub.2 nps (bottom trace), and the inventive SnO.sub.2 nanospheres calcined at 500° C.

[0021] FIG. 3. (A) Long-term durability performance of the inventive SnO.sub.2 nanospheres (upper data points), non-templated SnO.sub.2 nps, and commercially-available SnO.sub.2 nps at −1.2 V vs. RHE during several on/off cycles. (B) FE-SEM images of three electrodes before and after long-term electrolysis. (C) Synchrotron-based XRD patterns of the best-in-class SnO.sub.2 nanospheres collected after operating for various time periods at −1.2 V vs. RHE.

[0022] FIG. 4. FE-SEM images of SnO.sub.2 nanospheres calcined at (A, B) 300° C., (C, D) 400° C. and (E, F) 600° C. Higher thermal annealing temperatures resulted in broken spherical shells.

[0023] FIG. 5. HR-TEM images of SnO.sub.2 nanospheres calcined at 300° C., revealing the spherical shells were ca. 5 nm-thick and composed of 2-3.5 nm SnO.sub.2 nanocrystallites. Inset of (A) is the corresponding FFT diffraction pattern showing polycrystalline tetragonal rutile SnO.sub.2.

[0024] FIG. 6. HR-TEM images of SnO.sub.2 nanospheres annealed at 500° C., indicating thicker wall of 25-30 nm containing interconnected 6-9 nm SnO.sub.2 nanocrystals with distinct grain boundaries. The TEM-determined nanocrystal diameter of 6-9 nm is consistent with the average 7 nm crystallite size determined from X-Ray diffraction.

[0025] FIG. 7. Comparison of Faradaic efficiency for (A) formate, (B) CO, and (C) H.sub.2 vs. potentials for SnO.sub.2 nanospheres calcined from 300° C. to 600° C.

[0026] FIG. 8. Total geometric current densities for 3D SnO.sub.2 nanosphere series. The bottom trace is from the nanospheres calcined at 300° C.

[0027] FIG. 9. (A) Formate selectivity (referring to total CO.sub.2RR products) and (B) production rate of formate for SnO.sub.2 nanospheres calcined at 500° C.

[0028] FIG. 10. (A) XRD patterns and (B) Raman spectra of non-templated SnO.sub.2 nps and com-SnO.sub.2 nps compared with SnO.sub.2 nanospheres calcined at 500° C.

[0029] FIG. 11. Comparison of Faradaic efficiency for (A) formate (inventive is top trace), (B) CO, and (C) H.sub.2 vs. potentials of the best performing SnO.sub.2 nanospheres, non-templated SnO.sub.2 nps, and com-SnO.sub.2 nps.

[0030] FIG. 12. Double-layer capacitance measurement in CO.sub.2-purged 0.1 M KHCO.sub.3 electrolyte: (A-C) Cyclic voltammetry profiles measured in the non-Faradaic region with different scan rates. (D) Scan rate dependence of the current density for com-SnO.sub.2 nps, non-templated SnO.sub.2 nps and the inventive SnO.sub.2 nanospheres electrodes.

[0031] FIG. 13. Core-level (A) Sn 4p+Pt 4f, (B) Sn 4s+Pb 4f, (C) Sn MNN+Zn 2p, and (D) Sn 3p+Fe 2p spectra of SnO.sub.2 nanospheres electrode before and after long-term stability run. No distinguishable features of trace Pb, Zn, Fe, or crossover Pt elements indicates these potential trace contaminants were not deposited onto the catalyst surface during the 36-hour electrolysis. A similar lack of contaminants was observed for non-templated and commercially available SnO.sub.2 after long-term electrolysis.

[0032] FIG. 14. XRD crystallize size of starting SnO.sub.2 and reduced β-Sn as a function of time obtained from time-resolved XRD patterns of SnO.sub.2 nanospheres collected at −1.2 V vs. RHE.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Catalysts were synthesized using a tin-salt precursor dissolved in alcohol and citric acid. A polymer template was mixed with the starting catalyst precursor, dried in air and calcined at high temperatures to form the catalyst structures. We could control the resulting catalyst structure based on the synthetic conditions and calcination temperature. A preferred catalyst prepared at 500° C. comprises approximately 205-210 nm diameter and 25-30 nm wall thickness hollow spheres constructed from interconnected, about 10 nm SnO.sub.2 nanoparticles. X-ray photoelectron spectroscopy confirmed the composition and oxidation state of the metal, and X-ray diffraction confirmed the nanocrystallite SnO.sub.2 size of ˜7.5 nm.

[0034] 3D SnO.sub.2 nanospheres were prepared by a combined sol-gel and templating approach (FIG. 1A). Negatively charged tin (II) citrate complex was absorbed on the surface of positively-charged poly(methyl methacrylate) (PMMA) spheres (diameter of ca. 220 nm) through electrostatic interaction. The system underwent hydrolysis, condensation, nucleation, and self-assembly to create tin-containing coating layers on the surface of the PMMA spheres. Subsequent calcination in air between 300 to 600° C. converted these coating layers into SnO.sub.2 nanocrystals and removed the PMMA template to produce hollow SnO.sub.2 nanospheres (FIG. 1B and FIG. 4-6). A representative scanning electron microscope (SEM) image in FIG. 1B shows a SnO.sub.2 nanosphere sample calcined at 500° C. HR-TEM micrographs in FIG. 1C and FIG. 6 indicate the nanosphere walls were constructed from small, interconnected nanocrystals. The lattice fringes of 0.335 and 0.264 nm in FIG. 1C correspond to (110) and (101) planes of polycrystalline rutile SnO.sub.2.

[0035] The PMMA template fixed the nanosphere diameter at 205-210 nm for all calcination temperatures, and XRD and EXAFS confirmed a consistent SnO.sub.2 oxidation state and tetragonal rutile structure. Higher calcination temperatures produced sharper, more intense XRD peaks that indicate increased crystallinity and larger mean crystallite size, and FIG. 1D (circles) demonstrates that the SnO.sub.2 crystallite size scaled with calcination temperature. These characterizations reveal that both the size and crystallinity of the constituent SnO.sub.2 nanocrystals were well-controlled with post-treatment calcination temperature, but we found calcining at 600° C. produced nanosphere structures with severely fractured walls (FIG. 4).

[0036] Particle size of primary nanoparticles can be measured by electron microscopy techniques. Since the inventive particles are spherical, all diameters are assumed to be the same, but in the general case, the size is the minimum diameter through the center.

[0037] Electrochemical reduction of CO.sub.2 was conducted at room temperature in an aqueous electrolyte of 0.1M KHCO.sub.3. Typical experiments involved holding an electrochemical potential for a set amount of time in a gas-tight reactor cell. After a pre-determined amount of time the gaseous reaction products were measured with gas chromatography and liquid formate production was measured with ion chromatography.

[0038] A catalytic figure of merit is defined as the partial current density for formate production (j.sub.formate/mA cm.sup.−2). This value describes the amount of electrochemical current per geometric electrode area associated with formate production (FIG. 2). In the tested example, formate was produced at rates (partial current densities) approximately six times higher than commercially-available materials. Our formate production rates are also approximately two times higher than the best reported materials in scientific literature. Initial stability testing over several hours shows extremely stable performance and consistent product formation rates. Importantly, no other liquid products were formed. The only other byproducts were gaseous CO and H.sub.2 (syngas), which could be easily removed from the reactor and for other industrial applications (methanol synthesis, etc.).

Examples

Synthesis of Poly (Methyl Methacrylate) (PMMA) Latex

[0039] All chemicals were purchased from Sigma-Aldrich and used as received without further purification. PMMA latex was prepared by surfactant-free emulsion polymerization using a cationic free radical initiator. 875 mL of deionized water (DIW) and 100 g of methyl methacrylate (CH.sub.2═C(CH.sub.3)COOCH.sub.3) were mixed at room temperature under a nitrogen flow for 30 min and then maintained at 70° C. Subsequently, a solution containing 0.15 g of 2,2′-azobis (2-methylpropionamidine) dihydrochloride ([═NC(CH.sub.3).sub.2C(═NH)NH.sub.2].sub.2.2HCl) and 25 mL of DIW was quickly added under vigorous stirring to form a milky white suspension. The suspension was then stirred at 70° C. for 6 h to complete the polymerization. After cooling down to room temperature for 1 h, the concentration of obtained PMMA latex (size of ca. 220 nm) was 10 wt %. The latex was diluted with DIW to achieve 0.5 wt % for further use.

Synthesis of Hierarchical Hollow SnO.SUB.2 .Spheres

[0040] All chemicals were purchased from Sigma-Aldrich and used as received without further purification. Hierarchical hollow SnO.sub.2 spheres were synthesized by a combined sol-gel and templating method. Poly (methyl methacrylate) (PMMA) spherical template (diameters of ca. 210 nm) was prepared by surfactant-free emulsion polymerization using a cationic free radical initiator. In a typical procedure, 226 mg of tin (II) chloride dihydrate (SnCl.sub.2.2H.sub.2O) were dissolved in 5 mL of ethanol (C.sub.2H.sub.5OH, 200 proof) and 38 mg of anhydrous citric acid (C.sub.6H.sub.8O.sub.7) were separately mixed in 5 mL of ethanol. Citric solution was then added into tin precursor and sonicated for 15 min. 1.5 mL of tin-citric solute ion was dropwise added into 30 mL of aqueous PMMA latex template (0.5 wt %) under vigorous stirring at room temperature. After 30 min, the mixture was evaporated overnight in the oven at 60° C. to obtain the as-synthesized powders. Same stock tin-citric solution was used to make multiple batches of as-synthesized materials which were subsequently annealed in static air at 300, 400, 500 and 600° C. for 3 h with ramping rate of 1° C. min.sup.−1. The obtained powder was denoted as “SnO.sub.2 nanospheres”.

[0041] Non-hierarchical SnO.sub.2 nanoparticles were prepared using similar recipes, except using 30 ml of deionized water in lieu of PMMA dispersion. After evaporation at 60° C., the products were subsequently calcined in air at 500° C. with ramping rate of 1° C. min.sup.−1 for 3 h and named “non-templated SnO.sub.2 nps”. Commercial SnO.sub.2 nanopowder with ≤100 nm average particle size (Sigma, product number 549657) was also used as reference material and denoted as “com-SnO.sub.2 nps”.

Electrochemical CO.SUB.2 .Reduction Measurement

[0042] Electrochemical experiments were performed in a gas-tight, two-compartment H-cell separated by a Nafion 117 proton exchange membrane. Each compartment was filled with 60 mL of aqueous 0.1 M KHCO.sub.3 electrolyte (99.99%, Sigma-Aldrich) and contained 90 mL headspace. The ultra-pure deionized water with 18.3 MΩ cm.sup.−1 resistivity (Barnstead EASYpure LF) was used in all electrochemical experiments. The catholyte was continuously bubbled with CO.sub.2 (99.999%, Butler gas) at a flow rate of 20 mL min.sup.−1 (pH˜6.8) under vigorous stirring during the experiments. The counter and reference electrodes were Pt mesh and Ag/AgCl (saturated NaCl, BASK)), respectively. The catalyst ink was composed of 2.8 mg of the powder catalysts, 0.32 mg Vulcan VC-X72 carbon black, and 40 μL of Nafion® 117 solution binder (Sigma-Aldrich, 5%) in 400 μL of methanol. Working electrodes were fabricated by drop-casting the ink onto PTFE-coated carbon paper (Toray paper 060, Alfa Aesar) and N.sub.2-dried. The mass loadings were kept at 9.5±0.6 mg.sub.ink cm.sub.geo.sup.−2 and 5.4±0.3 mg.sub.SnO2 cm.sub.geo.sup.−2. Cyclic voltammetry (CV) was obtained in CO.sub.2-saturated KHCO.sub.3 in the potential window of +1 V and −1.3 V vs. RHE with scan rate of 20 mV s.sup.−1. All potentials were referenced against the reversible hydrogen electrode (RHE) (unless otherwise specified), typical uncompensated resistances were 40-50Ω, and the uncompensated ohmic loss (Rn) was automatically corrected at 85% (iR-correction) using the BioLogic instrument software in all electrochemical experiments.

[0043] CO.sub.2 electroreduction tests were performed at room temperature using a SP-300 potentiostat (BioLogic Science Instrument). The fresh catholyte was saturated with CO.sub.2 by continuously purging with CO.sub.2 (20 mL min.sup.−1) under vigorous stirring during the experiments. Short-term chronoamperometric experiments were conducted for 20 min at each applied potential between −0.6 V and −1.3 V vs. RHE and the products were collected every 20 min. Long-term chronoamperometric experiments were conducted over several days at −1.2 V vs. RHE. The testing was run for 5 hours per day and the products were collected every hour. After each cycle, the electrodes were discarded from electrolyte and naturally stored in polystyrene petri dish for next cycle. Fresh aqueous KHCO.sub.3 catholyte was used for each cycle. The total and partial current densities were normalized to the exposed geometric area (unless otherwise specified). Each data point is an average of at least three independent experiments on different fresh electrodes. The evolved gas products were collected in a Tedlar gas-tight bag (Supelco) and then quantified by PerkinElmer Clarus 600GC equipped with both FID and TCD detectors, using ShinCarbon ST 80/100 Column and He as a carrier gas. The liquid products collected from the catholytes at intervals of 20 min or 1 h were filtered with PES 0.22 μm filter and determined by Dionex ICS-5000+ ion chromatography using ED50 conductometric detector, ASRS suppressor in auto-generation mode, AS11-HC column and KOH eluent with a gradient of 0.4-30 mM in 45 min run.

[0044] Materials characterization. Scanning electron microscopy (SEM) imaging was performed on a FEI Quanta 600F microscope operated at 10-20 kV equipped with an energy-dispersive X-ray (EDX) detector. High-resolution transmission electron microscopy (HR-TEM) was carried out on a FEI Titan Themis G2 200 Probe Cs Corrected Scanning Transmission Electron Microscope operated at an accelerating voltage of 200 kV. The powder sample was suspended in ethanol, drop-casted onto a holey carbon supported Cu grid, and naturally dried in air. X-ray powder diffraction (XRD) patterns were collected on a PANalytical X'Pert Pro X-ray diffractometer using CuKα radiation (λ=1.5418 Å) at a scan rate of 0.2° min.sup.−1. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 VersaProbe III scanning XPS microprobe (Physical Electronics, ULVAC-PHI Inc) using Al Kα (1486.6 eV) radiation source and a hemispherical analyzer. All the binding energies were internally calibrated to the surface adventitious hydrocarbon feature (C 1s) at 284.6 eV.

[0045] Synchrotron X-ray diffraction measurements were conducted at beamline 17-BM-B (λ=0.24121 Å) of the Advanced Photon Source at Argonne National Laboratory. The post-reaction electrodes under the application of −1.2 V vs. RHE were collected in the H-cell as a function of electrolysis time. Two-dimensional diffraction patterns were collected by a Perkin Elmer amorphous silicon detector, data acquisition was performed with QXRD and the diffraction ring was integrated using GSAS-II freeware package.

[0046] Raman spectroscopy was performed on a LabRam HR-Evolution spectrometer (Horiba Scientific) with a 633 nm laser as an excitation source and 100× working distance objective, and in situ measurements were carried out using a custom-made electrochemical cell and a 50× long-working-distance objective. The composition of catalyst ink was identical to the one used in CO.sub.2RR H-cell tests with 5 μl, of the catalyst ink drop-casted onto a glassy carbon working electrode. A Pt wire and Ag/AgCl were used as counter and reference electrodes, and iR-correction was applied in all measurements. 5 mL of 0.1 M aqueous KHCO.sub.3 electrolyte was continuously purged with CO.sub.2 during the measurements and sequential Raman spectra were collected under open circuit and at −1.2 V vs. RHE.

[0047] Sn K-edge X-ray absorption spectroscopy (XAS) was collected at the 8-ID (ISS) beamline of the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory using a Passivated Implanted Planar Silicon detector and Sn foil for energy calibration (29.2 keV). All synthesized SnO.sub.2 samples, bulk SnO.sub.2 and bulk SnO powders were loaded into Kapton capillary and Sn K-edge data were collected in fluorescence modes and subsequently analyzed using IFEFFIT freeware package.

[0048] The XRD patterns of 3D SnO.sub.2 nanospheres calcined from 300 to 600° C. were indexed to pure tetragonal SnO.sub.2 rutile (JCPDS 41-1445) having the space group P4.sub.2/mnm. Increasing calcination temperature produced sharper, more intense, peaks that indicate increased crystallinity and crystallite size up to ˜10 nm. In addition, the Sn K-edge EXAFS results in show the presence of first nearest neighbor shell of Sn—O and second Sn—Sn coordination shell for all SnO.sub.2 sphere samples. Higher calcination temperature led to more intense amplitude of these features, further indicating increased crystallinity, particle size, and coordination numbers, with less disorder.

[0049] The symmetrical Sn 3d.sub.5/2 and Sn 3d.sub.3/2 doublet in core-level XPS corresponds to Sn.sup.4+ oxidation state in rutile SnO.sub.2. The SnO.sub.2 nanospheres showed up-shifted Sn 3d peaks compared with bulk SnO.sub.2, and lower calcination temperatures (smaller SnO.sub.2 nanocrystals) produced larger binding energy (BE) increases. Similar size-dependent BE shifts have also been observed for other small SnO.sub.2 nanoparticles,.sup.25 as well as nanoparticulate Au,.sup.26 Pd,.sup.27 and PbS.sup.28 systems. There was no evidence of Sn.sup.2+ or any tin-related impurity phases using other characterizations, including XRD and Raman results.

[0050] XRD results of non-templated SnO.sub.2 nanoparticles and commercial nanoparticles demonstrate tetragonal rutile SnO.sub.2 crystal structure. Non-templated SnO.sub.2 nps had almost identical crystallinity, orientation, crystallite size (˜7 nm) and structural defects as 3D hierarchical SnO.sub.2 nanospheres prepared at same temperature (500° C.). However, commercial SnO.sub.2 nanoparticles possessed 4.4 wt % orthorhombic SnO.sub.2 phase (JCPDS 78-1063, space group Pbcn), much larger crystal size (ca. 28 nm). Similarly, Sn K-edge EXAFS spectra also showed the first nearest neighbor shell of Sn—O and second Sn—Sn coordination shell for two nanoparticle samples. The Sn 3d doublets also indicated the presence of Sn.sup.4+ valence state in both non-hierarchical nanoparticle samples.

[0051] SnO.sub.2 nanospheres show characteristic Raman bands including A.sub.1g (symmetric Sn—O stretching), B.sub.2g (asymmetric Sn—O stretching), doubly degenerated E.sub.g modes (space group D.sub.4h), and broad E.sub.u and A.sub.2g scattering peaks, as previously noted. In situ Raman spectroscopy was conducted to determine the change in oxidation state during application of electrochemical potential relevant to CO.sub.2RR.

[0052] In situ time-dependent Raman spectra of SnO.sub.2 nanospheres calcined at 500° C. (on glassy carbon electrode) under CO.sub.2RR at −1.2 V vs. RHE showed that the A.sub.1g and smaller E.sub.g and E.sub.u peaks were still visible for the SnO.sub.2 nanosphere catalysts deposited on a glassy carbon electrode and held at open circuit in CO.sub.2 saturated electrolyte. Time-resolved Raman spectra collected at −1.2V vs. RHE showed the attenuation and then complete disappearance of characteristic Raman bands. This result is consistent with the time-dependent XRD shown in FIG. 3C and provides further evidence for the reduction of SnO.sub.2 into metallic Sn during CO.sub.2RR. No other peaks associated with reduced tin oxides and/or surface-bound intermediate species were observed in the wide region of 150-850 cm.sup.−1. Our observation is consistent with operando Raman results for reduced graphene oxide supported SnO.sub.2 reported by Dutta et al..sup.45 Oxide fingerprints completely disappeared at very negative potentials, particularly −1.55 V vs. Ag/AgCl, as the catalyst fully reduced to metallic Sn. The re-emergence of characteristic SnO.sub.2 Raman bands when the electrode was held at open circuit after electrolysis indicates re-oxidation of the metallic Sn into oxide species.

[0053] Electrochemical CO.sub.2 reduction performance in an aqueous H-cell. CO.sub.2RR activity was screened between −0.6 V to −1.3 V vs. RHE in an H-cell containing CO.sub.2-saturated 0.1 M KHCO.sub.3. All SnO.sub.2 electrocatalysts produced formate as a main product, along with smaller amounts of CO and H.sub.2 (FIG. 7), but SnO.sub.2 spheres calcined at 500° C. exhibited the highest FE and formate partial current density (j.sub.formate) at all potentials (FIG. 2A and FIG. 7-9). This 500° C. SnO.sub.2 nanosphere catalyst contained −7 nm primary nanocrystals, and FIG. 2B shows that it produced 71-81% formate FE between −0.9 V and −1.3 V vs. RHE and a maximum j.sub.formate of 73±2 mA cm.sub.geo.sup.−2, which are among the highest performance metrics reported for Sn-based electrocatalysts in aqueous H-Cells (Table 1). The FEs for C1 products reached >90% in the range from −0.8 V to −1.2 V and the H.sub.2 evolution reaction was strongly suppressed. It is worth mentioning that gaseous CO and H.sub.2 side-products (syngas) are easily separated from liquid formate for subsequent use in methanol or Fischer-Tropsch synthesis.

[0054] The results in FIG. 2A also show an apparent dependence on the size of the constituent SnO.sub.2 nanocrystals. It has been reported previously that grain boundaries,.sup.4,12,13,18,35,38,39-41 oxygen vacancies,.sup.37,42,43 and particle size.sup.31,35,37,43 of SnO.sub.2 can impact CO.sub.2RR activity and selectivity. In this study, we suggest that SnO.sub.2 nanospheres annealed at 500° C. likely produced an optimum balance between crystallinity and nanocrystal size that maximized formate selectivity and production rate.

[0055] We also compared the performance of SnO.sub.2 nanospheres with similar sized (˜7 nm), non-templated SnO.sub.2 nps and commercially available SnO.sub.2 nps (named com-SnO.sub.2 nps) with a heterogeneous particle size distribution between 5-150 nm (FIG. 10). Non-templated SnO.sub.2 nps were synthesized with an identical procedure except without the polymer template, and then calcined at 500° C. FIG. 2C and FIG. 11 show the SnO.sub.2 nanospheres demonstrated a 2˜6-fold improvement in formate partial current density, 20-30% higher formate FE, and reduced H.sub.2 evolution compared with the non-templated and commercial SnO.sub.2 nps. Capacitance-based electrochemical surface area (ECSA) measurements.sup.9,18,19,38 indicated the SnO.sub.2 nanospheres demonstrated approximately 1.5-3 times larger ECSA than the non-templated and commercial SnO.sub.2 nps (FIG. 12 and Table 2), but all three samples produced comparable ECSA-normalized formate partial current density (FIG. 2D). This result indicates the total amount of electrochemically active surface area was the dominant influence on geometric formate partial current density. In this regard, controlling the SnO.sub.2 nanosphere surface structure improved geometric-based performance over commercially available and non-templated SnO.sub.2 nps by maximizing ECSA.

TABLE-US-00001 TABLE 2 Double-layer capacitance (C.sub.dl) and electrochemical surface area (ECSA) for SnO.sub.2 nanospheres calcined at 500° C., non-templated SnO.sub.2 nps, and com-SnO.sub.2 nps. All measurements were carried out in CO.sub.2-purged 0.1M KHCO3 and all electrodes had equivalent SnO.sub.2 loadings of 5.4 ± 0.3 mg.sub.SnO2 cm.sub.geo.sup.−2 (total ink loading, including SnO.sub.2 and carbon black, was 9.5 ± 0.6 mg.sub.ink cm.sub.geo.sup.−2). Sample C.sub.dl [mF cm.sup.−2] ECSA [cm.sup.2] SnO.sub.2 nanospheres 14.52 51.3 Non-templated SnO.sub.2 nps 3.24 31.8 Com-SnO.sub.2 nps 2.67 16.8

[0056] The long-term durability of SnO.sub.2 nanospheres, non-templated SnO.sub.2 nps, and commercial SnO.sub.2 nps was evaluated in an H-Cell at −1.2 V vs. RHE with multiple start/stop cycles. As seen in FIG. 3A, formate partial current density for the SnO.sub.2 nanosphere catalysts stabilized at an average 45±5 mA cm.sub.geo.sup.−2 over 36 hours of operation with an average 68±8% FE. Non-templated SnO.sub.2 nps and com-SnO.sub.2 nps produced a smaller −20 mA cm.sub.geo.sup.−2 and similar ˜70% FE during steady state operation. Post-electrolysis SEM imaging in FIG. 3B revealed severe particle agglomeration or coalescence for the non-templated SnO.sub.2 nps and commercial SnO.sub.2 nps after 20 hours of electrolysis at −1.2V vs. RHE. This behavior has been observed before and agglomeration is a known deactivation mechanism for SnO.sub.2 electrocatalyts..sup.18,44 In contrast, no substantial particle agglomeration was observed for the SnO.sub.2 nanospheres, which may stem from the interconnected SnO.sub.2 nanocrystals within the nanosphere walls preventing severe particle growth under these conditions. No evidence of trace contaminant deposition on the electrode surface, such as Pt, Fe, Pb, or Zn, was detected on the electrode surface after long-term electrolysis (FIG. 13).

[0057] Time-dependent, synchrotron-based XRD of SnO.sub.2 nanospheres operated at −1.2 V vs. RHE revealed the reduction of SnO.sub.2 nanocrystals into metallic Sn through the emergence of body-centered tetragonal β-Sn diffraction peaks (FIG. 3C). These results indicate rapid transformation of SnO.sub.2 into metallic Sn and a slight increase in crystallite size to 23-24 nm under steady state operation (FIG. 14). Notably, this crystallite size remained stable over 30 h of operation and the XRD data agrees well with post-reaction SEM imaging that ruled out severe particle growth during long-term electrolysis. We also observed a minor residual oxide phase that likely resulted from re-oxidation upon air exposure. These results strongly support complementary in situ Raman spectroscopy experiments that showed SnO.sub.2 was reduced to metallic Sn during CO.sub.2RR at −1.2V, which is consistent with previous operando Raman results for other SnO.sub.2 CO.sub.2RR electrocatalysts..sup.45

Calculation of Faradaic efficiency and selectivity [0058] The Faradaic efficiency (FE) for product i is defined as the percentage of supplied electrons used to convert CO.sub.2 into product i and calculated as follows:

[00001]${\mathrm{FE}}_i=\frac{z_i*F*n_i}{I*t}=\frac{z_i*F*n_i}{Q}$

where z.sub.i is the number of electrons involved in the formation of product i (z=2 for formate, CO, and H.sub.2); F is the Faraday's constant (96485 C mol.sub.e.sup.−1); n.sub.i is the number of moles of product i formed (determined by GC and IC); I is the total current; t is electrolysis time; and Q is total charge in Coulombs passed across the electrode. [0059] The formate selectivity is defined as molar ratio of formate compared with the total CO.sub.2RR products:

[00002]$S_{\mathrm{formate}}=\frac{{\mathrm{mol}}_{\mathrm{formate}}}{{\mathrm{mol}}_{\mathrm{formate}}+{\mathrm{mol}}_{\mathrm{CO}}}$$\mathrm{or}:S_{\mathrm{formate}}=\frac{2*r_{\mathrm{formate}}}{2*r_{\mathrm{formate}}+2*r_{\mathrm{CO}}}$

[0060] where r is production rate for a reduced product, and 2 is the number of electrons involved in the formation of CO and HCOOH.

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TABLE-US-00002 TABLE 1 Comparison of CO.sub.2-to-formate performance of 3D hierarchical SnO.sub.2 spheres and other Sn based electrocatalysts reported for formic acid/formate production (excludes mixed metal oxides, alloys, and doped systems). HCOO− current Operating Faradaic density Catalysts Electrolyte potential [V] efficiency [%] [mA cm .sup.2] References 0.1M KHCO.sub.3 −1.2 V vs. RHE 81.1 50.4 This work 3D SnO.sub.2 nanospheres 0.1M KHCO.sub.3 −1.0 V vs. RHE 76.6 28.9 This work Hierarchical Sn dendrite 0.1M KHCO.sub.3 −1.36 V vs. RHE 71.6 12.2 Won et al., ChemSusChem 2015, 8, 3092 Nanoporous Sn foam 0.1M NaHCO.sub.3 −2.0 V vs. Ag/AgCl 90 20.7 Du et al., ChemistrySelect 2016, 1, 1711 Chainlike mesoporous SnO.sub.2 0.1M KHCO.sub.3 −1.06 vs. RHE 82 13.5 Bejtka et al., ACS Appl. Energy Mater. 2019, 2, 3081 SnO.sub.2 nanoparticles 0.1M KHCO.sub.3 −1.1 V vs. RHE 85 20.1 Daiyan et al., Adv. Sci. 2019, 6, 1900678 Grain boundary rich 1M KOH −0.73 V vs. RHE 74 51.8 Liang et al., J. Mater. Chem. SnO.sub.2 nanoparticles (<5 nm) A 2018, 6, 10313 Ultrathin sub-2 nm 0.1M KHCO.sub.3 −1.156 V vs. RHE 87.3 13.7 Liu et al., Angew. Chem. Int. SnO.sub.2 quantum wires Ed. 2019, 58, 8499 Nanoporous SnO.sub.2 0.1M KHCO.sub.3 −1.2 V vs. RHE 90 17.1 Liu et al., GreenChE. 2022, in press Ultra-small SnO 0.5M KHCO.sub.3 −0.9 V vs. RHE 66 13.2 Gu et al., Angew. Chem. Int. nanoparticles/carbon black Ed. 2018, 57, 2943 SnO.sub.2-carbon nanotubes 0.5M KHCO.sub.3 −0.77 V vs. RHE 76 4.6 Pavithra et al., Catal. Sci. Technol. 2020, 10, 1311 Hierarchical SnO.sub.2 0.5M NaHCO.sub.3 −1.6 V vs. Ag/AgCl 87 45 Li et al., Angew. Chem. Int. nanosheets/carbon cloth Ed. 2017, 56, 505 SnO.sub.2 0.5M KHCO.sub.3 −0.7 V vs. RHE 90 40 Zhang et al., Chem. Eng. J. nanosheets/N-doped carbon cloth 2021, 421, 130003