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Tuesday, June 6, 2023

Acidic CO2-to-HCOOH electrolysis with industrial-level present on part engineered tin sulfide

Determine 1 illustrates the evolution of π-SnS for acidic CO2-to-HCOOH electrolysis. By using part engineering technique, sturdy intrinsic binding energy between Sn and S is realized in π-SnS, which stabilizes larger S content material within the subsurface of Sn metals after CO2RR activation. Because of this, sturdy *OCHO adsorption and weak *H adsorption achieved on π-SnS-derived catalyst throughout CO2RR, and the excessive protection of *OCHO intermediate on the S-Sn websites may promote acidic CO2-to-HCOOH manufacturing and suppress HER at industrial-level present densities.

Fig. 1: Schematic of part engineering of SnS pre-catalysts for CO2RR.
figure 1

I Crystal construction of π-SnS pre-catalyst. II Derived S-doped Sn construction after CO2RR activation with larger S content material than typical part. III Sturdy *OCHO adsorption realized on S-doped Sn floor throughout CO2RR.

The π-SnS was ready by a hydrothermal technique (see Strategies), which is achieved by mixing SnCl2·2H2O, cysteine and polyvinylpyrrolidone (PVP). The sphere-emission scanning electron microscope (FESEM) photos (Fig. 2a, b) and X-ray diffraction (XRD) evaluation (Fig. 2c) reveal uniform rhombic dodecahedron construction of π-SnS with a particle measurement of 262 ± 37 nm (Supplementary Fig. 1). As well as, the decided π part in cubic crystal system (a, b, c = 11.83 Å) matches effectively with the simulated XRD sample of the π-SnS. It’s noteworthy that the π part has solely been found just lately as a brand new household of supplies. Controllable synthesis of π phase-based supplies stays drastically difficult, and only some research have developed small-scale π phase-based supplies synthesis25,26. On this work, we offered a easy and environment friendly technique to efficiently synthesize π-SnS with uniform rhombic dodecahedron morphology, which is uncommon amongst steel sulfides. For comparability, typical α part SnS (α-SnS) refers to particular layered constructions with an orthorhombic crystal system (Fig. 2c, Supplementary Fig. 2). The chemical compositions of π-SnS and α-SnS had been confirmed by the energy-dispersive X-ray spectroscopy (EDS) with an atomic ratio of ~1:1 between Sn and S (Supplementary Fig. 3). The high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and corresponding EDS mapping photos additional affirm the superb crystalline construction of π-SnS with the homogeneous distribution of Sn and S components (Fig. 2nd–f). The atomic constructions of π-SnS along with corresponding quick Fourier rework (FFT) patterns (Fig. 2nd, e) affirm the well-defined cubic construction with uncovered (110) side, which is in settlement with the simulated construction (Fig. 2e, g). In distinction, distinct stacking sequences are noticed in α-SnS (Supplementary Fig. 2).

Fig. 2: Structural characterization of π-SnS pre-catalyst.
figure 2

a,b FESEM photos of π-SnS. c XRD patterns of π-SnS (prime) and α-SnS (backside), and the inset of corresponding crystal constructions, tin in cyan and sulfur in yellow. d HAADF-STEM picture taken within the [101] route of π-SnS and the inset of FFT sample. e Magnified HAADF-STEM picture taken from the corresponding space in (d) (prime), and the simulated microscopic picture of π-SnS from [101] route (backside). Cyan, tin; yellow, sulfur. f EDS mapping of a π-SnS nanocrystal. g Atomic constructions of (110) side in π-SnS with prime view (prime) and facet view (backside). h FT-EXAFS spectra for π-SnS and α-SnS at Sn Ok-edge. i Contour plots of Sn Ok-edge WT-EXAFS for π-SnS (prime) and α-SnS (backside).

To disclose the completely different bonding strengths between Sn and S, we additional detect the coordination atmosphere in numerous phases by synchrotron-based X-ray absorption spectroscopy (XAS). The Fourier-transformed prolonged X-ray absorption superb construction (FT-EXAFS) spectrum of π-SnS illustrates a shorter Sn-S common bond size within the first shell than that in α-SnS (Fig. 2h), which is in keeping with the bond size of their mannequin constructions (Supplementary Fig. 4). The shorter Sn-S bond in π-SnS means a stronger Sn-S bond vitality than α-SnS27,28. X-ray absorption near-edge construction (XANES) spectra of Sn Ok-edge decide the same oxidation states of Sn in π-SnS and α-SnS (Supplementary Fig. 5), which excludes the impact of excessive oxidation states Sn (Sn4+) on the binding vitality. Wavelet-transform (WT)-EXAFS analyses (Fig. 2i) reveal a definite bigger k-value of π-SnS (11.0 Å−1) than that of α-SnS (7.3 Å−1). The elevated k-values recommend the next structural order of π-SnS than α-SnS29,30.

Intensive research affirm that the reconstitution of assorted steel compounds at discount potentials in CO2RR is a standard however difficult phenomenon. Derived catalysts usually show distinct constructions from their pre-catalysts, nonetheless, this reconstitution course of is commonly ignored. The identification of derived catalysts by way of reconstitution can contribute to a greater understanding of the origin of catalytic efficiency31,32,33. Right here, we examine the structural variations of the catalysts derived from π-SnS and α-SnS by CO2RR activation. We carried out the CO2RR discount for 900 s at −100 mA cm−2 to derive S-doped Sn from completely different phases of SnS (Supplementary Fig. 6). The main variations between the 2 derived catalysts are the quantity of residual S. We noticed larger content material of S-doping in Sn (Sn(S)-H) and decrease content material of S-doping in Sn (Sn(S)-L) derived from π-SnS and α-SnS pre-catalysts, respectively. First, metallic Sn peaks in XRD evaluation point out that each π-SnS and α-SnS are lowered to Sn metals with the principle uncovered (100) side (Fig. 3a). The weak peak at 26.6 diploma ought to be attributed to the Sn oxidation throughout XRD check (SnO2, JCPDS# 70-4177). HAADF-STEM (Fig. 3b) and corresponding elemental mapping photos (Fig. 3c, Supplementary Fig. 7) affirm the reconstitution of pre-catalysts with a hint quantity of S residual. Moreover, in situ Raman characterization was carried out to hint the structural evolution strategy of π-SnS and α-SnS pre-catalysts in −1.5 V vs. reversible hydrogen electrode (vs. RHE) (Fig. 3d, Supplementary Fig. 8). The clear Sn-S stretching sign (168 cm−1, 188 cm−1, 220 cm−1) remained effectively in Sn(S)-H after 7 h discount, suggesting the nice residual content material of S aspect after CO2RR activation22,34,35. Nevertheless, the Sn(S)-L derived from α-SnS exhibits undetectable Sn-S stretching solely after 3 h discount, indicating an entire discount of the fabric. We additional adopted synchrotron-based delicate X-ray near-edge X-ray absorption superb construction (NEXAFS) to check the S content material of catalysts derived from π-SnS and α-SnS. The same broad peaks round 2480.6 eV are attributed to the adsorbed SO42− and SO32− from the electrolyte, whereas the retention of S (Sn-S, round 2472.6 eV) in these two samples (Fig. 3e, Supplementary Fig. 8c) are distinctly completely different. It is because Sn(S)-H has the next S quantity after CO2RR activation than Sn(S)-L. X-ray photoelectron spectroscopy (XPS) outcomes for the samples derived from π-SnS CO2RR activation underneath excessive present density ( −400 mA cm−2) show larger S quantity, which accords effectively with NEXAFS information (Fig. 3f). The same proof of upper residual S quantity in Sn(S)-H may be present in different sulfur-free acidic electrolytes (Supplementary Fig. 9). Because of the complexity of the reconstitution to the derived catalysts, we additionally checked different potential elements similar to defects which may have an effect on the catalytic efficiency32,33. As no apparent defects had been noticed in each Sn(S)-H and Sn(S)-L (Supplementary Fig. 10), we affirm the principle distinction after reconstitution between Sn(S)-H and Sn(S)-L is the quantity of residual S. Such variations of residual S quantity in derived catalysts could be attributed to the part engineering technique on the pre-catalysts. The stronger binding energy of Sn-S in π-SnS results in the next S content material in Sn subsurface even underneath excessive reductive present density.

Fig. 3: Structural characterization of derived catalysts.
figure 3

a XRD patterns of derived catalysts on carbon paper after CO2RR activation. b,c Magnified HAADF-STEM picture of catalyst derived from π-SnS with the inset of FFT sample and corresponding EDS elemental mapping. d In situ Raman spectra of Sn(S)-H with growing discount time at −1.5 V (vs. RHE) in CO2-saturated 0.5 M Ok2SO4 resolution (pH = 3). e S Ok edge NEXAFS spectra of Sn(S)-H and Sn(S)-L. f Excessive-resolution S 2p XPS spectra of Sn(S)-H and Sn(S)-L after CO2RR activation at −400 mA cm−2. RHE: reversible hydrogen electrode.

Electrochemical CO2RR measurements for derived Sn(S)-H and Sn(S)-L had been performed in a regular three-electrode stream cell with a fuel diffusion electrode (GDE) because the working electrode (see Strategies). The pH worth of the electrolyte was adjusted to three.0 utilizing H2SO4 in 0.5 M Ok2SO4 (Supplementary Fig. 11). Linear sweep voltammetry (LSV) curves present each decrease onset potentials of Sn(S)-H ( −0.86 V vs. RHE) and Sn(S)-L ( −1.12 V vs. RHE) in CO2-saturated electrolyte than in Ar-saturated electrolyte, indicating a good CO2RR pathway for the 2 derived catalysts at low overpotentials (Supplementary Fig. 12)36. With growing utilized potential, HER on Sn(S)-L step by step dominates, whereas Sn(S)-H displays a greater HER suppression in Ar-saturated electrolyte with a bigger Tafel slope. As well as, Sn(S)-H has a considerably bigger present density in CO2-saturated electrolyte, suggesting a greater CO2RR efficiency (Supplementary Fig. 12). It’s well-known that the general catalytic exercise usually consists of two components: the variety of lively websites and the intrinsic exercise of catalysts37,38. The Sn(S)-H catalyst has a decrease worth of electrochemical floor space (ECSA), confirming the next intrinsic CO2RR exercise of Sn(S)-H in acidic medium than that of Sn(S)-L (Supplementary Fig. 13).

We additional quantified the FE of HCOOH within the derived catalysts in a chronopotentiometry mode with the utilized present densities from −0.1 to −1 A cm−2 (Supplementary Fig. 14). Gasoline and liquid merchandise had been decided utilizing fuel chromatography (GC) and 1H nuclear magnetic resonance (1H NMR), respectively (see Strategies). Sn(S)-H achieved the utmost FE of 92.15 ± 1.26% for HCOOH at −200 mA cm−2, along with over 85% FE for HCOOH in a variety of present densities (Fig. 4a), corresponding effectively with 1H NMR and GC ends in every present density (Supplementary Figs. 15–17). As compared, decrease FE (65–70%) of Sn(S)-L was obtained from −100 to −300 mA cm−2, accompanied by clearly elevated HER at excessive present densities, proven in Fig. 4b. Determine 4c highlights the variations in FE amongst Sn(S)-H, Sn(S)-L and pure Sn, reflecting a greater HER suppression efficiency of Sn(S)-H. Because of this, the Sn(S)-H achieves −730.2 mA cm−2 partial HCOOH present density and yields a excessive manufacturing charge of 13.7 mmol h−1 cm−2, which is healthier than most reported catalysts in alkaline or near-neutral medium (Supplementary Fig. 18, Supplementary Desk 1). It’s value noting that pure Sn steel additionally exhibits a dominated HER efficiency at excessive present densities (Fig. 4c, Supplementary Fig. 19), elucidating the numerous function of S dopants in Sn subsurface for environment friendly acidic CO2-to-HCOOH manufacturing. As well as, after we elevated the focus of hydronium ions at pH = 1, Sn(S)-H maintained good efficiency in suppressing HER for HCOOH manufacturing (Supplementary Fig. 20). For acidic CO2 discount, steel dissolution with associated long-term stability are vital points that may not be ignored32,33,39. Sn(S)-H exhibits a 85% FE of HCOOH manufacturing underneath −400 mA cm−2 in 13.5 h stability measurement (Fig. 4d). As well as, Sn(S)-H maintained metallic Sn construction with secure S-Sn bonding after stability measurement (Supplementary Figs. 21, 22). Whereas Sn(S)-L exhibited constantly lowering FE of HCOOH for six h with a distinctly larger Sn dissolving ratio than Sn(S)-H (Supplementary Fig. 23), which may very well be attributed to the dissolution of S-Sn websites.

Fig. 4: Electrochemical CO2RR efficiency in acidic medium.
figure 4

a,b FE worth of CO2RR merchandise for (a) Sn(S)-H and (b) Sn(S)-L underneath completely different present densities. c Partial HCOOH present densities and HCOOH manufacturing charges for Sn(S)-H, Sn(S)-L and pure metallic Sn catalysts. d Stability measurement of HCOOH manufacturing on the complete present density of −400 mA cm−2 for Sn(S)-H. e SPCEHCOOH and FE worth of HCOOH for Sn(S)-H at completely different CO2 stream charges underneath the present density of −400 mA cm−2. f Comparability of this work with different reported catalysts for CO2RR to supply HCOOH/HCOO, together with HCOOH partial present density, FE worth of HCOOH, HCOOH manufacturing charge, SPCE worth of HCOOH and pH of the electrolyte. Error bars correspond to the usual deviation of three impartial measurements.

Just lately, carbon effectivity of CO2RR has obtained fixed consideration as a result of over 50–75% of enter CO2 is consumed to supply CO32− or HCO3 in alkaline medium6,7,40, lowering the financial advantages. Due to this fact, single-pass carbon effectivity (SPCE) of activated Sn(S)-H catalysts was detected at −400 mA cm−2 at completely different CO2 stream charges (Fig. 4e, Supplementary Fig. 17b). Remarkably, 36.43% of SPEC was achieved (with partial SPCE for HCOOH being 34.54%) through the CO2 stream charge at 5 normal cc min−1 (sccm). Moreover, with the CO2 stream charge growing from 2 to 10 sccm at −400 mA cm−2, the numerous development of FE for HCOOH signifies the optimistic CO2RR pathway in opposition to HER of Sn(S)-H. Because the financial worth of HCOOH is $1000 USD per ton, which far exceeds that of CO2 of $70 per ton41,42, it’s discovered that the CO2-to-HCOOH course of on the stream charge of 10 sccm has the very best financial worth by balancing carbon effectivity and FE. As well as, the manufacturing charge of HCOOH on Sn(S)-H additionally exceeds most catalysts operated in alkaline/impartial options (Fig. 4f)16,21,43,44,45.

In situ surface-enhanced Raman spectroscopy (SERS) in stream cell was employed to disclose the response mechanism of CO2-to-HCOOH on Sn(S)-H. No apparent CO32− or HCO3 sign was noticed from 1000 to 1100 cm−1, indicating the suppression of carbonate manufacturing with a neighborhood acid atmosphere on the Sn(S)-H catalyst (Supplementary Fig. 24). The identical phenomenon was additionally noticed at excessive overpotentials from −1.4 to −2 V. The slim peak at 1550 cm−1 corresponds to uneven stretching vibration (νasO-C-O) of *OCHO46,47. Since no apparent *COOH adsorption peaks had been detected, *OCHO was decided as the first intermediate for acidic CO2-to-HCOOH in our work, which agrees with the principle intermediate of HCOO formation in impartial and alkaline media4,48,49.

We additional carried out in situ attenuated complete reflectance Fourier rework infrared spectroscopy (ATR-FTIR) to check the protection of *OCHO on Sn(S)-H and Sn(S)-L (Fig. 5a, Supplementary Fig. 25) with the utilized potentials from −0.5 to −1.9 V vs. RHE. Equally, no evident *CO (to CO product) appeared from 1900 to 2100 cm−1, which agrees with the in situ SERS outcomes. For 2 CO2-to-HCOOH intermediates, *OCHO is broadly thought-about extra environment friendly than *COOH for HCOOH manufacturing4,36. As proven in Fig. 5a, the distinct *OCHO sign round 1367 cm−1 on Sn(S)-H along with no apparent *OCHO hydrogenation intermediates detected in in situ ATR-FTIR experiments, affirm the environment friendly HCOOH technology of Sn(S)-H34,50. Nevertheless, the notable low peak space on Sn(S)-L are primarily attributable to aggressive HER on the catalyst (Supplementary Fig. 25). To additional reveal the intrinsic relationship between *OCHO adsorption protection and CO2RR efficiency, we in contrast the FTIR peak areas (round 1342–1380 cm−1) of Sn(S)-H and Sn(S)-L with the operate of overpotential (Fig. 5b). Each Sn(S)-H and Sn(S)-L exhibited no evident sign earlier than −0.7 V (vs. RHE) of CO2RR onset. In distinction, the speedy development of *OCHO peak space from −0.7 to −1.1 V (vs. RHE) on Sn(S)-H could be attributed to the onset of CO2-to-HCOOH manufacturing, which is constant effectively with the electrochemical information (Supplementary Fig. 12). After that, the elevated peak areas in Sn(S)-H on giant overpotential area recommend extra important *OCHO protection than that in Sn(S)-L, which explains the origin of the improved CO2-to-HCOOH efficiency on Sn(S)-H by chronopotentiometry assessments. Notice that the marginally decreased *OCHO peak space from −1.7 V to −1.9 V (vs. RHE) suggests the aggressive HER within the response, which is in accordance with the minor FE loss from −0.4 to −1 A cm−2.

Fig. 5: Mechanism investigations.
figure 5

a In situ ATR-FTIR spectra of Sn(S)-H. b FTIR peak areas of Sn(S)-H and Sn(S)-H. c Floor Pourbaix diagram of π-SnS (110) and α-SnS (110). The atomic construction (prime view) of the S-terminated, stoichiometric and Sn-terminated floor is proven as insets. Cyan sphere: Sn; yellow sphere: S. The working situation of U = −1.1 V (vs. SHE) and pH = 3 is roughly marked in black dashed field. d Adsorption free vitality (Gadvert) of *OCHO, *COOH and *H on Sn (100) and S-doped Sn (100). e Free vitality diagram of CO2 electroreduction to HCOOH underneath U = −1.1 V (vs. SHE) on Sn (100) and S-doped Sn (100). f Differential cost density of Sn (100) (facet view) with and with out S-doping. The yellow and blue contour represents electron accumulation and depletion, respectively. The isosurface stage is about to be 0.02 e/Bohr3. The ball-and-stick mannequin within the decrease layer exhibits the positions of atoms.

To grasp the formation mechanism of Sn(S)-H and Sn(S)-L catalysts, floor Pourbaix diagrams for the π-SnS and α-SnS had been analyzed by density purposeful concept (DFT) calculations. The (110) side is chosen for the calculation, comparable to the dodecahedral morphology of π-SnS pre-catalyst noticed within the SEM experiment (Fig. 2b). Essentially the most secure floor with the bottom floor free vitality underneath completely different potential (U) and pH circumstances is proven in Fig. 5c. In contrast with α-SnS (110), the stoichiometry of π-SnS(110) floor is secure over a wider vary of electrochemical circumstances (inexperienced space). Particularly, underneath the experimental circumstances of round U = −1.1 V vs. normal hydrogen electrode (SHE), pH = 3 (black dotted field), α-SnS(110) has fully eliminated the floor S to exhibit a Sn-rich floor, whereas π-SnS(110) begins the floor discount of S. This means that the S-atom bonding on the π-SnS floor layer is stronger than that of α-SnS. Due to this fact, it may be speculated that after the discount of π-SnS, S atoms usually tend to stay on the floor, comparable to the upper S atoms content material measured experimentally (Fig. 3e, f).

To additional reveal the CO2RR mechanism in derived Sn(S)-H and Sn(S)-L catalysts, the adsorption free vitality (Gadvert) of the competing intermediates on the derived S-doped Sn catalysts floor was then calculated51,52. Herein, numerous potential adsorption configurations had been thought-about, and probably the most secure adsorption construction for *OCHO was taken because the lively web site (Supplementary Fig. 26). For HCOOH manufacturing, the formation of *OCHO intermediates was thought-about as a figuring out step by way of proton-coupled electron switch (PCET) course of4,19. In contrast with the pure Sn floor, the binding of the carbon-containing species *OCHO is considerably stronger on S-doped Sn, whereas *H adsorption is far weaker (Fig. 5d), indicating the suppression of HER after S doping. This information can be in accordance with the excessive *OCHO protection on Sn(S)-H noticed within the in situ ATR-FTIR (Fig. 5a, b). As well as, Gadvert(*OCHO) is far decrease than Gadvert(*COOH), i.e., the intermediate to generate HCOOH is extra energetically favorable than that to generate CO, which is in keeping with the noticed sturdy peak for *OCHO on Sn(S)-H catalyst within the in situ SERS and in situ ATR-FTIR spectra (Fig. 5a, b).

The free vitality diagram exhibits the mechanism for CO2RR producing HCOOH (Fig. 5e). Beneath U = −1.1 V vs. SHE, the response is mainly a readily exothermic course of. The stronger adsorption of *OCHO on S-doped Sn effectively explains the excessive selectivity to HCOOH on the catalyst at excessive present density (Fig. 4a). The pathway by way of *OCHO is energetically extra favorable than *COOH (Supplementary Fig. 27), which confirms that the response selectively produces HCOOH quite than CO. To raised perceive the influences of the doping S atoms on numerous adsorption species, a differential cost density evaluation was carried out between Sn (100) floor with and with out S-doping (Fig. 5f). When a subsurface Sn atom is changed by S, the electron density on the floor is considerably lowered (blue space), indicating that the Sn bonded to S within the floor layer is positively charged. Due to this fact, such floor lively websites are extra inclined to work together with the electron-rich oxygen-containing teams, particularly the *OCHO with two O atoms binding with the floor, quite than the proton *H. The theoretical calculations above suggest that S-doped Sn promotes CO2RR to supply HCOOH however suppresses HER. Thus, the modestly elevated S quantity in Sn(S)-H demonstrates a greater CO2RR efficiency than Sn(S)-L.

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