Gold-Like Activity, Copper-Like Selectivity of Heteroatomic Transition Metal Carbides (M2C) for Electrocatalytic Carbon Dioxide Reduction Reaction

doi:10.21203/rs.3.rs-244769/v1

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Gold-Like Activity, Copper-Like Selectivity of Heteroatomic Transition Metal Carbides (M2C) for Electrocatalytic Carbon Dioxide Reduction Reaction

https://doi.org/10.21203/rs.3.rs-244769/v1

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published 20 Aug, 2021

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An overarching challenge of the electrochemical carbon dioxide reduction reaction (eCO₂RR) is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials. Here, we have studied the two-dimensional transition metal carbide (TMC) class of materials and found that di-tungsten carbide (W₂C) nanoflakes exhibit maximum methane (CH₄) current density of -421.63 mA/cm² and a CH₄ faradic efficiency of 82.7%±2% in a hybrid electrolyte of 3 M potassium hydroxide (KOH) and 2 M choline-chloride (CC). Powered by a triple junction photovoltaic cell, we have demonstrated a flow electrolyzer that uses humidified CO₂ to produce CH₄ in a 700-hours process under one sun illumination with a CO₂RR energy efficiency of about 62.3% and a solar-to-fuel efficiency of 20.7%. Density functional theory (DFT) calculations reveal that dissociation of water, chemisorption of CO₂ and cleavage of the C-O bond – the most energy consuming elementary steps in other catalysts such as copper – become nearly spontaneous at the W₂C surface. This results in instantaneous formation of adsorbed CO – an important reaction intermediate – and an unlimited source of protons near the tungsten surface sites that are the main reasons for the observed superior activity, selectivity, and small potential.

Chemical Engineering

Materials Engineering

Materials Chemistry

electrochemical carbon dioxide reduction reaction

two-dimensional transition metal carbide

Heteroatomic Transition Metal Carbides

The electrocatalytic carbon dioxide reduction reaction (eCO₂RR) driven by renewable energy has great potential for the sustainable production of chemicals and fuels at the gigaton scale that can be used any time, any place^1–4. It also offers a promising way to store energy in chemical bonds due to having nearly two orders of magnitude higher energy density compared to the most advanced battery technologies⁵. However, reducing CO₂to value-added chemicals is both costly and slow based on intrinsic thermodynamics and kinetics, making the goal of an effective and feasible process a real challenge^6–9.

Conventional pure metal catalysts such as gold (Au), palladium (Pd), silver (Ag) and newly developed transition metal dichalcogenides (TMDCs)^8,10–18 are known to exhibit high activities for the CO₂RR in different electrolyte solutions^19–26. However, these catalysts are mainly selective for carbon monoxide (CO), known as an intermediate product^8,27. Other catalysts such as copper (Cu) and Cu-based catalysts have the ability to reduce CO₂ to various chemicals such as methane (CH₄), ethylene (C₂H₄), formic acid (HCOOH), methanol (CH₃OH), and ethanol (C₂H₅OH)^28–36. Despite their good selectivity, these catalysts require high potentials –excess energy– to achieve suitable current densities –reaction rates– impeding their use for effective production of chemicals and fuels^37,38. Therefore, a novel catalyst with outstanding electronic properties needs to be developed to selectively produce hydrocarbons at high rates at relatively low potentials.

Heteroatomic transition metal carbide (TMC) catalysts, also known as MXenes, have recently received great attention for various electrocatalytic reactions due to their unique structural and electronic properties^39–42. In particular, M₂C (M denotes transition metals) stoichiometry of this class of two-dimensional materials forms layered structures of M-C-M where a plane of carbon atoms is sandwiched between two hexagonal planes of metal atoms. This structure provides a high density of active metal atoms at the surface breaking conventional scaling relationships that limit electrocatalytic performance of their counterparts such as TMDCs and pure metals⁴³. However, there is limited knowledge of their performance and characteristics as eCO₂RR catalysts under actual experimental conditions.

Here, we have studied the performance of di-tungsten carbide (W₂C), di-molybdenum carbide (Mo₂C) di-niobium carbide (Nb₂C) and di-vanadium carbide (V₂C) nanoflakes (NFs) as inexpensive, non-precious members of TMCs for eCO₂RR.

The TMC NFs i.e., W₂C, Mo₂C, Nb₂C and V₂C were synthesized using a carburization process followed by the liquid exfoliation technique (Supplementary section S1)^27,44,45. The electrocatalytic performance of TMC NFs with the similar crystallite sizes (25.4±5 nm) were then studied in a three-electrode cell and compared with Au and Cu nanoparticles (NPs), conventional catalysts for this reaction,⁴⁶ under identical experimental conditions (Supplementary section S2). To improve the CO₂RR performance in competing with hydrogen evolution reaction (HER), we have employed a mixture of 3 M potassium hydroxide (KOH) and 2 M choline-chloride (CC) solution (KOH:CC (3M:2M)) as the electrolyte in this study⁴⁷.

The linear sweep voltammetry (LSV) experiments and a real time product stream analysis show that CO₂RR on the W₂C surface starts at a potential of -122.7 mV vs reversible hydrogen electrode (RHE) by producing CO and H₂ and reach maximum CO₂RR current density (j_CO2RR) of -548.9 mA/cm² at -1.05 V vs RHE (Supplementary Figs. S2-4 and Fig. 1a). As shown in Fig. 1a, j_CO2RR of -419.9, -381.9 and -350.8 mA/cm² were observed for Mo₂C, Nb₂C, and V₂C NFs, respectively, at this potential (Supplementary section S3). However, Au and Cu NPs exhibit a j_CO2RR of -208.11 and -89.53 mA/cm²at -1.05 V vs RHE (Fig. 1a). The selectivity analysis also indicates that TMC NFs produce hydrocarbons (i.e., CH₄, C₂H₄, CH₃OH and C₂H₅OH) at a potential range of -0.45 to -1.05 V vs RHE for W₂C, Mo₂C and Nb₂C NFs and a potential range of -0.55 to -1.05 V vs RHE for V₂C NFs where CH₄ is identified as the main product (Supplementary section S3).

Figure 1b illustrates CH₄ formation current densities (j_CH4, mA/cm²) of the TMC NFs compared to Cu NPs, a conventional catalyst for hydrocarbon production. The partial current densities of different products (i.e., H₂, CO, CH₄, C₂H₄, CH₃OH and C₂H₅OH) were calculated by multiplying FEs and total current densities at different potentials (Supplementary section S3 and Fig. S3). As shown in Fig. 1b, a maximum j_CH4of -421.63 mA/cm² is obtained for W₂C NFs at a potential of -1.05 V vs RHE where Nb₂C NFs, Mo₂C NFs, and V₂C NFs show values of -219.16, -211.33, and -147.56 mA/cm², respectively, at this potential. We also compared the CH₄ formation activity of TMCs i.e., W₂C, Nb₂C, Mo₂C, and V₂C NFs with state-of-the-art catalysts in the literature (Supplementary Table S2)^46,48–54. Supplementary Table S2b indicates that the j_CH4of W₂C NFs is 3.6 and 4.2 times higher than recently studied La₂CuO₄ (-117 mA/cm² at -1.4 V vs RHE)⁵¹ and Cu-N (-100 mA/cm² at -1.0 V vs RHE)⁴⁸, respectively. The partial current densities of other hydrocarbon products i.e., C₂H₄, CH₃OH and C₂H₅OH are also shown in Fig. S3 (Supplementary section S3).

To evaluate the intrinsic activity of W₂C NFs, we measured CH₄ formation turnover frequency (TOF_CH4) by normalizing its activity to the number of active atoms at the surface using roughness factor method and compared it with the other catalysts in this study (Supplementary section S5). Our calculations indicate a TOF_CH4 of 10.42 s^-1 at a potential of -1.05 V vs RHE for W₂C NFs; by comparison, TOF_CH4 of 4.54, 3.74 and 2.79 s^-1 were calculated for Mo₂C NFs, Nb₂C NFs, and V₂C NFs, respectively. The calculated TOF_CH4 of W₂C NFs at the potential of -1.05 V vs RHE is about 2 orders of magnitude higher than that of Cu NPs (0.0736 s^-1) under identical experimental conditions (Supplementary Fig. S7a). Moreover, total CO₂RR turnover frequencies (TOF_CO2RR) of 19.09, 19.36, 17.82, 17.55 s^-1 were calculated for W₂C NFs, Mo₂C NFs, Nb₂C NFs, and V₂C NFs, respectively, where Au NPs and Cu NPs exhibit TOF_CO2RR of 4.35 and 0.1956 s^-1, respectively (Supplementary Fig. S7f). These results suggest the superior CH₄ selectivity of TMC catalysts compared to state-of-the-art catalysts^{48–51,54–57}.

Furthermore, we performed a comparative mechanistic study by calculating Tafel slopes for different products to gain insight about the eCO₂RR mechanism of the TMCs i.e., W₂C, Mo₂C, Nb₂C, and V₂C NFs in the two-compartment three-electrode electrochemical cell (Supplementary section S6, Fig. S8) ⁵⁸. Our Tafel plot analyses show that the TMC NFs possess steeper Tafel slopes, and therefore a weaker potential dependence compared with Cu NPs for the formed products (i.e., CO, CH₄ and C₂H₄) (Supplementary Fig. S8)⁵⁸. The Tafel plot analyses suggest a different CO₂RR mechanism for TMC NFs than that of Cu catalysts where C-O bond scission is the rate determining step⁵⁸.

To gain more insight to the remarkable performance of these catalysts for electrocatalytic CO₂RR, the structural and physicochemical properties of TMC NFs were characterized at molecular and atomic scales by performing X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) (Supplementary sections S7-9). At first, we have performed XPS experiments to analyze the surface chemistry of TMC NFs. XPS analysis (Supplementary Fig. S9) indicates that our NF samples contain metallic TMCs, with little or no evident surface oxidation. The results show that the chemical composition of the surface, the empirical formula of M₂C (M: transition metal, C: Carbide) and the oxidation state of +2 for the transition metals i.e., W, Mo, Nb, and V are similar in all synthesized TMCs (Supplementary section S7). The lattice structure and crystallite size of the TMC NFs were then studied by performing XRD experiments. The XRD pattern of W₂C NFs shows a sharp peak at 39.91º along with three pronounced peaks at 34.84º, 38.54º, and 52.65º corresponding to (101), (100), (002) and (102) crystal surfaces of W₂C, respectively. The XRD spectra of the TMCs show all Bragg peaks of W₂C, Mo₂C, Nb₂C, and V₂C NFs; verifying their homogenous and pure structures. The XRD results indicate a constant dominant lattice plane of (101) and a similar average crystallite size of 25.4±5 nm for all synthesized TMCs (Supplementary Fig. S10) ^59–61.

Furthermore, we performed atomic scale STEM experiments to study surface atom coordination, crystallite sizes and dominant plane structures of TMC NFs (Supplementary Fig. S11-18). Figures 2a-d show STEM results of W₂C NFs. Figures 2a and b indicate high-angle annular dark-field (HAADF) image and corresponding fast Fourier transforms (FFT) of W₂C NFs in the <101> zone axis. The atomic models of the <101> zone axis and bright field (BF) image of W₂C NFs are represented in Figs. 2c and d, respectively. Figure 2d indicates the carbon atomic columns in the red box and the intensity profile across the red box region showing that the distance between two carbon atoms is 2.55 Å. The STEM results of other TMCs i.e., Mo₂C, Nb₂C, and V₂C NFs are explained in Supplementary section S9. The STEM and XRD results of synthesized TMC NFs confirm that the structure of these materials is a perfect match with standard 1T structure, suggesting a tetragonal symmetry and octahedral coordination of the atoms (Fig. 2e) ^43,62. Figure 2e indicates the schematic of 1T structure TMC NFs showing tetragonal symmetry, one layer per repeat unit with octahedral coordination. The lattice constant a is in the range of 3.07 to 3.15 Å for synthesized TMC NFs. The stacking index b indicates the interlayer spacing which is in the range of 4.53 to 5 Å for studied TMCs. As shown in Fig. 2e, 1T atomic coordination provides metal-terminated surface atoms that are known to be favorable binding sites of adsorbed intermediates in eCO₂RR ^43,62. Our atomic and molecular scale structural analyses indicate that the synthesized TMC NFs have fairly similar structural properties e.g., (1T) crystalline structure with a dominant plane of (101), crystallite sizes, and atomic coordination.

To further discern the difference between the observed electrocatalytic performance of the TMCs, we have studied their electronic properties by performing electrochemical impedance spectroscopy (EIS) (Supplementary section S10) and, work function measurements using ultraviolet photoelectron spectroscopy (UPS) (Supplementary section S11)⁸. At first, we have employed the EIS experiments to compare the overall electron-transfer properties of the TMC catalysts in the double layer region (Supplementary section S10). To do this, TMC NFs with similar structural and physical properties e.g., sizes, shapes, and mass loadings (0.1 mg/cm²) coated on glass carbon were used as the working electrodes. This results in similar roughness, morphology, intrinsic capacitance, and exposed surface area of the studied samples confirmed by our characterization results (Supplementary sections S5-9). The EIS experiments have been performed at a potential of -310 mV vs RHE for all TMCs under identical experimental conditions (Supplementary section S10). Figure 2f shows the fitted EIS spectra of each TMC catalyst using Randles circuit model, indicating a smaller charge transfer resistance (R_ct) for W₂C NFs (~17 ohms) compared to the other TMCs, i.e., Mo₂C NFs (~25 ohm), Nb₂C NFs (~33 ohm), and V₂C NFs (~38 ohm)⁶³. The UPS method also was used to compare the surface work function of TMCs (Fig. 2g). The results indicate a lower work function for W₂C NFs (0.2 to 0.84 eV) compared to Mo₂C NFs (3.92 eV), Nb₂C NFs (4.44 eV), and V₂C NFs (4.55 eV). The charge transfer resistance obtained by EIS experiments and the surface work function value measured by UPS experiments suggest the superior activity of W₂C NFs compared to other TMCs in this study i.e., Mo₂C, Nb₂C, and V₂C NFs.

In addition to our experimental observations, we have performed density functional theory (DFT) calculations to gain more insight to the electronic and catalytic properties of M₂C compounds. The aim is to address the enhanced activity and selectivity of these TMCs and to explore both electrochemical (i.e., driven) and chemical (i.e., favorable or spontaneous) processes that distinguish them from other catalysts, such as Au and Cu.

With respect to activity, the electronic density of states (DOS) indicate that transition metal d states dominate at the Fermi level of these TMCs, much more so than elemental Au, another high activity catalyst. Bader charge calculations indicate that metal atoms at TMC surface are significantly more reduced compared to the bulk atoms (Fig. 2h, Supplementary Fig. S24). These results indicate the increased availability of electrons at metal-rich TMC surfaces, which may increase the catalytic activity of TMC NFs.

With respect to the increased selectivity of TMC NFs, especially for CH₄ production, we have explored the CO₂RR pathway on the W₂C (101) surface in detail by using DFT calculations. Focusing initially on electrochemical processes, we employed the computational hydrogen electrode (CHE) model^64–66 (Supplementary Tables S7 and S8) to explore the stepwise electronic reduction and protonation of adsorbed species in the low molecular coverage limit. The lowest free energy pathway to produce CH₄ with only electrochemical steps is shown in Fig. 3 (see a more detailed path in Supplementary Fig. S25), indicating limiting steps (at zero potential with respect to RHE) that involve protonation of adsorbed CO₂ and O, and, most importantly, the desorption of H₂O following protonation of adsorbed OH. The difficulty of this final step is not surprising, as W₂C (101) strongly adsorbs and spontaneously dissociates water ( , ) without electrochemical assistance. Similarly, our calculations indicate that W₂C (101) strongly chemisorbs CO₂ ( , bond length ) in contrast to normally weak physisorption on Cu ( ) and other catalyst surfaces^28,67–69. Furthermore, the (101) surface of W₂C enables favorable and unassisted dissociation of adsorbed CO₂ ( , , Supplementary Table S9) suggesting that C–O bond scission may take place in the early stages of CO₂ reduction, skipping the uphill production of adsorbed carboxyl. Based on these findings, we propose that W₂C (101) distinguishes itself as a catalyst due to an interplay between surface-assisted chemical steps, whose energetics will depend on the local chemical equilibrium at the surface and electrochemical steps that reduce pre-existing surface reagents and open up new pathways for the overall reaction to proceed. More detailed studies of such cooperative catalytic processes and their limiting steps may be encouraged based on the promise of W₂C as a high-performance CO₂ reduction catalyst. Here, we highlight the plausible cooperative effects of these steps, which set apart W₂C from conventional noble metal catalysts and the other TMCs, specially for CH₄ production. The immediate benefit of the favorable chemical processes mentioned above should be a higher surface coverage of CO₂ (and consequently CO) and an excess of surface protons. This may explain the high Faradaic efficiencies for production of both H₂ and CO at low potentials (see Supplementary section S3 and Table S1). However, once a limiting potential (-0.74 V estimate) is reached, the readily protonated products of adsorbed CO that produce CH₄ are no longer hindered by a build-up of adsorbed byproducts (O* then OH*), which can now be protonated and released from the surface.

We can divide the complex, multistep reaction into two key parts: initial conversion of adsorbed CO₂ to adsorbed CO, followed by conversion of adsorbed CO to CH₄ with the release of H₂O (see Fig. 3). As indicated in Fig. 3, the first part, generation of adsorbed CO, can be achieved by chemical or electrochemical means. We have direct and favorable chemical conversion of adsorbed CO₂ to adsorbed CO and O on W₂C (101), but also two electrochemical pathways: production of HO-CO* in a single step (+0.66 eV, Fig. 3) or an alternative initially favorable protonation to OCHO* followed by two uphill electrochemical steps producing first OCH₂O* followed by release of H₂ and the final product of HO-CO* with a similar free energy cost (+0.68 eV, Supplementary Fig. S25 and Table S9). A final electrochemically driven protonation of HO-CO* favorably releases H₂O and leaves CO*.

With chemically or electrochemically generated adsorbed CO, we can proceed to the second part of the overall reaction to produce CH₄ from CO₂, which involves multiple favorable protonation steps. The W₂C catalyst distinguishes itself here. The electrochemical activation of remains thermodynamically favorable (ΔG = -0.26 eV) on W₂C (101), whereas on other catalysts, such as Cu, this process is usually uphill with the potential ranging from -0.74 V to -0.97 V vs RHE^66,70. Moreover, due to the spontaneous water dissociation, the direct H* transfer step on W₂C could be even more favorable with a resultant ΔG= -0.433 eV (Supplementary Table S9). The next two electrochemical steps are downhill (ΔG = -0.04 and -0.58 eV): the first forming the unstable methoxy radical with oxygen attached to a surface W atom; the second leading to spontaneous dissociation into the methyl radical and a surface oxygen atom . The electrochemical conversion of the surface into CH₄ is favorable (ΔG = -0.43 eV) and the protonation of the surface oxygen is only slightly uphill (ΔG = +0.03 eV). As we already stated, for the overall reaction on W₂C (101) it is the final protonation of to release H₂O that is the limiting step (ΔG = +0.74 eV).

We also compared W₂C with the other TMCs studied by calculating the energies of adsorption of water and CO₂ as well as the potentials of the rate determining step (i.e., protonation of ) for Nb₂C, Mo₂C, and V₂C (Supplementary Table S10). Our calculations indicate that these TMCs also strongly chemisorb CO₂ with adsorption energies of -1.32, -1.62, and -0.96 eV, respectively. Moreover, Nb₂C also shows favorable C–O bond scission of adsorbed CO₂. Additionally, Nb₂C, Mo₂C, and V₂C strongly adsorb water with the energies of -1.87, -1.23 and -0.59 eV, respectively, where Nb₂C is the only other catalyst that dissociates water. In contrast to W₂C, the energies required for the protonation of are higher: +1.17, +1.25, +0.85 eV for Nb₂C, Mo₂C, and V₂C, respectively (Supplementary Table S10). Therefore, we can conclude that, within this set of four TMCs, W₂C possesses the optimal characteristics for efficient completion of CO₂RR: (1) sufficiently strong adsorption of CO₂, (2) spontaneous dissociation of water, and (3) the lowest limiting potential for OH* protonation. We conclude that the performance of Nb₂C is reduced due to its stronger water adsorption, resulting in the protonation of requiring more energy. We would expect Mo₂C to have a lower surface coverage of protons and higher costs for the protonation of . The weakest CO₂ adsorption on V₂C decreases its surface coverage, making it the worst TMC catalyst here, despite its relatively small limiting reaction potential of protonation of OH*.

As we mentioned before, for W₂C the realistic network of pathways towards CH₄ consists of a potential dependent combination of competing chemical and electrochemical steps with the actual limiting potential being in the range from -0.483 V to -0.744 V vs RHE (see full path. Supplementary Fig. S25), which is consistent with our three-electrode electrochemical experimental results (Supplementary section S3 and Table S1). A steeper Tafel slope for CH₄ formation on W₂C than other TMCs and Cu, (Supplementary Fig. S8) also indicates the competition between reactions for the active sites on the catalyst surface. Specifically, the spontaneous water dissociation on W₂C (101) explains the ease of the HER in our non-acidic electrolyte where the source of protons is normally water. A weak potential dependence of the partial CO current and its small overpotential also originate from the interplay between chemical and electrochemical steps (see Supplementary Material for details).

Experimentally, we have studied the effect of choline chloride (CC) on the activity and selectivity of the TMC catalysts. To do this, we have performed electrochemical CO₂RR in different choline chloride concentrations of i.e., 0.01, 0.1, 1, and 2 M mixed with 3 M KOH (Supplementary section S13). Figure 4 shows CO₂RR overall current density and different products (i.e., CH₄, C₂H₄, CO, alcohols-CH₃OH and C₂H₅OH- and H₂) partial current densities for W₂C NFs in different CC concentration electrolytes. Figure 4a indicates that by increasing the concentration of CC in the electrolyte the CO₂RR current density (j_CO2RR) increases and reaches a maximum value of -548.89 mA/cm² at a potential of -1.05 V vs RHE for 2 M of CC. The obtained value is about 32, 24, 17, 9% higher than that of 0, 0.01, 0.1, and 1 M of CC, respectively. Moreover, a maximum CH₄ formation current density (j_CH4) of -421.63 mA/cm² is obtained for 2M CC at a potential of -1.05 V vs RHE that is about 1.41, 1.29, 1.19 and 1.1 times higher than that of 0, 0.01, 0.1, and 1 M, respectively (Fig. 4b).

The results also indicate using W₂C NFs, maximum partial current densities of other products i.e., C₂H₄ (j_C2H4 of -35.84 mA/cm²), CO (j_CO of -78.48 mA/cm²), and alcohols (j_Alcohols of -12.81 mA/cm²; -6.84 mA/cm²for CH₃OH and -5.97 mA/cm² for C₂H₅OH) were obtained at the potential of -1.05 V vs RHE in 2M CC (Figs. 4b-d). In contrast, the measured H₂ partial current densities indicate that by adding higher concentration of CC to the electrolyte solution the rate of H₂ production decreases significantly where a minimum H₂ formation current density of -4.48 mA/cm² was obtained for 2 M CC at a potential of -0.85 V vs RHE that is 12.31, 8.97, 6.76, 3.23 times lower than that of 0, 0.01, 0.1 and 1 M CC, respectively.

These results suggest that adding choline chloride to the 3 M KOH electrolyte suppresses the competing hydrogen evolution reaction (HER) and increases the formation of CO₂RR products more specifically CH₄ ²⁷.

The stability of the choline chloride electrolytes was studied by conducting nuclear magnetic resonance (NMR) and ¹³CO₂ isotope experiments (Supplementary section S14 and S15) ^27,46,71. The ¹H and ¹³C NMR spectra reveal similar peak areas and chemical shifts for fresh and used electrolytes indicating no generation of new diamagnetic species or change in the choline chloride structure under applied potential of -1.05 V vs RHE (Supplementary Figs. S36 and S37). The ¹³CO₂ isotope experiments also show that the CO₂ gas present inside the electrolyte is the only source of the formed products in the electrochemical CO₂RR (Supplementary Fig. S39). These results confirm that choline chloride with different concentrations i.e., 0, 01, 0.1, 1 and 2 M remains stable at the range of applied potentials in the electrochemical CO₂RR experiments.

Next, we studied the performance of W₂C NFs in our developed solid polymer electrolyte flow electrolyzer for continuous electrochemical CO₂RR using this catalyst as the cathode (Supplementary section S16). The flow electrolyzer used in this study consists of a two-compartment electrochemical setup with active area of 5 cm² coated with W₂C NFs at the cathode and iridium oxide nanoparticles (IrO₂ NPs) as the anode and were then fed with humidified CO₂ and KOH:CC (3M:2M) electrolyte, respectively (Supplementary section S16).

To study the CO₂RR performance of W₂C NFs in the flow electrolyzer, we performed chronoamperometry (CA) experiments at different cell potentials ranging from -1.5 to -2.3 V for W₂C NFs (Supplementary section S16). As shown in Fig. 5a, the results show that at a cell potential of -1.5 V, hydrogen (H₂, FE of 54.9%±1.4) and CO (FE of 40.1%±1.8) are the dominant products. However, our measurements indicate that by increasing the cell potential a system becomes more selective for CH₄ formation with the maximum FE of 82.7%±2 at a cell potential of -2.1 V. At this potential, W₂C NFs slightly produce other products such as C₂H₄, CH₃OH, C₂H₅OH, CO, and H₂ with FEs of 5.6%, 1.4%, 1.2%, 6.1%, and 1.4%, respectively. Figure 5b shows the maximum CH₄, C₂H₄, CH₃OH, and C₂H₅OH current densities of -421.28, -27.31, -5.95, and -5.19 mA/cm² at the cell potential of -2.3 V, respectively, confirming high selectivity of W₂C NFs towards CH₄ as the main product.

Next, we coupled the electrolyzer to a triple junction photovoltaic (TJ-PV) cell with a maximum efficiency of 34.3% to determine the CO₂RR performance and energy efficiency of W₂C NFs in a solar-driven device (Supplementary section S17). The j-V characteristic curve of the TJ-PV cell under one sun illumination (100 mW/cm²) using a sun simulator light source is shown in Supplementary Fig. S46. The operating point is chosen to provide a photo current density of -394.3 mA/cm²at a potential of -2.1 V which has the maximum FE of CH₄ (82.7%±2) calculated in the flow electrolyzer (Supplementary Fig. S47).

Figure 5c shows the current density of the solar-driven electrolyzer for a 700-hour continuous process at a potential of -2.1 V. The results shown in Fig. 5c indicate a negligible decrease (approximately 2%) in the photo-current density of W₂C NFs over the 700-hour experiment while the corresponding photo-potential fluctuates between -2.08 to -2.12 V, confirming the high stability of W₂C NFs for CO₂RR.

The measured sun to CO₂RR products (CO, CH₄, C₂H₄, CH₃OH, and C₂H₅OH) as well as total solar-to-fuel efficiency (SFE) of W₂C NFs over a 700-hour process are shown in Fig. 5d (Supplementary section S17). As shown in this figure, an average sun to CH₄ production efficiency of 17.3% with negligible variation (2%) is achieved during the 700-hour continuous process. Considering other products, W₂C NFs show a SFE of 20.7%.

We also calculated the energy efficiency of CO₂RR in our developed flow electrolyzer and compared it with state-of-the-art catalytic systems in the literature (Supplementary section S16)^{29,30,37,57,72}. As shown in this figure (Supplementary Fig. S45), a maximum energy efficiency of 62.3% was obtained for our developed flow electrolyzer using W₂C catalyst that is about 67% and 73% more efficient than Cu_oh (37.4%)⁵⁰ and recently developed Cu-CIPH (36.1%)⁷² catalytic systems, respectively.

In summary, we have synthesized four members of TMCs with a formula of M₂C, i.e., W₂C, Mo₂C, Nb₂C, and V₂C NFs using the carburization method followed by the liquid exfoliation technique and tested their catalytic performance for eCO₂RR in KOH:CC (3M:2M) electrolyte. The electrocatalytic performance studies of TMCs shows these materials are mainly selective for CH₄ formation with W₂C NFs having the best CO₂RR activity compared to the studied catalysts. For instance, a CO₂RR current density of -548.89 mA/cm² and a maximum CH₄ current density of -421.63 mA/cm² at the potential of -1.05 V vs RHE were observed for W₂C NFs. Our electrochemical results also indicate that adding choline chloride to the electrolyte enhances the formation of CO₂RR products by suppressing the HER for all studied TMCs. Moreover, the NMR and ¹³CO₂ isotope experiments confirm that the choline chloride remains stable during the electrochemical experiments. Atomic and molecular scale characterizations such as XPS, XRD, and STEM indicate that all synthesized TMCs have similar lattice structure of 1T with a dominant plane of (101) and almost the same average crystallite size of 25.4 nm. Furthermore, the electronic property analyses of TMCs reveal superior electronic properties of W₂C NFs: low work function; small charge transfer resistance in the electrochemical double layer region; and heavily reduced tungsten atoms at the surface, which may lead to the observed outstanding activity. Computational results also indicate that the studied TMCs spontaneously chemisorb CO₂ and water as compared to Cu. However, among the TMCs studied, W₂C exhibits the optimal combination for CH₄ production, with favorable adsorption energies of water and CO₂ coupled with spontaneous dissociation, and less costly protonation of OH*, which is the limiting step, with a low limiting potential in the range of -0.483 V to -0.744 V vs RHE. Using W₂C NFs, we have demonstrated a solar-driven flow electrolyzer that can work up to 700 hours with a solar to CH₄ efficiency and a total SFE of 17.3 and 20.7%, respectively, under one sun illumination. The demonstrated solar-driven flow electrolyzer using a non-precious metal catalyst (W₂C NFs) in this study achieves a maximum efficiency of 62.3% making it a good candidate to approach the commercially relevant electrocatalytic CO₂RR. This opens a new direction toward a low cost, sustainable large-scale production of fuels from CO₂ that can be used any time any place.

Synthesis of TMCs: TMCs were prepared by carburization process in a dual zone tubular furnace with a controlled flow of CH₄ and H₂ mixture (volumetric ratio CH₄:H₂ of 1:9) at a temperature of 973 K. The obtained bulk powders were then collected and ground to fine powders in a mortar and pestle. Next, a certain amount of TMC powders were processed in isopropyl alcohol using an ultrasonic liquid processor (Sonics VibraCell VCX-130) to obtain a solution of TMC nanoflakes (NFs). The resulting solution was further centrifuged at 1000 rpm for 30 min and the top two-third of the solutions were collected and stored as the TMCs in a vial for cathode electrode preparation. The detailed explanation is provided in Supplementary section S1.

Electrochemical setup: A two-compartment three-electrode electrochemical cell was used to perform the fundamental study for cathodic half-cell reaction using the synthesized W₂C, Mo₂C, Nb₂C, and V₂C NFs and compared them with Au and Cu NPs. In the three-electrode cell study, the working electrode was prepared by drop-casting the catalysts (mass loading of 0.1 mg) on a glassy carbon electrode with a geometric surface area of 1 cm². The catalyst loading on the electrode was precisely controlled to be 0.1 mg/cm² on the glassy carbon electrode. Platinum (Pt) gauze 52 mesh (Alfa Aesar) and Ag/AgCl (BASi) were used as counter and reference electrodes, respectively. The cathode and anode parts of the cell were separated through an anion exchange membrane (Sustainion X37-50 Grade RT, Dioxide Materials). All experiments were performed in CO₂ saturated KOH:CC (3M:2M) electrolyte with a pH of 14.5 ± 0.1. A two-compartment zero-gap solid polymer electrolyte flow electrolyzer was used to study the electrochemical performance where the working and counter electrodes are separated using an anion exchange membrane. Working electrodes (cathode) were prepared by brush-coating the solution of studied catalysts (W₂C NFs, Au NPs, and Cu NPs) on gas diffusion layer (GDL, Sigracet 39 BC, Fuel Cell Store) electrodes with a geometrical surface area of 5 cm². The counter electrode (anode) was prepared using a similar procedure where IrO₂ powder (Sigma Aldrich) was used as the catalyst solution. The actual loadings of 0.1 ± 0.01 mg/cm² were determined by weighing the dry GDLs before catalyst deposition and coated GDLs after being dried in a vacuum-oven overnight. As a separator in our experiments, we used an anion exchange membrane (Sustainion X37-50 Grade RT, Dioxide Materials) which was treated in 1 M KOH overnight at 75°C and then washed with deionized water prior to use. Anolyte flow of KOH:CC (3M:2M) with a flow rate of 20 ml/min was fed to the anode compartment using a peristaltic pump (Masterflex, Cole-Parmer). A mass flow controller (SmartTrak 50, Sierra, calibrated with CO₂ gas) connected to the CO₂ humidifier kit, was used to feed the cathode compartment with a flow rate of 50 ml/min.

PV cell characterization

A solar-powered flow cell was assembled by connecting the solid polymer electrolyte flow electrolyzer to a triple-junction photovoltaic (TJ-PV) solar cell. The TJ-PV cell was characterized at different sun illuminations using a custom-made sun simulator light source and an InGaAs photodiode (Thorlabs, FDG03-CAL) with a known responsivity calibration curve. Our results indicated a maximum efficiency of 34.32 % under one sun illumination used in our study.

Electrochemical characterization

Electrochemical experiments were performed using a Biologic Potentiostat SP-150. The chronoamperometry (CA) technique was used to study the performance of TMC NFs i.e., W₂C, Mo₂C, Nb₂C, and V₂C NFs and compared them with that of Au and Cu NPs. The CA experiments were carried out in the range of -0.45 to -1.05 V vs RHE potentials. All experiments were performed under identical experimental conditions. The linear sweep voltammetry (LSV) technique was used to study the fundamentals of the cathodic half-cell reaction in the three-electrode cell setup. LSV curves were obtained by sweeping the potential between + 0.2 and − 1.05 V vs. reversible hydrogen electrode (RHE) with scan rate of 20 mV/s. The conversion of Ag/AgCl reference electrode potential to RHE scale was calculated using Nernst equation considering the pH of solution (pH = 14.5).

Product characterization

A gas chromatography system (GC, SRI, 8610C) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) was used to detect and quantify the electrochemical CO₂RR products. Ultra-high purity helium (He) and nitrogen (N₂) gases (UHP 99.99%, Airgas) were used as the carrier gas to identify any possible type of product. the signal response of the FID and TCD to each gaseous product (e.g., H₂, CO, CH₄, C₂H₄) was calibrated by analyzing a series of standard gas mixtures with known compositions prior to the experiments. To study the products, 1 mL sample of the headspace of the cell was injected to the GC system using a lock-in syringe (Hamilton). Moreover, an in-situ differential electrochemical mass spectrometer (DEMS, Hiden Analytical, HPR-40) was used to validate the obtained information from the GC system by continuously detecting all possible products, even at trace amounts (partial pressure of 1 x 10^− 13 Torr), during the electrochemical CO₂RR (CA experiment), resulting in a more precise measurement. The signal responses of the DEMS instrument for different products (H₂, CO, CH₄, C₂H₄, CH₃OH, and C₂H₅OH) were calibrated by feeding standard samples into the mass spectrometer. An electron energy of 70 eV was used for ionization of all species, with an emission current of 500 µA. All mass-selected product cations were detected by a secondary electron multiplier with a detector voltage of 1200 V for maximizing the signal to noise ratio of the products.

X-ray diffraction (XRD)

The XRD technique was used to identify the phase purity and crystallinity of all studied catalysts (W₂C, Mo₂C, Nb₂C, V₂C NFs, Au NPs, and Cu NPs) using a Bruker D2 PHASER diffractometer in Bragg–Brentano geometry employing a Ni filtered Cu Kα radiation (1.5405 Å). The X-ray diffraction (XRD) patterns were obtained using a LynxEye linear position sensitive detector and a step width of 0.2 °2θ with a counting time of 1 s/step.

X-ray photoelectron spectroscopy (XPS)

A Thermo-Scientific ESCALAB 250Xi instrument equipped with an electron flood and scanning ion gun was used to identify the oxidation states of the W₂C NFs. All obtained spectra were analyzed using Thermo-Avantage software, considering the standard carbon peak at 284.8 eV and relative sensitivity factors.

Ultra-violet photoelectron spectroscopy (UPS)

Surface work function measurements were carried out using the UPS technique. All UPS data were acquired by a Thermo-Scientific ESCALAB 250Xi instrument using He I (21.2 eV) ultraviolet radiation and a pass energy of 8.95 eV.

Scanning transmission electron microscopy (STEM)

W₂C NFs were characterized at atomic scale using a spherical aberration corrected JEOL JEM-ARM 200CF STEM with a cold field emission gun operating at 200 kV. High-angle annular dark-field (HAADF) detector with 22 mrd inner-detector angle and bright field (BF) detector were utilized to obtain the atomic resolution images.

Theoretical study

We performed a comparative DFT analysis for the observed catalytic activity and reactivity of W₂C NFs with Au and other TMCs using SIESTA package, with Perdew–Burke–Ernzerhof functional with a double-zeta with polarization (DZP) localized basis set and the norm-conserving Troullier-Martins pseudopotentials. Calculations of density of states for bulk and slab geometries of Au and TMCs were performed using the Effective Screening Method (ESM)⁷³ for Brillouin zones of the unit cells sampled by Monkhorst-Pack k-point grids of size 9 x 9 x 9 and 1 x 9 x 9, respectively, together with a plane-wave cutoff of 300.0 Ry. The optimization of the atomic positions and cell parameters were carried out using a conjugate-gradient algorithm until a maximum atomic force tolerance of 0.04 eV/Å and a maximum stress component along each periodic direction of lower than 1 GPa were achieved. The Vienna ab initio Simulation Package (VASP, version 5.4.4) with PAW (projector augmented wave method) and Perdew–Burke–Ernzerhof exchange-correlation functionals were used to analyze the adsorption free energies of various molecular species on the (101) surface of M₂C (M = W, V, Mo, Nb). All the VASP calculations were performed for neutral non spin-polarized systems and dipolar electrostatic correction was used along the normal to the surface of the slab. Next, we used the tetrahedron method with Blöchl corrections and 1 x 3 x 3 Monkhorst-Pack grid k-point sampling for the calculations of total electronic energy (smearing σ = 0.1). The adsorption free energies were then used within the computational hydrogen electrode (CHE) model^64–66 to evaluate the lowest free energy pathways and the limiting reaction potentials.

Acknowledgments

Mohammad Asadi’s work was supported by Illinois Institute of Technology start-up funding, Wanger Institute for Sustainable Energy Research (WISER) Institute for Sustainable Energy Research (WISER) seed fund (262029 221E 2300), American Institute of Architects (AIA) Upjohn Development Research Grant (387523 240M 2301) and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-152205) funding at Northwestern University. This work was also supported by the Molecular Foundry and its compute cluster (vulcan), managed by the High-Performance Computing Services Group, at Lawrence Berkeley National Laboratory (LBNL), and by the National Energy Research Scientific Computing Center (NERSC). Resources are provided by the Office of Science of the U.S. Department of Energy under contract No. DE-AC02-05CH11231. Reza Shahbazian-Yassar efforts were supported by NSF (DMR-1809439). We acknowledge the EPIC facility (NUANCE Center, Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the Nanoscale Science and Engineering Center (NSF EEC−0647560) at the International Institute for Nanotechnology; and the State of Illinois, through the International Institute for Nanotechnology. The authors acknowledge Rao Tatavarti from Micro-Link Device, Inc. at Chicago for providing the triple junction PV cell. This work made use of instruments in the Electron Microscopy Service (Research Resources Center, UIC). The acquisition of the UIC JEOL JEM-ARM200CF was supported by an MRI-R2 grant from the National Science Foundation (Award No. DMR-0959470).

Author Contributions

M.A. and M.E. conceived the idea of the work. M.E. synthesized the nanostructured materials. M.E. and R.A. designed and fabricated the experimental devices. M.E., A.K., A.R.B., P.N.M.D., and J.P. performed electrocatalysis experiments and data analysis. D.P., A.B., A.S.M. and J.Q carried out DFT calculations and CHE model analysis. M.E., K.K., and C.U.S., did the XRD characterization and analysis. M.E. and A.K. did the XPS, UPS and NMR characterizations. B.S., M.T.S. and R.S.Y. performed the STEM characterization. M.A. supervised M.E., A.K., A.R.B., P.N.M.D. and J.P. efforts. All authors discussed the results and assisted with manuscript preparation.

Competing financial interests

M.A., M.E., A.K., and A.R.B. filed a patent application. The other authors declare no competing financial interests.

Data and materials availability

The data supporting and findings of the study are available in the paper and its supplementary materials.

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Gold-Like Activity, Copper-Like Selectivity of Heteroatomic Transition Metal Carbides (M2C) for Electrocatalytic Carbon Dioxide Reduction Reaction

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