A Co₃O₄-CDots-C₃N₄ three component electrocatalyst design concept for efficient and tunable CO₂ reduction to syngas

Abstract

Syngas, a CO and H₂ mixture mostly generated from non-renewable fossil fuels, is an essential feedstock for production of liquid fuels. Electrochemical reduction of CO₂ and H⁺/H₂O is an alternative renewable route to produce syngas. Here we introduce the concept of coupling a hydrogen evolution reaction (HER) catalyst with a CDots/C₃N₄ composite (a CO₂ reduction catalyst) to achieve a cheap, stable, selective and efficient route for tunable syngas production. Co₃O₄, MoS₂, Au and Pt serve as the HER component. The Co₃O₄-CDots-C₃N₄ electrocatalyst is found to be the most efficient among the combinations studied. The H₂/CO ratio of the produced syngas is tunable from 0.07:1 to 4:1 by controlling the potential. This catalyst is highly stable for syngas generation (over 100 h) with no other products besides CO and H₂. Insight into the mechanisms balancing between CO₂ reduction and H₂ evolution when applying the HER-CDots-C₃N₄ catalyst concept is provided.

Introduction

Syngas, a mixture of H₂ and CO, is a critical feedstock for production of synthetic fuels and industrial chemicals via well-established industrial processes such as the Fischer–Tropsch process (commercialized by Sasol and Shell)^{1, 2}. The H₂/CO ratio in syngas is of a great significance for meeting the requirements for specific products: H₂/CO = 2:1 for methanol and H₂/CO = 1:1 for dimethyl ether for example. The conventional production approach of syngas is based on reforming non-renewable fossil fuels (e.g., coal, petroleum coke, and natural gas)³, which increases the consumption of fossil fuel and aggravates the energy crisis. Synthesizing syngas with a controlled H₂/CO ratio by reduction of CO₂ not only contributes to the solution of the energy crisis, but at the same time reduces the amount of greenhouse gases (CO₂).

CO₂ reduction to CO and hydrogen evolution reactions (HER) per se are two independent major and important fields. Electrochemical (EC) and photoelectrochemical (PEC) methods integrating CO₂ reduction reaction and HER are key components of prospective technologies for renewable syngas⁴. Different types of semiconductors have been combined with an efficient catalyst for CO₂ reduction to produce syngas by the PEC approach^5,6,7,8. Cu-ZnO/GaN/n⁺⁻p Si was recently reported as a highly efficient PEC catalyst to produce syngas with a tunable H₂/CO ratio (between 1:2 and 4:1)⁵. Metal or metal-based composites, including Ag⁹, Cu¹⁰, Ru(II) polypyridyl complex¹¹, Ag/C₃N₄¹², and Re-functionalized graphene oxide¹³, have been investigated for EC reduction of CO₂ and H⁺/H₂O to syngas. The different methods used to tune the ratio of H₂/CO include altering the CO₂ flow rate¹⁴ and pressure¹⁵, the reaction temperature¹⁶, and the applied potential⁹. Different crystalline sites of Au catalyze different reaction channels (edge sites initiate CO generation and corner sites H₂ generation)^{17, 18}. A novel pulsed-bias technique using Cu as the catalyst was recently applied to tune the H₂/CO ratio in syngas between ~32:1 and 9:16 by using different pulse times for the same working potential. The selectivity is however limited and CH₄ and C₂H₄ by-products affect the purity of syngas¹⁰. Using Ag/C₃N₄¹², the H₂/CO ratio in the produced syngas can be tuned from 100:1 to 2:1 by controlling the applied potential and the Ag loading on graphitic carbon nitride but the total current density is lower than 1 mA/cm² at −0.6 V. The previous EC attempts to synthesize renewable syngas are still characterized by an unsatisfactory performance including some of the following disadvantages: a high onset overpotential necessary to initiate the CO₂ reduction reaction, a low CO + H₂ generation current density, a small selectivity of CO production, and a poor stability of the generation current density and the Faradaic efficiency (FE) of H₂ and CO.

We hereby propose a design concept of a cheap composite EC catalyst for a tunable, stable, selective, and efficient production of syngas, made of three components: a HER catalyst, a CO₂ reduction catalyst toward CO, and a catalyst which stabilizes the active hydrogen (H•) necessary to trigger both HER and the CO₂ reduction reactions. For HER, we choose several known catalysts (Co₃O₄, Pt, MoS₂, and Au). For CO₂ reduction, we apply graphitic carbon nitride (C₃N₄) since carbon-bonded nitrogen groups including pyridinic N, pyrrolic N, and graphitic N have recently been proposed as active sites for CO₂ reduction to CO^19,20,21. C₃N₄ has a porous structure and is shown as a good substrate for dispersion of catalytic nanoparticles^22,23,24,25. The selected catalyst for stabilization of active hydrogen (H•) are carbon dots (CDots)^{26, 27}, which possess significant adsorption capabilities for H^{+28, 29} and CO₂³⁰ and exhibit excellent ability of electron transfer^{31, 32} necessary for H• generation (H⁺+e⁻→H•). CDots also improve the conductivity of C₃N₄.

Here we found that the Co₃O₄-CDots-C₃N₄ is the best EC catalyst for syngas production studied within the present work. We thus first describe the structural characterization of this catalyst. We then present the design concept of the three-component catalyst for syngas production and detail its working mechanisms. We follow by showing that this concept is valid and that Co₃O₄-CDots-C₃N₄ is indeed an efficient, tunable, stable, selective, and cheap EC catalyst for syngas production. It initiates CO₂ reduction to CO in aqueous solutions at a low overpotential (0.17 V vs. reversible hydrogen electrode (RHE)) and the total current density of H₂ and CO generation may reach up to 15 mA/cm² at a potential of −1.0 V vs. RHE. The H₂/CO ratio of syngas generated by Co₃O₄-CDots-C₃N₄ is tunable from 0.07:1 to 4:1 by controlling the applied potential. Dedicated experiments highlight the different mechanisms by which syngas production applying Co₃O₄-CDots-C₃N₄ is controlled and manipulated. Finally, the generality of the catalyst design concept applying Pt, MoS₂, and Au as the HER catalyst is demonstrated. The significance of this work is thus twofold. First, it presents Co₃O₄-CDots-C₃N₄ as an efficient, cheap, tunable, and stable catalyst for syngas production on one hand. Second, it provides an avenue to design catalysts for controlled and tuned production of syngas in particular and other chemicals of interest to the chemical industry in general.

Results

Characterization of Co₃O₄-CDots-C₃N₄

Transmission electron microscopy (TEM) image (Fig. 1a) reveals that the Co₃O₄-CDots-C₃N₄ consist of nm-sized Co₃O₄ nanoparticles (NPs) and CDots dispersed on the C₃N₄ matrix. The two different types of nanocrystals shown in the inset of Fig. 1a are identified as Co₃O₄ NPs with a d-spacing of 0.24 nm consistent with Co₃O₄ (311)³³ and CDots with an interplanar spacing of 0.33 nm (see Supplementary Fig. 1)³⁴. Figure 1b displays a scanning transmission electron microscopy (STEM) micrograph and its corresponding chemical maps of C-K, N-K, O-K, Co-K, and Co-L for the Co₃O₄-CDots-C₃N₄. Co-K, Co-L, and O-K cover the entire C₃N₄ area monitored, indicating that Co₃O₄ NPs are evenly distributed on the C₃N₄ sheet. X-ray diffraction (XRD) shows (Supplementary Fig. 2) the typical diffraction lines of Co₃O₄ and C₃N₄. Besides the average size of Co₃O₄ NPs derived from the XRD spectrum of the Co₃O₄-CDots-C₃N₄ using the Debye−Scherrer equation (Supplementary Table 1), is about 10 nm, which is consistent with the size of Co₃O₄ NPs in the TEM image. Energy dispersive X-ray absorption (EDX) analysis of the Co₃O₄-CDots-C₃N₄ (Supplementary Fig. 3) reveals an elemental atom composition of C (46.65 at. %), N (48.74 at. %), O (3.01 at. %), and Co (1.60 at. %). X-ray photoelectron spectroscopy (XPS) of the composite (Supplementary Fig. 4) shows C 1s, N 1s, O 1s, and Co 2p peaks with a similar elemental composition to that deduced by EDX (C (39.9 at. %), N (52.5 at. %), O (5.8 at. %), and Co (1.8 at. %)) (Supplementary Table 2). The Co 2p_3/2 peak was fitted (Supplementary Fig. 4e) by using two synthetic peaks positioned at binding energy (BE) = 780.0 and 781.3 eV (Co³⁺ and Co²⁺, respectively) and the Co 2p_1/2 peak was fitted (Supplementary Fig. 4e) by using two synthetic peaks positioned at BE = 795.0 and 797.3 eV (Co³⁺ and Co²⁺, respectively). Figure 1c shows type IV N₂ adsorption–desorption isotherms of the Co₃O₄-CDots-C₃N₄ with a H3-type hysteresis loop³⁵, indicating the formation of a porous structure (the pore-size distribution was derived from the isotherms; for more details, see Supplementary Methods) is presented in Fig. 1c inset; specific surface area ~160 m²/g). The porous Co₃O₄-CDots-C₃N₄ was found to adsorb a large amount of CO₂ (~0.33 mmol/g at 1.2 atm, Fig. 1d). This strong CO₂ adsorption capability is mainly due to CO₂ molecules adsorbed on the porous C₃N₄ surfaces, which constitute more than 90 wt.% of the composite catalyst (Supplementary Fig. 5a). It is nevertheless enhanced in a synergistic way by the incorporation of CDots (Supplementary Fig. 5b shows that the CO₂ adsorption on Co₃O₄-CDots-C₃N₄ and on CDots-C₃N₄ is about the same, while that on Co₃O₄-C₃N₄ is much lower). The adsorption of H⁺ (Fig. 1e) is very rapid in the first 15 min, then it becomes slower with contact time, stopping additional absorption after 30 min. The amount of adsorbed H⁺ (for more details, see Supplementary Methods) on Co₃O₄-CDots-C₃N₄ and CDots-C₃N₄ is about 13.8 mg/g, while the amount of adsorbed H⁺ is only 5.11 mg/g for the Co₃O₄-C₃N₄ (Supplementary Fig. 6b). The incorporation of CDots thus significantly increases the adsorbed capacity of H⁺. All these results indicate that the present electrocatalyst has a strong adsorption capacity for both H⁺ and CO₂, which is significantly important for both processes of CO₂ reduction reaction and HER.

Fig. 1

Characterization of the Co₃O₄-CDots-C₃N₄. a TEM image of a grain of the Co₃O₄-CDots-C₃N₄ and a HRTEM image of Co₃O₄-CDots-C₃N₄ (inset), scale bar 20 nm and 5 nm (inset). b STEM micrograph and the corresponding elemental mapping of C-K, N-K, O-K, Co-K, and Co-L for the Co₃O₄-CDots-C₃N₄, scale bar 50 nm. c N₂ adsorption–desorption isotherm and the corresponding pore-size distribution of Co₃O₄-CDots-C₃N₄ (inset). d CO₂ adsorption isotherm of Co₃O₄-CDots-C₃N₄. e The time-course adsorption of H⁺ by the Co₃O₄-CDots-C₃N₄. The adsorption is 13.8 mg/g. Experiments were performed in triplicates and results are shown as mean ± standard deviation

The electrocatalyst design and reaction mechanism

We first present the design concept of the ternary electrocatalyst and highlight its operation mode to produce syngas in a controllable, tunable way. The proposed electrocatalyst consists of three parts: an electrocatalyst for HER (e.g., Co₃O₄), an electrocatalyst for CO₂ reduction to CO (e.g., C₃N₄), and an electrocatalyst which triggers both reaction channels (e.g., CDots by trapping H⁺ and e⁻ and generating H•). A schematic diagram of the Co₃O₄-CDots-C₃N₄ ternary electrocatalyst syngas generation is presented in Fig. 2. CDots first trap H⁺ from the solution and e⁻ from the glassy carbon electrodes and combine them to form H•. The two other catalyst components compete for these H• species. They may diffuse to C₃N₄ (the CO₂ reduction catalyst due to its different N active sites^19,20,21) and reduce CO₂ to CO. Alternatively they may diffuse to Co₃O₄ (the HER catalyst) and produce H₂. We thus suggest that CDots are the generation site of H•, C₃N₄ is the generation site of CO, and Co₃O₄ is the generation site of H₂. Syngas with tunable H₂/CO ratio can be thus achieved by balancing the CO₂ reduction channel and the HER channel. The different catalyst components have additional functions. Introduction of CDots to the composites enhances the adsorption of both CO₂ and H⁺. Incorporation of CDots in the electrocatalyst improves its conductivity (Supplementary Fig. 7). C₃N₄ serves as the highly porous substrate in which the CDots and Co₃O₄ nanoparticles are incorporated. C₃N₄ thus provides a large surface area which enhances the reaction activity and also guarantees a proximity between the different generation sites (CDots for H• generation, C₃N₄ for CO generation, and Co₃O₄ for H₂ generation). This proximity is essential for an efficient reaction. Experimental evidence substantiating this proposed mechanism will be given in the following sections.

Fig. 2

Schematic diagram of the reaction mechanism induced by Co₃O₄-CDots-C₃N₄. CDots are the generation site of H•, Co₃O₄ the generation site of H₂, and C₃N₄ the generation site of CO

The electrocatalytic performance of Co₃O₄-CDots-C₃N₄

We now show that the suggested design concept of the Co₃O₄-CDots-C₃N₄ indeed provides a tunable and stable production of syngas. The electrocatalytic performance of Co₃O₄-CDots-C₃N₄ for syngas production was tested in an airtight three electrodes electrochemical H-type cell. The gaseous reduction products monitored by a gas chromatography (GC) system were CO and H₂ while no other reduction liquid products were found by H¹ NMR. Figure 3a shows the current densities of CO (j_CO, red trace) and H₂ (j_H2, black trace) vs. different potentials acquired in a CO₂-saturated 0.5 M KHCO₃ (pH = 7.2) solution. The curves indicate a significant generation of both CO and H₂ but the j_CO/j_H2 ratio varies with the potential applied. The j_CO curve shows that the CO₂ electrocatalytic reduction starts at an initial potential of −0.28 V vs. RHE (all potentials reported throughout this paper are with respect to this reference). This operating voltage corresponds to a low overpotential of 0.17 V (the equilibrium potential of CO₂/CO is at −0.11 V). Up to −0.45 V, both j_CO and j_H2 are very small and above −0.45 V both currents increase. Between −0.45 and −0.75 V, j_CO is larger than j_H2, indicating that the CO₂ reduction reaction predominates. For potentials larger than −0.75 V, the HER channel becomes more significant than the CO₂ reduction reaction. The increase of j_H2 with potential is about constant (the j_H2 curve is a straight line). In contrast, the slope of the j_CO curve decreases with the potential applied. We attribute this behavior to the consumption of CO₂ at high CO generation rates due to its low solubility or slow mass transfer. Indeed (inset of Fig. 3a), bubbling CO₂ into the electrolyte increases j_CO but the increase rate of j_CO with potential is still lower than that of j_H2. Figure 3b shows the FEs of CO and H₂ generation vs. the applied cathodic potentials. The FE of CO generation increases with the applied potential and reaches a maximum of 89% at −0.6 V. For higher potential, it decreases to ~40% with increasing potential to −0.8 V, most probably due to the limited mass transport of CO₂ in the electrolyte. In contrast, the FE of H₂ generation is about 86% for a low applied potential, decreases with potential to about 5% at −0.6 V and then rises again reaching 55% at −0.8 V. The total FE of CO and H₂ combined reaches up to 95%. Consequently, syngas with different H₂/CO ratios can be obtained by altering the applied potential as shown in Fig. 3c. The volume ratio between H₂ and CO can be tuned from 4:1 to 0.07:1 and syngas with H₂/CO = 1:1 can directly be generated at the potentials of −0.45 V (total current density is 0.25 mA/cm²) and −0.75 V (total current density is 5.78 mA/cm²). Note that the catalyst mass activity for syngas generation reaches a high value of 10 A/g_catalyst at a potential of −0.6 V. The stability of the Co₃O₄-CDots-C₃N₄ catalyst for electrocatalytic CO₂ reduction and HER (Fig. 3d) was studied as well. A negligible decay in current density was observed when operating the system for 100 h at the applied potential of both −0.6 V (Fig. 3dΙ, black trace, CO₂ reduction reaction predominates) and −0.75 V (Fig. 3dΙ, red trace, HER and CO₂ reduction are equal). It reveals that the Co₃O₄-CDots-C₃N₄ maintains high electrocatalytic stability during the 100-h test. An additional experiment validated the stability of both the current density and the FE for 30 h (it is very likely that the FE is stable for 100 h as well).

Fig. 3

Electroreduction of CO₂ and H⁺/H₂O to syngas with adjustable H₂/CO ratio. a The current density for HER (j_H2, black trace) and for CO₂ reduction, (j_CO, red trace) vs. the applied potential, catalyzed by Co₃O₄-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte. The inset shows the same experiment conducted with bubbling of CO₂ to the solution to overcome CO₂ consumption during the experiment. b The FEs of the reduction of CO₂ to CO (red points) and H⁺ to H₂ (black points) catalyzed by Co₃O₄-CDots-C₃N₄ vs. the applied potential. Experiments were performed in triplicates and results are shown as mean ± standard deviation. c The volume ratio between H₂ and CO vs. the applied potential. The H₂/CO volume ratio is about 1:1 at the potential of −0.45 V or −0.75 V vs. RHE. d The stability of the performance of Co₃O₄-CDots-C₃N₄ for producing syngas: operated at potentiostatic potential of −0.6 and −0.75 V for 100 h (dΙ); operated at potentiostatic potential of −0.6 V for 30 h (dII): current density vs. time (left axis) and FEs of CO production vs. time (right axis)

Investigation of the catalytic mechanisms

A series of controlled experiments were carried out to validate and further understand the catalytic mechanism operating with the Co₃O₄-CDots-C₃N₄ proposed in a previous section (Fig. 2). Issues investigated included: the generation sites of H•, CO, and H₂; the role of H⁺; the roles of CDots, C₃N₄, and Co₃O₄; the role of the proximity between the CDots, C₃N₄, and Co₃O₄; ways in which the relative significance of the different reaction channels and the resulting CO/H₂ volume ratio can be manipulated; other HER-CDots-C₃N₄ systems with different HER materials. We first studied the linear sweep voltammetry (LSV) curves of different combinations of the three components comprising the ternary Co₃O₄-CDots-C₃N₄. The comparison of the different LSV curves (Fig. 4a) and the complementary study of the CO and H₂ composition generated during the electrocatalytic processes catalyzed by the different components (Fig. 4b) enabled a clear determination of both the generation sites of the CO and H₂ and the role of the three basic components used. We first discuss the LSVs showing the apparent current density (current density per geometrical area), which is also equivalent to the mass activity of the catalysts (current per gram catalyst) as shown in Fig. 4a. Further on, we also give the corresponding turn on frequencies (TOFs) and the real current densities derived from the real surface areas of the components of the different combinations of catalysts in Fig. 4a. LSVs of C₃N₄ (Fig. 4a, curve 1, black trace, morphology of C₃N₄ is shown in Supplementary Fig. 8a), CDots (Fig. 4a, curve 2, cyan trace, morphology of CDots is shown in Supplementary Fig. 1), Co₃O₄ (curve 3, gray trace, morphology of Co₃O₄ is shown in Supplementary Fig. 8b), CDots-C₃N₄ (curve 4, blue trace, morphology of CDots-C₃N₄ is shown in Supplementary Fig. 8c), Co₃O₄-C₃N₄ (curve 5, green trace, morphology of Co₃O₄-C₃N₄ is shown in Supplementary Fig. 8d), Co₃O₄-CDots (curve 6, purple trace, morphology of Co₃O₄-CDots is shown in Supplementary Fig. 8e), and Co₃O₄-CDots-C₃N₄ (curve 7, red trace) were performed using a CO₂-saturated 0.5 M KHCO₃ (pH = 7.2) solution (Fig. 4a). The LSVs of C₃N₄ (curve 1, black trace), CDots (curve 2, cyan trace), Co₃O₄ (curve 3, gray trace), CDots-C₃N₄ (curve 4, blue trace), and Co₃O₄-C₃N₄ (curve 5, green trace) show poor electrocatalytic performances evident by their low current densities and high onset potentials (Fig. 4a). Only the Co₃O₄-CDots (curve 6) and the Co₃O₄-CDots-C₃N₄ (curve 7) exhibit high current densities and low onset potentials (Fig. 4a). CDots generate H₂ only, but not CO (Fig. 4b). We explain it by their trapping and stabilizing H•, which may generate a small amount of H₂ even in the absence of a HER catalyst. The LSV of Co₃O₄ (curve 3, gray trace) shows a relatively small activity producing only H₂ (Fig. 4b). In comparison, Co₃O₄-CDots (curve 6, purple trace) produces a much larger amount of H₂ compared to pure CDots or pure Co₃O₄. We attribute it to the effect of the CDots, which greatly enhance the electrocatalytic performance of Co₃O₄ by providing H•, which is necessary for generation of H₂. We thus conclude that Co₃O₄ is the H₂ generation site while CDots are the generation site of H• and both are needed for a large generation rate of H₂. Now, we prove that the generation site of CO is C₃N₄. Figures 3a, b and 4a, b indicate that significant amounts of CO are generated by either the CDots-C₃N₄ or the Co₃O₄-CDots-C₃N₄. Co₃O₄ was shown to generate H₂ only, which leaves CDots-C₃N₄ as the producer of CO. CDots per se produce only H₂ as previously discussed. They however are essential for CO generation by CDots-C₃N₄ since they significantly enhance the CO₂ adsorption, adsorb H⁺, and stabilize H•. C₃N₄ per se catalyzes only a very small (negligible) current (Fig. 4a, curve 1) so that the very little amount of gas produced by pure C₃N₄ (Fig. 4b) contains more H₂ than CO. The supply of H• by the CDots is necessary to promote the generation of CO in C₃N₄. Figure 4a shows that each catalyst component adds to the activity by enhancing one of the three reactions (H• generation, H₂ generation, and CO generation) but the complete three components composite Co₃O₄-CDots-C₃N₄ is necessary for intense generation of syngas. We further studied the effect of the type of mixing of the different catalyst components on the catalytic activity. The LSVs of physical mixtures of catalysts (Co₃O₄ + C₃N₄, CDots + C₃N₄, and Co₃O₄ + CDots + C₃N₄) were compared to those of composites of the same components chemically blended (Co₃O₄-C₃N₄, CDots-C₃N₄, and Co₃O₄-CDots-C₃N₄). The chemically prepared composites had much larger activities (current densities) than their corresponding physical mixtures (Supplementary Fig. 9). We attribute the large activity of the chemically prepared composites to the close proximity between the different catalysts (active sites) in the composite materials. In contrast, physical mixing does not provide such a proximity so that the large distance between the active sites hinders the activity of the physically mixed catalysts.

Fig. 4

Electrochemical experiments providing insight into the electrochemical processes. a The linear sweep voltammetry curves (LSVs) for C₃N₄ (black trace, curve 1), CDots (cyan trace, curve 2), Co₃O₄ (gray trace, curve 3), CDots-C₃N₄ (blue trace, curve 4), Co₃O₄-C₃N₄ (green trace, curve 5), Co₃O₄-CDots (purple trace, curve 6), and Co₃O₄-CDots-C₃N₄ (red trace, curve 7) in CO₂-saturated 0.5 M KHCO₃ electrolyte, 10 mV/s. Current density on left y axis and mass activity on right y axis. The comparison of the curves allows determination of the role of the different catalyst components in the electrochemical reactions. b The FEs of the reaction products at −0.5 and −0.8 V, using C₃N₄, CDots, Co₃O₄, CDots-C₃N₄, Co₃O₄-C₃N₄, Co₃O₄-CDots, and Co₃O₄-CDots-C₃N₄, respectively, as catalysts. Note that only the C₃N₄ and CDots-C₃N₄ composites produce significant amounts of CO. c Total current density vs. time curves of CO₂ reduction to CO and HER at the potential of −0.6 V in CO₂-saturated KHCO₃ solution (0.5 M, pH = 7.2) and phosphate buffer (PB) solution (pH = 7.2), respectively. d LSVs for the Co₃O₄-CDots-C₃N₄ in N₂- (black trace) and CO₂-saturated (red trace) MeCN containing 0.5 M [BMIM]PF₆, 10 mV/s. The inset shows the total current density vs. time curves of CO₂ reduction at the potential of −0.6 V in CO₂-saturated MeCN containing 0.5 M [BMIM]PF₆. The current in the inset is much smaller than in c and no reaction products are detected indicating that H⁺ is essential for both HER and CO₂ reduction to CO. e Current density for HER (j_H2, black trace) and current density for CO₂ reduction, (j_CO, red trace) vs. the applied potential, catalyzed by 0.04 M Co₃O₄-CDots-C₃N₄ in a CO₂-saturated 0.5 M KHCO₃ electrolyte. f Current density for HER (j_H2, black trace) and current density for CO₂ reduction, (j_CO, red trace) vs. the applied potential, catalyzed by 0.02 M Co₃O₄-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte; note that the reduction of the amount of the HER catalyst component shifts the balance of gas generation toward enhanced CO generation

The comparison between the LSVs of the different catalyst components and their composites (Fig. 4a) should be done carefully. The catalyst areal density was kept the same (0.127 mg/cm²) for all (3 mm in diameter) electrodes. Since the composition of the Co₃O₄-CDots-C₃N₄ was 6 wt% Co₃O₄, 1 wt% CDots, and 93 wt% C₃N₄, it follows that the amount of CDots or Co₃O₄ in the CDots, Co₃O₄, or Co₃O₄-CDots electrodes was much larger than in the C₃N₄ containing composite electrodes. This means that the reaction activity per catalyst mass of curves 2, 3, and 6 is very low compared to that of the Co₃O₄-CDots-C₃N₄ electrode (Fig. 4a, curve 7). Since we deal with three different catalysts with different functions, we should calculate the mass activity (current per gram catalyst) for each component separately. This was done in Supplementary Fig. 10. The reaction activity per mass of a single catalyst component (Co₃O₄ or CDots) of Co₃O₄-CDots (Fig. 4a, curve 6), e.g., is actually lower by more than an order of magnitude than the activity per mass of a single catalyst component of that of the Co₃O₄-CDots-C₃N₄ catalyst (Fig. 4a, curve 7) though curve 6 appears to indicate (Fig. 4a) a larger activity than curve 7 (compare Fig. 4a to Supplementary Fig. 10). Supplementary Fig. 10 clearly shows that starting from a single catalyst component, the addition of each of the two other components increases the activity of syngas production, i.e., all components are necessary for an optimized syngas generation. Similarly, we calculate the turn on frequencies (TOFs) of the catalysts compositions of Fig. 4a. TOF = (number of reacted electrons per time/number of catalyst active sites). We approximate the number of active sites by the number of catalyst atoms. Since we have three different catalyst components with three different functions, we calculate the TOFs per each catalyst component (CDots, Co₃O₄, and C₃N₄). Supplementary Fig. 11 shows (similar to Supplementary Fig. 10) that the addition of each single component increases the syngas production activity and all components are necessary for the optimal performance. We further measure the BET and the electrochemical real surface areas of the catalyst components of each combination shown in curves 1–7 (Supplementary Tables 3–4). We use the electrochemical surface areas (ECSAs) to calculate the real current densities related to the specific catalyst components that constitute the seven combinations shown in curves 1–7. Supplementary Fig. 12 shows that similar to the mass activities and the TOFs curves, the real current densities of one component increase with the addition of a second component and are the largest when all three catalyst components combine to a three-component catalyst composite. Figure 3 shows that the ternary concept design indeed yields an optimal performance of its different components balancing between CO₂ reduction and HER. This explains the high value of 10 A/g_catalyst obtained at −0.6 V. The CO₂ and H⁺ adsorption measurements (Supplementary Figs. 5 and 6) of the individual different catalyst components (CDots, Co₃O₄, and C₃N₄) and their composites (CDots-C₃N₄, Co₃O₄-C₃N₄, CDots-Co₃O₄, and CO₃O₄-CDots-C₃N₄) reveal another effect which improves the ternary composite catalyst performance. The synergism of the three components acting simultaneously enhances the adsorption of the ternary composite by a factor of 2–3 with respect to the adsorption of the individual components. Real surface area measurements (BET and electrochemical) of the catalyst compositions (Supplementary Table 3) indicate that the inclusion of the C₃N₄ component results in a high surface area ((S_real/S_geometrical) = 200 for BET and 30 for electrochemical) while it is lower by an order of magnitude for the nanoparticle catalysts (CDots, Co₃O₄, or CDots-Co₃O₄). The dispersion of the catalysts nanoparticles on the C₃N₄ surface seems to explain the synergistic adsorption behavior.

To study the role of H⁺ in syngas generation, we investigated the electrocatalytic performance of Co₃O₄-CDots-C₃N₄ in aqueous solutions with the same pH value (HER in phosphate buffer solution, pH = 7.2; syngas reaction in CO₂-saturated 0.5 M KHCO₃ solution, pH = 7.2) and in an ionic liquid (without H⁺). The current density–time curves of pure HER and syngas reactions at the potential of −0.6 V (at −0.6 V the syngas reaction produces ~90% CO) are shown in Fig. 4c. A stable current density (~−1.21 mA/cm²) for syngas is observed at −0.6 V while −1.25 mA/cm² for HER is reached at the same potential. Then, ionic liquid was used as the electrolyte solution to eliminate H⁺. The electrocatalytic performance of Co₃O₄-CDots-C₃N₄ was tested at the potential of −0.6 V in a CO₂-saturated MeCN solution containing 0.5 M [BMIM]PF₆. The current density is only about 0.61 mA/cm² in a CO₂-saturated ionic liquid (Fig. 4d), which is much lower than that obtained for CO₂ reduction in CO₂-saturated KHCO₃ solution (−1.21 mA/cm²). Notably, no gaseous products of CO reduction could be detected suggesting that H⁺ plays an important role for CO₂ reduction to CO in the present Co₃O₄-CDots-C₃N₄ catalyst system.

It was shown that the H₂/CO volume ratio obtained using the Co₃O₄-CDots-C₃N₄ catalyst system is determined by the balance between the HER channel and the CO₂ reduction channel to CO. This ratio was tuned by modifying the potential (Fig. 3). Another plausible tuning method is the decrease of the amount of the HER catalyst component (Co₃O₄) thus reducing the HER activity and increasing the CO₂ reduction activity. We therefore measured the electrocatalytic activities of Co₃O₄-CDots-C₃N₄ produced using lower amounts of Co (0.04 and 0.02 M Co₃O₄ loadings with respect to the standard one of the present work (0.05 M)). Figures 3a and 4e, f show the j_H2 and j_CO obtained at different applied potentials for 0.05, 0.04, and 0.02 M Co₃O₄-CDots-C₃N₄, respectively. It is obvious that the CO/H₂ ratio was strongly affected by the change of the Co₃O₄ HER catalyst amount.

Finally, the design concept of the HER-CDots-C₃N₄ ternary catalyst was applied for three additional HER active electrocatalysts: Pt, MoS₂, and Au to form Pt-CDots-C₃N₄, MoS₂-CDots-C₃N₄, and Au-CDots-C₃N₄. The structural characterization of these catalysts is given in Supplementary Fig. 13. The electrocatalytic activity of these three ternary composite catalysts for producing syngas were tested under the same conditions as for Co₃O₄-CDots-C₃N₄. Applying the Pt-CDots-C₃N₄ electrocatalyst (Fig. 5a), only hydrogen was detected in the gas phase and no reduction products from CO₂ reduction were observed. Pt is considered as the most efficient electrocatalyst to facilitate HER³⁶. The application of Pt-CDots-C₃N₄ as a catalyst shifts the balance between HER and CO₂ reduction toward H₂ generation, increases the intensity of the efficient HER channel and completely suppresses the CO₂ reduction to CO. The MoS₂-CDots-C₃N₄ electrocatalyst is still sufficiently HER active to produce close to 90% j_H2, but a small amount of j_CO (a few percent of the total current) is nevertheless observed. The FEs of CO are no more than 10% for the MoS₂-CDots-C₃N₄ (inset of Fig. 5b). The Au-CDots-C₃N₄ electrocatalyst (Fig. 5c) still exhibits a higher HER activity than the CO₂ reduction activity, but the amount of CO is much larger than for the MoS₂-CDots-C₃N₄ catalyst (the FE of CO production reaches 25%, inset of Fig. 5c). It can be thus concluded that the concept of the ternary HER-CDots-C₃N₄ is general and valid for HER catalysts different than Co₃O₄. Achievement of a relatively high amount of CO/H₂ however requires the application of a HER catalyst with only a medium activity pushing the balance between HER and CO₂ reduction toward CO₂ reduction. The smaller the HER activity, the larger relative amount of CO is obtainable. This conclusion was directly checked by reducing the amount of Au in the ternary Au-CDots-C₃N₄ electrocatalyst. The electrochemical tests (Fig. 5d) show that decreasing the amount of Au by a factor of two enabled the generation of syngas with a CO to H₂ volume ratio larger than one. A striking property of the h-Au-CDots-C₃N₄ catalyst is its extremely high mass activity for producing syngas, i.e., >700 A/g_Au for CO production and >700 A/g_Au H₂ production (~1500 A/g_Au for the total current) at −0.7 V for the catalyst shown in Fig. 5d. This activity is two orders of magnitude larger than previously reported for efficient Au electrodes for CO production^{17, 18}. This high mass activity allows a reduction of the electrode cost when precious catalysts (e.g., Au) are applied.

Fig. 5

Catalytic activity of composite catalysts with different HER catalysts. a Current density for HER (j_H2, black trace) vs. the applied potential, catalyzed by Pt-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte. b Current density for HER (j_H2, black trace) and current density for CO₂ reduction to CO, (j_co, red trace) and FEs of H₂ and CO (inset) vs. the applied potential, catalyzed by MoS₂-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte. c Current density for HER (j_H2, black trace) and current density for CO₂ reduction, (j_co, red trace) and FEs of H₂ and CO (inset) vs. the applied potential, catalyzed by Au-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte. d Current density for HER (j_H2, black trace) and current density for CO₂ reduction, (j_co, red trace) and FEs of H₂ and CO (inset) vs. the applied potential, catalyzed by Au-CDots-C₃N₄ in CO₂-saturated 0.5 M KHCO₃ electrolyte. The amount of Au in d is half of that in c (marked as h-Au-CDots-C₃N₄) leading to an increase of the CO/H₂ ratio in d with respect to that in c

Discussion

The design concept of the HER-CDots-C₃N₄ EC catalyst for syngas generation was introduced and its electrocatalytic performance for syngas production in aqueous solutions was studied, applying Co₃O₄, MoS₂, Au, and Pt as the HER catalyst component. The Co₃O₄-CDots-C₃N₄ electrocatalyst was found the most efficient for syngas production among the composite combinations investigated. The Co₃O₄-CDots-C₃N₄ is capable of controlling the balance between the HER channel and CO₂ reduction channel. The Co₃O₄-CDots-C₃N₄ initiates the reaction of CO₂ reduction to CO in aqueous solutions at a low overpotential (0.17 V) while the total current density reaches up to 15 mA/cm² at a potential of −1.0 V. The mass activity of the Co₃O₄-CDots-C₃N₄ is ~10 A/g_catalyst at −0.6 V when the total mass of the catalyst is considered and 1–2 orders of magnitude larger when the mass of the HER catalyst is considered (which is ~0.5–5% of that of the total catalyst weight). The Co₃O₄-CDots-C₃N₄ induces high FEs (95%) and is characterized by a stable production of syngas (over 100 h). Notably, the H₂/CO ratio of syngas produced applying Co₃O₄-CDots-C₃N₄ is tunable from 0.07:1 to 4:1 by controlling the applied potential. The H₂/CO may be also tuned by varying the amount of Co₃O₄ in the Co₃O₄-CDots-C₃N₄.

Dedicated experiments validated the catalyst design concept and provided additional insight to the syngas generation processes. C₃N₄ and Co₃O₄ are the activity sites for CO₂ reduction reaction and HER, respectively. CDots are the generation site of H• needed to trigger both the reduction of CO₂ to CO and the HER. The three-component catalyst concept is a general one and may be applied to a host of other materials. The versatility of the three components composite design may open a powerful pathway for the development of high-performance catalysts for syngas production as well as for other chemicals generation. Such an efficient and cost-effective electrocatalytic system has a high potential to be employed for the large-scaled production of syngas and controlled mixtures of other chemicals from CO₂.

Methods

Instruments

Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and scanning TEM (STEM) images were obtained using a FEI/Philips Tecnai G2 F20 TWIN transmission electron microscope. The energy dispersive X-ray spectroscopy (EDS) analyses were taken on a FEI-quanta 200 scanning electron microscope with an acceleration voltage of 20 kV. The crystal structure of the resultant products was characterized by X-ray diffraction (XRD) using an X’Pert-ProMPD (Holland) D/max-γAX-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). X-ray photoelectron spectroscopy (XPS) was obtained by using a KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with a monochromatized Mg Kα X-ray source (hν = 1283.3 eV). The electrocatalysis reactions were tested by a Model CHI 660C workstation (CH Instruments, Chenhua, Shanghai, China). The electrochemical impedance spectroscopy (EIS) measurements were obtained applying a CHI 832 electrochemical instrument (CHI Inc., USA).

Materials

KHCO₃ (99.7%) and Nafion perfluorinated resin solution (5 wt.%) were purchased from Sigma-Aldrich; hydrogen (99.999%), nitrogen (99.999%), and carbon dioxide (99.999%) were purchased from Airgas; Nafion®212 membrane was purchased from Dupont; Toray Carbon Paper (TGP-H-60) was purchased from Alfa Aesar; All chemicals were purchased from Sigma-Aldrich unless specifically stated. Milli-Q ultrapure water (Millipore, ≥18 MΩ/cm) was used throughout the work.

Fabrication of electrocatalysts

CDots were synthesized by our previously reported electrochemical etching method³⁴. After 30-days reaction, a dark yellow solution containing CDots was formed in the reaction cell. It was then purified and concentrated to form a CDots solution of 3 mg/mL. For C₃N₄ fabrication, 10 g of melamine powder was put into an alumina crucible with a cover and then heated to 550 °C at a rate of 0.5 °C per min in a muffle furnace and maintained at this temperature for 3 h. The yellow powder (C₃N₄) was obtained after cooling down to room temperature. Co₃O₄ NPs was synthesized by hydrothermal method. Twenty mL Co(NO₃)₂ (0.01 M) solution was added into an alumina crucible with a cover and then heated to 550 °C at a rate of 0.5 °C per min in a muffle furnace and maintained at this temperature for 3 h. The black powder (Co₃O₄ NPs) was obtained after cooling down to room temperature. For preparation of Co₃O₄-C₃N₄, CDots-C₃N₄ or Co₃O₄-CDots-C₃N₄, 10 g of melamine powder was mixed with 10 mL solution containing 0.05 M Co(NO₃)₂, CDots, CDots and 0.05 M Co(NO₃)₂, respectively. Then, the mixture was put into an alumina crucible with a cover and heated to 550 °C at a rate of 0.5 °C per min in a muffle furnace and maintained at this temperature for 3 h. For preparation of Co₃O₄-CDots, 10 mL solution containing 0.05 M Co(NO₃)₂ and CDots (3 mg/mL) was put into an alumina crucible with a cover and heated to 550 °C at a rate of 0.5 °C per min in a muffle furnace and maintained at this temperature for 3 h.

Synthesis of Pt-CDots-C₃N₄

An aliquot of 0.3 g CDots-C₃N₄ (obtained by heating melamine at 550 °C for 3 h) was added into 10 mL H₂PtCl₆ (2 mM) aqueous solution and stirred for 12 h. After centrifuging, the precipitate was irradiated by UV light for 10 h. The resulting product was collected by centrifugation and dried in a vacuum at 60 °C for 12 h.

Synthesis of MoS₂-CDots-C₃N₄

An aliquot of 0.3 g CDots-C₃N₄ (obtained by heating melamine at 550 °C for 3 h) was added into 15 mL aqueous solution containing Na₂MoO₄ (0.0625 g) and L-cysteine (0.1 g). The mixed solution was stirred 3 min. After that, the mixture was poured into a Teflon-lined stainless steel autoclave, and heated at 180 °C for 24 h. After the autoclave was cooled down to room temperature, the resulting black sediments were collected by centrifugation (10,000 rpm, 10 min) and washed with deionized water and ethanol for several times, and then dried in a vacuum oven at 80 °C for 12 h³⁷.

Synthesis of Au-CDots-C₃N₄ and h-Au-CDots-C₃N₄

An aliquot of 0.3 g CDots-C₃N₄ (obtained by heating melamine at 550 °C for 3 h) was added into 10 mL HAuCl₄ (2 mM or 1 mM) aqueous solution and stirred for 12 h. After centrifuging, the precipitate was irradiated by UV light for 2 h. The resulting product was collected by centrifugation and dried in vacuum at 60 °C for 12 h³⁸.

Electrocatalysis activity test

Electrocatalysis activity test experiments were performed using a standard three-electrode configuration. A platinum wire was used as an auxiliary electrode and a saturated calomel electrode (SCE) was used as a reference electrode. The working electrode was either a catalyst-modified carbon fiber paper electrode (CFPE for short, 0.7 cm × 0.7 cm), or a catalyst-modified glassy carbon disk electrode (GCE for short, 3.0 mm diameter). For product analysis and constant-potential electrolysis experiment, the CFPE working electrode was a catalyst-modified carbon fiber paper electrode (0.7 cm × 0.7 cm). The preparation of the CFPE working electrode is as follows. An aliquot of 1.3 mg of electrocatalyst was ground with 0.1 mg polyvinylidene fluoride (PVDF) with a few drops of 1-methyl-2-pyrrolidone (MP) added to the produced mixture. The mixture was added into 10 mL 0.5% Nafion solution. After sonication, 1 mL dispersed solution was dropped directly onto the two sides of a 0.7 cm × 0.7 cm carbon fiber paper (the two sides of the carbon paper were modified by the catalyst). The bulk electrolysis was performed in an airtight electrochemical H-type cell with three electrodes. H-type cell consists of two compartments (volume of each part is 115 mL) separated by a Nafion®212 anion exchange membrane with 75 mL 0.5 M KHCO₃ electrolyte in each chamber and. Besides, LSV experiments were used with the catalyst-modified GCE as the working electrode. The preparation of the GCE working electrode is as follows. 6 mg electrocatalyst was added into 2 mL 0.5% Nafion solution. After sonication, 3 μL dispersed solution was dropped on GCE. The mass density of catalyst was 0.127 mg/cm². The electrochemical tests of different catalysts combinations were performed with full loading, i.e., the mass of the composite catalyst was 9 µg (Supplementary Table 4). The electrochemical surface area (ECSA) test of single catalyst components were performed using partial loading, i.e., 1% of 9 µg CDots and 6% of 9 µg Co₃O₄ (Supplementary Table 4). For LSVs experiments, initially, polarization curves for the modified electrode were carried out under an inert N₂ (gas) atmosphere. After this, the solution was purged with CO₂ (99.999%) for at least 30 min (CO₂-saturated high purity aqueous 0.5 M KHCO₃) and the electrocatalytic CO₂ reduction was measured.

Data availability

The data that support the findings of this study within the paper and its Supplementary Information file are available from the corresponding authors on request.