Large‐Scale Synthesis of Multifunctional Single‐Phase Co2C Nanomaterials

Abstract Achieving scalable synthesis of nanoscale transition‐metal carbides (TMCs), regarded as substitutes for platinum‐group noble metals, remains an ongoing challenge. Herein, a 100‐g scale synthesis of single‐phased cobalt carbide (Co2C) through carburization of Co‐based Prussian Blue Analog (Co‐PBA) is reported in CO2/H2 atmosphere under mild conditions (230 °C, ambient pressure). Textural property investigations indicate a successful preparation of orthorhombic‐phased Co2C nanomaterials with Pt‐group–like electronic properties. As a demonstration, Co2C achieves landmark photo‐assisted thermal catalytic CO2 conversion rates with photo‐switched product selectivity, which far exceeds the representative Pt‐group‐metal–based catalysts. This impressive result is attributed to the excellent activation of reactants, colorific light absorption, and photo‐to‐thermal conversion capacities. In addition to CO2 hydrogenation, the versatile Co2C materials show huge prospects in antibacterial therapy, interfacial water evaporation, electrochemical hydrogen evolution reaction, and battery technologies. This study paves the way toward unlocking the potential of multi‐functional Co2C nanomaterials.

indicated that both the CO2 and H2 gas were indispensable for Co2C formation. Figure S2c indicated that the traditional metal/metal oxides precursors could not be converted to Co2C, in the same carbonization condition, reflecting the advantage of Co-PBA precursor for Co2C production. As a special class of coordinated frameworks, the Co atoms in PBA could be replaced partially or entirely by other common transition metals and noble metals, [1] and  Figure S4. XRD patterns of the Co2C materials prepared using CO/H2 at different temperatures. For example, the 250°C-CO/H2-10h described the sample prepared at 250 °C with CO/H2 flow rates of 10/10 sccm for 10 hours.
7 Figure S5. The MS spectrum of the collected CO2 gas samples from Co2C oxidation at 400 °C after switching 13 CO2/H2 flow to O2 for 10 min.

Notes:
The Carbon-13 isotope tracing experiments were conducted in the miniature photo-assisted thermal catalytic micro reactor. To figure out the source of the carbon element in Co2C, the Co-PBA was directly added into the photo-assisted micro reactor and heated to 300 °C under light irradiation and maintained for 10 h in an Ar atmosphere to remove the surface interfering molecule.
Subsequently, a mixture of 13 CO2/H2 (4/16 sccm) was introduced into the reactor and the compositions of gas samples from the outlet of reactor were analyzed by an online GC. After GC spectra stabilized for 5 h (considered that the Co-PBA has entirely transformed into Co2C, confirmed in Figure S2a) carbon dioxide gas. The CO2 samples were collected as well. Figure S5 indicated that the carbon element in the in-situ formed Co2C was dominantly 13 C labelled, confirming the carbon element in Co2C originated from both 13 CO2 gas and Co-PBA ( 12 C) precursor.

Notes:
In terms of carbon element, as shown in Figure S6a, the reactant was 13 CO2.
However, after hydrogenation process, the 12 C labelled products were more dominant compared with the 13 C labelled products ( Figure S6b). Significantly, the CO2 redundant from the reactor outlet was primarily 12 C labelled (m/z = 44) rather 13 C labelled (m/z = 45), meaning that a dynamic chemical exchange process between Co2 12 C and Co2 13 C existed during the reaction. As the carburization proceeds, the 12 C element in Co2C was gradually substituted by 13 C element, and the results were also in line with the observations in Figure   S5. 14 Figure S8. XRD pattern of (NH4)CO2(OH) product collected after the Co2C was prepared at the outlet of the reactor.

Notes:
We scratched the white powders at the outlet of the reactor after Co2C preparation and analyzed them using XRD to confirm the compositions. It is found that (NH4)CO2(OH) was produced accompanied with the formation of Co2C.

Binding energy C 1s
Co-C C-C 284.8 eV Figure S12. C 1s XPS spectrum of Co2C.

Notes:
As depicted in Figure S19, the prepared Co2C powder exhibited superparamagnetic behavior without saturation magnetization, which agrees with previous reports, [2] further demonstrating the high purity of Co2C materials in this work and excluding the existence of metallic Co and Co3C species in the samples.

Notes:
In this work, for the photo-assisted thermal catalytic process, heat was provided by the furnace, and the corresponding set reactor temperature was denoted as Te. The actual catalyst surface temperature, denoted as Tc, was measured by the thermocouple, which inserted into the middle of catalyst layer (ca. 0.25 mm 24 to the catalysts surface). The inner dimensions of the quartz flow reactor were 24 mm × 24 mm × 2 mm, and the diameter of the thermocouple was 1.5 mm.
In dark, Tc = Te. Under light illumination, owing to the photothermal effect of the catalyst, Tc was always higher than Te.
25 Figure S21. GC spectrums of products during photo-assisted thermal CO hydrogenation over Co2C: a) FID 1 and b) FID 2 under different temperature with/without light illumination.

Notes:
The photo-assisted thermal CO hydrogenation performance over Co2C is shown in Figure S21. Under light irradiation, the Co2C catalyst efficiently catalyzed CO hydrogenation to CH4 with high production selectivity, confirming the feasibility of the hydrogenation of adsorbed CO (*CO) to CH4 under illumination.   Figure S25. Molecular-level mechanism from DFT calculations for CO2 hydrogenation into methane.

Notes:
As shown in Figure S26a, under 8 suns light irradiation, the Co2C materials achieved nearly 100% inactivation against S. aureus and E. coli within 15 min.
Photo-to-thermal conversion tests in Figure S26b

Notes:
As shown in Figure S27b

Notes:
As the electrocatalyst for hydrogen evolution reaction, Co2C materials exhibited high electrocatalytic activity. As shown in Figure S28a,  selected discharge/charge profiles of the Co2C cathodes with fixed specific capacities of 500 mAh g −1 from cycling performance; and f) cycling performance of Co2C cathodes at 100 mA g −1 .

Notes:
As shown above, pristine Co2C exhibited a large overpotential as a Li-O2 battery cathode material. The CV curves in Figure S29a with a large current density and integral area indicate good electrocatalytic activity of Co2C materials, and the EIS plots in Figure S29b with a small semicircle diameter indicate good electronic conductivity. As shown in Figure S29c, the initial full discharge/charge profiles confirmed the good output capacity, and at a current density of 75 mA g −1 , Co2C delivered a specific capacity of 7814 mAh g −1 , which is close to that of the reported MXene materials. [4] The rate performances of Co2C at a fixed specific capacities of 500 mAh g −1 in Figure S29d suggested that with the rise in current density from 50 to 400 mA g −1 , the charge potential plateau of Co2C increases from 4.17 to 4.42 V and recovers back to 4.16 V when the current is switched back to 50 mA g −1 , reflecting the favorable reversibility. The Co2C cathode also delivered excellent cycling performance, with stable terminal voltages for 60 cycles, suggesting good electrocatalytic stability (Figure S29e-f).

Notes:
As shown in Figure S30a, the profile of the initial cycle in CV with peaks was different from those of the subsequent ones because of the formation of solid electrolyte interface (SEI) films with the preliminary decomposition of the electrolyte and other irreversible reactions. [5] During the subsequent anodic scan, the overlapped cathodic and anodic peaks indicated abundant redox  Figure S30b). The EIS plot with a small semicircle diameter indicates good electronic conductivity of the Co2C materials ( Figure S30c). Co2C achieved a superior rate performance of 0.05 to 2 A g −1 .
As depicted in Figure S30d,
[b] In the batch reactors.
[e] CO production rate, the unit is mmolCO g -1 cat h -1 .