Interface Catalysts of In Situ-Grown TiO₂/MXenes for High-Faraday-Efficiency CO₂ Reduction

Abstract

Climate change and the global energy crisis have led to an increasing need for greenhouse gas remediation and clean energy sources. The electrochemical CO₂ reduction reaction (CO₂RR) is a promising solution for both issues as it harvests waste CO₂ and chemically reduces it to more useful forms. However, the high overpotential required for the reaction makes it electrochemically unfavorable. Here, we fabricate a novel electrode composed of TiO₂ nanoparticles grown in situ on MXene charge acceptor 2D sheets with excellent CO₂RR characteristics. A straightforward solvothermal method was used to grow the nanoparticles on the Ti₃C₂T_x MXene flakes. The electrochemical performance of the TiO₂/MXene electrodes was analyzed. The Faradaic efficiencies of the TiO₂/MXene electrodes were determined, with a value of 99.41% at −1.9 V (vs. Ag/AgCl). Density functional theory mechanistic analysis was used to reveal the most likely mechanism resulting in the production of one CO molecule along with a carbonate anion through ∗CO, ∗O, and activated CO₂²⁻ intermediates. Bader charge analysis corroborated this pathway, showing that CO₂ gains a greater negative charge when TiO₂/MXene serves as a catalyst compared to MXene or TiO₂ alone. These results show that TiO₂/MXene nanocomposite electrodes may be very useful in the conversion of CO₂ while still being efficient in both time and cost.

1. Introduction

Climate change and the global energy crisis have led researchers to turn to CO₂ reduction as a means of greenhouse gas remediation to produce a clean energy source. The process of harvesting waste CO₂ and chemically reducing it to a more useful form promises to address both issues but is energy-intensive. For this reason, efficient photo-, electro- and photoelectro-catalytic processes to promote the CO₂ reduction reaction (CO₂RR) are of great interest. The use of TiO₂ as a catalyst is broad and multifaceted, which has led to many reviews on the process [1,2,3,4,5,6,7,8,9,10,11]. Of particular interest are the catalytic processes occurring at the TiO₂ surfaces [5,10,11]. Over 50 years ago, TiO₂ was demonstrated as a catalyst in water splitting [12]. By nature, the surface characteristics of a heterogeneous catalyst play a crucial role in its catalytic abilities. To this end, TiO₂ surface structures and properties have been studied extensively with high-resolution scanning tunneling microscopy (STM) [13,14], scanning electron microscopy (SEM) [15,16], and transmission electron microscopy (TEM) [17,18]. TiO₂ has found a role as a catalyst in the reduction of CO₂ as well [19]. However, the catalytic reduction of CO₂ to form methane or methanol on TiO₂ nanoparticles is not without flaws: the close proximity of the reduction and oxidation half-reactions can lead to unwanted carrier recombination, and the reaction pathways proposed for the catalysis are complex and still debated [20,21,22]. As a potential solution, cocatalysts have been introduced to assist in the catalytic CO₂RR by TiO₂. Systems such as TiO₂/Pt, MgO/Pt/TiO₂, Ag/TiO₂, and many more have been investigated for their catalytic potential in CO₂RR and their variable performances have been cataloged [23]. One such cocatalyst, Ti₃C₂ (MXene), shows particular promise due to the high performance of the combined TiO₂/Ti₃C₂ material and the absence of precious materials [24].

MXenes encompass a broad family of 2D layered materials composed of transition metal carbides and nitrides, with applications spanning from environmental remediation to photonics [25,26,27]. Specifically, Ti₃C₂T_x (T=F, O, or OH) was first fabricated in 2011 by Naguib et al. by selectively etching away the Al layer in MAX phase materials (Ti₃AlC₂) [28,29]. Although carbide MXenes take the general form of Ti_n+1C_nT_x, further references to MXene in this work refer specifically to Ti₃C₂T_x. MXenes have found uses in flexible light-emitting diodes (LEDs) [30], hydrogel sensors [31], brain activity recording [32], and adjustable-focus ocular lens devices [33]. Of particular interest are the energy storage capabilities of MXenes and their electrochemical uses [34,35,36,37]. Such charge storage capabilities have been ascribed to the layered structure of MXenes, where it has been shown that Na⁺ ions are reversibly transported into and out of the interlayer space resulting in expansion and contraction, respectively, in aqueous electrolytes [38]. Such intercalation of cations by MXenes has also been demonstrated for other species, both spontaneously and electrochemically, where the 2D sheets become capacitors [39]. In fact, MXene-based solid-state supercapacitors have been successfully deposited onto conductive polymer threads, advancing the usefulness of the material to a flexible system [40]. Due to these properties, MXene is an excellent host material for TiO₂ photocatalysts acting as a charge acceptor and scaffold.

TiO₂/MXene composite materials were developed some time ago and boasted increased performance over the individual components [41,42,43,44,45]. The first demonstration of a TiO₂/MXene catalyst was for the photocatalytic CO₂RR, and was achieved by calcining Ti₃C₂ [24]. This work monitored the CO₂RR to methane and concluded that the mechanism involved the MXene scaffold preventing fast charge recombination at the TiO₂ surfaces and emphasized that the in situ growth of TiO₂ on MXene was crucial in enhancing charge separation. In the same year, a solvothermal process for the fabrication of TiO₂/MXene was published and analyzed for its photocatalytic potential [46], finding that TiO₂/MXene readily degraded carbamazepine through ·OH and ·O₂ species. Electrocatalytically, TiO₂/MXene composites were tested for their ability to reduce N₂ to NH₃, showing superior efficiency under acidic conditions and excellent durability [47]. In an interesting approach, TiO₂ was married to a hybrid Ti₃C₂/Ru cocatalyst which provided enhanced charge separation that promoted the hydrogen evolution reaction (HER) [48]. Increased catalytic activity of TiO₂/MXene over TiO₂ in the oxidative dehydrogenation of ethane has also been observed, where the defects in the composite surface relative to TiO₂ were shown to contribute to a 4× increase in performance [49]. With specific regard to CO₂ reduction, attempts at designing TiO₂/MXene-based materials have been made including C₃N₄/Ti₃C₂T_x/TiO₂ [50,51], Ru-Ti₃CN/TiO₂ [52], TiO₂/Ti₃CN [53], and even tailoring the anatase/rutile moieties of TiO₂ in TiO₂A/Ti₃C₂/TiO₂R MXene [54]. Due to the photocatalytic activity of TiO₂, most of the work to date covering the catalytic CO₂RR by TiO₂/MXene has focused on the photocatalytic process. However, this has led to the direct electrocatalytic counterpart to be overlooked.

Here, we fabricate a novel electrode composed of TiO₂ nanoparticles grown in situ on MXene charge acceptor 2D sheets with excellent electrocatalytic CO₂RR characteristics. A straightforward solvothermal method was used to grow the nanoparticles on the Ti₃C₂T_x MXene flakes for electrochemical CO₂RR. After thorough characterization of the composite material, we used experimental and theoretical analyses to hypothesize a mechanism to explain the high Faradaic efficiency of the CO₂RR.

2. Results and Discussion

TiO₂/MXene catalysts were fabricated as described below and illustrated in Figure 1. To characterize our samples, the XRD scattering patterns of TiO₂/MXene, MXene, and pristine MAX are presented in Figure 2A. After etching, a shift of the (002) peak from 9.64° to 7.28° and the elimination of most intense peaks at 39.06° indicated removal of Al layers in MAX and the formation of exfoliated MXene [55]. After hydrothermal processing, there were two new peaks at 25.76° and 39.14° in TiO₂/MXene, which correspond to the (101) and (004) facets of anatase TiO₂ [56]. Furthermore, the peak shift from 7.28° to 6.42° indicated an increase in the interlayer distance from the exfoliation of TiO₂/MXene after the solvothermal treatment.

Next, we investigated the chemical state and composition of TiO₂/MXene and pristine MXene by X-ray photoelectron spectroscopy (XPS). The presence of Ti, O, C, F, and Cl is shown in Figures 2B–D, S2 and S3. The F and Cl elements present on the surface of MXene are a product of HF etching. For the Ti 2p region, the peaks at 458.3, 459.7, and 464.2 eV correspond to the Ti–O 2p_3/2, Ti–F/Cl, and Ti–O 2p_1/2, respectively. The C 1s region was fitted with three peaks at 284.6, 285.9, and 288.7 eV, ascribed to the C-C, C-O, and O-C-O bonds, respectively [6]. The O 1s region spectra can also be divided into three subpeaks at 529.9, 531.2, and 532.5 eV, which are ascribed to oxygen in TiO₂ (Ti–O–Ti), oxygen in the surface hydroxyl group (Ti–O–H), and C–O–Ti, respectively. Compared with MXene [57], all signals in TiO₂/MXene displayed negative shifts, indicating an increased electron transfer from MXene to TiO₂. As a result, a strong interfacial interaction can be achieved at the TiO₂/MXene interface, which may enhance the performance toward electrocatalyzing the CO₂ reduction.

The morphology and microstructure of the synthesized TiO₂/MXene were characterized by TEM and HRTEM. As shown in Figure 3A, the exfoliated MXene nanosheets showed single (or few)-layered and flake-like morphology using TEM. TiO₂ nanoparticles with a size of 10–20 nm were also observed to be uniformly distributed on the surface of MXene nanosheets. As shown in Figure 3B, lattice fringes with a spacing of 0.244 nm were observed, which were ascribed to the (103) plane of anatase TiO₂. Energy-dispersive X-ray (EDX) characterization was also conducted and the atomic ratio of Ti:O was found to be 2.96:1 (Figure S4). The elemental mapping results of the TiO₂/MXene further confirm that the distribution of Ti and O elements of the in situ formation of TiO₂ was monodisperse on the MXene surface, as shown in Figure 3C. TiO₂ nanoparticles were grown in situ on MXene surfaces, endowing robustness to the microstructure and morphology.

The electrochemical conversion of CO₂ to CO was investigated by cyclic voltammetry (CV) and controlled potential coulometry (CPC). Figure 4A shows the CVs collected using the TiO₂/MXene electrocatalyst in the electrolyte saturated with N₂ (black) or CO₂ (red) gas. Here, we see a clear difference between the CVs upon introduction of CO₂ during both increasing and decreasing potentials. In the N₂-purged solution, TiO₂/MXene showed a typical capacitive profile due to the CP support. The peak at −1.3 V is attributed to reduction from Ti⁴⁺ to Ti³⁺ on the TiO₂ surface. With the CO₂-saturated electrolyte, an apparent cathodic current was observed, corresponding to efficient CO₂ conversion. Figure 4B shows the CPC scans of the N₂-(black) or CO₂-purged (red) electrolyte at a constant potential of −1.9 V. As such, the TiO₂/MXene nanocomposites show significant potential-dependent CO₂RR catalytic properties.

To investigate the performance of the TiO₂/MXene catalyst during CO₂RR, the Faradaic efficiency for CO production (FE_CO) was calculated for various potentials. As shown in Figure 4C, it was observed that CO production started at potentials more negative than −1.4 V and peaked at 99.41 ± 0.50% at −1.9 V. A further increase in potential led to a decrease in efficiency due to hydrogen evolution occurring at high potentials. It also is important to note that CO was not produced in the N₂-saturated electrolyte (Figure S2), indicating that the generated CO originated from the electrocatalytic reduction of CO₂, rather than from ACN or MXene decomposition.

To ensure that TiO₂/MXene was responsible for CO₂RR and not its components, we compared the FE_CO of TiO₂/MXene, MXene, and commercial TiO₂ at −1.9 V. As shown in Figure 4D, the TiO₂/MXene electrode exhibits a near 100% FE_CO; however, almost no CO₂ electroreduction resulted from electrodes fabricated using pure MXene or commercial TiO₂. These results indicate the efficient CO₂ reduction arises from the TiO₂/MXene composite material. To confirm this hypothesis, the catalytic performance of the TiO₂/MXene nanocomposite using different in situ growth times was studied (Figure S6). We found that increasing growth times up to 12 h produced higher FE_CO values, but excessive growth time reduced FE_CO. We hypothesize that the observed decrease in FE_CO was due to poor conductivity caused by the excess synthesis of TiO₂ on MXene, as confirmed by the electrochemical impedance spectroscopy (EIS) results in Figure S7. To demonstrate the stability of our TiO₂/MXene system, CO₂RR was performed at increasing timescales from 30 min to 3 h. As depicted in Figure S8, after 3 h of electrocatalysis, FE_CO only decreased by 13%, demonstrating certain stability for potential high-throughput applications. Overall, our results suggest that the in situ growth of TiO₂ on MXene sheets produces efficient catalytic sites for CO₂RR with high Faradaic efficiency and selectivity, which is an improvement compared to the other non-precious electrocatalyst materials reported, such as Bi nanoparticles [58], TiS₃ [59], and TiO₂ [60], as shown in Table S1.

To best understand the CO₂RR mechanism on TiO₂/MXene, density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package with the Perdew–Burke–Ernzerh functional [61,62,63,64]. The projector-augmented wave pseudopotentials were implemented for the exchange correlation function and pseudopotential treatment [65,66]. The structure of the TiO₂/MXene used in these considerations is shown in Figure 5A. The calculations were conducted on three systems: (1) TiO₂ nanoparticles, (2) MXene, and (3) TiO₂/MXene. The kinetic cutoff energy was 600 eV throughout the simulation. The Brillouin zone was sampled by a Monkhorst–Pack grid centered at the γ point with a k-point mesh of 3 × 3 × 3 for TiO₂ and 3 × 3 × 1 for MXene and TiO₂/MXene. All structures analyzed by DFT were fully optimized, and computations were continued until they met the convergence thresholds of having a total energy of 10⁻⁶ eV and atomic forces not exceeding 0.05 eV/Å. Other model details, e.g., the sizes of MXene (112 atoms) and TiO₂ nanoparticles (42 atoms), the construction of slab models, and the nanoparticles (Figure S9), are included in the Supplementary Materials. The size of systems used for modeling was selected as a balance between computational efficiency and accuracy [61]. The analysis suggests that the (100) direction of TiO₂ nanoparticles is most likely to be perpendicular to the (110) direction of MXene (Figure S10). Furthermore, the strong affinity between the TiO₂ nanoparticles and MXene is primarily governed by the interaction involving oxygen atoms of TiO₂ and titanium atoms of MXene, facilitated through oxygen dangling bonds. The resulting TiO₂/MXene model is illustrated in Figure 5A and serves as the system upon which subsequent calculations are based.

To perform the adsorption energy analysis of CO₂ onto the TiO₂/MXene, we examined five geometrically unique adsorption configurations, as shown in Figure 5B, where the negative shift in adsorption energy indicates that the process is favorable. Interestingly, the adsorption free energy magnitude is comparatively weaker than that of other catalytic systems, placing TiO₂/MXene within the optimal range for catalytic adsorption of CO₂ [67,68]. From these results, we can ascertain that the CO₂ molecules are adsorbed above the 17th and 12th Ti atoms, which are selected for further analysis. It is worth noting that the pattern of adsorption energy does not exist when CO₂ is adsorbed onto MXene or TiO₂ (Figures S11 and S12), where the elevated adsorption energy could potentially poison the adsorbent.

A Gibbs free energy change analysis was conducted to understand the energetics of intermediates while the reaction proceeds. The analysis was performed following three possible reaction paths for CO₂ reduction in an aprotic environment. Step (R1) is the adsorption of the first CO₂ molecule and reaction. Step (R2) is the successive adsorption of the second CO₂ molecule onto the catalytic system. The reactions given in steps (R3a–c) represent the possible reductive paths of the two adsorbed CO₂ molecules. Figure 5C depicts the Gibbs free energy changes for reactants and intermediates present in the reaction on TiO₂/MXene, compared with pure MXene and TiO₂ nanoparticles.

$$\ast +\mathrm{C}{\mathrm{O}}_2\to \ast \mathrm{C}{\mathrm{O}}_2$$

(R1)

$$\mathrm{C}{\mathrm{O}}_2+\ast \mathrm{C}{\mathrm{O}}_2$$

(R2)

$$2\ast \mathrm{C}{\mathrm{O}}_2\to \ast \mathrm{C}\mathrm{O}+\ast \mathrm{O}+\ast \mathrm{C}{\mathrm{O}}_2\to \ast \mathrm{C}\mathrm{O}+\mathrm{C}{\mathrm{O}}_3^{2-}$$

(R3a)

$$2\ast \mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to 2\ast \mathrm{C}{\mathrm{O}}_2^{2-}\to \ast \mathrm{o}\mathrm{x}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{e}({\mathrm{C}}_2{\mathrm{O}}_4)$$

(R3b)

$$2\ast \mathrm{C}{\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O}+{\mathrm{O}}_2$$

(R3c)

Our free energy analysis leads to three unique systems for the potential catalysts, as shown in Figure 5C. (1) In the case of pure MXene (brown lines), we observe a gradual change in free energy as CO₂ molecules are successively adsorbed. This incremental change suggests that reductive reactions on pure MXene surfaces are less likely to occur. (2) In the case of pure TiO₂ nanoparticles (orange lines), the free energy change undergoes a significant negative shift upon adsorption of the first and second CO₂ molecules. These adsorption processes lead to the formation of an oxalate group, accompanied by a substantial negative change in free energy (−9.20 eV). This oxalate group eventually dissociates into two CO₂ molecules. (3) The case of the TiO₂/MXene catalytic system is more nuanced. Initially, there is a slight positive change in free energy (0.07 eV) upon the adsorption of the first CO₂ molecule, which reduces it to 0.02 eV when the second CO₂ molecule is adsorbed near the first. In this scenario, three possibilities arise: (R3a) the two dimerized CO₂ molecules can form one oxalate, (R3b) two CO molecules and one O₂ molecule may be generated, and R3c) the formation of one CO molecule along with a carbonate anion (CO₃²⁻) via intermediates such as ∗CO, ∗O, and activated CO₂²⁻. Of these options, the first and second cases exhibit positive changes in free energy (0.06 eV and 6.64 eV, respectively), making these reactions less favorable. However, in the third scenario, the formation of one CO molecule and one CO₃²⁻ shows a negative change in free energy (−0.51 eV), indicating that the CO₂RR is energetically favorable in this direction. Further support for these findings comes from the Bader charge analysis shown in Figure 5D, where the net charge on adsorbed CO₂ molecules reveals that they gain a higher negative charge when TiO₂/MXene serves as a catalyst compared to pure MXene or TiO₂. The results of the Bader analysis are visualized in Figure 5E, where the yellow shading indicates areas of charge accumulation, and cyan shows those of charge depletion. This suggests a significant charge transfer from TiO₂/MXene to CO₂ molecules. In the future, other product analyses may be performed including in situ spectroscopy [69], and possibly interfacial intermediate analyses through sum-frequency generation spectroscopy. Additionally, while our TiO₂/MXene electrocatalyst showed high FE for the CO₂RR to CO, it is at the cost of a high overpotential of 1.9 V vs. Ag/AgCl. Future strategies to reduce this overpotential include composite materials such as TiI₄ to modify the electronic structure and promote more marketable reaction conditions [70].

3. Experiment

Materials. Ti₃AlC₂ (MAX) powders were purchased from Forsman (Beijing, China). Carbon paper (HCP020N, CP) was purchased from Hesen (Shanghai, China). Nickel foam (NF) was provided by Hefei Kejing Materials Technology (Hefei, Anhui, China). Nafion@117 solution was purchased from Sigma-Aldrich (St. Louis, MO, USA). Acetonitrile was purchased from Fisher Chemical (Pittsburgh, PA, USA). Ethanol and hydrochloric acid were purchased from Pharmco (Brookfield, CT, USA). Lithium fluoride was purchased from Alfa Aesar (Ward Hill, MA, USA). Potassium hexafluorophosphate was purchased from Oakwood Chemical (Estill, SC, USA). Deionized (DI) (18.2 MΩ cm) water was produced by using a Milli-Q ultrapure system (Thermo Scientific Barnstead E-Pure, Waltham, MA, USA).

Preparation of MXene. Ti₃C₂T_x MXene was obtained via an HF etching method [71]. A total of 100 mg of LiF was added into 2 mL of 9 M HCl solution in an ice bath before 100 mg of MAX was added into the solution. After etching at 35 °C for 24 h, the obtained MXene was washed 8 times with DI water and centrifuged at 3000 rpm for 1 h. The collected MXene was freeze-dried and stored in a vacuum desiccator.

Preparation of TiO₂/MXene. As illustrated in Figure 1, 20 mg of Ti₃C₂T_x MXene was dispersed in 2 mL ethanol and sonicated at 400 W for 2 h. The suspension was transferred into a 40 mL PTFE autoclave for solvothermal treatment at 120 °C for 12 h. Afterward, the as-prepared suspension was centrifuged at 3000 rpm for 20 min. The obtained solid was freeze-dried and stored in a vacuum desiccator.

Characterizations. X-ray diffraction (XRD) patterns were analyzed by using a Rigaku Miniflex II X-ray diffractometer (Rigaku Americas, The Woodlands, TX, USA) with Cu Kα radiation (λ = 1.5406 Å). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) images, and elemental mapping analysis were collected using a FEI Tecnai G2 F20 FE-TEM (FEI, Hillsboro, OR, USA) microscopy system. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra instrument (Kratos Analytical Ltd., Manchester, UK).

Preparation of the Working Electrode. A total of 4 mg of th TiO₂/MXene catalyst was suspended in a mixture of 960 μL Di-water and 40 μL Nafion@117 solution, followed by a 2 h sonication to form a homogeneous ink. A total of 100 μL of the ink (0.4 mg of catalyst) was deposited onto a carbon paper strip with an area of 1 cm² and dried in a vacuum desiccator.

Electrocatalytic CO₂ Reduction. All potentials are reported with respect to the Ag/AgCl reference electrode. Electrochemical CO₂ reduction was performed using a Gamry Interface 1000 electrochemical workstation. All electrochemical measurements were conducted in a three-electrode gas-sealed single-compartment glass cell, as shown in Figure S1. TiO₂/MXene supported on CP (0.4 mg/cm²) was used as the working electrode while Ag/AgCl (0.1 M KPF₆ in acetonitrile) and nickel foam (NF) (2 × 1 cm) electrodes were used as the reference and counter electrodes, respectively, and conducted in a 0.1 M KPF₆ acetonitrile solution. The electrolyte solution was purged with CO₂ for 30 min before measurement. Cyclic voltammogram (CV) scans were performed at 10 mV/s with IR compensation. Electrochemical impedance spectroscopy (EIS) analysis was performed at −1.9 V vs Ag/AgCl in the frequency range of 10⁴–10⁻² Hz. Controlled potential coulometry was performed at −1.9 V for 30 min. Gas products produced from electrolysis were measured by an SRI 8610C (SRI Instruments, Torrance, CA, USA) gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID), using argon as the carrier gas. Faradaic efficiency (FE) for CO was calculated by

$$\epsilon ={\displaystyle \frac{e\times F\times M}{Q}}\times 100\%,$$

(1)

where ε is Faradaic efficiency, $F=96485 \mathrm{C}/\mathrm{m}\mathrm{o}\mathrm{l}$ is Faraday’s constant, $e=2$ is the number of transferred electrons for CO₂RR, M is the amount of CO (mol) and is obtained from the corresponding peak area of the FID curve from GC experiments, and Q is the charge obtained from the reaction.

4. Conclusions

In this work, we demonstrated the catalytic ability of novel TiO₂/MXene nanocomposite electrodes for an efficient electrochemical CO₂RR. The nanocomposite TiO₂/MXene electrodes were easily fabricated using cost-effective solvothermal and drop-cast techniques. Electrochemical characterization of the electrode through cyclic voltammetry showed that it behaved as a capacitor in the absence of CO₂ but a cathodic current in its presence indicated a reduction reaction at the electrode. The electrochemical performance of the CO₂RR to produce CO showed an excellent FE of 99.41% at −1.9 V (vs. Ag/AgCl), by far outperforming the individual components. A DFT-level analysis of the CO₂RR on TiO₂/MXene electrodes revealed the likely mechanism producing CO and a carbonate anion through ∗CO, ∗O, and activated CO₂²⁻ intermediates, as well as two less likely pathways which resulted in an oxalate or two CO molecules with one O₂. Subsequent Bader charge analysis showed that TiO₂/MXene behaves as a catalyst in the CO₂RR compared to its TiO₂ or MXene components alone. Additional work is still needed to confirm the proposed reaction mechanism by quantifying reaction products through mass-spectrometry and monitoring intermediate formation through sum-frequency generation spectroscopy. In sum, easy-to-produce TiO₂/MXene nanocomposite electrodes may hold great utility in the harvesting and use of atmospheric CO₂ through an efficient CO₂RR to combat the pressing global climate and energy crises.