Device design for contact-electro-catalytic CO2RR
The schematic structure of TENG for contact-electro-catalytic CO2RR is shown in Fig. 1a. As the electropositive tribolayer of TENG, quaternized ammoniated cellulose nanofibers (CNFs) have a strong ability to adsorb CO2. Furthermore, because of the abundance of hydroxyl groups, quaternized CNFs can form hydrogen bonds with water molecules in high-humidity environments, allowing them to fix water molecules and participate in triboelectrification, hence improving TENG electrical output performance in high-humidity conditions.31,32 The preparation process of quaternized CNF, physical photo and TEM image are shown in Fig.S1. The electronegative tribolayer of TENG is electrospun PVDF loaded with single Cu atoms-anchored polymeric carbon nitride (Cu-PCN). Cu-PCN is supported by PVDF fibers, allowing the catalyst to make complete contact with the catalytic substrate. In addition, the combination of PCN with excellent piezoelectric properties33–36 and PVDF will boost the electron density of the electronegative tribolayer and accelerate the progress of CO2RR. The schematic diagram of the contact-electro-catalytic reaction of CO2 on Cu-PCN surface is shown in Fig. 1b. Initially, CO2 molecules adsorb on the electrode surface, activating them and increasing their reactivity. CO2 molecules are then gradually reduced to\({\text{C}\text{O}}_{2}^{-}\), an intermediate species with a negative charge, thanks to the controlled electron transfer. Following the deoxygenation process of \({\text{C}\text{O}}_{2}^{-}\), an oxygen atom is released, catalyzing the generation of CO molecules as the primary end product. The protons required for this course of action are produced by the ionization of water on the tribolayer's surface. The specific reaction process of CO2 on the electrode surface of TENG is shown in Fig.S2.
The preparation process of Cu-PCN is shown in Fig.S3. In order to distribute Cu-PCN evenly on PVDF fibers, ultrasonically dispersed Cu-PCN was thoroughly mixed with PVDF solution before electrospinning. The optimal mass ratio of catalyst to PVDF is 1:100, which ensures that the catalyst will not agglomerate due to excessive concentration. The physical photo, SEM images and EDS of Cu-PCN@PVDF are shown in Fig. 1c, S4a and S5a-4f, and the photo of excess Cu-PCN agglomeration is shown in Fig.S4b. The inset of Fig. 1c shows the TEM image of Cu-PCN@PVDF, indicating that Cu-PCN is tightly attached to the PVDF fiber surface. In order to improve the utilization rate and selectivity of copper atoms in the catalyst, the copper supported in PCN has a single-atom structure. Figure 1d shows the spherical aberration electron micrographs of single-atom copper in Cu-PCN. The abundant luminous dots evident in the image align with the profoundly dispersed configuration of the single-atom copper catalyst.37–39 This observation underscores the catalyst's existence as solitary copper atoms firmly situated on the catalyst substrate. Moreover, the even distribution of these luminous points signifies the catalyst's commendable dispersion, an attribute pivotal to ensuring steadfast catalytic performance. To further demonstrate the existence of single-atom copper in Cu-PCN, extended X-ray fine structure spectroscopy (EXAFS) spectra and fourier-transformed EXAFS spectra of Cu-PCN were tested, as shown in Fig. 1e and 1f. The absorption edge for Cu-PCN locates between those for CuO and Cu2O, indicating the coexistence of Cu+ (which is the predominant oxidation state) and Cu2+ in Cu-PCN.40 The structural parameters were obtained by fitting the EXAFS data at the Cu K-edge (Fig.S6 and Table S1). Magnitudes of the FT of the EXAFS data at Cu K-edge are shown in Fig. 1f. The main symmetrical peak in Cu foil at ≈ 2.2 Å results from the Cu-Cu bond, which cannot be observed in Cu-PCN. Instead, a peak at ≈ 1.6 Å is visible that can be ascribed to the Cu-N bond.
The moisture resistance of TENG is the key to ensure the successful progress of CO2RR. In previous studies, we reported the moisture-resistant mechanism of bio-based TENG, that is, the hydroxyl groups on the surface of the material can combine with water molecules in the environment to participate in triboelectrification and enhance the electrical output of TENG in high-humidity environments.41 As a hydroxyl-rich bio-based material, quaternized CNF also has outstanding moisture resistance performance, as shown in Fig. 1g and S5. The TENG can generate a current of 18.7 µA and a voltage of 405 V at 99% RH. The electrical output of TENG increases with humidity (Fig.S7a), and the infrared spectrum at 100°C shows a red-shift of the hydroxyl absorption peak of the quaternized CNF after water absorption (Fig.S7b), indicating the formation of hydrogen bonds. Furthermore, the introduction of positively charged quaternary ammonium salts42 and PCN with piezoelectric properties increased the electrical output performance of the quaternized CNF-Cu-PCN@PVDF-based TENG compared with pure CNF-PVDF-based TENG, as shown in Fig.S8. Based on the high output under high humidity, TENG can reduce CO2 to CO under normal conditions of pressure, temperature, and high humidity. Comparison of CO signal peaks before and after CO2RR tested by Gas chromatography (GC) is shown in Fig. 1h. Before the test, the electrical output of the TENG was accumulated for hours (4–8 h) under pure argon atmosphere to saturate the charge (current about 18.5 µA). Then, 1 LCO2 gas was introduced into the airtight box. The airtight box was filled with PTFE blocks to keep its effective volume at 5 L. Simultaneously, GC measured the concentration of CO2 as it was injected. At the moment when CO2 was just injected, the GC did not detect the signal peak of CO, indicating that there was no additional CO in the airtight box. After a 5-hour duration of the reaction, GC analysis revealed a distinct peak signal of CO, suggesting the effective reduction of CO2 within the sealed environment, ultimately yielding CO as the resulting product. Underlining the significance is the GC analysis, which showcased a noticeable reduction in CO2 levels before and after the reaction. However, it is essential to acknowledge the influence of high humidity, as CO2 can be converted to carbonic acid, possibly affecting its detectable levels in tightly controlled humid conditions.
Performance of contact-electro-catalytic CO2RR
The Cu-PCN catalyst is essential for promoting electron transport and accelerating the CO2RR reaction. Various Cu amounts and catalyst loadings were evaluated on the product yield to completely analyze the influence of Cu-PCN on contact-electro-catalytic CO2RR, as shown in Fig. 2a and 2b. The results show that when Cu content increases, it has a higher catalytic efficiency in CO generation. In contrast, since the TENG generates a high voltage of 405 V, which is sufficient to drive CO2RR, the PCN control device (without Cu) could also generate a small quantity of CO in contact-electro-catalytic CO2RR. However, due to the lack of electron-enriching effect, the electrons transferred to CO2 will be very limited, which leads to an insignificant yield in CO production. In addition, the yield of CO increases with the higher loading of Cu-PCN. Likewise, TENG without Cu-PCN loading can generate an extremely small amount of CO, which is consistent with previous observation. Even while the CO yield of the electrospun PVDF-based TENG with 1.5% Cu-PCN loading is greater than 1%, the excessively high catalyst content causes Cu-PCN to aggregate, which negatively impacts the fibers' formability and strength. Subsequently, we calculated the number of transferred charges involved in CO2RR, as shown in Fig. 2c. The transferred charge is obtained by real-time current integration during the CO2RR process, and the peak shape of the transferred charge at the beginning and end of the real-time current is shown in the inset. The results show that the amount of transferred charge in the process of contact-electro-catalytic CO2RR is 31.5 mC.
To identify the origin of CO, isotopically pure 13CO2 gas was used as the source in the contact-electro-catalytic CO2RR, in which the products were detected by GC-MS. As shown in Fig. 2d, after 13CO2 was triboelectrically reduced for 5 h in a closed box, the product was detected by GC. The results showed that there were signal peaks of CO2 (6.8 min retention time) and CO (21.5 min retention time) in the GC spectrum. Since the airtight box is filled with argon gas, and the volume of injected 13CO2 is 1 L, after the reaction, the main components of the gas in the box are Ar, 13CO2 and 13CO. Subsequently, GC-MS was employed to further investigate the source of CO. In order to enhance the detection sensitivity of target compounds, selected ion monitoring (SIM) mode was adopted to detect the presence of 13CO (m/z = 29) in the mixed gas. The results showed that the absorption peak of CO was detected at the retention time of 1.43 min (Fig. 2e). CO outputs three primary peaks in the mass spectrum (Fig. 2f). The strongest MS signal at m/z = 29.1 belonged to 13CO which generates two other fragments at m/z = 13 (13C) and m/z = 16.1 (O). These results clearly suggest that CO is derived from contact-electro-catalytic CO2RR. It should be noted that O at m/z = 16.1 in the MS spectrum has a dominant signal, which may be caused by ambient oxygen carried over by the gas mixture when it was injected into the GC.
Additionally, an excellent stability in a 35-h cyclic experiment of contact-electro-catalytic CO2RR was observed in Fig. 2g. The used Cu-PCN@PVDF electrode was dried by a blower before using in each cycle of experiments. After 7 cycle runs, the yield of the reduced products remained high at about 240 molg− 1, suggesting a continuous production of electrons on the surface of Cu-PCN@PVDF for CO2RR under contact-separation. The output current of the TENG was recorded during each cycle run, as shown in Fig. 2h. The results show that the initial current value (saturation current) of TENG is about 18.6 µA in 7 cycles, and the current after catalytic reaction for 5 h is about 16.5 µA, which shows the stability of the electrical output during CO2RR. Based on the calculation of the transferred charge for contact-electro-catalytic CO2RR, we calculated the CO Faradaic efficiency. The specific calculation process is shown in Table S2. The CO Faradaic efficiency of contact-electro-catalytic CO2RR is calculated as 96.24% compared with conventional electrocatalysis, as shown in Fig. 2i. The contact-electro-catalytic CO2RR stands out as a notably elevated achievement among all the reported literature touting CO Faradaic efficiencies over 90%.43–49
Mechanism of contact-electro-catalytic CO2RR
In order to explore the mechanism of CO2RR using contact-electro-catalysis, we analyzed the adsorption performance of the materials and the catalytic process. First, the temperature programmed desorption (TPD) of CO2 adsorption by quaternized CNF and Cu-PCN@PVDF was tested, as shown in Fig. 3a. The results show that Cu-PCN@PVDF only has a TCD signal peak at 78.8°C, indicating the physical adsorption between Cu-PCN and CO2. In the case of quaternized CNF, TCD signal peaks manifest at both 92.4°C and 205°C, highlighting robust chemical adsorption between the quaternary amino functional group and CO2. This distinctly underscores the pivotal role of the quaternized CNF surface as the primary locus for CO2 adsorption during the contact-electro-catalytic CO2RR process. Fig.S9 displays the results of the thermogravimetric analysis (TGA) on both the pure and quaternized CNF membranes. According to the findings, the weight of the quaternized CNF film is reduced by 10% at a temperature of 314°C. Therefore, the quaternized CNF will not degrade at 205°C. Furthermore, we tested the CO2 vapor adsorption of quaternized CNF and Cu-PCN@PVDF at normal temperature and pressure, as shown in Fig. 3b. We chose high concentration (20%) and low concentration (0.02%) CO2 as gas source and Ar as supplementary gas. Assessing adsorption under varying concentrations of CO2 allows for a comprehensive evaluation of the CO2 adsorption capabilities of both quaternary ammoniated CNF and Cu-PCN@PVDF materials which is crucial in identifying the specific adsorption sites that play a pivotal role in the contact-electro-catalytic CO2RR. The results show that when the adsorption reaches equilibrium, quaternized CNF always has a greater adsorption capacity than Cu-PCN@PVDF, regardless of CO2 concentration, which is consistent with the CO2-TPD test results. Furthermore, to examine the parameters affecting the CO2/N2 selectivity, Henry's law CO2/N2 selectivity was tested using TG-MS, as shown in Fig.S10. When the gas mixture (50% CO2 and 50% N2) was desorbed under Ar vapor, the MS recorded the area ratio of CO2 to N2 to determine the CO2/N2 selectivity. The results show that the Henry's law CO2/N2 selectivity of the quaternized CNF is about 80%. Figure 3c presents a comprehensive comparison between quaternized CNF and other materials exhibiting exceptional CO2 adsorption capabilities.50–52 These materials are ranked based on various metrics that are crucial for practical applications, encompassing adsorption capacity, adsorption rate, thermal stability, adsorption selectivity, and cycle stability. Remarkably, quaternized CNF emerges as the superior performer among its counterparts. The ability to achieve strong adsorption is crucial to enable contact-electro-catalytic CO2RR.
The output current of TENG in the contact-electro-catalytic CO2RR process will exhibit distinctive fluctuations under the influence of the two tribolayers on CO2 adsorption. As shown in Fig. 3d, the output current shows an initial fall followed by a climb when there are high CO2 levels present. This observation is caused by the 20% extra CO2 concentration. Under this condition, CO2 is first adsorbed by the electropositive quaternary ammoniated CNF, leading to a decrease in current. This is owing to the electron-withdrawing effect of the carbonic acid double bond, which will withdraw electrons during the triboelectrification process, resulting in a drop in the hole density of the electropositive quaternized CNF.53,54 As the reaction proceeds, surplus CO2 gradually finds residence within the interstices of PVDF electrospun fibers. This gradual accommodation imparts heightened electronegativity to the Cu-PCN@PVDF tribolayer, thereby engendering a commensurate augmentation in electrical output. However, when we eliminated the effect of the quaternary amino functional groups in the quaternized CNFs, the current showed a single changing trend, as shown in Fig. 3e. After the electropositive tribolayer quaternized CNF was replaced by pure CNF, CO2 was only adsorbed on the Cu-PCN@PVDF tribolayer, which led to an increase in the electron density on the electronegative tribolayer and a consequent increase in the electrical output. Nevertheless, with a reduction in CO2 concentration to 0.02%, a distinct shift in the output current pattern emerged. Precisely, when employing the electropositive quaternary ammoniated CNF as the tribolayer in TENG, predominant adsorption takes place upon the quaternary ammoniated functional groups, leading to an ensuing continuous decrement in current. Similarly, upon substitution of quaternary ammoniated CNF with pure CNF, the locus of CO2 adsorption transitioned to Cu-PCN@PVDF, consequentially fostering an elevation in the output current. Given that the adsorbed quantity of CO2 at diminished concentrations markedly contrasts with that at higher concentrations, a corollary emerges for pure CNF-based TENG. In this context, the surge in current at heightened concentrations surpasses the increment observed at lower concentrations.
Subsequent experimentation delved into the impact of varying CO2 concentrations on the yield of contact-electro-catalytic CO2RR, as elegantly illustrated in Fig. 3f. The results indicate that, as the CO2 concentration decreases from 20–0.02%, the yield of CO product in the contact-electro-catalytic CO2RR catalyzed by both quaternized CNF-based and pure CNF-based TENGs diminishes. Remarkably, the quaternized CNF-based TENG outperforms its pure CNF-based counterpart across both high and low concentration ranges. This phenomenon can be elucidated through two distinct avenues. Primarily, the TENG's current output escalates as the CO2 concentration rises after reacting for 5 hours, ultimately culminating in an augmented charge transfer for contact-electro-catalysis, which facilitates an upsurge in the production of CO. Secondly, in contrast to lower CO2 concentrations, the surface of quaternized ammoniated CNF exhibits heightened CO2 adsorption at elevated concentrations. This translates to an increased involvement of reaction substrates in the contact-electro-catalytic process, consequently leading to an augmentation in CO production. A noteworthy achievement emerges from this study-successful contact-electro-catalytic CO2RR under experimental conditions featuring even lower CO2 concentrations than those present in ambient air (approximately 0.04%). This accomplishment paves the path for further explorations into the catalytic prowess of TENG in real-world atmospheric conditions.
Based on the study of CO2 adsorption performance, we further proposed the mechanism of contact-electro-catalytic CO2RR, the schematic diagram of which is shown in Fig. 3g. During the contact charging process of TENG, quaternized CNF effectively adsorb CO2 molecules from the surrounding environment. Simultaneously, within the Cu-PCN@PVDF material, single-atom copper plays a crucial role in accumulating electrons. When these two tribolayers come into contact, the electrons, accumulated by the presence of single-atom copper, which may facilitate the electron transfer to the adsorbed CO2 molecules, thereby driving and completing a catalytic reaction. To deeply study the CO2 adsorption and electron enrichment, DFT calculations were carried out to calculate the binding energy of CO2 to quaternized CNF and the electron enrichment effect of Cu-PCN. As shown in Fig. 3h, the quaternized CNF can capture the CO2 effectively through the hydrogen bond between the CO2 and hydrogen from quaternized CNF. The binding energy is calculated to be -0.5 to -0.7 eV indicating a chemical binding. Furthermore, DFT calculations were conducted to investigate the role of electron enrichment from Cu-PCN. The analysis of the density of states (DOS) shows that pure PCN exhibits typical semiconductor characteristics, as well established by other study.55,56 Upon the introduction of the Cu atoms into the PCN structure, a strong hybridized peak emerges between Cu and the PCN substrate, indicating the strong interaction that stabilizes Cu on the PCN substrate. Of greater significance, the incorporation of Cu leads to the formation of a defect state near the conduction band edge (indicated by the arrow in Fig. 3i), resulting in a reduction of the band gap that will facilitate the electron injection. This defect state primarily arises from the presence of Cu, suggesting that upon injection into Cu-PCN, electrons will accumulate around the Cu atoms. This is further supported by the electron distribution calculation as shown in Fig. 3j. In light of these DFT calculations, we propose that such electron accumulation may positively influence its catalytic performance.
Contact-electro-catalytic CO2RR from ambient air
As the driving force behind TENG's generation of contact-electro-catalytic charges predominantly originates from ambient mechanical energy, the triboelectrically induced interfacial CO2RR holds promise for eventual application in atmospheric settings, effecting the reduction of excess CO2 in our environment. Moreover, the exceptional CO2 adsorption performance of quaternized CNF lays a strong foundation for CNF based TENG application in catalyzing the reduction reaction of CO2 in air. Schematic illustration of contact-electro-catalytic CO2 reduction in air is shown in Fig. 4a. In a fictional scenario, TENG can be attached to the sole of a shoe to convert mechanical energy into electrical energy during exercise, thereby catalyzing CO2RR in the air. To realize contact-electro-catalytic CO2RR in air under experimental conditions, we used compressed air as the gas source instead of the CO2/Ar mixed gas. The experiment was conducted in a confined box under normal temperature and pressure conditions with 99% humidity maintained by distilled water, as shown in Fig. 4b. First, a fixed-bed CO2 breakthrough experiment was conducted on the quaternized CNF utilizing ambient air feed, which contained approximately 400 ppm of CO2. This experiment was aimed to assess the CO2 adsorption capabilities of the material within ambient air environment, as illustrated in Fig. 4c. The tests were carried out under flowing air conditions to simulate real-world conditions more realistically. The calculated fixed bed breakthrough capacities were 6.25 mlg− 1 and 5.3 mlg− 1 under dry and wet conditions, respectively (Fig. 4d). The observed breakthrough curves were steep and sharp indicating fast mass transfer in the fixed-bed. The high CO2 adsorption capacities of quaternized CNF at low CO2 concentrations under both dry and humid conditions indicate that quaternized CNF would be a good candidate for capturing carbon dioxide from ultra-dilute gas streams (i.e. ambient air).
Based on the excellent CO2 adsorption performance of quaternized CNF in air, quaternized CNF-Cu-PCN@PVDF-based TENG can catalyze the reduction reaction of CO2 in air. After the reaction for 10.5 h, 235.65 nmol of CO is produced in the airtight box, and its GC spectrum is shown in Fig. 4e. Furthermore, real-time current changes during the reaction were recorded, as shown in Fig. 4f. The entire catalytic reaction process lasted for 10.5 h, and the current showed a trend of increasing first and then decreasing. In the first stage of the reaction, TENG is mainly dominated by charge accumulation. At the same time, contact-electro-catalytic CO2RR occurs in the system. Saturation is reached when the current accumulates to about 20 µA. Subsequently, in the second stage of the reaction, the current starts to decrease due to the work done by the electrons (electrons transfer from Cu-PCN to CO2 for the catalytic reaction to proceed) in the contact-electro-catalytic process. However, when the compressed air in the system is replaced with Ar, an interesting phenomenon is observed: initially, the current tends to increase before reaching a stable level, and notably, the current does not exhibit a subsequent decrease, as shown in Fig.S11. This observation indicates that the contact-electro-catalytic reaction takes place within the air environment, while such a reaction does not manifest when Ar is used instead. Furthermore, we calculated the transferred charges with the CO2RR reaction, as shown in Fig. 4g. Similar to the method for calculating the transferred charge, the transferred charge with CO2RR was obtained by current integration. The calculated transfer charge of contact-electro-catalytic CO2RR in air is 48.4 mC. Finally, we compared the CO yield of contact-electro-catalytic CO2RR in air compared with conventional photocatalysis, as shown in Fig. 4h and Table S3. The CO yield of contact-electro-catalytic CO2RR is 33 µmol g− 1h− 1, which is a new record for the yield of CO produced by catalytic CO2RR in air.57–61 The innovative utilization of quaternized CNF in conjunction with the unique properties of the quaternized CNF-Cu-PCN@PVDF based TENG opens up exciting avenues for efficient and sustainable CO2 reduction within air environments, holding significant promise for advancing carbon capture and utilization technologies.