Contact-electro-catalytic CO₂ reduction from ambient air

The study addresses the challenge of sustainably converting CO₂ into value-added chemicals with high selectivity and low energy input. Conventional electrocatalytic CO₂ reduction can be efficient but often relies on costly catalysts and significant electrical energy. Triboelectric nanogenerators (TENGs) harvest mechanical energy via contact electrification and have been explored for driving chemical reactions; however, prior contact-electro-catalysis typically proceeded through free-radical pathways (e.g., H₂O₂ generation, dye degradation), which suffer from multi-step energy conversion, inefficiencies, and poor selectivity. Additional barriers include the inertness of CO₂, insufficient adsorption/activation sites on tribolayers, and humidity-induced degradation of TENG performance despite water being needed as a proton source. The authors propose a direct electron-transfer contact-electro-catalytic platform that leverages quaternized cellulose nanofibers (CNF) for robust CO₂ capture and water immobilization under high humidity, and an electrospun PVDF tribolayer loaded with single-atom Cu on polymeric carbon nitride (Cu-PCN) to enrich and transfer electrons to adsorbed CO₂. The goal is high-selectivity CO production from CO₂ under ambient conditions, including from dilute CO₂ in air, using mechanically harvested energy without requiring a conventional external circuit.

The paper situates its contribution within prior work on electrocatalytic CO₂ reduction and TENG-driven catalysis. Earlier contact-electro-catalysis demonstrations focused on free-radical-mediated reactions (e.g., H₂O₂ synthesis and organic dye degradation), which involve indirect conversion of mechanical to electrical energy and then to reactive species, leading to energy losses and limited selectivity/control. In contrast, interfacial mechanisms with direct electron participation can improve efficiency and selectivity. The authors also reference materials strategies for CO₂ capture and TENG operation in high humidity, including hydroxyl-rich biobased tribolayers, and advances in single-atom catalysis and piezoelectric carbon nitride, highlighting gaps in achieving selective CO₂-to-CO conversion directly from ambient air via contact-electro-catalysis.

Device and materials: A contact-separation TENG comprises an electropositive quaternized cellulose nanofiber (CNF) tribolayer and an electronegative electrospun PVDF tribolayer loaded with single-atom Cu anchored on polymeric carbon nitride (Cu-PCN). Quaternized CNF provides strong CO₂ adsorption sites and immobilizes water via hydrogen bonding at 99% RH, enabling proton supply. Cu-PCN@PVDF enriches surface electrons during contact electrification and facilitates electron transfer to adsorbed CO₂. Synthesis and fabrication: Cu-PCN single-atom catalyst was prepared per Supplementary procedures. For Cu-PCN@PVDF, Cu-PCN was ultrasonically dispersed in DMF and mixed with PVDF in acetone (0.7 g PVDF in 2.9 g acetone; Cu-PCN 3.5–10.5 mg in 1.9 g DMF), stirred 24 h, then electrospun (15 kV, 1 mL h⁻¹, 10 cm tip-to-collector, 4 h; collector area 165 cm²). Optimal catalyst:PVDF mass ratio was 1:100 to avoid aggregation. Quaternized CNF films (~100 µm) were prepared and laminated with Al electrodes; electrospun Cu-PCN@PVDF (~297 µm) was similarly backed by electrodes. 4 cm × 4 cm pieces were assembled face-to-face with double-sided tape to form the TENG. CO₂RR testing: Experiments were conducted in a sealed chamber at room temperature and 99% RH (maintained by distilled water). The chamber was first purged with Ar and the TENG operated to reach a stable saturation current (~18.5 µA). For concentrated CO₂ tests, 1 L CO₂ was injected into a 5 L effective volume box; reaction proceeded 5 h with hourly GC sampling (1 mL). For ambient air tests, compressed air (~400 ppm CO₂) was fed into a fixed-bed for breakthrough characterization and into the sealed box for CO₂RR (10.5 h). Control: a 410 V DC field without TENG showed no CO formation after 5 h, indicating products arise from contact-electrocatalysis, not high-voltage dissociation. Characterization and analytics: Output voltage/current measured via NI DAQ (100 MΩ load) and SR570 amplifier at 5 N, 5 Hz. Electron structure and single-atom dispersion assessed by HAADF-STEM and Cu K-edge XANES/EXAFS. CO₂ adsorption/desorption assessed by TPD (Quantachrome autosorb-iQ) and TG-MS, with Henry’s law CO₂/N₂ selectivity from MS area ratios. CO₂ vapor adsorption capacities measured at room temperature at high (20%) and low (0.02%) CO₂. Breakthrough capacities measured with a Vaisala GMP343 probe under dry and 99% RH air feeds. Products quantified by GC (Agilent 8890, FID/TCD, Ar carrier) and isotope tracing by GC–MS (Agilent 7890A/5975C, SIM mode for m/z 29). Calculations: Real-time current was integrated to obtain transferred charge Q during CO₂RR. Faradaic efficiency FE = Z·n·F / Q × 100%, where Z = electrons per CO (2), n = moles of CO, F = 96485 C mol⁻¹. CO yield normalized to catalyst mass: the 4 × 4 cm tribolayer contained 0.68 mg Cu-PCN (from 7 mg in 0.7 g PVDF electrospun onto 11 × 15 cm area). DFT (VASP, PBE-GGA, PAW, 450 eV cutoff, 3×3 PCN model with 3×3×1 k-mesh) analyzed DOS and charge distribution for Cu-PCN and PCN; Gaussian computed adsorption energies of CO₂, O₂, N₂ on quaternized CNF. Charge density difference Δρ = ρtotal − ρCO₂ − ρCu-PCN evaluated for CO₂/Cu-PCN at varying separations. Operating conditions and variables: Humidity up to 99% RH; evaluation of catalyst Cu content and loading (0, 0.5, 1, 2 Cu in synthesis; 0–1.5 wt% Cu-PCN in PVDF). Effects of tribolayer thickness and area on current and CO yield were investigated. Cyclic stability over seven 5 h runs (total 35 h) recorded both CO production and current decay from ~18.6 µA to ~16.5 µA per cycle.

- High selectivity and efficiency: Contact-electro-catalytic CO₂RR achieved CO Faradaic efficiency (FECO) of 96.24% (calculated via integrated transferred charge during reaction). A wind-driven TENG variant achieved FECO ≈ 92% versus 93.95% for motor-driven. - Operation under high humidity: The TENG delivered 18.7 µA current and 405 V at 99% RH, benefiting from quaternized CNF’s hydroxyl-mediated water immobilization and enhanced contact electrification. - From concentrated CO₂: Clear CO formation observed by GC after 5 h in a 5 L sealed box following injection of 1 L CO₂ (none at t = 0). Transferred charge during CO₂RR was 31.5 mC (from current integration). Isotope tracing confirmed product origin: SIM GC–MS detected ¹³CO at retention time 1.43 min with dominant m/z 29.1 (¹³CO), and fragments m/z 13 (¹³C) and 16.1 (O). - From ambient air: With compressed air (~400 ppm CO₂) at 99% RH, 235.65 nmol CO formed in 10.5 h; integrated transferred charge was 48.4 mC. Normalized CO yield is 33 µmol g⁻¹ h⁻¹ (based on 0.68 mg Cu-PCN loading). - Stability: Over seven consecutive 5 h runs (35 h), CO production remained ~240 µmol g⁻¹ per cycle, with stable initial saturation current (~18.6 µA) decreasing to ~16.5 µA after each 5 h run. - Role of Cu-PCN loading: Increasing Cu content and catalyst loading enhanced CO yields; however, excessive loading (≥1.5 wt%) caused Cu-PCN aggregation and compromised fiber integrity. Subsequent studies used 1 wt% loading for optimal device performance. - Geometry effects: CO yield decreased with increased tribolayer thickness (due to inactive embedded catalyst) and scaled with device area via increased current. - Adsorption and selectivity: CO₂-TPD showed Cu-PCN@PVDF had a physical adsorption peak (~78.8 °C), whereas quaternized CNF exhibited strong adsorption/desorption features (~92.4 °C and 205 °C) indicating robust chemical adsorption via quaternary ammonium sites. CO₂ vapor adsorption at both 20% and 0.02% CO₂ showed higher capacity for quaternized CNF than Cu-PCN@PVDF. Henry’s law CO₂/N₂ selectivity for quaternized CNF was ~80%. Fixed-bed breakthrough capacities with ambient air were 6.25 mL g⁻¹ (dry) and 5.3 mL g⁻¹ (99% RH), with steep breakthroughs indicating fast mass transfer. - Mechanistic evidence: Quaternized CNF is the primary CO₂ adsorption locus; electron-enriched Cu sites on Cu-PCN facilitate rapid electron transfer to adsorbed CO₂ upon contact. DFT showed stronger adsorption energy on quaternized CNF for CO₂ (−0.7 eV) versus O₂ (−0.41 eV) and N₂ (−0.08 eV). DOS analysis revealed Cu-induced defect state near PCN’s conduction band and electron density localized on Cu atoms, consistent with electron enrichment and efficient transfer when CO₂ approaches within <4 Å of Cu on Cu-PCN. - TENG current behavior: Under high CO₂ (20%), current initially decreased (CO₂ adsorption on quaternized CNF reduces hole density) then increased (CO₂ occupies PVDF interstices, increasing electronegativity of Cu-PCN@PVDF). At low CO₂ (0.02%), patterns depend on which tribolayer adsorbs CO₂, consistent with mechanistic assignments.

The findings demonstrate that a TENG-driven, direct electron-transfer mechanism can selectively reduce CO₂ to CO with very high Faradaic efficiency, overcoming the inefficiencies and poor selectivity of radical-mediated contact-electro-catalysis. Quaternized CNF ensures strong and selective CO₂ adsorption, even at sub-ambient concentrations, and immobilizes water under high humidity to supply protons. The Cu-PCN@PVDF tribolayer enriches electrons at single-atom Cu sites and provides a high interfacial potential, enabling rapid electron transfer to adsorbed CO₂ during contact. Together, these functions decouple adsorption and electron delivery, enabling operation in ambient air and under 99% RH. The system achieves competitive or superior performance to state-of-the-art approaches for air-based CO₂RR, and maintains stability over multiple cycles. The mechanistic insights (adsorption energetics, DOS, and charge transfer proximity) underscore the crucial role of single-atom Cu for electron accumulation and transfer, and of quaternized CNF for capture and selectivity, providing a blueprint for designing future contact-electro-catalytic platforms.

This work establishes a contact-electro-catalytic platform for CO₂ reduction powered by mechanical energy, achieving FECO of 96.24% and CO yields up to 33 µmol g⁻¹ h⁻¹ from ambient air. The synergistic design—quaternized CNF for strong CO₂ capture and proton supply under high humidity, coupled with electron-enriched single-atom Cu on PCN for efficient interfacial electron transfer—enables selective CO formation under ambient conditions. Beyond demonstrating performance and stability, the study provides mechanistic validation via adsorption analyses and DFT. Future directions include integrating diverse single-atom catalysts and porous coordination networks, optimizing catalytic site density and selectivity toward higher-value multi-carbon or liquid products, and translating the platform into wind-driven or wearable devices for distributed carbon capture and utilization.

- High-humidity requirement: The device relies on 99% RH to immobilize water and supply protons; performance at lower humidity was not detailed and may diminish. - Geometry and loading constraints: Excessive Cu-PCN loading leads to catalyst aggregation and weakened PVDF fibers; increased tribolayer thickness traps inactive catalyst, reducing yield, indicating scale-up must manage architecture carefully. - Enclosed-system testing: Core demonstrations used sealed chambers and controlled gas feeds; real-world, open-air operation may introduce mass transport and environmental variability (e.g., fluctuating humidity, airflow) affecting adsorption and desorption. - Product scope: The study focuses primarily on CO; extension to more reduced or multi-carbon products remains to be demonstrated. - Gas flow effects: Wind-driven operation showed slightly lower FECO than motor-driven, suggesting sensitivity to flow-dependent adsorption/desorption dynamics.