Photoelectrode design, photovoltaic characteristics and devised assemblyIn solar-driven reactions, the efficiency of the photoelectrode emerges as a critical determinant of overall performance. However, the most reported photoelectrodes incite severe charge recombination owing to their inherent photovoltage limitations and inadequate passivation layers, ultimately decreasing the efficacy35,36. The 3-J photoanode with a sufficient power output that can catalyze the CO2RR without external bias is utilized in this study. At first, the rear side of the 3-J photoanode is integrated with Nickel (Ni)-foil, which serves dual purposes—stabilizing the device against photocorrosion and functioning as an efficient heat dissipation layer to perform PEC-CO2RR under aqueous conditions37. This layer ensures the 3-J photoanode bottom and Ni interface an ohmic contact, unlike the Schottky barrier or anolyte-induced depletion layer over the Ni surface38. To fulfill the photovoltage prerequisites of PEC-CO2RR, low-onset potential ruthenium oxide (RuOx) is further deposited over the Ni-foil to catalyze the water oxidation at a low applied potential. This 3-J|Ni/RuOx design of the PEC cell presents the distinct advantage of effectively decoupling the optical absorption and electrocatalytic interfaces, extending its longevity and operational range (Fig. 1a).Fig. 1: Schematics of the PEC assembly for CO2-to-liquid fuel conversion and photovoltaic characteristics of the 3-J photoanode.a Mechanistic demonstration of the CO2Rcat||3-J|Ni/RuOx PEC device during CO2RR; enlarged view of 3-J|Ni/RuOx photoanode on the right-hand side and HAADF-STEM mapping of CO2Rcat (Sn-Bi2O3) on the left-hand side. b HAADF-STEM of 3-J photoanode with elemental mapping of In, Ga, P, As, and Ge, with 1 µm size, respectively. c Operating current-voltage characteristics, and (d) EQE% of the 3-J photoanode. ARC: Anti Reflection Coating, RE: Reference electrode (Ag/AgCl), BPM: Bi-polar membrane.The photovoltaic characteristics of the 3-J photoanode under dry conditions were analyzed under AM 1.5 G illumination to validate the photovoltage requirements for PEC-CO2RR. The performance metrics of the 3-J photoanode, as indicated by the current–voltage (J–V) profile, including open-circuit voltage (VOC) of 2.6 V, a short-circuit current density (JSC) of 13.97 mA cm–2, a fill factor of 83.5%, and a power conversion efficiency standing at 29.8% (Fig. 1c). The J–V outcomes confirm that the photovoltages generated by 3-J photoanode surpass the potential requirements of PEC-CO2RR (~2.2 eV, see Fig. 2a). To illustrate the light-absorption properties further and describe the integral role of 3-J photoanode in PEC performance, external quantum efficiency (EQE) with the absorption edge of each constituent sub-cell was evaluated (Fig. 1d). The uppermost sub-cell (GaInP, Eg = ~1.85 eV) exhibits efficient photon absorption within the 300–600 nm λ spectrum. Subsequently, photons that traverse the upper layer and reach the middle sub-cell (GaAs, Eg = ~1.40 eV) are absorbed within the 600–900 nm λ spectrum. Finally, the lowermost sub-cell (Ge, Eg = ~0.67 eV) captures high-frequency photons within the λ spectrum within the 900–1800 nm spectrum. The consistency between these EQE calculations and the JSC values reveals the sufficient light-harvesting and effective charge carrier separation capabilities of the 3-J photoanode. Subsequently, using elemental mapping, the layered structure of the 3-J photoanode was confirmed with high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). With a structural composition incorporating semiconductors such as InGaP, GaAs, Ge, and layers coated with Ni and RuOx, the cumulative thickness of the device was estimated to be ~6.2 μm. The illuminated surface of the 3-J photoanode was precisely characterized via TEM further, which presents numerous quantum wells distribution within the mediating region (Fig. S1). In terms of elemental dispersion, a ~500 nm thick layer of InGaP resides at the bottom structure, succeeded by a substantial ~5 μm thick layer of GaAs, interspersed with multiple quantum wells in the intermediary region, and concluded by a ~400 nm thick layer of Ge at the top, as elucidated in Fig. 1b.Fig. 2: Three-electrode PEC-CO2RR operation under AM 1.5 G illumination with CO2Rcat||3-J|Ni/RuOx PEC device.a Current–voltage characteristics of 3-J photoanode with electrochemically activated currents: anodic RuOx with Ag/AgCl Reference Electrode and cathodic Sn-Bi2O3 NCs catalyst in the dark. b Calculated FE% (left-hand axis) and obtained HCOOH and H2 production rates (right-hand axis) with Sn-Bi2O3 NCs cathode. The error bars indicate the results from ten individual tests. c Quantified Electrical Energy efficiency (ηEE%, right-hand axis) and operating photovoltages (left-hand axis) during 100 h of PEC-CO2RR with 3-J photoanode and Sn-Bi2O3 cathode. d ηSTF% of the cathodic products (right-hand axis) with the operating anodic photocurrent density (left-hand axis) during 100 h of PEC-CO2RR using 3-J photoanode and Sn-Bi2O3 cathode. The setup recorded 2.14 ±  0.013 Ω resistance, and cell voltage readings have not been iR-corrected.In addition to the above considerations, an efficient, stable, and cost-effective system is a prerequisite for efficient PEC performance. To address this, an artificial photosynthesis-inspired, unassisted, two-electrode PEC-CO2RR setup employing a single photoelectrode to catalyze both the cathodic reduction and anodic oxidation is devised. Figure 1a elucidates the schematics insights of the unassisted PEC-CO2RR assembly, comprising 3-J|Ni/RuOx photoanode with Sn-Bi2O3 (CO2Rcat)-cathode. The photogenerated electrons of the 3-J photoanode drift towards the front collection point, which is directly connected to the Sn-Bi2O3-cathode (separated by the bipolar membrane) to initiate CO2RR, and holes transfer to the rear side of the photoanode on the Ni/RuOx to facilitate water oxidation. Moreover, the physical representation of this PEC-CO2RR setup is further depicted in Fig. S2.Photoelectrochemical CO2 reductionThe bias-free PEC system, operated by a single-photoelectrode, exemplifies a promising, cost-effective, and efficient approach to upscaling energy conversion processes. During PEC operation, the current-voltage characteristics of the 3-J|Ni/RuOx photoanode (red) under 1-sun illumination with the electrochemically activated currents: anodic RuOx (black) and cathodic Sn-Bi2O3 (green) under dark in aqueous conditions are measured first as displayed in Fig. 2a. Here, the red-green intersection curve of Sn-Bi2O3 signify the operating point for CO2R at –0.73 V vs RHE and 12.99 mA cm–2 current density. Similarly, a potential of 1.46 V vs RHE is required to initiate water oxidation (indicated by the red-black intersection of RuOx). Additionally, the PEC system experiences external voltage losses of ~0.4 V due to the bipolar membrane, as detailed in Supplementary Note 1 and Figs. S4–S5. Considering this, the PEC-CO2RR necessitates ~2.59 V to derive the complete PEC-CO2RR.During PEC-CO2RR, liquid HCOOH was a major product and gaseous H2 (Supplementary Note 2) was analyzed as a minor product. Before PEC operation, 3-J|Ni/RuOx photoanode in 0.5 M aqueous KOH (pH ~14) anolyte with CO2-saturated CO2Rcat in a 0.5 M aqueous potassium bicarbonate (KHCO3) (pH ~7.4) catholyte was immersed separately. Within the three-electrode PEC-CO2RR setup, the potential at the counter electrode (Sn-Bi2O3) was recorded first, where the system stabilized 0.25 V potential for 100 h (Fig. S3a). On hourly three-electrode PEC-CO2RR operation, the CO2Rcat||3-J|Ni/RuOx setup exhibited 94 ± 1.9% FE, corresponding to a significant production amount of 20.3 ± 1.3 mmol L–1 h–1 (Fig. S6). Subsequently, this PEC assembly stabilized an average 90 ± 2.1% FE with a consistent yield of 19.7 ± 1.7 mmol L–1 h–1 for 100 h (Fig. 2b). The hourly nuclear magnetic resonance (NMR) performance for HCOOH is quantified further to confirm the amount of obtained liquid products (Fig. S7). To stimulate artificial photosynthesis-inspired, unassisted two-electrode PEC setup was executed further to synthesize the storable and economically viable liquid fuel synthesis. Prior to unassisted PEC operation, the J–V characteristics of the 3-J|Ni/RuOx photoanode in two-electrode configurations under standard AM 1.5 G illumination in aqueous conditions were analyzed. In an unassisted two-electrode setup, this 3-J|Ni/RuOx photoanode maintained a JSC of 10.5 mA cm–2 (Fig. S8). Subsequently, in a two-electrode PEC-CO2RR setup, the counter electrode (Sn-Bi2O3) recorded 0.25 V potential during long-run continuous operation of 100 h (Fig. S3b). In the initial hourly evaluation, the unassisted CO2Rcat||3-J|Ni/RuOx PEC device demonstrated 91 ± 2.3% FE with 17.9 ± 1.9 mmol L–1 h–1 product generation (Fig. S6). Subsequently, long-run 100 h of operation maintained a consistent FE of 88 ± 3.2% and delivered an average production of 17.3 ± 1.9 mmol L–1 h–1 (Fig. 3a). Additionally, the performance metrics of PEC-CO2RR utilizing individual Ni-foil as an OER catalyst were assessed, and findings are thoroughly detailed in the Supplementary Note 3 and Figs. S9–11. Moreover, the half-anodic oxygen evolution reaction in both the three- and two-electrode configurations, occurring on the rear-side of the 3-J|Ni/RuOx photoanode under AM 1.5 G illumination, are demonstrated in Supplementary Movie 1 and 2.Fig. 3: Two-electrode PEC-CO2RR operation under AM 1.5 G illumination with CO2Rcat||3-J|Ni/RuOx PEC device.a Obtained FE% (left-hand axis) and production yields (mmol L–1 h–1, right-hand axis) of HCOOH and H2 using CO2Rcat||3-J|Ni/RuOx PEC device. The error bars indicate the results from ten separate measurements. b Calculated ηEE% (right-hand axis) and operating photovoltages (left-hand axis) during 100 h of PEC-CO2RR with 3-J photoanode and Sn-Bi2O3 cathode. c ηSTF% of the cathodic products (right-hand axis) and operating anodic photocurrent density (left-hand axis) during 100 h of PEC-CO2RR using 3-J photoanode and Sn-Bi2O3 cathode. 3-E: Three-electrode, 2-E: Two-electrode. The setup recorded 2.14 ±  0.013 Ω resistance, and cell voltage readings have not been iR-corrected.Electrical energy efficiency (ηEE), which reveals the energy difference during CO2 conversion into targeted products, is a key factor for further scalability estimations is measured. Based on the obtained selectivity and generated photovoltage, CO2Rcat||3-J|Ni/RuOx PEC device demonstrated a remarkable average ηEE of 57.2 ± 1.5% for HCOOH during a three-electrode and 60.0 ± 1.9% with an unassisted two-electrode assembly during 100 h of PEC-CO2RR (Figs. 2c, 3c). During 100 h of PEC-CO2RR, the generation of photovoltages demonstrated a commendable consistency, thereby substantiating the stability of the PEC device within aqueous electrolytic conditions (Figs. 2c, 3c). These ηEE results confirm that an appropriate potential is crucial for targeting the desired products. In the PEC-CO2RR, the ηSTF mark is state-of-the-art, indicating that the incident light induces the formation of the chemical bonds of various fuels. Based on obtained selectivity and photocurrent, CO2Rcat||3-J|Ni/RuOx PEC device in a three-electrode system exhibits remarkable ηSTF of 15.5 ± 0.7% during hourly and recorded an average ηSTF of 15.1 ± 0.9% for HCOOH throughout 100 h of experimentation (Fig. 2d). Operating under identical conditions, artificial leaf-inspired, unassisted two-electrode CO2Rcat||3-J|Ni/RuOx PEC device in hourly investigation recorded 12.5 ± 0.6% of ηSTF, which was recorded at an average of 12 ± 0.9% during 100 h operation (Fig. 3d).The lack of stability inherent to the photoelectrode represents a major challenge, effectively limiting the practical implementation of PEC devices. In the initial investigation, a 3-J photoanode without a protective layer (Ni-foil) was employed, which exhibited limited durability of <1 h within an aqueous electrolyte, thereby confirming its chemical instability and susceptibility to corrosion (Fig. S12). Consequently, the rear-side protected PEC device with Ni-foil demonstrated an enduring photocurrent for 100 h of CO2R operation (Figs. 1e, 3c). Owing to the instability of the cathodic catalyst beyond 100 h in PEC-CO2RR, the integrated 3-J|Ni/RuOx photoanode with Pt-foil as a cathode was further experimented to validate its durability under aqueous circumstances. This individual PEC device exhibited robust operation over 500 h, with a negligible irreversible variation of 0.5% in photocurrent (Fig. S13). A slight deviation in the J–V characteristics under aqueous condition measurements was observed before and after 500 h of operation (Fig. S14). This indicates that the 3-J|Ni/RuOx photoanode is durable for long-run experimentation. This exceptional performance confirms the Ni foil’s efficient electrolyte permeation, thereby improving the chemical stability under aqueous conditions. Overall, passivation by integrating the Ni-foil as a protection layer efficiently maintains chemical stability, dissipating heat and preventing corrosion. Furthermore, using RuOx as a low-onset potential catalyst effectively catalyzes water oxidation at a lower potential.Significance of liquid-fuel and comparison analyses of PEC assemblyAmong CO2 converted liquid fuels, HCOOH is an exceptional choice due to its unique electron-per-mole advantage and high energy density39 Annually, ~800,000 metric tons of HCOOH are produced and employed in diverse applications, subsequently considered a remarkable H2 storage medium due to rapid catalyst-aided dehydrogenation40. Its salient features in the liquid phase at ambient environmental conditions exhibit low toxicity and offer high H2 storage volumetric density (approximately 53 g per litter)40. Inspired by artificial photosynthesis, PEC-CO2 conversion into HCOOH is a compelling scheme for cost-effective commercial viability, given that the resulting liquid products can be precisely stored, seamlessly transported, and directly utilized5,16,22. Despite numerous PEC-CO2RR-based gaseous generations, liquid fuel synthesis remains a significant concern due to high energy demands. Such systems consistently undergo several limitations, including inadequate photovoltages, chemical instability, high onset potential and catalyst durability constraints, thereby limiting these implementations on their road to commercial viability.Considering PEC as a renewable method and HCOOH as a high-energy density liquid fuel, our research demonstrates a standalone bias-free CO2Rcat||3-J|Ni/RuOx device. Under AM 1.5 G illumination, unassisted CO2Rcat||3-J|Ni/RuOx PEC device, marked by high selectivity and long-run production yield, leads to significant ηSTF over 100 h. Remarkably, the selectivity, operational durability, and recorded ηSTF outperform those of previously reported photoanode- and photocathode-promoted PEC-CO2RR setups (Table S1). The assembly, strategically segregating anodic and cathodic compartments within the PEC assembly, facilitates independent reaction adjustment further, thereby preventing the need for electrolyte flow, concentrated solar irradiance, gas-diffused photoelectrodes and thermoelectric systems22,41. The significant performance achieved in this work is credited to the multi-layered photoelectrodes design, which generates sufficient photovoltage, and the implementation of an artificial photosynthesis-inspired unassisted PEC assembly to induce bias-free CO2RR. Progressing further on this achievement, catalyst selection and engineering emerge as crucial factors in long-run operations.Electrochemical CO2 reductionTo validate the selectivity, production yields and durability of cathode catalysts, the electrochemical performance was performed. The linear sweep voltammetry (LSV) of the cathode-catalysts (Bi2O3 and Sn-Bi2O3) revealed higher negative currents under CO2 saturation than Argon, indicating strong interactions between CO2 and electrode surfaces (Fig. S15a). Combining Sn (i.e., 1 to 4 wt.%) with Bi2O3 maximizes the current density, as LSV curves, selectivity, and production yields are presented in Fig. S16a, b and Table S2. With 3% Sn loading, the Sn-Bi2O3 demonstrated a FE of 95 ± 2.3% and 3.1 ± 1.2% with a yield rate of 20.7 ± 1.2 and 0.6 ± 0.8 mmol L–1 h–1 for HCOOH and H2 at –0.73 V vs RHE potential, respectively (Fig. S15b). Under the same operation conditions, the Bi2O3 presented FE of 85 ± 1.7% and 5 ± 0.9% with a yield of 13.6 ± 1.1 and 1.4 ± 1.1 mmol L–1 h–1 for HCOOH and H2, respectively (Fig. S15b). During 100 h of operation at –0.73 V vs RHE, the average FE of 90 ± 1.9% and 3.5 ± 1.3% with a production amount of 20.1 ± 1.2 and 0.7 ± 0.5 mmol L–1 h–1 corresponding to HCOOH and H2 is achieved with Sn-Bi2O3, respectively. Moreover, the Bi2O3 performed an average of 80 ± 1.3% and 7.9 ± 1.1% FE with 11.9 ± 0.9 and 1.8 ± 0.8 mmol L–1 h–1 production relative to HCOOH and H2, during 100 h of operation (Fig. S17). These enhancements in performance at a low applied potential and positive shift in onset potential are attributed to the appropriate loading of Sn onto Bi2O3. This considerable improvement primarily originates from the synergistic effects between the catalytically active sites of Bi and Sn. During this metal-semiconductor interface, the Sn facilitates electron transport to Bi2O3 at the interface, thereby increasing the number of active sites and stabilizing the reaction intermediates, thus improving the overall catalytic performance33,42,43.The hourly current densities in the −0.6 to −1.2 V vs RHE range with a decrease of −0.1 V in each run are further investigated (Fig. S18a, b). The Sn-Bi2O3 displays over 90 ± 2.5% FE towards HCOOH in the potential range from –0.7 to –1.1 V, which is restricted to ~82 ± 2.9% while utilizing the Bi2O3. This ultimate FE at a more negative potential is due to the enriched adsorption capacities of adsorbed CO2•− and OCHO*. Moreover, when the potential of −1.2 V is applied to the Sn-Bi2O3, the FE of HCOOH drops to 80 ± 3.8%, while that of H2 improves to 18 ± 2.8%. Conversely, when a Bi2O3 NSs was employed, the FE for HCOOH decreased to 60 ± 5.1%, with a substantial rise of H2 to 24 ± 3.8%. This decrease in the FE of HCOOH at a more negative potential indicates dominance of H2 over HCOOH, as demonstrated by the product distribution (Fig. S19).To further validate the enhanced catalytic CO2R activity and active site number, the double-layer capacitance (Cdl) principle was employed in the non-Faradaic region to measure the electrochemical surface area (ECSA). Linear fitting of the cyclic voltammetry curves at varied scan rates indicated that the Cdl of Sn-Bi2O3 at 2.23 mF cm−2 (Fig. S20a, c), which is notably higher than Bi2O3 (0.76 mF cm−2) (Fig. S20d, c). The calculated ECSA results indicate that the Sn-Bi2O3 demonstrates a three times higher active surface area than Bi2O3. This increase in ECSA is associated with the decoration of Sn onto Bi2O3, which improves catalytic active sites. To further verify the catalytic performances, the partial current densities (j) at various potentials were analyzed (Fig. S21a). The highest jHCOOH of 33.6 ± 1.2 mA cm–2 was observed with Sn-Bi2O3, 1.5 times that of Bi2O3 (22.4 ± 1.6 mA cm–2). A comparison analysis of this study with earlier reported Bi-based catalysts towards HCOOH is summarized in Table S3. Moreover, electrochemical impedance spectroscopy (EIS) was investigated to validate the charge transfer process. Detailed insights are provided through Nyquist plots and equivalent electronic circuits (inset), as depicted in Fig. S21b. Notably, the Nyquist plots demonstrated a charge transfer resistance (Rct) for Bi2O3 at 4.13 Ω, which was further reduced to 2.17 Ω for Sn-Bi2O3. These EIS outcomes support the accelerated faradic process in EC-CO2RR, indicating an enhanced capability of hydrated CO2 molecules to accept electrons, thus readily forming CO2•− intermediates. The Sn-Bi2O3 potentially strengthens electrical conductivity, facilitating faster charge transfer at the catalyst surface and reducing impedance. This leads to a product distribution towards HCOOH, which is associated with a rise in current density under the same potential conditions. The decreased adsorption at more negative potential is further confirmed through CO2 adsorption studies (Fig. 22a). Isotherms substantiate the superior adsorption capacity of Sn-Bi2O3 (0.192 ± 0.05 mmol g–1) than Bi2O3 (0.118 ± 0.03 mmol g–1). Moreover, Tafel slope analyses shed light on the reaction kinetics of CO2RR to validate the selectivity towards HCOOH (Fig. 22b). The low Tafel slope for Sn-Bi2O3 NCs at 141 ± 1.7 mV dec–1 is lower than the Bi2O3 (223 ± 3.3 mV dec–1), implies a faster conversion of CO2 to HCOOH. The determined slope for Bi2O3 indicates that the chemical rate-limiting step entails the primary electron transfer. It is important to note that Tafel slopes increase towards more negative potentials, likely attributed to mass-transport restrictions.Morphological, crystal and electronic structure analysesUsing micro- and spectroscopic techniques, the morphology of Bi2O3 nanosheets (NSs) and Sn-Bi2O3 NCs were characterized first. Scanning electron microscopy (SEM) image reveals the flower-like vertical growth of Bi2O3 NSs with uniform thickness (Fig. 4a). However, no noticeable morphological alteration was observed by SEM when Sn was incorporated over the Bi2O3 NSs (Fig. 4b). Further, the HAADF-STEM image of Bi2O3 NSs displays the flower-like morphological structure of the NSs. Subsequently, while characterizing the Sn-Bi2O3 NCs, Sn with uniform distribution was observed over Bi2O3 NSs (Fig. 4c, d). Further, Energy-Dispersive spectroscopy (EDS) was performed in conjunction with HAADF-STEM to reveal the elemental distribution of the Sn, confirming homogenous distributions of Bi, O, and Sn elements in Sn-Bi2O3 (Figs. 4g–j and S23a–c). The Bi2O3 NSs exhibit a consistent layered morphology before and after Sn modification, as confirmed via TEM, favouring the appropriate exposure of active sites to perform CO2RR (Fig. S23d–e). Furthermore, the High-Resolution TEM (HRTEM) image of Bi2O3 NSs reveals lattice fringes with d-spacings of 0.380 and 0.320 nm, assigned to Bi(110) and Bi(111) facets (Fig. S24), in agreement with the X-ray diffraction (XRD) pattern. In the HRTEM image of Sn-Bi2O3 NCs, the interplanar distances of 0.335 and 0.274 nm relate to the Sn(110) and Sn(200) facets, with spacings of 0.380 and 0.320 nm associated with Bi(110) and Bi(111) planes, respectively (Fig. 4e). Selected-area electron diffraction (SAED) patterns were examined to validate the structural properties of Bi2O3 NSs and compositionally fine-tuned Sn-Bi2O3 NCs. Crystal lattices with Bi(110) and Bi(111) planes at respective interplanar distances of 0.380 and 0.320 nm (Figs. 4e and S24 insets) and Sn(110) and Sn(200) planes at respective d-spacings of 0.335 and 0.274 nm (Fig. 4f) was confirmed through SAED pattern in Sn-Bi2O3 NCs.Fig. 4: Morphological studies of Bi2O3 NSs and Sn-Bi2O3 NCs.a Field emission scanning electron microscopy images (FE-SEM) of pristine Bi2O3 NSs and (b) Sn-Bi2O3 NCs. c, HAADF-STEM images of Bi2O3 NSs and d Sn-Bi2O3 NCs. e High-resolution TEM image of Sn-Bi2O3 NCs. f The selected area electron diffraction pattern (SAEED) of Sn-Bi2O3 for Bi and Sn planes. g–j HAADF-STEM image of Sn-Bi2O3 NCs and elemental mapping of Sn, Bi, and O, respectively.Moreover, the crystal structure of synthesized Bi2O3 NSs and Sn-Bi2O3 NCs was investigated using XRD. The sharp diffraction peaks of Bi2O3 align with the standard cubic pattern of Bi2O3 NSs (JCPDS no. 27-0052, Fig. S25)44. However, in the Sn-Bi2O3 pattern, no Sn-containing crystalline phases were detected, possibly attributable to the low Sn concentration or the superposition of broad peaks, indicating that Sn modification does not disrupt the crystal structure of Bi2O3. Moreover, X-ray photoelectron spectroscopy (XPS) was performed to investigate the catalysts’ electronic structure, phase compositions, and valence states. The Bi 4f spectrum of Bi2O3 displays two significant peaks relating to Bi 4f5/2 and Bi 4f7/2 at 164.11 and 158.88 eV, which confirms negative shifts of 0.16 eV in the binding energy of Sn-Bi2O3 NCs, verifying electron transfer, which modulates electronic structures (Fig. 26a). The Bi2O3 also displayed an O 1s spectrum with two distinct peaks corresponding to the lattice oxygen in Bi–O bonds and on the adsorbed surface at 530.35 and 531.41 eV; thus, a negative shift of ~0.17 eV in the O 1s peak was observed in the Sn-Bi2O3 (Fig. S26b). The commercial metallic Sn displays two Sn0 corresponding peaks of 3d3/2 and 3d5/2 at 493.63 and 485.05 eV, and compared to this, positive shifts of 0.31 eV in the binding energies of Sn0 with a noticeable change in peak shapes are observed in Sn-Bi2O3 NCs (Fig. S26c). This positive shift in the Sn0 peaks authenticates its partial oxidation, confirming an electron interaction between Sn and Bi2O3. Overall, a noticeable downshift in the binding energies of Bi 4f, O 1s and an obvious upshift in Sn 3d confirms electron transfer from Sn to Bi2O3 due to the difference in electronegativities, indicating charge redistribution, thereby improving the CO2R rate. Finally, the survey spectra verify the presence of Sn, Bi, and O in the Sn-Bi2O3 and Bi and O in Bi2O3 (Fig. S26d).Significance of Sn on charge transfer modulationFor better performance, adjusting the work function by increasing the number of electronic states near the Fermi level is essential for semiconductors. With this objective, metallic Sn was interfaced with semiconducting Bi2O3 to tune catalytic conversion efficiency. At first, the Mott-Schottky analyses for pristine Bi2O3 and Sn-Bi2O3 NCs were investigated, which displayed negative slopes, indicating their p-type semiconducting nature (Fig. S27a). The analysis further revealed that the flat band potential of Sn-Bi2O3 NCs was calculated at −0.187 V, notably higher than the −0.279 V value measured for Bi2O3. To further understand the influence of electronic structure modulation, contact potential difference (VCPD) and the work function (Φ), pivotal factors in composites’ interfacial charge transfer dynamics were further investigated using kelvin probe force microscopy (KPFM). The KPFM analysis revealed that metallic Sn demonstrates a lower VCPD of −0.180 mV than the 330 mV value measured for the Bi2O3, resulting in the Φ of 4.93 eV for Bi2O3 relatively higher than metallic Sn at 4.42 eV (Fig. S27b, Eq. 12)45,46. This discrepancy in Φ confirms that Sn possesses a lower electronic Fermi level relative to Bi2O3, which can facilitate charge transfer through metal-semiconductor interactions. The KPFM measurements further indicated that metal-semiconductor interfaced Sn-Bi2O3 NCs exhibited a VCPD of 70 mV (Fig. S27b). This deviation in the VCPD of Sn-Bi2O3 indicates alterations in the work function compared to the Bi2O3, predominantly attributed to the charge transfer between metal-semiconductor interfaces. Subsequently, using the VCPD value, the Φ of Sn-Bi2O3 was recorded to be 4.67 eV, which is comparatively lower than Bi2O3 (Fig. S27c, Eq. 12). This adjustment in Φ generates an electric field at the interfaces that drive charge carrier separation at their contact layers (Fig. S27b). This favorable band alignment promotes efficient electron transfer from Sn to Bi2O3, enhancing the electron density at the semiconductor interface, which is advantageous for CO2RR. Notably, the Sn-Bi2O3 NCs modulate charge transfer capabilities relative to pristine Bi2O3, facilitating local junction characteristics that align well with Mott-Schottky analysis outcomes (Fig. S27b). The impedance analysis matches well with these findings, which shows that Sn-Bi2O3 has reduced charge transfer resistance compared to Bi2O3, indicating superior charge transfer kinetics for CO2RR (Fig. S22b). The measured electronic properties and band alignments for Sn-Bi2O3 and Bi2O3 are comprehensively summarized in Table S4.Computational studiesThis study further investigated the density functional theory (DFT) calculations to validate the experimental findings. At first, the Gibbs free energy calculation via HCOO* and OCHO* pathways are examined separately to validate how Sn modification affects Bi2O3 during HCOOH formation. During CO2RR, the CO2 molecule first adsorbed over the catalyst surface, thereby reducing into CO2•− and converting into the HCOO*/OCHO*. During the initial protonation, carbon atoms of HCOO*/OCHO* bond with the O-bidentate atom on the catalyst surface by positioning the protonated hydrogen atom upwards, whereas oxygen atoms are bonded with either Bi or Sn surface atoms. During the HCOO* conversion pathway, the required formation energy of Sn-Bi2O3 reduced from 2.20 eV to 1.45 eV at the Sn site and 0.91 eV at the Bi sites compared to the Bi2O3, presenting notable energy differences (ΔE) of 0.75 eV at the Sn sites and 1.29 eV at the Bi sites (Fig. S28a). Conversely, through the OCHO* pathway, the formation energy of Sn-Bi2O3 lowers from 1.48 to 1.15 eV at the Sn sites and 0.61 eV at the Bi sites, demonstrating ΔE of 0.87 eV on the Bi site and 0.34 eV on Sn site (Fig. S29a). This initial protonation reaction validated a significant Gibbs free energy difference (ΔG) between Bi2O3 and Sn-Bi2O3, validating Bi-site in Sn-Bi2O3 favorable for HCOO* or OCHO* formation. Notably, these significant energy barriers in HCOO* and OCHO* formation validate the initial proton-coupled electron transfer as a possible rate-limiting step, consistent with Tafel slope observations (Fig. S22b). Subsequently, in the second protonation of HCOO*/OCHO* on its oxygen atom, these adsorbed species react with an electron-proton pair to form stable HCOOH* with a notable decrease in ΔE from the catalysis surface. Overall, the higher ΔE on the Sn-Bi2O3 surface confirms the species’ favorable formation due to alterations in the electronic structure after Sn modification. Consequently, bonds of HCOO* or OCHO* with HCOOH* are cleaved due to intrinsic scaling relation limitations when Bi2O3 is modified with Sn47. Overall Gibbs energy calculation results confirm CO2 to HCOOH conversion via the OCHO* pathway more favorable than HCOO*, as the HCOO* path demands a higher potential, leading to a thermodynamically challenging reaction.To further ensure the conversion of CO2 to HCOOH, the adsorption energy of HCOO* and OCHO* on the catalyst surface was examined (Figs. S28b and S29b). For the HCOO* intermediate, the Bi-site of Sn-Bi2O3 exhibits a higher adsorption energy of −3.83 eV compared to its Sn-site (−4.39 eV) and the Bi-site (−5.59) of Bi2O3. Similarly, for the OCHO* intermediate, the Bi-site of Sn-Bi2O3 demonstrates more substantial adsorption energy of −3.14 eV than its Sn-site (−3.40 eV) and the Bi-site of Bi2O3 (−4.79 eV). Findings confirm that the negative adsorption energy of HCOO* and OCHO* are favorable for HCOOH formation at the Bi and Sn sites of Sn-Bi2O3 relative to Bi2O3, confirming facile adsorption and desorption during HCOOH formation. To further gain deeper insight into the root cause of favorable adsorption energies on the Sn-Bi2O3 surface, p-projected density of states (p-DOS) analyses via HCOO* and OCHO* paths were performed (Fig. S30a, b). Following both the HCOO* and OCHO* adsorption, the p-DOS of Sn-Bi2O3 compared to Bi2O3 shifts towards the Fermi level (Ef), confirming the stronger binding affinity to HCOO* and OCHO*. However, the p-DOS associated with OCHO* adsorption analysis reveals a higher Ef than HCOO*, thereby validating that OCHO* exhibits superior binding affinity over the HCOO*. Moreover, a significant upward shift in the Ef of the p-DOS of the Bi and Sn sites in Sn-Bi2O3 is observed via OCHO* and HCOO* intermediates. This alteration causes an increase in the antibonding state and decreases the bonding states, resulting in a stronger binding affinity towards intermediates48. Hence, an effective HCOOH yield is associated with a shift in adsorption energy and the synergistic effect of Sn and Bi2O3. These computed DOS findings agree with the Gibbs free energy simulation outcomes. This is the case even at a high negative potential range; CO2 and consecutive intermediates possess adequate adsorption capabilities on the catalyst surface.Post-reaction characterizationsThe TEM, XRD, and XPS analyses of the 100 h post-reacted Sn-Bi2O3 NCs were conducted to investigate the alterations in surface morphology, structural properties, and chemical state. The post-electrolysis morphological characteristics of Sn-Bi2O3 NCs were examined using TEM and HRTEM. Post-reaction TEM analysis shows that the catalyst’s surface maintains its initial NSs morphology significantly. However, a notable alteration in the shape of the post-reacted sample’s NSs is observed, where initial smooth NSs demonstrate a rougher shape. This morphological transformation is likely a result of surface reconstruction processes occurring within the catalyst during the electrolysis period49. Moreover, the HRTEM analyses show that Bi2O3 partially reduced to metallic Bi while the core retained its original state (Fig. S31a, b). However, Sn undergoes oxidation while partially retaining its metallic state. Observing the consistent selectivity trends in the initial and final hours of CO2RR, it is estimated that the number of accessible active sites per electrode area primarily influences the partial variation in overall electrode activity. These structural arrangements maintain the CO2 reactants and by-products through their active sites. Moreover, the XRD analysis of the 100 h post-reacted catalyst is characterized, in which diffraction patterns revealed the conversion of the Bi2O3 into the Bi2O2CO3 phase, consistent with JCPDS No. 41-1448 (Fig. S32)50. The emergence of the Bi2O2CO3 phase is attributed to aqueous electrolyte-mediated conversion by reaction with HCO3− of KHCO3 electrolyte51. This factor plays a pivotal role in influencing the CO2 conversion pathway, featuring the critical impact of electrolyte conditions on the reaction mechanism50. These XRD results highlight that the Bi2O2CO3 does not compromise the catalyst’s fundamental core structure, affirming its stable crystal structure during long-run CO2RR. Following 100 h of post-CO2RR, XPS analysis of the Bi 4f spectrum of the post-reacted Sn-Bi2O2CO3 catalyst presented two distinct peaks corresponding to Bi 4f5/2 and Bi 4f7/2, which validate the presence of Bi3+ (Fig. S33a). Simultaneously, in the post-reacted catalyst, the rise of two additional energy peaks at 161.2 and 156.5 eV attributed to Bi 4f5/2 and Bi 4f7/2 confirms the formation of metallic Bi0 (Fig. S33a), consistent with reported literature52. The dominant presence of the Bi3+ peak in the post-reacted catalyst indicates partial conversion to the metallic Bi0. In the subsequent analysis, the O 1s spectra of the post-treated sample demonstrated a higher oxygen concentration than the pre-treated sample, indicating increased oxygen adsorption (Fig. S33b). This rise of the additional oxygen peak is attributed to the CO3− formed when the Bi2O3 reacts with aqueous HCO3− and is converted into the Bi2O2CO3. Subsequently, in the post-reacted Sn-Bi2O3, Sn species exhibited oxidation, with valence states ranging from Sn0 to Sn+4, further authenticating electronic interactions between the Sn and the Bi2O3 (Fig. S33c). This observation confirms that the strong interfacial interaction between Sn and Bi2O3 causes a significant alteration in the material’s electronic structure. Despite the partial conversion of Bi3+ to Bi0 and Sn0 to Sn4+, the retention of peaks corresponding to both Bi3+ and Sn0 at their respective positions confirms the structure’s integrity of the 100-h post-reacted catalyst. Overall, the post-morphology, crystal structure, and chemical states remain intact except for the change of the crystal phase, partial transitions of Bi3+ to metallic Bi, and oxidation of Sn0. Furthermore, this partial conversion does not hinder the catalyst’s durability and selectivity towards HCOOH, aligning with findings reported during CO2RR50,52,53,54,55. Post-reaction ECSA was performed further to confirm the electrocatalytic active sites. The cyclic voltammetry curves, linearly fitted at various scan rates, showed that the Cdl value for Bi2O3 NSs post-reaction was recorded at 0.68 mF cm−2, which slightly decreased from its pre-reaction value of 0.76 mF cm−2 (Fig. S20a–c). In contrast, the Sn-Bi2O3 NCs also preserved their ECSA performance after reaction and recorded Cdl at 2.11 mF cm−2, which is minimally below the pre-reaction value of 2.23 mF cm−2 (Fig. S20d–f). These post-reaction ECSA results show that catalysts maintained their ECSA performance with a minimal decrease in the active surface area during 100 h of continuous operation. However, this slight decrease in ECSA can be associated with blocking some active sites owing to the strong adsorption of some contaminants onto the catalyst surface. These post-reaction measurements indicate that Bi2O3 and Sn-Bi2O3 maintain their catalytic durability and are consistent with post-reaction morphological and chemical state results.Catalytic reconstruction mechanismBased on the post-reaction findings, a catalytic reconstruction mechanism is proposed (Figs. S34–S35). During CO2RR, when Bi2O3 were immersed into the 0.5 M aqueous KHCO3 electrolyte, the HCO3− ions reacted with catalysts, which underwent an electrolyte-mediated transition and reconstructed a metastable Bi2O2CO3 phase as confirmed by XRD analyses (Fig. S32). Further, during CO2RR, when a negative potential is applied to the Sn-Bi2O2, a partial conversion from Bi3+ to metallic Bi0 in the post-reacted sample is confirmed by XPS analysis (Fig. S33). Subsequently, oxidation in the Sn species of the post-reacted sample, with valence states ranging from Sn0 to Sn+4, was further confirmed. These transitions confirm strong electron coupling between Sn and Bi2O3, which promotes electron transfer at their interfaces and changes the electronic structure of active sites, thereby improving the CO2 conversion rate. During CO2RR, the reaction environment and negative applied potential are the main driving forces to phase and valance state conversions. Overall, the conversion of Bi2O3 into the Bi2O2CO3 phase and partial conversion to metallic Bi with oxidation of Sn0 do not affect the post-reacted catalyst’s core structure.
