Activating SnO_x-coated Cu₂O photocathodes for efficient photoelectrochemical CO₂ reduction and unassisted tandem device integration

Abstract

Efficient and selective photoelectrochemical CO₂ reduction remains a significant challenge for solar fuel production. Here, we fabricated a heterostructured Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ photocathode featuring a pristine SnO₂ layer deposited via atomic layer deposition. Upon photoelectrochemical activation, the SnO₂ layer transformed into uniformly distributed SnO_x nanoparticles across the photocathode surface. Systematic characterization revealed the SnO₂ layer's structural evolution. The Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode significantly enhanced the charge transfer characteristics and PEC CO₂ reduction performance, delivering a current density of −2.5 mA cm⁻² at −0.2 V vs. RHE and a formate faradaic efficiency of 75.4% at −0.1 V vs. RHE. Coupling with a semitransparent BiVO₄/NiCo LDH photoanode enables unassisted, solar-driven CO₂ reduction and ethylene glycol (EG) oxidation, achieving a current density of 0.11 mA cm⁻² and producing 0.13 µmol cm⁻² formate yield in the first 0.5 hour. This work presents a scalable and earth-abundant material-based strategy for efficient PEC CO₂ reduction and underscores the potential of tandem architectures for sustainable solar fuel generation.

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1. Introduction

The production of solar fuels from CO₂ and water with sunlight, often termed artificial photosynthesis, represents a promising and innovative approach to address global energy and environmental challenges.^1,2 Photoelectrochemical (PEC) CO₂ reduction (CO₂R) utilizes photoelectrodes to harness solar energy directly, driving CO₂ reduction reactions through photo-generated electrons and converting CO₂ into valuable carbon-based fuels or chemcials.^3,4 However, photocathodes used in PEC-CO₂R systems face key limitations, including low energy efficiencies and poor product selectivity.^5,6

The requirement for the conduction band level to be higher than the equilibrium potential of CO₂ reduction limits the number of semiconductors suitable for CO₂ conversion.⁷ Prominent examples include p-GaP,⁸ p-InP,⁹ p-Si^10–12 and p-Cu₂O.^13,14 Among them, p-Cu₂O, a metal oxide semiconductor with a band gap of approximately 2.0 eV, offers a theoretical photovoltage of 1.6 V and a theoretical photocurrent density of −14.7 mA cm⁻² under the AM 1.5 G illumination, rendering it an attractive choice for PEC-CO₂R.¹⁵ Highly efficient heterostructure Cu₂O photocathodes for the PEC hydrogen evolution reaction (HER) have been developed with a buried p–n junction, consisting of the Cu₂O photo-absorber, buffer layer and protection layer.^16–18 However, the application of such heterostructures in CO₂R remains underexplored.

Grätzel and co-workers demonstrated PEC CO₂ reduction using a Cu₂O-based heterostructure photocathode modified with an electrochemically deposited Sn/SnO_x catalyst layer, achieving faradaic efficiencies (FEs) of 42.9% for H₂, 22.7% for CO, and 34.4% for formate.¹⁹ Despite this progress, two major challenges hinder the practical implementation of Cu₂O-based PEC-CO₂R systems: (1) the thermodynamic competition between HER and CO₂ reduction pathways, and (2) inadequate control over product selectivity. Advancing the viability of these systems requires strategic improvements in charge carrier dynamics and rational interface engineering to enhance catalytic selectivity.

Beyond photocathode development, constructing a solar-driven, unassisted PEC device necessitates pairing with a suitable photoanode or dark anode for the oxidation half-reaction.^20,21 However, water oxidation suffers from sluggish kinetics. In contrast, ethylene glycol (EG) offers a favorable alternative, with a significantly lower oxidation potential (∼0.2 V vs. RHE),^22,23 thereby reducing the anodic energy barrier and facilitating overall device operation.

In this study, we developed a heterostructure Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ photocathode featuring the SnO₂ pristine catalyst layer deposited by atomic layer deposition (ALD). After activation, the homogeneous SnO₂ pristine catalyst layer transformed into SnO_x, resulting in a densely and uniformly dispersed nanoparticle layer across the photoelectrode surface. The structural evolution of the SnO₂ layer and its structure–property correlation with PEC performance were unambiguously elucidated. The Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode demonstrated superior charge transfer characteristics and enhanced photoelectrochemical performance, delivering a current density of −2.5 mA cm⁻² at −0.2 V vs. RHE. Notably, its FE of formate reached 75.4% at −0.1 vs. RHE.

Furthermore, we integrated the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode with a semi-transparent BiVO₄ photoanode to construct a tandem PEC device. This system achieved a solar-to-fuel (STF) efficiency of 0.074% for the co-production of formate, CO, and H₂. Operating in a mixed electrolyte of 1 M KHCO₃ and 1 M EG under AM 1.5 G illumination, CO₂ reduction to formate on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode and EG oxidation to formate on the BiVO₄ photoanode occurred simultaneously. The device sustained an average current density of 0.11 mA cm⁻² and yielded 0.13 µmol cm⁻² of formate within the first 0.5 hour. These results underscore the potential of unassisted all-oxide tandem Cu₂O-BiVO₄ PEC devices for efficient, solar-driven CO₂ conversion and sustainable valuable chemical production.

2. Results and discussion

The heterostructure Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂ photocathodes without a catalyst layer were fabricated according to the previous report (refer to the SI for detailed information).¹⁷ Then the SnO₂ pre-catalyst layer was deposited onto it by atomic layer deposition (ALD). The morphology and structure of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathodes were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As illustrated in Fig. 1a, the distinct structure of the assembled cubic particles of Cu₂O was obscured by multiple overlayers. TEM imaging of focused ion beam (FIB)-sliced cross-sections and corresponding energy-dispersive X-ray (EDX) mapping confirmed a well-defined layered architecture (Fig. 1b–d, S1a–i and S2). The SnO₂ film conformed well to the Cu₂O photocathode surface, and its thickness was precisely controlled by the number of ALD cycles. Unless otherwise specified, the “Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ photocathode” referred to a sample with a 60 nm SnO₂ layer.

Fig. 1 (a) SEM image, (b) TEM image in profile view, (c) corresponding layered EDX mapping and (d) EDX mapping of Sn of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode. (e) SEM image, (f) TEM image in profile view, (g) corresponding layered EDX mapping and (h) EDX mapping of Sn of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode. (i) TEM image in profile view of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode, (j) magnified image of white square area in image (j) and (k) HR-TEM image of nanoparticles squared in image (j). (l) GIXRD patterns of Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) and Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathodes collected under 1° incident angle of X-ray. (m) XPS spectra of Sn 3d and (n) Ti 2p of Cu₂O photocathodes.

The SnO₂ layer on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathodes functioned as a pre-catalyst layer, requiring additional activation. To activate it, 10 cycles of linear sweep voltammetry (LSV) were conducted over a potential range of 0.3–0 V vs. RHE in CO₂-saturated 1 M KHCO₃ under AM 1.5 G light illumination. As illustrated in Fig. S3, small nanoparticles emerged on the surface of the Cu₂O photocathode after two LSV cycles, and their size gradually increased to an average of 100–200 nm as the number of LSV cycles increased. During the LSV treatment, the SnO₂ layer was reduced and reconstructed into nanoparticles, which became densely and uniformly dispersed across the Cu₂O photocathode surface (Fig. 1e). FIB-TEM images and corresponding EDX mapping demonstrated that the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode retained the same well-defined layer structure as the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode (Fig. 1f, j and S1j–r), but the continuous SnO₂ layer had transformed into nanoparticles after activation (Fig. 1g and h). The TEM images further demonstrated that these nanoparticles were evenly dispersed on the surface of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode (Fig. 1i).

Grazing incidence X-ray diffraction (GIXRD) was performed to evaluate structural changes. Diffraction peaks observed in the pattern of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode at 0.1° incident angle were attributed to metallic Sn (Fig. S4). However, no peaks appeared in the pattern of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode under the same test conditions, as the SnO₂ synthesized via ALD was amorphous in nature. When the incident angle increased to 1°, diffraction peaks corresponding to Au and Cu₂O were present in both samples, but metallic Sn peaks were only observed in the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode (Fig. 1l). Furthermore, the lattice spacing of the nanoparticles on the photocathode, as shown in the HR-TEM image, was consistent with the (101) and (200) planes of metallic Sn (Fig. 1k). X-ray photoelectron spectroscopy (XPS) was carried out to confirm the chemical states of Sn. In the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode, only Sn⁴⁺ was detected, while in the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode, Sn existed in the metallic Sn⁰, Sn²⁺ and Sn⁴⁺ states (Fig. 1m). Consequently, the SnO₂ layer originally present on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode underwent transformation into mixed-valence SnO_x (x = 0, +2 or +4) following photocathode activation. As demonstrated by Mayer et al., SnO₂ retains its characteristic oxidation states even at applied potentials as negative as −0.9 V vs. RHE.²⁴ The Sn²⁺ and Sn⁴⁺ species detected on the activated Cu₂O photocathode originated partly from residual unreduced SnO₂ and partly from the atmospheric oxidation of metallic Sn during air exposure. The full coverage of 60 nm SnO₂ on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode surface masked the underlying TiO₂ layer, preventing detection of the Ti 2p peak in the XPS spectrum (Fig. 1n). After activation, the continuous SnO₂ layer transformed into discrete nanoparticles, exposing the underlying TiO₂ layer and resulting in a clear Ti 2p peak corresponding to Ti⁴⁺ (Fig. 1n).

Kelvin probe force microscopy (KPFM) was employed to spatially resolve the surface potential of the photocathode samples under dark and illuminated conditions. Atomic force microscopy (AFM) images of the pristine and activated Cu₂O photocathodes revealed a textured surface with smooth corners, and further revealed the presence of small nanoparticles on the surface of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode (Fig. 2a and d), confirming the findings from SEM analysis. The average contact potential difference under dark and light conditions ranged from 250 to 312 mV on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode, whereas it increased from 170 to 589 mV on Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathodes (Fig. 2b, c, e and f). The 6.7-fold increase in the contact potential difference on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode compared to the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode indicated more efficient charge separation and transfer on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode under light illumination. Interestingly, the contact potential difference of the valleys on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode was higher than that of the peaks. In contrast, the contact potential difference of valleys was significantly lower than that of the SnO_x nanoparticles on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode under light illumination conditions, suggesting that electrons accumulate in the valleys of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode and on the SnO_x nanoparticles of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode (Fig. 2g and h). Photoelectrochemical impedance spectroscopy (PEIS) measurements performed at 0.1 V vs. RHE in CO₂ saturated 1 M KHCO₃ under AM 1.5 G light illumination revealed a reduced semicircle diameter for the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode compared to the pristine one (Fig. S5), indicating improved charge transfer efficiency and enhanced carrier separation capability in the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode. In summary, the SnO_x nanoparticles on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode enhance charge separation and facilitate electron transfer to the photocathode surface.

Fig. 2 (a) AFM image, KPFM image collected under (b) dark conditions and (c) light illumination conditions of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode. (d) AFM image, KPFM image collected under (e) dark conditions and (f) light illumination conditions of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode. Schematic images of charge carrier transfer on the (g) Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode and (h) Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode.

The performance of PEC CO₂ reduction on Cu₂O photocathodes was evaluated in an H-type two-chamber cell using a three-electrode configuration (Fig. 3a). The LSV curves of pristine and activated Cu₂O photocathodes, measured in CO₂-saturated 1 M KHCO₃ under chopped-light illumination, are shown in Fig. 3b. The Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode exhibited an onset potential of 0.4 V vs. RHE and a plateau photocurrent density of −2.5 mA cm⁻², better than the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode. The Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode showed significant degradation in CO₂ reduction performance, yielding a FE for formate of only 33.0–41.6% and a total FE of 65.6–68.0% at 0–0.1 V vs. RHE (Fig. S6a). Under these applied potentials, two main reactions occurred on the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ (pristine) photocathode: the reduction of SnO₂ and the CO₂ reduction reaction. The competition between these reactions for available electrons led to the low overall product FE. In contrast, the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode exhibited enhanced CO₂ reduction performance, with the FE for H₂ as low as ∼10%, while the formate FE reached 67.8–75.4% across a broad potential range of −0.1 to 0.2 V versus RHE (Fig. 3c). The production rate of formate reached 28 µmol cm⁻¹ h⁻¹, with an average current density of −1.7 mA cm⁻² at −0.1 V vs. RHE (Fig. 3d). Chronoamperometry measurements revealed a stark contrast in stability: the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathodes showed a notable stability (Fig. 3e), whereas the pristine one degraded at 0 V and 0.1 V vs. RHE (Fig. S6b). We ascribe this instability to the sparse dispersion of SnO_x nanoparticles on the photocathode surface (Fig. S7), a feature particularly pronounced at 0 V vs. RHE and distinct from the activated samples (Fig. S12), as it likely impeded efficient electron transport. The efficiency of incident photon-to-current efficiency (IPCE) of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode reached 33% under 375 nm light illumination, with the integrated current density of approximately 2.25 mA cm⁻² over the whole spectrum range.

Fig. 3 (a) Schematic illustration of PEC CO₂ reduction on Cu₂O photocathodes in a two chamber H-type cell. (b) LSV curves of pristine and activated Cu₂O photocathodes in CO₂-saturated 1 M KHCO₃ under chopped-light illumination. (c) FE of reduction products at different applied potentials, (d) production rate of formate and average current density and (e) chronoamperometry of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode at different applied potentials under light illumination. (f) The IPCE and integrated current density of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode.

The Cu₂O photocathodes with varying SnO₂ layer thicknesses were further investigated. Samples with 20 nm SnO₂ and 40 nm SnO₂ were denoted as Cu₂O-20 and Cu₂O-40 photocathodes, respectively. The Cu₂O photocathode and activated Cu₂O photocathode represented the samples with 60 nm SnO₂ and derived from 60 nm SnO₂, respectively. The LSV curves for Cu₂O photocathodes during activation are illustrated in Fig. S8. Compared with Cu₂O-20 and Cu₂O-40 photocathodes, the Cu₂O photocathode exhibited a more positive onset potential, higher current density and increased fill factor, suggesting higher activity in the PEC CO₂ reduction reaction. LSV measurements of both pristine and activated photocathodes with different SnO₂ thicknesses further demonstrated that the 60 nm SnO₂ layer led to higher catalytic activity (Fig. S9a, b and 3b). The PEC CO₂ reduction performance of activated Cu₂O-20 and Cu₂O-40 photocathodes was assessed under the same test conditions as the activated Cu₂O photocathode, with FE analysis confirming the superior activity of the activated Cu₂O photocathode over its counterparts (Fig. S10). SEM analysis revealed that increasing SnO₂ thickness correlated with enlarged dimensions and higher population density of SnO_x nanoparticles on activated photocathodes (Fig. S11a, b and 1e). However, the SnO_x nanoparticles on the activated Cu₂O photocathode were more densely dispersed. The KPFM results identify these SnO_x nanoparticles as essential electron transfer sites, thus SnO₂ thickness consequently determines the electron transfer capability and the resultant CO₂ reduction performance. SEM imaging combined with EDX analysis demonstrated that the activated Cu₂O photocathode retained enhanced structural stability after 1800 s of testing in CO₂-saturated 1 M KHCO₃, likely attributed to the improved retention of SnO_x particles on its surface compared to Cu₂O-20 and Cu₂O-40 counterparts (Fig. S9c, d, S11c, d, S12 and Table S1).

To identify the active phase of Sn on the photocathode, the Sn XPS spectrum of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photoelectrode after CO₂ reduction testing was collected. As shown in Fig. S13, Sn exhibits a mixed phase of metallic Sn⁰, Sn²⁺, and Sn⁴⁺ species, a phase composition similar to that of the fresh Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode (Fig. 1m). The previously reported studies systematically indicated that Sn–O (Sn^δ+) species are the active sites for formate production.^25,26 Mayer et al. also confirmed the persistence of SnO_δ (δ = +2 or +4) even at applied potentials as negative as −0.9 V vs. RHE by in situ X-ray absorption spectroscopy.²⁴ Therefore, we conclude that the Sn^δ+ species is the active phase for the formate production in our work.

A prolonged stability test was conducted on the activated Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x photocathode at 0 V vs. RHE. As shown in Fig. S14, the current density retained about 77% of its initial value after 1800 s but decreased to approximately 38% by 3600 s. To investigate the origin of this performance degradation, we performed SEM and XRD characterization on the sample after the 3600 s test. The SEM image (Fig. S15) shows that the SnO_x nanoparticles became sparsely dispersed on the photocathode surface, indicating a substantial loss of active sites essential for electron transfer and CO₂ reduction. This observation was further corroborated by the XRD results (Fig. S16), in which the characteristic peaks corresponding to Sn species became nearly negligible after 3600 s of operation. Consequently, we attribute the performance degradation primarily to the loss of SnO_x species.

For unassisted solar-driven PEC operation, the Cu₂O photocathode was paired with a semitransparent BiVO₄/NiCo LDH photoanode²⁷ (Fig. 4a). To match the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode of CO₂ reduction, the semitransparent BiVO₄ photoanode decorated with NiCo LDH catalysts (BiVO₄/NiCo LDH) was prepared for the anodic reaction (Fig. S17). The BiVO₄/NiCo LDH photoanode was first investigated for its potential to drive water oxidation. The LSV curves of these two photoelectrodes were collected using three-electrode configuration in CO₂-saturated 1 M KHCO₃. The BiVO₄/NiCo LDH photoanode was back-illuminated during LSV testing, while the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode was positioned posterior to the photoanode and irradiated by transmitted light passing through the BiVO₄/NiCo LDH (Fig. 4a). As shown in Fig. 4b, the BiVO₄/NiCo LDH photoanode exhibited a positive onset potential and sluggish current rise in LSV, indicating inefficient water oxidation kinetics. The LSV curves of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode and BiVO₄/NiCo LDH photoanode intersected at a point characterized by very low current density and high potential, highlighting an energy-mismatched operating point in the tandem system. Under AM 1.5 G light illumination, the Cu₂O-BiVO₄ tandem device operated in the CO₂-saturated 1 M KHCO₃ electrolyte for 1 h, exhibiting an average current density of 0.03 mA cm⁻² (Fig. 4c). The FE of formate reached 31.1% during the initial 0.5 h. However, formate production became negligible in the latter half hour, with hydrogen evolution dominating the product distribution (Fig. 4b and S18). The solar-to-fuel (STF) efficiency for the first half hour was calculated to be 0.022%, 0.029%, and 0.078% accounting for formate alone, formate combined with CO, and the total sum of formate/CO/H₂ products, respectively (Fig. 4d).

Fig. 4 (a) Schematic illustration of the solar-driven unassisted Cu₂O-BiVO₄ tandem device. LSV curves of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode and BiVO₄/NiCo LDH photoanode for (b) water oxidation and (e) EG oxidation. (c) Current density and FE of formate, (d) STF for the Cu₂O-BiVO₄ tandem device with water oxidation as the anodic reaction. (f) Current density and yield of formate for the Cu₂O-BiVO₄ tandem device with EG oxidation as the anodic reaction.

To lower the energy consumption of the anodic reaction, EG oxidation was further examined as a substitute for water oxidation. The LSV curves of the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode for CO₂ reduction and BiVO₄/NiCo LDH photoanode for EG oxidation were collected in a CO₂-saturated mixture of 1 M KHCO₃ and 1 M EG. The LSV curve of the BiVO₄/NiCo-LDH photoanode exhibited a more negative onset potential and a significantly steeper current density increase during ethylene glycol (EG) oxidation compared to water oxidation, demonstrating its superior catalytic activity toward EG oxidation (Fig. 4e). The LSV curves intersected at 0.3 V vs. RHE (0.16 mA cm⁻²), demonstrating the potential of the Cu₂O-BiVO₄ tandem device for co-driven CO₂ reduction and EG oxidation. The EG oxidation activity of the BiVO₄/NiCo LDH photoanode was further measured in a CO₂-saturated mixture of 1 M KHCO₃ and 1 M EG in a H-type two-chamber cell with three-electrode configuration. The FEs of formate from EG oxidation were 9.4% and 11.0% at 0.3 V and 0.4 V vs. RHE, respectively (Fig. S19a), accompanied by relatively stable current density (Fig. S19b). As reported by Yan et al., the electrooxidation of EG exhibits high formate selectivity in alkaline media, whereas glycolaldehyde becomes the dominant product under weakly alkaline to neutral conditions.²⁸ Specifically, at pH = 8, the FE of glycolaldehyde exceeds 60%, while the FE for formate drops below 10%. In this work, 1 M KHCO₃ was adopted as the electrolyte, which under CO₂-saturated conditions reaches a pH of approximately 7.8. This explains why the BiVO₄/NiCo LDH anode exhibited a low formate FE of only about 10%. The FE of formate for the Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode achieved 48.2% at 0.3 V vs. RHE in a mixture of 1 M KHCO₃ and 1 M EG (Fig. S19c). The BiVO₄/NiCo LDH photoanode and Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO_x (activated) photocathode both demonstrated formate selectivity at the intersection potential of the LSV curves (0.3 V vs. RHE), confirming the feasibility of simultaneous CO₂ reduction and EG oxidation in the Cu₂O-BiVO₄ tandem device.

The Cu₂O-BiVO₄ tandem device retained stable operation for three hours in a CO₂-saturated mixture of 1 M KHCO₃ and 1 M EG under AM 1.5 G light illumination, yielding a time-dependent formate production of 0.13 µmol cm⁻² within the initial 0.5 h and a cumulative yield of 0.34 µmol cm⁻² by the end of the test (Fig. 4f). The average current density of the tandem device was 0.105 mA cm⁻² during the first 0.5 hours of operation and stabilized at 0.071 mA cm⁻² over 3 hours. As shown in Fig. S20, the FE of H₂ progressively increased, which may indicate catalyst degradation. The Cu₂O-BiVO₄ PEC tandem device successfully demonstrated the feasibility of driving CO₂ reduction and EG oxidation for the production of formate under solar illumination (Fig. 4a). However, further improvements are required to enhance the device's efficiency and stability.

3. Conclusions

We developed a heterostructure Cu₂O photocathode with an ALD-deposited SnO₂ overlayer that transforms into uniformly distributed SnO_x nanoparticles upon activation. This structural evolution enhances interfacial charge transfer and boosts CO₂ reduction performance, achieving −2.5 mA cm⁻² at −0.2 V vs. RHE and a 75.4% formate faradaic efficiency at −0.1 V vs. RHE in CO₂-saturated 1 M KHCO₃. Integration with a semitransparent BiVO₄/NiCo-LDH photoanode enabled an unassisted tandem PEC device with 0.074% solar-to-fuel efficiency. In a KHCO₃/EG electrolyte, the system produced 0.13 µmol cm⁻² of formate in 0.5 h under AM 1.5 G illumination, coupling CO₂ reduction with ethylene glycol oxidation. This work highlights a scalable strategy for activating earth-abundant photocathodes and establishes a viable tandem platform for solar-driven CO₂ utilization and value-added chemical synthesis.

4. Experimental

4.1. Preparation of the activated Cu₂O photocathodes

The Cu₂O-x samples were pre-activated by running 10 cycles of linear sweep voltammetry (LSV) at a scan rate of 10 mV s⁻¹ in the potential range of 0.3 V vs. RHE to 0 V vs. RHE in CO₂-saturated 1 M KHCO₃ (99.99% metal basis, Aladdin) under AM 1.5 G light illumination. The LSV measurements were performed with a scan rate of 10 mV s⁻¹ in the range of 0.6 V vs. RHE to −0.2 V vs. RHE on the samples before and after LSV activation when necessary. The Cu₂O-x samples after activation were denoted as activated Cu₂O-x. The activated Cu₂O photocathode represented the photocathode derived from the 60 nm SnO₂ photocathode unless otherwise indicated.

4.2. Materials characterization

The morphologies of the samples were imaged by SEM (JSM-7800F), and corresponding EDX spectroscopic mapping was collected when required. FIB-TEM images were acquired on an FEI Talos F200X G2 operated at a 300 kV accelerating voltage and collected corresponding EDX mapping when required. GIXRD was performed on an Aeris X-ray diffractometer (Malvern Panalytical) over a 2θ range of 20–80° at a scan rate of 10° min⁻¹. XPS was performed on a Thermo Scientific ESCALAB 250 Xi XPS spectrometer, with an Al Ka X-ray source operating at 1486.6 eV. The spectra were obtained with an analyser pass energy of 100 eV with an energy spacing of 1 eV. AFM and KPFM measurements were performed on a Bruker Dimension Icon under dark and illumination conditions. The contact potential difference (CPD) measured by KPFM represents the difference between the probe's and the sample's surface potential.

4.3. Photoelectrochemical CO₂ reduction

Photoelectrochemical CO₂ reduction was carried out in a two-chamber H-type cell separated by an anion exchange membrane. The activated Cu₂O-x (around 0.25 cm²), platinum gauze (1 cm²) and Ag/AgCl electrode were adopted as the working electrode, counter electrode and reference electrode, respectively. 15 mL of CO₂-saturated 1 M KHCO₃ (pH 7.8) was used as an electrolyte for both chambers. CO₂ gas was bubbled into the electrolyte with a flow rate of 10 sccm during the test continuously. A Xe lamp with a light intensity of 100 mW cm⁻² illuminated the activated Cu₂O-x during the chronoamperometry tests in the potential range of 0.2 to −0.1 V vs. RHE. Each test was carried out for 30 min and repeated three times.

The gas products were detected using an online GC (GC9790 Plus, Fuli Instruments) equipped with two flame ionization detector (FID) and one thermal conductivity detectors (TCD), which enables the detection of hydrogen, carbon monoxide, methane, ethane and ethylene. The gaseous products were automatically sampled every 10 min. The liquid products were quantified by high performance liquid chromatography (HPLC, Agilent 1260) equipped with a refractive index detector (RID), a variable wavelength ultraviolet detector (VWD) and an Aminex HPX-87H Column (Bio-rad, 300 × 7.8 mm). The catholyte was collected after performance tests, and 900 µL catholyte and 100 µL 4.5 M H₂SO₄ were mixed to balance the pH of the catholyte and 5 mM H₂SO₄ was utilized as a carrier liquid.

Photoelectrochemical impedance spectroscopy (PEIS) was carried out on the pristine and activated Cu₂O photocathode in CO₂ saturated 1 M KHCO₃ under light illumination conditions at 0.1 V vs. RHE. The IPCE was measured by comparing the photoresponse of the activated Cu₂O photocathode and silicon diode at every 10 nm in the range of 300–700 nm under light from a xenon lamp through a monochromator (7IS151W, Newport).

4.4. Photoelectrochemical ethylene glycol oxidation

Photoelectrochemical ethylene glycol (EG) oxidation was carried out in a two-chamber H-type cell separated by an anion exchange membrane. The BiVO₄/NiCo LDH (1 cm²), platinum gauze (1 cm²) and Ag/AgCl electrode were adopted as the working electrode, counter electrode and reference electrode, respectively. 15 mL of CO₂ saturated 1 M KHCO₃ containing 1 M EG was used as the anolyte and 1 M KHCO₃ was used as the catholyte. CO₂ gas was bubbled into the electrolyte with a flow rate of 10 sccm during the test continuously. A Xe lamp with a light intensity of 100 mW cm⁻² illuminated the BiVO₄-NiCo LDH from the backside during the chronoamperometry tests in the potential range of 0.3 to 0.4 V vs. RHE. Each test was carried out for 30 min. The anolyte was collected after each test for liquid product detection. HPLC was used to quantify the mass of formate.

4.5. Construction of the solar-driven unassisted PEC device

The LSV curves for the activated Cu₂O photocathode and the BiVO₄/NiCo LDH photoanode were recorded in a single-chamber quartz cell. During the LSV measurement of the BiVO₄/NiCo LDH photoanode, the BiVO₄/NiCo LDH served as the working electrode, while a platinum gauze (1 cm²) and an Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. Light illumination was applied from the backside of the BiVO₄/NiCo LDH photoanode. For the LSV measurement of the activated Cu₂O photocathode, a three-electrode configuration was used. The BiVO₄/NiCo LDH photoanode was placed in front of the activated Cu₂O to replicate the light illumination conditions of the tandem device. Light passing through the BiVO₄/NiCo LDH illuminated the activated Cu₂O photocathode. CO₂-saturated 1 M KHCO₃ was used as the electrolyte for water oxidation measurements. For the evaluation of EG oxidation, a CO₂-saturated mixture of 1 M KHCO₃ and 1 M EG was utilized.

The activated Cu₂O photocathode and BiVO₄/NiCo LDH photoanode were coupled as an unassisted device for the tandem device. The activated Cu₂O and BiVO₄/NiCo LDH were connected using a copper conductor, and two electrodes were fixed face-to-face with a distance of 5 mm. 15 mL of CO₂-saturated 1 M KHCO₃ or 15 mL of a CO₂-saturated mixture of 1 M KHCO₃ and 1 M EG were used as the electrolyte for water oxidation and EG oxidation, respectively, and CO₂ gas was continuously bubbled into the electrolyte with a flow rate of 10 sccm during the test. A Xe lamp with a light intensity of 100 mW cm⁻² illuminated on the back side of BiVO₄/NiCo LDH, and then the light that penetrated the photoanode further illuminated the activated Cu₂O during the running process. Photocurrent was recorded using an electrochemical workstation under bias-free conditions. The electrolyte was collected every 30 min during the device running, and the liquid product was detected by HPLC. The gas products H₂ and CO were detected by online GC.

4.6. Calculation of the solar-to-fuel (STF) efficiency

where J is the photocurrent density of the Cu₂O-BiVO₄ tandem device; E_a is the equilibrium potential of water oxidation; E_b is the equilibrium potential of CO₂ reduction to formate; FE is the faradaic efficiency of formate on the activated Cu₂O photocathode; P is the incident irradiance in mW cm⁻².

Conflicts of interest

The authors declare no competing conflicts.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: detailed description of systhesis method of Cu₂O photocathode and semitransparent BiVO₄ photoanode. SEM, TEM and correspongding EDX mapping images, XRD patterns, PEIS curves, XPS spetra, LSV and I–t curves, FE of reduction or oxidation products and tables of elements percentage. See DOI: https://doi.org/10.1039/d5ta07811c.

Acknowledgements

L. W. acknowledges the funding support from the National Natural Science Foundation of China (22209081) and the start-up funding of Inner Mongolia University (No. 10000-23112101/131). J. L. acknowledges the funding support from the National Key Research and Development Program of China (Grant No. 2019YFE0123400), the National Natural Science Foundation of China (Grant No. 52072187 and 22122903), and the Tianjin Distinguished Young Scholar Fund (Grant No. 20JCJQJC00260). D. D. acknowledges the funding support from the National Natural Science Foundation of China (22309089).

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Footnote

† These authors contributed equally to this work.

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