A CO₂ adsorption-enhanced semiconductor/metal-complex hybrid photoelectrocatalytic interface for efficient formate production

Abstract

In photoelectrochemical CO₂ conversion, the concentration of fixed CO₂ on the photocathode surface is of primary concern. Herein, a CO₂ adsorption-enhanced semiconductor/metal-complex hybrid photoelectrocatalytic interface was established by utilizing a carbon aerogel as the CO₂ fixation substrate. In CO₂ reduction photoelectrocatalysis, Co₃O₄ was employed as the light harvester, and Ru(bpy)₂dppz was utilized as the electron transfer mediator and CO₂ activator. The CO₂ surface concentration exhibited a 380-fold increase on this hybrid interface than that on Co₃O₄/FTO. The CO₂ conversion to formate occurred at an onset potential of −0.45 V (vs. normal hydrogen electrode, NHE) under photoelectrochemical conditions, 160 mV more positive than its thermodynamic redox potential. At an applied potential of −0.60 V (vs. NHE), the selectivity of the formate yield reached 99.95%, with a production rate of approximately 110 μmol cm⁻² h⁻¹ and a Faradaic efficiency of 86%. Such a conversion has an electron transfer rate of 2.94 × 10⁻³ cm s⁻¹. The CO₂ conversion to formate was confirmed to be an instantaneous proton-coupled electron transfer process, originating from the rapid photoelectrochemical activation of bpy and dppz in Ru(bpy)₂dppz as well as the synergic effect of the promoted CO₂ adsorption and the applied molecular catalysis.

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Broader context

In natural photosynthesis, CO₂ is fixed by ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCo) and then reduced by a regulated proton-coupled electron transfer process. Thus, there are three aspects to achieve a highly effective process of CO₂ conversion to fuels by mimicking natural photosynthesis: enhanced CO₂ adsorption, rapid electron transfer, and decreased energy input. For a high CO₂ surface concentration, molecular catalysts, such as Ru(II) bipyridyl complexes, are promising candidates due to their unique properties in CO₂ binding affinity. The application of photoelectrochemical methods that integrate photocatalysis with electrocatalysis provides a promising strategy for CO₂ reduction at a low overpotential. This study demonstrates the use of highly porous and adsorptive carbon aerogels (CA) to promote CO₂ fixation as well as the employment of a robust visible-light harvester Co₃O₄ photoelectrocatalyst and Ru(bpy)₂dppz as a molecular catalyst to accelerate electron transfer for CO₂ reduction. This interface enables high CO₂ adsorption, a rapid electron transfer rate and a high yield of formate, an important industrial C1 stock. This reduction occurred at a relatively low onset potential under photoelectrochemical conditions and with a moderate Faradaic efficiency, originating from the multiple synergies among the enhanced CO₂ adsorption from CA, the distinguished photoelectrocatalytic activity of Co₃O₄ and the rapid electrochemical kinetics of Ru(bpy)₂dppz.

Introduction

Natural photosynthesis converts CO₂ into a series of intermediate products (e.g., malate and pyruvate) and finally to glucose with the assistance of solar light.¹ In such a carbon recycling process, CO₂ is fixed by ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCo). Solar light is harvested, and the electrons are generated by cytochrome. A proton-coupled electron transfer is then regulated and mediated by nicotinamide adenine dinucleotide phosphate hydride, leading to the photochemical reduction of CO₂.¹ Unfortunately, such a process has a low conversion efficiency. Processes mimicking natural photosynthesis, so-called artificial photosynthesis, have been developed to increase this conversion efficiency and provide more pathways to produce valuable chemicals using CO₂ as the carbon stock.^2–8

CO₂ adsorption is of the greatest concern during the CO₂ reduction process because the reduction kinetics are highly correlated with the CO₂ concentration. However, the free CO₂ concentration in aqueous solution is only approximately 0.034 M, which severely deteriorates the aqueous heterogeneous reduction. Although various CO₂ absorptive materials have been proposed,^9–11 their CO₂ adsorption performance and mechanism during catalytic processes have not been investigated in-depth. Moreover, the obtained efficiency for the CO₂ reduction still remains to be further increased.

Semiconductor-based photoelectrochemical methods are currently popular and have been successful for CO₂ reduction in that the synergetic conjunction of photoelectrocatalysis (namely, electrocatalysis and photocatalysis) facilitates the separation of photo-induced electrons and holes under applied electric fields.^6,7 Light irradiation-induced band bending compensates the required overpotential for CO₂ reduction, causing the CO₂ conversion to occur at relatively positive potentials. Surprisingly, the majority of semiconductors exhibited non-specific CO₂ surficial binding. In other words, the CO₂ concentration on the surface of the semiconductors was rather low. Therefore, only slow reduction kinetics were reported.^6,7 In addition, the majority of semiconductors have poor conductivities, which deteriorated the electron transfer rate as well. Thus, photoelectrochemical CO₂ reduction still occurred at relatively negative potentials in these systems, for example, at −1.0 V for MoS₂/TiO₂ and InP/TiO₂ (vs. normal hydrogen electrode, NHE, the same as below),^12,13 at −1.1 V for ZnTe,⁵ at −1.2 V for p-GaP and GaAs,¹⁴ and at −0.9 V for Mg–CuFeO₂.³ Furthermore, inevitable hydrogen evolution was observed, leading to relatively low Faradaic efficiency.

As an alternative to those semiconductors, molecular catalysts, such as Ru(II) or Re(II) bipyridyl complexes, have been employed because they present unique properties in CO₂ binding affinity, and high CO₂ surface concentrations are expected.^15,16 Furthermore, the formation of a chemical bond between CO₂ and the metal centre will lead to a high turnover rate and high selectivity of CO₂ conversion during a catalytic cycle.^15,16 The main drawback of these catalysts is that additional pathways are required to activate these molecular catalysts. In most cases, an extremely negative potential is required. For example, potentials of −1.52, −1.6, and −1.73 were applied for Ru(bpy)(tpy)(NCCH₃),¹⁷ Re(tBu-bpy)(CO)₃Cl,¹⁸ and Re(pbn)(CO)₃Cl,¹⁹ respectively, to achieve the catalytic reduction of CO₂. These molecular catalysts encountered destabilization (e.g., dimerization or dissolution) during a long-term electrolysis process, resulting in rapidly reduced photoelectrochemical activity for CO₂ conversion.^20,21

The complementary combination of a semiconductor and a molecular catalyst was then developed for photoelectrochemical CO₂ conversion.^14,22–24 These combinations accelerated electron transfer generation and lowered the activation barriers for both molecular catalysts and CO₂ binding. The highly efficient and highly selective generation of hydrocarbon fuels, such as methanol and formic acid, has been reported on these systems.^14,22–24 However, these systems have shown significant flaws, such as high overpotentials (e.g., −1.0 V on H–Si–Re(tBu-bpy)(CO)₃Cl),¹⁴ poor selectivity of reduction products (e.g., the simultaneous formation of HCOOH, H₂ and CO on Ag/TaON–Ru(bpy)₃–(CH₂)₂–Ru(bpy)(CO)₂Cl₂, while HCOOH occupies only 56.5%),²³ and low Faradaic efficiency (e.g., only 62.3% on [Ru(dcbpy)₂(CO)₂]_n–p-InP–Zn).²⁴

Thus, the aim of this work is to construct an adsorptive photoelectrochemical interface for CO₂ reduction with better performance. The constructed interface is schematically shown in Scheme 1. In our concept, CO₂ fixation is promoted by utilizing an adsorptive substrate, a conductive, micropore-dominated three-dimensional carbon aerogel (CA) with a high surface area (up to 1815 m² g⁻¹); solar light is efficiently harvested by Co₃O₄ micro-flowers, a visible-light driven and robust photoelectrocatalyst. These highly index-faceted Co(III)-enriched {121̄} structures are grown epitaxially inside CA networks. An enzyme-mimicking molecular catalyst, Ru(bpy)₂dppz, is immobilized on Co₃O₄/CA to accelerate and regulate the electron transfer for CO₂ reduction. Using such an interface, the CO₂ conversion to formate has been achieved at an onset reduction potential under photoelectrochemical conditions as low as −0.45 V, with a yield of approximately 110 μmol cm⁻² h⁻¹, a selectivity of 99.95%, and a Faradaic efficiency of 86% at −0.60 V.

Scheme 1 Schematic plots of the CO₂ adsorption-enhanced Ru(bpy)₂dppz-Co₃O₄/CA interface together with its energy level diagram and the possible reaction pathways for CO₂ conversion on this photocathode. Such a photoelectrocatalytic interface is composed of CA as the CO₂-adsorption substrate, Ru(bpy)₂dppz as the molecular catalyst, and Co₃O₄ as the photoelectrocatalyst.

Experimental section

Fabrication of the Ru(bpy)₂dppz-Co₃O₄/CA photocathode

Monolith bulky CA was synthesized via an ambient pressure resorcinol-formaldehyde (RF) drying method. The detailed synthesis process of CA is shown as Scheme S1 in the ESI.† A solvothermal reaction was utilized to synthesize Co₃O₄/CA. In a 50 mL acetone–water mixture (V_acetone : V_water = 5 : 45), 1.4552 g of Co(NO₃)₂·6H₂O, 0.2593 g of NH₄F, and 1.4019 g of hexamethylenetetramine were added. Magnetic stirring of this mixture for 10 min led to the formation of a pink transparent solution. The solution was then transferred to a Teflon-lined stainless steel autoclave, where one slide of CA (4 × 1.5 cm²) was located. The solvothermal reaction was conducted at 95 °C for 24 h. After cooling down to room temperature, deposition (pink to violet colour) occurred on the CA. The as-prepared sample was carefully rinsed with acetone and then dried under vacuum at 60 °C for 2 h, followed by a calcination process in a N₂ atmosphere at 450 °C for 2 h. The ramping rate was 10 °C min⁻¹. After such a calcination process, the Co₃O₄/CA sample was synthesized. The amount of Co₃O₄/CA loading, averaged over the geometric area of the CA, was typically 8 mg cm⁻². A similar procedure was applied for the preparation of the control sample (Co₃O₄/FTO). The loading density of Co₃O₄ on FTO was 5 mg cm⁻². In this case, a piece of fluorine-doped tin oxide glass electrode (FTO, 4 × 1 cm²) was put into the autoclave instead of CA.

Chemical polymerization was used to decorate the Co₃O₄/CA with Ru(bpy)₂dppz (the detailed synthesis procedure for Ru(bpy)₂dppz is provided in the ESI†). Briefly, 0.0040 g of Ru(bpy)₂dppz (5.75 μmol) was dissolved in 1 mL of acetonitrile solution, denoted as solution A. Next, 0.0650 g of Fe(NO₃)₃·9H₂O and 8 μL of pyrrole were dissolved in 1 mL of ethanol, and the mixture was denoted as solution B. The stock solution for the electrode coating was obtained by mixing solutions A and B (with equal volumes) and shaking. One slide of Co₃O₄/CA was then coated with 200 μL of the stock solution in a dropwise manner, and the composite was dried at 60 °C for 5 min. After repeating the coating procedure 10 times, Ru(bpy)₂dppz-Co₃O₄/CA was obtained.

Electrochemical quartz crystal microbalance

To calculate the amount of CO₂ adsorbed onto the photocathodes, electrochemical quartz crystal microbalance (EQCM) experiments were conducted on a CHI440A (CH Instruments Inc., USA) with a Au-coated AT-cut quartz crystal (a fundamental frequency of 8 MHz) as the working electrode.

Approximately 10 mg of ground CA powder was ultrasonically dispersed in 1 mL of H₂O along with 20 μL of 2% Nafion-117 film solution (Alfa Aesar). After intense ultrasonication for 5 min, 10 μL of the dispersed solution was dip-coated onto the Au electrode and dried in air. The CA-modified electrode was further used in the EQCM studies.

In situ IR spectroelectrochemical experiments

For the in situ IR spectroelectrochemical experiments, the mercury–cadmium–telluride detector was cooled down to 77 K by liquid nitrogen. A self-made four-bottleneck cell with a CaF₂ window (2 mm thickness, 25 mm in diameter) was used as the electrochemical cell. A Pt disk electrode (5 mm in diameter), a Ag/AgCl (filled with saturated KCl) electrode, and a Pt foil (1 × 1.5 cm²) were used as the working, reference and counter electrodes, respectively. A mixture of 0.010 g mL⁻¹ Ru(bpy)₂dppz and 2% Nafion-117 was stirred in a sonication bath for 1 min. This mixture (10 μL) was dip-coated onto the Pt disk electrode and dried under ambient conditions. One hundred IR spectra were collected with a spectrum resolution of 8 cm⁻¹ and subsequently averaged at each potential. For these experiments, the solution was purged with CO₂ for at least 30 min to completely remove the dissolved oxygen and saturate the solution with CO₂.

Photoelectrochemical characterization and reduction of CO₂

To evaluate the performance of the Ru(bpy)₂dppz-Co₃O₄/CA catalyst, constant potential photoelectrolysis of CO₂ was conducted in order to evaluate the performance of Ru(bpy)₂dppz-Co₃O₄/CA, which was conducted in a home-made H-type cell with a maximum volume of 100 mL. The as-prepared Ru(bpy)₂dppz-Co₃O₄/CA working electrode and the Ag/AgCl (filled with saturated KCl) reference electrode were placed in the cathodic chamber, while the counter electrode, a graphite plate (4 × 1 cm²), was placed in the anodic chamber. The two chambers were connected with 0.1 M NaHCO₃ but separated with a Nafion-117 proton exchange membrane (Dupont). Prior to the experiments, the electrolyte in the cathodic chamber was purged with high-purity CO₂ (99.99%) gas for more than 30 min at a flow rate of 20 mL min⁻¹. Negative potentials (0.0, −0.2, −0.4, −0.6, −0.8, −1.0 V) were applied to the photocathode through the electrochemical workstation. An APLS-SXE300 xenon lamp with a UV cutoff (λ > 420 nm, light intensity at 9 mW cm⁻²) was used as the light source and illuminated on the Ru(bpy)₂dppz-Co₃O₄/CA photocathode upon the addition of negative potential.

After such constant potential photoelectrolysis for 8 h, the reduction products were collected and quantitatively determined by HPLC and GC using the same procedure as described previously.²⁵ For the products in the aqueous phase, 0.2 mL of the liquid sample was collected and transferred into a 10 mL test-tube. The pH of the sample was adjusted to neutral by adding 0.2 mL of pH 7.6 phosphate buffer solution. Subsequently, 2,3,4,5,6-pentafluorobenzyl bromide (20 g L⁻¹, 1.0 mL) was added. The mixture was shaken for 1 min and then kept at 60 °C for another 1 h. The esterification product was extracted with 2.0 mL of n-hexane and centrifuged at 3000 rpm for 5 min. The upper layer was the organic phase, which was filtered through a 0.45 μm membrane. A C18 column was used with a mobile phase consisting of 65% methanol and 35% H₂O at a flow rate of 1.0 mL min⁻¹. The detection wavelength was 225 nm. For gaseous products, 1.0 mL of the gas sample was collected through a syringe. The detection conditions were an injection inlet temperature of 130 °C, an oven temperature of 80 °C, a detector temperature of 150 °C, N₂ carrier gas, and a gas flow rate of 0.2 L min⁻¹.

Prior to an isotopic ¹³C experiment, ¹³CO₂ (¹³C enrichment 98%) was purged into 0.1 M NaH¹³CO₃ (¹³C enrichment 98%) electrolyte solution for at least 30 min in order to fully expel oxygen and other impurity gases. The photoelectrochemical reduction of ¹³CO₂ saturated NaH¹³CO₃ (0.1 M) was identical to the procedure described in the Experimental section, holding the constant potential at −1.2 V vs. Ag/AgCl. Blank experiments using nitrogen purged Na₂SO₄ (0.1 M) were also conducted using an identical procedure. After reduction, 0.5 mL of catholyte solution was mixed with 0.1 mL of D₂O (Sigma Aldrich) containing 0.5 μL of DMSO as the internal standard. A one-dimensional ¹H nuclear magnetic resonance (NMR) spectrum was recorded with water suppression using a pre-saturation method.

Results and discussion

Enhanced CO₂ adsorption

For photoelectrochemical CO₂ conversion, the surface concentration of CO₂ on the photocathode, namely, CO₂ fixation, is a chief concern. A high surface concentration of CO₂ on the photocathode accelerates the photoelectrochemical kinetics. Herein, the surface concentration of CO₂ (Γ_ads), the normalized amount of adsorbed CO₂ with the electrochemical active surface area (S_EASA), was adopted as the parameter to evaluate the efficiency of CO₂ fixation on the photocathodes.

The results in Table S1 (ESI†) demonstrate the advantage of using a CA substrate for CO₂ fixation. This statement is further supported by the EQCM results of CA in a CO₂-saturated electrolyte under negative potentials, as shown in Fig. S1 (ESI†). The mass addition is 6–10 times heavier on activated CA than that on an Au electrode. For example, at −0.4 V, a mass addition of 174 ng cm⁻² was obtained on a CA substrate, whereas it was barely observed on an Au quartz substrate. At −0.6 V, the mass addition on the CA substrate was 8.2-fold larger than that on the Au quartz substrate. At −0.9 V, the mass addition reached 143.5 ng cm⁻² on the CA substrate. When the applied potentials were more negative than −0.9 V, the mass addition on the CA substrate was even larger than that on the Au quartz substrate. CA is not capable of reducing CO₂ electrochemically; thus, the mass addition on the CA substrate is mainly ascribed to the promoted electro-sorption of carbonaceous species (e.g., CO₂ and HCO₃⁻) from the electrolyte. The carbonaceous species are significantly promoted because the micropores contributed a high Brunauer–Emmett–Teller surface area (S_BET) of up to 1815 m² g⁻¹ (Fig. S2, ESI†).

This high S_BET offers numerous sites for CO₂ adsorption, as indicated by Fujishima et al. and Yaghi et al.,^21,26 as CO₂ molecules tend to be adsorbed in the micropores of a material. This value is also higher than that of common porous carbon materials, such as ordered mesoporous carbon (812.3 m² g⁻¹), commercially available Vulcan (237.9 m² g⁻¹) and carbon nanocoil (233 m² g⁻¹).²⁷ The superior performance of CA supports the notion that CO₂ adsorption on other high-surface-area carbon materials (e.g., activated carbon, carbon nanotubes) might result from the following aspects. First, the microporous feature of CA (Fig. S2, ESI†) offers numerous sites for CO₂ adsorption. Second, the activated CA possesses a relatively high surface area (1815 m² g⁻¹), which provides many sites for Co₃O₄ loading. According to our measurements, the Co₃O₄ loading amount was increased to 8 mg cm⁻² compared to 5 mg cm⁻² on FTO. The S_BET of Co₃O₄/CA of 713 m² g⁻¹ (Fig. S2a, ESI†) is considerably higher than that of commonly designed porous inorganic semiconductor electrodes.^28,29 This result is further supported by the well-maintained micropore domination of the electrode (Fig. S2b, ESI†). Based on the voltammetry (Fig. S3, ESI†) and chronocoulometry (Fig. S4, ESI†) for Co₃O₄/CA, Γ_ads exhibits a 20-fold increment (0.25 pmol cm⁻²) compared to that (0.01 pmol cm⁻²) of Co₃O₄/FTO. The electrochemically active surface area (S_EASA) was determined to be 8015 cm² (Table S1, calculated from Fig. S5, ESI†) with respect to that of Co₃O₄/FTO (316 cm²). Experimental tests on the effects on CO₂ fixation of other porous carbon-based substrates are currently in progress.

As shown in the scanning electron microscopic (SEM) images of Co₃O₄/CA (Fig. S6, ESI†), the CA backbone is clearly visible even after the solvothermal growth of Co₃O₄ micro-flowers. The XRD patterns of both CA and Co₃O₄ in Co₃O₄/CA can also be clearly observed (Fig. S7, ESI†). In other words, such an epitaxial growth pattern fully exposes the adsorption sites to CO₂, which results in an increase in the Γ_ads for CO₂. Moreover, the 3D structure of CA allows for the penetration of electrolytes into the pores of the electrode, leading to an increase in the interfacial area between the electrode and the electrolyte. Finally, CA possesses a high conductivity (electrical resistivity < 40 mΩ), similar to those of carbon nanotubes and graphene.

Γ _ads further increases to 3.79 pmol cm⁻² when Co₃O₄/CA is coated with a molecular catalyst, Ru(bpy)₂dppz, which is 15.2 and 300 times larger compared to those on Co₃O₄/CA and Co₃O₄/FTO, respectively. However, the S_EASA of Co₃O₄/CA is approximately 4 times larger than that of Ru(bpy)₂dppz-Co₃O₄/CA, likely due to the blockage of Co(III) active sites by immobilized Ru(bpy)₂dppz during the chemical polymerization. Nonetheless, the active area of this electrode is maintained at 1.64 × 10⁻² cm² (Table S1 and Fig. S3, ESI†). The existence of the Ru(bpy)₂dppz molecular catalyst promotes CO₂ fixation on the photocathode with an enhancement factor of 300. Γ_ads was also normalized by the catalyst weight (Table S1, ESI†) and exhibited the same trends as that normalized by S_EASA. Hence, the enhancement of CO₂ adsorption and photoelectrochemical density is in fact the synergic effect of multiple factors: the electrosorption from the CA substrate, the Co(III) sites from the high index faceted Co₃O₄ on CA and the electrochemical activity of Ru(bpy)₂dppz. These results confirm that CA, Co₃O₄ and Ru(bpy)₂dppz co-promote CO₂ fixation and lead to a high CO₂ surface concentration on such an interface. Thus, accelerated kinetics are expected for photoelectrochemical CO₂ conversion.

Photoelectrochemical CO₂ conversion

The reactivity of Ru(bpy)₂dppz as a homogenous catalyst toward electrochemical CO₂ conversion was first studied in CO₂-saturated NaHCO₃ solution. As shown in Fig. S8 (ESI†), the CO₂ reduction potential remains at −0.40 V in the current aqueous solution, which is at least 950 mV more positive than those reported in organic solvents.³⁰ This is the first time that a molecular catalyst, polybipyridyl Ru(II), was observed to convert/reduce CO₂ at such a low potential, indicating the high electron transfer ability of Ru(bpy)₂dppz toward CO₂ conversion.

CO₂ conversion on Ru(bpy)₂dppz-Co₃O₄/CA was investigated in detail using a potentiodynamic mode. As seen in Fig. 1a, CO₂ conversion/reduction occurs at an onset potential of −0.45 V under photoelectrochemical conditions (red line). This potential is an “underpotential” of approximately 160 mV with respect to the thermodynamic redox potential for CO₂ to formic acid.³¹ The peak potential is approximately −0.61 V, considerably lower than other reported values, e.g., −1.33 V on p-Cu₂O immobilized by Re(tBu-bipy)(CO)₃Cl³² and −0.6 V on p-InP–Zn decorated by [Ru(dcbpy)₂(CO)₂]_n²⁴ On Co₃O₄/CA, the reduction potential is approximately −0.62 V with an onset potential of −0.52 V under photoelectrochemical conditions (dashed lines). Moreover, a second peak appears at −0.73 V. On Co₃O₄/FTO, the onset potential is −0.67 V (dotted lines), indicating an entirely different photoelectrochemical reduction pathway. The positive shift of the CO₂ reduction potential on Ru(bpy)₂dppz-Co₃O₄/CA is likely due to the increased surface concentration of CO₂ and the catalytic role of Ru(bpy)₂dppz. Based on these peak potentials, the heterogeneous electron transfer rate constant (k_s) for CO₂ reduction was calculated using a method provided in the literature.³³k_s is 2.94 × 10⁻³ cm s⁻¹ for Ru(bpy)₂dppz-Co₃O₄/CA, which is 26% larger than that (2.19 × 10⁻³ cm s⁻¹) for Co₃O₄/CA.

Fig. 1 (a) Linear sweep voltammograms (LSVs) of Co₃O₄/FTO (dotted line, with a magnification factor of 10), Co₃O₄/CA (dashed line) and Ru(bpy)₂dppz-Co₃O₄/CA (solid line) in CO₂-saturated 0.1 M NaHCO₃ at a scan rate of 50 mV s⁻¹ under illumination conditions; (b) LSVs of Ru(bpy)₂dppz-Co₃O₄/CA in N₂-purged 0.1 M Na₂SO₄ (dotted line), N₂-purged 0.1 M NaHCO₃ with (dashed line)/without (dash-dotted line) light irradiation, and CO₂-saturated 0.1 M NaHCO₃ without (solid line) light irradiation at a scan rate of 50 mV s⁻¹; (c) amperometric i–t curves on Co₃O₄/FTO (dotted line), Co₃O₄/CA (dashed line) and Ru(bpy)₂dppz-Co₃O₄/CA (solid line) in CO₂-saturated 0.1 M Na₂SO₄ at a potential of −0.40 V. The lines were shifted in the Y-axis direction for comparison.

This higher k_s value suggests an accelerated and enhanced electron transfer on Ru(bpy)₂dppz-Co₃O₄/CA. Such a method to evaluate the efficiency of CO₂ conversion is entirely new. According to our knowledge, this method has not been presented in the literature to date. Considering the CO₂ reduction potential and k_s on both electrodes, the catalytically active centre for CO₂ reduction is Ru(bpy)₂dppz-Co₃O₄/CA, not Co(III). This statement is further demonstrated from the Tafel plots described below.

The current density reaches approximately 8.1 mA cm⁻² on Ru(bpy)₂dppz-Co₃O₄/CA, which is considerably more intense than sole Ru(bpy)₂dppz and those reported using similar molecular catalyst–semiconductor composites. For instance, on Ru(dcbpy)₂(CO)₂/p-InP and CO-dehydrogenase/p-NiO, the photocurrent density reached only the level of μA cm⁻² at similar potentials. A photocurrent density larger than −2.0 mA cm⁻² was obtained on a Re(tBu-bpy)(CO)₃Cl/Cu₂O photocathode, but a potential of −2.0 V was applied.³² So, photoelectrochemical CO₂ conversion on Ru(bpy)₂dppz-Co₃O₄/CA occurs at a low overpotential but with a high photocurrent density.

The current densities shown in this paper were obtained using the geometric areas of the electrodes. Due to their porous structures, CA-based photoelectrodes will have a higher S_EASA value. Their S_EASA values were determined using surface-sensitive redox probes of Fe(CN)₆^3−/4− (Table S1 and Fig. S3, ESI†). A 2.7-fold enhancement was observed after loading the same amount of Co₃O₄ onto FTO. The current densities were then re-calculated using their S_EASA values. The magnitude of the current densities followed the same trend, namely, in the order of Co₃O₄/FTO < Co₃O₄/CA < Ru(bpy)₂dppz-Co₃O₄/CA, the same as that obtained from the current densities normalized by the geometric area. To compare our results with those obtained using other porous materials presented in the literature, the current densities shown throughout the paper were then calculated using the geometric areas of the photoelectrodes, the most frequently applied approach for electrochemical and photoelectrochemical CO₂ reduction.

Fig. 1b shows the LSVs on Ru(bpy)₂dppz-Co₃O₄/CA in 0.1 M NaHCO₃ solution purged with N₂ or CO₂. The pH value of 0.1 M NaHCO₃ is in the range of 8.3–8.5 after being purged with N₂. Once it was saturated with CO₂, the pH value decreased to the range of 6.5–7.0.³⁴ Such a weak acidic environment provides a favourable protic environment for CO₂ reduction using a Ru(bpy)₂dppz catalyst.^2–5 Notably, the peak potential for CO₂ reduction is 50 mV more positive than that of its N₂-purged counterpart, with the peak current density doubled. These results clearly confirm the involvement of protons in CO₂ reduction. Fig. 1b shows the effect of light irradiation on CO₂ conversion as well. On Ru(bpy)₂dppz-Co₃O₄/CA without light irradiation, the peak current density of the CO₂ reduction decreases by a ratio of 35% along with a 50 mV negative shift in the peak potential compared to the case when light irradiation is applied. Similar tendencies were observed on Co₃O₄/CA and Co₃O₄/FTO (Fig. S9, ESI†). This phenomenon could be due to the photoelectrocatalytic properties of the Ru(bpy)₂dppz-Co₃O₄/CA electrode.

An amperometric photocurrent response was further investigated at a fixed potential of −0.40 V. As shown in Fig. 1c, the steady photocurrent density reaches approximately 1.5 mA cm⁻² on Ru(bpy)₂dppz-Co₃O₄/CA, with a stabilizing time of only 40 s, whereas on Co₃O₄/CA, the peak photocurrent intensity decreases by approximately 7 times compared to Ru(bpy)₂dppz-Co₃O₄/CA. However, the peak photocurrent intensity is 8–10 times higher than that on Co₃O₄/FTO. The stabilization times for Co₃O₄/CA and Co₃O₄/FTO are 150 and 200 s, respectively. The potentiodynamic voltammograms in 0.1 M Na₂SO₄ also reflected that Ru(bpy)₂dppz-Co₃O₄/CA showed a distinguished photoelectrochemical activity. The difference between the photocurrent (j_PEC) and dark current (j_EC), denoted as j_PEC − j_EC, is summarized in Fig. S10 (ESI†). j_PEC − j_EC is approximately 10 times higher on Ru(bpy)₂dppz-Co₃O₄/CA than on FTO. This result could be explained by several reasons. First, the {121̄} crystal facet of Co₃O₄ (with an inter-planar spacing of 0.286 nm) being finely exposed (Fig. S11, ESI†), which enriches triply uncoordinated Co(III) sites. Such a high-index-facet Co₃O₄ facilitates the photoelectrochemical reduction of CO₂, as our previous work has indicated.⁴ Second, the epitaxial growth manner of high-index-facet Co₃O₄ on CA provided a direct electron transfer channel. Photo-induced holes could rapidly transfer to the highly conductive CA network. Electrons and holes are efficiently separated. Finally, the ingenious merging of photocatalysis and electrocatalysis on a single surface has been found to efficiently separate the photo-induced carriers, resulting in a reduction of the overpotential for CO₂ conversion.^2,4 The applied negative potential creates a more upward bending for Co₃O₄, a p-type semiconductor. Then, the driving force for photoelectrons to cross the semiconductor–electrolyte junction is enlarged, resulting in an enhanced photocurrent density. In the meantime, the light-induced upward band-bending of Co₃O₄/CA lifts the Fermi level, which compensates part of the required applied negative potential under dark conditions, yielding a reduced overpotential for CO₂ reduction. All of these effects reveal the essential role of Co₃O₄ as the photoelectrocatalyst in a photoelectrochemical strategy for CO₂ conversion.

Comparing the cyclic voltammograms of different photoelectrodes, the lowest overpotential and the highest photocurrent density for CO₂ conversion were obtained on Ru(bpy)₂dppz-Co₃O₄/CA. Apart from the respective contributions from the enhanced adsorption on CA and distinguished photoelectrochemical properties of Co₃O₄/CA, the molecular catalyst Ru(bpy)₂dppz also exhibits excellent electrochemical reductive activity toward CO₂ reduction, as suggested by the above-mentioned k_s. The synergistic effect among those effects exerted on Ru(bpy)₂dppz-Co₃O₄/CA has superiority in magnifying the photocurrent density, reducing the overpotential, and accelerating and regulating the electron transfer pathway for the photoelectrochemical CO₂ reduction process.

The photoelectrochemical CO₂ conversion on Ru(bpy)₂dppz-Co₃O₄/CA was further studied in CO₂-saturated 0.1 M NaHCO₃ by varying the applied potential. Its variation with the potentials applied is summarized in Fig. 2a. The amplitude of j(CO₂) − j(N₂) increases when the potential is more negative, following the order of Co₃O₄/FTO < Co₃O₄/CA < Ru(bpy)₂dppz-Co₃O₄/CA. A sharp increase in the amplitude of j(CO₂) − j(N₂) occurs at −0.4 V. At a reduction potential of −0.6 V, the net photocurrent reaches −0.35 mA cm⁻². This value is twice as high as that on Co₃O₄/CA and nearly 10 times higher than that on Co₃O₄/FTO.

Fig. 2 (a) Variation of j(CO₂) − j(N₂), on Co₃O₄/FTO (grey), Co₃O₄/CA (cyan) and Ru(bpy)₂dppz-Co₃O₄/CA (red) with the applied potential; (b) variation of the yield rates of formate on the photocathode of Co₃O₄/FTO (gray triangle), Ru(bpy)₂dppz/CA (green rhombus), Co₃O₄/CA (cyan square) and Ru(bpy)₂dppz-Co₃O₄/CA (red circle) with the applied potentials. The dashed lines are for guiding the eyes. (c) ¹H NMR spectra of the reduction product in N₂-purged Na₂SO₄(I), CO₂-saturated NaHCO₃(II) and ¹³CO₂-saturated NaH¹³CO₃(III). The concentrations of these solutions were 0.1 M.

Under identical photoelectrolysis conditions, the yield rate of formate on these four photocathodes is in the order of Ru(bpy)₂dppz-Co₃O₄/CA > Co₃O₄/CA > Ru(bpy)₂dppz/CA > Co₃O₄/FTO at the same applied potentials, as shown in Fig. 2b. For example, 869.8 μmol formate was produced at a potential of −0.6 V on Ru(bpy)₂dppz-Co₃O₄/CA. This value is considerably higher than that presented previously (384 μmol) on hierarchical Co₃O₄, even at −0.7 V.⁵ If the electrode area is considered, the predicted yield rate of formate on Ru(bpy)₂dppz-Co₃O₄/CA is approximately 450 μmol cm⁻² at the onset potential of −0.45 V under photoelectrochemical conditions. At −0.7 V, the yield rate of formate reaches approximately 900 μmol cm⁻². Considering the reduction time span further, the yield rate of formate reaches nearly 110 μmol cm⁻² h⁻¹, even at a reduction potential of −0.6 V. The yield rate of formate on Ru(bpy)₂dppz-Co₃O₄/CA is higher than most reported values, e.g., nearly 29-, 5.2-, and 2-fold higher than that on Ru(II)–Re(I) multinuclear complexes,²² Ru(II) electro-polymerized p-InP–Zn,²⁴ and Cu₂ZnSnS₄,³⁵ respectively. The estimated Faradaic efficiency for the yield of formate is approximately 86%, which is comparable to that on semiconductor–molecular photocatalytic systems as well as on other semiconductor-based photocatalytic systems, such as Mg-doped CuFeO₂³ and Au/ZnO/ZnTe.⁵ Notably, formate yield rates on the Ru(bpy)₂dppz/CA were lower than that on the Co₃O₄/CA at each investigated potential, the reason is described below.

To confirm the carbon source for the reduction product of formate, isotopic measurements were conducted using ¹³CO₂-saturated NaH¹³CO₃ (0.1 M) electrolyte solution.^36,37 Control experiments were also performed in N₂-purged Na₂SO₄ (0.1 M) solutions. These related ¹H-NMR spectra are shown as Fig. 2c. The spectrum obtained from the N₂-purged Na₂SO₄ (0.1 M) solutions is featureless (Fig. 2c-I) in the chemical shift range of 9.2 to 7.7, indicating that no formate is produced when no carbonaceous species (e.g., CO₂, HCO₃⁻) are present. In Fig. 2c-II, a singlet peak at the chemical shift value of 8.42 is detected, likely resulting from the yield of HCOO⁻ from CO₂-saturated NaHCO₃. When ¹³CO₂-saturated NaH¹³CO₃ solution is used, a doublet peak is shown at the chemical shift values of 8.64 and 8.25 (Fig. 2c-III). This doublet peak indicates the production of H¹³COO⁻.³ In other words, formate is obtained from the ¹³C species in the electrolyte rather than from the impurities in the electrolyte or on the surface of the photocathode. Photoelectrochemical CO₂ reduction thus occurs on the adsorption-enhanced molecular catalyst–semiconductor hybrid interface.

To investigate the energy conversion efficiency, the turnover number (TON) and turnover frequency (TOF) for photoelectrochemical CO₂ reduction were estimated at different potentials on this interface. These values are tabulated in Tables S2 and S3 (ESI†). Co₃O₄/CA has a 3- to 5-fold increment in both TON and TOF compared to Co₃O₄/FTO, resulting from the effect of improved CO₂ adsorption. This result further demonstrates the importance of the primary CO₂ fixation process. On the Ru(bpy)₂dppz molecular catalyst immobilized interface, the magnitudes of both TON and TOF were increased by approximately 1 order. This trend was also proven by the results from [Zn(II)TRP]⁴⁺/[SiW₁₂O₄₀]^4− 38 and COF-367-Co,²¹ both of which possess a high surface area, thus resulting in a high TON value. The TON value was increased with the negative shift of the potential. For example, at −0.6 V vs. NHE, the TON value already reached 978.7, with a TOF value of 122. In Table 1, these results are further compared with those shown in the literature. The higher TON value (978.7) in our case compared to those reported (including those of similar semiconductor–molecular complex model systems (e.g., InP/[MCE2-A + MCE4]–TiO₂/Pt,³⁹ the NiO–RuRe complex⁴⁰ and Ru(dcbpy)/N-Ta₂O₅⁴¹)) demonstrates the distinguished photoelectrochemical performance of the concurrent Ru(bpy)₂dppz-Co₃O₄/CA system.

Table 1 TON and TOF compared to results presented in the literature

Catalyst	Condition	Product	TON (TOF)	Ref.
Ru(bpy)2dppz-Co3O4/CA	−0.6 V vs. NHE, 9 mW cm^−2 Xe lamp, 8 h	HCOO^−	978.7 (122)	This work
Ru(bpy-H2PO3)2(CO)2Cl2–C3N4	400 W Hg lamp (λ > 400 nm), 20 h	HCOOH	1100 (55)	42
Ru(bpy)3–(CH2)2–Re(CH3-bpy)(CO)3Cl	Hg lamp (λ > 500 nm), 24 h	HCOOH	25 (1.04)	43
Ru(H4P2O6–C2H4-bpy)(CO)2Cl2-mpg C3N4	450 W Hg lamp (λ > 400 nm), 20 h	HCOOH	∼210 (10.5)	44
InP/[MCE2-A + MCE4]	AM 1.5G, 24 h	HCOO^−	>17 (0.7)	39
Ru(dcbpy)/N-Ta2O5	Xe lamp (410 < λ < 750 nm), 20 h	HCOO^−	90 (4.5)	45
NiO–RuRe complex	−1.2 V vs. Ag/Ag^+, 300 W Xe lamp, 5 h	CO	32 (6.4)	40
Zn–TPP–Re complex	200 W Hg lamp, λ > 375 nm, 50 h	CO	12.8 (0.26)	46
Re(bpy)(CO)3Cl	−1.25 V vs. SHE, 14 h	CO	300 (22)	47
[Co(CR)Cl2](ClO4)	LED light strip (λ > 460 nm), 22 h	CO	268 (12.18)	48

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Hydrogen, as the only side product detected, only occupied 0.05% of the total products, which is indicative of the high selectivity of Ru(bpy)₂dppz-Co₃O₄/CA toward CO₂ reduction. On Co₃O₄/CA, the hydrogen production is 3 times (Fig. S12b, ESI†) higher, but the yield rate of formate is approximately 1.5 times lower (Fig. 2b). On Ru(bpy)₂dppz/CA (Fig. S12c, ESI†), lower amounts of H₂ were detected after applying a potential of −0.9 V. On Co₃O₄/FTO (Fig. S12d, ESI†), the yield rate of formate is the lowest, although no hydrogen is detected. The selectivity for Ru(bpy)₂dppz-Co₃O₄/CA is higher than those of Ru(II)-electropolymerized p-InP–Zn²⁴ and Cu₂ZnSnS₄,³⁵ in which non-negligible amounts of H₂ and CO were generated. Therefore, the selectivity of the CO₂ conversion to formate is extremely high on Ru(bpy)₂dppz-Co₃O₄/CA (99.95%).

The hydrogen evolution potentials were further estimated based on the potential-dependent hydrogen evolution profiles shown in Fig. 2b and Fig. S12a–d (ESI†). The potentials on Ru(bpy)₂dppz-Co₃O₄/CA, Co₃O₄/CA, Ru(bpy)₂dppz/CA and Co₃O₄/FTO are −0.6 V, −0.7 V and −0.9 V vs. NHE, respectively. However, there are no clear cathodic peaks shown in Fig. 1 for hydrogen evolution because the Ru(II) centre on the molecular catalyst Ru(bpy)₂dppz has fully coordinated to the nitrogen atoms from two 2,2′-bipyridine and dppz ligands. Then, both the bipyridine ligand and dppz are not able to dissociate from Ru(II) during the electrochemical processes. In other words, there will be no opportunity to generate any catalytic wave for hydrogen evolution. These results indicate that this unwanted reaction has not been involved in our system during CO₂ reduction. Thus, an improved conversion efficiency and high selectivity are expected.

The stability of the proposed photoelectrocatalytic interface was examined by recording the XRD patterns and cyclic voltammograms of Ru(bpy)₂dppz-Co₃O₄/CA before and after a long-term photoelectrochemical CO₂ reduction. The XRD patterns of both CA and Co₃O₄ do not vary after the 8 h photoelectrochemical reduction of CO₂ (Fig. S13, ESI†). The peak current density of Ru(bpy)₂dppz-Co₃O₄/CA is slightly enhanced after 100 cycles of cyclic voltammetry (Fig. S14, ESI†). These facts confirm the high stability of our molecular catalyst–semiconductor-assembled photocathode.

The underlying mechanism of such a synergic effect, the high selectivity for formate and its relation to the photoelectrochemical CO₂ reduction is discussed. Prior to this discussion, the interfacial energetics between the two components were analysed. The bandgap (Fig. S15, ESI†) and the flat-band potential (Fig. S16, ESI†) of Co₃O₄/CA were estimated to be 1.87 V and 0.37 V, respectively. Provided that the valence band of Co₃O₄ is 0.1 V more positive than the flat band potential, the valence band of Co₃O₄ was estimated to be 0.47 V. So the conduction band of Co₃O₄/CA was estimated to be −1.40 V vs. NHE (E_g = |E_CB − E_VB|). In contrast, the energy difference between the HOMO and the LUMO for Ru(bpy)₂dppz is approximately 2.29 eV, as calculated using the intersection wavelength at 540.8 nm from its normalized UV-vis and fluorescence spectra in acetonitrile (Fig. S17a, ESI†). According to the Ru(III)/Ru(II) redox potential (Fig. S17b, ESI†), the LUMO energy level of Ru(bpy)₂dppz was calculated to be −0.71 V. Ru(bpy)₂dppz's LUMO energy level is then 0.69 V more positive than the conduction band of Co₃O₄/CA. Therefore, the photo-induced electrons on Co₃O₄ transfer to the LUMO of the molecular catalyst, as confirmed by various reports.^{24,32,35,40,41} Although the value of the conduction band and the redox potential of the molecular complex vary with the pH value of the electrolyte, the shift of the redox potential of a similar molecular complex [Ru(phen)₂(ptpbβ)]²⁺ by a rate of −62 mV pH^−1 49 is in line with the typical −59 mV pH⁻¹ for semiconductors.

Electrochemical characterization data can also support such an electron transfer process. The onset potential of electrochemical CO₂ reduction using Ru(bpy)₂dppz was at −0.4 V vs. NHE, which is similar to that for photoelectrochemical CO₂ reduction using Ru(bpy)₂dppz-Co₃O₄ (−0.45 V vs. NHE). Moreover, the cathodic current density of electrochemical CO₂ reduction on Ru(bpy)₂dppz (ca. 0.63 mA cm⁻²) was far smaller than that on Ru(bpy)₂dppz-Co₃O₄ (ca. 15.0 mA cm⁻²). From in situ IR spectroelectrochemical spectra (Fig. 3a), the upward IR peaks at 1419 and 1446 cm⁻¹, assigned to the A1 mode of the C–C–H deformation bending vibration on bpy˙⁻ and dppz˙⁻,^50,51 validate such a statement. A relatively strong broad upward peak at 1716 cm⁻¹ displays the same trend. This peak likely arises from the stretching of C=O from the as-formed formate.^52,53 These facts suggest that the electrons are transferred from the excited Co₃O₄ to Ru(bpy)₂dppz, and then take part in photoelectrochemical CO₂ reduction. As a result of such a photo-induced electron transfer to the LUMO of Ru(bpy)₂dppz, CO₂ conversion/reduction occurs in the adsorptive substrate CA, the photocatalyst Co₃O₄ and the molecular catalyst Ru(bpy)₂dppz-Co₃O₄/CA, exactly as illustrated in Scheme 1. Such an electron transfer process is the core of such an interface. Note that Ru(bpy)₂dppz in its activated form is capable of reducing CO₂ into formate with a two-electron process. The potential of its singly reduced form, Ru(bpy)₂(dppz˙⁻), is −0.95 V vs. NHE in MeCN.³⁰ Its doubly reduced state, Ru(bpy)(bpy˙⁻)(dppz˙⁻), is possible to be obtained by merging Ru(bpy)₂dppz and Co₃O₄. This is because the electrons at the conduction band of Co₃O₄ bear only an energy of −1.40 V. In the presence of Co₃O₄, the formation of a band-alignment interface based on Ru(bpy)₂dppz and Co₃O₄ allows the direct injection of photo-induced electrons to the LUMO of Ru(bpy)₂dppz, which greatly suppresses the quenching of the molecular catalyst in the pure water phase.⁴⁹ Eventually, the photocatalytic activity of Ru(bpy)₂dppz is remarkably improved. This is further supported by the fact that the rate yield of formate, along with the TON/TOF values of the Ru(bpy)₂dppz-Co₃O₄/CA after 8 h of reduction was almost 20 times higher than that on the Ru(bpy)₂dppz/CA (Fig. 2b). And on the basis of the Marcus–Gerischer model,^54,55 the negative bias applied to the Ru(bpy)₂dppz-Co₃O₄ system suppresses the back electrons transferred from Ru(bpy)₂dppz to Co₃O₄. The utilization of the electrons is then greatly improved, and a high heterogeneous electron transfer rate of 2.94 × 10⁻³ cm s⁻¹ is reached.

Fig. 3 (a) In situ photoelectrochemical IR spectra of Ru(bpy)₂dppz within the wavenumber ranges of 4000–3400 cm⁻¹ and 2000–650 cm⁻¹; (b) Koutecky–Levich fitting of the steady-state current on a Ru(bpy)₂dppz-coated glassy carbon electrode. (c) Tafel plots of Ru(bpy)₂dppz-Co₃O₄/CA (circles) and Co₃O₄/CA (squares) at a scan rate of 50 mV s⁻¹. All of these electrochemical measurements were conducted in CO₂-saturated 0.1 M NaHCO₃.

It also noteworthy that the absence of the stretching vibrations of Ru–H, N–H, and C–H in the IR spectra indicates that the protons are not directly bonded to Ru(bpy)₂dppz.^56,57 A broad peak at approximately 3691 cm⁻¹ emerges from the featureless IR spectra when the applied potential is higher than −0.6 V vs. NHE (Fig. 3a). This newly emerged upward peak is assigned to the stretching vibration of a non-hydrogen bonded hydroxyl. Water molecules are attracted and nearly dissociate from their bulky form due to the existence of localized electrons on the bpy ligand in Ru(bpy)₂dppz, thus causing the appearance of the peak at 3691 cm⁻¹. Hydrodynamic voltammograms using rotating glassy carbon disk electrodes were recorded using the electrode coated with Ru(bpy)₂dppz (Fig. S18a, ESI†) and Co₃O₄ (Fig. S18b, ESI†). Fig. 3b shows the related Koutecky–Levich plot for the electrode coated with Ru(bpy)₂dppz, indicating its non-linear features. This non-linearity is quite helpful for CO₂ conversion in that it indicates that CO₂ and the surrounding water fix into the outer sphere of Ru(bpy)₂dppz, apart from its diffusion and surface reaction with Ru(bpy)₂dppz under hydrodynamic conditions.⁵⁸ Taking a Langmuir–Hinshelwood bimolecular reaction mechanism as a model,⁵⁹ these results clearly confirm the enzyme-mimicking role of the molecular catalyst, Ru(bpy)₂dppz, for CO₂ conversion. In contrast, a linear reaction is observed on the electrode coated with Co₃O₄ (Fig. S18c, ESI†), indicating different electrokinetics between Co₃O₄ and Ru(bpy)₂dppz.

The rate-determining step of the CO₂ conversion was estimated through Tafel analysis (Fig. 3c). For Ru(bpy)₂dppz-Co₃O₄/CA, a Tafel slope of 145 mV dec⁻¹ is determined. Due to the porous feature of CA, it is slightly deviated from its classical value of 118 mV dec⁻¹.^60,61 Therefore the rate-determining step of photoelectrochemical CO₂ conversion on Ru(bpy)₂dppz-Co₃O₄/CA is the single electron transfer step rather than the proton transfer step. This result is in agreement with our previous study regarding the photoelectrochemical CO₂ conversion on hierarchical Co₃O₄.⁴ This finding also supports the notion that the protonation of the reaction intermediate occurs rapidly in CO₂-saturated solution. Namely, faster electrode kinetics for CO₂ conversion were obtained with the help of Ru(bpy)₂dppz. In contrast, a Tafel slope of 191 mV dec⁻¹ is observed for Co₃O₄/CA, which originates from the porous structure of CA and is indicative of its slow photoelectrochemical kinetics in the absence of Ru(bpy)₂dppz. In addition, the activation energy of the CO₂ reduction in our system, defined as the energy required to activate CO₂ into its radicals,^62–64 was calculated to be 1.047 kJ mol⁻¹ (see the ESI† for details). This value is presented for the first time in this study and therefore further density functional-based theoretical calculations are required to confirm this value.

Based on the above analysis, the activation of Ru(bpy)₂dppz is accomplished only by the photoelectrochemical process from the irradiated Co₃O₄/CA. Ru(bpy)₂dppz is actually “catalysed” by Co₃O₄/CA rather than merely an electron conductor. Therefore, the reduction performance is markedly enhanced with the assistance of the photoelectrocatalyst of Co₃O₄. Therefore, as schematically shown in Scheme 1, the possible pathways for CO₂ conversion on Ru(bpy)₂dppz-Co₃O₄/CA involve the transfer and mediation of electrons, protons, and the activated form of Ru(bpy)₂dppz.

Conclusions

In summary, an adsorption-enhanced Co₃O₄/Ru(bpy)₂dppz hybrid photoelectrocatalytic interface has been established. The CO₂ adsorption is remarkably increased through the utilization of a high-surface-area and micropore-dominated carbon aerogel substrate. The epitaxial growth of high-index-facet Co₃O₄ along with the intrinsic highly conductive network of the carbon aerogel enables an elevated electron transfer rate with the assistance of a molecular catalyst of Ru(bpy)₂dppz. Eventually, such an interface bears a unique proton-coupled electron-transfer reactivity and rapid electron kinetics toward CO₂ reduction. Through the simultaneous activation of the bpy and dppz ligands of the Ru(II) molecular catalyst, the photoelectrochemical conversion of CO₂ to formate has been realized on such a photocathode with an onset potential as low as −0.45 V vs. NHE. At a potential of −0.6 V vs. NHE, the yield rate of formate reaches approximately 110 μmol cm⁻² h⁻¹, with a selectivity of 99.95% and a Faradaic efficiency of 86%. The mechanism of such a rapid photoelectrochemical electron transfer process is explained as the synergistic effect of the photoelectrochemical activation of bpy and dppz inside Ru(bpy)₂dppz as well as the remarkably promoted CO₂ adsorption on Co₃O₄/CA. Although future work on the effect of the morphology of Co₃O₄/CA (e.g., the size, facets, and shape of Co₃O₄ on CA) and the loading amount of Co₃O₄/CA (e.g., distribution and density) as well as the light density on the efficiency of the CO₂ conversion must be conducted, such an adsorption-enhanced molecular catalytic photoelectrocatalytic interface has potential for application in the production of useful chemicals from CO₂ carbon stock in the future.

Acknowledgements

The authors acknowledge Dr Sanjun Zhang (State Key Laboratory of Precision Spectroscopy, East China Normal University) for his assistance with spectral data analysis and discussion on the electron transfer mechanism. G. Z. acknowledges the financial support from the National Natural Science Foundation of China (NSFC) under projects No. 21537003 and 21477085. N. Y. acknowledges the financial support from the German Research Foundation (DFG) under project YA344/1-1.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental methods, SEM and TEM characterization, chronocoulometry, EQCM results, BET and BJH results, XRD patterns, CVs, UV-DRS, Tauc plots, Mott–Schottky plot, and RDE results. See DOI: 10.1039/c6ee00968a

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