CO2 Synergistic Reduction in a Photoanode-Driven

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CO2 synergistic reduction in a photoanode-driven photoelectrochemical cell with a Pt-modified TiO2 nanotube photoanode and a Pt-RGO electrocathode Meng Zhang, Jun Cheng, Xiaoxu Xuan, Jun-hu Zhou, and Ke-fa Cen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00909 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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ACS Sustainable Chemistry & Engineering

CO2 synergistic reduction in a photoanode-driven photoelectrochemical cell with a Pt-modified TiO2 nanotube photoanode and a Pt-RGO electrocathode Meng Zhang, Jun Cheng∗ ∗, Xiaoxu Xuan, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

ABSTRACT CO2 synergistic reduction in a photoanode-driven photoelectrocatalytic (PEC) cell was conducted with a Pt-modified TiO2 nanotube (Pt-TNT) photoanode and a Pt-modified reduced graphene oxide (Pt-RGO) electrocathode to reduce energy consumption and increase CO2 PEC reduction efficiency. The carbon atom conversion rate of CO2 reduction under PEC conditions was 2.3 times higher than that of the total rate under photocatalytic and electrocatalytic conditions. Synergistic CO2 reduction in the PEC cell was mainly due to the use of the photoanode, which played a dual role during CO2 reduction: (1) anode photovoltage compensated and conferred more negative cathode potential for CO2 reduction; and (2) anode water decomposition provided protons and electrons for cathode CO2 reduction. System current density, product generation rate of CO2 reduction, and carbon atom conversion rate increased first and then decreased with increasing deposition amount of Pt on TNT. The optimal photocatalytic activity of the Pt-TNT anode was obtained with a Pt loading amount of 5%, which resulted in the highest system current density of 4 mA/cm2 and carbon atom conversion rate of 1250 nmol/(h cm2) under the catalysis of the Pt-RGO cathode. Keywords: carbon dioxide; synergistic photoelectrocatalysis; photoanode-driven photoelectrochemical cell; Pt-modified TiO2 nanotube; Pt-modified reduced graphene oxide ∗ Corresponding author: Prof. Dr. Jun Cheng, State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. Tel.: +86 571 87952889; Fax: +86 571 87951616. E-mail: [email protected]

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1. Introduction CO2, as a major component of greenhouse gas, is a potential carbon energy source. However, CO2 transformation and utilization technologies require high energy input because of the stable chemical property of CO21. Hence, novel CO2 transformation and utilization technologies with low energy consumption must be developed. Photoelectrocatalytic (PEC) reduction is one of the most promising techniques for CO2 transformation and utilization2,3. It uses solar energy for stimulation of semiconductors to produce photo-generated electron–hole pairs and induction of redox reactions to produce hydrocarbons by using CO2 and H2O. Different configurations of the PEC system exist depending on which electrode, i.e., anode, cathode, or both, acts as photoelectrode: (1) Photocathode-driven PEC systems: a photocathode made of p-type semiconductor and an anode made of a metal. (2) Photoanode-driven PEC systems: a photoanode made of n-type semiconductor and a cathode made of a metal. (3) (PN junction)-driven PEC systems: a photoanode made of n-type semiconductor and a photocathode made of p-type semiconductor.

Photocathode-driven PEC systems,

with a metal anode for O2 evolution, have been widely studied because of their high conduction band energy suitable for CO2 reduction4-8. However, CO2 reduction on p-type semiconductors requires a high bias potential because the valence band potentials of these materials are not sufficiently positive to oxidize water. Moreover, two-electron compounds, such as CO and HCOOH are the main products of CO2 reduction on a bare p-type photocathode3. In contrast to photocathode-driven PEC cell, a photoanode-driven PEC cell with an n-type semiconductor and an electrocathode possesses high feasibility and flexibility for CO2 reduction. This cell type could improve CO2 reduction product selectivity over efficient electrocatalysts and reduce energy input over the photoanode catalyst9,10. CO2 reduction in a photoanode-driven PEC cell depends on two components, cathode catalysts


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and photoanode activity. N-type TiO2 nanotube (TNT) synthesized via anodization of titanium is one of the most promising photocatalytic materials because of its excellent stability against photocorrosion and charge transfer characteristics under band gap illumination in water photoelectrolysis11,12. TiO2 materials exhibits two disadvantages: easy recombination of photo-generated electron–hole pairs restricts the photocatalytic activity of the material; and a small UV fraction of around 4% solar light can be utilized to sensitize TiO2 because of its large band gap of 3.20 eV. Various methods, including decorating noble metal nanoparticles, doping non-metal anions or transition metal cations, and coupling other semiconductors, have been applied to improve the photocatalytic activity and solar energy utilization rate of TiO213-17. Deposition of noble metal nanoparticles has been demonstrated as one of the most promising ways to depress the recombination of photo generated electron-hole pairs in photocatalytic process. Noble metals with low Fermi level are expected to trap electrons and inhibit the recombination of photo-generated electron–hole pairs13-15. Platinum with the lowest Fermi level is one of the best candidates of co-catalyst for trapping electrons14. CO2 adsorption activation is a key step during catalytic reduction of CO2. CO2 adsorption capacity and activation on the catalyst affect and even determine catalytic reduction routine, final reduction products, and reduction efficiency18. Efficient CO2 reduction catalysts should possess large specific surface area and excellent electric properties. Graphene, a one-atom-thick sheet of carbon, has gained wide attention from researchers. Graphene exhibits excellent electrical, thermal, and mechanical properties and is used in fuel cells, super capacitors, organic pollutant removal, and so on19-22. Our previous studies showed that graphene-based catalysts can significantly increase the efficiency CO2 reduction and improve the distribution of CO2 reduction products9,10,23. Therefore, CO2 reduction on graphene-based catalysts deserves further study.



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In this study, a photoanode-driven PEC cell with Pt-modified TiO2 nanotubes (Pt-TNT) as photoanode and Pt-modified reduced graphene oxide (Pt-RGO) as electrocathode was used for synergistic CO2 reduction. The synergistic catalytic mechanisms between photocatalysis and electrocatalysis in the PEC cell induced by the photoanode were experimentally verified and theoretically analyzed. The effect of Pt deposition amount on TNT on the performance of the PEC system and CO2 reduction efficiency was further investigated. 2. Experimental 2.1. Preparation of anode electrode TNT anode was prepared through anodization of Ti foil (0.025 mm thick, 99.6% purity; Baoji Yunjie Metal Production Co., Ltd., China) at 50 V for 3 h with an ethylene glycol solution (2 vol% H2O and 0.3 wt% NH4F) used as electrolyte. Prior to anodization, the Ti foil was mechanically polished through successive ultrasonic degreasing in isopropanol and ethanol. After anodization, the samples were cleaned with deionized water and annealed in a muffle furnace at 450 °C for 3 h to transform amorphous TiO2 to the anatase phase (heating and cooling rate is 2 °C/min). Pt was electrodeposited onto TNT under a constant current of 2 mA/cm2 to improve the photocatalytic performance. TNT was used as working electrode, a nickel sheet was used as counter electrode, and an aqueous solution of 1 g/L H2PtCl6·6H2O was used as electrolyte. 2.2. Preparation of cathode electrode Graphene oxide (GO) was prepared using high-purity flake graphite (G) (Nanjing Pioneer Nanometer Material Technology Co., Ltd., China) as raw material through modified Hummers’ method9. Pt-RGO was prepared through ethylene glycol reduction24. In a typical procedure, 0.8 g of GO was mixed with 200 ml of ethylene glycol, 0.028 g of H2PtCl6·6H2O, and 55 ml of deionized water.


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The mixture was stirred for 2 h and then sonicated for 1 h. The resulting mixture was transferred into a reactor lined with a Teflon inner layer and subjected to reduction at 120 °C for 24 h under constant stirring. The obtained Pt-RGO was washed with ethanol and deionized water and then dried in a freeze dryer. The cathode electrode was prepared using the combination of a Ni foam matrix (90 mesh, Inco Hi-tech Material Co., Ltd., Dalian) and a Pt-RGO catalyst layer. The Pt-RGO catalyst (4 mg/cm2) was mixed with Nafion solution (25 µL/cm2) and deionized water (25 µL/cm2) and then applied onto one side of the Ni foam. The formed cathode was dried in a vacuum oven at 25 °C for 24 h. 2.3. Catalyst characterization X-ray diffraction (XRD) analysis was performed using X’Pert PRO diffractometer (PANalytical, Netherlands) with Cu Kα radiation to determine the crystal phases of the combined electrode. The morphology of the catalysts was characterized by field-emission scanning electron microscopy (FSEM, SU-70, Hitachi, Japan), transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) and atomic force microscopy (AFM, Multimode, Veeco Instruments, USA) analyses. Pt loading amount on TNT or RGO was determined through energy dispersive X-ray spectroscopy (EDX) analysis on the SEM and TEM systems equipped with an energy dispersive X-ray analytical system. 2.4. PEC reduction of CO2 The reaction of CO2 PEC reduction was conducted in a homemade H-type reactor, which was divided into two chambers by a proton exchange membrane (Nafion 115, Dupont, USA). The active surface areas of the cathode and anode were 1 and 6 cm2, respectively. Briefly, 0.5 M H2SO4 and 0.5 M NaHCO3 solutions were used as electrolytes in anodic and cathodic compartments, respectively. A constant voltage of 2V was applied between the anode and the cathode. The photoanode was



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illuminated with a 300 W Xe arc lamp (PLS-SXE300/UV, Perfect Light, China), and the wavelengths of light ranged from 320 nm to 410 nm, with a maximum intensity at 365 nm. A potentiostat (CH instruments, CHI660D) was used for PEC measurements. Water decomposition and O2 evolution were conducted on the photoanode under band-gap illumination and biased voltage. Protons generated on the photoanode passed through the Nafion membrane, and electrons were collected and reached the cathode through an external wire. Protons reacted with CO2 in the presence of electrons under the catalysis of Pt-RGO or recombined with electrons to provide H2 on the cathode. Figure 2 shows the schematic of the PEC system. Product analysis was performed offline. CO2 reduction products, including acids and alcohols, were immediately sampled through the catholyte after CO2 reduction. Hydrogen was collected using a reservoir bag linked with the headspace of the sealed cathode chamber. Alcohols produced were analyzed using a gas chromatography system (GC; Agilent 7820A, USA) equipped with a flame ionization detector and a column of DB-FFTP (φ50 m × 0.32 mm × 0.5 µm). Hydrogen was analyzed using the same GC with a thermal conductivity detector and Porapak Q column (6Ft 1/8 2 mm, Porapak Q 80/100 SST). Acids were analyzed on an ion chromatography system (ICS 2000, Dionex, USA) equipped with a conductivity detector and IonPac AS11-HC analytical column (φ4 mm × 250 mm). Product analysis under each condition was performed in triplicate, and the results were averaged to minimize measurement errors. 3. Results and discussion 3.1. Anode catalyst characterization TNT prepared by anodization of the Ti foil was used as anode catalyst for water decomposition to produce intermediates for CO2 reduction. Fig. 3 (a) and (b) show the SEM pictures of the TNT anode


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with a uniform diameter and smooth wall. The TNTs feature 100 nm diameter, 10 nm thickness, and 6 µm length. TNT prepared with large diameter and thin wall thickness possesses a large specific surface area and is predicted to perform excellent photocatalytic performance. Fig. 3(c) shows the XRD patterns of TNT before and after calcination. The diffraction peaks of the TNT samples without annealing correspond to the characteristic peaks of the Ti matrix; hence, the original TNT prepared by anodization exhibits an amorphous structure. After annealing at 450 °C, sharp and strong diffraction peaks of anatase TiO2 were observed, indicating a good crystallization of TNT. The quantum efficiency of the TNT photocatalyst is very low because of easy recombination of the photo-generated electron–hole pairs13. Pt nanoparticles were subjected to galvanostatic deposition on the TNT to prepare Pt-TNT/Ti composite electrode for the resolution of the aforementioned limitations. By controlling the galvanostatic deposition time, Pt-TNT samples with 1.1%, 5%, 10.6%, and 15.8% Pt loading amount were prepared (Fig. 4 (a), (b), (c), and (d), respectively). At the early stage of galvanostatic deposition, Pt nanoparticles were detected on partial areas on the TNT surface with correspondingly low Pt loading. The amount of Pt particles increases and are evenly dispersed on the surface of TNT with uniform nanoscale size with prolonged deposition. With further extension of galvanostatic deposition time, Pt nanoparticles begin to aggregate on the TNT surface and even block the TNT nanotubes. The aggregation size of Pt particles continuously increases with increasing electrical deposition time. The largest aggregation size of Pt on the TNT surface reaches 300 nm when the loading amount of Pt is 15.8%. 3.2. Cathode catalyst characterization X-ray diffraction analysis was applied to GO and Pt-RGO, and the results are shown in Fig. 5. A sharp and strong diffraction peak at 2θ = 10° was observed for single GO; this peak could be attributed



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to the (002) reflection of the few-layer graphene structure9. After reduction with ethylene glycol, the diffraction peak of GO (002) in Pt-RGO completely disappears. The suppression of GO peak indicates that the conjugated graphene network (sp2 carbon) is reestablished because of reduction24. A lower and wider peak between 18°and 30° was detected, which is assigned to the characteristic structure of RGO (002) and indicates relatively low crystallization of RGO. A significant sharp diffraction peak of Pt was detected for Pt-RGO at 2θ = 39.6°, which corresponds to the (111) plane of Pt9. The mean crystallite size of Pt on RGO is 3.5 nm, as calculated from the broadening of the Pt (111) plane by using Scherrer’s equation25. The SEM characteristic image of Pt-RGO indicates that RGO exhibits a typical lamellar structure; the size of a single chip can reach tens of microns [Fig. 6 (a)]. The single layer and multilayer flake graphene was clearly observed in the TEM image of Pt-RGO [Fig. 6 (c)]. Uniformly dispersed Pt particles on RGO were detected in both the TEM and enlarged SEM images of Pt-RGO. A thin sheet of RGO (about five to six layers thick) was found at the edge in the high-magnification TEM image [Fig. 6 (d)]. In addition, 0.23 and 0.2 nm spacings displayed in Fig. 6 (e) are assigned to the Pt (111) and (200) planes, respectively26. The loading amount of Pt on RGO is approximately 2 wt%, as confirmed by EDX analysis. According to the statistical measurement, the average aggregation size of the Pt nanoparticles is about 3.6 nm [Fig. 6 (f)], which is consistent with the XRD analysis results. Fig. 7 (a) and (b) show the AFM images of GO and Pt-RGO, respectively. GO was exfoliated into a single layer, with a thickness of about 1.2 nm. The single layer thickness of GO is significantly higher than that of graphite (0.34 nm) because of numerous oxygen-containing functional groups (such as C=O, C−OH, and −COOH) on GO27. The existence of these groups leads to a certain degree of folding and bending of the structural layer of the GO crystal, thereby increasing the distance among graphite


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layers. With the reduction of oxygen-containing functional groups, the single-layer thickness of RGO (0.85 nm) is lower than that of GO but higher than that of graphite because of incomplete reduction27. Highly raised bright spots on RGO are aggregated Pt nanoparticles, with 3–5 nm size, in accordance with the results of XRD and TEM characterization. Raman spectra of pristine graphite, GO and Pt-RGO were shown in Fig.8. The D band of GO and Pt-RGO presented at about 1350 cm-1 due to disorder induced features caused by lattice defect20,28. The G band at about 1600 cm-1 represented the first-order scattering of the E2g vibrational mode within aromatic carbon rings29. The highly ordered pristine graphite showed no D-band. I(D)/I(G) intensity ratios of GO and Pt-RGO were 0.94 and 1.27, respectively. The I(D)/I(G) intensity ratio of Pt-RGO increased, compared with that of GO. This indicated an increase of C/O ratio and effective reduction of GO into RGO 28. 3.3. CO2 synergistic reduction in the photoanode-driven PEC cell In the Pt-TNT photoanode-driven PEC cell, water decomposition and O2 evolution were conducted on the photoanode under band-gap illumination and biased voltage. Protons generated on the photoanode passed through the Nafion membrane and reached the cathode chamber, whereas electrons were collected and reached the cathode through an external wire by a potentiostat. Protons react with CO2 in the presence of electrons under the catalysis of Pt-RGO or recombine with electrons to provide H2 on the cathode. Figure 9 (a) shows the system currents with N2 or CO2 bubbling through the catholyte under conditions with and without light. Two competitive reactions, namely, CO2 reduction and H2 evolution, simultaneously occur on the Pt-RGO/Ni foam cathode. The system current with CO2 bubbling is the sum of the current of CO2 reduction and H2 production, whereas that with N2 bubbling is only the



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current of H2 evolution. The difference between system currents with CO2 bubbling and N2 bubbling can be regarded as the current of CO2 reduction, ignoring the differences in H2 evolution with or without CO2 reduction27. System current increases with CO2 bubbling (3 mA/cm2) compared with that with N2 bubbling (2.5 mA/cm2) [Fig. 9 (a)], indicating the excellent performance of the PEC cell for CO2 reduction. The current density of the PEC cell under illumination is larger than that in the dark either with N2 or CO2 bubbling. For example, system current density of the PEC cell under illumination is 23 times higher than that under dark with CO2 bubbling. Thus, photoanode coupling could significantly increase the performance of the PEC cell. In addition, current density minimally changes under illumination in the experiments, thereby indicating the photochemical stability of the Pt-TNT photoanode. Different catalytic conditions were conducted to evaluate CO2 reduction in the PEC cell; these conditions include photocatalytic reduction (PC), electrocatalytic reduction (EC), and PEC reduction. PC experiments were performed without applied voltage. The cathode and photoanode are connected through an external wire. EC experiments were conducted without anode illumination. Fig. 9 (b) and (c) show the system current density, product generation rates of CO2 reduction, and carbon atom conversion rates in the PEC cell under various conditions. The system current densities under PC, EC, and PEC are 0.065, 0.15, and 3mA/cm2, respectively. The EC properties of the PEC cell is improved compared with the PC performance. When PC was coupled with EC, a high current density was obtained, which is higher than that of the total density under EC and PC processes (0.215 mA/cm2). This finding indicates a synergistic effect between photocatalysis and electrocatalysis in the PEC cell. Changes in the product generation rate of CO2 reduction and carbon atom conversion rate under different catalytic conditions are consistent with those in system current density. Gas chromatography


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and ion chromatography analyses show that CO2 reduction products mainly include formic acid, methanol, acetic acid, and ethanol. The carbon atom conversion rate of CO2 reduction under PEC condition [935 nmol/ (h cm2)] is 2.3 times higher than that of the simple sum under PC [150 nmol/ (h cm2)] and EC [255 nmol/ (h cm2)] conditions. Application of the photoanode induces the synergistic catalytic effect and considerably improves the performance of the PEC cell for CO2 reduction. During cathode CO2 reduction, Pt-TNT photoanode plays dual roles of (1) anode water decomposition, which provides protons and electrons for cathode CO2 reduction; and (2) anode photovoltage compensates and induces the cathode potential to be more negative for CO2 reduction. Water electrolysis and photolysis simultaneously occur on the anode to provide protons and electrons for cathode CO2 reduction. Oxygen evolution reaction through water decomposition occurs at a high potential, as shown in Eq. (1) 30. When the semiconductor photoanode was applied, a strong oxidation photo potential for water splitting is generated under illumination, which results in the occurrence of water decomposition at relatively low potentials, as shown in Eq. (3)12,30. Thus, a potential compensation is applied to the cathode negative potential for CO2 reduction because the potential difference between the anode and cathode is constant (2 V in this study). Therefore, CO2 can easily passed through the energy barrier for reduction (Fig. 10). The ingenious use of the anode photovoltage confers the synergistic effect between photocatalysis and electrocatalysis and remarkably improves the performance of the PEC system and the efficiency of CO2 reduction. H2O electrolysis: H2O – 2 e- = 1/2 O2 + 2 H+

+1.36 V vs. NHE

(1)

H2O photolysis: hv = h+ + e–

(2)



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H2O + 2 h+ = 1/2 O2 + 2 H+

+0.72 V vs. NHE

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(3)

To evaluate the performance of a PEC cell for CO2 reduction, analyses of CO2 reduction efficiency and energy input should simultaneously be taken into consideration. It is not appropriate to directly compare the cathode potential since it cannot essentially represent the energy input. The energy input in a PEC device is determined by the voltage applied through the cell, i.e. potential difference between the working electrode (WE) and counter electrode (CE) denoted as (EWE - ECE). Hence it is more relevant to compare the values of voltage applied through the PEC devices to assess the relative energy requirements. To maintain the same cathode potential for CO2 reduction, (EWE - ECE) of the photoanode-driven PEC system is less than that of a photocathode-driven system due to anode photovoltage compensation on cathode potential. Thus, the energy efficient photoanode-driven PEC cell can make an efficient CO2 reduction with relatively low energy input. 3.4. Optimization of Pt loading amount on TNT photoanode to improve CO2 reduction on Pt-RGO In contrast to that in the photocathode-driven systems, the overall efficiency of CO2 reduction in the photoanode-driven system depends on two components, i.e., cathode electrocatalysts for CO2 reduction and photoanode. Photoanode is mainly involved in achieving the synergistic effect in the PEC cell. Thus, the photocatalytic activity of Pt-TNT directly affects cathode CO2 reduction to a significant extent. Pt nanoparticles were subjected to galvanostatic deposition on the TNT anode to inhibit the recombination of photo-generated electron–hole pairs and improve its photocatalytic activity31,32. The effect of TNT Pt loading amount on the performance of the PEC system and CO2 reduction efficiency was comprehensively studied. Fig. 11 shows the system current density using TNT photoanode with different Pt loading amounts


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under conditions with and without light. With increasing Pt loading amount on the TNT electrode, the system current density in the dark slightly increases because of the improved electrical conductivity of the Pt-TNT photoanode. Conversely, system current density under illumination tends to increase first and then decrease with increasing Pt loading amount. This finding could be due to the fact that at the early stage of increasing Pt loading, Pt nanoparticles can effectively accumulate the photo-generated electrons and inhibit the recombination of photo-generated electron–hole pairs33. This phenomenon efficiently improves the photocatalytic activity of TNT anode and thus increases the current density of the PEC cell. With further increase in Pt loading, Pt nanoparticles start to aggregate on the TNT surface, thereby decreasing the active specific surface area. Moreover, the aggregation of Pt particles on the TNT surface blocks the nanotubes and reduces the photocatalytic active area of the TNT31. Therefore, the light response performance of the Pt-TNT anode is weakened, leading to decreased system current density. The highest system current density (4 mA/cm2) under illumination was obtained using TNT photoanode with a Pt loading amount of 5%. As shown in Fig. 12, changes in the product generation rate of CO2 reduction and carbon atom conversion rate with increasing Pt loading amount on the TNT photoanode are consistent with that in the system current density; these changes follow the trend of increasing first and then decreasing. As discussed in section 3.3, Pt-TNT photoanode was not solely used for water decomposition to produce intermediates for CO2 reduction. More importantly, photovoltage was ingeniously used to compensate cathode CO2 reduction potential and reduce CO2 reduction energy barrier. With initial increase in Pt loading amount on TNT, the photocatalytic activity of TNT anode is improved, photovoltage increases, and water decomposition is enhanced32. As a result, cathode CO2 reduction potential shifts to a more negative position, and the amount of intermediates used for CO2 reduction increases; hence, CO2



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reduction is improved. However, if Pt loading amount continuously increases, then the photo response of the TNT anode is reduced, photovoltage decreases, and water decomposition is weakened. Correspondingly, cathode CO2 reduction potential shifts to a more positive position, and the amount of intermediates used for CO2 decreases; consequently, the product generation rate of CO2 reduction and carbon atom conversion rate decrease. The highest carbon atom conversion rate [1250 nmol/(h cm2)] was obtained using the TNT photoanode with a Pt loading amount of 5%, in accordance with the results in system current density. Under Pt-RGO catalysis, CO2 reduction products mainly exist in liquid form, with CH3COOH and C2H5OH as main products. The reduction of CO2 to hydrocarbons, which are highly sensitive to material properties, reaction conditions, and applied potential, is a complex multistep reaction involving shared intermediates and multiple pathways. However, it is often described as a reaction between CO2 and H+ to form intermediates and C1 products first, such as: CO2 + 2H+ + 2e- = HCOOH

(4)

CO2 + 6H+ +6e- = CH3OH + H2O

(5)

CO2 + 6H+ +6e- = (:CH2) + 2H2O

(6)

C2 products are generally considered as the further coupling of C1 intermediate products with the surface adsorbed methylene groups (:CH2) during the reaction34, as the following: HCOOH + (:CH2) = CH3COOH

(7)

CH3OH + (:CH2) = C2H5OH

(8)

In this study, C1 intermediate products of CO2 reduction tend to further react by C-C coupling, instead of desorption from the Pt-RGO/Ni foam cathode; this finding could be due to the fact that RGO was used as the carrier of Pt nanoparticles and the 3D Ni foam was used as the cathode matrix. The Ni


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foam is regarded the optimal catalyst carrier and electrode matrix because of its well-defined porosity, large specific surface area, and excellent electrical conductivity35,36. Graphene, as a one-atom-thick sheet of carbon, possesses large specific surface area (theoretically, 2630 m2/g

37

). On the one hand,

graphene can benefit uniform dispersion of Pt nanoparticles. On the other hand, graphene provides numerous adsorption sites for CO2 reduction intermediates and facilitates further reduction. CO2 adsorption activation is a key step during the catalytic reduction of CO2. CO2 adsorption capacity and activation on the catalysts affect and even determine catalytic reduction routine, final reduction products, and reduction efficiency. Furthermore, graphene exhibits high electron transport mobility (about 15000 cm2/(V s) at room temperature37), which favors rapid combination of electrons, protons, and CO2 reduction intermediates. Although Pt-RGO possesses a good catalytic effect on CO2 reduction, the selectivity of CO2 reduction products is low and must be further improved. Competition reaction (hydrogen evolution) is serious on cathode during CO2 reduction. This is mainly caused by two reasons. On one hand, low hydrogen overpotential of Ni foam matrix lead to strong hydrogen evolution. On the other hand, water decomposition, especially water electrolysis, produces lots of protons in the reaction. However, CO2 reduction was limited to charge transfers and the contents of reactants adsorbed on the catalysts. Therefore, lots of protons combine with electrons to produce hydrogen. Catalysts and electrode matrix with affinity for CO2 reduction to hydrocarbons must be developed in the further study to get a better catalytic performance for CO2 reduction. 4. Conclusion The combination of the Pt-TNT photoanode and the Pt-RGO/Ni foam cathode resulted in synergistic reduction of CO2 in the PEC cell. The carbon atom conversion rate of CO2 reduction under PEC condition reached 2.3 times higher than that of the simple sum under photocatalytic and



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electrocatalytic conditions. Photoanode played a dual role in the PEC cell. On one hand, anode photovoltage compensated cathode CO2 reduction potential; on the other hand, anode water decomposition provided protons and electrons for CO2 reduction. Galvanostatic deposition of Pt nanoparticles on TNT efficiently improved photocatalytic activity and increased the PEC performance of the PEC cell. System current density, product generation rate of CO2 reduction, and carbon atom conversion rate first increased and then decreased with increasing Pt loading amount on TNT and peaked with a Pt loading amount of 5%. Further study is required to elucidate CO2 reduction mechanisms and increase CO2 reduction efficiency.

Supporting Information Results of Pt loading amount on TNT and RGO tested by EDX, and stability investigation of the PEC cell.

Acknowledgements This study was supported by the National Natural Science Foundation-China (51476141), Zhejiang Provincial Natural Science Foundation-China (LR14E060002), National Key Technology R&D Program-China (2015BAD21B01).

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List of figures: Fig.1 Synthesized schematics of (a) Pt-modified TiO2 nanotube (Pt-TNT) and (b) Pt-modified reduced graphene oxide (Pt-RGO). Fig. 2 Schematic of the photoelectrocatalytic system used for CO2 conversion. Fig. 3 SEM images of TiO2 nanotubes (TNT): (a) vertical view, (b) side view; (c) XRD patterns of TNT before and after annealing. Fig. 4 SEM images of Pt-modified TiO2 nanotube (Pt-TNT) with different Pt loading amount: (a) 1.1%; (b) 5%; (c) 10.6%; (d) 15.8%. Fig. 5 XRD patterns of graphene oxide (GO) and Pt-modified reduced graphene oxide (Pt-RGO). Fig. 6 SEM images of Pt-modified reduced graphene oxide (Pt-RGO) (a, b); TEM image of Pt-RGO (c), inset of (c): the SAED pattern of Pt–RGO; HRTEM images of Pt–RGO (d, e); Pt nanoparticle size distribution on Pt–RGO (f). Fig. 7 AFM images of graphene oxide (a) and Pt-modified reduced graphene oxide (b). Fig. 8 Raman spectra of graphite, graphene oxide (GO) and Pt-modified reduced graphene oxide (Pt-RGO). Fig. 9 (a) Current–time (i–t) curve with N2 or CO2 bubbling under illumination and dark conditions, (b) Current-time (i-t) curves and (c) product generation rates in the PEC cell with Pt(1.1%)-modified TiO2 naotube photoanode and Pt–modified reduced graphene oxide cathode under various catalytic conditions. Fig. 10 Schematic of photovoltage compensation for cathode potential in a TiO2 nanotube photoanode-driven PEC cell. Fig. 11 Current–time (i–t) curve with CO2 bubbling under illumination and dark conditions using TiO2 nanotube (TNT) with different Pt loading amount as photoanode and Pt-modified reduced graphene oxide/Ni foam as electrocathode for CO2 reduction. Fig. 12 CO2 reduction product generation rate and carbon atom conversion rate in the PEC cell using Pt-modified reduced graphene oxide/Ni foam as electrocathode and TiO2 naotube (TNT) with different Pt loading amount as photoanode.



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Fig.1 Synthesized schematics of (a) Pt-modified TiO2 nanotube (Pt-TNT) and (b) Pt-modified reduced graphene oxide (Pt-RGO).


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Fig. 2 Schematic of the photoelectrocatalytic system used for CO2 conversion.



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Fig. 4 SEM images of Pt-modified TiO2 nanotube (Pt-TNT) with different Pt loading amount: (a) 1.1%; (b) 5%; (c) 10.6%; (d) 15.8%.



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Fig. 7 AFM images of graphene oxide (a) and Pt-modified reduced graphene oxide (b).


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Fig. 9 (a) Current–time (i–t) curve with N2 or CO2 bubbling under illumination and dark conditions, (b) Current-time (i-t) curves and (c) product generation rates in the PEC cell with Pt(1.1%)-modified TiO2 naotube photoanode and Pt–modified reduced graphene oxide cathode under various catalytic conditions. Note: photocatalytic (PC); electrocatalytic (EC); photoelectrocatalytic (PEC). PC experiments were performed without applied voltage, the cathode and photoanode were connected through an external wire. EC experiments were conducted without anode illumination.


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Fig. 10 Schematic of photovoltage compensation for cathode potential in a TiO2 nanotube photoanode-driven PEC cell.



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CO2 synergistic reduction in a photoanode-driven photoelectrochemical cell with a Pt-modified TiO2 nanotube photoanode and a Pt-RGO electrocathode Meng Zhang, Jun Cheng, Xiaoxu Xuan, Junhu Zhou, Kefa Cen

TABLE OF CONTENTS (TOC) GRAPHIC

CO2 synergistic reduction resulted by anode photovoltage compensation on cathode potential in a photoelectrochemical cell with a Pt-modified TiO2 nanotube photoanode and a Pt-RGO electrocathode.

 Corresponding author: Prof. Dr. Jun Cheng, State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. Tel.: +86 571 87952889; Fax: +86 571 87951616. E-mail: [email protected]


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CO2 Synergistic Reduction in a Photoanode-Driven

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