Continuous conversion of CO2 to alcohols in a TiO2 photoanode-driven photoelectrochemical system

BACKGROUND The recycling of CO2 by photo-electrochemical reduction has attracted wide interest due to its potential benefits when compared to electro-, and photo-catalysis approaches. Among the different available semiconductors, TiO2 is the most employed material in photo-electrochemical cells. Besides, Cu is a well-known electrocatalyst for alcohols production from CO2 reduction. RESULTS In this study, a photo-electrochemical cell consisting on a TiO2 photoanode Membrane Electrode Assembly (MEA) and a Cu plate are employed to reduce CO2 to methanol and ethanol continuously under UV illumination (100 mW·cm-2). A maximum increment of 4.3 mA·cm-2 in current between the illuminated and dark conditions is achieved at -2 V vs. Ag/AgCl. The continuous photo-electrochemical reduction process in the filter-press cell is evaluated in terms of reaction rate (r), as well as Faradaic (FE) and energy (EE) This article is protected by copyright. All rights reserved. efficiencies. At -1.8 V vs. Ag/AgCl, a maximum reaction rate of r= 9.5 μmol·m-2·s-1, FE= 16.2 % and EE= 5.2 % for methanol, and r= 6.8 μmol·m-2·s-1, FE= 23.2 % and EE= 6.8 % for ethanol can be achieved. CONCLUSIONS The potential benefits of the photoanode-driven system, in terms of yields and efficiencies, are observed when employing a TiO2-based MEA photoanode and Cu as dark cathode. The results demonstrate first the effect of UV illumination on current density, and then the formation of alcohols from the continuous photoreduction of CO2. Increasing the external applied voltage led to an enhanced production of methanol, but decreases ethanol formation. The system outperforms previous photoanode-based systems for the CO2-to-alcohols reactions.


INTRODUCTION
The concentration of CO2 in the atmosphere has increased to worrying levels in recent years, mainly due to fossil fuels burning. This has accelerated research activities to obtain value added products from residual CO2, instead of discard it as a residue. 1 There are different possibilities to convert CO2 into valuable products. Among the different technologies available, the electrochemical reduction of CO2 has attracted great interest due to the potential economic and environmental benefits. This technology, apart from allowing CO2 dissociation at ambient conditions using electricity, is also an excellent alternative to store the intermittent energy produced from renewables in the chemical This article is protected by copyright. All rights reserved.
bonds. 2 Moreover, the integration of a light source in electrochemical reduction devices (a photoelectrochemical approach, PEC) has attracted an increasing interest recently because it may allow avoiding the interconnections between devices, reducing, in principal, system capital costs and electricity losses. 3 Compared to photocatalysis, the applied bias can cause band bending and help the oriented transfer of the photogenerated electrons, decreasing the recombination of the photogenerated electron-hole pairs.
Besides, photocatalytic materials with unfavorable band positions for CO2 reduction and H2O oxidation can be still used in photoelectrocatalytic systems when applying an external bias.
There are different electrode configurations for the PEC systems on dependence on which electrode (i.e., anode, cathode or both) acts as photoelectrode. 4 Photocathode-dark anode has been the most employed configuration. [5][6][7][8][9] In this case, a p-type semiconductor is employed as photocathode and a metal as anode. Unfortunately, with the p-type semiconductors two-electron products are usually obtained and the system efficiency is low. On the other hand, CO2 reduction in a photoanode-dark cathode PEC is a simpler configuration and depends on the photoanode and the cathode activity independently [10][11][12] , and offers the advantage of reducing the external energy requirements of the process. 13 In this configuration, H2O oxidation in the photoanode provides electrons and protons for CO2 reduction in the cathode compartment. Besides, the negative potential voltage generated in the photoanode by the light, supplies an additional negative potential for CO2 reduction in the cathode. 4 Another PEC photoelectrodes configuration is the combination of a photocathode for CO2 reduction with a photoanode to deals with H2O oxidation (as represented in Figure 1). [13][14][15] In contrast to the previous configurations, the main advantage is that with some pairs of materials an external electrical energy supply for the redox reactions may, ideally, not be needed. However, in most cases, the voltage generated by the light source is not enough and a continuous external power supply is required. 16,17 Figure 1. PEC cell in a phocathode-photoanode configuration. Edited from [2].
Moreover, the formation of alcohols from CO2 has received increasing attention due to their proper integration in the actual fuel infrastructure, besides been considered an advantageous energy storage and a precursor to synthesize other products. [18][19][20] Methanol (CH3OH) is considered an excellent energy intermediate due to the stable storage properties and its high energy density. In addition to the application as a fuel, CH3OH is an intermediate to other bulk chemicals employed in everyday life products like plastics and paints. 21 Ethanol (C2H5OH) is an important raw material with high heating value usually employed in disinfectants and organics material. 22 It can also replaces fossil fuels in a key sector such as the transport industry. 23 Thus, both products offer an alternative to deal with climate change, This article is protected by copyright. All rights reserved. reducing our dependence on fossil fuels and allowing CO2 recycling in a neutral carbon cycle. 18,24 TiO2 is the most employed semiconductor in photo-assisted processes. [25][26][27][28] It is a n-type semiconductor that possesses a wide band gap (3.0 eV) and mainly absorb UV light. 29 This semiconductor has been considered a cheaper and more environmental friendly material 30 since the first report in 1979. 25 Besides, Cu has demonstrated to possess the capacity to produce hydrocarbons and alcohols from CO2. 31 In particular, previous works from our group employed a Cu plate for CO2 electroreduction, with a reaction rate of CH3OH of r= 8.7 μmol·m -2 ·s -1 and a FE= 4.6 % at -1.3V vs. Ag/AgCl. These values were subsequently enhanced by using Cu oxide 31,32 and metal-organic frameworks (MOFs)-based electrodes at different experimental conditions in continuous operation mode. 18,[31][32][33] Thus, the aim of the present work is coupling a TiO2-based photoanode in an electrochemical filter press cell for the continuous conversion of CO2 to alcohols (i.e. CH3OH and C2H5OH) in order to reduce the requirements of external energy, and so reduce energy consumption. A Cu plate is used as dark cathode for designing a reliable photoanode-dark cathode configuration for CO2 photoreduction. The specific objectives are as follows: (1) to prepare and optimize a TiO2-based photoanode, (2) to adapt an electrochemical cell for the PEC conversion of CO2 to alcohols, and finally (3) to analyze the effect of voltage on process performance. The results are considered a step further in the development of continuous CO2 conversion processes under the sun.

EXPERIMENTAL SECTION
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TiO2 photoanode preparation
A TiO2 photoanode is fabricated by air-brushing an ink composed by a mixture of TiO2 (Sigma Aldrich, P25), Nafion (Alfa Aesar 5wt%) as binder and isopropanol (99.5%, Sigma Aldrich) as vehicle, with a 70:30 wt% TiO2/Nafion ratio in a 3 wt.% of solid in the final isopropanol dispersion onto a Toray carbon paper (TGP-H60). An ultrasound bath is used to homogenize the mixture. The TiO2 loading varies from 1 to 3 mg·cm -2 . The TiO2 photoanode is coupled by hot-pressing with a Nafion membrane (Nafion 117), previously activated in HCl for 30 minutes and rinsed with deionized water before use, forming a Membrane Electrode Assembly (MEA) able to enhance H + transport, reducing mass transfer limitations. 33, 35

Photoelectroreduction cell and experimental conditions
The continuous PEC reduction of CO2 is carried out using a commercial filter press cell reactor (Electrocell, Denmark), employing a Cu plate as cathode and the prepared MEA as photoanode, which also separates the cell compartments. A 1 M KOH aqueous solution is used as catholyte and anolyte and a cold UV LED light (100 mW·cm -2 ) to illuminate the photoactive area (10 cm 2 ) of the TiO2 photoanode. In the cathode side, there are two inputs: the catholyte and the CO2 gas; and one output: the catholyte with the reaction products (liquid and gaseous). The Cu plate works as working electrode, the TiO2 phoelectrode as counter electrode and Ag/AgCl as reference electrode. The CO2 reduction is conducted at ambient conditions. Figure 2 shows a scheme of the experimental setup, while Figure 3 shows a detailed view of the cell configuration. The experimental system This article is protected by copyright. All rights reserved.
includes four tanks for inlet and outlet electrolyte solutions, two peristaltic pumps to circulate the liquid at a flow rate of 10 ml·min -1 , two pressure indicators and mass-flow controllers to fix the CO2 inlet flow rate at 180 ml·min -1 . Moreover, a potentiostat (AutoLabPGSTAT 302N) is used. The PEC behavior of the TiO2 photoanodes is evaluated in a one-compartment glass at a scan rate of 50 mV·s −1 after 20 cycles with and without light illumination. This article is protected by copyright. All rights reserved.

H +
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area and time (µmol·m -2 ·s -1 ) and the FE is defined as the selectivity of the reaction to form each products, calculated according to the Ec 1: where z is the theoretical number of e − exchanged to form the desired product, n is the number of moles produced, F is the Faraday constant (F = 96,485 C·mol −1 ) and q is the total charge applied in the process. Moreover, the energy efficiency, EE, which indicates the total energy used toward the formation of the desired products can be calculated according to the following equation (Ec 2): Where E is the real potential applied in the system and ET the theoretical potential needed for the formation of CH3OH (-0.58 V vs. Ag/AgCl) and C2H5OH (-0.53 V vs. Ag/AgCl). where an optimal reduction of CO2 to alcohols can be expected. 34,37,38 It should be noted This article is protected by copyright. All rights reserved.

Photoelectrochemical behavior of the TiO2 photoelectrode
that the TiO2-based photoanodes showed no changes in response (current) during on-off cycles when illuminated with a visible LED light (100 mW·cm -2 ). This article is protected by copyright. All rights reserved.
be beneficial for alcohols production. 36 These results may be taken as a first indication for an enhanced PEC process performance as evaluated hereafter.

Process performance of the TiO2/Nafion/Cu plate system
The products obtained for the continuous CO2 reduction PEC reduction employing a TiO2-based MEA as photoanode and a Cu plate as cathode are alcohols (i.e. CH3OH and C2H5OH) together with CO and C2H4. Their formation, and so process performance, is evaluated in terms of r and FE. Figure 5 initially shows an example of the evolution of j during the experimental time (UV illumination).

Time (min)
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commonly caused by the direct input of CO2 gas into the cathode compartment, as well as the production of gaseous products (bubbles) on the electrode surface. The results also give an idea of the stability of the system, although, of course, longer-term tests would be needed in order to assess the feasibility of the system for real applications.
Among the different key variables of the process, the TiO2 catalyst loading can have a remarkable impact on process performance. 35 . In principle, the higher the semiconductor surface, the greater the photocurrent generated as more electrons are excited in the photoanode.
Therefore, figure 6 shows the FE values at three different TiO2 catalyst loadings in the MEA (i.e.1, 2 and 3 mg·cm -2 ).

TiO 2 catalyst loading (mg·cm -2 )
This article is protected by copyright. All rights reserved. can be achieved. At higher catalyst loadings than 3 mg·cm -2 , Seger et al. 34 found that the light absorption limits the photocurrent generation due to particle agglomeration which hampers the accessibility of the light to the catalytic surface. Consequently, a TiO2 loading of 3 mg·cm -2 is recommended to get the best performance in the developed TiO2 photoanode MEA-driven system for CO2 PEC conversion and is applied hereafter.
Then, the values for alcohols (i.e. CH3OH and C2H5OH) formation in a voltage range from -1.2 to -2 V vs. Ag/AgCl are presented in Table 1, while Figure 7 shows the FE to all liquid and gas-phase products detected as a function of the applied voltage.
The values in Table 1 are normalized by the total charge q (C) passing through the system in order to properly analyze the PEC activity. The values are also compared with our previous findings for the electrochemical conversion of CO2 (in the dark) using a Cu plate. 31 Table 1. Production rates, r, for alcohols at different E employing a Cu plate as cathode. First, the data shows that the current densities values in the PEC system are significantly reduced (for a comparable production of alcohols) to those at a Cu plate and a Pt anode in an electrochemical system (in the dark), which might initially indicate an enhancement in energy efficiency. Then, the reaction rate values, r, show similar ranges for CH3OH

E (V) j (mA·cm -2 ) q (C) r (µmol·m -2 ·s -1 ) r · C -
for the same voltage level in electrochemical and PEC experiments. The r·C -1 values obtained in this work, however, are clearly superior in PEC (r= 9.3 µmol·m -2 ·s -1 ·C 1 for CH3OH at -1.8 V vs. Ag/AgCl) in comparison to those achieved in the electrochemical experiments (r= 2.6 µmol·m -2 ·s -1 ·C 1 for CH3OH) at similar voltage level, showing the benefits of the PEC. The PEC system seems also to be beneficial for the formation of C2H5OH, which requires a higher number of electrons and C-C coupling. This enhancement of CO2 reduction to alcohols can be generally associated with two phenomena: first, the cathode potential becoming more negative (higher supply of e -) and then, the large amount of H + generation from water the photo-electrolysis, that seem to be available for CO2 conversion to alcohols in the cathode. 11 The FE to both alcohols notably decrease at -2 V vs. Ag/AgCl, which can be ascribed to an enhanced formation of C2H4 (Figure 7).
This article is protected by copyright. All rights reserved.  Furthermore, Table 2 summarizes EEs achieved for alcohols as a function of the applied potential.

CONCLUSIONS
Coupling light to electrochemical devices for CO2 conversion is an interesting approach to obtain value added products under the sun, solving the issues related to global warming at the same time. When the continuous photoelectrochemical transformation of CO2 is carried out in a filter-press cell employing a TiO2 MEA photoanode, a maximum increase This article is protected by copyright. All rights reserved. of 4.3 mA·cm -2 in current density is observed under UV illumination, denoting the potential benefits of the developed photoanode-driven system for an enhanced energy efficiency. Methanol and ethanol, as well as hydrogen, carbon monoxide and ethylene, are detected as liquid and gas-phase products in the system employing a Cu plate as cathode. At -1.8 V vs. Ag/AgCl, the maximum production of methanol is 9.5 μmol·m -2 ·s -1 , the Faradaic efficiency is 16.2 % and the energy efficiency 5.2 %. In case of ethanol, a reaction rate of 6.8 μmol·m -2 ·s -1 , a Faradaic efficiency of 12.1 % and energy efficiency of 6.8 % can be achieved for a TiO2 loading of 3 mg·cm -2 in the photoanode. In general, methanol production results are enhanced at higher voltages, while ethanol production seems to be negatively affected. All in all, the developed TiO2 photoanode-driven system is able to enhance the production of alcohols from CO2 under UV light, reducing the energy required in the process compared to an electrochemical system.

ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) through the projects CTQ2016-76231-C2-1-R and Ramón y Cajal programme (RYC-2015-17080).

REFERENCES
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