Continuous Production of Ethylene from Carbon ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Continuous Production of Ethylene from Carbon Dioxide and Water Using Intermittent Sunlight Dan Ren,†,‡,# Nicholas Wei Xian Loo,†,# Luo Gong,† and Boon Siang Yeo*,†,‡ †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, 117543, Singapore Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, 117574, Singapore

‡

S Supporting Information *

ABSTRACT: The large-scale deployment of efficient artificial photosynthesis systems to convert carbon dioxide (CO2) into carbon-based fuels and chemical feedstocks holds great promise as a way to ensure a carbon neutral cycle. While catalysts have been developed for the pertinent half-reaction of CO2 reduction to C2 molecules, an integrated system for this purpose has never been designed and built. In this work, we demonstrate an energetically efficient formation of ethylene directly from CO2 and water (H2O) using solar energy at room temperature and pressure. A two-electrode cell (electrolyzer) was designed, and cell parameters such as electrolyte and voltage were optimized. Oxide-derived copper (Cu) and iridium oxide (IrOx) were used as electrocatalysts respectively in the cathode and anode. Coupling this electrolyzer with silicon solar panels under laboratory 1 sun illumination (100 mW/ cm2), we show that CO2 could be facilely reduced to ethylene with a faradaic efficiency of 31.9%, partial current density of 6.5 mA/cm2, and a solar-to-ethylene energy efficiency of 1.5%. When liquid fuels such as ethanol and n-propanol were included, the total solar-to-fuel efficiency was 2.9%. These outstanding figures-of-merits are the state-of-the-art. We also introduced insoluble chelating agents in the electrolyte to capture contaminants such as dissolved iridium ions, and thus significantly improved the longevity of the electrolyzer. Compared to previously reported solar-to-fuel setups which were only tested under simulated sunlight, our system, when coupled with a rechargeable battery, could run and produce ethylene continuously using only intermittent natural sunlight. KEYWORDS: Artificial photosynthesis, Ethylene, Carbon dioxide, Solar energy, Electrocatalysis

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USD per MWh in 2018).4 These considerations make a tandem PV-electrolyzer system highly attractive. Previous photoelectrochemical systems have mainly focused on the formation of formic acid or carbon monoxide (Table 1).5−12 For example, Zhou et al. has recently reported a 10% solar to formic acid efficiency using Pd/C and Ni as the electrocatalysts, with a bipolar membrane (BPM) in between.8 The formation of higher hydrocarbons or alcohols, such as ethylene (C2H4), using PV-electrolyzer systems has, however, yet to be accomplished. This could be attributed, in part, to difficulties in developing catalysts with the required functionality and stability. Challenges such as matching the PV system with the optimized formation of ethylene, which is highly potentialdependent, also need to be addressed. Being the basic building block for polyethylene, the global production of C2H4 reached ∼173 million tons in 2015 and is expected to exceed 220 million tons in 2020.13 Current industrial production of ethylene employs the steam cracking of

INTRODUCTION Carbon capture is a key step within the natural carbon cycle and it is mainly achieved via photosynthesis, in which green vegetation uses solar energy to convert carbon dioxide (CO2) and water (H2O) to carbohydrates and oxygen (O2). Because the rate of CO2 emissions exceeds that of carbon capture, there has been a steady increase in the atmospheric concentration of CO2. This has been attributed as a major cause of global warming which leads to undesirable environmental changes.1 Mimicking nature, artificial photosynthesis holds great promise as a clean and sustainable technology to produce fuels and chemical feedstocks, and also serves to reduce the carbon footprint. Artificial photosynthesis can be accomplished by integrating an electrolyzer with a photovoltaic (PV) cell, which can be overall more energetically efficient and stable than a direct photocatalytic system.2 In this system, a PV is first used to convert solar energy to electricity, and the electricity then powers the electrolyzer to produce chemicals from CO2 and H2O.3 The levelized cost of electricity (LCOE) produced by PV plants entering service in 2018 is estimated to reach 53.5 USD per MWh, which is about half the LCOE generated from conventional combustion turbines (LCOE estimated to be 92.6 © 2017 American Chemical Society

Received: June 27, 2017 Revised: August 11, 2017 Published: August 31, 2017 9191

DOI: 10.1021/acssuschemeng.7b02110 ACS Sustainable Chem. Eng. 2017, 5, 9191−9199

Research Article

ACS Sustainable Chemistry & Engineering

Table 1. A Summary of the Previous Artificial Photosynthesis Systems (Photoelectrochemical) from Carbon Dioxide and Water photovoltaics Si Si Si + InGaN SiGe GaAs/InGaP Si Cu(InxGa1‑x)(SySe1‑y)2 perovskite Si a

cathode Cu2Oderived Cu In In Ru-based polymer Pd/C Au Au Au WSe2

anode

electrolyte

product

solar to fuel efficiency (%)

partial current density (mA/cm2)a

ref

IrOx

0.2 M KHCO3

C2H4

1.5

6.5

this work

IrOx Ni−O IrOx

1 M KHCO3 3 M KHCO3 0.1 M phosphate buffer (K2HPO4:KH2PO4 = 1:1) 2.8 M KHCO3/BPM/1.0 M KOH 0.5 M KHCO3 0.5 M KHCO3 0.5 M NaHCO3 50% EMIM-BF4 in water (cathode)/potassium phosphate buffer (anode)

HCOOH HCOOH HCOOH

1.4−1.8 0.97 4.6

N.R.b ∼ 0.4 ∼ 0.1

White et al.5 Sekimoto et al.6 Arai et al.7

HCOOH CO CO CO CO

10 2.0 4.23 6.5 4.6

∼8 ∼ 1.5 N.R. ∼1.4 N.R.

Zhou et al.8 Sugano et al.9 Jeon et al.10 Schreier et al.11 Asadi et al.12

Ni CoOx Co3O4 IrO2 Co−O/OH

Partial current density was calculated using the geometric surface area of the cathode. bN.R.: not reported.

Figure 1. Characterization and electrochemistry of the electrocatalysts. (a) SEM image of Cu2O before reduction; (b) SEM image, (c) TEM (inset HR-TEM) images, and (d) SAED pattern of Cu2O after 1 h reduction at −0.95 V vs RHE; (e) faradaic efficiencies of ethylene and liquid fuels as well as the total current density obtained from 1 h chronoamperometry on oxide derived Cu cathode (area = 0.385 cm2) at selected potentials in a threeelectrode configuration; (f) SEM image of IrOx before linear sweep voltametry; (g) SEM image, (h) TEM image, and (i) SAED pattern of IrOx after linear sweep voltametry; (j) a linear sweep voltammogram of IrOx (area = 5 cm2) at a scan rate of 10 mV/s (inset shows 12-h stability of IrOx at −1.92 mA/cm2) in a three-electrode configuration.

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Figure 2. Artificial photosynthesis from carbon dioxide and water under simulated 1 sun (100 mW/cm2). (a) Schematic design of the two-electrode electrolyzer coupled with silicon solar panels; (b) anodic reaction and representative cathodic reactions occurring on the surfaces of the catalysts (the reactions are not balanced); (c) I−V curve of the solar panels and the I−V curve of the electrolyzer shown in (a); (d) cathodic current density and faradaic efficiency of ethylene as a function of time over 1 h electrolysis.

naphtha or saturated hydrocarbons at 750−950 °C. This process requires enormous energy inputs which account for ∼8% (including both ethylene and propylene) of the total primary energy consumption in the chemical industry.14 Furthermore, this process generates ∼2 tons of CO2 per ton of ethylene produced. As a result, a more sustainable technology to produce ethylene is urgently in need. On the basis of the above discussion, the direct synthesis of ethylene from CO2 and H2O using solar energy via a tandem PV-electrolyzer system is highly appealing on the grounds of environmental and industrial sustainability. The pertinent electrochemical half-reactions and overall reaction at standard conditions are15

In this work, to achieve the solar-driven reduction of CO2 to ethylene, we have designed a system coupling solar panels with a highly efficient electrochemical cell consisting of an oxidederived Cu cathode and an IrOx anode. The electrodes were characterized and the working potential was optimized. The device configuration, stability, and practical usage of the system have also been investigated in detail. Under laboratory conditions, the solar to ethylene efficiency is comparable with the efficiency of general natural photosynthesis. A customdesigned circuit that contains solar panels, a rechargeable lithium polymer battery, and a voltage regulator is further incorporated to allow the electrochemical cell to run continuously, even with intermittent solar radiation.

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anodic reaction, oxygen evolution reaction:

RESULTS Characterization and Electrochemistry of the Electrocatalysts. Cu2O-derived Cu and IrOx were respectively used as ethylene-producing and O2-evolving electrocatalysts.16,17 Their catalytic activities were first optimized by varying their deposition parameters (only data from the optimized materials will be shown). Freshly prepared Cu2O films consisted of smooth polyhedron particles, which were several hundred nanometers in size (Figure 1 and Supporting Information (SI)

2H 2O ⇌ O2 + 4H+ + 4e−

cathodic reaction, CO2 reduction to ethylene: 2CO2 + 12e− + 8H 2O ⇌ C2H4 + 12OH−

overall reaction: 2CO2 + 2H 2O → C2H4 + 3O2

(ΔG = + 1323 kJ/mol) 9193


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ACS Sustainable Chemistry & Engineering Figure S1). After the films were used as catalysts for CO2 reduction, their surfaces were found decorated with 10−100 nm-sized nanoparticles (Figure 1b). These oxide-derived copper nanoparticles, which were polycrystalline (Figure 1c,d), have a rough and highly defective surface morphology. These surface features are believed to be crucial in binding C1 intermediates such as CO and facilitating their C−C coupling to give higher hydrocarbons or alcohols.18−20 The electrocatalytic activity of oxide-derived Cu film toward CO2 reduction was characterized in a three-electrode configuration21 using chronoamperometry at potentials from −0.85 to −1.10 V vs RHE, with the products formed quantified by gas chromatography, headspace gas chromatography, and high performance liquid chromatography (Figure S2−S3, SI). Aqueous 0.2 M KHCO3 electrolyte was used. KHCO3 solutions are frequently employed as electrolytes for the reduction of CO2 to hydrocarbons and alcohols.22 A more concentrated KHCO3 solution will reduce the catalyst’s selectivity toward ethylene formation (due to increased buffering which lowers the local pH at the surface of the electrode), although it will help to decrease the overall resistance.23 Twelve carbonaceous products were detected, and this can be attributed to the formation of multiple C1−C3 intermediates, which could react in different ways.24 The total faradaic efficiencies of the carbonaceous products and H2 add up to 89.2−103.7% (Table S1, SI). This demonstrates that our experimental protocol is comprehensive and the instruments are well calibrated. The selectivity of CO2 reduction to ethylene was optimized at −0.95 V. Here, the faradaic efficiency of ethylene was 29.7% (Figure 1e), with very little deactivation even after 4 h electrolysis (Figure S4, SI). When liquid fuels such as ethanol and n-propanol were included, the total faradaic efficiency was 50.8%. The current density obtained at this optimum potential was −24.9 mA/cm2 (the current was −9.6 mA). We note here that the optimized faradaic efficiency of ethylene is slightly lower than our previously reported values (38.8% of ethylene at −0.99 V) measured in 0.1 M KHCO3 electrolyte.25 Insights into the selectivity16,19,23,25−27 and high overpotential28,29 of oxide-derived Cu films toward ethylene formation have been reported previously and will not be discussed further. However, we do highlight previous reports of oxide-derived Cu films that gave CO and formic acid selectivity instead.30−32 Considerable optimization of the catalyst preparation method, film thickness, and applied potential was therefore necessary in this work to obtain the high ethylene selectivity. The IrOx catalysts consisted of agglomerates of 10−100 nmsized nanoparticles (Figure 1f−h, Figure S5, SI).17 These particles were shown by selected area electron diffraction (SAED) to be structurally amorphous (Figure 1i). Linear sweep voltammetry (LSV) of the anode in CO2-saturated 0.2 M KHCO3 electrolyte indicated that a small overpotential of ∼300 mV was required to achieve a current of 9.6 mA, which corresponded to a current density of 1.92 mA/cm2 (Figure 1j). This is the optimum current for the cathodic reaction (Figure 1e). The anodic peak at 1.17 V in the LSV could be assigned to the oxidation of Ir3+ to Ir4+. This is consistent with the increase of O:Ir atom ratio in the anode (x increased from 1.8 to 2.1) after the LSV (Figure S6, SI). There was no observable deactivation of the anode over a 12-h electrolysis (Figure 1j). We also confirmed the inertness of the anode toward the oxidation of ethylene by both repeated LSV scans and chronopotentiometry at 1.92 mA/cm2 in C2H4-saturated 0.2 M KHCO3 (Figure S7, SI). Reactivity of the anode toward the

oxidation of liquid products was also found negligible (Table S2, SI). We note that the inertness of IrOx toward products oxidation has not been investigated before. These critical findings will enable our system to operate without the need for a membrane to separate the anodic and cathodic compartments. This will in turn lower the resistance between two electrodes and hence improve the overall energetic efficiency. Artificial Photosynthesis Device and Efficiency Testing Using Solar Simulator. Our custom-built two-electrode electrolyzer powered by photovoltaics is shown schematically in Figure 2a (Figure S8, SI). An aqueous 0.2 M KHCO3 solution was used as the electrolyte, with CO2 reactant continuously flowing through it. The distance between the cathode and anode was 5 mm. To decrease the resistance between the two electrodes, the membrane separating them was excluded (Table S3, SI). This lowered the overall resistance from ∼102 to ∼87 Ω. The anodic and representative cathodic reactions occurring at the electrodes are shown in Figure 2b (Table S4, SI). We note here that the current required for this setup could be slightly different from the values found earlier, due to different cell configurations.33 Thus, the current for the electrolyzer was further optimized using chronopotentiometry and was determined to be ∼7.7 mA, with the voltage of ∼3.3 V (inclusive of voltage drop due to solution resistance and the overpotentials for two half reactions). A PV system that matched these voltage and current requirements was then selected (Figure S9, SI). In order to achieve a high efficiency, the matching point should also be as close as possible to the maximum power of the solar panel. Our chosen system was based on two Si solar panels connected in parallel (effective illuminated area of 1.92 cm2). Its I−V curve was measured under simulated terrestrial 1 sun (100 mW/cm2) illumination (Figure 2c). This curve crossed the I−V curve of the electrolyzer at the point where its cathodic current and voltage were respectively 7.8 mA (∼20.3 mA/cm2) and 3.3 V. During 1 h of electrolysis powered by simulated solar radiation, the cathodic current density of the cell was stable at ∼20 mA/cm2 (Figure 2d). The faradaic efficiency of ethylene stabilized at ∼32% after ∼20 min, and its partial current density was 6.5 mA/cm2. It should be noted that this optimized FE of ethylene was similar to the one obtained using the threeelectrode setup (∼30%). This indicates that the presence of O2 evolving at the anode does not affect the production of ethylene at the Cu cathode. This contrasts with the recent work of Engelbrecht et al., which found that 10−60% O2 in the reactant gas (CO2) could enhance the reduction of CO2 to ethylene on a Cu2O electrode.34 Adding up ethylene and the liquid products, the total faradaic efficiency reached up to 60.2%. The rest of the products were mainly H2 (Table S6, SI). The solar-to-electricity efficiency of the solar panels (ηS‑E), electricity-to-fuel efficiency (ηE‑F) and solar-to-fuel efficiency (ηS−F) for each product (depicted as X) can be calculated:35 ηS − E =

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Icell × V Isolar × A

(1)

ηE − F(X ) =

E(X ) × FE(X ) V

(2)

ηS − F(X ) =

E(X ) × Icell × FE(X ) Isolar × A

(3)


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Overall Reactions and the Efficiency of the PV-Electrolyzer System overall reactiona 2CO2 + 2H2O → C2H4 + 3O2 (E = 1.15 V) 2CO2 + 3H2O → C2H5OH + 3O2 (E = 1.14 V) 2CO2 + 2H2O → 2HCOO− + 2H+ + O2 (E = 1.46 V) 6CO2 + 8H2O → 2C3H7OH + 9O2 (E = 1.02 V) 3CO2 + 3H2O → C2H5COH + 4O2 (E = 1.09 V) 3CO2 + 3H2O → C3H5OH + 4O2 (E = 1.12 V) 4CO2 + 4H2O → 2CH3COH + 5O2 (E = 1.18 V) subtotal a

production rate (μmol cm−2 h−1)

Faradaic efficiency (%)

electricity to fuel efficiency (%)

ethylene

19.89 ± 0.55

31.92 ± 0.75

11.12 ± 0.26

1.49 ± 0.04

ethanol

5.99 ± 0.14

9.60 ± 0.27

3.32 ± 0.09

0.45 ± 0.01

formate

32.79 ± 3.87

8.77 ± 1.06

3.88 ± 0.47

0.52 ± 0.06

n-propanol

2.16 ± 0.11

5.20 ± 0.30

1.61 ± 0.09

0.22 ± 0.01

propionaldehyde

1.20 ± 0.17

2.58 ± 0.38

0.85 ± 0.13

0.11 ± 0.01

allyl alcohol

0.63 ± 0.03

1.35 ± 0.08

0.46 ± 0.03

0.062 ± 0.004

acetaldehyde

0.60 ± 0.06

0.80 ± 0.08

0.29 ± 0.03

0.039 ± 0.004

60.2 ± 1.2

21.5 ± 0.6

2.9 ± 0.1

reduction product

solar to fuel efficiency (%)

E represents the equilibrium potential at standard conditions.

Figure 3. Artificial photosynthesis using real solar flux. (a) Circuit design for a noninterruptible and constant voltage output for the electrochemical cell with and without solar flux. (b) A photograph taken on one of the days when experiments were performed. The inset shows the actual solar radiation during the experiment (131 W/m2 = 13.1 mW/cm2) on the spot where the solar panels were mounted. (c) Cathodic current density and ethylene formation as a function of time during 4 h electrolysis: 2 h with real solar flux and 2 h without solar flux.

where Icell is the current passing through the electrolyzer (or the output current of the solar panels); V is the working potential of the electrolyzer (or the output of voltage of the solar panels); Isolar is the intensity of solar radiation; A is the effective illuminated area of two parallel-connected solar panels; E(X) is the equilibrium potential for the overall electrochemical reaction for product X at standard conditions; and FE(X) is the faradaic efficiency of product X. The application of Icell = 7.8 mA, V = 3.3 V, Isolar = 100 mW/ cm2, and A = 1.92 cm2 to eq 1 gives ηS‑E = 13.4 ± 0.1% (the standard deviation is calculated from three independent measurements). This is very close to the maximum efficiency of the solar panels (14.7%). It is extremely important to match the requirements of the electrochemical cell with the solar panels, that is, the matching point (Figure 2c) should both give

the optimum output of ethylene and be close to the maximum efficiency of the solar panel. These requirements were met in this work. Further applying E(X) and FE(X) values (Table 2) to eqs 2 and 3 gives the ηE‑F and ηS−F for ethylene as 11.1% and 1.5%, respectively. With the oxygenates included, the overall efficiency of solar-to-fuels could reach 2.9%. These figures-ofmerit, including the production rates of the molecules, are summarized in Table 2. Compared with previous works (Table 1), this is the first report of ethylene and higher alcohols being directly produced with such remarkable efficiencies from CO2 and H2O using solar electricity. The high industry value and partial current density of ethylene obtained (6.5 mA/cm2) also make this study stand out among most of the previously reported systems. It is noteworthy that a 1.5% solar to ethylene 9195


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battery was able to drive the electrochemical reactions without interruption. During the 4 h electrolysis, ethylene was formed continuously with a FE of 26 ± 3% (Figure 3c). We note here a slight decrease in ethylene formation after 4 h. This could be attributed to factors not remedied by the chelating agent, for example, deposition of graphitic carbon on the working electrode or the dissolution of the Cu catalyst itself.23,38 Further optimizations to extend the stability of the Cu catalyst will be pursued.

energetic efficiency is close to the efficiency of general natural photosynthesis for producing carbohydrates (3−6%).36 Continuous Artificial Photosynthesis of Ethylene Using Intermittent Natural Sunlight. Beyond the aforementioned work, we developed an artificial photosynthesis system that is durable and powered exclusively by natural sunlight. The stability of the electrolyzer was first investigated using 4 h chronopotentiometry with a cathodic current of 20 mA/cm2. We found that the faradaic efficiency of ethylene decreased gradually over this time period from ∼30 to ∼15% (Figure S10−S11, SI). SEM and XPS of the cathode revealed the presence of Ir on the cathode after electrolysis. This suggests that the deactivation could have been caused by the partial dissolution of Ir from the anode and its redeposition on cathode (a known drawback of using IrOx anodes).37 Introducing an anion exchange membrane would prevent the crossover of Ir, but would increase the energy loss due to higher resistance between the two electrodes (Table S3, SI). We thus added insoluble chelating agents (a styrene-divinylbenzene copolymer, Figure S10) instead into the electrolyte to trap the dissolved Ir ions before they could deactivate the cathode. The output of ethylene was then found to be stable at 28% and no Ir could be detected on the cathode’s surface after electrolysis. Another important consideration is that the optimized formation of ethylene from CO2 reduction only occurs at a specific potential (Figure 1e).25 However, the intermittent nature of solar radiation would result in the PV generating unstable potentials. When the solar flux is high and hence the potential generated by the solar panel is higher than required, H2 gas will be the dominant product (Table S1). If the solar intensity is low (for example, on a cloudy day), the output potential will be lower than optimum, and will result in a decreased formation of ethylene (Figure 1e). Moreover, a solar panel would not be able to drive the electrochemical cell at night. To address these practical issues, we designed a circuit that gave a constant voltage output with only solar photons as the external energy source (Figure 3a): (A) A voltage regulator was used to ensure a constant output voltage of 3.3 V even when the solar panels were under excessive sunlight; (B) A battery was connected in parallel with the solar panel for the storage of excess energy. Thus, when the solar intensity is strong and offers more than what the electrolyzer optimally needs, 3.3 V will still be offered to the electrolyzer while the battery is being charged simultaneously. When the solar flux is insufficient to provide a voltage of 3.3 V, the battery will automatically drive the electrochemical cell without any interruption. This design allows electrolysis to be performed continuously, which is not possible for systems powered only by photovoltaics. It is also more efficient than systems directly powered by batteries that are in turn charged by solar photovoltaics (due to energy losses from the charge−discharge process). To assess the usability of the whole system under natural sunlight, two commercialized solar panels were mounted in an area in Singapore that received intermittent sunlight (intensity ranges from 8−25 mW/cm2, Figure S12) for the first 2 h (Figure 3b). The panels were then disconnected to mimic night conditions (solar flux is 0) and the battery was used as energy source for another 2 h. With the proper selection of solar panels and battery (Table S7), we were able to charge the battery and power the electrochemical cell during the first 2 h, even when the solar flux fluctuates because of cloud cover and movement of the sun (Figure 3c). More significantly, after 2 h when there was no solar flux, the

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DISCUSSION A recent operando Raman spectroscopy study by our group has shown the formation of CO intermediates during CO2 reduction on a Cu cathode.39 The adsorbed CO could possibly be further reduced to CHxO (x = 1,2) species. These C1 species could then couple to form C2 intermediates, which are further reduced to ethylene.40−42 The optimized reduction of CO2 to ethylene typically requires large overpotentials in excess of >1 V.43 This is due to the high energy barrier associated with the formation of the C−C bond between two C1 intermediates such as CO. 44 In contrast, the hydrogenation of C 1 intermediates is more kinetically facile. The poor solubility of CO2 in aqueous electrolytes (∼33 mM at 298 K) and general ease of forming a H−H bond on Cu electrodes also leads to significant amounts of H2 side-product. Overall, these factors render the selective and efficient formation of ethylene far more challenging than that of producing carbon monoxide or formic acid (Table 1). Here, we have employed state-of-the-art electrocatalysts (oxide-derived Cu and IrOx), optimized the electrolysis parameters, and judiciously selected the solar panels to match the electrolyzer. The inertness of the IrOx anode toward ethylene oxidation is also very critical (Figure S7, SI). These key features of our setup have enabled us to fine-tune the solar to ethylene conversion efficiency to 1.5%. CO2 reduction driven by photocatalysts, such as TiO2, or TiO2 modified with metals or their oxides, phosphides, etc., typically produced C1 products such as carbon monoxide, methane, and methanol.45−48 An energetic efficiency of 2.5% toward methanol formation using Cu/TiO2 under ultraviolet illumination has been reported.46 However, under an actual solar spectrum, the energetic efficiencies of these photocatalysts are generally low.49,50 For example, CO2 reduction occurred on Pt/Cu immobilized on nitrogen-doped TiO2 nanoarrays with only an energetic efficiency of 0.01% under AM 1.5 sunlight. Oxidized Cu based materials such as Cu2O, CuO, CuFeO2/ CuO catalyst have also been studied as photocatalysts.51−53 Using CuFeO2/CuO as the photocatalyst under AM 1.5G solar radiation, Kang et al. demonstrated a 1% energetic efficiency (solar-to-fuel) for the conversion of CO2 to formate.53 Herein, our system has produced high-value molecules including ethylene, ethanol, and n-propanol with a much higher energetic efficiency of 2.9% under AM 1.0 solar flux. More strikingly, it was demonstrated to be able to produce ethylene continuously even under intermittent natural sunlight. Our timely study has provided new insights into the practical design of a PVelectrolyzer system for the artificial photosynthesis of ethylene, which we believe, will narrow the gap between its development in the laboratory and deployment in the industry. In summary, we have developed a prototype artificial photosynthesis system targeted for ethylene production from carbon dioxide and water. The careful selection and optimization of state-of-the-art electrocatalysts, electrolysis parameters, and solar PVs to match the electrolyzer enabled 9196



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the electricity-to-ethylene efficiency of our system to reach as high as 11.1%. The solar-to-ethylene energy efficiency was 1.5%. We also showed that the dissolution of the Ir anode gave rise to contaminants in the electrolyte to deactivate the cathode. Adding chelating agents to the electrolyte to trap the ions mitigated this issue. Nonetheless, more efforts should be invested in development of more stable IrOx anodes. The importance of a stable voltage output for the electrolyzer was also emphasized. Considering the intrinsic fluctuations and intermittencies associated with the use of natural sunlight, we developed a circuit that can give a constant and stable voltage output. As a result, the production of ethylene was observed even as the solar flux fell below the required intensity. We believe that our work has helped to solve many key challenges faced in the implementation of a global artificial photosynthesis system, and represents a major step forward in the important field of solar energy utilization.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02110. Preparation of the electrocatalysts; characterization of the electrocatalysts; the two-electrode electrolyzer; and solardriven CO2 reduction under solar simulator and under natural sunlight (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dan Ren: 0000-0003-3738-6421 Luo Gong: 0000-0002-1127-6008 Boon Siang Yeo: 0000-0003-1609-0867 Author Contributions

EXPERIMENTAL SECTION

#

D.R. and N.W.X.L. contributed equally to this work.

Preparation and Characterization of the Electrodes. Cu2O films were electrodeposited onto mechanically polished Cu substrates (99.99%, Goodfellow) at a constant current density of −2.1 mA/cm2 (Autolab PGSTAT30) for 600 s. These parameters give Cu2O films with optimum performance for ethylene formation.25 IrOx was electrodeposited onto Ti foils using 100 cycles of cyclic voltammetry scans from 0.32 to 1.48 V vs RHE. The morphology and chemistry of the catalysts were characterized by scanning electron microscopy (JEOL JSM-6710F, 5 kV, 10 μA), transmission electron microscopy with selected area electron diffraction and energy dispersive X-ray spectroscopy (JEOL 3010, 300 kV, 112 μA), and X-ray photoelectron spectroscopy (Kratos AXIS UltraDLD, Al Kα emission source). For the TEM analysis, the films were scraped off the substrate and suspended in ethanol. The suspension was then drop-coated onto Au grids coated with lacey carbon (Electron Microscopy Sciences, LC-300 Au, 300 mesh), and dried under a lamp. Electrochemistry of the Electrodes. The activity of the Cu2O cathode for CO2 reduction was tested in a three-electrode cell.39 The cathodic and anodic compartments were separated by an anion exchange membrane (Asahi Glass). CO2 (99.999%, Linde) was continuously flowed through both compartments at a flow rate of 20 cm3/min. A Ag/AgCl (Pine, Saturated KCl) and Pt wire were respectively employed as reference and counter electrodes. A 0.2 M KHCO3 (>99.7%, Merck) aqueous solution was used as the electrolyte, and 60 min chronoamperometry with iR compensation (Gamry Reference 600) at selected potentials was applied to the cathode. The gaseous products were analyzed by gas chromatography (Agilent 7890A), and the liquid products were quantified by headspace gas chromatography (Agilent 7890B) and high performance liquid chromatography (Agilent 1260). The activity of the IrOx anode toward oxygen evolution was tested using linear sweep voltammetry at a scan rate of 10 mV/s in CO2-saturated 0.2 M KHCO3 aqueous solution. All the currents and production rates were normalized using the geometric surface areas of the electrodes. Solar Driven Carbon Dioxide Reduction. A two electrode setup was used. For experiments using simulated solar radiation, we used a solar panel purchased from IXYS (KXOB22-01X8F). The effective illuminated area of one such panel is 0.96 cm2. AM 1.0 one sun (100 mW/cm2, Sun 2000 Solar Simulator, ABET Technologies) illumination was used since Singapore lies only 1.5 degree north of the equator. For the experiments under real solar flux, two larger silicon solar panels (0.5 W Solar Panel 55*70, Seeed Studio) were connected in parallel. A rechargeable lithium polymer battery (3.7 V, 500 mAh) was used for the storage of excess energy. The current was measured by a potentiostat (Gamry Reference 600).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by a research grant from the Ministry of Education, Singapore (R-143-000-683-112). D.R. and L.G. acknowledge Ph.D. research scholarships from the Ministry of Education, Singapore. We thank Dr. Souradip Malkhandi (NUS) and Prof. Christian Kurtsiefer (NUS) for helpful discussions regarding the design of circuits, and Prof. Wee Shong Chin (NUS) for access to a solar simulator.

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REFERENCES

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