Toward a Practical Solar-Driven CO2 Flow Cell

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Towards Practical Solar-Driven CO FlowCell Electrolyzer: Design and Optimization Gowri Manasa Sriramagiri, Nuha Ahmed, Wesley W Luc, Kevin Dobson, Steven Hegedus, and Feng Jiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02853 • Publication Date (Web): 24 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Sustainable Chemistry & Engineering

Towards Practical Solar-Driven CO2 Flow-Cell Electrolyzer: Design and Optimization Gowri M. Sriramagiri, Jiao*

†, ‡

Nuha Ahmed,

†, ‡

§

†

Wesley Luc, Kevin D. Dobson, Steven S. Hegedus,

†, ‡

Feng

§

†

Institute of Energy Conversion, University of Delaware, 451 Wyoming Road, Newark, DE 19716, USA. Dept. of Electrical and Computer Engineering, Evans Hall, University of Delaware, 139 The Green, Newark, DE 19716, USA.

‡

§

Dept. of Chemical and Biomolecular Engineering, Colburn Laboratory, University of Delaware, 150

Academy Street, Newark, DE 19716, USA. * Corresponding author. Email Address: [email protected] ABSTRACT: A first-of-its-kind solar CO2 flow-cell electrolyzer is reported here with a solar-to-fuel efficiency (SFE) of 6.5% at high operating currents, >1 A, orders of magnitude greater than other reported solar-driven devices which typically operate at currents of a few milliAmps. The approach of solar moduledriven electrolysis, compared to monolithic photoelectrochemical cells, allows simpler manufacture, use of commercially-available components, and enables optimization of the power-transfer between the photovoltaic and the electrochemical systems. Employing commercial high efficiency crystalline silicon 2

solar cells with a large area flow-cell CO2 electrolyzer (25 cm ), we present a procedure to optimize the SFE of a decoupled photovoltaic electrolyzer by impedance-matching the source and the load using their independent current-voltage characteristics. The importance of the voltage-dependent Faradaic efficiency of the electrolyzer on device performance and optimization is highlighted. KEYWORDS: Solar electrolysis, artificial photosynthesis, photoelectrochemical cell, solar-to-fuel efficiency, carbon dioxide

■ SYNOPSIS: A procedure for performance optimization of a solar CO2 electrolyzer is presented through modelling and practical demonstration, where a peak solar to fuel efficiency of 6.5% is achieved, making it the first-reported high-current solar electrolysis system that is both practical and readily deployable for CO2 reduction.

■ INTRODUCTION The extent of recent human intervention in the ecosphere’s carbon cycle is unprecedented, causing irreversible damage to the planet’s environment. Extensive reforestation alone doesn’t suffice to tackle the problem, considering the scale of greenhouse gas emissions and the low efficiencies of natural 1

photosynthetic processes. Artificial photosynthesis approaches for H2 and carbon-based fuel production 1

ACS Paragon Plus Environment


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are viable solutions and have gained a considerable amount of interest in recent times.

2-13

Several

lectrochemical and photoelectrochemical strategies for water splitting and CO2 reduction have been reported,

2-6

and the underlying mechanisms are well understood.

7- 19

However, preliminary techno-

economic analyses of these systems indicate that despite decades of research, further efforts are required for efficient scaling of electrolyzer technology and reduction of system costs.

14

Additionally,

direct atmospheric absorption of CO2 by existing solar electrolysis technologies has been shown to be impractical due to mass transport limitations and low atmospheric concentrations.

20

However, solar-

driven CO2 electrolyzers to reduce CO2 to CO, based on the capture of CO2 emissions from fossil-fuel fired power plants and other manufacturing facilities, provide a promising solution for mitigating the increase in greenhouse gases. Additionally, CO is a precursor to produce transportation fuels, which can be used as energy-dense storage for renewable energy generation. Many aspects of H2 generation by water electrolysis have been extensively studied and optimized ranging from electrode materials to scaling and technology integration,

21, 22

and much of this technology can be

transferred to CO2 reduction system design. Several architectures for solar electrolytic systems, ranging from photoelectrochemical cells (PEC) to photovoltaic electrolyzers (PV-EC), have been reported.

22

A

PEC is generally designed as a monolithic device utilizing a photo-absorber that doubles as the electrode, often coated with a catalyst and immersed in the electrolyte, creating an integrated package. A PV-EC on the other hand, consists of a PV source and electrolyzer load separated from each other, working independently, connected through cables. PEC-type devices, which have been developed for over 45 years, have had many well-documented issues including: (i) limitations in obtaining photovoltages suitable to drive desired reactions from a singlejunction device; (ii) poor photo-stability of narrow bandgap semiconductors which required the use of wide bandgap photocatalysts, thus, decreasing device photoresponse in the visible region of the spectrum, (iii) the need to immerse the photoactive components in potentially corrosive electrolyte solution, which complicates device design and manufacturability and necessitates the use of protective layers and/or compatible electrocatalysts, and (iv) optical losses due to inherent design and fabrication limits. Point (iii) limits the materials selection available for device application, and materials processing must be compatible with the device structure. Additionally, multi-junction III-V PV cells have been used in order to achieve the required high voltages and with high electrolysis efficiencies, however, such devices are difficult to manufacture, expensive, and are not yet commercially available PV-EC device design, however, offers many advantages over PECs since the separation of the optical and electrical components allows a greater selection of materials and eliminates concerns of processing compatibilities and solution stability of the light active components, as well as allowing the use of the high quality electrocatalysts and commercially available components that can easily be incorporated into the design. In particular, low-cost and reliable silicon-based PV cells and modules are already widely available and can be configured to provide the needed current and voltage independently. For example, 2


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the application of a series-connected array to obtain high photovoltages to drive the electrolytic reactions have recently been demonstrated for water splitting

23

and CO2 reduction,

24

where the photovoltage is the

sum of the voltages of the total number of series-connected PV devices. This can be provided by devices ranging from lab-bench solar cells to commercial PV modules, allowing suitably high voltages to be obtained to drive reactions. These could previously only be obtained for PEC or other photoelectrochemical approaches with extremely expensive (and unconcentrated) multijunction photoactive components. Similarly, the area of the solar device(s) (APV) can be much greater than that of the electrolyzer enabling higher current densities than those of the PEC structure. The PV-EC system with suitable electrocatalysts has the promise for more efficient solar driven manufacture of different species, including fuels and other industrially-relevant chemicals. Some of the PEC-type devices reported so far for CO2 electrolysis outperform PV-EC type devices. However, the PV-EC type architecture holds promise for increased efficiency devices in the future, since its scalability is easier and more direct compared to the PEC architectures. This is because of PV-EC’s simpler design, where the electroactive component and photoactive components of a PV-EC type device are separated, and therefore, can be optimized independently. When commercially available PV devices are used in a PV-EC configuration, the interconnection between the source and load must be carefully designed, based on the current-voltage characteristics of the individual components. This allows for optimization of the voltage, in terms of the number of solar cells in series and of the current, based on number of cells in parallel, or conversely, the individual solar cell areas. Furthermore, the coupling of different configurations can be simulated and used to calculate the solar-to-fuel efficiency (SFE), the key figure-of-merit, as well as to determine the relationships of the individual parameters on the final device performance. Such an approach would ensure optimum utilization of power available at the source, enabling enhanced SFE’s. Compared to the PV-EC device, the performance of a PEC, is simpler to model since the photoabsorber generating the potential for reduction is coated on the electrode where the catalytic reaction simultaneously occurs, making their surface areas equal and avoiding voltage drops through wired connections. However, this approach does not allow for optimization of coupling efficiency beyond what is dictated by the individual current density – voltage (J-V) characteristics of the photoactive and catalyst layers. While H2-generating PEC’s have shown performance of 30%,

25

the highest efficiency reported for

a CO2-reduction PEC is ~10%, where a bipolar membrane was used with separated electrolytes for CO2 reduction reactions and oxygen evolution reactions (OER), using a multijunction photoabsorber of small electrode area.

26

In another report, a SFE of 6.5% was demonstrated for a PV-EC-type device for CO2

reduction, employing a thin film perovskite solar cell array.

24

In this work, we designed and implemented a practical PV-EC system, using commercial silicon solar cells and a large-area flow-cell CO2 electrolyzer.

27

The power-transfer between the source and the load

was modeled to optimize the system design for maximum SFE. The experimental results of this effort 3



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were presented previously without detail to the procedure for developing the model.

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28

In this work, the

modeling of performance was explained comprehensively and validated based on proof-of-concept demonstration of several system configurations. Herein, we described a deployment-ready PV-driven CO2 flow-cell electrolyzer, utilizing commercially available solar modules, with SFE potentially exceeding 6.5%. System Design. Figure 1 shows a schematic of the PV-EC system that was investigated, where an independently operating custom-built solar array drove a directly-connected flow-cell electrolyzer. Fabrication of the individual components of the PV-EC device are described in the experimental methods section of the Supporting Information. The known operating parameters and conditions of these devices were used for modeling experiments. An experimental matrix was designed which included variables such as the solar cell illumination area, hence output current, and number of cells in series, hence output voltage. Solar cell areas greater than and less than the predicted optimum were studied experimentally to demonstrate the dependency of SFE and other key parameters on the solar cell illumination area.

hν

O2

Figure 1. Schematic of PV-EC system with photovoltaic and CO2 electrolyzer components. The performance figure-of-merit for the device, SFE, which depends on PV and electrolyzer operational parameters is given by the following equation:

23

(1)

where FEco is Faradaic efficiency of reducing CO2 to CO, JOP is operating current density, PIN is the input 2

solar power density with standard 1-Sun illumination of 100 mW/cm , and µTH is 1.34 V which is the 4


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overall thermodynamic voltage for CO2 electrochemical reduction to CO.

23

The overall reaction is as

follows: +

-

Cathode: CO2(g) + 2H +2e → CO(g) + H2O(l) (-0.11 V vs RHE) +

-

Anode: H2O → ½ O2(g) + 2H +2e

Overall: CO2(g) → CO(g) + ½O2(g)

(+1.23 V vs RHE) (µTH = 1.34 V)

(2) (3) (4)

where V vs RHE is potential measured against the reversible hydrogen electrode. Equation (2) is the halfreaction for CO2 reduction to CO while Equation (3) is the half-reaction for water oxidation at the cathode and anode, respectively. In addition, when CO2 reduction occurs in aqueous solution, the competitive and undesirable hydrogen evolution reaction (HER) also occurs at the cathode. FEco describes the efficiency with which charge converts CO2 into the desired CO product. In order to minimize the competitive HER and to avoid formation of stable carbonates in alkaline conditions, the electrochemical reaction was carried out in near neutral conditions. The operating current density was calculated by measuring the operating current in the circuit and dividing it by the total solar cell illumination area of all solar cells in series in the array and is as follows:

# ! " #$"

(5)

The components of the PV-EC system were first studied individually to determine the operating voltages and currents of all the possible configurations. The CO2 electrolyzer was studied under constant potential experiments using an Autolab PGSTAT128N potentiostat with a 10 A booster and the overall performance is summarized in a linear voltammagram curve as shown in Figure 2(a), where total current and CO Faradaic efficiency in the span of 30-minute operation were plotted versus cell potential. The cell potential included all the voltage losses within the device due to the internal resistance and transport and kinetic limitations. In short, the electrolyzer was a sandwich-type flow-cell reactor, with a large area (25 2

cm ) nanoporous-silver (np-Ag) and iridium-coated catalyst membrane (Ir-CCM) as cathode and anode, respectively.

27

It has been recently shown that a np-Ag catalyst can efficiently facilitate the catalytic

conversion of CO2 to CO where the highly curved internal surfaces of the nanoporous structure can significantly increase the total catalytic surface area while possessing large numbers of step sites that can stabilize key reaction intermediates.

29

(refs are the same) As for the anode, Ir was selected as the water

oxidation catalyst due to its high activity as well as stability in a wide-range of pH’s. As shown, the CO2 electrolyzer was able to achieve over an Amp of current with high selectivity towards converting CO2 to CO. Moreover, the only observed CO2 reduction product was CO, and the remaining charge balance was attributed to the competing HER. As shown in Figure 2(a), the electrolyzer required a minimum of 2.4 V to 5



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drive the reaction of interest. More details of the CO2 electrolyzer and operating conditions can be found in the Supporting Information. To power the CO2 electrolyzer, SunPower interdigitated back contact crystalline silicon solar cells were investigated, owing to their high current density. The current-voltage behavior of these solar cells was o

measured using OAI solar simulator under standard testing conditions (25 C operating temperature and 2

100 mW/cm irradiation) and the performance of one SunPower solar cell is summarized in Figure 2(b). a)

b)

Figure 2. (a) Independently measured performance of the CO2 electrolyzer. (b) Independently measured I-V plot of one of the SunPower cells, with the average parameters of all available cells in the inset. Note: On the I-V curve of any solar cell, there is current-voltage point at which the solar cell output power peaks which is the maximum power point (MPP). The current and voltage at this point are called its maximum power voltage (VMP) and maximum power current (IMP).

The solar array design was performed by determining the number of identical solar cells in series which is based on the voltage output of individual solar cells and the desired operating voltage of the electrolyzer. When similar solar cells are connected in series, the resulting output voltage is the sum of the voltages of individual solar cells, and the resulting current is the same as the current output from a single solar cell, and vice versa when connected in parallel. To overcome voltage losses within the electrolyzer, a suitable overpotential must be applied in order for the electrochemical reaction to proceed. Considering that the solar cells used in this study had a maximum power voltage output (VMP) of ~0.6 V, and that the electrolyzer required at least 2.4 V for its operation; it can be deduced that a solar array containing between 4 or 5 identical solar cells connected in series was needed. However, due to voltage drops in the circuit and temperature losses in the system, an array of at least 5 solar cells, giving an array VMP ~3 V, was used to operate the electrolyzer. As such, the voltage requirement of the electrolyzer was satisfied by choosing the number of identical solar cells in the array connected in series. However, since the output current of these solar cells at maximum power point (IMP) of ~2.8 A was much higher than the operating

6


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current of the electrolyzer (~1 A), the area of all the individual solar cells in the array in series was reduced to lower the array current output. Next, in order to match the current requirement for the PV-EC system, the ideal area of the solar cells was investigated through modeling. Modeling and Optimization. After performing voltage-matching by determining the number of identical cells in the array, current-matching was conducted by determining the optimum area of the individual solar cells such that the power transfer between the source (solar array) and the load (electrolyzer) was maximized. The “solar cell area” mentioned here onwards refers to the illumination area of each of the identical solar cells connected in series, which when changed, influenced the output current of the array. Therefore, the total solar array area is this “solar cell area” multiplied by the number of cells in series. This way, we used the individual solar cell area as the second control parameter in power optimization. The current versus potential (I-V) curves of each of the several possible solar array configurations of varying solar cell area were constructed by scaling their measured current densities to this solar cell area. For each of these configurations, the operating voltage (VOP) and current (IOP) of the resulting PV-EC device were determined from where the CO2 electrolyzer’s I-V curve intersected with that of each of the solar array I-V curves. The result of these calculations for the 5-cell solar array configuration are given in Figure 3(a). The dependence of the operating parameters (VOP and JOP), selectivity (FECO) and the figure-of-merit (SFE) of the resulting PV-EC device on these control parameters (the number of solar cells and the individual solar cell area) was then evaluated from this calculation. For the 5-cell configuration, this dependence is shown as plots of VOP, JOP, FECO, and SFE versus the solar cell area in Figure 3(b) and 3(c). Figure 3(b) shows how JOP decreased while VOP increased with increasing illuminated cell area. Figure 3(c) shows that SFE, which is directly calculated from the product of JOP and FECO, had very similar shape and peaks at same point as FECO. Figure 3(c) also shows that larger illumination areas tend to provide lower SFEs due to lower JOP (Figure 3b). However, even though VOP does not directly affect SFE, it does influence FECO. This dependency of SFE on JOP and FECO creates a tradeoff between JOP and VOP for identifying the configuration to maximize SFE. Thus, in each PV array configuration with a given number of cells connected in series, the SFE peaks at an optimum value of solar cell illumination area. Consequently, this peak-SFE area is one for which the PV-EC operating point lies closest to the maximum power point (MPP) on the solar array I-V curve, ensuring the optimum power transfer between the source and load. Using the parameters of solar cell and selected electrolyzer components, the ideal SFE, with no losses was predicted to be >7% for the 5-cell array with each PV cell having an individual solar cell illuminated 2

area of ~25 cm (Table 1) connected in series.

a)

7



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b)

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c)

Figure 3. (a) Modeled I-V curves of 5-cell PV array under different solar cell areas and I-V curve of CO2 electrolyzer. Note: “Solar Cell Area” refers to the illumination area of individual and identical solar cells in the 5-cell array. The total solar array area is 5 times that of the “Solar Cell Area”. The intersection points give the IOP and VOP for the device setup. The diamonds on each of the solar array I-V curves are its maximum power points (MPP’s), whereas the green triangles are the intersecting points of the solar array I-V curve and the electrolyzer I-V curve, which give the operating points (IOP and VOP) (b) The resulting JOP and VOP values for configurations with varying solar cell area. (c) The extrapolated FECO and calculated SFE for the solar cell areas considered.

Table 1. Results from modeling 5-cell configuration PV array. A (cm²)

VMP (V)

IMP (mA)

JMP (mA/cm²)

VOP (V)

IOP (mA)

J'OP (mA/cm²)

JOP (mA/cm²)

FECO (%)

SFE (%)

10 14 25

2.85 2.85 2.85

350 490 876

35.0 35.0 35.0

2.43 2.59 2.75

376 523 903

37.6 37.3 36.1

7.52 7.47 7.23

56.1 55.1 75.9

5.7 5.5 7.4

8


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33 2.85 1143 35.0 2.87 1129 34.6 6.92 40.2 3.7 77 2.85 2686 35.0 3.12 1719 22.4 4.48 8.5 0.5 Note: The values in red highlight the configuration giving best predicted SFE. J’OP is the operating current density of individual cells. In order to limit the different possible source configurations, only PV arrays comprising of 5 or 6 cells in series were modeled as these gave VOP in approximately the right range as discussed above. The modeled parameters of the 6-cell configuration and its results are discussed in the Supporting Information, where it is shown that their SFE values never exceeded those obtained with a 5-cell array under the same conditions. With the increasing number of cells in the array, the MPP of the PV array moved to larger voltages and further from the electrolyzer operating (or intersection) point. In this way, the developed model was used to optimize the PV-EC performance by using the number of solar cells in series and the individual solar cell area as the control parameters. In short, a directly coupled PV-EC devices benefits from careful voltage and current matching between the source and the load such that optimum SFE can be obtained.

■ EXPERIMENTAL RESULTS AND DISCUSSION Based on the modeling results, a 5-cell-PV array-driven CO2 electrolyzer setup as shown in Figure 1 was implemented to experimentally validate the dependence of SFE on solar cell area by varying illumination 2

2

area from 14 cm to full area, 77cm , using adjustable shadow masks. The conditions of these experiments are shown in Table 2. Table 2. Experimental matrix with varying solar cell illumination areas PV Area Duration 2 Configuration Mask Dimensions (hrs.) Illuminated (cm )

Comment

1 2 3

4.1 cm x 3.4 cm 7.6 cm x 3.4 cm 9.6 cm x 3.4 cm

14 26 33

1 1 2

< Optimal PV Area = Modelled Optimum > Optimal PV Area

4

No Mask- Full Area

77

1

> Optimal PV Area

The results of each of these experimental conditions are summarized in Table 3. The current readings were logged every minute in each of the configurations and are shown in Figure S2 of the Supporting Information. The plots confirmed that the performance of the test system and its components were stable for the experimental duration. Figure 4(a) shows plots of IOP and VOP values measured in the circuit for each of the experimental conditions, against the cell area, from which JOP was determined using Equation 5. FECO was calculated every 15 minutes for each configuration and the average and peak values are listed in Table 3. Figure 4(b) shows the measured FECO (peak values) and the resulting SFE of each of the experimental

9



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conditions, plotted against the solar cell area. The measured currents varied from the modeled values due to voltage discrepancies between the terminals of the PV array and the electrolyzer. It can be seen 2

2

that the determined optimum area has shifted from 25 cm to 33 cm due to these voltage losses in the 2

circuit. A peak SFE of 6.5% was obtained with the 33 cm solar cell area in the 5-cell array, approaching the predicted value of 7.3%. Table 3. Summary of PV-EC experimental results PV VPV ∆V Area VEC (V) R (Ω) (V) (V) (cm²)

IOP (mA)

J’OP

JOP 2

2

FECO

SFE

(mA/cm )

(mA/cm )

(%)

(%)

32.75

6.55

55.5

4.9

Average Values 14

2.59

2.51

0.08

0.164

457

26

2.80

2.70

0.10

0.121

855

33.08

6.62

66.2

33

2.90

2.76

0.14

0.133

1024

31.37

6.27

75.9

5.9 6.4

77

3.00

2.83

0.17

0.137

1234

16.09

3.22

51.86

2.2

Peak Values (Corresponding to Peak FECO) 14

2.59

2.51

0.08

0.173

456

32.71

6.54

58.3

5.1

26

2.80

2.69

0.11

0.128

861

33.32

6.66

67.9

6.1

33

2.90

2.77

0.13

0.127

1021

31.28

6.26

78.1

6.5

77

3.00

2.83

0.17

0.137

1240

16.17

3.23

52.7

2.3

Predicted Values from Modeling 14

2.53

2.53

0.00

0.000

522

37.34

7.47

55.5

5.6

26

2.77

2.77

0.00

0.000

932

35.86

7.17

75.9

7.3

33

2.87

2.87

0.00

0.000

1128

34.58

6.92

51.9

4.8

77

3.01

3.01

0.00

0.000

1463

19.08

4.48

51.9

3.1

Note: PV area is the illuminated solar cell area, VPV is the voltage at the PV module, VEC is the voltage at the electrolyzer, ∆V is the difference between the VPV and VEC, R is the resistance calculated from the differences in voltage between PV and electrolyzer divided by operating current IOP, J’OP is the operating current density per cell, JOP is the overall operating current density and actual value used in calculating SFE, and FECO is the CO Faradaic efficiency.

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a)

b)

Figure 4. (a) JOP and VOP versus solar cell area, and (b) SFE and FECO versus solar cell area obtained from results of practical demonstration of PV-driven CO2 electrolyzer using 5-cell array.

Comparing Calculated and Measured Results. The solar array I-V curves calculated without additional losses were compared with those measured under the solar simulator (Figure 5) for the case of 33 cm

2

illumination area to verify the validity of the developed model. The measured I-V behavior of the solar array was expected to be slightly inferior to the predicted results, since the array suffered from voltage losses during experiment due to higher cell temperatures under the solar simulator (which left the cells o

working at ~40 C due to radiative heating), and parasitic series resistance (RS) from wiring and tabbing. More details of the experimental setup are given in the Supporting Information. Generally good agreement between the as-measured and the modeled I-V curves is apparent in Figure 5(a). Figure 5(b) expands the voltage axis around the MPP and shows how successive corrections for RS and temperature losses moved the measured curve much closer to the modeled curve. This agreement validates the procedure used here for modeling the solar array I-V curve generation and shows the relative impact of realistic losses. Additional voltage drops measured in the circuit were incorporated back into the model to improve its accuracy. These drops, incurred due to parasitic circuit resistances, affected the experimental results more significantly than the I-V curve discrepancies shown in Figure 5. These were integrated into the analysis by reproducing the load curve with a resistor of the measured circuit resistance in series. This resulted in the calculated SFE values approaching the measured values within error smaller than 3%. The results are summarized in Table 4 and confirmed the validity of the developed model. While the practical 2-electrode system used in the CO2 electrolyzer in this work suffered from significant voltage drops due to parasitic circuit resistances, we emphasize that this is one of the very few large-area solar electrolyzers to be reported. In summary, the results predicted from initial modeling were modified after experimentation,

11



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by incorporating the unexpected empirical losses, such as the higher operating temperature and parasitic circuit resistances. When the model was corrected for these experimental losses sequentially, the modified results agreed well with the experimental results. a)

b)

Figure 5. (a) Comparison of solar-array I-V curves generated from our model with those measured under the home-made solar simulator. (b) The same plot with x-axis expanded to better emphasize the difference in the curvature of each of the plots.

Table 4. Average values of SFE predicted from modeling when corrected for measured parasitic circuit resistive losses Modeled SFE (%) Area Measured SFE (%) ∆SFE (% abs.) ∆SFE (% rel.) 2 (cm ) with ∆V correction 14 26

5.00 5.96

4.87 5.87

0.13 0.09

2.60 1.54

33

6.40

6.38

0.02

0.31

77

2.23

2.24

-0.01

-0.33

Summary of Proposed Optimization Procedure. The modeling process performed in this work that led to successful prediction of SFE values and verified through experimentation, can be concisely summarized as follows. To optimize the SFE of a PV-EC device, where the current can be controlled by the solar cell area, a tradeoff is set between the interaction of JOP, a function of cell area, and FECO, a function of VOP. Based on this, the ideal solar cell area for an array with a determined number of cells in series, was that which produced sufficient voltage, current density and the respective FECO to produce the maximum possible SFE for the device. The decrease in JOP and increase in FECO with increasing cell area established the trade-off. This is until the FECO (V) curve reached its inflection point beyond which both the parameters began to decrease. A converse method can be employed in cases where the solar cell area is fixed but the electrode surface area is adjustable. In that case, if it is safe to assume that the I-V curve of the electrolyzer scales linearly with the surface area, we can similarly balance the trade-off between the JOP and FECO, using electrode surface area. Voltage drops arising due to several parasitic

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contact and cable resistances, can also be incorporated in modeling, if their values are known. This has been shown to give accurate predictions of SFE’s within 1% relative error. Using this model, it is shown in the Supporting Information that SFEs as high as 14% are possible for CO2 electrolysis (including resistive losses), employing high efficiency triple junction solar cell technology mentioned in reference 25, with the flow-cell electrolyzer described in this work.

■ CONCLUSION We have shown that developing a solar fuel production system using the PV-EC architecture facilitated independent design of the power source and electrolyzer components. This eliminated the challenges of materials compatibility and excess optical losses associated with integrated PEC’s. The primary benefit was that PV-EC enabled optimization of the power-transfer between the PV and the EC systems, targeting SFE maximization using rigorous design and modeling. A directly-coupled PV-driven large-area CO2 flow-cell electrolysis system was designed, modeled, optimized and implemented in this work using high efficiency commercial silicon solar cells with a sandwich-type flow cell electrolyzer. The predicted trend of SFE dependence on illuminated cell area based on modelling was verified with experiments, designed to span from sub-optimal to beyond optimal cell areas. Unexpected voltage drops in the circuit decreased the peak SFE and shifted the best cell area to higher values than predicted from initial calculations. Modeled results were then corrected for higher operating temperature of the solar cells and measured parasitic circuit resistance. Following this correction, the predicted results exhibited an excellent agreement with the empirical values, to within 8% SFE’s.

■ ASSOCIATED CONTENT Supporting Information Results for 6-cell configuration, potential for high SFE’s with multijunction tandem solar cells under concentration and experimental methods, including CO2 electrolyzer, photovoltaic power source (the solar cells used), solar simulator construction for illumination source and system operation

■ ACKNOWLEDGEMENTS

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This work was partially supported by University of Delaware Energy Institute. In addition, W. L. and F. J. would like to acknowledge the University of Delaware Research Foundation and National Science Foundation CAREER Program (Award No. CBET-1350911) for financial support.

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■ For Table of Contents Use Only. 1.

TOC Graphic:

2.

Synopsis: A procedure for optimization of a high-current solar electrolyzer for CO2 reduction is

presented, and a solar to fuel efficiency of 6.5% is achieved. A scaled-up version of such a system deployed in field has a potential to curb carbon emissions by enabling conversion of CO2 to fuels, thereby completing the carbon cycle.

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Toward a Practical Solar-Driven CO2 Flow Cell

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