Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10959-10966
Toward a Practical Solar-Driven CO2 Flow Cell Electrolyzer: Design and Optimization Gowri M. Sriramagiri,†,‡ Nuha Ahmed,†,‡ Wesley Luc,§ Kevin D. Dobson,† Steven S. Hegedus,†,‡ and Feng Jiao*,§ †
Institute of Energy Conversion, University of Delaware, 451 Wyoming Road, Newark, Delaware 19716, United States Department of Electrical and Computer Engineering, University of Delaware, Evans Hall, 139 The Green, Newark, Delaware 19716, United States § Department of Chemical and Biomolecular Engineering, University of Delaware, Colburn Laboratory, 150 Academy Street, Newark, Delaware 19716, United States
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‡
S Supporting Information *
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 those of other reported solar-driven devices that typically operate at currents of a few milliamperes. The approach of solar module-driven electrolysis, compared to monolithic photoelectrochemical cells, allows simpler manufacture, use of commercially available components, and optimization of the power transfer between the photovoltaic and the electrochemical systems. Employing commercial high-efficiency crystalline silicon solar cells with a large area flow cell CO2 electrolyzer (25 cm2), we present a procedure for optimizing 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
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INTRODUCTION
which can be used as energy-dense storage for renewable electricity 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 photoabsorber 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 more than 45 years, have had many well-documented issues, including (i) limitations in obtaining photovoltages suitable for driving desired reactions from a single-junction device; (ii) poor photostability of narrow bandgap semiconductors that required the use of wide bandgap photocatalysts, thus, decreasing the
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 does not suffice to tackle the problem, considering the scale of greenhouse gas emissions and the low efficiencies of natural photosynthetic processes.1 Artificial photosynthesis approaches for H2 and carbon-based fuel production are viable solutions and have gained a considerable amount of interest recently.2−13 Several electrochemical 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 because of 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 gas levels. Additionally, CO is a precursor for the production of transportation fuels, © 2017 American Chemical Society
Received: August 17, 2017 Revised: September 20, 2017 Published: September 24, 2017 10959
DOI: 10.1021/acssuschemeng.7b02853 ACS Sustainable Chem. Eng. 2017, 5, 10959−10966
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ACS Sustainable Chemistry & Engineering
Figure 1. Schematic of the PV-EC system with photovoltaic and CO2 electrolyzer components.
When commercially available PV devices are used in a PVEC 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 the 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, and 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 that of the PV-EC device, the performance of a PEC is simpler to model because 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 losses 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 30% performance,25 the highest efficiency reported for a CO2-reduction PEC is ∼10%, where a bipolar membrane was used with separated electrolytes for the CO2 reduction reaction and the oxygen evolution reaction (OER), using a multijunction photoabsorber with a 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 PVEC 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 were presented previously without details about the procedure for developing the model.28 In this work, the modeling of performance was explained comprehensively and validated on the basis of a proof-of-concept demonstration of several system configurations. Herein, we described a deployment-ready PVdriven CO2 flow cell electrolyzer, utilizing commercially available solar modules, with SFE potentially exceeding 6.5%.
device photoresponse in the visible region of the spectrum; (iii) the need to immerse the photoactive components in a 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, multijunction III−V PV cells have been used to achieve the required high voltages and with high electrolysis efficiencies; however, such devices are difficult to manufacture, expensive, and not yet commercially available. PV-EC device design offers many advantages over PECs because 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 high-quality electrocatalysts and commercially available components that can easily be incorporated into the design. In particular, lowcost 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, a seriesconnected array has been applied to obtain high photovoltages to drive the electrolytic reactions for water splitting23 and CO2 reduction,24 where the photovoltage is the sum of the voltages of the total number of series-connected PV devices. Suitably high voltages can be similarly obtained by devices ranging from lab-bench solar cells to commercial PV modules. Such high voltages for PEC are only possible by using extremely expensive tandem-junction photoactive components. Additionally, the area of the solar device(s) (APV) in a PV-EC device can be much greater than that of the electrolyzer, enabling current densities higher than those of the PEC structure. The PV-EC system with suitable electrocatalysts shows 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 higher-efficiency devices in the future, because its scalability is easier and more direct compared to those of the PEC architectures. This is because of the PVEC’s simpler design, in which the electroactive component and photoactive components of a PV-EC-type device are separated and, therefore, can be optimized independently. 10960
DOI: 10.1021/acssuschemeng.7b02853 ACS Sustainable Chem. Eng. 2017, 5, 10959−10966
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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 that on the I−V curve of any solar cell, there is a 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), respectively.
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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 is 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, and hence output current, and the number of cells in series, hence the output voltage. Solar cell areas larger than and smaller than the predicted optimum were studied experimentally to demonstrate the dependency of SFE and other key parameters on the solar cell illumination area. The performance figure of merit for the device, SFE, which depends on PV and electrolyzer operational parameters is given by the following equation:23 μ SFE = JOP × TH × FECO PIN (1)
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. To minimize the competitive HER and to avoid formation of stable carbonates under alkaline conditions, the electrochemical reaction was performed under near-neutral conditions. The operating current density was calculated by measuring the operating current in the circuit and dividing it by the total illumination area of all solar cells in the array as follows: JOP =
cathode: CO2 (g) + 2H+ + 2e− → CO(g) + H 2O(l) (2)
anode: H 2O(l) →
1 O2 (g) + 2H+ + 2e− ( +1.23 V vs RHE) 2 (3)
overall:
CO2 (g) → CO(g) +
(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 2a, where the total current and CO Faradaic efficiency during a 30 min 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 cm2) nanoporous silver (np-Ag) and iridium-coated catalyst membrane (Ir-CCM) as the 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 As for the anode, Ir was selected as the water oxidation catalyst because of its high activity and stability over a wide range of pH values. As shown, the CO2 electrolyzer was able to achieve >1 A of current with high selectivity toward 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 2a, the electrolyzer required a minimum of 2.4 V to 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, because of their high current density. The current−voltage behavior of these solar cells was measured using an OAI solar
where FECO is the Faradaic efficiency of reducing CO2 to CO, JOP is the operating current density, PIN is the input solar power density with a standard 1 sun illumination of 100 mW/cm2, and μTH is 1.34 V, which is the overall thermodynamic voltage for electrochemical reduction of CO2 to CO.23 The overall reaction is as follows:
( −0.11 V vs RHE)
IOP no. of cells in the module × individual cell area
1 O2 (g) (μ TH = 1.34 V) 2 (4)
where V versus RHE is the potential measured against the reversible hydrogen electrode. Equation 2 is the half-reaction for the reduction of CO2 to CO at the cathode, while eq 3 is the half reaction for the water oxidation at the anode. In addition, when CO2 reduction occurs in aqueous solution, the 10961
DOI: 10.1021/acssuschemeng.7b02853 ACS Sustainable Chem. Eng. 2017, 5, 10959−10966
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Figure 3. (a) Modeled I−V curves of a five-cell PV array under different solar cell areas and I−V curve of the CO2 electrolyzer. Note that “Solar Cell Area” refers to the illumination area of individual and identical solar cells in the five-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) Resulting JOP and VOP values for configurations with varying solar cell areas. (c) Extrapolated FECO and calculated SFE for the solar cell areas considered.
simulator under standard testing conditions (25 °C operating temperature and 100 mW/cm2 irradiation), and the performance of one SunPower solar cell is summarized in Figure 2b. 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 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, we deduced that a solar array containing four or five identical solar cells connected in series was needed. However, because of voltage drops in the circuit and temperature losses in the system, an array of at least five solar cells, giving an array VMP of ∼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, because the output current of these solar cells at the maximum power point (IMP) of ∼2.8 A was much higher than the operating current of the electrolyzer (∼1 A), the area of all the individual solar cells in the array in series was reduced to decrease the array current output. Next, 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 voltage matching had been performed 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 hereafter 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, the individual solar cell area was used 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 areas 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 the point at which the CO2 electrolyzer’s I−V curve intersected with that of each of the solar array I−V curves. The results of these calculations for the five-cell solar array configuration are given in Figure 3a. The dependence of the operating parameters (VOP and JOP), selectivity (FECO), and 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 these calculations. For the five-cell configuration, this dependence is shown as plots of VOP, JOP, FECO, and SFE versus the solar cell area in panels b and c of Figure 3. Figure 3b shows how JOP decreased while VOP increased with an increase in the illuminated cell area. Figure 3c shows that SFE, which is directly calculated from the product of JOP and FECO, had a shape very similar to that of FECO and peaks at same point.Figure 3c also shows that larger illumination areas tend to provide lower SFEs due to lower JOP values (Figure 3b). However, even though VOP does not directly affect SFE, it does influence FECO. This dependency of 10962
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ACS Sustainable Chemistry & Engineering Table 1. Results from Modeling a Five-Cell Configuration PV Arraya
a
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 33 77
2.85 2.85 2.85 2.85 2.85
350 490 876 1143 2686
35.0 35.0 35.0 35.0 35.0
2.43 2.59 2.75 2.87 3.12
376 523 903 1129 1719
37.6 37.3 36.1 34.6 22.4
7.52 7.47 7.23 6.92 4.48
56.1 55.1 75.9 40.2 8.5
5.7 5.5 7.4 3.7 0.5
The bold values highlight the configuration giving best predicted SFE. J′OP is the operating current density of an individual cell.
SFE on JOP and FECO creates a trade-off between JOP and VOP for identifying the configuration that maximizes 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 five-cell array with each PV cell having an individual solar cell illuminated area of ∼25 cm2 (Table 1) connected in series. To limit the different possible source configurations, only PV arrays comprising five or six cells in series were modeled as these gave VOP values in approximately the correct range as discussed above. The modeled parameters of the six-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 five-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 device benefits from careful voltage and current matching between the source and the load such that the optimum SFE can be obtained.
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. The plots confirmed that the performances of the test system and its components were stable for the experimental duration. Figure 4a 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 eq 5. FECO was calculated every 15 min for each configuration, and the average and peak values are listed in Table 3. Figure 4b shows the measured FECO (peak values) and the resulting SFE of each of the experimental conditions, plotted versus 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 that the determined optimum area has shifted from 25 to 33 cm2 due to these voltage losses in the circuit. A peak SFE of 6.5% was obtained with the 33 cm2 solar cell area in the five-cell array, approaching the predicted value of 7.3%. 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 a 33 cm2 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, because the array suffered from voltage losses during the experiment due to higher cell temperatures under the solar simulator (which left the cells 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 modeled I−V curves is apparent in Figure 5a. Figure 5b 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 to model 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 because of 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 an error of 8% SFE’s.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02853. Results for a six-cell configuration and potential for high SFE’s with multijunction tandem solar cells under concentration and experimental methods, including the CO2 electrolyzer, photovoltaic power source (the solar cells used), solar simulator construction for illumination source, and system operation (PDF) 10965
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