pH-Tuning a Solar Redox Flow Battery for Integrated Energy

pH-Tuning a Solar Redox Flow Battery for Integrated Energy Conversion and Storage William D. McCulloch, Mingzhe Yu, and Yiying Wu* Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: The intermittent nature of renewable energy sources such as solar and wind requires an energy storage method for future viability. Integrated solar energy conversion and storage devices such as solar redox flow batteries offer an innovative approach to this problem. Herein, we demonstrate electrolyte pH to be a valuable and tunable parameter for optimization of aqueous solar redox flow batteries. This can be accomplished by utilizing a pH-dependent redox anolyte and pH-independent catholyte to effectively tune the cell voltage by varying the operating pH, which allows direct integration of a dye-sensitized photoelectrode. A quinone−iodine redox flow battery can achieve high columbic efficiency over ∼90% for 50 cycles under mild pH conditions (pH ∼ 2−8). Furthermore, a pH-tunable solar redox flow battery can be charged using only solar illumination, thus allowing for integrated energy conversion and storage within a single device edox flow batteries (RFBs) have been identified as the leading electrical energy storage option to enhance grid efficiency and allow for the storage of renewable energy.1 The direct integration of a photoelectrode into a RFB system allows for solar energy harvesting and in situ storage. Therefore, solar redox flow batteries (SRFBs) represent an attractive approach in addressing the intermittent nature of solar illumination. Because common redox electrolytes used in RFBs overlap with those used in regenerative photoelectrochemical solar cells, the integration can be seamless. In the current literature there are a few examples of all-aqueous SRFBs.2 A tandem p-Si/n-Si photocharging module was recently applied to a redox flow battery system originally proposed by Huskinson et al. which utilizes highly acidic conditions.3,4 Also, a hematite-based photoanode was applied to a redox flow battery originally presented by Lin et al. which operates under highly basic conditions.5,6 For a more comprehensive review there are two recent review articles on the subject.7,8 Electrolyte pH is an important parameter in tuning the potentials, charge-transfer kinetics, and stability of the photoelectrode and redox couples. However, it has not been utilized in prior aqueous SRFB research. The electrolyte is typically heavily basic or acidic depending on the redox couple.9 These conditions are much too harsh for the application of many types of photoelectrodes such as dye-sensitized photoelectrodes which are usually stable in a mild pH range of 2−8.10 We employ a pH-independent redox catholyte (I3−/I−) and a pH-

R

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dependent anolyte (anthraquinone-2,7-disulfonic acid, AQDS). This combination allows the cell to achieve a large voltage while maintaining the ability to optimize the photoelectrode performance. When compared to current RFBs, our SRFB operates under mild conditions (ie. pH ∼ 2−8) which allows the application of a dye-sensitized photoelectrode, as described in Scheme 1. For the pH-dependent redox anolyte, a quinone-based redox molecule was selected because of its low toxicity and cost.4 Yang et at. also published a paper in 2014 utilizing different redox active quinone molecules as both anolyte and catholyte which exhibit fast electron transfer rates on carbon electrodes.11 For the pH-independent redox catholyte, the I3−/I− redox couple was chosen because of its performance in aqueous dyesensitized solar cells. On the basis of the iodine Pourbaix diagram, the I3−/I− redox couple is pH-independent from pH 0−9. The electrochemical processes are shown in eqs 1−6 for the proposed photocharging and discharging. The photocharging process is the most complex and is represented in Scheme 1. First, the sensitized dye is photoexcited (Dye*) and injects an electron into the conduction band of TiO2 (CB-e−). This electron is shuttled through the external circuit to reduce the AQDS on the counter electrode. The photo-oxidized dye Received: July 25, 2016 Accepted: August 19, 2016

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ACS Energy Letters

based electrolytes.12 The photoelectrode consisted of a TiO2 film sensitized with a ruthenium-based dye (Z907). This dye was selected because the long hydrophobic chains limit its dissolution in the aqueous flowing electrolyte. Because a photoanode was selected, this requires that the redox electrolyte in contact with the photoelectrode must be the catholyte of the SRFB. A three-electrode photoelectrochemical cell was used to probe the performance of the photoanode (TiO2/ Z907 sensitized)−catholyte (I3−/I−) interface. Linear sweep voltammetry (LSV) was used to obtain the J−V response curve of the photoelectrode, which is shown as Figure 1a. The electrolyte consisted of 2 M NaI, 20 mM I2, and 0.5 M guanidine thiocyanate (GuSCN) in water saturated with chenodeoxycholic acid (CDCA) with 0.30 M citrate buffer. The Nernst potential of this electrolyte was calculated to be 0.460 V vs NHE assuming an equilibrium constant K1 (I− + I2 ↔ I3−, K1) to be 723.10 CDCA was also added to the dye solution as a cosensitizer. GuSCN and CDCA were added to the electrolyte; both have been shown to enhance photoelectrode performance because of improved surface wetting.13 The results from three cells are shown in Figure S1. In Figure 1a, under 1 sun AM 1.5G illumination the photoelectrode shows pH-dependent performance. Under more acid conditions pH ∼ 2, the open-circuit potential (Eoc) is more positive, and as pH was increased, the Eoc was pushed more negative. The open-circuit potential is a representation of the quasi Fermi level of the mesoporous TiO2.14 It is welldocumented that surface protonation of the TiO2 semiconductor causes a positive shift in the conduction band edge. With increasing pH a negative shift in the open-circuit potential of the photoelectrode has been observed (∼60 mV/ pH unit, Figure 1b).15,16 Although at higher pH (4.5−6) there is a decrease in photocurrent which would shift the quasi Fermi level more positive; thus, any negative shift in the conduction band above pH 4.5 is offset by the decreased photocurrent.14,17,18 The influence of pH on AQDS redox potential was investigated to accurately determine the theoretical cell voltage across a range of pH values and moreover to give a method to calculate optimum operating pH conditions. From eqs 4 and 5,

Scheme 1. Photocharging Process and pH Tunability of the AQDS-Iodine

(Dye+) is then regenerated by iodide ions in solution to complete the cycle, thus converting and storing solar energy. Photocharging Process Dye(Z907) + hυ → Dye*(Z907) Dye*(Z907) →

+ Dye(Z907)

(Photo‐excitation)

+ CB‐e

−

(Charge Injection)

+ 2Dye(Z907) + 3I− → 2Dye(Z907) + I3−

(1) (2)

(Dye Regeneration) (3)

−

+

2CB‐e + AQDS + 2H → AQDSH 2

(Anolyte Reduction) (4)

Discharge Process AQDSH 2 → 2e− + AQDS + 2H+ 2e− + I3− → 3I−

(Catholyte)

(Anolyte)

(5) (6)

For solar redox flow batteries in general, the photoelectrode−electrolyte interface is a critical factor for the entire system. The thermodynamics and kinetics at this interface govern the performance of the SRFB. Here, a dye-sensitized photoanode was chosen due its performance in aqueous iodide-

Figure 1. (a) Three-electrode J−V response curve of TiO2 sensitized with Z907 dye under 1 sun conditions. (b) Variation of open-circuit potential (Eoc) with pH. (c) Experimental photocharging power as a function of different pH conditions. 579

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ACS Energy Letters the reduction−oxidation of AQDS involves a proton and therefore carries a pH dependence. Cyclic voltammetry was used to determine the relationship between AQDS redox potential and pH (Figure S2). Under basic conditions, the formal reduction potential shifts more negative (left) and under acidic conditions, more positive (right). As shown in Figure 2,

Figure 2. pH influence on the redox potential of AQDS (left) and theoretical cell voltage (right).

Figure 3. Discharge−charge profiles of AQDS−iodine RFBs at different pH conditions.

the formal reduction potential of AQDS can be effectively tuned from acidic to basic conditions with a slope of ∼ −50 mv/pH unit. A linear fit of these pH-dependent redox potentials allows for the calculation of the theoretical cell voltage at a given pH if the I3−/I− redox couple (0.536 V vs NHE) is used as the positive electrolyte, and this is shown as the right axis in Figure 2. According to the Pourbaix diagram of iodine, the pH can be adjusted from 0 to 9, after which iodates are the dominant redox species.19 For these reasons we limit our pH range to 0−9. With the linear fit in Figure 2 and the J−V response in Figure 1a, an experimental photocharging power curve was calculated with respect to different pH conditions. Here, because of the low photocurrent density and large surface area of the anode, the polarization of the anolyte is considered negligible. The power curve is plotted in Figure 1c and was generated by multiplying the theoretical cell voltage at a given pH by the photocurrent at the AQDS redox potential for the same pH conditions. When the AQDS−iodine RFB is converted to a SRFB, a pH between 3 and 4 should yield the highest photocharging power. This is the point where the Eoc was shifted most negative without any noticeable decrease in the photocurrent. To demonstrate a variable voltage RFB, an AQDS−iodine battery is assembled under different pH conditions with emphasis on RFBs in the pH range 3−4. In Figure 3 the voltage profiles are represented on one voltage axis with the higher voltage batteries appearing under more basic conditions, as expected. For these experiments, the catholyte was 0.8 M sodium iodide and the anolyte was 0.05 M AQDS in 0.3 M buffer. A large excess of I− was used in order to suppresses precipitation of I2. The batteries in Figure 3 have the anolyte compartment as the limiting side with a loaded amount of 53 mAh (assuming a 2 electron reduction of AQDS). The trend in overpotential of the RFBs follows that of the cyclic voltammo-

grams (Figure S2). The average cell voltage for each flow battery is represented as a dashed line, and this value is compared back to the expected cell voltage from Figure 2, which agree reasonably well. This confirms that the AQDS− iodine RFB is capable of operating in a wide pH range as compared to current RFBs which typically employ extremely acidic or basic solution. Another important factor for a RFB or SRFB is its cycling ability. The RFB operating pH was tuned to investigate its effect on cycling ability. The anolyte and catholyte were buffered to pH 2.90, 3.90, and 8.55, and a flow battery was cycled at a current of 50 mA. Panels a, b, and c of Figure S3 show discharge−charge curves and Coulombic efficiencies for RFBs buffered at pH 2.90, 3.90, and 8.55, respectively. At pH 2.9 and 3.9, AQDS−iodine RFB displays high Coulombic efficiencies above 90% capacity after 50 cycles. At pH 8.55, the RFB reaches a stable Coulombic efficiency of 89% for 50 cycles. However, from Figure S3d, it appears that there is significant capacity fading at higher pH values, with pH 2.90 displaying the highest capacity retention of 91% after 50 cycles. Here, the capacity fade is likely due to iodate formation, which is favored under higher pH conditions. Finally, in order to convert the AQDS−iodine RFB to a SRFB, an in-house two-electrode photoelectrochemical flow cell was used. The solutions were buffered to pH 2.90, and the same concentrations were used as in the RFB. The flow cell and cell setup are depicted in Figure S4. The electrolytes flow from the reservoir into a commercial Scribner flow cell where the open-circuit voltage (OCV) is monitored. The electrolytes then flow into the photocharging module where the photocurrent is monitored with 0 V of applied bias. Then electrolytes return to the reservoir for recirculation. The electrolytes were loaded in the discharged state (AQDS/I−) and then photocharged under 1 sun illumination. In Figure 4a the red curve represents the OCV between the two electrolytes during photocharging and 580

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Figure 4. (a) Photocharging and discharging of a AQDS−iodine solar redox flow battery. (b) Coulombic efficiency.

the application of solar illumination at the photocharging module. As expected the cell voltage increases as the iodide is oxidized to triiodide and AQDS reduced to AQDSH2 (eqs 1−4). Here, the photocharging process was cut off after ∼1 h of photocharging in order to cycle the SRFB in a reasonable amount of time. The photocurrent does decrease with charge time, which can be attributed to the increased Nernst potential of the electrolytes due to increased state of charge. The photocurrent also decreases with cycle number but stabilizes as well. This may in part be because initial loading of the flow battery involves only discharged species, but upon cycling a slight amount of each form (oxidized/reduced) will be present; thus, lower photocurrent would be observed. The blue curve represents the discharge reactions as shown in eqs 5 and 6. Here, the discharging is performed at 1 mA using the commercial scribner cell with no application of light. This confirms that the anolyte and catholyte may be photocharged and then subsequently discharged from the photochemical production of redox species. The integration of the photocurrent curve (orange) represents the amount of charge passed during photocharge, and the integration of the discharge current curve (green) represents the amount of charge extracted during discharge (Figure 4b). The storage−extraction Coulombic efficiency increases with cycle number to a value of ∼70% after the first 4 cycles. The low efficiently of the first cycle may be due to undischarged species remaining (Figure S5). The overall limiting factor in this system is the solar energy conversion due to the lesser developed aqueous dye-sensitized solar cell (DSSC) field with efficiencies generally less than 5%12,20 compared to nonaqueous DSSCs which have much higher efficiencies of >10%. Previously, the extreme conditions typically employed in aqueous RFBs have deterred use with DSSCs.10 In conclusion, the pH of a SRFB can be used to effectively tune the performance with respect to the photoelectrode. With the mild conditions demonstrated here and the enhanced flexibility of a variable voltage flow battery, we hope to enhance motivation for further development of aqueous dye-sensitized solar cells and SRFBs. The pairing of a pH-dependent and pHindependent redox couple allows for manipulation of cell voltage in aqueous RFBs by changing pH conditions and in

general can be further applied to other aqueous based redox systems allowing for improved solar redox flow batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00296. Experimental details, cyclic voltammograms, SRFB test cell scheme, and cycling performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the U.S. Department of Energy (Award DE-FG02-07ER46427). REFERENCES

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