High Energy Efficiency and Stability for Photoassisted Aqueous Lithium−Iodine Redox Batteries Georgios Nikiforidis,†,‡ Keisuke Tajima,§ and Hye Ryung Byon*,†,‡,∥ †
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Byon Initiative Research Unit (IRU), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Emergent Functional Polymer Research Team, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ∥ KAIST Institute NanoCentrury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: We demonstrated photoassisted lithium−iodine (Li−I2) redox cells integrated with a hematite photoelectrode that are applicable to energy storage systems (ESSs). The hematite photoelectrode presents low cost, light absorption in the visible light region, and inertness to aqueous electrolytes, which allow for stable production of photocurrent under illumination. In the aqueous Li−I2 redox cells, the harnessing of photoenergy generates photocarriers that promote the I− oxidation process without electrolysis of the aqueous solution. The energy efficiency for the photoassisted charge process is ∼95.4%, which is ∼20% higher than that in the absence of illumination at a current rate of 0.075 mA cm−2. The hematite is profoundly stable in aqueous I−/I3− catholyte and exhibits over 600 h of cycling without noticeable performance decay and photocorrosion. This achievement highlights photoinduced ESSs with improved energy efficiency.
L
delivered high capacity (∼98% of the theoretical capacity), Coulombic efficiency, and excellent cycling stability, which holds great promise for enhancing the energy density in ESSs by using aqueous I−/I3− catholyte.
iquid electrolyte solution-based rechargeable redox batteries are one of the most promising stationary electrochemical storage systems (ESSs).1 The redox couples dissolved in the electrolyte reservoirs can store electricity converted from intermittent and sustainable energy sources and can provide power during electricity shortage emergencies. Typically, commercial redox batteries consist of enormous electrolyte tanks and circulators in order to be scalable to the required energy capacity. For establishing economically viable ESSs, however, downsizing electrolyte tanks and implementing low-cost electrolytes are crucial; these objectives can be realized through the development of advanced redox couples possessing high redox voltage and solubility.2−4 We previously demonstrated iodine (I2)-based catholytes (i.e., electrolyte solution working at positive electrode side) composed of the iodide/triiodide redox couple (I−/I3−) in aqueous solution.5,6 The addition of I2 to excess iodide produces I3− (xI2(s) + yI− ⇄ xI3− + (y − x)I−) with a maximum solubility of approximately 8 M. When metallic lithium (Li) is employed with the negative electrode, a suitable redox voltage of I−/I3− redox couple (∼3.5 V vs Li+/Li) was demonstrated (eq 1). The redox lithium−iodine (Li−I2) cells © XXXX American Chemical Society
2Li + I3− ⇄ 2Li+ + 3I−,
E 0 ≈ 3.5 V vs Li+/Li
(1)
For the next strategy, the addition of solar power leads to increased energy efficiency during the charge process (i.e., I− oxidation) in aqueous Li−I2 redox cells.7−11 Previous reports demonstrating dye-sensitized solar cells (DSSCs) incorporated with Li−I2 redox cells highlighted the viable benefit of photodriven battery performance.12 However, the critical intrinsic challenge of poor solar-to-electricity conversion efficiency (∼5%)13 limits the solar-driven charge process; thus, an additional electrical source is still nessessary.7,12,14 In addition, because of the large band gap of the TiO2 electrode (∼3.2 V), the DSSC restricts the absorption region of the solar spectrum while also requires additional Pt electrode and Received: August 15, 2016 Accepted: September 21, 2016
806
DOI: 10.1021/acsenergylett.6b00359 ACS Energy Lett. 2016, 1, 806−813
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
Figure 1. Photoresponse of aqueous Li−I2 cells integrated with hematite photoelectrode at OCV. The aqueous catholyte is composed of 1 M KI, 0.03 M LiI, and 0.08 M I2. (a) Top-view SEM and digital (inset) images of hematite. (b) Voltage and temperature profiles at OCV (opencircuit voltage) without (blue area) and with (yellow area) illumination. (c) Nyquist plots from electrochemical impedance spectroscopy (EIS) measurements at a frequency range from 600 kHz to 200 mHz and an AC amplitude of 10 mV. Inset indicates the corresponding equivalent circuit. (d) Tafel plots at a sweeping rate of 0.5 mV s−1.
substrate) by simple anodic electrodeposition followed by annealing (Experimental Methods and Figure S1 in the Supporting Information).23 In a three-electrode cell setup, an aqueous FeCl2 solution (pH 4.1) formed an amorphous ferric oxy-hydroxide (γ-FeOOH) coating via electrochemical oxidation of Fe2+ (Fe2+ → Fe3+ + e−) and electroplating (Fe3+ + 2H2O → FeOOH + 3H+) at 78 °C for 2 min. Optical and scanning electron microscopy (SEM) images (Figure S2) show the light-orange color of the γ-FeOOH film composed of dense and uniform nanoparticles deposited on the FTO substrate. The γ-FeOOH on FTO substrates was then transferred to a tube furnace for annealing at 550 °C in air and converted to αFe2O3 structure displaying a deeper orange color. This hematite film has a rhombohedral structure as verified by X-ray diffraction (XRD) pattern (Figure S3a) and seven Ramanactive optical modes (2A1g + 5Eg, Figure S3b).24−26 X-ray photoelectron spectroscopy (XPS, Figure S3c) also confirms Fe 2p3/2 and 2p1/2 peaks along with Fe3+ satellite from α-Fe2O3.27 The film consists of granular nanoparticles with an average diameter of ∼33 nm (Figure 1a) and a total film thickness of ∼90 nm (Figure S4). Ultraviolet (UV)−visible absorption spectroscopy and Tauc plots revealed an optical band gap (Eg) of ∼2.15 eV (Figure S3d), indicating significant absorption of the visible light.28 All these characteristics attest to the successful preparation of a photoactive hematite semiconductor. Then, the hematite/FTO substrate was used as a positive photoelectrode in the photoassisted Li−I2 cells including the aqueous I−/I3− catholyte and a Li electrode/nonaqueous medium (1 M LiTFSI in EC/DMC) as the negative electrode. The positive and negative electrodes were separated by ceramic membranes (Li2O−Al2O3−TiO2−P2O5, LATP) that prevent crossover of aqueous/nonaqueous media while allowing for the exchange of Li+ ions for charge balance.29 The Li−I2 cell configuration and photoresponse process along with the corresponding energy diagram are illustrated in
expensive ruthenium-based dyes in the electrolyte solution, which complicates the fabrication of practical solar-powered redox batteries. Alternatively, the strategy for the incorporation of a photoelectrode into the Li−I2 cell is attractive. The photointegration can be achieved simply by replacing the carbon electrode with a semiconductor possessing a small band gap, which enables us to maintain a two-electrode configuration giving increased versatility in line with the target services and required performance in cost-savings. This photoelectrode is responsible for absorbing light as well as promoting charge transfer;15 thus, the energy levels of conduction (CB) and valence bands (VB) should be suitable for carrying out the Li− I2 electrochemical reaction in the absence of dye or catalyst. Among several candidates, hematite (α-Fe2O3) has been known to be one of the most promising photoelectrodes in solardriven energy conversion technologies, owing to (1) small optical band gap (1.9−2.2 eV) allowing for absorption of visible light;16,17 (2) excellent chemical stability in neutral and basic aqueous media; (3) abundance, low cost, and environmental inertness; and (4) ease of preparation and handling.18−22 Herein, we demonstrate aqueous Li−I2 redox cells through the integration of a hematite electrode, which allows for the facile charge process with the aid of harnessed photoenergy. The photogenerated charge carriers in the hematite promote the I− oxidation process without electrolysis of the aqueous solution, which results in the production of photocurrent with low overpotential and the enhancement of the total energy efficiency by around 20% in Li−I2 redox cells. Hematite also contributed to high rate capability and excellent stability in aqueous I−/I3− catholyte under sunlight, leading to negligible capacity fading for ∼600 h of cycling, which holds great promise for the construction of robust photoassisted battery systems. The hematite photoelectrodes were prepared on fluorinedoped tin oxide (FTO)-coated glass (denoted hereon as FTO 807
DOI: 10.1021/acsenergylett.6b00359 ACS Energy Lett. 2016, 1, 806−813
Letter
ACS Energy Letters
constant; aI3− and aI− are the activity of I3− and I−, respectively, both assumed to be equal to the corresponding concentrations. In contrast, the OCV falls to ∼3.41 V under sunlight (AM 1.5G, 100 mW cm−2). A voltage difference of ∼160 mV arising from the generation of charge carriers at the hematite photoelectrode was retained for over 50 h under chopped illumination (Figure S5). It is noted that illumination somewhat increases the cell temperature to ∼32 °C. To decouple the photoresponse from the temperature effect, charge-transfer resistances at the electrode interface were evaluated through electrochemical impedance spectroscopy (EIS) at different temperatures. The Nyquist plots as fitted according to the plausible equivalent-circuit model (Figure 1c and Figure S6) reveal a charge-transfer resistance at the hematite/FTO substrate interface of ∼7 kΩ cm2 with illumination, which is around half of that under dark conditions at 25 °C (∼18 kΩ cm2) and also 32 °C (∼15 kΩ cm2, Table S1). Tafel plots further support superior exchange current density (j0) upon illumination (∼15 μA cm−2) to that under dark conditions regardless of temperature increase (4−5 μA cm−2, Figure 1d), in agreement with suppressed polarization resistance (Figure S7 and Table S2). Therefore, it is concluded that the significant enhancement of hematite conductivity by photogenerated charge carriers overwhelms the temperature effect. More importantly, the photoenergy facilitates the I − oxidation when an electrical load drives the charge process (Scheme 1). The e− excited to the CB edge flows toward the Li electrode. In the meantime, free h+ accumulates at the quasiFermi level of hematite in the depletion layer and diffuses through the interface of the hematite photoelectrode to carry out the electrochemically most favorable reaction. In thermodynamic terms, the h+ is scavenged to carry out water electrolysis in aqueous catholytes (E0 ≈ 1.23 V vs SHE (standard hydrogen electrode) that is converted to ∼4.27 V vs Li/Li+), but its slow kinetics31 profoundly leads to a bypass for I− oxidation, in turn prompting the predominant production of I3− (eq 6) instead of water electrolysis.
Scheme 1. Upon illumination, the photon (hv) absorbed by the hematite generates the charge carriers of electron (e−) and hole Scheme 1. Illustrations of (a) Photoassisted Charge Process in Aqueous Li−I2 Cells Using a Hematite Photoelectrode (αFe2O3/FTO Substrate) and (b) the Corresponding Energy Diagram Describing Different Open Circuit Voltage (OCV) under Light and Dark Conditions
(h+) (eq 2). The majority carrier of e− at the quasi-Fermi level of hematite jump to the CB edge under the sunlight (eq 3) while the quasi-Fermi level for the minority carrier of h+ aligns with the voltage of I−/I3− redox reaction. Therefore, the photoresponse first monitored by open-circuit voltage (OCV) depicts the CB of hematite relative to Li by e− excitation under light conditions. This is evidently distinguishable from the OCV appearing in the absence of light (denoted as dark, eq 4). α‐Fe2O3 + hv → α‐Fe2O3 + (e− + h+) −
−
(e )quasi‐Fermi level ⇄ (e )CB 3I− ⇄ I3− + 2e−
(6)
The promotion of I− oxidation in the photoassisted Li−I2 redox cells is indicated by anodic linear sweep voltammetry (LSV). The photoassisted process shifts the onset oxidation voltage to ∼3.4 V, which is ∼450 mV lower than in the absence of photoassistance (∼3.85 V, Figure 2a). The reward of illumination is further manifested by the comparison of current densities (j), revealing ∼12 times higher current density at 4.0 V (∼0.24 vs ∼0.02 mA cm−2 under light and dark conditions, respectively). In the same sense, voltage gain is apparent at the galvanostatic mode (applying constant current rate) when examining the Li−I2 redox cell performance. The deep discharge−charge profiles in Figure 2b and Figure S8 show a high charge voltage of ∼4.13 V for the I− oxidation under dark conditions at a current rate of 0.075 mA cm−2, which is markedly reduced to ∼3.43 V with illumination and stably repeated after discharge (I3− → 3I− + 2e+, carried out under dark conditions). The large overpotential under dark conditions is attributed to the poor electronic conductivity of hematite/ FTO (Table S1), which requires a great internal energy to drive the oxidation reaction. In contrast, the photogenerated charge carriers reduce the overpotential by ∼660 mV, translating to a voltage efficiency (ηV) gain of ∼20% and in turn an energy
(2) +
E ≈ 3.4 V vs Li /Li
E 0 ≈ 3.5 V vs Li+/Li
3I− + 2h+ → I3−
(3) (4)
Figure 1b and Figure S5 show OCVs of Li−I2 cells containing 0.08 M I2, 1 M KI, and 0.03 M LiI in aqueous catholyte (pH 7.9) by chopping illumination. In dark conditions, the OCV at ∼3.57 V is solely attributed to an equilibrium voltage of I−/I3− at the electrode interface which is very close to the estimated redox voltage (∼3.55 V) stemming from the Nernst equation (eq 5)30 RT a I− E = E0 − ln , E 0 = 3.58 V vs Li+/Li nF a I−3 (5) where E is the redox reaction voltage, E0 the standard cell voltage, R the gas constant, T the absolute temperature, n the number of moles of electrons transferred, and F the Faraday 808
DOI: 10.1021/acsenergylett.6b00359 ACS Energy Lett. 2016, 1, 806−813
Letter
ACS Energy Letters
Figure 2. Photoassisted I− oxidation to I3− in Li−I2 redox cells with aqueous catholyte of 1 M KI, 0.03 M LiI, and 0.08 M I2 using potential sweep and galvanostatic modes. (a) Photocurrent gain at low oxidation potential with illumination in anodic linear sweep voltammetry (LSV) at a sweeping rate of 0.5 mV s−1. (b) Deep discharge−charge voltage profile under dark conditions for the first cycle (blue area) followed by illumination for the second and third cycles (yellow area) at a current rate of 0.075 mA cm−2. All discharge processes were conducted under dark conditions. (c) UV−visible spectra of as-prepared (top), fully discharged (middle), and fully charged with illumination (bottom) aqueous catholyte. Inset shows the optical color of aqueous catholyte (in dotted circle) covering the hematite electrode (background of deep orange color) in the Li−I2 cell.
efficiency (ηE) gain of ∼21% in comparison with the one under dark (total ηE = 95.4% under light, eqs 7 and 8). The capacity of ∼192 mAh g−1 is marginally reduced for ∼200 h of cycling with over 99% Coulombic efficiency (ηC, eq 9). This result indicates minor depletion of the hematite photoelectrode and negligible loss of I−/I3− through long-term cycling under sunlight exposure. It is worth noting that sunlight-induced temperature increase (32 °C) enhances the Li+ hopping rate in the LATP membrane. A symmetric LATP cell exhibits ∼1.35 times higher ionic conductivity at 32 °C under dark conditions compared to 25 °C (Figure S9a). However, the charge voltage in the Li−I2 redox cell is still considerably higher (∼4.1 V) under dark conditions (Figure S9b−c), suggesting a primary contribution of photogenerated charge carriers to the overall cell performance. Furthermore, there is no evidence of water electrolysis in the aqueous catholyte during charge, demonstrated by ∼100% Faradaic efficiency (eq 10) along with >99% Coulombic efficiency. The UV−visible absorption spectra for the aqueous catholyte reveal the appearance of the I− band at 226 nm32 after discharge while the I3− bands at 287 and 353 nm are apparent after charge (Figure 2c). The dark brown color displayed from the as-prepared and charged catholytes implies inclusion of high concentration of I3−, which is distinct from the transparent discharged catholyte containing I−. The discharge−charge characteristics with increasing current rates indicate a delay in voltage rise and capacity decrease by photoassistance. The photogenerated h+ efficiently participates in the production of I3− even at higher current rates, as evidenced by the 26% difference in capacity retention from dark conditions and 17% in charge voltage at 0.1 mA cm−2 (Figure 3a). Figure 3b exhibits voltage efficiencies greater than 90% at 0.05−0.1 mA cm−2 with illumination, which reliably repeat for a total of 600 h of deep discharge−charge cycling (Figure S10).
Polarization profiles display a linear rise in voltage along with the corresponding volumetric and areal power densities as the current rate is increased up to 0.5 mA cm−2 (Figure 3c,d and Figure S11). The maximum volumetric power density (∼18.7 W L−1 at 4.3 V) under illumination is far greater than that in the case of dark conditions (∼4.5 W L−1). The photoassisted Li−I2 redox cells are also stably cycled at high current rates when using a high concentration of I−/I3− in the aqueous catholyte (0.2 M I2 and 2 M KI, Figure 3e). At a current rate of 0.3 mA cm−2 applied for the latter 22 cycles, the capacity lingers at ∼180 mAh g−1 with >99% retention and >99% Coulombic efficiency. The gravimetric energy density was estimated to ∼0.6 kWh kg−1 based on the mass of active material (I2), which is comparable to that of aqueous Li−I2 cells we reported previously with a carbon electrode.33,34 In addition, the hematite photoelectrode did not display any evidence of corrosion after long-term cycling, revealing negligible change of XRD pattern, Raman optical mode, and surface morphology (Figure S12). Post-mortem analysis corroborates the highly robust and chemically stable nature of hematite toward the aqueous I−/I3− catholyte and also to photoresponse, pointing to the great prospect of these photoelectrode−integrated redox cells. It is worth noting that the photoassisted charge process enhances the energy efficiency to ∼95.4% at a current rate of 0.075 mA cm−2, similar to that of a carbon electrode (∼96%), the latter having higher conductivity (Figure S13). We believe that further engineering of the hematite by the introduction of a conducting underlayer such as a gold thin film between hematite and FTO (Figure S14) as well as a decrease in the hematite thickness can mitigate the challenges of low electrical conductivity and e−−h+ recombination,35,36 which in turn can 809
DOI: 10.1021/acsenergylett.6b00359 ACS Energy Lett. 2016, 1, 806−813
Letter
ACS Energy Letters
Figure 3. Cyclic performance and polarization curves of photoassisted Li−I2 redox cells. Cyclic performance using 1 M KI, 0.03 M LiI, and 0.08 M I2 with (a) deep discharge−charge profiles under dark and light conditions at a current rate of 0.1 mA cm−2 and (b) corresponding voltaic efficiency (ηV) and capacity profiles at 0.1 and 0.05 mA cm−2. Blue and yellow areas indicate dark and light, respectively. Photoeffects with increasing current rate: (c) polarization curve and (d) volumetric power density profiles using 2 M KI and 0.2 M I2. The cell was fully discharged prior to measurement. Cyclic performance using 2 M KI and 0.2 M I2 at high current rates: (e) 30-times deep discharge and charge (in light) curves and (b) corresponding capacity and Coulombic efficiency (ηC) profiles. All discharge processes were carried out under dark conditions.
■
EXPERIMENTAL METHODS Electrodeposition of Hematite Film on FTO Glass. The electrodeposition of the hematite film on fluorine-doped tin oxidecoated glass was conducted according to the report by Spray and Choi.23 In short, a FTO-coated glass (2 mm thick (ϕ); 2.5 cm × 2.5 cm; resistivity (R) = ∼10 Ω; PV Tech, Japan) was sonicated in water, acetone, and ethanol followed by drying under a N2 stream. An aqueous plating solution consisting of 0.025 M iron(II) chloride tetrahydrate (FeCl2 4H2O, Wako Chemicals) was prepared with a pH of 4.1 (DI water (Merck Millipore, Direct-Q UV3), resistivity (R) = 18.2 MΩ). Then, the FTO glass was put in the plating aqueous solution with a platinum mesh as a counter electrode (Als.co, Ltd.) and an Ag/ AgCl (4 M KCl, Als.co, Ltd.) reference electrode. The electrodeposition was carried out at 1.2 V vs Ag/AgCl using a CHI Instruments Electrochemical Analyzer (704 E Als.co Ltd.) while the temperature of the aqueous electrolyte was maintained at 78 °C (±2 °C) using a jacketed cell equipped with a thermostated water bath (AS One, LTB-125). The deposition process resulted in the oxidation of Fe2+ to Fe3+ ions
condense the electric power consumption for future photoinduced ESS systems. We successfully demonstrated a photoassisted Li−I2 cell with high energy efficiency and stability. The hematite is advantageous as a photoelectrode owing to low cost, visible light absorption, and incredible stability, which in turn ameliorate the energy efficiency. In these proof-of-concept aqueous Li−I2 cells integrated with a hematite photoelectrode, the photoassisted charge process has evidently been beneficial in decreasing the charge voltage because the photogenerated charge carriers promote the I− oxidation reaction by modulating the voltage to the energy level of the conduction band edge of hematite. In addition, superior current densities can be employed, yielding greater power and energy densities compared to those in the absence of illumination. The highly stable hematite repeatedly carried out photoelectrochemical reaction over 600 h of deep discharge−charge cycles without noticeable performance decay and photocorrosion. 810
DOI: 10.1021/acsenergylett.6b00359 ACS Energy Lett. 2016, 1, 806−813
Letter
ACS Energy Letters (Fe2+ → Fe3+ + e−) followed by the precipitation of Fe3+ as amorphous ferric oxyhydroxide (Fe3+ + 2H2O → FeOOH + 3H+) onto the FTO due to the limited solubility of Fe3+ (
