The current total global energy consumption is about 1.04×108 GWh per year.The majority of this high energy supply is provided by fossil fuels, which comprise of about 82%.
The utilization of fossil fuels results in emission of about 10 gigatons a year of carbon di oxide. Carbon di oxide is a greenhouse gas and hence causes global warming. In addition to this, the swiftly depleting nature of these fossil fuels is cause of concern. Therefore, increasing use of renewable energy sources such as solar and wind is to be implemented. But the intermittent nature of the renewables is another problem that needs to be addressed. Hence, it is important to develop large scale energy storage devices. Redox flow batteries are one among such devices. Redox flow batteries convert and store multi-Megawatts of electrical energy into chemical energy and then they convert back the chemical energy into electrical energy whenever the necessity arises,. Solar rechargeable redox flow batteries are those devices which store the solar energy into electrical energy. Since the solar energy is very abundant and is readily available, they are considered as promising candidates.
Solar Energy are those in which the radiant heat or light energy is being harnessed by solar heating, solar thermal energy, photovoltaics, solar architecture and artificial photosynthesis.The large magnitude of solar energy available makes it a highly appealing source of electricity. According to the United Nations Development Programme in its 2000 World Energy Assessment, it is stated thatd that the annual potential of solar energy was 1,575–49,837 exajoules (EJ). This is magnitudes of times larger than the total world energy consumption, which was 559.8 EJ in 2012. Solar power is the conversion of solar energy into electricity, either directly using photovoltaics(PV) or indirectly by use of concentrated solar power. PV convert light into power by photoelectric power.
Solar Cells are categorized into three different generations. The first generation utilize silicon wafers and their efficiency is about 15-25%. These have good performance and high stability. But they have disadvantages like rigidity, high cost of production, low efficiency and they lose efficiency at high temperatures. The second generation utilized thin film technologies. These solar cells utilize lesser amounts of materials and can be manufactured at low cost compared to the first generation solar cells. However the 2nd generation solar cells have significant drawbacks, including lower overall efficiency compared to 1st generation and the toxicity of the component materials used. 3rd generation solar cells, are based on nanomaterials and they consist of purely organic or a mixture of organic and inorganic components, thus allowing for a vast and inexhaustible choice of materials.
Dye-sensitized solar cells fall into the third generation solar cells and have received great attention for their simple fabrication process, low production costs and, relatively high conversion efficiency9.The operating principle of a dye-sensitized solar cell (DSSC) is explained as follows: The schematic band diagram of DSSC is represented in Figure 1. As depicted in the
Figure 1, a DSSC consists of four components:
Figure 1. Schematic Band diagram of DSSC
- Photo-anode (semiconducting metal oxide deposited on the surface of transparent conducting oxide (TCO) substrate, typically fluorine-doped tin oxide (FTO) glass);
- Electrolyte; and
- Counter electrode
The photo-anode is also known as working electrode in DSSC. The photo-anode usually consists of semiconducting wide band gap metal oxide deposited on the surface of TCO substrate, typically fluorine-doped tin oxide (FTO) glass. The functions of TCO substrate in DSSC are to support the semiconducting layer and to collect the current. FTO is the most widely used oxide in DSSC application owing to its excellent electrical conductivity and optical transparency. In DSSC fabrication, the preparation of photo-anode involves the deposition and sintering of TiO2 paste on TCO substrate at high temperature(~450 º C) in order to improve electrical contact. However, the sheet resistance of ITO glass will increase when subjected to thermal treatment at around 300 ºC, which will lead to poorer efficiency. Therefore, FTO substrate is preferred over ITO substrate in DSSC application
In DSSC, the dye/sensitizer is usually anchored on the surface of metal oxide. Dye is one of the key components in DSSC as it is responsible for light harvesting and generation of photoexcited electrons. An ideal dye should possess several requirements: a. High molar extinction coefficient in visible and near-infrared region. b. Appropriate lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital levels for charge injection into conduction band of the semiconducting metal oxide and dye regeneration from the electrolyte respectively and c. Good solubility and photo stability. The dye can inﬂuence the cell efﬁciency inthree different ways, namely the efﬁciency in absorbingthe incident photons, efﬁciency in converting the incidentphoton to electron–hole pairs and lastly, the efﬁciency incharge transfer process. Ruthenium-basedorganometallic dyes (e.g. N3, N719, and black dyes) arecurrently the most efﬁcient dyes due to their superior lightabsorption, durability and most importantly, the metal–ligand charge transfer transition that allows the photo-generated charges to be injected into TiO2efﬁciently. Both the charge transfer process and photon-to-electron conversion are very efﬁcient in ruthenium dyes,leaving little room for improvement. Extensiveresearch focused on enhancing the light-harvestingproperty of ruthenium dyes. Besides the organometallicdyes, organic dyes such as porphyrin, TP6CADTS, RK1, and ADEKA-1 have been utilized inDSSC applications. Natural sensitizers or chlorophyll dyeextracted from leaves such as Pandannusamaryllifoliusand papaya have been investigated as well.
The electrolyte functions as an electrically conducting medium that transports electronic charge between working electrode and counter electrode, as well as allowing the regeneration of oxidized dye. The most popularly used electrolyte in DSSC is iodide/triiodide redox couple in an organic solvent, normally acetonitrile. The excellent performance of iodide/triiodide) based liquid electrolyte is attributed to its several interesting properties, namely low recombination loss, extremely fast dye regeneration and slow penetration into semiconducting metal oxide film. However, some undesirable intrinsic properties also exist in liquid electrolyte, which affect the long-term stability of DSSC. The main concern is the evaporation of volatile iodide ions that will decrease the charge carrier concentration, resulting in cell degradation. Besides, the leakage of toxic organic solvent will also lead to environmental pollution. In order to overcome the disadvantages of liquid electrolyte, other types of electrolytes such as quasi-solid state, aqueous based, solid state and room temperature ionic liquid electrolytes have been investigated for DSSC applications.
Though efficiencies of up to 12% have been achieved, they have intrinsic disadvantages like
flammability, toxicity and high cost. Water-based solar cell electrolytes have advantages like non-toxicity, non-flammability, non-volatility, low vost and ability to dissolve many potential redox mediators. Traditionally used Redox mediator is I/I3– couple as reported by Law et al.,
Federico Bella et al.,in their review presented developments in aqueous
dye sensitized solar cells. The table below shows the I-V parameters of different aqueous electrolytes for DSSCs.
Table 1. PV-performance of DSSCs assembled with Aqueous electrolytes11
It seems that the main factor that negatively affects the efficiency of aqueous DSSC when compared to those based on organic solvents is the quality of photoanode/electrolyte interface. Excessive hydrophilicity of the dye sensitized surface favours the desorption of the sensitizer molecule, hence decreasing the photocurrent and the stability over time. On the other hand, highly hydrophobic dyes do not allow the complete wettability of the electrode which, in turn, results in a less effective regeneration process for aqueous electrolytes. The optimum between these two extremes has not yet been achieved. The creation of intimate photoanode/ electrolyte interface will be the key area of research in the years to come.
Electrocatalytic property of the counter electrode is important in governing the PV performance of DSSC. Without the catalytic layer, the TCO substrate has a very high charge transfer resistance (> 106 Ω/cm2) in iodide/triiodide electrolyte, which makes it a very poor counter electrode. Platinum is the standard catalyst deposited on the counter electrode due to its high catalytic activity, ability to reduce the overpotential for redox reaction and high resistance to corrosion against electrolyte. Deposition of platinum on the TCO substrate can be implemented by using a wide range of methods, namely spray pyrolysis, sputtering and doctor blading technique. Although platinum is the most efficient catalyst for counter electrode to date, its expensiveness makes it unsuitable for low-cost approach of DSSC. Other alternatives such as carbon black and conducting polymer such as poly(3,4-ethylenedioxythiophene) doped with toluenesulfonate anions were employed as catalyst for the realization of platinum-free DSSC.
The operating principle of a dye-sensitized solar cell (DSSC) is explained as follows:
- The light strikes the dye molecules
- The photons will excite an electron from the dye, if they have sufficient energy and the electron is transmitted into the conduction band of the TiO2.
- The counter electrode transfers the electrons back by the cycling current into the cell
- The electrons are transferred to the dye from the electrolyte, restoring its original state
- The original state of the electrolyte is restored by the electrons coming from the counter electrode
Figure 2. Working principle of a DSSC8
There are three main techniques used to characterize DSSC, viz. photocurrent-voltage (I-V) measurement, incident photon to current conversion efficiency (IPCE) spectroscopy and electrochemical impedance spectroscopy (EIS).
Photocurrent-voltage (I-V) measurement
Figure 3. Typical I-V curve of dye-sensitized solar cell.9 [Colour figure can be viewed at wileyonlinelibrary.com]
The main function of I-V measurement is to determine the electrical output power of DSSC under standard illumination condition. The standard irradiance spectrum used for DSSC measurement is known as air mass 1.5, which specifies cell temperature of 25 ° C and the total power density of solar irradiation to be 1000 W/m2. Four important parameters of DSSC can be obtained conveniently from I-V curve as shown in Figure 3, namely open circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and power conversion efficiency (PCE, η). Open-circuit voltage (Voc) is defined as the maximum voltage that a solar cell can supply to external circuit, which is obtained from the separation of the hole and electron quasi-Fermi levels. Voc is proportional to the difference between the Fermi level of photo-anode and electrochemical potential of redox couple. Voc is measured under open-circuit condition, for which no current can flow. As shown on the I-V curve in Figure 3, Voc is derived from I = 0 A intercept. Voc is independent of the cell area and is always constant under the identical illumination condition regardless of cell area.
Power conversion efficiency (η) of a solar cell is the ratio of maximum generated power (Pmax) to the incident power (Pin) generated by the solar cell divided by the incident power on representative cell area under standard air mass 1.5 illumination condition. It can be seen that higher values of Voc, Isc and FF will lead to increased η. The mesoporous network of photo-anode directly influences those parameters, and hence, reducing the loss in the mesoporous network is essential to fabricate DSSC with higher η.
BabakPashaeiet al., in their review presented different redox shuttles
based on transition metal complexes for DSSCs. The introduction of novel transition metal complex mediators has led to a deeper understanding of dye regeneration processes and also improved the control of OCP. The key issue in improving the photovoltaic properties of DSSCs is developing a suitable photosensitizer to match the redox couples. Major advantage of transition metal complexes as redox mediators is the tenability of their redox potentials by modifying the ligands.
Batteries convert chemical energy into electrical energy. A battery consists of a certain number of voltaic cells. Each cell consists of two half cells connected in series by a conductive electrolyte containing anions and cations. One half cell consist of electrolyte and the negative electrode, the electrode to which anions (negatively charged ions) migrate and the other half-cell includes electrolyte and the positive electrode to which cations (positively charged ions) migrate. Cations are reduced (electrons are added) at the cathode during charging, while anions are oxidized (electrons are removed) at the anode during charging. The reverse process takes place while discharging.
Batteries are further classified into primary and secondary batteries. Primary batteries are designed to be used until they are exhausted of energy and then discarded. Their chemical reactions are generally not reversible, so they cannot be recharged. Secondary batteries can be charged and recharged and used again multiple times.
A redox flow battery (RFB) is shown in a generic form in the figure below. An RFB is a type of energy storage device capable of providing reversible conversion between electrical and chemical energy. It consists of two soluble redox couples contained in external electrolyte tanks. Unlike traditional batteries that store energy in electrode materials, RFBs are called regenerative fuel cells as energy is stored in the form of dissolved redox couples that convert into electricity at the electrodes. Since the redox reactions are reversible, they are qualified in to the secondary batteries system. The electrolytes that flow into cathode and anode are often referred to as anolyte and catholyte.
Figure 4. The generic form of a Redox Flow Battery17
Types of RFBs:
Weber et al., in their review presented the various redox systems that have been investigated.
The development of the modern RFB, traces its origin back to the iron/chromium (Fe/Cr) in the 1970s at NASA, which demonstrated a 1 kW/ 13 kWh system for photovoltaic system. The Fe/Cr system is based upon an aqueous solution of a ferric/ferrous redox couple at the positive electrode and the negative electrolyte is a mixture of chromic and chromous ions. They use hydrochloric acid as the supporting electrolyte,.
Fe2+ Fe3+ + e– E0 = 0.77V vs RHE
Cr2+Cr3+ + e– E0 = -0.41 V vs RHE
The system can operate with an Ion exchange membrane/ separator and low cost carbon-felt electrodes.
The bromine/polysulfide RFB was patented by Remick, then extensively studied by Regenesys Technology. In the bromine/polysulfide system, the positive electrolyte is sodium bromide and the negative electrolyte is sodium polysulfide, but the counter-ion could be replaced with another cation. The important characteristics of the system are that the species that comprise the two electrolytes are abundant and reasonably inexpensive and they are highly soluble in aqueous electrolytes
At the positive electrode,
3Br– Br3– + 2 e– E0 = 1.09 V vs RHE
At the negative electrode, the sulfur in solution is shuttled between polysulfide and sulphide
E0= – 0.265 V vs RHE
In both the systems described above, the main concern is the incompatibility between and sensitivity of the two electrolyte streams to contamination from the other. If a species crosses over and reacts irreversibly with the elements in the opposite stream, it comprises not just an efficiency loss on that particular charge/discharge cycle, but a loss of capacity and degradation in the overall performance of the system, which may result in expensive electrolyte separation and reactant recovery. So it would be helpful to develop a system a system with more than two oxidation states of the same element. This is addressed by the employment of the V(II)/V(III) redox couple at the negative electrode and V(IV) and V(V) redox couple at the positive electrode.
The following reactions occur at the cathode and anode:
While exploratory research on vanadium as a redox couple began at NASA, the all- Vanadium redox battery was invented and developed by Maria Skyllas-Kazacos and co workers at the University of New South wales,,. Further research has continued on this technology since then.
Since there is no limit of how much vanadium that can be stored in the solution in the VRB systrm, some of the researchers who pioneered the work on VRB cell noted that solubility can be enhanced in the presence of halide ions. Herein, the bromide ions in the positive half-cell undergo oxidation to form a polyhalide ion Br2Cl-®. The higher solubility of vanadium bromide results in higher energy densities (35-70 Wh/L) compared to VRB systems (25-35 Wh/L). However the toxic bromine vapor emissions can be a problem.
A fuel cell takes a fuel ( normally hydrogen) and an oxidant (typically air) and produces electricity and water. For a Fuel cell, Hydrogen oxidizes at the anode according to the reaction
2H24H+ + 4e- E0 = 0.00V vs RHE
And at the cathode, oxygen is reduced
4H+ + O2 + 4e– 2H2O E0 = 1.229 V vs RHE
If one were to design a system where the fuel cell acts in both the charge and discharge directions, then an RFB system would exist. Such an RFB system has been examined both with the same and different stacks for charge and discharge,,,.
There are other battery configurations that have the same development heritage and issues as that of a RFB. Here, the active material can be introduced to or removed from the electrochemical cell without dissembling the cell. These do not store all the active materials in liquid or gas form. They are therefore considered as semi-flow cells with more complex electrochemical reactions than simple shuttling between the oxidation states of a single species.
A typical semiflow RFB is the Zinc/Bromine system. In this system, the electrolyte
isstored in the external tanks and circulated through each cell in the stack. However, the zinc reaction doesn’t only involve dissolved species in aqueous phase. On the positive electrode side, bromide ions are converted into bromine and back, as shown in the reaction below. Here relatively high concentrations of Br- and Br2 are used; hence there is an enhancement in both reaction kinetics and energy density.
3Br– Br3– + 2 e– E0 = 1.09 V vs RHE
The toxicity and corrosive nature of bromine are limitations. At the negative electrode Zinc metal is dissolved and redeposited. The metal negative electrode ensures compact electrode, thus increasing the energy density.
Zn Zn2+ + 2e- E0 = -0.76 V vs RHE
In addition this system has high cell voltage, good reversibility and low material costs. But, the limitations are material corrosion, dendrite formation and electrical shorting. RedFlow Ltd successfully demonstrated this batter unit up to MW size with an energy efficiency of nearly 74% in Australia. The derivatives of the Zn/ Bromine are other halogens like chlorine forming the Zn/Chlorine battery, with similar performance and limitations.
A soluble form of the lead-acid battery has also been considered. The charge
transfer reactions are written the same as the lead-acid battery configuration.Lead-acid batteries do not shuttle the same ion between the negative and positive electrode; that is, Pb2+ is introduced and removed from solution at the negative electrode as lead is dissolved and plated. At the negative electrode,
Pb Pb2+ + 2e- E0 = -0.13V vs RHE
At the positive electrode, lead ions combine with water to produce lead dioxide and protons.
Pb2+ + 2H2OPbO2+4H++ 2e- ;E0 = 1.49V vs RHE
As long as the solid forms of lead and lead dioxide are maintained at the negative and positive electrodes, circulation of electrolyte can maintain the open-circuit potential of the battery and allow greater specific cell performance than with sealed or flooded lead-acid cells, assuming minimal weight and volume of the external storage tank. As with other semi-solid flow configurations, there are risks associated with maintaining the morphology of the solid phase as material can detach or grow across the separator gap to cause short-circuit problems.
Similar to the all-vanadium RFB, this system includes only a single element where at
One electrode Fe(II) goes to Fe(II) and on the other plating of iron takes place.
Fe2+ Fe3+ + e– E0 = 0.77V vs RHE
Like other hybrid systems the problems are obtaining a uniform plating of the metal, which therefore requires precise pH control and supporting electrolyte. However there are no dendrite problems like Zinc based batteries. The advantages are materials are nonhazardous and inexpensive.
The use of non-aqueous electrolytes in RFB configurations has been considered because of their higher cell potentials that are possible because unlike aqueous electrolytes which breakdown, they are not. But, electrolyte conductivities, stability and cost limit are the challenges of non-aqueous RFBs. As an example Zinc/Cerium cell has been developed by Plurion Limited. The negative electrolyte dissolves and plates Zinc.
Zn Zn2+ + 2e- E0 = -0.76 V vs RHE
At the positive electrode, cerium is shuttled between Ce(III) and Ce(IV) as given below.
Ce3+ Ce4+ + e- E0 = 1.75 V vs RHE
These batteries have a cell potential of approximately 2.5V on charging, but it drops below 2V during discharge with an energy density of 37.5 to 120 Wh/L,. The high operating potential window is achieved by using methane sulfonic acid rather than pure water as the solvent, thus minimizing decomposition of water into hydrogen and oxygen as well as helping in Zinc plating. The limitations being that redox reaction of Cerium ions is kinetically slow and Ce(III) has a low diffusivity. Other similar batteries like Vanadium instead of Zn are developed.
While non-aqueous electrolytes are of higher costs than aqueous and vetted for environmental and chemical compatibility, the increase of operating potential window is attractive, as the potential difference has an impact on the amount of power delivered at a specific current density.
Other examples of nonaqueous RFBs include that being developed by Matsuda et al., which is based on the redox system based on [Ru(bpy)3]2+/[Ru(bpy)3]3+ as the anolyte and [Ru(bpy)3]+/[Ru(bpy)3]2+ as the catholyte where bpy is bipyridine, with acetonitrile (CH3CN) with tetraethylammoniumtetrafluoroborate (TEABF4) as the supporting electrolyte. This system has an OCP of 2.6V with energy efficiency of 40%.
Other similar types are presented by Chakrabartiet al.,, where Ruthenium acetylacetonate was used giving a OCP of 1.77 V, Yamamura et al.,, demonstrated a system based on several Uranium beta-diketonates with OCP of 1 V. Recently, Thompson et al.,,, in their work developed redox – flow system using M(acac)3 (M= V, Cr, or Mn and acac is acetylacetonate) with atleast three different oxidation states. The vanadium and chromium acetylacetonate systems showed higher OCPs , 2.2 and 3.4V respectively compared to 1.26V for aqueous VRB system. In a review by Feng Pan et al.,presented a series of metal complexes and metal free organic compounds for Redox flow battery a summary is given in Table 1.
Table 2. Summary of metal complexes and metal-free organic compounds.40
Gao et al., fabricated a solar rechargeable redox flow battery with Li2WO4 as anode in aqueous electrolyte, LiI as cathode in organic electrolyte, and LISICON film as membrane to separate liquid anode/cathode active species as shown in the figure 5.
Figure 5.Scheme of the configuration and working mechanism of the solar
rechargeable redox flow battery in dual-phase electrolytes41
The following reactions take place in the anode and the cathode. The initial discharge capacity of the battery is 0.0153 mA h mL-1, with the cut-off voltage at 0.2 V.A discharge capacity of0.0151 mA h mL-1is retained after 10 cycles, demonstrating the good stability of the solar rechargeable flow battery system.
3I- I-3 + 2e-
xLi+ + Li2 WO4 + xe- Li2+xWO4
In another study, Yang et al., investigated a system consisting of a DSSC and a RFB connected through two electrolyte circuit of a DSSC and a RFB, which are connected through two electrolyte circuitloops in which two soluble redox couples enter the RFB separately for electrochemical discharge, and then to pump the discharged redox couples into the DSSC for photoregeneration. After the photoregeneration, the two redox couples are stored in two individual reservoirs for subsequent RFB applications. The two cells are separated by a lithium-ionic conducting glass ceramic membrane to prevent the crossover of the redox couples. The schematic of such a system is shown in Figure 6. As a proof of concept experiment they chose a redox-active I3–/I– and [Fe(C10H15)2]+/Fe(C10H15)2(denoted as DMFc+/DMFc) redox couples because of their electrochemical reversibility and suitable redox potentials. They showed that the discharge capacities were increased from 3.86 mAh at a flow rate of 1 mLs-1to 13.3 mAh at 20 mLs-1. The photocharge capacity and electrochemical discharge capacity in the third cycle for the redox couples were 33.1 and 20.4 mAhg-1(108.0 mAhL-1and 66.6 mAhL-1), respectively.
Figure 6.Schematic of a RFB system that can store solar energy. O1/R1and O2/R2denote the oxidized/reduced states of the two redox couples42
Peimanifard et al., evaluated the performance of a vanadium photo-electrochemical cell with GC/MWCNT/CdS as a photoanode. The following reactions take place at the electrodes:
They achieved applied bias photon-to-current efficiency (ABPE) for both two and three-electrode configurations. The glassy carbon/multi walled carbon nanotube/cadmium sulphide yields high maximum ABPE of 2.6% and 2.12% in three and two-electrode setups, respectively. Figure 7 shows the schematic of their cell configuration.
Figure 7.Model for vanadium photoelectrochemical cell with GC/MWCNT/CdS photoanode43.
M. Yu et al., reported an aqueous Lithium-iodine solar flow battery (SFB), which integrated a Li-I redox flow battery and a DSSC by linking a I3−/I−catholyte for simultaneous conversion and storage of solar energy. The battery showed an average discharging voltage of 3.35 V and a charging voltage of 3.55 V at a current density of 0.50 mA cm-2, which matches well with the reported redox potential of the iodide couple in water (~3.5 V). The overpotential is small compared to other Lithium based batteries due to rapid kinetics. They reported that at a acutoff voltage of 3.6 V, the solar battery with 0.100 mL of catholyte (2.00 M LiI, 0.50 M GuSCN in saturated Cheno aqueous solution) is able to be photocharged to a volumetric capacity of 32.6 Ah L−1in 16.80 h, which is 91 percent of its theoretical capacity (i.e., 35.7 Ah L−1for the catholyte with 2.00 M LiI). Figure 8 shows the details of the battery
Figure 8. (a) Schematic of a Li−I SFB device with the three-electrode configuration; (b) energy diagram for the photoassisted charging process; (c) photoelectrochemical half-reactions44.
P. Liu et al., constructed a solar rechargeable battery by the use of a hybrid TiO2/poly(3,4-ethylenedioxythiophene, PEDOT) photo-anode and a ClO4− doped polypyrrole counter electrode. Here, the dye-sensitized TiO2/PEDOT photoanode serves for positive charge storage and a p-doped PPy counter electrode acts for electron storage in LiClO4electrolyte. The proposed device demonstrated a rapid photo-charge at light illumination and a stable electrochemical discharge in the dark, realizing an in situ solar-to-electric conversion and storage. The discharge capacity calculated from the amount of the loaded PEDOT on the hybrid photo-anode is about 8.3 mAh g−1. Figure 9 shows the schematic and the working principle of the battery
Figure 9. Schematic demonstration of the configuration and working mechanism of the
solar rechargeable battery45.
S. Liao et al., reported an efficient SRFC based on a dual-silicon photoelectrochemical cell and a quinone/bromine redox flow battery for in situ solar energy conversion and storage. Using narrow bandgap silicon for efficient photon collection and fast redox couples for rapid interface charge injection, the device showed an optimal solar-to-chemical conversion efficiency of ~5.9% and an overall photon–chemical– electricity energy conversion efficiency of ~3.2%, which, outperformed previously reported SRFCs. The proposed SRFC can be self-photocharged to 0.8V and delivers a discharge capacity of 730mAh L-1. Figure 10 shows the schematic of the SRB. The cell reactions are shown below:
F Figure 10.Schematic configuration of the proposed SRFC46.
McKone et al., demonstrated a solar flow battery based on anthroquinone/iodide
redox couples along with n-type WSe2 photoelectrodes. The cell was discharged by carbon auxiliary electrodes with an overall round trip efficiency as high as 2.8%. The device operates stably for at least 12 hours.
Chakrabarti et al., presented a redox flow battery system for solar energy storage. They employed an undivided reactor configuration with porous graphite felt electrodes and ruthenium acetylacetonate as electrolyte in acetonitrile solvent. They determined the limiting current densitites for 0.02 M and 0.1 M ruthenium acetylacetonate. For 0.02 M ruthenium acetylacetonate the charging current density was determined to be 7 mA/cm2 and discharge current density as 2 mA/cm2.They obtained an power output of 35 mW for discharge current density of 2.1 mA/cm2. The increase in current density of 5 fold was obtained for concentration of 0. 1M.
Azevedo et al., used a photoelectrochemical cell with the configuration:l(CdS(s)|V3+, VO2+||V3+, V2+|Carbon Felt(s), E0=0.6 VNHE). They demonstrated that CdSphotoanode exhibited competitive photocurrents, when compared to other photo-electrochemical devices. They obtained a photovoltage of 1.3 V. The schematic of their cell configuration is shown in Figure 11
Figure 11. Schematic diagram of (a) all vanadium solar redox flow battery charged with a CdSphotoanode and (b) energy diagram of the system, including the standard redox reactions49.
Li et al., in their work, developed an electrochemical storage device integrating regenerative silicon solar cells and 9,10-anthraquinone-2,7-disulfonic acid (AQDS)/1,2-benzoquinone-3,5- disulfonic acid (BQDS) RFBs. This device can be directly charged by solar light without external bias, and discharged like normal RFBs with an energy storage density of 1.15 WhL -1 and a solar-to-output electricity efficiency (SOEE) of 1.7 percent over many cycles. The schematic is shown in the figure below.
Figure 12.The integrated PEC-RFB device using AQDS/BQDS redox couples in catholyte/anolyte50
Cheng et al.,showed that the combined photovoltages exceeding 1.4 V can be obtained using a Ta3N5 nanotube photoanode and a GaN nanowire/Si photocathode with high photocurrents (>5 mA cm–2). The photoelectrode system makes it possible to operate a 1.2 V alkaline anthraquinone/ferrocyanide redox battery with a high ideal solar-to-chemical conversion efficiency of 3.0% without externally applied potentials. Importantly, the photocharged battery is successfully discharged with a high voltage output. A schematic of this redox battery is shown in the figure below
Figure 13. Schematic illustration of the design and the corresponding energy band diagrams. a) The battery consists of two electrolytes (K4[Fe(CN)6] in the positive compartment and 2,6-DHAQ in the negative compartment; see main text for full chemical names) and two photoelectrodes (Ta3N5 as the photoanode and GaN/Si as the photocathode). The discharge takes place on a separate set of carbon paper electrodes. b) The energy band diagram under illumination showing the charge separation and flow charts of the system51.
In another study, Wedege et al.,developed a solar aqueous alkaline redox flow battery using lowcost and environmentally safe materials. The electrolytes consist of the redox couples ferrocyanide and anthraquinone-2,7-disulphonate in sodium hydroxide solution, yielding a standard cell potential of 0.74 V.They demonstrated photo voltage enhancement strategies for the ferrocyanide-hematite junctionby employing an annealing treatment and growing a layer of a conductive polyaniline polymer on the electrode surface ,which decreases electron– hole recombination. The surface-modified sample shows a higher fill-factor and thus higher photocurrent of 0.15 mA cm-2. The solar-to-chemical-energy conversion efficiency of the combined system is ca. 0.05–0.08%.
Figure 14. Energy diagram of the PEC/RF cell for solar charging of electrolytes connected to a RF cell for discharge. Desired electron-hole pathways under light exposure are shown with full blue arrows. Undesirable back-electron transfer is shown with red dotted arrow52.
In another study, Wang et al., demonstrated a dye-sensitized solar cell (DSC) with in situ energy storage capacity using a lead–organohalide electrolyte CH 3 NH 3I·PbCl2 (LOC) to replace the conventional I − /I 3 − electrolyte. The coupling of lead and iodine in one electrolyte creates a dual-function rechargeable solar battery that combines the working processes of photoelectrochemical cells with electrochemical batteries. Optimization of the H + concentration in the electrolyte leads to increased photocharging efficiency and storage. The power conversion efficiency of the LOC–DSC was 8.6% under one sun illumination (AM 1.5, 100 mW cm −2 ) as a DSC. When operating as a battery, Faraday efficiency can be achieved as high as 81.5% using a bromide-based CH 3 NH 3Br·PbBr2 (LOB) electrolyte in a DSC configuration. This new cell design suggests a means of combining photovoltaic energy conversion and electrical energy storage. The schematic diagram is shown in Figure 15.
Figure 15.a) The working mechanism of photons’ conversion process by a typical DSC under illumination; b) The redox reactions happened at two electrodes of the LOC–DSC under charging and discharging processes53.
Lei et al.,in their work integrated the hydrogen production, storage and utilization into a hybrid system of the dye-sensitized solar cell and electrochemical cell with the dye-sensitized TiO2 as photo-anode, LiI as the cathode active material in organic electrolyte, AB5-type hydrogen storage alloy as anode in alkaline solution, and PEDOT-modified Nafion membrane as separator. Here, the photo-generated electrons in organic electrolyte pass to the AB5-type hydrogen storage alloy to split water in alkaline aqueous electrolyte for generating hydrogen, which is in situ stored into AB5-type hydrogen storage alloy. Subsequently, the hydrogen stored in the AB5-type hydrogen storage alloy can be oxidized by electrochemical way to generate electricity, coupled with LiI cathode in organic electrolyte. The solar rechargeable battery demonstrates a new solution of the solar energy conversion, hydrogen production, storage, and utilization, achieving the new energy conversion and storage from solar energy to chemical energy, and further to electrical energy. They obtained a photo conversion efficiency (PCE) of 4.75% and a discharge capacity of 26.7 mAh g−1, which is calculated based the alloy as active material. After 5 cycles, the discharge capacity is increased gradually to 33.9 mAh g−1. The Schematic is shown in the Figure 16.
Figure 16.(left) Schematic of the configuration and working principle of the solar rechargeable battery system. (right) Diagram of the hydrogen storage and oxidation in photo-charge andelectrochemical discharge processes54.
Herein, LiI is worked as cathode in organic electrolyte (0.1 mol L−1LiI and 0.5 mol L−1TBP in PC), AB5-type hydrogen storage alloy is used asanode in alkaline solution (5 mol L−1KOH and 1 mol L−1LiOH), and PEODT modified Nafion film is used as membrane to separate the dual-phase electrolyte53.
Liao et al., demonstrated a solar rechargeable flow cell with photoelectrochemical regeneration of vanadium redox species. Here TiO2 and MWCNT/acetylene black composite served as the photoanode and counter electrode respectively, with all Vanadium redox couples VO2+ /VO2 + and VO 2 + /V 3 + , as solar energy storage media. The cell can be photocharged under a bias as low as 0.1 V, which is much lower than the discharge voltage of ∼0.5 V. The cell delivered discharge energy of 23.0 mWh/L. This prototype enables further study for cost-effective solar energy storage devices. Figure 17 shows the schematic of the cell.
Figure 17.The schematic of the solar rechargeable flow cell with photoelectrochemical regeneration of Vanadium redox species55.
In our recent study we presented a solar rechargeable redox battery with I − /I 3– and S42-/S22- . A stable discharge voltage is shown with high areal energy storage capacity of
180 mW h cm-2. The following reactions take place:
At Cathode (Pt mesh)
At Anode (Ni Foam)
Both static and flow batteries were studied. The energy density and the charge capacity of the flow battery were 2.8 WhL-1 and 6.5 AhL-1. The areal energy density of the static battery was 180 µWh cm-2,which is best in the literature.
In an another review, Liuyue Cao et al.,presented a range of developments in photo electrochemical cells (PECs) and SRFBs. A summary of the progresses is presented in the table below:
Table 3.Photoelectrochemical rechargeable batteries since 200957.
The developments in the solar rechargeable redox flow batteries are in the early stages. So, it is important to understand the redox chemistries of the electrolytes and the nature of dyes that are suitable for dye sensitized solar cell based SRFBs, before attempting to build robust large scale energy storage devices. This project serves to address this need. It is also important to design and fabricate novel 3D architectures that suit the purpose.
The aims of the project involve
- Identification of aqueous based positive and negative redox electrolytes
- Identification of dyes that is compatible with the positive electrolyte.
- Selection of appropriate membranes separating positive and negative electrolytes.
- Design and development of dye sensitized solar cells(DSSCs) integrated with redox flow batteries, forming the Solar Rechargeable redox flow batteries(SRFBs).
- Then the SRFBs are tested with different redox couples and dyes and for their performance by electrochemical measurements.
- Improving the energy efficiency of the SRFBs by choosing appropriate cell design and alternative dye-electrolyte combination
Though efficiencies of up to 12% have been achieved, they have intrinsic disadvantages like
flammability, toxicity and high cost. Water-based solar cell electrolytes have advantages like non-toxicity, non-flammability, non-volatility, low vost and ability to dissolve many potential redox mediators. Traditionally used redox mediator is I/I3– coupleas reported by Law et al.,
The following electrolyte composition:1 M NaI,0.1 M I2,1 M NaClO4 was investigated with film conditions:2.5 um T-Dyesol, 3um Scattering layer and N719 Dye in 1:1 solution of acetonitrile:tert-butanol. The following results were obtained:
Figure 18. I-V Characteristics of Iodine/Iodide Based Aqueous DSSCs
Table 4. I-V parameters for Iodine/Iodide based aqueous DSSCs
Use of Fe(CN)64-/3− can overcome some limitations of I−/I3−, such as iodate formation and
corrosiveness. Ferrocyanide and ferricyanide are non-corrosive and barely colored and, thus, do not compete with the dye for incident photons. Since Fe(CN)64-/3− has a similar redox potential to I−/I3−existing semiconductors and dyes can be utilized.With the electrolyte composition of 0.4 M Potassium Ferrocyanide K4[Fe(CN)6], 0.04 M Potassium Ferricyanide K3[Fe(CN)6], 0.1 M KCl, 50mMTrizma hydrochloride(pH 8) and 0.1 (v/v)% Tween 20 and MK-2 Dye in 1:1:1 acetonitrile, toluene, tert-butanol, the following photovoltaic parameters were obtained. Efficiency of up to 1.56% was obtained.
Figure 19. Structure of MK-2 Dye
Table 5. I-V parameters for Fe(CN)64-/3- based aqueous DSSCs
Figure 20. I-V characteristic Fe(CN)64-/3- Based Aqueous DSSCs
The application of a cobalt(II)/(III) tris(2,2’-bipyridine) based electrolyte has recently
resulted in a benchmark efficiency of >12%.This result has accelerated the transition to non-iodide based electrolytes, which are less corrosive. the photoinstability of [Fe(CN)6]3represents a major drawback of this electrolyte.Electrolyte composition 0.2 M Co(bpy)3(NO3)2, 0.04M Co(bpy3(NO3)3, 0.1M KCl and 0.7M N-methylbenzimidazole ; with film conditions 2.5 um T-Dyesol, 3um Scattering layer. The dye used is MK-2 Dye in 1:1:1 solution ofacetonitrile:toluene:tert-butanol. The following photovoltaic parameters were obtained.
|Cell Number||Initial||After 30 mins||After 1 day|
Table 6. I-V parameters for aqueous DSSC electrolytes based on the cobalt(II)/(III) tris(bipyridine) redox couple
Figure 21.I-V characteristicof cobalt(II)/(III) tris(bipyridine) based aqueous systems
The DSSC part of the SRFB was tested and the following I-V characteristic was obtained
Figure 22. I-V Characteristics of DSSC part of SRFB
I-V Characteristics of the Solar side of SRFB
The following photocharging vs time data was recorded
Figure 23.Photocharging current of the DSSC part of RFB
Figure 24. Photocurrent when Light is turned on and off periodically
Hydrogels based on polyurethane are capable of dispersing redox electrolytes that are water based. The hydrogels are to be dispersed with different electrolytes and the DSSCs are fabricated and the photovoltaic parameters are to be investigated.
Different redox couples in aqueous electrolytes are to be tested based on the literature, with standard procedure for fabrication of DSSCs and tested. Those which show high efficiencies are to be incorporated in the SRFBs. Also, novel redox couples are to be tested and their performance in DSSCs are to be investigated. The rational for the selection of redox couples lies in electrochemical potential of the species in the cathode and anode compartment, as shown in the figure below.
Figure 25.Electrode potentials of the Redox couples with the potential of the Titania.
A Novel solar rechargeable redox flow battery based on LiI/Li2S4 redox couples and comparewith the previously developed one
– A LiBr/Li2S4 battery to be built and compared the performance to that already existing in the literature
– AMgBr/Mg2S4 to be built
– A MgI/Mg2S4 to be built and compared the performance to that already existing in the literature
– A NaBr/Na2S4 to be built and all compared to that previously developed SRFBs
SRFBs with aqueous electrolytes based on the cobalt(II)/(III) tris(bipyridine) redox couple are to be built since they show high efficiency. Other redox couple that is to be tested is the Fe(CN)64-/3- Based Aqueous DSSCs.
It is also proposed to investigate other bipyridine complex based redox couples such as nickel and iron. Other acetyl acetonate based redox couples are also proposed to be investigated.
The dyes that are compatible for the redox couples investigated in the previous
step are to be studied. In particular, amphiphilic homologues of the pioneering ruthenium based N-3 dye have been developed. These amphiphilic dyes display several advantages compared to the N-3 dye such as:
- a higher ground state pKa of the binding moiety thus increasing electrostatic binding onto the TiO2 surface at lower pH values,
- the decreased charge on the dye attenuating the electrostatic repulsion in between adsorbed dye units and thereby increasing the dye loading,
- increasing the stability of solar cells towards water-induced dye desorption,
- the oxidation potential of these complexes is cathodically shifted compared to that of the N-3 sensitizer, which increases the reversibility of the ruthenium III/II couple, leading to enhanced stability.
Figure 26. Structure of MK-2 dye
Figure 27. Structure of the N3 Dye
Figure 28.Structure of N719 Dye
Figure 29.Structure of Z907 dye
The design of the solar rechargeable redox flow battery lies in the integration of the DSSC and the redox flow battery. Highly efficient DSSCs are to be coupled with the RFBs.
A novel redox rechargeable redox flow battery was designed as shown in the figures below
Exploded view of the Solar Redox Flow Battery
Fluid pathway and gasket
Cross sectional View 1 Cross sectional View 2
Figure 30. Design of the SRFB
However the above design had difficulties in leakages arising from the electrical connectors used, so an alternative design is proposed.
The proposed design incorporates the use of platinized gaskets for electrical connections as shown in the figures below:
- Solar cell side of the SRFB
- Battery Side of the SRFB
- The Whole cell configuration of the SRFB.
Figure 31.Alternative design of the SRFB.
Another design of the SRFB is presented in the figure below.
Figure 32. Proposed Alternative design of the SRFB
After the SRFB is fabricated, the following electrochemical measurements are to be performed:
- Photocharging current(I) vs time
- Discharge voltage(V) vs time
- Cell Voltage (V) vs discharge rate (mA/cm2)
- Energy density (Wh/L) vs discharge rate (mA/cm2)
Then the experiment is to be repeated with different flow rates such as 1µL/min, 3 µL/min, 5 µL/min, 10µL/min and 20 µL/min.
With the appropriate selection of the redox shuttles, electrolyte
composition and dyes, it is intended to develop SRFBs with high energy density and power density. Herein, it is also necessary to vary the design parameters of the SRFBs. The performances of the developed SRFBs are to be compared with those existing in the literature.
The project will serve to understand the redox flow battery chemistry driven by the solar energy. It will help us to understand what design parameters and redox couples will enhance the energy density and the power density of such batteries. It will also enable us to gain knowledge and explore the value of 3D printing for SRFBs.
Started PhD in March 2017
|Area of Focus||Semester
|Fabrication of Dye-sensitized Solar Cells(DSSCs)|
|Design of Solar Redox Flow Battery|
|Development of Aqueous Electrolytes for DSSCs|
|Fabrication and Testing of SRFBs with Iodine/Iodide and sulphide/polysulfide redox couples|
|Identification of Highly Efficient Redox couples for SRFBs|
|Identification of compatible Dyes for the Redox couples|
|Fabrication and testing of SRFBs with Alternative redox couples|
|Development of High Energy and High Power Density
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