Development of photovoltaic redox flow battery using aqueous iodide/triiodide electrolyte

2014 Fiscal Year Research-status Report

• Project/Area Number: 26810132
• Research Institution: The Institute of Physical and Chemical Research
• Principal Investigator: BYON HYERYUNG 独立行政法人理化学研究所, Byon国際主幹研究ユニット, 国際主幹研究員 (70606409)
• Project Period (FY): 2014-04-01 – 2016-03-31
• Keywords: Li-I2 battery / solar stimulatation / flow battery

Outline of Annual Research Achievements

The studies on the existing Li-I2 aqueous battery focused on two scopes; 1) enhancing the I-/I3- electrochemical behavior 2) linking it to a solar source. Two major findings were stemmed from this study:
I.The aqueous I-/I3- couple exhibits a quasi-reversible behavior. Higher concentrations of I2 favour the reduction reaction but hinder oxidation, especially on the glassy carbon electrode. The additive LiI enhancing ionic conductivity does not significantly influence the I-/I3- reaction.
II.The overpotential loss of the platinum electrode is smaller compared to the glassy carbon.
Electrolytes without LiI showed comparable performance to the cells containing LiI. Electrolytes containing only LiI/I2 exhibited higher specific capacities by a factor of 3. Yet, issues regarding cycleability are currently under investigation. Mixing of Pt with carbon paste should prompt the I3- reduction reaction. Enhanced voltage efficiency by 10% was found.
For the development of the proposed prototype system, FTO glass electrode was tested (instead of DSSC as originally planned due to its instability in aqueous solution) showing inferior reversibility and specific capacity. Therefore, Fe2O3 layer was deposited acting as a photocatalyst, which has suitable bandgap to harvest the visible part of the solar spectrum. Electrodeposition is used but further optimization (temperature, electroplating duration, electrodeposition potential) is currently underway. Preliminary results on the battery system under illumination during charging have shown a great enhancement in the voltaic efficiency.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

1)The starting of the project was delayed as the project member joined later than the starting date.
As this battery system was new for the candidate, some time was spent in learning both the theoretical (through literature) and technical aspects (how to devise and construct the cells) of this battery system. The first goal was to recreate previous reported results and then investigate more in detail the existing system. This was accomplished rather smoothly due to the previous experience of the candidate especially in the half-cell studies (cyclic voltammetry). The next goal involved further studies on the battery system by examining various parameters such as electrolytes and carbon collectors. This stage is almost completed and the findings are being processed. No serious issues/problems have been presented so far, therefore the progress in this matter is rather smooth.

2)Change of the original plan regarding the solar module – Fe2O3 electrodeposited FTO instead of DSSC.
This change was due to a) the instability of DSSC in aqueous solution and b) low LUMO value of DSSC, which is close to the H2 evolution potential in aqueous medial leading to low energy efficiencies. The modification of the FTO substrate and its integration with the solar simulator is currently underway. The preliminary results are promising and more work is to be done towards the completion of this system for a better understanding of how to improve this system.

Strategy for Future Research Activity

Future work includes the proof of concept of photovoltaic redox flow battery on the basis of hematite photoelectrodes. To achieve this, the following issues have to be addressed.
a)Optimize Fe2O3 electrodeposition conditions on FTO in order to achieve a i) smooth (no dendrites) and ii) uniform (thickness: 400-500 nm) photoactive Fe2O3 layer that will promote its photoelectrochemical properties.
b)Determine the effect of sunlight during charge and discharge of the Li-I2 aqueous battery under static conditions (no flow of the electrolyte). Initial results indicated minimization of overpotential loss during charging (from 3.9 V to 3.3 V) but also low specific capacities. Further studies to determine the mechanism in the presence of sunlight are to be conducted.
c)Integrate flow conditions at the cathode of the system (using a pump). In this case, the fuel is replenished and therefore is expected to behave in a different manner than in b). By integrating flow conditions we increase the energy density of the system considerably as the energy is stored in the electrolyte and not inside the battery. Optimization of the operating conditions are to be determined.

Causes of Carryover

As the project member joined this project later than starting date (because I looked for (recruiting) the proper person after acceptance of this proposal due to budget problem), the total schedule is slightly delayed. In this financial year, we push more research progress with the carry-over and this year's budgets.

Expenditure Plan for Carryover Budget

The carry-over budget will be used for article cost and travel expense. The project member will attend the conference (Annual meeting of international society of electrochemistry, 4-9/Oct 2015 in Taiwan) to give a presentation of his achievement.