Development and Optimization of Integrated Photoelectrochemical Energy Storage Cells Using Ion Soft-Landing
Authors: Prabhakaran Venkateshkumar , Romo Joelle , Norberg Zachary , Kalb David , Johnson Grant , Laskin Julia 
Published: February 2020

The development of efficient energy conversion and storage technologies is important to meet increasing demand. Solar energy is a valuable renewable and sustainable resource utilized in numerous energy technologies. However, the inherent variability of solar energy necessitates the addition of efficient energy storage components (e.g. batteries, flow cells, etc.) to achieve a continuous supply of electricity. Development of integrated systems that combine solar energy conversion and storage in a single electrochemical device is a promising approach that may potentially improve the efficiency of solar energy utilization.

A photoelectrochemical system capable of converting and storing solar energy relies on photo-induced reactions of redox-active species.(1, 2) Polyoxometalates (POMs) are a class of stable anionic metal oxides clusters with multi-electron redox activity.(3) POMs undergo multiple redox transitions by utilizing either electrons or photons and exhibit a color change during certain transitions (photo- and electro chromic).(4) For example, the photo-induced reduction of POM as a photo catalyst has been used in water oxidation and hybrid fuel cell applications.(5, 6) In this work, we developed an integrated photo electrochemical energy storage (IPES) cell with the ability to convert and store solar energy in a single device using POM’s photochromic and redox properties, and explored factors that affect the performance of the IPES cell. Electrode surfaces prepared using conventional solution based methods often contain both electrochemically active ions and inactive counter ions that can hinder electron transfer processes. Previously, we demonstrated that a complete elimination of strongly coordinating counter cations substantially improved the performance of redox-supercapacitors prepared by soft landing of mass-selected POM anions.(7) Ion soft landing enables deposition of uniform layers of electroactive ions with well-defined charge state, composition and kinetic energy onto electrode surfaces, something not possible using other preparation techniques.(8, 9) Herein, we prepared IPES cells with POM anions deposited onto the anode using either ion soft landing or solution based deposition for comparison.

The IPES cells consist of (i) Keggin POMs deposited on an indium-tin oxide (ITO) photoanode using either spin casting and or ion soft landing (ii) an ionic liquid membrane that contains ferrocene methanol as a model redox molecule (iii) a glassy carbon cathode. Our preliminary results with the IPES cells irradiated by direct solar light confirmed that the IPES cell generates useful current and potential due to the photo-induced redox conversion of POM at the anode and the counter redox reaction at the cathode. Additionally, in-situ UV-Vis spectroscopy confirmed the photo chromic transition of POM in the IPES cell from a clear to dark blue color upon irradiation. The design of the IPES cell will be presented and its performance under different experimental conditions will be discussed.

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