TiO2-Photoanode-Assisted Direct-Solar-Energy Harvesting and

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TiO2-photoanode-assisted direct solar energy harvesting and storage in a solar-powered redox cell using halides as active materials Shun Zhang, Chen Chen, Yangen Zhou, Yumin Qian, Jing Ye, Shiyun Xiong, Yu Zhao, and Xiaohong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04314 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

TiO2-Photoanode-Assisted Direct Solar Energy Harvesting and Storage in a Solar-Powered Redox Cell Using Halides as Active Materials Shun Zhang,†# Chen Chen,†# Yangen Zhou,† Yumin Qian,† Jing Ye, ‡ Shiyun Xiong, † Yu Zhao,*† and Xiaohong Zhang*† †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, 199 Renai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China. ‡

Analytical and Testing Center, Soochow University, 199 Renai Road, Suzhou Industrial Park,

Suzhou, Jiangsu 215123, P. R. China. #

These authors contribute equally to this work.

Corresponding authors [email protected] (Y.Z.) [email protected] (X.Z.) Keywords energy conversion, energy storage, solar energy, redox-flow cell, halide, photocatalyst

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Abstract

The rapid deployment of renewable energy is resulting in significant energy security, climate change mitigation, and economic benefits. We demonstrate here the direct solar energy harvesting and storage in a rechargeable solar-powered redox cell, which can be charged solely by solar irradiation. The cell follows a conventional redox-flow cell design with one integrated TiO2 photoanode in the cathode side. Direct charging the cell by solar irradiation results in the conversion of solar energy in to chemical energy. While discharging the cell leads to the release of chemical energy in the form of electricity. The cell integrates energy conversion and storage processes in a single device, making the solar energy directly and efficiently dispatchable. When using redox couples of Br2/Br- and I3-/I- in the cathode side and anode side, respectively, the cell can be directly charged upon solar irradiation, yielding a discharge potential of 0.5V with good round-trip efficiencies. This design is expected to be a potential alternative towards the development of affordable, inexhaustible and clean solar energy technologies.


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Introduction

Solar energy is one of the most important intermittent renewable energy resource and has been singled out as perhaps the most crucial need for the secure energy future. The rapid expansion of photovoltaics and photocatalysis in the past decade demonstrate great success in rational utilization of solar energy,

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the former converts solar irradiation into electricity and the latter

stores solar irradiation as energy carriers like hydrogen and hydrocarbon fuels. Recent research suggests that it’s possible to integrate the intrinsically independent energy conversion and storage process into a single device, a solar-powered redox cell (SPRC).

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operating mode, SPRC can be categorized into two types (Figure 1a): biased 13−18

According to the 9−12

and unbiased.

The former needs externally applied bias to facilitate the excited electrons moving from the

conduction band of the photoanode to the redox couples in the anolyte. The potential during charge can be lowered as a result of the assistance of solar irradiation. However, the biased SPRCs require additional equipment as an external charging source to fully charge the cell, which does not make them an energy self-sufﬁcient integrated design. Compared with biased ones, unbiased SPRCs usually yield lower discharge potential due to the band structure of the photoelectrode. However, the unbiased SPRCs, especially those with single photoelectrode configuration, greatly simplify the device architecture and can be solely charged by solar irradiation, which may pose profound impact on practical application in consideration of manufacturing process and material cost. Among the reported unbiased SPRCs, either a splittype cell design is used or a layer of dye molecules is needed for the photoanode in an integrated design. The former uses a photoelectrochemical cell to converts solar irradiation into chemical energy and a redox-flow cell to convert the chemical energy into electricity. While the latter has proven difficult to improve the stability of the dye molecules under long-term solar illumination


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and to eliminate expensive materials, notably platinum and ruthenium, as in dye-sensitized solar cells. 20 SPRCs that have significantly lower costs and simpler architecture than the present ones would be of considerable interest for a fully solar-based future energy production and storage. In previous investigations on photoelectrochemical systems with combined energy storage, the cell structure usually consists of a Pt counter electrode and an illuminated n-type photoanode such as single-crystalline MoSe and GaAs. 21–24 The poor corrosion stabilities of the photoanode together with the electrolyte contamination resulting from the cross-over of active materials in the cathode and anode make these photoelectrochemical cells barely rechargeable unless transport the electrolytes into a separate redox flow cell for electricity generation. Such systems synergized from multiple individual devices would lead to complex system design and post maintenance, and the use of strong acidic media limits the selection of a wide range of light absorbing materials. Besides, the high cost of photoanodes and noble metal electrode is another concern for grid-scale applications. To simplify the system architecture and to achieve energy self-sufﬁcient integrated design, we demonstrate here an integrated, rechargeable SPRC, which can be directly charged solely by solar irradiation. The SPRC is constructed from a conventional redox-flow battery with an additional photo-sensitized electrode in the cathode side. The energy storage and conversion can be achieved in weak acidic halides solutions as in the form of Nernst potential shift of redox couples of Br2/Br− and I3−/I−. The demonstrated SPRC shows good cyclability and high cycling efficiency. A volumetric capacity up to ca. 6 Ah·L−1 has been achieved which is among the highest values achieved in the reported SPRCs. 25 Results & discussion


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The SPRC was built based on a redox-flow cell configuration with a carbon electrode and a rutile-phase TiO2 photoanode immersed in an aqueous solution of KBr/LiH2PO4 at the cathode side, and a carbon counter electrode immersed in an aqueous KI/I2/LiH2PO4 solution at the anode side (Figure 1b and Figure S1, Supporting Information). The cathode and the anode were separated by a Li+-ion conductive membrane. The photoanode was insulated from the carbon electrode to avoid self-discharge upon charging. Such a cell design allows electrolytes flow through the electrodes then stored in external reservoirs so as to increase the total energy capacity. The TiO2 photoanode and carbon electrode (marked as 1 and 3, respectively, Figure 1b) were connected to allow the cell being charged solely by solar irradiation, while the other carbon electrode (marked as 2, Figure 1b) and anode were connected with an external load to discharge the cell. Upon illumination of the photoanode, photo-excited carriers were collected at the TiO2−electrolyte interface and used to oxidize Br− via reaction (1). Simultaneously, electrons move to the anode side reducing I2 via reaction (2).

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This process produced Br2 and I3− that

correspond to charged species in the redox flow cell and could be directly converted into electricity during the discharge process. ℎ : 2 + 2ℎ → = 1.09 .

(1)

: !" + 2 → 3! = 0.54 .

(2)


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Figure 1. (a) Schematic illustration of the photo-charging process in a biased and unbiased SPRC. (b) Schematic cross-sectional view and (c) energy diagram of the unbiased SPRC with single photoelectrode.

Such kind of device can be treated as an enlarged Donor–Bridge–Acceptor systems with strong coupling over the bridge (Figure 1c). During the photo-charging process, the whole electron transfer process is governed by the long-range bridge mediated donor-acceptor electron transfer. 27

Due to the energy level alignment, the photon excited electrons diffuse to the donor acceptor

and reduce I3− at the anode, while holes hop to the donor and oxidize Br−. This process continues until the cell reached the maximum state-of-charge. Due to the large distance along the electron transfer path way, the electron-transfer process shall not result from a tunneling process by the super-exchange effect since there is no direct contact between TiO2 and I− ions. 28 The only way


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for the electron transfer is through thermally activated electron hopping. The redox potential of I3−/I− lies nearly ∆EC=1.0 eV below the conduction band of TiO2. Such energy difference results in electrons transferring from TiO2 to the anode without using external bias. Meanwhile, the electrons are trapped in the anode because of the energy differences. During the discharge process, the external circuit link the cathode and anode directly, the electrons flow from anode to cathode with the bias Eocv.

Figure 2. Characterizations of the photoanode. (a) SEM image, (b) XRD pattern, (c) UV-vis absorption spectrum, and (d) linear sweep voltammogram of the photoanode.

To demonstrate the SPRC, we first studied the individual components. Rather than using TiO2 thin film composed of nanoparticles, the vertically-aligned TiO2 one-dimensional nanostructures should exhibit short diffusion distance for photogenerated minority carriers and relatively large


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number of active sites for electron-hole pair generation,

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which is expected to facilitate the

charge separation and reduce the loss by electron-hole recombination. The morphology of the assynthesized photoanode appeared to be vertically aligned nanorod arrays with average diameter and length approximately 200 nm (Figure 2a) and 1.5 µm (Figure S2, Supporting Information), respectively. X-ray diffraction (XRD) analysis suggested the nanorod array was composed of phase-pure rutile TiO2 (Figure 2b). High-resolution transmission electron microscope (HR-TEM) further revealed that the nanorod was constructed from secondary single-crystalline nanorod bundles grown along direction of rutile TiO2 (Figure S3, Supporting Information). The TiO2 photoanode absorbed incident lights in the near ultraviolet region as shown in the UV-vis absorption spectrum (Figure 2c). The band gap was estimated to be ca. 3.2 eV according to the corresponding Tauc plots (Figure S4, Supporting Information). It’s worth noting that only a small fraction (λ

TiO2-Photoanode-Assisted Direct-Solar-Energy Harvesting and

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