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Simultaneous enhancement in charge separation and onset potential for water oxidation in a BiVO4 photoanode by W-Ti codoping
Nanyang Technol Univ, Sch Mat Sci & Engn, 50 Nanyang Ave, Singapore 639798, Singapore.
Nanyang Technol Univ, Sch Mat Sci & Engn, 50 Nanyang Ave, Singapore 639798, Singapore;Northwest Univ, Sch Chem Engn, Xian 710069, Shaanxi, Peoples R China.
Nanyang Technol Univ, Sch Phys & Math Sci, Singapore 637371, Singapore.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-3548-133x
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2018 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 6, no 35, p. 16965-16974Article in journal (Refereed) Published
Abstract [en]

Efficient charge separation of photo-generated electrons and holes is critical to achieve high solar to hydrogen conversion efficiency in photoelectrochemical (PEC) water splitting. N-type doping is generally used to improve the conductivity by increasing the majority carrier density and enhance the charge separation in the photoanode. However, minority carrier transport is also very important in the process of charge separation, especially in materials that possess inadequate minority carrier mobility. Herein, we take a BiVO4 PEC water splitting cell as an example to demonstrate how to analyze the limiting factor and to formulate the corresponding solutions to improve the hole mobility. The benefits and problems caused by n-type doping (W-doping here) of BiVO4 are analyzed. Codoping with Ti further enhances the charge separation by improving the hole transport and leads to a cathodic shift of the photocurrent onset potential. A high charge separation efficiency (79% at 1.23 V-RHE) in a compact BiVO4 photoanode has been achieved without any nanostructure formation. Theoretical results show that W-Ti codoping has decreased the hole polaron hopping activation energy by 11.5% compared with mono-W doping, and this has resulted in a hole mobility increase by 29%. The calculated adsorption energy and reaction Gibbs free energies indicate that the Ti site is energetically more favorable for water splitting. Moreover, the Ti site possesses a lower overpotential in the W-Ti codoped sample compared with the mono-W doped sample. The current study indicates that in order to improve the solar energy conversion efficiency, there should be a balanced charge transport of both majority and minority charge carriers. This can be achieved by simply choosing appropriate codoping elements.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2018. Vol. 6, no 35, p. 16965-16974
National Category
Physical Chemistry Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-366730DOI: 10.1039/c8ta05491fISI: 000445218000025OAI: oai:DiVA.org:uu-366730DiVA, id: diva2:1269905
Funder
Swedish Research CouncilSwedish Energy AgencyAvailable from: 2018-12-11 Created: 2018-12-11 Last updated: 2018-12-19Bibliographically approved
In thesis
1. Materials Modelling for Energy Harvesting: From Conversion to Application through Storage
Open this publication in new window or tab >>Materials Modelling for Energy Harvesting: From Conversion to Application through Storage
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this Ph.D. thesis, ab initio density functional theory along with molecular dynamics and global optimization methods are used to unveil and understand the structures and properties of energy relevant materials. In this connection, the following applications are considered: i. electrocatalyst for solar fuel production through water splitting, ii. hybrid perovskite solar cell for generation of electrical energy and iii. Battery materials to store the electrical energy. The water splitting mechanism in terms of hydrogen evolution and oxygen evolution reactions (HER and OER) on the catalytic surfaces has been envisaged based on the free energy diagram, named reaction coordinate, of the reaction intermediates. The Ti-functionalized two-dimensional (2D) borophene monolayer has been emerged as a promising material for HER and OER mechanisms as compared to the pristine borophene sheet. Further investigation in the series of this noble metal free monolayer catalyst is 2D Al2C monolayer both in form of pristine and functionalized with nitrogen (N), phosphorous (P), boron (B), and sulphur (S). It has been observed that only B substituted Al2C shows very close to thermoneutral, that could be the most promising candidate for HER on functionalized Al2C monolayer. The adsorption of O* intermediate is stronger in S-substituted Al2C, whereas it is less strongly adsorbed on N-substituted Al2C. The subsequent consideration is being the case of n-type doping (W) along with Ti codoped in BiVO4 to enhance the efficiency of BiVO4 photoanode for water splitting. The determined adsorption energy and corresponding Gibbs free energies depict that the Ti site is energetically more favorable for water splitting. Moreover, the Ti site possesses a lower overpotential in the W–Ti codoped sample as compared to the mono-W doped sample. We have also explored the effect of mixed cation and mixed anion substitution in the hybrid perovskite in terms of structural stability, electronic properties and optical response of hybrid perovskite crystal structures. It has been found that the insertion of bromine (Br) into the system could modulate the stability of the Guanidinium lead iodide (GAPbI3) hybrid perovskite.  Moreover, the band gap of the mixed hybrid perovskite is increased with the inclusion of smaller Br anion while replacing partially the larger iodine (I) anion. Finally the electrochemical storage mechanism for Sodium (Na) and lithium (Li) ion insertion has been envisaged in inorganic electrode (eldfellite, NaFe(SO4)2) as well as in more sustainable organic electrode (di-lithium terephthalate, Li2TP). The full desodiation capability of the eldfellite enhances the capacity while the activation energies (higher than 1 eV) for the Na+ ion diffusion for the charged state lower the ionic insertion rate. The key factor as the variation of Li-O coordination in the terephthalate, for the disproportionation redox reaction in Li2TP is also identified.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 96
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1760
Keywords
Materials Modelling, DFT, Energy Materials, Photocatalysis, HER and OER, Hybrid Perovskite Solar Cells, Stability, Thermodynamics and Kinetics in Na-ion battery, Organic Crystal Battery
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-369695 (URN)978-91-513-0544-8 (ISBN)
Public defence
2019-02-15, 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-01-24 Created: 2018-12-19 Last updated: 2019-02-18

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Banerjee, AmitavaChakraborty, SudipAhuja, Rajeev

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