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Photosensitised Hydrogen Evolution on a Floating Electrocatalyst Coupled to Electrochemical Recycling Astrid J. Olaya, Jonnathan C. Hidalgo-Acosta, Terumasa Omatsu, and Hubert H. Girault J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06729 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 1, 2018
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Photosensitised Hydrogen Evolution on a Floating Electrocatalyst Coupled to Electrochemical Recycling Astrid J. Olaya‡, Jonnathan C. Hidalgo-Acosta‡,Terumasa Omatsu†, and Hubert H. Girault*,‡ ‡
Laboratory of Physical and Analytical Electrochemistry, EPFL Valais Wallis, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland. † Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Kyoto, Japan. Supporting Information Placeholder ABSTRACT: Photoexcited protonated tetrathiafulvalene
(HTTF+) was found to act as photosensitizer injecting electrons into Pt microparticles (floating electrocatalysts) to produce H2 and TTF•+ in acidic acetonitrile. In addition, TTF•+ was electrochemically reduced back to TTF on a carbon electrode, to be further protonated to continuously produce H2 photochemically. The onset potential for the electrochemical recycling of TTF•+ on carbon was set at a potential 500 mV more positive than the potential required for the direct reduction of protons. HTTF+ showed no signs of decomposition after 51 h of continuous recycling and photo-induced production of H2 proving stability and reversibility.
Hydrogen evolution, oxygen reduction, and water oxidation are key reactions in the development of electrochemical energy storage systems. 1-8 Metal−molecule interactions play a crucial role in such multi-electron charge-transfer reactions.9 In a recent perspective article, we introduced the term “redox electrocatalysis” as the catalysis of electron-transfer reactions on electrically floating conductive particles. 9 Spiro 10 and Bard 11 pioneered redox electrocatalysis. The first practical examples involved the generation of free radicals in aqueous solutions to charge and discharge colloidal metallic NPs (Pt, 12-15 Ag, 16-17 Au 18-19), inducing changes in their Fermi level accordingly. The resulting charged metallic NPs act as polarized floating “microelectrodes” or “nanoelectrodes”, 20-21 which have been used to catalyze dark and photo-induced hydrogen evolution reaction (HER), 11-12, 14, 19, 22-25 by means of sacrificial (irreversible) molecules. Several examples of HER on floating electrodes based on Pt, Pd, and Pt-CNT among others, or supported on liquid/liquid interfaces have been published 26-27, however, none of those examples has shown the successful recycling of the redox electron donors.
We report here that tetrathiafulvalene (TTF, Scheme 1) when protonated in presence of Pt microparticles (floating electrocatalysts) and under illumination with visible light (455 nm) is capable of continuously reducing protons to H2 at Pt microparticles, while TTF•+ is electrochemically reduced on a carbon electrode back to protonated TTF. After 51 h of photo-HER, the electron donor/photosensitiser HTTF+ did not show evident signs of decomposition. Scheme 1. Chemical structure of TTF, HTTF+ and TTF•+
Batch H2 photogeneration on Pt microparticles as floating electrocatalysts: Photo-HER by HTTF+ was achieved by illumination of suspensions composed of TTF, triflic acid (TA) and Pt microparticles (< 10 µm) in extra dry acetonitrile (MeCN) (Sigma Aldrich, Section SI1). All the experiments were performed in a glove box. The optimal conditions of the photochemical reaction such as: wavelength, power of the Light Emitting Diode (LED), initial concentration of TTF, initial concentration of TA and load of Pt microparticles were found to be: 455 nm, 7.3 mW·cm–2, 3 mM and 1 mg, respectively (Section SI2). The products of the photoreaction were quantified by UV-Visible absorption (UVVis) and gas chromatography (GC) (details in Section SI1). The inset in Figure 1a shows the UV-Vis spectrum of the initial mixture of reaction (diluted 60 times) before irradiation. This spectrum displays maxima at 248, 270 and 465 nm, corresponding to HTTF+. No TTF is observed (TTF lmax= 316 nm) indicating that in presence
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of TA, TTF is completely protonated, and protonation is the first step of the photochemical HER (Eq. 1, Scheme 2).
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action is typical of heterogeneous reactions in which the catalyst forms adducts with the adsorbed product saturating the catalyst, indicating that the desorption of H2 from the catalyst (Eq. 4, Scheme 2) is the limiting step of the global reaction. The photo-HER does not reach 100% efficiency due to the consumption of HTTF+ by the competition reaction between HTTF+ and TTF (Eq. 2, Scheme 2). Scheme 2. Chemical reactions and energy diagram for photochemical HER by HTTF+ in MeCN a
Figure 1. Photocatalytic HER. Initial conditions: 3 mM TTF, 0.1 M TA, 1 mg Pt microparticles, illumination at 455 nm (7.3 mW·cm–2). (a) Evolution of the UV-Vis spectrum over time. Inset: close up of the HTTF+ spectrum before reaction. (b) Evolution of H2 quantified by GC as a function of time of photoreaction.
Two control experiments were conducted: i. TTF and TA in absence of Pt and under illumination, ii. TTF and TA in presence of Pt, without illumination. The UV-Vis absorption spectra of these controls (Section SI2, Fig. SI5a and SI5b) show the disappearance of HTTF+ and the gradual appearance of signals centred at 335, 436 and 582 nm, corresponding to TTF•+.28-29 However, H2 was not detected, indicating that in the controls the oxidation of TTF is triggered by the reaction between HTTF+ and TTF (Eq. 2, Scheme 2). 30-32 In contrast, the photochemical reaction between TTF and TA in presence of Pt yielded both TTF•+ (Fig. 1a) and H2 (Fig. 1b), indicating that the HER by HTTF+ requires both, illumination and Pt.. Considering that TTF•+ is formed from two competition reactions (Eqs. 2 and 3, Scheme 2), the quantification of TTF•+ cannot be used to determine the efficiency of the photo-activated HER over time. Figure 1b shows the evolution of H2 as a function of time (Eq. 3, Scheme 2), which stops when 90 % of the stoichiometric expected amount of H2 was reached, with a rate of reaction equal to 0.008 mM·h–1 and pseudo-zero order of reaction, which is in line with the results found when studying the effect of the catalyst load on the reaction’s rate (Section SI2). The apparent zero order of re-
a
The energy data are on the absolute vacuum scale, and the standard redox potentials on the SHE scale. The Fermi levels of TTF and HTTF+ in MeCN were calculated from the redox potentials in Figure 2a. The data vs SHE was obtained by using the calibrations TTF vs Fc in MeCN 28-29 and Fc vs SHE in MeCN 33.
A control in absence of TTF (only TA, Pt and light, details in Section SI1) produced H2 that accounts for less than 5% of the H2 evolved during the photochemical reaction between TTF and TA on Pt. This is due to lightinduced decomposition of TA, and it was therefore subtracted from the total amount of H2 reported in Figure 1b. In accordance with the concept of redox electrocatalysis on electrically floating metallic particles, 9 the most plausible mechanism for the photo-HER by excited HTTF+ is shown in the energy diagram in Scheme 2: In the dark, the Fermi level of the electrons on the Pt microparticles is determined by the Nernst potential of the redox species in excess in solution, i.e. HTTF+/TTF•+, (bottom line in Scheme 2), which is too low to drive H+ reduction. However, upon irradiation, the photo-excited
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HTTF+* is capable of transferring electrons to Pt microparticles (top line in Scheme 2). Electrons are then injected above the dark Fermi level leading to the reduction of adsorbed protons. Continuous H2 photogeneration: Although the photoHER by HTTF+ and Pt is slow, TTF does not undergo decomposition during the photocatalytic reaction (Figure 1a). In order to prove its potential as reversible electron donor/photosensitiser, the electrochemical recycling of TTF was carried out in-situ while continuously producing H2 photochemically by using the cell illustrated in Scheme 3.
the photo-electrochemical reaction was not reached, due to the excess of protons and the stability of TTF. The faradaic efficiency of the recycling was found to be 98%±5%.
Scheme 3. Continuous photo-HER on Pt microparticles during in situ electrochemical recycling of TTF•+ a .
a
Initial composition of the electrochemical cell: 3 mM TTF, 0.1 M TA and, 1 mg Pt particles (
