Skip to main content
NCBI home page
JACS Au logoLink to JACS Au
. 2021 Nov 11;1(12):2294–2302. doi: 10.1021/jacsau.1c00408

Visible-Light-Driven Water Oxidation on Self-Assembled Metal-Free Organic@Carbon Junctions at Neutral pH

Astrid J Olaya †,*, Julieta S Riva †,‡, Dominika Baster †, Wanderson O Silva †, François Pichard †, Hubert H Girault †,*
PMCID: PMC8715488  PMID: 34977899

Abstract

graphic file with name au1c00408_0008.jpg

Sustainable water oxidation requires low-cost, stable, and efficient redox couples, photosensitizers, and catalysts. Here, we introduce the in situ self-assembly of metal-atom-free organic-based semiconductive structures on the surface of carbon supports. The resulting TTF/TTF•+@carbon junction (TTF = tetrathiafulvalene) acts as an all-in-one highly stable redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven water oxidation reaction (WOR) at neutral pH, eliminating the need for metallic or organometallic catalysts and sacrificial electron acceptors. A water/butyronitrile emulsion was used to physically separate the photoproducts of the WOR, H+ and TTF, allowing the extraction and subsequent reduction of protons in water, and the in situ electrochemical oxidation of TTF to TTF•+ on carbon in butyronitrile by constant anode potential electrolysis. During 100 h, no decomposition of TTF was observed and O2 was generated from the emulsion while H2 was constantly produced in the aqueous phase. This work opens new perspectives for a new generation of metal-atom-free, low-cost, redox-driven water-splitting strategies.

Keywords: water oxidation, hydrogen evolution, photochemistry, sustainable, redox shuttles

Introduction

The increase of the global energy demand and the negative environmental impact triggered by the overexploitation of fossil fuels accentuate the need for the development of clean, efficient, and sustainable energy technologies.1−18 Hydrogen is rapidly becoming the energy vector of choice, and advanced water electrolysis is one of the most efficient approaches to produce high-quality green H2 from renewable energy sources despite their intermittencies.1−3 To carry out what is often referred as artificial photosynthesis to produce hydrogen and oxygen directly from water, one usually combines an antenna such as a dye to absorb the light, a semiconductor to carry out the charge separation, and some electrocatalysts to drive both water reduction and the water oxidation reaction (WOR).1−18 As of today, the photosensitized oxidation of water to oxygen under visible-light irradiation is still the bottleneck of artificial photosynthesis hindering large-scale developments. Light-driven water oxidation on semiconductor photoelectrodes has been extensively studied for nearly five decades with slow progress due to the difficulty of finding electrode materials exhibiting high chemical stability, suitable optoelectronic properties, and high catalytic efficiencies. Materials, such as titanium dioxide, absorbs only a very small part of the solar spectrum.1−18 The sensitization of semiconductors has been significantly improved by using a wide range of metal complexes, organic dyes, and porphyrins.4,12,14,17,19−26 Photocatalytic junctions based on BiVO4, Fe2O3, WO3, and Ag3PO4, among others, have been widely studied for water oxidation in both suspensions and photoelectrochemical devices.8,10,27−33 Metal–molecule interactions play a key role in the water oxidation reaction.34−36 However, the need for sacrificial electron acceptors such as Ag+, Fe3+, and peroxodisulfate (S2O82–) restricts its scalability.37 Silicon and III–V compounds (GaAs, GaP, etc.) exhibit excellent optoelectronic properties but due to their instability under illumination in aqueous solutions, they require high-quality protective layers to prevent photocorrosion.2

On the molecular side, many organometallic compounds have been proposed as antennae and redox catalysts, but none have yet found industrial applications.17 Nonetheless, it can also be noted that some fully organic compounds have been found to be electrocatalytically active for water oxidation on glassy-carbon or Pt working electrodes in the dark.38−40

Due to their strong self-assembly properties, the radical cation of the organic electron donor tetrathiafulvalene (TTF•+) and its derivatives are frequently used as building blocks for the synthesis of highly electrically conducting structures.34,41−43 Such properties have been extensively exploited to develop organic conductors, superconductors, photovoltaic cells, and solar cells, among others.41−47 Metal–ligand coordination strategies have been widely used to create self-assembled three-dimensional TTF-based architectures where the TTF•+ sites enhance conductivity. Indeed, TTF-based metal–organic frameworks (MOFs) show an extended charge transport pathway within the rigid MOF structure,48,49 exhibiting semiconductive properties comparable to that of common organic semiconductors.48,49 Some examples include Zn2(TTFTB) (10–6 S cm–1), Co2(TTFTB) (10–6 S cm–1), Mn2(TTFTB) (10–5 S cm–1), and Cd2(TTFTB) (10–4 S cm–1), TB stands for tetrabenzoate.48,49

(TTF)3(BF4)2 crystals are nonstoichiometric, with TTF moieties composed of TTF° monads interspersed with (TTF+)2 dyads stacked in parallel columns alternating with BF4– layers.34,49,50 This salt behaves as an organic semiconductor, showing a conductivity of 2 × 10–5 S cm–1.34,49,50

To overcome the efficiency, scale-up, and cost-related roadblocks of the conversion of solar energy into H2 through photosensitized water splitting, the development of low-cost and highly stable photosensitizers/redox shuttles and photocatalysts is crucial. Furthermore, a system operating with water at neutral pH and without ion-exchange membranes is highly desirable.

Here, we introduce an efficient and highly stable visible-light-driven WOR by in situ self-assembled semiconductive tetrathiafulvalene-based structures (TTF/TTF•+) deposited on a solid carbon support. A water|butyronitrile emulsion was used as a platform for the spontaneous separation of the generated lipophilic TTF from the generated hydrophilic protons, with O2 evolving as a gas. Such a separation allows the subsequent effective reduction of the protons to H2 and the electrochemical recycling of the electron acceptor TTF•+.

Experimental Section

Synthesis of TTF/TTF•+BF4–

TTF/TTF•+BF4– was synthesized entirely inside a glovebox purged with nitrogen by the stoichiometric oxidation of TTF (99%, Acros, structure in Scheme 1) with dry nitrosonium tetrafluoroborate (NOBF4, 98%, Alfa Aesar) in dry butyronitrile (butyronitrile, 99.9%, Across). Prior to the reaction, butyronitrile was degassed in a Schlenk line and dried with molecular sieves (3 Å, Fisher Chemical) during 3 days inside of a glovebox. For the photochemical experiments, an excess of 5% of TTF was used in the synthesis of TTF•+ in order to consume any excess of oxidant, therefore avoiding oxidation of TTF•+ to TTF2+. The resulting product in solution (Figure SI1) is further referred to as TTF/TTF•+BF4–. After the completion of the reaction, the solution was thoroughly degassed inside of a glovebox in order to evacuate the NO released during the reaction. The TTF/TTF•+ BF4– solution was always kept inside the glovebox and protected from the light.

Scheme 1. Chemical Structures of TTF, HTTF+, and TTF•+.

Scheme 1

Open in a new tab

Cyclic Voltammetry (CV)

A three-electrode configuration composed of a commercial 3 mm diameter glassy-carbon electrode as the working electrode, Pt wire as the counter electrode, and an Ag/AgCl/3 M KCl double-junction reference electrode was used to analyze by CV 1 mM of the synthesized TTF/TTF•+ BF4– in 50 mM tetrabutylammonium tetrafluoroborate (TBABF4, 99%, Acros) in butyronitrile. The scan rate was 10 mV s–1. The CV (Figure SI1) was recorded inside a glovebox with a Metrohm Autolab PGSTAT 302N (Herisau, Switzerland) potentiostat.

For the electrochemical recycling of TTF•+ depicted in Scheme 2, CVs were recorded before and after the recycling to determine and verify the potential to be applied at the anode (WE). A glassy-carbon electrode (D = 3 mm) was used as the WE, and the scan rate was 25 mV ·s–1.

Scheme 2. Visible-Light-Driven WOR by TTF/TTF•+@RVC or TTF/TTF•+@GF Junctions upon In Situ Electrochemical Recycling of TTF•+ and Continuous Reduction of Protons by V2+/Mo2C.

Scheme 2

Open in a new tab

Anodic compartment (left): organic phase (butyronitrile), 1 mM TTF/TTF•+BF4– in 100 mM TBABF4; aqueous phase, pure water. Anode, RVC or GF. Cathodic compartment (right): 100 mM VCl2 in water, neutral pH, 40 mg of Mo2C microparticles. Cathode: Pt mesh. The contents of the septum-sealed compartments were vigorously stirred, and the anodic compartment was illuminated with a 455 nm LED. Constant anode potential electrolysis of TTF: 0.6 V vs an Ag/AgCl/3 M KCl double-junction reference electrode. Bridge: glass wool plug to avoid butyronitrile from entering the cathodic compartment, while the two aqueous phases were kept in contact.

Visible-Light-Driven Water Oxidation by TTF•+*@Carbon Junctions during Constant Anode Potential Electrolysis of TTF Back to TTF•+ and Proton Extraction to Water for Reduction by V2+/Mo2C

The visible-light-driven WOR by TTF/TTF•+@carbon and the in situ electrochemical regeneration of TTF•+ were performed in the anodic compartment of the H-cell depicted in Scheme 2 (left side). The anode was illuminated with a 455 nm LED, and stirred inside of a glovebox filled with N2. The protons resulting upon the WOR were spontaneously transferred to the aqueous phase and then to the cathodic compartment where they were reduced by V2+/Mo2C (Scheme 2, right side) by redox electrocatalysis.36 The two septum-sealed compartments of the H-cell were separated by a glass wool plug, to avoid the mixing of TTF•+ and VCl2. The glass wool plug ensured that the cathodic compartment was purely aqueous and that of the anodic compartment was a biphasic system kept as an emulsion during vigorous stirring.

The emulsion in the anodic compartment (left) was composed of 6 mL of 1 mM TTF/TTF•+BF4– in 100 mM TBABF4 in butyronitrile (organic phase) and 10 mL of pure water (aqueous phase). The pH of the emulsion was initially neutral. Two different carbon anodes (working electrode (WE)) were studied independently: reticulated vitreous carbon (RVC, Duocel) and graphite foil (GF, high-purity flexible graphite foil, SIGRAFLEX).

The cathodic compartment (right) contained 16 mL of 100 mM VCl2 in water at neutral pH and 40 mg of Mo2C microparticles (<10 μm). A Pt mesh (Sigma-Aldrich) was used as the cathode (counter electrode (CE)).

The reference electrode was an Ag/AgCl/3 M KCl double-junction electrode connected to the anodic compartment. The recycling potential of TTF•+ was set at 0.6 V vs the Ag/AgCl/3 M KCl double-junction reference electrode.

The gas products of the visible-light-driven WOR on the anodic side and the HER on the cathodic side were analyzed every 15 min by gas chromatography (GC-TCD-MS). Electrochemical impedance spectroscopy (EIS) under illumination was also performed every 15 min. To avoid overpressure of the system upon gas evolution, which in fact has a detrimental effect in the progress of the WOR and the HER, the recycling and illumination were paused after each GC and EIS analysis, and the cell was opened and degasified with N2. Once the anode and the cathode did not show remanent O2 or H2, respectively, the cell was septum-sealed again and the WOR/electrochemical recycling/proton reduction was resumed.

Three control experiments were performed following the same methodology:

  • Control 1.

    without TTF/TTF•+BF4– in the anodic compartment but with V2+/Mo2C in the cathodic compartment; GF as the anode and Pt mesh as the cathode.

  • Control 2.

    with TTF/TTF•+ BF4– in the anodic compartment but without V2+/Mo2C in the cathodic compartment; GF as the anode and Pt mesh as the cathode.

  • Control 3.

    carbon electrode replaced by platinum mesh with TTF/TTF•+ BF4– in the anodic compartment and with V2+/Mo2C in the cathodic compartment

For all three controls, the reference electrodes and all the other conditions were the same as for the system in Scheme 2. After every experiment the glass H-cell was washed with water, acid, and isopropanol. The carbon electrodes were new for each experiment.

Gas Chromatography (GC)

The headspaces of both septum-sealed compartments of the H-cell shown in Scheme 2 were sampled every 15 min by using a lock-in syringe with a push–pull valve (SGE Analytical Sciences). The gas was then injected into a TRACE 1300 GC equipped with a thermal conductivity detector (TCD) and an ISQ single quadrupole (mass spectrometry detector, MSD) from Thermo Fisher Scientific (GC-TCD-MSD). The chromatographic column was 5 Å molecular sieves, 80/100 mesh. Oxygen was detected with the MSD, using helium as the carrier gas, and H2 was detected with the TCD, using argon as the carrier gas. The instrument was calibrated for O2 and H2 by using 8.0% and 0.8% O2 in N2 or H2 in N2 standards (99%, Carbagas). Prior to injection, the injection line was purged with N2 and the exit of the line was connected to a water trap in order to avoid suction of air during the injections.

Electrochemical Impedance Spectroscopy (EIS)

Each EIS spectrum was recorded after every 15 min of electrolysis of TTF to TTF•+. Prior to impedance spectra recording, stirring was turned off such that the electrode was immersed only in the organic phase. Stirring was then resumed to continue the electrolysis. Conditions: 10 mV amplitude perturbation, 100 kHz to 100 mHz, frequency step 10 points per decade. All experiments were recorded in the dark and under visible-light illumination at 0.6 V vs an aqueous Ag/AgCl/3 M KCl reference electrode.

More details of the methods are given in the Supporting Information.

Results and Discussion

In 2018, we demonstrated that the in situ self-assembly of tetrathiafulvalene (TTF), TTF•+, and BF4– or PF6– on the surface of Pt microparticles in wet acetonitrile is active for the WOR upon visible-light irradiation.34 In short, we had demonstrated that TTF•+* is highly oxidizing and that the molecular assembly TTF/TTF•+BF4–, once photoexcited, can provide the four electrons required to oxidize water when it is dissolved in an organic solvent. However, this system is short-lived because it suffers from a gradual increase in the proton concentration in the wet organic phase and subsequent protonation of TTF in the molecular assemblies, triggering competing reactions such as proton and oxygen reduction34 and preventing the electrochemical recycling of TTF back to TTF•+.

In this work, we eliminate the use of metallic catalysts for the WOR, replacing them with a low-cost, and highly stable, TTF/TTF•+BF4–@carbon junction, which works as an all-in-one redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven WOR at neutral pH. Thus, we take advantage of the possibility of adjusting the Gibbs energy transfer across a polarized water|butyronitrile interface, to separate in situ and in a spontaneous manner the hydrophilic protons from the lipophilic TTF formed upon the visible-light-driven WOR by TTF/TTF•+BF4– assemblies. The goal of such a separation is to prevent TTF from being protonated and therefore allow its electrochemical (in situ) reoxidation to perform continuous photosensitized WOR. Subsequently, the protons generated are chemically consumed in the cathodic compartment by redox electrocatalysis. Indeed, the standard redox potential of the V3+/V2+ couple is −0.26 V and is therefore reducing enough to generate hydrogen on an electrocatalyst such as molybdenum carbide. Here, the strategy is to set up a “proton pump” to extract the photogenerated protons in the anodic compartment toward the cathodic compartment, where they are directly reduced to produce hydrogen. The reason to choose the V3+/V2+ couple lies in the long experience that our group has in producing hydrogen on a very large scale using a vanadium redox flow battery (200 kW–400 kWh).51−53 However, on consideration that our long-term objective is to develop a fully environmentally friendly WOR-HER system, the next step is to improve the HER part of the cell, replacing vanadium by a nonmetallic hydrogen evolution system, which is a work in progress in our laboratory.

Because the primary objective of this work is to demonstrate long-lived water photo-oxidation by TTF/TTF•+BF4– assemblies on carbon electrodes without any metal catalyst, we chose to operate the electrochemical regeneration of TTF•+ from TTF at the carbon electrode by applying a constant anode potential using a potentiostat where the carbon electrode is the working electrode (see Scheme 2). The electrode in the aqueous compartment acts as a counter electrode, and its potential cannot be directly controlled. It is, of course, the locus of a reduction reaction such as the reduction of V3+ to V2+.

Scheme 2 schematically shows the H-cell used to test the visible-light-driven WOR by TTF/TTF•+@RVC and TTF/TTF•+@GF junctions (anodes), coupled to the in situ electrochemical regeneration of TTF•+ in the anodic compartment, and the reduction of protons by V2+/Mo2C in the cathodic compartment, using Pt mesh as a cathode.

Once the visible-light-driven WOR is triggered by illumination of the anodic compartment, the protons that formed are transferred to the aqueous phase of the emulsion, while TTF is transferred to the organic phase, as it is poorly soluble in water, the salt TTF•+BF4– being soluble in both phases. As the two septum-sealed compartments are physically connected by an aqueous glass wool plug, the protons are extracted to the cathodic compartment, where they are reduced by V2+/Mo2C to produce H2. The glass wool plug avoids a convective mixture of TTF•+ and VCl2. Indeed, the transfer of VCl2 to the anodic compartment results in the quenching of the visible-light-driven WOR and, therefore, in the inhibition of oxygen evolution and proton production, as the oxidation of V2+ by TTF•+ is thermodynamically much more feasible than the oxidation of water.

In accordance with the CV recorded prior to the experiment (Figure 1), the onset current of the electrochemical WOR on the carbon electrode is observed beyond 1.15 V, and the electrochemical oxidation of TTF is observed above 0.4 V vs an Ag/AgCl/3 M KCl double-junction reference electrode. Therefore, we set the continuous recycling of TTF•+ at 0.6 V vs Ag/AgCl/3 M KCl, which is about 600 mV less positive than the potential required for the electrochemical WOR on the bare carbon electrode under the conditions of the experiment (Figure 1). It should also be noted that, in the absence of stirring, the two phases separate and the anode is then immersed only in the butyronitrile phase and that at this potential (0.6 V vs Ag/AgCl/3 M KCl) basically no aqueous species, including water, can be oxidized.

Figure 1.

Figure 1

Open in a new tab

The black plot shows the cyclic voltammogram of the organic phase of the anodic compartment shown in Scheme 2 (1 mM TTF•+) prior to the experiment. The green plot corresponds to the CV obtained after 9 h of experiment for control 2: with TTF•+ in the anodic compartment but without V2+/Mo2C in the cathodic compartment and GF as the anode and Pt mesh as the cathode. The reference electrode and all the other conditions were the same as for the system in Scheme 2. A glassy-carbon electrode (D = 3 mm) was used as the WE, with 50 mM TBABF4 as the supporting electrolyte. Scan rate: 25 mV s–1.

As shown in Figure 2, the GC-MSD analyses of the gas accumulated in the headspace of the anodic compartment proved the continuous evolution of O2 upon visible-light-driven WOR by the TTF/TTF•+@GF junction coupled to the electrochemical recycling of TTF•+. Similarly, the GC-TCD analyses of the headspace of the cathodic compartment showed the continuous evolution of H2 upon reduction of the protons produced in the anodic compartment after they were transferred to the cathodic compartment. As expected for the stochiometric WOR, the experiment described in Scheme 2 produced on average 1 mol of O2 per 2 mol of H2. It is also important to note the high purity of the gases formed in both compartments. Indeed, the GC analyses showed that only O2 was produced in the anodic compartment and only H2 was produced in the cathodic compartment and they do not cross to the adjacent compartment during the experiment (Figure SI2).

Figure 2.

Figure 2

Open in a new tab

Analysis of the gas accumulated in the headspace of the two septum-sealed compartments of the H-cell depicted in Scheme 2 (anode, GF; cathode, Pt mesh; RE, Ag/AgCl/3 M KCl double junction). (a) Anodic compartment: production of O2 upon visible-light-driven WOR by TTF/TTF•+@GF coupled to in situ constant anode potential electrolysis of TTF. (b) Cathodic compartment: production of H2 upon continuous reduction of the protons produced in the anodic compartment. The left inset shows the gas measured during control 2. Each peak corresponds to the gas produced in each compartment every 15 min during a 150 min experiment (sampling time).

Figure 3a shows the total number of H2 moles produced and the yield of the WOR defined as YH2,WOR = 2n150 min(H2)/n0(TTF•+) × 100, achieved after 150 min of visible-light driven WOR, constant anode potential electrolysis of TTF, and proton reduction (for more details on the calculation of the efficiencies see eqs SI3–SI6 in the Supporting Information). Figure 3a compares the activity of TTF/TTF•+@RVC and TTF/TTF•+@GF junctions. The TTF/TTF•+@RVC and the TTF/TTF•+@GF junctions achieved an impressive YH2,WOR = 0.5 × 105% and 2.2 × 105% efficiencies, respectively, accounting for a recycling of TTF•+ of 530 and 2200 times, respectively. These results prove that the TTF/TTF•+@carbon systems alone act as a redox shuttle/photosensitizer/catalyst triad for the visible-light-driven WOR, without the need for any metallic catalyst.

Figure 3.

Figure 3

Open in a new tab

Analysis of the efficiency and stability of the TTF/TTF•+@RVC and TTF/TTF•+@GF (and TTF/TTF•+@Pt as control) junctions on the visible-light-driven WOR upon in situ constant anode potential electrolysis of TTF and continuous reduction of protons to H2 (Scheme 2). (a) Overall H2 production in the cathodic compartment. (b) Anode current vs time and charge vs time plots recorded during 16 h of the experiments performed with the TTF/TTF•+@Pt and TTF/TTF•+@GF junctions. When GF was used as the anode, the experiment was turned off every 5 h to change the anode for a new GF. Afterward, the experiment was turned back on. The time depicted in the plot corresponds only to the time during which the light and the recycling were on. (c) Production of H2 quantified every 15 min during a 3 h long experiment for the three systems described in (a). The last column corresponds to an individual quantification after 16 h of recycling.

As a control, a Pt mesh that yielded the same initial current as the GF electrode (Figure 3b) was also tested as an anode. This TTF/TTF•+@Pt junction reached half of the efficiency of the TTF/TTF•+@GF junction (Figure 3a). In addition, as shown in Figure 3b, the TTF/TTF•+@GF junction is electrochemically stable for as long as 5 h, while the stability of the TTF/TTF•+@Pt junction starts to decrease after 1 h of reaction.

The SEM-EDX analyses of the GF (Figure 4a–i) after constant visible-light-driven WOR and anode potential electrolysis of TTF show the self-assembly of micrometer-sized (0.3–2 μm) cauliflower-like arrays, composed by sulfur and fluorine (Figure 4b–i), indicating the self-assembly of TTF/TTF•+BF4– on the surface of the GF. These arrays are likely to be the nuclei of the microtubes (0.1–0.5 μm) observed in Figure 4c–i, which are similar to the microtubes observed in our previous work on Pt microparticles in wet acetonitrile.34 The size of the microtubes is consistent with the smaller diameter of the seminal graphite filaments (ca. 0.3 μm, Figure 4a). Indeed, Favier et al.54 showed the slow electrocrystallization of TTF(Br)x conductive microtubes on Pt nanoparticles deposited on a GF electrode, whose diameters were directly proportional to the diameter of the Pt particles.

Figure 4.

Figure 4

Open in a new tab

SEM analysis. (a) New GF cleaned with water and butyronitrile (ETD SE). (b–f) GF as the anode after 150 min of experiment (Scheme 2); T1D: bright area, higher atomic number (TTF); gray area, lower atomic number (carbon). (g) ETD SE: bright area, self-assembly; dark area, support (GF). (h) EDX composition: sulfur in red (TTF). (i). EDX: composition, fluorine in blue (BF4). (j) Pt mesh as the anode after 150 min of constant anode potential electrolysis (Scheme 2). T1D: bright area, higher atomic number (Pt); gray area, lower atomic number (TTF-based film). (k) ETD SE: bright area, covering film; dark area, Pt mesh.

The growth of conductive TTF/TTF•+ microtubes on the surface of the GF observed by SEM explains the decrease in resistance of the electron transfer through the GF observed by EIS as the visible-light-driven WOR and electrochemical recycling of TTF•+ advanced (Figure 5).

Figure 5.

Figure 5

Open in a new tab

Electrochemical impedance spectra (EIS) recorded during the recycling of TTF•+ (constant anode potential electrolysis upon visible-light-driven WOR, Scheme 2). Each spectrum corresponds to 30 min of constant anode potential electrolysis starting from the experiment without light (dark). Prior to recording of the impedance spectra the stirring was turned off, and after the analysis the stirring was turned back on. Conditions: 10 mV amplitude perturbation, 100 kHz to 100 mHz, frequency step 10 points per decade. All experiments were recorded under visible-light illumination at 0.6 V vs an aqueous Ag/AgCl/3 M KCl reference electrode.

In contrast, for the control where a Pt mesh was used as the anode instead of carbon, the SEM-EDX analyses showed the growth of a TTF-based film on the surface of Pt (Figure 4j,k), which correlates with the increase in capacitance of the TTF/TTF•+@Pt junction as the WOR advances (Figure 5). The EIS and SEM analyses show that the filamentary structure of the GF (electric conductivity 9.1 × 104 S m–1)55 offers countless micrometer-sized seeds for the growth of TTF-based arrays, explaining the better performance of the TTF/TTF•+@GF junction in comparison with the flat Pt mesh that, although it exhibits a much higher conductivity (9 × 106 S m–1), does not offer the micrometer-sized seeds required to grow the TTF-based conductive arrays.

After 5 h of reaction, both the current recorded during the electrochemical recycling of TTF to TTF•+ (Figure 3b) and the efficiency of the visible-light-driven WOR by the TTF/TTF•+@GF junction (Figure 3c) started to decrease due to passivation of the GF. At this point, we switched the light off and stopped the electrochemical process to replace the anode by a clean GF. Once the light and the constant anode potential were turned back on, the current and efficiency recovered to their initial values (Figure 3b,c). The time depicted in Figures 3b,c corresponds to the time during which the light and the recycling were on. The used GF was then cleaned up with butyronitrile and used again, showing a total recovery of the activity.

This process was repeated three times up to 16 h (Figure 3b), exhibiting a stable current and a decrease of 15% in the efficiency of the WOR (YH2,WOR = 1.1 × 106%), which is mainly due to the depletion of the V2+ solution, which was also replaced twice during the 16 h reaction. The fact that the whole system can be turned off and on without evident loss of efficiency and stability is a competitive advantage if the system is to be connected to an intermittent supply of energy. The stability of the TTF/TTF•+@GF junction upon continuous visible-light WOR and in situ constant anode potential electrolysis of TTF was further tested up to 100 h. The results of this experiment showed the constant production of gases, a constant current, and a constant charge increase (Figure SI3) proving the high stability of the photocatalytic system, which could indeed run further, as it did not show signs of decomposition.

The main advantage of the electrochemical recycling of TTF•+ over the use of sacrificial electron acceptors is that no waste products which could decompose and eventually quench and slow down the WOR are produced.37 Attempts of electrochemical recycling of other electron donors/acceptors have been unsuccessful due to their long-term instability.16,56

The UV absorption spectrum (Figure SI4) and MS (Figure SI5) show no evidence of decomposition of the TTF species, showing that the cell can operate on a long-term basis.

To prove that the visible-light-driven WOR is triggered by the TTF/TTF•+@GF junction alone and no other factor was taking place, two control experiments were performed.

Control 1 was made without TTF•+ in the anodic compartment, with V2+/Mo2C in the cathodic compartment, and using GF as the anode and Pt mesh as the cathode (all other conditions were the same as for Scheme 2). This experiment produced neither O2 in the anodic compartment nor H2 in the cathodic compartment, proving that, without the TTF/TTF•+ assemblies, the visible-light-driven WOR is not triggered in the system described in Scheme 2 and therefore no protons were produced to be reduced by V2+/Mo2C in the cathodic compartment, explaining the absence of H2.

Control 2 was made with TTF•+ in the anodic compartment, without V2+/Mo2C in the cathodic compartment, and using GF as the anode and Pt mesh as the cathode (all the other conditions were the same as for Scheme 2). This blank produced a small amount of O2 and H2 in the anodic and cathodic compartments, respectively, accounting for a recycling of TTF•+ of only 35 times, which is far below the recycling achieved when V2+/Mo2C is used for the proton reduction (2200 times). In this blank, the protons obtained upon visible-light-driven WOR by the TTF/TTF•+@GF junction in the anodic compartment were reduced by the Pt mesh alone in the cathodic compartment; however, the pH was not sufficiently low to trigger efficient electrochemical reduction of protons on the Pt mesh at the potential applied during the recycling (0.6 V). The CV recorded after control 2 was performed (green plot in Figure 1) shows the total protonation of TTF upon accumulation of the protons formed during the visible-light-driven WOR by TTF•+ in the anodic compartment.

This explains why the recycling ceases. The oxidation potential of HTTF+ is close to 1.05 V vs the Ag/AgCl/3 M KCl double-junction reference electrode, which is too high in comparison to the potential set for the oxidation of the photogenerated TTF to TTF•+ (0.6 V). Therefore, the protonation of TTF also stops the anode potential electrolysis, the current dropping to almost zero after only 30 min of the experiment. This control proves that, under the conditions of the experiment, the chemical reduction of protons by V2+/Mo2C is much more efficient than that of Pt alone.

The mechanism of the WOR by TTF•+ assemblies at neutral pH (eqs 1 and 2) is the reverse mechanism of that previously proposed for the slow four-electron reduction of oxygen to water by TTF assemblies in acidic pH (see ref (57)). Indeed, in ref (57) it was computationally established that the self-assembly of TTF-TTFH+ dimers could provide the four electrons required to reduce oxygen to water, which is in fact the opposite reaction to the four-electron oxidation of water to O2 studied in this paper.57 However, in ref (57) the self-assembly of TTF was not proved experimentally; therefore, this manuscript offers experimental proof of the computational mechanism proposed in 2011.57 The mechanism proposed in ref (57) involves the helical tetramer [TTF4H2]2+ formed by the interaction of two TTF-TTFH+ dimers [TTF4H2]2+ (eq 1), which delivers the four electrons needed for the reduction of oxygen to water.57

graphic file with name au1c00408_m001.jpg 1

Electrochemical recycling of TTF•+ on GF:

graphic file with name au1c00408_m002.jpg 2

Indeed, [TTF4H2]2+ binds O2 to form a [TTF4H2···O2]2+ intermediate that is converted into the more stable [TTF4H···HO2]2+ complex (eq 1).57 The highly reactive HO2• and TTF•+ moieties of the [TTF4H···HO2]2+ complex combine and dissociate from the tetramer and get the needed protons from the solution to form two molecules of water.57 Counteranion–cation complexes such as (TTFH+)2(BF4–)2 exhibit cation–anion electrostatic interactions rather than direct interactions between the two TTFH+ units.57 This paper offers evidence of the formation of such a TTF-based assembly.

Conclusions

A water|butyronitrile emulsion was successfully used as a platform for the spontaneous separation of the products of the visible-light-driven WOR by self-assembled TTF/TTF•+@GF junctions at neutral pH. Such a separation allowed the successful and efficient electrochemical recycling of TTF back to TTF•+ at 0.6 V vs. Ag/AgCl/3 M KCl in the organic phase, while simultaneously performing the molecular photosensitized WOR. Moreover, the protons partitioned to the aqueous phase were reduced to H2 by V2+ acting as electron donors on Mo2C microparticles acting as redox electrocatalysts. This work shows that, as long as the TTF•+/TTF assemblies have an appropriate substrate with a small grain size to grow on, they do not need any metal or organometallic catalyst to perform the photosensitized WOR efficiently. In fact, the TTF/TTF•+@GF junction showed 2 times the efficiency and 5 times the stability of a TTF/TTF•+@Pt analogue. During 16 h of recycling, no decomposition of TTF was observed and H2 was consistently produced, achieving a yield of H2 production of 1.1 × 106% in relation to the initial concentration of TTF•+. Constant gas production, constant current, and constant increase of the charge were recorded during 100 h of visible-light-induced WOR and in situ constant anode potential electrolysis of TTF, and no signs of decomposition were observed, indicating that the recycling could be continued further. In conclusion, we have demonstrated a sustainable photosensitized WOR system operating in an emulsified system, where a TTF/TTF•+@GF junction performs as a photosensitizer/molecular-catalyst/redox-shuttle triad with the total absence of metallic or organometallic catalysts and sacrificial electron acceptors. This work opens new perspectives for sustainable metal-atom-free visible-light-driven water splitting in biphasic systems at neutral pH.

Acknowledgments

The authors wish to acknowledge the Swiss Science Foundation SNSF grants 200021_175745, Photo Induced Charge Transfer Reaction at Molecular Interfaces, and 20SC-1_193608, Photoproduction of hydrogen in biphasic systems with electron donor recycling. We also thank Dr. Natalia Gasilova for her help with the MS analysis, and Danick Reynard for his help with the SEM analysis.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00408.

  • Methods, mechanism of separation of the products of the visible-light-driven WOR by TTF•+, details on the role of vanadium in the proton reduction reaction, an analysis of the headspace of the two compartments of the H-cell depicted in Scheme 2 after 15 min of experiment, a 100 h study of the stability of the TTF/TTF•+@GF junction upon continuous visible-light WOR and in situ constant anode potential electrolysis of TTF, and the study of the stability of TTF by UV–vis absorption and mass spectrometry (PDF)

The authors declare no competing financial interest.

Supplementary Material

au1c00408_si_001.pdf (1.1MB, pdf)

References

  1. Moniz S. J. A.; Shevlin S. A.; Martin D. J.; Guo Z.-X.; Tang J. Visible-Light Driven Heterojunction Photocatalysts for Water Splitting - a Critical Review. Energy Environ. Sci. 2015, 8 (3), 731–759. 10.1039/C4EE03271C. [DOI] [Google Scholar]
  2. Reece S. Y.; Hamel J. A.; Sung K.; Jarvi T. D.; Esswein A. J.; Pijpers J. J. H.; Nocera D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334 (6056), 645–648. 10.1126/science.1209816. [DOI] [PubMed] [Google Scholar]
  3. Rausch B.; Symes M. D.; Chisholm G.; Cronin L. Decoupled Catalytic Hydrogen Evolution from a Molecular Metal Oxide Redox Mediator in Water Splitting. Science 2014, 345 (6202), 1326–1330. 10.1126/science.1257443. [DOI] [PubMed] [Google Scholar]
  4. Aratani N.; Kim D.; Osuka A. Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna. Acc. Chem. Res. 2009, 42 (12), 1922–1934. 10.1021/ar9001697. [DOI] [PubMed] [Google Scholar]
  5. Barber J. Photosynthetic Energy Conversion: Natural and Artificial. Chem. Soc. Rev. 2009, 38 (1), 185–196. 10.1039/B802262N. [DOI] [PubMed] [Google Scholar]
  6. Bard A. J.; Fox M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28 (3), 141–145. 10.1021/ar00051a007. [DOI] [Google Scholar]
  7. Abe R.; Sayama K.; Arakawa H. Significant Effect of Iodide Addition on Water Splitting into H2 and O2 over Pt-Loaded TiO2 Photocatalyst: Suppression of Backward Reaction. Chem. Phys. Lett. 2003, 371, 360. 10.1016/S0009-2614(03)00252-5. [DOI] [Google Scholar]
  8. Abe R.; Sayama K.; Domen K.; Arakawa H. A New Type of Water Splitting System Composed of Two Different TiO2 Photocatalysts (Anatase, Rutile) and a IO3-/I- Shuttle Redox Mediator. Chem. Phys. Lett. 2001, 344 (3–4), 339–344. 10.1016/S0009-2614(01)00790-4. [DOI] [Google Scholar]
  9. Abe R.; Sayama K.; Sugihara H. Development of New Photocatalytic Water Splitting into H2 and O2 Using Two Different Semiconductor Photocatalysts and a Shuttle Redox Mediator IO3-/I. J. Phys. Chem. B 2005, 109 (33), 16052–16061. 10.1021/jp052848l. [DOI] [PubMed] [Google Scholar]
  10. Sivula K.; Formal F. L.; Grätzel M. WO3-Fe2O3 Photoanodes for Water Splitting: A Host Scaffold, Guest Absorber Approach. Chem. Mater. 2009, 21 (13), 2862–2867. 10.1021/cm900565a. [DOI] [Google Scholar]
  11. Sivula K.; Zboril R.; Le Formal F.; Robert R.; Weidenkaff A.; Tucek J.; Frydrych J.; Grätzel M. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. J. Am. Chem. Soc. 2010, 132 (21), 7436–7444. 10.1021/ja101564f. [DOI] [PubMed] [Google Scholar]
  12. Moon S. J.; Baranoff E.; Zakeeruddin S. M.; Yeh C. Y.; Diau E. W. G.; Grätzel M.; Sivula K. Enhanced Light Harvesting in Mesoporous TiO2/P3HT Hybrid Solar Cells Using a Porphyrin Dye. Chem. Commun. 2012, 47 (29), 8244–8246. 10.1039/c1cc12251g. [DOI] [PubMed] [Google Scholar]
  13. Yu Z.; Li F.; Sun L. Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components. Energy Environ. Sci. 2015, 8 (3), 760–775. 10.1039/C4EE03565H. [DOI] [Google Scholar]
  14. Yu J.; Mathew S.; Flavel B. S.; Johnston M. R.; Shapter J. G. Ruthenium Porphyrin Functionalized Single-Walled Carbon Nanotube Arrays - A Step toward Light Harvesting Antenna and Multibit Information Storage. J. Am. Chem. Soc. 2008, 130 (27), 8788–8796. 10.1021/ja801142k. [DOI] [PubMed] [Google Scholar]
  15. Yu Z.-Y.; Duan Y.; Gao M.-R.; Lang C.-C.; Zheng Y.-R.; Yu S.-H. A One-Dimensional Porous Carbon-Supported Ni/Mo2C Dual Catalyst for Efficient Water Splitting. Chem. Sci. 2017, 8 (2), 968–973. 10.1039/C6SC03356C. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Symes M. D.; Cronin L. Decoupling Hydrogen and Oxygen Evolution during Electrolytic Water Splitting Using an Electron-Coupled-Proton Buffer. Nat. Chem. 2013, 5, 403. 10.1038/nchem.1621. [DOI] [PubMed] [Google Scholar]
  17. Das B.; Thapper A.; Ott S.; Colbran S. B. Structural Features of Molecular Electrocatalysts in Multi-Electron Redox Processes for Renewable Energy - Recent Advances. Sustain. Energy. Fuels 2019, 3 (9), 2159–2175. 10.1039/C9SE00280D. [DOI] [Google Scholar]
  18. Chen L.; Dong X.; Wang Y.; Xia Y. Separating Hydrogen and Oxygen Evolution in Alkaline Water Electrolysis Using Nickel Hydroxide. Nat. Commun. 2016, 7 (1), 11741. 10.1038/ncomms11741. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Adachi M.; Okada I.; Ngamsinlapasathian S.; Murata Y.; Yoshikawa S. Dye-Sensitized Solar Cells Using Semiconductor Thin Film Composed of Titania Nanotubes. Electrochemistry 2002, 70 (6), 449–452. 10.5796/electrochemistry.70.449. [DOI] [Google Scholar]
  20. Alivisatos A. P. Semiconductor Clusters, Nanocrystals, and Quantm Dots. Science 1996, 271, 933. 10.1126/science.271.5251.933. [DOI] [Google Scholar]
  21. Chen X. B.; Shen S. H.; Guo L. J.; Mao S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503. 10.1021/cr1001645. [DOI] [PubMed] [Google Scholar]
  22. Hisatomi T.; Kubota J.; Domen K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43 (22), 7520–7535. 10.1039/C3CS60378D. [DOI] [PubMed] [Google Scholar]
  23. Kamat P. V.; Tvrdy K.; Baker D. R.; Radich J. G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells. Chem. Rev. 2010, 110 (11), 6664–6688. 10.1021/cr100243p. [DOI] [PubMed] [Google Scholar]
  24. Campbell W. M.; Jolley K. W.; Wagner P.; Wagner K.; Walsh P. J.; Gordon K. C.; Schmidt-Mende L.; Nazeeruddin M. K.; Wang Q.; Grätzel M.; Officer D. L. Highly Efficient Porphyrin Sensitizers for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2007, 111 (32), 11760–11762. 10.1021/jp0750598. [DOI] [Google Scholar]
  25. Chen K. S.; Liu W. H.; Wang Y. H.; Lai C. H.; Chou P. T.; Lee G. H.; Chen K.; Chen H. Y.; Chi Y.; Tung F. C. New Family of Ruthenium-Dye-Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar-Energy-Conversion Efficiency. Adv. Funct. Mater. 2007, 17 (15), 2964–2974. 10.1002/adfm.200600985. [DOI] [Google Scholar]
  26. Feldt S. M.; Gibson E. A.; Gabrielsson E.; Sun L.; Boschloo G.; Hagfeldt A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132 (46), 16714–16724. 10.1021/ja1088869. [DOI] [PubMed] [Google Scholar]
  27. Fu X.; Xie M.; Luan P.; Jing L. Effective Visible-Excited Charge Separation in Silicate-Bridged ZnO/BiVO4 Nanocomposite and Its Contribution to Enhanced Photocatalytic Activity. ACS Appl. Mater. Interfaces 2014, 6 (21), 18550–18557. 10.1021/am505651d. [DOI] [PubMed] [Google Scholar]
  28. Ho-Kimura S.; Moniz S. J. A.; Handoko A. D.; Tang J. Enhanced Photoelectrochemical Water Splitting by Nanostructured BiVO4-TiO2 Composite Electrodes. J. Mater. Chem. A 2014, 2 (11), 3948–3953. 10.1039/c3ta15268e. [DOI] [Google Scholar]
  29. Hong S. J.; Lee S.; Jang J. S.; Lee J. S. Heterojunction BiVO4/WO3 Electrodes for Enhanced Photoactivity of Water Oxidation. Energy Environ. Sci. 2011, 4 (5), 1781–1787. 10.1039/c0ee00743a. [DOI] [Google Scholar]
  30. Kudo A.; Ueda K.; Kato H.; Mikami I. Photocatalytic O2 Evolution under Visible Light Irradiation on BiVO4 in Aqueous AgNO3 Solution. Catal. Lett. 1998, 53 (3), 229–230. 10.1023/A:1019034728816. [DOI] [Google Scholar]
  31. Pihosh Y.; Turkevych I.; Mawatari K.; Asai T.; Hisatomi T.; Uemura J.; Tosa M.; Shimamura K.; Kubota J.; Domen K.; Kitamori T. Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting. Small 2014, 10 (18), 3692–3699. 10.1002/smll.201400276. [DOI] [PubMed] [Google Scholar]
  32. Bignozzi C. A.; Caramori S.; Cristino V.; Argazzi R.; Meda L.; Tacca A. Nanostructured Photoelectrodes Based on WO3: Applications to Photooxidation of Aqueous Electrolytes. Chem. Soc. Rev. 2013, 42 (6), 2228–2246. 10.1039/C2CS35373C. [DOI] [PubMed] [Google Scholar]
  33. Bi Y.; Ouyang S.; Cao J.; Ye J. Facile Synthesis of Rhombic Dodecahedral AgX/Ag3PO4 (X = Cl, Br, I) Heterocrystals with Enhanced Photocatalytic Properties and Stabilities. Phys. Chem. Chem. Phys. 2011, 13 (21), 10071–10075. 10.1039/c1cp20488b. [DOI] [PubMed] [Google Scholar]
  34. Olaya A. J.; Omatsu T.; Hidalgo-Acosta J. C.; Riva J. S.; Bassetto V. C.; Gasilova N.; Girault H. H. A Self-Assembled Organic/Metal Junction for Water Photo-Oxidation. J. Am. Chem. Soc. 2019, 141 (16), 6765–6774. 10.1021/jacs.9b02693. [DOI] [PubMed] [Google Scholar]
  35. Olaya A. J.; Hidalgo-Acosta J. C.; Omatsu T.; Girault H. H. Photosensitized Hydrogen Evolution on a Floating Electrocatalyst Coupled to Electrochemical Recycling. J. Am. Chem. Soc. 2018, 140 (32), 10149–10152. 10.1021/jacs.8b06729. [DOI] [PubMed] [Google Scholar]
  36. Peljo P.; Scanlon M. D.; Olaya A. J.; Rivier L.; Smirnov E.; Girault H. H. Redox Electrocatalysis of Floating Nanoparticles: Determining Electrocatalytic Properties without the Influence of Solid Supports. J. Phys. Chem. Lett. 2017, 8 (15), 3564–3575. 10.1021/acs.jpclett.7b00685. [DOI] [PubMed] [Google Scholar]
  37. Schneider J.; Bahnemann D. W. Undesired Role of Sacrificial Reagents in Photocatalysis. J. Phys. Chem. Lett. 2013, 4 (20), 3479–3483. 10.1021/jz4018199. [DOI] [Google Scholar]
  38. Mirzakulova E.; Khatmullin R.; Walpita J.; Corrigan T.; Vargas-Barbosa N. M.; Vyas S.; Oottikkal S.; Manzer S. F.; Hadad C. M.; Glusac K. D. Electrode-Assisted Catalytic Water Oxidation by a Flavin Derivative. Nat. Chem. 2012, 4 (10), 794–801. 10.1038/nchem.1439. [DOI] [PubMed] [Google Scholar]
  39. Li H.; Xie F.; Zhang M.-T. Metal-Free Electrocatalyst for Water Oxidation Initiated by Hydrogen Atom Transfer. ACS Catal. 2021, 11 (1), 68–73. 10.1021/acscatal.0c04606. [DOI] [Google Scholar]
  40. Datta A.; Kapri S.; Bhattacharyya S. Carbon Dots with Tunable Concentrations of Trapped Anti-Oxidant as an Efficient Metal-Free Catalyst for Electrochemical Water Oxidation. J. Mater. Chem. A 2016, 4 (38), 14614–14624. 10.1039/C6TA04737H. [DOI] [Google Scholar]
  41. Bigot J.; Charleux B.; Cooke G.; Delattre F.; Fournier D.; Lyskawa J.; Sambe L.; Stoffelbach F.; Woisel P. Tetrathiafulvalene End-Functionalized Poly(N-Isopropylacrylamide): A New Class of Amphiphilic Polymer for the Creation of Multistimuli Responsive Micelles. J. Am. Chem. Soc. 2010, 132 (31), 10796–10801. 10.1021/ja1027452. [DOI] [PubMed] [Google Scholar]
  42. Giffard M.; Gorgues A.; Riou A.; Roncali J.; Alonso P.; Uriel S.; GarÌn J.; Nguyen T. P. Generation of Radicals and Formation of Conducting Materials by Protic Doping of Tetrahiafulvalenes. Synth. Met. 1995, 70, 1133–1134. 10.1016/0379-6779(94)02787-Y. [DOI] [Google Scholar]
  43. Akutagawa T.; Kakiuchi K.; Hasegawa T.; Noro S.-I.; Nakamura T.; Hasegawa H.; Mashiko S.; Becher J. Molecularly Assembled Nanostructures of a Redox-Active Organogelator. Angew. Chem., Int. Ed. 2005, 44 (44), 7283–7287. 10.1002/anie.200502336. [DOI] [PubMed] [Google Scholar]
  44. Halling M. D.; Bell J. D.; Pugmire R. J.; Grant D. M.; Miller J. S. Solid-State NMR Spectra and Long, Intra-Dimer Bonding in the (-[TTF]22+ (TTF = Tetrathiafulvalene) Dication. J. Phys. Chem. A 2010, 114 (24), 6622–6629. 10.1021/jp910509f. [DOI] [PubMed] [Google Scholar]
  45. Sun W.; Xu C.-H.; Zhu Z.; Fang C.-J.; Yan C.-H. Chemical-Driven Reconfigurable Arithmetic Functionalities within a Fluorescent Tetrathiafulvalene Derivative. J. Phys. Chem. C 2008, 112 (43), 16973–16983. 10.1021/jp804628u. [DOI] [Google Scholar]
  46. Kobayashi Y.; Yoshioka M.; Saigo K.; Hashizume D.; Ogura T. Hydrogen-Bonding-Assisted Self-Doping in Tetrathiafulvalene (TTF) Conductor. J. Am. Chem. Soc. 2009, 131 (29), 9995–10002. 10.1021/ja809425b. [DOI] [PubMed] [Google Scholar]
  47. Wenger S.; Bouit P.-A.; Chen Q.; Teuscher J.; Censo D. D.; Humphry-Baker R.; Moser J.-E.; Delgado J. L.; Martin N.; Zakeeruddin S. M.; Grätzel M. Efficient Electron Transfer and Sensitizer Regeneration in Stable t-Extended Tetrathiafulvalene-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132 (14), 5164–5169. 10.1021/ja909291h. [DOI] [PubMed] [Google Scholar]
  48. Sun L.; Campbell M. G.; Dincǎ M. Electrically Conductive Porous Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2016, 55 (11), 3566–3579. 10.1002/anie.201506219. [DOI] [PubMed] [Google Scholar]
  49. Jana A.; Bähring S.; Ishida M.; Goeb S.; Canevet D.; Sallé M.; Jeppesen J. O.; Sessler J. L. Functionalised Tetrathiafulvalene- (TTF-) Macrocycles: Recent Trends in Applied Supramolecular Chemistry. Chem. Soc. Rev. 2018, 47 (15), 5614–5645. 10.1039/C8CS00035B. [DOI] [PubMed] [Google Scholar]
  50. Legros J.-P.; Bousseau M.; Valade L.; Cassoux P. Crystal Structure of a Non-Conductive Non-Stoichiometric Tetrathiafulvalenium Salt: (TTF)3(BF4)2. Mol. Cryst. Liq. Cryst. 1983, 100 (1–2), 181–192. 10.1080/00268948308073731. [DOI] [Google Scholar]
  51. Peljo P.; Vrubel H.; Amstutz V.; Pandard J.; Morgado J.; Santasalo-Aarnio A.; Lloyd D.; Gumy F.; Dennison C. R.; Toghill K. E.; Girault H. H. All-Vanadium Dual Circuit Redox Flow Battery for Renewable Hydrogen Generation and Desulfurisation. Green Chem. 2016, 18 (6), 1785. 10.1039/C5GC02196K. [DOI] [Google Scholar]
  52. Amstutz V.; Toghill K. E.; Powlesland F.; Vrubel H.; Comninellis C.; Hu X.; Girault H. H. Renewable Hydrogen Generation from a Dual-Circuit Redox Flow Battery. Energy Environ. Sci. 2014, 7 (7), 2350–2358. 10.1039/C4EE00098F. [DOI] [Google Scholar]
  53. Reynard D.; Girault H. Combined Hydrogen Production and Electricity Storage Using a Vanadium-Manganese Redox Dual-Flow Battery. Cell Reports Physical Science 2021, 2, 100556. 10.1016/j.xcrp.2021.100556. [DOI] [Google Scholar]
  54. Favier F.; Liu H.; Penner R. M. Size-Selective Growth of Nanoscale Tetrathiafulvalene Bromide Crystallites on Platinum Particles. Adv. Mater. 2001, 13, 1567–1570. . [DOI] [Google Scholar]
  55. Castañeda L. F.; Walsh F. C.; Nava J. L.; Ponce de León C. Graphite Felt as a Versatile Electrode Material: Properties, Reaction Environment, Performance and Applications. Electrochim. Acta 2017, 258, 1115–1139. 10.1016/j.electacta.2017.11.165. [DOI] [Google Scholar]
  56. Norris M. R.; Cossairt B. M. CdSe on a Mesoporous Transparent Conducting Oxide Scaffold as a Photocathode. J. Mater. Chem. A 2015, 3 (28), 14585–14591. 10.1039/C5TA03910J. [DOI] [Google Scholar]
  57. Olaya A. J.; Ge P.; Gonthier J. F.; Pechy P.; Corminboeuf C.; Girault H. H. Four-Electron Oxygen Reduction by Tetrathiafulvalene. J. Am. Chem. Soc. 2011, 133 (31), 12115–12123. 10.1021/ja203251u. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

au1c00408_si_001.pdf (1.1MB, pdf)

Articles from JACS Au are provided here courtesy of American Chemical Society

RESOURCES