Enhanced Hydrogen Production with Chiral Conductive Polymer

Article pubs.acs.org/JPCC

Enhanced Hydrogen Production with Chiral Conductive PolymerBased Electrodes Published as part of The Journal of Physical Chemistry virtual special issue “Veronica Vaida Festschrift”. Francesco Tassinari,†,§ Koyel Banerjee-Ghosh,†,§ Francesca Parenti,‡ Vankayala Kiran,† Adele Mucci,‡ and Ron Naaman*,† †

Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, Modena 41125, Italy

‡

S Supporting Information *

ABSTRACT: Efficient photo-electrochemical production of hydrogen from water is the aim of many studies in recent decades. Typically, one observes that the electric potential required to initiate the process significantly exceeds the thermodynamic limit. It was suggested that by controlling the spins of the electrons that are transferred from the solution to the anode, and ensuring that they are coaligned, the threshold voltage for the process can be decreased to that of the thermodynamic voltage. In the present study, by using anodes coated with chiral conductive polymer, the hydrogen production from water is enhanced, and the threshold voltage is reduced, as compared with anodes coated with achiral polymer. When CdSe quantum dots were embedded within the polymer, the current density was doubled. These new results point to a possible new direction for producing inexpensive, environmentally friendly, efficient water-splitting photoelectrochemical cells.

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INTRODUCTION

radicals may approach each other on the singlet surface, resulting in the formation of the singlet hydrogen peroxide. Although reducing the overpotential and avoiding byproducts are important steps toward producing efficient photovoltaic cells for hydrogen production, they are not enough. It is essential to have a high current density that will make the process applicable for producing hydrogen. Typically, when the anode in the cell is coated with organic molecules the current density is reduced dramatically as compared with bare titania. To overcome the problem of low current density, we developed a photo-electrochemical cell in which the anode is coated with a chiral conductive polymer. The concept of a conductive polymer-coated anode was proposed before,15,16 since the conductive polymers have several clear advantages. They are relatively inexpensive, they are nontoxic, and they absorb light in the visible range and therefore can be combined with a solar-driven process. The conductivity of the polymer serves both for absorbing light, since the band gap energy corresponds to absorption in the visible range, and for allowing low resistance between the solution and the titania electrode. The chiral conductive polymer, applied in the present study, has another benefit: the chirality of the polymer ensures the low

The search for an efficient method for photoassisted hydrogen production has been the focus of intensive studies.1 The goal is to find an inexpensive catalyst, to use electric potential as close as possible to the thermodynamic limit, and to have high current density.2 Several conclusions were reached after years of studies. The first is the observation that typically one needs a relatively high overpotential, of ∼0.6 V, for producing hydrogen from water.3−9 It has been proposed that magnetic electrodes, in which the electron spins are coaligned, might overcome this problem.10Indeed, several catalysts were found that allow the process to occur at a lower overpotential. Those catalysts typically contained heavy metals or ferromagnetic materials.11 The other problem found is the production of damaging byproducts, like hydrogen peroxide.12 Recently we found that, when the anode, in the photoelectrochemical cell, is coated with chiral molecules, the overpotential is reduced,13 and the production of hydrogen peroxide is eliminated.14 These observations were rationalized by the requirement to have electron spin correlation in the OH radicals formed at the anode. Since these radicals combine to form the oxygen molecule in its triplet ground state, the spins of the two unpaired electrons on two OH radicals must be aligned parallel to each other to form the oxygen on the triplet potential energy surface. If the spins are not correlated, the two © XXXX American Chemical Society

Received: May 3, 2017 Revised: June 21, 2017 Published: June 26, 2017 A

DOI: 10.1021/acs.jpcc.7b04194 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (A) Molecular structure of the achiral polymer (AP) and chiral polymer (CP). (B) SEM image of the cross section of a photoanode in which a CP monolayer is adsorbed on TiO2−FTO glass. The average thickness of the TiO2 layer is ∼4 μm. The fact that it is not possible to see aggregates on top of the TiO2 confirms the uniformity of the thin polymer layer.

studied by applying conducting probe atomic force microscopy (AFM).22,23 Fabrication of Photoelectrodes. To prepare the anode, TiO2 nanoparticles (Sigma-Aldrich, diameter < 25 nm) were deposited onto a fluorine-doped tin oxide (FTO)-coated glass surface. The deposition was performed using the electrophoretic deposition (EPD) technique reported before.13 The TiO2 electrodes were functionalized with CP and AP (Figure 1A), taking advantage of the reactivity of the carboxylic acid moiety present in the polymer’s side chains with the hydroxyl groups on the TiO2 surface,24 thus forming a self-assembled monolayer (Figure 1B). The monolayers were fabricated by immersing the TiO2 electrodes into a 0.7 mg/mL polymer solution, using dimethyl sulfoxide (DMSO) as solvent, for 24 h. Spin selectivity in the transport through the CP was obtained only for the case in which the polymer formed a monolayer. When a thicker layer was formed, using spin coating, no spinselective transport could be detected. Preparation of CdSe-Functionalized Photoelectrodes. Hybrid polymer/CdSe QD-coated anodes were prepared in two different structures. In the first, the CdSe QDs were embedded into the polymer matrix (CdSe@CP and CdSe@ AP). To prepare the composites of CdSe QDs and CP or AP, 2.5 mg of CdSe QDs was dissolved in 2.5 mL of tetrahydrofuran (THF). Then, 2.5 mg of polymer and a few drops of DMSO were mixed in the solution and were sonicated for 1 h. This solution was then spin-coated on top of the TiO2 electrodes at 2000 rpm for 40 s, and the samples were dried with N2. In the second configuration, CdSe QD-functionalized photoanodes (CdSe-CP and CdSe-AP) were produced by dipping the polymer-functionalized TiO2 electrodes in a 2.5 mM toluene solution of CdSe QDs for 3 h, followed by rinsing with toluene to remove the loosely bound QDs and then drying

overpotential and avoids the formation of undesired byproducts. In the present study, we aimed at combining the advantages of having a chiral anode with those of the conductive polymers, to obtain high current density, low overpotential, and byproduct-free hydrogen production. To this end, we studied a photoanode prepared using titania (TiO2) nanoparticles coated with a chiral poly(fluorene-co-thiophene) (CP), whereas the relative achiral polymer (AP) was used as the reference material. To additionally improve the current densities, we conducted a series of experiments, whereby the polymer was decorated with CdSe quantum dots (QDs) to further increase the absorption window of the polymer. A composite of the polymer and CdSe quantum dots was prepared, and photoelectrochemical activity was studied. In this work another study was conducted, where the polymer layer was modified by making a chemisorbed layer of CdSe quantum dots on the polymer surface. Here, we used a mixture of 0.35 M Na2SO3 and 0.25 M Na2S aqueous solution (pH = 9.5) as the electrolyte. The Na2S sacrificial reagent plays the role of a hole scavenger, and it is oxidized to S22− to prevent the photocorrosion of CdSe.13−21

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EXPERIMENTAL SECTION Polymer Synthesis and Characterization. The polymers were synthesized following the procedures given in the Supporting Information and structurally characterized using nuclear magnetic resonance (NMR) spectroscopy and gelpermeation chromatography. The analysis confirms that the chemical structure and composition of the two polymers are basically the same, so that the differences seen in the photoelectrochemical behavior can only be related to the fact that one polymer has a chiral side-chain, whereas the other does not. The spin transport properties through the polymers were B


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electrochemical hydrogen production was measured using chronoamperometry at an applied potential of 1.7 V versus Ag/AgCl. In addition, the TiO2 electrodes functionalized with the CdSe-polymer composite (CdSe@CP and CdSe@AP) were characterized by EIS. The measurements were performed in 0.1 M Na2SO4 solution as the electrolyte in the potential range from −0.8 to 0.4 V versus Ag/AgCl with a scan rate of 2 mV/s and an imposed bias of 20 mV both in dark and under illumination. The current density as a function of voltage was measured for the CdSe-polymer composite-coated TiO2 electrode in the potential range from −0.4 to 0.7 V with a scan rate of 20 mV/s in dark and under illumination. Finally, EIS measurements were also performed on the TiO2 electrodes having CdSe QDs adsorbed on the surface of the polymeric monolayer (CdSe-CP and CdSe-AP), using a mixture of 0.35 M Na2SO3 and 0.25 M Na2S aqueous solution (pH 9.5) as the electrolyte. The measurements were performed for the CP- and AP-functionalized TiO2 electrodes before and after modification with CdSe QDs in the potential range from −1.2 to 0.4 V with a scan rate of 2 mV/s and with an imposed potential of 20 mV. The current density was measured as a function of voltage versus Ag/AgCl in the potential range from −1 to 0.5 V and from −1 to 1.5 V with a scan rate of 20 mV/s. Here the sulfide ion (S2−) plays the role of the sacrificial electrolyte and inhibits the photocorrosion of CdSe. The addition of Na2SO3 results in the reduction of disulfides to sulfides and enhances the hydrogen generation.27 The gas evolution using CdSe QDs-CP functionalized TiO2 was measured at an applied potential of 0.1 and 0.4 V, whereas that of the CdSe-AP-functionalized TiO2 electrodes was measured at an applied potential of 0.4 V. The photoelectrochemical measurement was performed in both dark and under illumination.

the surfaces with N2. UV−vis absorption spectroscopic measurements (Figure S3) show that similar amount of CdSe QDs were loaded into both polymers electrodes. Cyclic Voltammetry. Electrochemical measurements were performed to verify the chiral properties of the anode coated with CP.25 The chirality was verified by using either (R)- or (S)-N,N-dimethyl-1-ferrocenylethylamine (Sigma-Aldrich) as the redox couple in solution.26 We used a three-electrode electrochemical cell, with a Pt wire as the counter electrode, a Ag/AgCl (saturated KCl) reference electrode, and a selfassembled layer of CP on gold as the working electrode. The working electrodes were prepared by incubating the gold surfaces in the polymer solution (2.5 mg/mL, DMSO) for 24 h. Electrolyte solutions were prepared using the redox probes (R)or (S)-N,N-dimethyl-1-ferrocenylethylamine in 0.1 M phosphate buffer solution (pH 7.2) with the same concentration (0.1 mM). The measurements were performed in the potential range from 0 to 0.6 V versus Ag/AgCl at a scan rate of 100 mV/s. For comparison, the cyclic voltammetry was also performed using a bare gold surface as the working electrode under identical experimental conditions. Photolectrochemistry. Mott−Schottky electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate how the functionalization of the TiO2 electrodes affected the polymers (see the Supporting Information). The results show that the flat band potential (EFB) of the photoanodes has the same value for both polymers, confirming the similar nature of the two polymers and their similar interaction with the TiO2 surfaces. Figure 2 schematically

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RESULTS AND DISCUSSION Figure 3A shows the cyclic voltammetry characteristics of (R)and (S)-N,N-dimethyl-1-ferrocenylethylamine using a CP selfassembled layer on a gold substrate and a bare gold substrate (Figure 3B). The cyclic voltammograms (CVs) show that the anodic peak potential and the peak separation for the two enantiomers are the same when we used a bare gold substrate as the working electrode. However, when a CP-coated gold surface was used as the working electrode, the anodic peak potential and the peak separation are different for the two enantiomers. This difference in the CV indicates that the CPcoated gold electrode is enantioselective, thus demonstrating that the chiral group in the side chain of the polymer influences its electrochemical properties. The conducting probe AFM study (Figure 3C) also shows the transport through the CP is spin-dependent, with a spin ratio reaching 1:2 at 1 V. Initially we tested the photo-electrochemical process by coating the anode either with CP or AP polymers. Measurements were conducted both in dark and under illumination from a sun-simulating light source (xenon lamp, 80 mW/cm2 intensity). Figure 4A presents the current density measured, where the blue and red curves correspond to the CP and AP coating, respectively. The dotted and solid lines refer to the current density measured in dark and when the cell was illuminated, respectively. It is clear from the results that both in dark and with light the threshold voltage is lowered by ∼0.2 V in the case of CP as compared with AP. Accordingly, the current density is much higher in the case of CP versus AP,

Figure 2. Schematic diagram of the photo-electrochemical cell illustrating the reduction processes, H2 production, at the counter electrode and oxygen evolution at the photoanode, which is composed of CP or an AP polymer-modified TiO2 surface (working electrode).

presents the three electrodes of the photo-electrochemical cell used in the experiments. A Pt wire was used as the counter electrode, and Ag/AgCl (saturated KCl) was the reference electrode. The measurements were conducted in 0.1 M Na2SO4 solution (pH 6.5) in the potential range from −0.8 to 0.4 V versus Ag/AgCl with a scan rate of 2 mV/s and with an imposed bias of 20 mV both in dark and under illumination. With the same experimental configuration, the photo-electrochemical activity of the polymer-modified TiO2 was observed by measuring the current density as a function of voltage versus Ag/AgCl using linear sweep voltammetry (LSV) in the potential range from −0.2 to 0.7 V with a scan rate of 20 mV/s, both in dark and under illumination. The photoC


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Figure 3. (A) The CV characteristics of (R)- and (S)-N,N-dimethyl-1-ferrocenylethylamine in 0.1 M phosphate buffer solution, pH 7.2, using a CP self-assembled layer on gold substrate as the anode. (B) The CV measured on the same system but with bare gold substrate as the anode. (C) Average I−V curves obtained for CP-modified Ni/Au substrates for magnet up (blue) and down (red). (inset) A schematic representation of a conducting probe AFM setup used for spin-dependent transport measurements. N and S represent the magnetic poles of the permanent magnet used. A conducting tip (Pt-coated Si) was used with CP- or AP-coated Ni film (120 nm) covered with Au (8 nm) film.

Figure 4. (A) Current density as a function of applied potential vs Ag/AgCl with a scan rate of 20 mV/s. The measurements were performed in dark (dotted lines) and under illumination (solid lines) in 0.1 M Na2SO4 (pH = 6.5) aqueous solution for anodes coated with CP and AP, respectively. (B) Photo-electrochemical hydrogen production as a function of time for anodes coated with CP and AP molecules (blue and red lines, respectively) at an applied potential of 1.7 V in 0.1 M Na2SO4 (pH = 6.5) aqueous solution.

cm−2, as was observed before for organic molecule-coated electrodes.12 In an attempt to increase the current density of the process, CdSe QDs were added to the polymers (CdSe@CP and CdSe@AP) that coated the anode, following two different strategies. In the first, the CdSe QDs were added to the solution of the polymers, and the mixture was spin-coated together (see the Experimental section). Here a large percentage of the QDs are typically not in direct contact with

with a ratio of ∼4 at the potential of 0.2 V. Consistently with the higher current density, the hydrogen production rate is also ∼4 times larger for the CP-coated electrode versus the APcoated one (Figure 4B). During the hydrogen production the stability of the current density versus time for 2 h using both CP- and AP-coated electrode are shown in the Supporting Information (Figure S10A). Despite the effect of CP, the current density obtained is still low, which is in the range of μA D


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Figure 5. Current density as a function of applied potential vs Ag/AgCl with a scan rate of 20 mV/s when the anode is coated with CP (A) or AP (B) both with and without embedded CdSe. The measurements were performed in dark (dotted lines) and under illumination (solid lines) in 0.1 M Na2SO4 (pH = 6.5) aqueous solution.

Figure 6. Current density as a function of applied potential vs Ag/AgCl with a scan rate of 20 mV/s. The measurements were performed in dark (dotted lines) and under illumination (solid lines) in aqueous solution containing both 0.25 M Na2S and 0.35 M Na2SO3 (pH = 9.5). (A, B) The current density in the potential range from −1 to 0.5 V for CP and AP, respectively, with and without the CdSe nanoparticles. The scale in (A) is an order of magnitude larger than in (B).

magnitude as compared with the current densities measured at a low pH with no sacrificial reagent. Figure 7 shows the photo-electrochemical hydrogen production as a function of time for CdSe-modified CP and AP in 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution. CdSe-CP starts to produce hydrogen at an applied potential of 0.1 V with an evolution rate of 1.45 μL/min, whereas CdSe-AP starts to produce hydrogen only at 0.4 V with a rate of 0.89 μL/ min. The rate of hydrogen production for CdSe-CP at 0.4 V is

the solvent in the cell, but rather, are embedded in the polymer matrix. This is evident from the fact that the QDs are not decomposed in the electrochemical cell even after several hours. Figure 5 presents the current density measured in a solution of 0.1 M Na2SO4 (pH 6.5) when the anode is coated with the polymer with and without the embedded QDs. Evidently, the threshold potential is reduced, upon adding the QDs, by ∼300 mV for both CP and AP, and the current density for CP is approximately twice as high as for AP. The CdSe QDs clearly enhance the current density for both polymers by about a factor of 2. In the second configuration, the CdSe QDs were deposited on the polymer only after the polymer was deposited on the anode. Here the QDs were introduced by incubating the electrodes in a 2.5 mM CdSe solution in toluene for 3 h. The QDs are exposed to the electrolyte, and to avoid their corrosion, the pH had to be increased, and Na2S was used as a sacrificial reagent. Figure 6 presents the current density as a function of the potential versus Ag/AgCl when the photo-electrochemical measurements were performed in a mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution, both in dark and under illumination. The measurements were performed before and after modifying the CP and AP layers adsorbed onto the TiO2 with CdSe quantum dots. The results clearly indicate the enhancement in the current upon introducing the CdSe quantum dots on top of the polymer, by ∼2 orders of

Figure 7. Photocatalytic hydrogen production as a function of time in a mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution (pH = 9.5). E


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The Journal of Physical Chemistry C 2.2 μL/min. The current density stability with respect to time for CdSe-CP at 0.1 V and CdSe-AP at 0.4 V for first 1 h is shown in the Supporting Information (Figure S10B). Figure 8 summarizes the current density in the potential range from −1 to 1.5 V as measured for all the photo-

the chirality by other means, for example, by introducing thin chiral solids.30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04194. Synthesis of the polymers. CD spectra of the polymers. EDS analysis of the cross section of the photoanode. Mott−Schottky measurements. Details of the spindependent transport measuements. Current density stability plots during hydrogen production. UV−vis absorption spectra of the CdSe in solution and adsorbed on CP and AP (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Figure 8. Summary of the current vs voltage results, as measured for all photo-electrochemical cells investigated.

Adele Mucci: 0000-0003-3303-8761 Ron Naaman: 0000-0003-1910-366X Author Contributions

electrochemical cells. For all cells containing the CP-coated anode, the current density is higher than for the cells with AP. The present study confirms the former results that indicate the enhancement of hydrogen production from water in the case of a chiral anode. It extends the observation to a simple and quite inexpensive coating of the anode, the chiral conductive polymer. This effect can be explained by the spinselective transport through the chiral layer,28,29 inducing relative spin alignment in the oxidized radicals. The role of the spin control can be verified using spin-dependent transport studies, in which electron transfer through CP and AP was probed using conductive probe atomic force microscopy (for details, see the Supporting Information). As evident from Figure 3C, a clear difference exists in the current−voltage (I− V) curves when the field is applied either in an UP or DOWN direction, suggesting the spin filtering ability of the CP polymer. Similar experiments were performed for AP-modified substrate. For AP, no magnetic field orientation effect exists. These results confirm former studies on the role of the chiral structure in controlling the spin in the water-splitting process. The chiral conductive polymers are relatively inexpensive, they do not contain heavy metals, and reduce the overpotential needed; as compared with the common catalysts, however, they, by themselves, do not have high enough conductivity and a light absorption cross section to allow to obtain high current densities. When CdSe quantum dots are embedded in the polymer, the photocurrent increases. This increase can in principle result from two effects: either enhancing the visible absorption of the anode’s coating or enhancing the conduction. Interestingly, the embedded QDs also reduce the threshold potential for the oxidation process. This may indicate that enhanced conduction is responsible for this effect. When CdSe QDs are adsorbed on top of the polymer, the current densities reach values approaching milliamps in a cell containing the sacrificial reagent at low pH values. This effect is similar to that observed before with CdSe,16,21 but here we also see the contribution of the chiral polymer. Hence, there is still a need to improve the concept of the chiral anode and to make it a candidate for implementation. This will require introducing

§

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013)/ERC Grant No. [338720] and from ERC POC-2017, Grant No. 764203-watersplit.

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REFERENCES

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Enhanced Hydrogen Production with Chiral Conductive Polymer

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