Control of Electrons' Spin Eliminates Hydrogen Peroxide Formation during Water Splitting

Article abstract of DOI:10.1021/jacs.6b12971

The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

Full text of DOI:10.1021/jacs.6b12971

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Article
Control of Electrons’ Spin Eliminates Hydrogen
Peroxide Formation During Water Splitting
Wilbert Mtangi, Francesco Tassinari, Kiran Vankayala, Andreas Vargas Jentzsch, Beatrice
Adelizzi, Anja R.A. Palmans, Claudio Fontanesi, Egbert Willem Meijer, and Ron Naaman
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12971 • Publication Date (Web): 30 Jan 2017
Downloaded from http://pubs.acs.org on January 30, 2017
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Control of Electrons’ Spin Eliminates Hydrogen Peroxide Formation
During Water Splitting
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Wilbert Mtangi1, Francesco Tassinari1, Kiran Vankayala1, Andreas Vargas Jentzsch2,
Beatrice Adelizzi2, Anja R.A. Palmans2, Claudio Fontanesi3, E.W. Meijer*2, and Ron
Naaman*1
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1Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel;
2Institute for Complex Molecular Systems, Eindhoven University of Technology,
Eindhoven, the Netherlands;
3Department of Engineering, University of Modena and Reggio Emilia, Via Vivarelli 10,
41125 Modena, Italy
ABSTRACT: The production of hydrogen through water splitting in a
photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In
addition, hydrogen-peroxide formation is identified as a competing process affecting the
oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode
with chiral organic semiconductors from helically-aggregated dyes as sensitizers; Zn-
porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed,
while the overall current through the cell, correlating with the water splitting process, is
enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented
by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-
porphyrins are compared. These findings contribute to our understanding of the
underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave
the way towards better chiral dye-sensitized photoelectrochemical cells.
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INTRODUCTION
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Since it has no carbon, has the highest specific enthalpy of combustion of any chemical
fuel, and generates water as its oxidation product, hydrogen has been referred to as the
fuel of the future.1 Although significant progress has been made over the past decades,2
the generation of hydrogen by green, sustainable methods on a global scale remains in the
future.3-8 While technologies exist for the electrolysis of water and photo-electrochemical
generation of hydrogen from water,9 the processes involve significant overpotentials and
the formation of peroxides and superoxide radical by-products. These by-products have
the tendency to adsorb onto the photo-catalyst, poisoning it, thereby reducing its stability
and lifetime.10 Although specific catalysts are proposed to use the peroxides as
intermediates,111 this path requires higher voltage. Therefore, a fundamental solution for
the off-pathway products is essential.
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Water splitting is a four-electron process that generates hydrogen molecules
having singlet ground states and oxygen molecules having triplet ground states.
Commonly, the artificial water splitting process requires an overpotential of about 0.6 V
vs. normal hydrogen electrode (NHE), to drive the oxygen evolution reaction.12-14 The
importance of electron-spin correlation of electrons in generating O2 has been debated for
biological photosynthesis. Particularly the chemistry associated with the oxygen
evolution reaction in photosystem II has been examined.15-17 Nevertheless, the details of
the mechanism of the O=O bond formation remain unresolved.18-20 Although the spin
state of the electrons involved is rarely discussed in works exploring artificial
photosynthesis, recent theoretical studies suggest that the overpotential required to split
water is linked to restrictions on the electrons’ spin in generating a ground state triplet
oxygen molecule.21,22 In recent experimental work, it has been shown that when the
anode in the water splitting cell is coated with chiral molecules, the overpotential is
reduced.23 It has been proposed that the effect is due to spin filtering occurring when
electrons are conducted through chiral systems.24 However and very importantly, the
possible role of the spin control in suppressing the formation of hydrogen peroxide has
not been discussed nor experimentally addressed.
Here, we hypothesize that hydrogen peroxide is produced due to uncontrolled spin
alignment25 and greatly contributes to the high overpotentials. Hence, controlling the spin
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state of the electronic potential on which the reaction occurs should result in more
efficient oxygen production and limited production of hydrogen peroxide. Although, we
do not strive here to present the highest production of hydrogen and oxygen – optimizing
the cells is ongoing – the results presented show an unprecedented control of chemical
kinetics through spin selection.
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RESULTS AND DISCUSSION
To first control the spin state, the ITO anode in the photoelectrochemical cell was
coated with two families of organic semiconductors, Zn porphyrins and tri(pyrid-2-
yl)amine trisamide, TPyA, both in their chiral (using enantiomerically pure side chains)
and achiral (using achiral side chains) versions (Fig. 1A, B). By modifying the side
chains of the molecules, we control the resulting helical supramolecular assemblies into
either a bias for one helical sense or a racemic mixture of both helical senses,
respectively. Thus, in the present study, electrons are transmitted into the TiO2 substrate
through the same molecular system, which only differs in its molecular organization;
either one helical sense or a mixture of both. In the case of Zn-porphyrins, the 390 nm
Soret band confirms the formation of helical supramolecular structures in solution (Fig.
1C),26,27 which are retained when transferred to the surface (inset of Fig. 1C). In a similar
manner, the 317 nm band is indicative of the formation of a supramolecular assembly for
the TPyA molecules. For both molecules, the chiral analogues show a strong CD
response which is not observed in the achiral version (Fig. 1E, F). The achiral molecules
most probably aggregate in equal amounts of left- and right-handed helices (racemic
mixture), while the chiral molecules prefer one of both only.
Insert Figure 1 here
Magnetic conducting atomic force microscopy (mc-AFM) measurements were
conducted to verify the spin selectivity of electron transmission through the Zn-porphyrin
stacks.28 Fig. 2B shows the dependence of the current on the orientation of the magnetic
field at the tip of the mc-AFM for the stack of chiral porphyrins, which confirms that one
spin is preferred over the other. It is important to realize that the ratio between the two
spin currents is affected by the non-ideal spin injection from the magnetic tip. Since the
spin polarization of the tip is only about 35%, it means that the actual spin filtering of the
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molecular system is about 4:1, in other words only about 20% of the electrons conducted
through the chiral molecular aggregates in this experiment have the “wrong” spin. From
Fig. 2C, it can be observed that for the achiral compounds, the magnitude of the current
measured as a function of applied bias does not depend on the orientation of the magnetic
field; hence the conduction through this system with equal amount of right and left
handed helices is not spin specific. These results indicate that the supramolecular
structures formed from the chiral porphyrins can efficiently filter spins and are consistent
with previous reports in which chiral molecules have been observed to be good spin
filters.29
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Insert Figure 2 here
Next, photoelectrochemical measurements were conducted in a three-electrode
cell, with the Ag/AgCl (saturated KCl) as the reference electrode and a Pt wire as the
cathode (Fig. 3). A 0.1 M Na2SO4 (pH = 6.56) aqueous solution was used as the
electrolyte. TiO2 substrates fabricated and functionalized as outlined in the
supplementary material, were used as photo-anodes. In these cells, the magnitude of the
measured current is correlated with the amount of oxygen bubbles produced at the anode
and hydrogen bubbles at the cathode.23
Higher photocurrent densities are observed for photoelectrodes functionalized
with helical aggregates of chiral molecules with preferred helicity, compared to those
coated with racemic aggregates of achiral ones. This is remarkable, given that the
chemical compositions of the chiral and achiral molecules for the two sets used in this
study are very similar, except for the stereo-center present in the chiral molecules. In
general, the magnitude of the photocurrents obtained with the Zn-porphyrins are typically
low, as has also been observed by Moore et al.30 With the TPyA molecules, the
photocurrent densities are reasonably high, since UV light is used for illumination of the
cell. The activity of the photoelectrodes is known to depend strongly on their electronic
properties, therefore Mott Schottky measurements were performed to characterize the
electronic properties of TiO2 electrodes modified with aggregates of Zn porphyrin and
TPyA molecules. A flat-band potential, Vbi of -0.51V vs. Ag/AgCl was obtained in the
dark for both the chiral and achiral Zn-porphyrins (Fig. 3E), while a value of
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approximately -0.70V vs. Ag/AgCl was obtained for the TPyA molecules (Fig. 3F), an
indication that the difference in the photocurrents, for chiral and achiral species, is not
related to the modification of the electronic properties of the photoelectrodes. Thus, the
differences in the photocurrents must be attributed to the chirality of the molecules.
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Insert Figure 3 here
After showing the difference in water splitting for the chiral versus racemic
aggregates, the beneficial effect of electrons’ spin control is highlighted by its effect on
hydrogen peroxide formation. An indirect quantification of the H2O2 produced during
photoelectrochemical water splitting was conducted. Spectrophotometric titration of the
used electrolytes were performed using o-tolidine as redox indicator.31,32 The amount of
peroxide formed has been quantified through Ellms-Hauser method calibrating the
system with commercial H2O2 (see Figure S12-S14). In the presence of H2O2, a yellow
color appears with an absorption peak at about 436 nm. This peak is characteristic for the
complete two-electron oxidation product of o-tolidine formed by the reaction with
hydrogen peroxide.33
The electrolyte obtained from the bare TiO2 and electrodes functionalized with
achiral dyes showed the characteristic peak at 436 nm, indicating the production of H2O2
during water oxidation (see Table S1). Fascinatingly, no detectable amount of hydrogen
peroxide was observed for electrodes with the chiral molecules physisorbed (Fig. 4A, B).
Noticeably, after 40 mins of irradiation, 43 5 mmolL-1 of hydrogen peroxide have been
produced with the achiral Zn-porphyrin functionalized system while non-detectable
levels of peroxide have been found using the chiral analogue. The TPyA functionalized
systems show low absorbance intensity at 436 nm. The relatively low level of H2O2 in the
case of A-TPyA may result for performing the electrochemistry measurements under
illumination with UV light, which might have led to the disproportionation of the
produced peroxide before titration. However, also in this case, much less H2O2 is
produced with the chiral molecules (S-TPyA) then with the achiral ones (A-TPyA).
Additional evidence is obtained by using achiral 3-mercaptopropionic acid and the chiral
oligopeptide [(COOH)-(Ala-Aib)7-NH-(CH2)2-SH] (see Figure S13). Only the latter
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decreases the H2O2 production. These results further indicate that the observed
elimination of the hydrogen peroxide production is general for all chiral molecules.
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Insert Figure 4 here
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The results presented here in the quantification of H2O2 production together with
the electrochemistry data show a strong correlation between the overpotential, the
formation of H2O2, and the electron’s spin alignment control. During water splitting, two
OH- species must combine to form molecular oxygen in its triplet ground state. In the
process, an electron from each OH- is transferred to the anode. This leaves the two OH•
radicals in their doublet ground state, namely each OH• has one unpaired electron. When
there is no spin control and the interaction electronic potential has a singlet character, the
formation of H2O2 is possible (Fig. 4). However, when the electron’s spins are aligned in
a parallel fashion, the two electrons interact on the triplet potential surface which
correlates with the formation of the ground state molecular oxygen and on which the
formation of H2O2 is symmetry forbidden.
The formation of oxygen and hydrogen peroxide are anti-correlated; i.e., the
formation of one is coming at the expense of the other. This is what the present results
indicate. However, it is difficult to draw a quantitative relation between the change in the
current and the hydrogen peroxide formation, since the change in the current reflects the
change in the threshold potential for the process while the decrease in the hydrogen
peroxide formation is an indication for the extent of spin polarization. Of course, the
substrate (the catalyst) may induce spin-orbit coupling that will mix the spin states of the
OH radical. However, for TiO2 and other relatively light materials, the spins of
physisorbed molecules are expected to be conserved, as observed by XPS studies.34 Note
that substrate-induced spin-orbit coupling may explain the low overpotential observed for
some oxides,35-37 magnetic catalysts38-41 or catalysts made from heavy atoms. For these
systems, the side products are not significantly suppressed. Indeed, no investigations
were performed in the past aimed at controlling the formation of H2O2.
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CONCLUSIONS
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The experimental results observed in the current study provide new insights into
the mechanism behind oxygen formation in the oxygen evolution reaction and provide a
new path for improving the efficiency of the water splitting process. Obviously, much
work has to be done to optimize the system with other and more effective chiral dyes,
chiral semi-conductors, and chiral catalysts, but a proof-of-principle to make another
counter-intuitive approach is presented. The control of electrons’ spin in the chemical
kinetics during the photochemical water splitting will also rejuvenate the field of
magnetic field effects in chemical kinetics and related phenomenon as reviewed by
Steiner and Ulrich many years ago, following the discovery and understanding of
electron spin polarized phenomena during chemical reactions.42 This is the more of
importance due to the recent interest in photoredox catalysis in organic chemistry with
exciting enatioselectivities.43 In addition, the work points to the importance of chirality
and spin selectivity in multiple electron reactions in biology.
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ASSOCIATED CONTENT
Supplementary information
The Supporting Information is available free of charge on the ACS Publications website
at DOI:XXXXX.
Experimental details, synthesis, preparation and functionalization of TiO2 electrodes.
Additional details SEM, spectroscopic data, photo-electrochemical measurements, AFM
mc-AFM and spectrophotometric titration of H2O2
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AUTHOR INFORMATION
Corresponding authors
ron.naaman@weizmann.ac.il
e.w.meijer@tue.nl
Notes
The authors declare no competing financial interest
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ACKNOWLEDGEMENTS
RN acknowledges discussion with Profs. Andrew Rapp and David Waldeck and
acknowledges partial support from the European Research Council under the European
Union's Seventh Framework Program (FP7/2007-2013) / ERC grant agreement n°
[338720]. The Eindhoven laboratory acknowledges financial support of EURO-
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SEQUENCES (H2020-MSCA-ITN-2014, No 642083) and the FMS Gravitation Program
(No 024.001.035).
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Fig. 1 Molecules used as photosensitizer and their supramolecular aggregation. (A) The chiral
and achiral Zn porphyrins, (C and E) their absorption spectra and CD spectra of the aggregated
state in solution (1.7 x 10-5 M in methylcyclohexane) and when adsorbed on the surface (inset)
(B) the chiral and achiral TPyA, (D and F) their absorption and CD spectra of the aggregated
state in solution and when adsorbed on the surface (inset). The red curves represent the chiral
molecules, while the blue represent the achiral ones.
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Fig. 2 Magnetic conducting atomic force microscopy (mc-AFM) measurements on stacks of
chiral and achiral Zn-porphyrins. (A) The experimental setup. The current as a function of the
applied voltage obtained from the chiral (B) and achiral (C) Zn-porphyrin molecules. The insets
show corresponding histograms of currents obtained at 3V for the two opposite magnetic
orientations of the tip.
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Fig. 3 The photoelectrochemical cell and the current density as a function of the potential. In A a
schematic representation of the photo-electochemical cell. The potential is given vs. the Ag/AgCl
electrode, when the TiO2 electrode is coated with self-assembled achiral (blue lines) or chiral (red
lines) molecules. In (B) and (C), Zn-porphyrins were used while in (D) and (E), TPyA molecules
were employed. The measurements were performed in the dark (solid lines) and under
illumination (dotted lines). In (C) and (E) measurements were performed while chopping the
light. All measurements were performed at a scan rate of 10 mVs-1. The flat-band potentials in the
dark for Zn-porphyrin and TPyA molecules were obtained from Mott-Schottky plots (F and G,
respectively) at a frequency of 1.99 kHz and oscillation voltage of 20 mV.
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Fig. 4 Control of the hydrogen peroxide production. UV-vis absorption spectra from the titration
of the used electrolyte (Na2SO4) with o-tolidine of bare TiO2 and TiO2 electrodes coated with (A)
self-assembled Zn-porphyrins of either achiral (A-Zn) or chiral (S-Zn) and (B) TPyA molecules.
The control refers to the titration of unused Na2SO4 with o-tolidine. (C) When the electrons
transfer to the anodes is non spin specific the spins of the unpaired electrons on the two OH• are
aligned antiparallel, hence the interaction between the two OH• is on a singlet surface that
correlates with the production of hydrogen peroxide (H2O2). (D) When the electron transfer to the
anode is spin specific, the spins of the two electrons are aligned parallel to each other, hence the
two OH• interact on a triplet surface that forbids the formation of H2O2 and facilitates the
production of oxygen in its ground state.
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