Photoelectroreduction of Building-Block Chemicals

Article abstract of DOI:10.1002/anie.201701764

Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO₂ into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

Full text of DOI:10.1002/anie.201701764

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701764
German Edition: DOI: 10.1002/ange.201701764
Photoelectrochemical Synthesis
Hot Paper
Photoelectroreduction toward Chemical Building Blocks
Fengjiao Chen, Wei Cui, Jie Zhang, Yeyun Wang, Junhua Zhou, Yongpan Hu, Yanguang Li,* and
Shuit-Tong Lee
Abstract: Conventional photoelectrochemical cells utilize
solar energy to drive the chemical conversion of water or
CO2 into useful chemical fuels. Such processes are confronted
with general challenges, including the low intrinsic activities
and inconvenient storage and transportation of their gaseous
products. A photoelectrochemical approach is proposed to
drive the reductive production of industrial building-block
chemicals and demonstrate that succinic acid and glyoxylic
acid can be readily synthesized on Si nanowire array photo-
cathodes free of any cocatalyst and at room temperature. These
photocathodes exhibit a positive onset potential, large satu-
ration photocurrent density, high reaction selectivity, and
excellent operation durability. They capitalize on the large
photovoltage generated from the semiconductor/electrolyte
junction to partially offset the required external bias, and
thereby make this photoelectrosynthetic approach significantly
more sustainable compared to traditional electrosynthesis.
portation would incur considerable extra cost, rendering the
use of these solar fuels less attractive.[5,6]
We reason that, in addition to splitting water or reducing
CO2, photogenerated carriers can also be used to drive the
electrochemical transformation of organic molecules and
produce value-added chemicals that have industrial signifi-
cance.[3,7] The ideal scenario would be to combine a photo-
anode and -cathode, and make use of both the anodic and
cathodic processes with zero external bias. Recently, Choi
et al. reported the PEC oxidative transformation of 5-
hydroxymethylfurfural to 2,5-furandicarboxylic acid on
a BiVO4 photoanode with a high yield and Faradaic effi-
ciency.[7] However, as far as we are aware, there is no
demonstration on the possible PEC reductive production of
industrial building-block chemicals.
An experimental proof of concept is provided, which
shows that two industrially significant molecules—succinic
acid and glyoxylic acid—can be selectively produced on PEC
photocathodes. Succinic acid is listed as one of the top value-
added chemicals by the US Department of Energy.[8,9] It is the
precursor of many key chemicals (including 1,4-butanediol,
tetrahydrofuran, 2-pyrrolidinone, and N-methylpyrrolidi-
none) and an important ingredient in food and pharmaceut-
ical products, surfactants and detergents, green solvents, and
biodegradable plastics.[9] The annual production volume of
succinic at present is between 30 and 50 million tons, and its
global market potential is projected to reach 7–10 billion
USD per year.[10] In industry, succinic acid is mainly produced
from the liquid-phase hydrogenation of maleic anhydride at
a temperature ranging from 120 to 1808C under a hydrogen
pressure between 5–40 atm.[11] This method is typically
energy-intensive and costly. A milder approach is to electro-
chemically reduce maleic acid in acidic medium on high
hydrogen overpotential metals such as lead and titanium
(Equation 1).[12] The second target molecule—glyoxylic
acid—is also an important intermediate in pharmaceutical,
perfumery, and fine chemical industries.[13,14] Its production by
electrochemical reduction of oxalic acid has been known for
about a century (Equation 2). Even though the electrosyn-
thesis of both compounds has been well-documented, their
PEC synthesis using renewable solar energy is unexplored.
P
hotoelectrochemical synthesis is the process where photo-
generated electrons and holes in semiconductor electrodes
are utilized for chemical fuel production.[1,2] The process
mimics what Mother Nature does through photosynthesis,
and may potentially enable efficient solar-to-chemical con-
version on a wide scale. In typical PEC devices, photo-
generated electrons are used to drive the reduction of water
to H2 or the reduction of CO2 to organic feedstocks (for
example, CO, formic acid, and CH4) at the cathode; photo-
generated holes are used to drive the oxidation of water to O2
at the anode.[1,2] Most light-adsorbing semiconductor materi-
als (such as Si) have limited intrinsic electrocatalytic activities
toward the aforementioned reactions. They often require the
assistance of cocatalysts (sometimes made of precious metals)
to expedite the charge transfer kinetics at the surface.[3,4]
However, the introduction of an additional semiconductor/
cocatalyst interface not only adds new design complexity to
existing devices, but may also risk creating adverse surface
states that trap electrons or holes. Another well-recognized
challenge with photoelectrochemical (PEC) water splitting or
CO2 reduction is that the products (for example, H2 or CO)
are often in the gaseous state. Their storage and trans-
HOOCꢀHC¼CHꢀCOOH þ 2Hþ þ 2eꢀ
HOOCꢀCH2ꢀCH2ꢀCOOH
!
[*] F. J. Chen, W. Cui, J. Zhang, Y. Y. Wang, J. H. Zhou, Y. P. Hu,
Prof. Dr. Y. G. Li, Prof. Dr. S. T. Lee
ð1Þ
ð2Þ
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu
Key Laboratory for Carbon-Based Functional Materials and Devices
Soochow University
HOOCꢀCOOH þ 2Hþ þ 2eꢀ ! HOOCꢀCOOH
Suzhou 215123 (China)
E-mail: yanguang@suda.edu.cn
Herein, we report for the first time the PEC reductive
transformation of maleic acid to succinic acid and oxalic acid
to glyoxylic acid. We use chemically etched Si nanowire arrays
without any cocatalyst as the model photocathode, and study
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201701764.
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their potential-dependent reaction rate and Faradaic effi-
ciency. A positive onset potential up to approximately 0.2 V
versus a reversible hydrogen electrode (RHE), a large
saturation photocurrent density of about 37 mAcmꢀ2, and
a Faradaic efficiency close to 100% were successfully
attained.
P-type Si was selected as the light-absorbing semiconduc-
tor electrode material for its relatively low cost and commer-
cial availability. Herein, large-area vertically aligned Si nano-
wire arrays (Si NWs) were fabricated by metal-assisted
chemical etching of p-type (100) Si wafers (see Supporting
Information for details).[15,16] By carefully controlling the
reaction time between 10 to 60 min, we obtained Si NWs with
varying lengths up to approximately 3 mm. Figure 1a shows
Figure 1. Vertically aligned Si NWs from the metal-assisted chemical
etching of Si wafers. a) SEM image of 2.8 mm Si NWs. b) TEM images
of a Si NW at different magnifications.
Figure 2. PEC reduction of maleic acid to succinic acid. a) Schematic
showing the PEC process on the Si NW photocathode. b) Polarization
curve of PEC maleic acid reduction on Si NWs in the light (‚) or dark
(&) in comparison with that of PEC maleic acid reduction on planar Si
*
*
in the light ( ), HER on Si NWs ( ) and electrochemical maleic acid
reduction on p++-Si ( ). c) Polarization curves of electrochemical
&
the scanning electron microscopy (SEM) image of Si NWs
etched to a specific length of 2.8 mm. They grew vertically with
respect to the substrate underneath. When examined under
transmission electron microscopy (TEM), these NWs were
revealed to have diameters in the range of 80–150 nm. High-
resolution transmission electron microscopy (TEM) analysis
confirmed the single-crystalline nature of each NW (Fig-
ure 1b). Their long-axial orientation was determined to be
along the [100] direction, consistent with the surface orienta-
tion of starting Si wafers.
maleic acid reduction on Ti/TiO2 and lead electrodes in comparison
with that of PEC maleic acid reduction on Si NWs in the light.
d) Chronoamperometric responses of the Si NW photocathode at
selected potentials as indicated. e) Potential-dependent Faradaic effi-
ciency for succinic acid on Si NW photocathode in comparison with
Ti/TiO2 and lead electrodes. f) Concentration increase of succinic acid
in the catholyte for the first 6 h during the long-term stability study.
Gray curve indicates the prediction assuming 100% Faradaic efficiency.
Inset: the corresponding chronoamperometric response of Si NW
photocathode held at 0 V.
Compared to planar electrodes, previous studies have
established that NW array electrodes are advantageous for
PEC applications in that they possess long optical paths for
efficient light absorption (because of substantial internal light
scattering) and short transport distances across the NW
diameter for efficient charge collection.[17,18] Moreover, the
enlarged surface area of NW arrays dilutes the distribution of
photogenerated electron/hole flux over their entire surfaces,
and therefore relaxes the electrocatalytic turnover require-
ment of the electrode and lowers the reaction overpoten-
tial.[18] This becomes particularly important if photogenerated
electrons or holes are used to drive kinetically sluggish
reactions.[19] We envisaged that a process similar to PEC water
splitting took place on the Si NW photocathode (Figure 2a).
Immediately following light absorption, photogenerated
electrons migrated to the NW/electrolyte interface, as
driven by the built-in potential, and were subsequently
injected into nearby organic electron acceptors in the electro-
lyte. Meanwhile, photogenerated holes moved through the
outer circuit to the counter electrode and participated in an
oxidation reaction. In this process, the hydrogen evolution
reaction (HER) presented an obvious competing reaction
channel. Depending on their relative interfacial charge
transfer rates, if photogenerated electrons werenꢀt rapidly
scavenged by organic electron acceptors, they would instead
be captured by solution protons, resulting in a lower reaction
Faradaic efficiency.
We first studied the PEC reductive transformation of
maleic acid to succinic acid. The electrochemical reduction of
maleic acid to succinic acid is a two-electron process with
a standard reduction potential of 0.62 V (vs. RHE). Lead and
titanium are common electrode materials for the electrosyn-
thesis of succinic acid with typical Faradaic efficiencies in the
range of 70–90%.[12,20] Our PEC experiments were carried out
in a custom-built two-compartment electrochemical cell using
the standard three-electrode system (Supporting Information,
Figure S1). The electrolyte was 0.3m Na2SO4 and 0.3m maleic
acid (pH 2.0). As displayed in Figure 2b, no current flowed in
dark conditions. Under 100 mWcmꢀ2 of simulated AM 1.5 G
2
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illumination, the J–V curve of 2.8 mm Si NWs exhibited an
onset potential (defined as the potential to reach 1 mAcmꢀ2
reductive transformation at substantially more positive
potentials (ca. 700 mV based on Vj=10), thereby reducing the
)
of 0.18 V versus RHE, beyond which the cathodic photo-
current density quickly took off and reached a saturation
value of approximately 37 mAcmꢀ2 at about ꢀ0.6 V. The
observed cathodic photocurrent density quantitatively corre-
sponds to the reduction of maleic acid to succinic acid (see
proceeding text). The large saturation photocurrent density of
the Si NW electrode agreed well with the ideal value for Si
photoelectrodes, suggesting that there was no optical loss and
required external bias.
Subsequently, we analyzed the potential-dependent
reduction product and its Faradaic efficiency on the Si NW
photoelectrode. To do so, bulk photoelectrolysis was per-
formed at a few selected potentials between ꢀ0.9 and 0.1 V.
All the recorded chronopotentiometric curves (j ~ t) remained
highly stable for at least 1 h (Figure 2d). At the conclusion of
each photoelectrolysis experiment, the liquid product in the
catholyte (ca. 125 mL) was collected and characterized by
nuclear magnetic resonance (NMR). Succinic acid was
determined to be the only liquid product (Supporting
Information, Figure S4). Based on its concentration in the
catholyte, we calculated the Faradaic efficiency for succinic
acid (Figure 2e). Faradaic efficiency was close to 100% within
the entire potential range, suggesting that HER was largely
overwhelmed by maleic acid reduction on the Si NW photo-
cathode. In stark contrast, electrochemical reduction of
maleic acid on both Ti/TiO2 and lead was accompanied by
significant cogeneration of H2. The Faradaic efficiency for
succinic acid on Ti/TiO2 was around 90% between ꢀ0.9 and
ꢀ0.4 V, and within the range of 70–90% on lead, which is
consistent with previous studies.[12,22] The very high Faradaic
efficiency for the quantitative transformation of maleic acid
to succinic acid on the Si NW photocathode therefore
presents another advantage over conventional electrosyn-
thesis.
Furthermore, we assessed the operation durability of the
Si NW photocathode. Its photocurrent density at 0 V was
monitored continuously for about 17 h, and there was no sign
of activity loss (Figure 2 f inset). The slight initial photo-
current decline was probably caused by the decreasing local
concentration of maleic acid, and a gradual rise in photo-
current thereafter was probably due to increasing wetting of
the Si NW photocathode. Postmortem SEM and TEM
examinations showed that there was no obvious change in
the electrode morphology after prolonged photoelectrolysis
(Supporting Information, Figure S5). Additionally, we
extracted the catholyte every 30 min for the first 6 h during
photoelectrolysis and analyzed the concentration of succinic
acid. The measured concentrations were in perfect accord-
ance with what was predicted by assuming all the cathodic
photocurrent resulted from the two-electron reduction of
that all the photogenerated electrons were fully utilized.[21]
A
control experiment with a planar Si electrode revealed a more
negative onset potential (0.06 V) and a smaller saturation
photocurrent density (ca. 34 mAcmꢀ2). We also compared the
PEC maleic acid reduction performance of Si NW electrodes
with different NW lengths (Supporting Information, Fig-
ure S2). Moreover, electrochemical reaction of maleic acid on
degenerately doped planar Si wafer (p++-Si, not p++-Si NWs
because their NWs could not be prepared by the same metal-
assisted chemical etching process) showed that the reaction
did not initiate until a very negative potential of ꢀ0.60 V was
reached on the Si surface. To a certain degree, the approx-
imately 770 mV difference in the onset potential between Si
NWs and p++-Si reflected the considerable photovoltage
generated from the Si NW/electrolyte junction under 1 sun
illumination.
To shed light on and contrast the different interfacial
charge transfer rates between maleic acid reduction and
HER, PEC experiments in the absence of maleic acid were
also carried out. The electrolyte was 0.3m Na2SO4 with its pH
adjusted to 2.0 with H2SO4. Under these circumstances, only
HER could possibly occur in the cathodic potential regime.
Mott–Schottky analysis indicated a similar flat band (ca.
0.17 V; Supporting Information, Figure S3). However, the
onset potential of the Si NW electrode was displaced to
ꢀ0.07 V, and its saturation photocurrent density was not
reached until about ꢀ1.2 V (Figure 2b). In the presence or
absence of maleic acid, the required potential to sustain
a photocurrent density of 10 mAcmꢀ2 (Vj=10) differed by
approximately 460 mV. Such a drastic difference emphasizes
the much facilitated reduction rate of maleic acid over the
competing HER process. As a result, we can reasonably
deduce that HER was fully suppressed in the presence of
maleic acid (see proceeding discussion).
&
To highlight the advantage of PEC photosynthesis relative
to conventional electrosynthesis, we subsequently investi-
gated the electrochemical reduction of maleic acid on heat-
treated titanium (Ti/TiO2)[22] and lead—two common elec-
trode materials used widely in previous studies with reported
Faradaic efficiencies typically in the range of 70–90%.[12,20] As
depicted in Figure 2c, the polarization curves on Ti/TiO2 and
lead exhibited more negative onset potentials of ꢀ0.21 V and
ꢀ0.30 V, respectively; and their potentials required to reach
a cathodic current density of 10 mAcmꢀ2 were ꢀ0.68 V and
ꢀ0.48 V, respectively. The Si NW photocathode displayed
markedly improved performance in solar-assisted maleic acid
reduction relative to Ti/TiO2 and lead. By capitalizing on the
large photovoltage generated at the semiconductor/electro-
lyte junction, the Si NW photocathode was able to drive this
maleic acid (Figure 2 f, ). These results suggest that the
Faradaic efficiency for succinic acid is retained at about 100%
during prolonged photoelectrolysis.
The aforementioned results corroborate that the conver-
sion of maleic acid to succinic acid was efficiently achieved on
Si NW photocathode under light illumination. Furthermore,
we demonstrated that the idea of PEC reductive transforma-
tion was not specific to succinic acid, but also applicable to
other industrial building-block chemicals. In the proceeding
text, we describe a second example in which glyoxylic acid
was photoelectrosynthesized on a Si NW electrode.
The electrochemical reduction of oxalic acid is a well-
established approach to synthesize glyoxylic acid in indus-
try.[13,23] The process has a standard reduction potential of
ꢀ0.08 V, which is very close to that of HER. As a result,
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reaction selectivity was a general concern. Herein, the PEC
reduction of oxalic acid was carried out in the same
experimental setup. The electrolyte was 0.3m K2SO4 and
0.1m oxalic acid (pH 1.85). Under 1 sun illumination, the
polarization curve of 2.8 mm Si NW electrode exhibited an
onset potential of ꢀ0.063 V and reached a saturation photo-
current density of approximately 37 mAcmꢀ2 at about ꢀ1.2 V
(Figure 3a). Both the onset potential and curve fill factor
were noticeably inferior to those of maleic acid reduction, but
still slightly better than those of HER (control experiment in
0.3m K2SO4 with pH adjusted to 1.85 by H2SO4). Considering
that the light absorption and charge separation steps were
essentially identical for all the three PEC reactions, the
aforementioned difference therefore reflected their dissimilar
interfacial charge transfer rates; oxalic acid reduction was not
as facile as maleic acid reduction, but still more kinetically
favorable than HER on the Si surface. Ion chromatography
(IC) was used to confirm glyoxylic acid as the sole reduction
product in the catholyte (Supporting Information, Figure S6).
Quantitative analysis showed that the Faradaic efficiency for
glyoxylic acid on the Si NW electrode was between 70–80%
within the potential range of ꢀ0.9 to ꢀ0.3 V (Figure 3b). At
more negative potentials, it is probable that the PEC
reduction of oxalic acid became diffusion-limited, and
consequently Faradaic efficiency was slightly decreased. The
operation durability of the Si NW photocathode for oxalic
acid reduction at ꢀ0.5 V was monitored for 6 h (Figure 3c).
Photocurrent density initially decreased and then leveled off
at approximately 14 mAcmꢀ2. Slight fluctuations were due to
the interference of H2 bubbles generated on the electrode
surface. Glyoxylic acid concentration was analyzed every
30 min (Figure 3d). The black and gray curves in this Figure
present theoretical predictions that assume 100% and 75% of
the cathodic photocurrent was used for oxalic acid reduction,
respectively. The measured concentrations of glyoxylic acid
were in quantitative agreement with the latter, suggesting the
PEC reductive transformation of oxalic acid proceeded at
about 75% Faradaic efficiency.
In summary, we used chemically etched Si NW arrays as
the model photocathode, and for the first time demonstrated
the photoelectrosynthesis of succinic acid and glyoxylic acid
with positive onset potential, large saturation photocurrent
density, high reaction selectivity and excellent operation
durability. This PEC process was substantially more advanta-
geous compared to conventional electrosynthesis because the
large photovoltage (ca. 700 mV) supplied from the semi-
conductor/electrolyte junction partially offset the required
external bias, rendering such
a photoelectrosynthetic
approach significantly more sustainable. Reduction products
could be isolated following established industrial protocols. In
the present study, Si was used as a model system for
a conceptual demonstration; it is far from the ideal p-type
semiconductor material since its valence band edge is too high
in energy. If other semiconductor materials (for example,
GaAs and CdS) were developed, they might reduce maleic
acid at an even more positive potentials. When combined with
proper photoanodes, they may achieve spontaneous photo-
synthesis of a wide range of valuable organic compounds.
Acknowledgements
We acknowledge support from the National Natural Science
Foundation of China (51472173 and 51522208), the Natural
Science Foundation of Jiangsu Province (BK20140302 and
SBK2015010320), the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions and Collabo-
rative Innovation Center of Suzhou Nano Science and
Technology.
Conflict of interest
The authors declare no conflict of interest.
Keywords: building-block chemicals · glyoxylic acid ·
photoelectrosynthetic synthesis · reduction · succinic acid
Figure 3. PEC reduction of oxalic acid to glyoxylic acid. a) Polarization
curves for PEC oxalic acid reduction on Si NWs in the light (‚) or
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Manuscript received: February 17, 2017
Final Article published: && &&, &&&&
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Communications
Photoelectrochemical Synthesis
A photoelectrochemical approach was
used to drive reductive production of
industrial building-block chemicals, such
as succinic and glyoxylic acids, on Si
nanowire array photocathodes. No coca-
talyst was required and the process was
performed at room temperature with
a positive onset potential, large satura-
tion photocurrent density, high reaction
selectivity, and excellent operational
durability.
F. J. Chen, W. Cui, J. Zhang, Y. Y. Wang,
J. H. Zhou, Y. P. Hu, Y. G. Li,*
S. T. Lee
&&&& — &&&&
Photoelectroreduction toward Chemical
Building Blocks
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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