Lignin-fueled photoelectrochemical platform for light-driven redox biotransformation

Article abstract of DOI:10.1039/d0gc01521k

The valorization of lignin has significant potential in producing commodity chemicals and fuels from renewable resources. However, the catalytic degradation of lignin is kinetically challenging and often requires noble metal catalysts to be used under harsh and toxic conditions. Here, we report the bias-free, solar reformation of lignin coupled with redox biotransformation in a tandem structure of a BiVO4 photoanode and perovskite photovoltaic. The tandem structure compensates for the potential gap between lignin oxidation and biocatalytic reduction through artificial Z-schematic absorption. We found that the BiVO4-catalyzed photoelectrochemical oxidation of lignin facilitated the fragmentation of higher molecular weight lignin into smaller carboxylated aliphatic and aromatic acids. Lignin oxidation induced photocurrent generation at the photoanode, which enabled efficient electroenzymatic reactions at the cathode. This study successfully demonstrates the oxidative valorization of lignin as well as biocatalytic reductions (e.g., CO2-to-formate and α-ketoglutarate-to-l-glutamate) in an unbiased biocatalytic PEC platform, which provides a new strategic approach for photo-biocatalysis using naturally abundant renewable resources.

Full text of DOI:10.1039/d0gc01521k

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Park, Green Chem., 2020, DOI: 10.1039/D0GC01521K.
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Lignin-Fueled Photoelectrochemical Platform for
Light-Driven Redox Biotransformation†
a
a
b
a
b
a
Ding Wang,‡ Sahng Ha Lee,‡ Seunghyun Han, Jinhyun Kim, Nguyen Vu Thien Trang,
a
b
a
a
Kayoung Kim, Eun-Gyu Choi, Passarut Boonmongkolras, Yang Woo Lee, Byungha Shin,
Yong Hwan Kim,* and Chan Beum Park*
b
a
aDepartment of Materials Science and Engineering, Korea Advanced Institute of Science and Technology
(
KAIST), 335 Science Road, Daejeon 305-701, Republic of Korea. E-mail: parkcb@kaist.ac.kr
b
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST) 50
UNIST-gil, Ulsan 44919, Republic of Korea. E-mail: metalkim@unist.ac.kr
†
‡
Electronic supplementary information (ESI) available: Additional Figs and tables. See DOI: 10.1039/xxx.
These authors contributed equally to this work.
Abstract
The valorization of lignin has a significant potential in producing commodity chemicals and
fuels from renewable resources. However, the catalytic degradation of lignin is kinetically
challenging and often requires noble metal catalysts to be used under harsh and toxic conditions.
Here, we report on the bias-free, solar reformation of lignin coupled with redox
biotransformation in a tandem structure of BiVO photoanode and perovskite photovoltaic. The
4
tandem structure compensates for the potential gap between lignin oxidation and biocatalytic
reduction through artificial Z-schematic absorption. We found that the BiVO -catalyzed
4
photoelectrochemical oxidation of lignin facilitated the fragmentation of high molecular weight
lignin into smaller carboxylated aliphatic and aromatic acids. Lignin oxidation induced
photocurrent generation at the photoanode, which enabled efficient electroenzymatic reactions
at the cathode. This study successfully demonstrates the oxidative valorization of lignin as well
as biocatalytic reductions (e.g., CO -to-formate and α-ketoglutarate-to-L-glutamate) in an
2
unbiased biocatalytic PEC platform, which provides a new strategic approach for photo-
biocatalysis using naturally abundant renewable resources.
1

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Introduction
Lignin is one of the major components of lignocellulosic biomass, which comprises about 20–
0% of the solid weight of plants. Despite lignin’s potential as a renewable resource, it has
3
been treated as a waste from pulp processing and has been underutilized because of its
recalcitrant nature, stemming from its complex and irregular polymeric structure.1, 2 Due to
lignin’s high resistance against degradation, the catalytic depolymerization of lignin often
involves chemical oxidants used under harsh conditions, such as extreme temperature/pressure
and high pH.3, 4 The electrochemical oxidation of lignin operates without any additive but is
still energy-intensive, requiring high current density, substantial bias, and costly bulk electrode
5
5
6, 7
8
9, 10
11
materials (e.g., Pt , Au , IrO , Ru/V oxides , Pb , and Ni ). Solar energy is a sustainable
2
alternative for lignin oxidation with efficiencies in both energy and cost. For example,
heterogeneous photocatalysis has been applied to solar lignin degradation, which involved
semiconductors under ultraviolet (UV) or visible-light irradiation (e.g., rare earth doped-
TiO /UV , ZnO/UV13, Fe O -Nb O /visible-light , and In S /visible-light ). The
1
2
14
15
2
2
3
2
5
2 3
photocatalytic degradation of lignin typically occurs via the radical-induced oxidation pathway,
•
-•
including the light-driven generation of hydroxyl (OH ) and superoxide (O ) radicals on the
2
edge of bandgaps. However, combining oxidation and reduction reactions in a one-pot system
hampers overall efficiency due to the recombination of photogenerated electron-hole pairs and
causes photocatalyst inactivation.16, 17 On the other hand, the photoelectrochemical cells offer
a greener option for cost‐effective, safe, and sustainable approach to draw electrons from lignin
for solar-to-chemical production by utilizing renewable sources like sunlight to reduce the
energy required for electrochemical method.18, 19
Here, we report on an unbiased photoelectrochemical (PEC) platform that couples
lignin oxidation with biocatalytic reductions under visible light. As depicted in Scheme 1, the
PEC system consists of three components: (1) a bismuth vanadate (BiVO ) photoanode used
4
2

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to photoelectrochemically oxidize lignin; (2) a triple-cation perovskite photovoltaic (PV) used
to form a tandem structure with BiVO and provide supplementary photovoltage driving redox
4
+
reactions under zero external bias; and (3) a carbon cloth cathode to reduce NAD to NADH
for enzymatic redox conversions (e.g., CO -to-formate, α-ketoglutarate-to-L-glutamate
2
reactions). Solely driven by light, the photoelectrochemical oxidation of lignin could be
achived under environmentally benign conditions, while boosting the green synthesis of value-
added chemicals by redox enzymes.
Experimental section
Chemicals and materials
All chemicals, including lignosulfonate, alkali lignin, bismuth(III) nitrate pentahydrate
[
Bi(NO ) ·5H O], potassium iodide (KI), nitric acid (HNO ), hydrochloric acid (HCl), p-
3 3 2 3
benzoquinone, vanadyl acetylacetonate [VO(acac) ], 1,4-dioxane, pyridine, tetrahydrofuran
2
(
THF), acetic anhydride, dimethyl sulfoxide-d (DMSO-d ), dimethyl sulfoxide (DMSO),
6 6
potassium hydride (KOH), magnesium sulfate heptahydrate (MgSO ), potassium bicarbonate
4
(KHCO3),
ethanol,
acetone,
isopropanol,
ethyl
acetate,
N,O-
Bis(trimethylsilyl)trifluoroacetamide with 1 % trimethylchlorosilane (BSTFA/TMCS), β-
+
nicotinamide adenine dinucleotide hydrate (NAD ), and sodium formate were purchased from
Sigma-Aldrich (St. Louis, MO, USA). These chemicals were used without further purification.
+
The rhodium (III)-based electron mediator (M = [Cp*Rh(bpy)H O] , Cp* =C Me , bpy = 2,2’-
2
5
5
2
0
bipyridine) was synthesized according to the previous literature. Formate dehydrogenase
from Thiobacillus sp. KNK65MA (TsFDH) was cloned, expressed, and purified according to
the previous paper.21
Synthesis and characterization of BiVO photoanode
4
3

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A fluorine-doped tin oxide (FTO) coated glass (TEC-7, Pilkington) was thoroughly washed in
.0 M KOH aqueous solution by ultrasonication for 30 min, followed by rinsing in deionized
DI) water and ethanol. To electrochemical deposit of bismuth oxyiodide (BiOI), the plating
solution was prepared by dissolving Bi(NO ) ·5H O (0.04ꢀM) in 100 mL KI (0.4ꢀM) aqueous
1
(
3
3
2
solution adjusting the pH to 1.7 using HNO , followed by the addition of 40 ml ethanol solution
3
containing p-benzoquinone solution (0.23ꢀM). The precursor solution was stirred for 1 h. A
conventional three-electrode configuration that consist of FTO as a working electrode,
Ag/AgCl3ꢀM NaCl as a reference electrode, and Pt mesh as the counter electrode was conducted
with a potentiostat/galvanostat (WMPG 1000, Wonatech Co., Korea) for the electrodeposition;
2
the FTO glass (2 × 3 cm ) was immersed in the as‐prepared precursor solution, and a cathodic
bias of −0.1 V (vs Ag/AgCl) was applied for 4 min. Afterward, the dark orange BiOI layer on
the FTO was washed with DI water and dried under ambient condition. To synthesis BiVO4
from the BiOI, a 200 uL of DMSO solution containing VO(acac) (0.2 M) was dropped onto
2
−
1
the BiOI layer, followed by heating at 723 K for 2ꢀh with a ramping rate of 1 Kꢀmin under
air. Subsequently, the excess V O on the electrodes was removed by soaking the
2
5
as‐synthesized BiVO electrodes in a NaOH (1 M) aqueous solution for 5 min. The morphology
4
of BiVO electrode was observed with scanning electron microscopy (SEM) using an S-4800
4
field microscope ((Hitachi High-technologies Co., Japan) at an acceleration voltage of 10 kV.
The crystal structure of BiVO was attained from X-ray diffraction (XRD) patterns by using a
4
D/MAX-RC thin film X-ray diffractometer (RIGAKU Co. Japan) equipped with a nickel filter
-1
under a scan speed of 5° min and Cu Kα radiation (λ = 1.5418 Å) in the range of 10−60°.
UV–vis absorption and transmittance spectra (range from 300 nm to 700 nm) were obtained
using a V-650 UV/visible spectrophotometer (JASCO Inc., Japan), in which the electrode was
placed in the center of an integrating sphere to measure all light reflected and transmitted to
accurately assess the absorbance.
4

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Fabrication of carbon cloth cathode
A carbon cloth (Zoltek Co., USA) was thoroughly washed by ultrasonication in acetone,
isopropanol, and DI water, and then dried in the oven at 353 K for 1 h, followed by treated with
ultraviolet ozone for 10 min. The clean carbon cloth was cut into small pieces and was wired using
silver paste and the edges were covered by an insulating epoxy resin (Hysol 9460), leaving an
active area of 2 cm2.
Photoelectrocatalytic degradation of lignin by BiVO4
Voltammetric experiments for lignin oxidation by BiVO were performed with a single-cell
4
compartment with a 3-electrode system: BiVO (working electrode), Ag/AgCl
4
3ꢀM NaCl
(reference electrode), and Pt wire (counter electrode) in connected to a potentiostat/galvanostat.
To examine the photooxidation of lignin on the BiVO photoanode, linear sweep voltammetry
4
(
LSV) was performed in KHCO buffer (10ꢀmM, pH 8.2) with different concentration of lignin
3
−
1
at a scan rate of 50ꢀmVꢀs under illumination using a 450 W Xe lamp equipped with a 400 nm
-2
cut-off filter. The light intensity was calibrated at 100ꢀmWꢀcm on the surface of photoanode
using an ILT 1400-A radiometer (International Light Technologies Co., USA). The
photoelectrocatalytic reaction was further conducted in the 3-electrode system mentioned
above with applied different anodic bias range from 0 mV to 800 mV (vs Ag/AgCl).
Characterization of lignin photooxidation and products
1
13
2
D H- C HSQC NMR analysis was carried out by dissolving 20 mg freeze-dried reaction
samples in 0.5 mL DMSO-d , which were then transferred to NMR tubes. NMR spectrum from
6
the lignin sample was acquired at 25℃ on a 400 MHz FT-NMR (Bruker) instrument with a 5
mm BBO NMR probe. The central DMSO solvent peak was used as an internal reference
5

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(H/C 2.50/39.52 ppm). The HSQC parameters and NMR chemical assignments followed the
2
2
previously published paper. For the gel permeation chromatography (GPC) analysis, 10 mg
freeze-dried reaction samples were acetylated by 0.5 mL pyridine and 0.5 mL acetic anhydride
at 50℃ for 16 h. Modifying lignin with acetyl and propyl groups improved its incorporation
into the matrix due to a correspondingly higher solubility. Therefore, lignin samples need to be
derived (e.g., acetylation, methylation) to enhance the solubility in organic solvent before
analysis.23, 24 Acetylated reaction samples were obtained by centrifugation with deionized
water and dried under vacuum at 35℃. Acetylated samples were dissolved in THF at 4 mg
-1
mL and filtrated by a PTFE membrane filter (0.2 µm pore size) before GPC analysis. The
average molecular weight distribution of lignin sample was measured by GPC (Agilent 1200S
equipped with an Optilab rEX (RI) detector). PLgel MIXED-C 5 µm, 7.5 × 300 mm and PLgel
MIXED-E 3 µm, 7.5 × 300 mm columns were used with THF as an eluent with a flow rate 1.0
mL/min at 30℃. Calibration curve for molecular weight was generated using polystyrene
standards. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were
collected using a Nicolet iS50 FTIR (Thermo Scientific Inc., USA) spectrometer with one
bounce reflectance diamond ATR crystal accessory, which was used to determine a
modification in internal structure of lignin before and after oxidation. The oxidation products
were further analyzed by using gas chromatography–mass spectrometry (GC-MS) and high
resolution mass spectra (HR-MS). Aqueous samples (20 ml) were acidified to pH 2–3 with
HCl and thoroughly extracted three times with ethyl acetate. We acidified the lignosulfonate
samples to protonate the products before extraction, which helps homogenizing and recovering
the possible phenolic products from the aqueous phase.25, 26 The organic layer of the extracts
were collected and dried over anhydrous MgSO , followed by rotary evaporation. Then the
4
residue was dissolved in 100 μl dioxane and 10 μL pyridine, followed by derivatization with
6

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5
0 μL of BSTFA/TMCS in a water bath at 353 K for 45 min under constant shaking, and the
resultant was used for GC-MS analysis. 4.00 μL of silylated samples was injected into a TG-5
MS column (30 m × 0.25 mm × 0.25 μm) and separation performed using a Trace 1300 GC
instrument with ISQ QD single quadrupole mass analyzer (Thermo Scientific Inc., USA). The
-1
analyses were performed using a splitless mode (split flow 52.2ꢀmL min ) with helium at
-1
1
.740ꢀmL min . The inlet temperature was maintained at 573 K, MS transfer line at 553 K, and
ion source at 553 K (electron ionization). The temperature gradient started at 318 K and was
-1
held there for 5ꢀmin, then raised to 553 K at 10 K min , and held there for a further 10ꢀmin.
The MS was operated with electron ionization mode (70 eV). For the analysis of oxidation
products from alkali lignin and milled wood lignin, the samples were extracted with
dichloromethane and analyzed using a GC–MS (Agilent) with a DB-5MS (60 m × 250 μm ×
0
.25 μm) column. The oven temperature was elevated from 323 K to 553 K. The product mass
spectrum was identified using an authentic library of standards. The mass analyzer range was
set to 5–1050 amu (scan time 0.2 sec) with positive ion polarity. The High Resolution Mass
Spectroscopy (HR-MS) was carried out with a microTOF-QⅡ instrument (Bruker Corp.,
Germany) using the technique of electrospray ionization (ESI). Instrumental conditions were
set as follows: spray voltage (IS) 4.5 kV, nebulizer temperature 180 °C, sheath gas (nitrogen)
flow rate, ca. 4 L/min. The electrospray source was heated to 50 °C.
NADH regeneration and biocatalytic reduction in lignin-fueled, unbiased PEC system
The BiVO /perovskite solar cell tandem configuration was constructed based on a two-
4
electrode system according to the literature.27, 28
The photoelectrochemical NADH
regeneration and biocatalytic reduction in lignin-fueled PEC system was conducted in the two-
cell compartment connected with a 3 M KCl salt bridge without an applied bias (0ꢀV) under
illumination using a 450 W Xe lamp equipped with a 420 nm cut-off filter (light intensity: 100
7

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-2
mW cm ). BiVO photoanode was immersed in a KHCO (10 mM, pH 8.2) containing certain
4
3
amount of lignin in a homemade large glass cell, whereas carbon cloth cathode was immersed
+
in a phosphate buffer (100 mM, pH 6.5) containing M (0.5 mM) and NAD (2 mM) in a
-1
disposable cuvette (2 mL). In full cell experiments, 8 mL of lignin solution (0-5 mg mL ) was
placed in a cuboid glass reactor. The distance between the perovskite solar cell and BiVO4
photoanode was 1.5 cm in the tandem structure, and the space was filled with lignin solution.
The regenerated amount of NADH was calculated from the absorbance increase at 340 nm
using a V-650 UV/visible spectrophotometer. For biocatalytic CO reduction to formate
2
-1
coupled with lignin oxidation, TsFDH (10 U ml ) was added to the above-mentioned cathodic
component. CO was purged to the cathodic solution for at least 1 h before and continuously
2
-1
purged during the whole reaction (flow rate: 50 mL min ). Formate concentration was
determined by calculating the area of formate peak from the sample solution (200 μL) diluted
by 2M H SO (20 μL) for inactivation of the TsFDH, and the peak was detected using
2
4
Prominence LC-20A modular liquid chromatography (Shimadzu Corp., Japan) equipped with
an Aminex® HPX-87H ion exclusion column. For GDH enzymatic reaction coupled with
lignin oxidation, the reaction medium for the cathodic component consisted of
+
NAD (2ꢀmM), M (0.5ꢀmM), α-ketoglutaric acid disodium salt dihydrate (50ꢀmM),
(
NH ) SO (250ꢀmM), and GDH (40ꢀU) in 3ꢀml of phosphate buffer (0.1ꢀM, pH 7.5). The L-
4 2 4
glutamate concentration was analyzed by using a 1260 Infinity liquid chromatography system
Agilent Technologies, U.S.A.).
(
Data treatment
All photocatalytic experiments were carried out as triplicates, and the derived performances
are drawn as mean values ±ꢀstandard deviation.
8

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Results and discussion
Characterization of nanoporous BiVO photoanode
4
2
9
We prepared nanoporous BiVO through a two-step process according to the literature, which
4
involved (1) electrochemical deposition of bismuth oxyiodide (BiOI) onto a fluorine-doped tin
oxide (FTO) substrate (Fig. S1†) and then (2) thermal transformation of BiOI into BiVO in
4
the presence of vanadyl acetylacetonate precursor. The resulting BiVO film exhibited a
4
nanoporous network structure with agglomerated particles averaging 120ꢀnm in diameter (Fig.
1
a). Approximately 2.5 eV of optical bandgap was estimated from the Tauc plot of UV-vis
absorbance for the nanoporous BiVO (Fig. 1b). Our X-ray diffraction analysis confirmed the
4
monoclinic scheelite structure of crystalline BiVO (PDF file No. 14-0688) (Fig. 1c). We
4
examined the capability of thus-synthesized BiVO to oxidize lignin under illumination using
4
a 450 W Xe lamp equipped with a 420 nm cut-off filter. According to our analysis using a
linear sweep voltammogram (LSV) (Fig. 1d and S2†), we observed a significant increase in
the oxidation current from -0.2 V (vs. Ag/AgCl) as well as a cathodically shifted oxidation
onset potential (≈50 mV) in the presence of lignins (e.g., alkali lignin, lignosulfonate) under
illumination, which we attributed to lignin’s capability to capture photogenerated holes in
BiVO and facilitate electron-hole separation. The oxidative photocurrent exhibited a
4
-1
proportional relationship to lignin concentration (0 to 5 mg mL ) (Fig. S3†), implying that
lignin oxidation is more favorable than water oxidation on BiVO electrodes. In contrast, other
4
non-lignin residues from cellulosic biomass, such as cellulose and xylan, had negligible effect
on the photocurrent generation by BiVO4.
Photoelectrochemical oxidation of lignin by BiVO4
We further investigated photoelectrochemical oxidation of alkali lignin using a three-electrode
configuration that consisted of BiVO as a working electrode, Ag/AgCl3ꢀM NaCl as a reference
4
9

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electrode, and Pt wire as a counter electrode. We analyzed the changes in the chemical
structures of lignin during the photoelectrochemical oxidation by using 2D-NMR spectroscopy.
As displayed in Fig. 2 and S4†, the characteristic linkages such as -O-4 (blue colored A), -
5
(green colored B), - (cyan colored C) were observed and progressively disappeared during
the prolonged photoelectrochemical oxidation. The results suggest that oxidation of lignin
occurred through the linkages with low bond dissociation energies (BDEs) such as -O-4 (54-
-1
-1
7
2 kcal mol ) and -5 (54-63 kcal mol ), rather than the one with relatively higher BDE (-,
-1 30
8
1 kcal mol ). According to our analysis using gel permeation chromatography (Fig. S5†), a
dramatic decrease in high molecular weight of lignin polymer was observed over time, which
suggested the depolymerization of lignin into small molecules. The major fraction of untreated
lignin was polymers of over 51,000 Mw and the photoelectrochemical reaction caused a
significant decrease of the molecular weight to less than 15,000 Mw after prolonged reaction
of 12 hours, which clearly showed fragmentation and degradation of lignin macromolecules.
The photoelectrochemical oxidation of lignin exhibited different oxidation
characteristics depending on the structure of original lignin. The photoelectrochemical
oxidation of lignosulfonate occurred through a ring-opening oxidation of polymeric lignin with
unspecific fragmentation into smaller substances. According to our analysis using FTIR (Fig.
-1
S6†), the absorption bands at 1420, 1512, and 1595 cm corresponding to aromatic skeletal
3
1
vibrations in lignosulfonate disappeared after 3 hours, indicating the cleavage of the aromatic
-1
structure. We observed increased absorption at around 1644 and 1719 cm , which corresponds
to the C=O stretch in conjugated and unconjugated carbonyl/carboxyl groups, respectively. The
results imply the conversion of aromatic ring structures into carbonyl (or carboxyl) groups. On
the other hand, the oxidation of alkali lignin exhibited an increase in C=O stretch at around
-1
-1
1
700 cm , while maintaining its aromatic ring vibrations at 1448, 1537 and 1625 cm . Gas
chromatography–mass spectrometry (GC-MS) analysis further showed that the majority of the
1
0

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oxidation product from alkali lignin was carboxylated aromatic compound of diethyl phthalate
(Fig. S7 and Table S1, †), which is in good agreement with the FTIR results. It indicates that
the aromatic ring structure of alkali lignin could be preserved during the photoelectrochemical
oxidation process. The oxidation of other lignin species (lignosulfonate and milled wood lignin)
yielded different kinds of carboxylated aliphatic and aromatic compounds, such as butanedioic
acid, glycolic acid, benzoic acid, N-methoxymethyl-N-methylformamide, 1,1,3,3-
tetrachloroacetone, and N-methyl-N(2-methylpropyl)formamide (Fig. S7, S8 and Table S2,
S3†). We speculate that the oxidation of lignin may differ from sources and pretreatment
methods, which significantly affect the nature (e.g., chemical structure and physicochemical
properties) of the respective lignin substrate. The mass spectra from high-resolution mass-
spectrometry (HR-MS) analysis further confirmed the fractionation of high-molecular weight
lignin (Fig. S9†). On the other hand, it is worth noting that prolonged over-oxidation reaction
may further decompose the products into CO as the simplest form of oxidized carbonaceous
2
materials. According to the literature,2, 32, 33 photocatalytic oxidation of lignocellulosic biomass
could generate CO , which is sequestered as carbonate. We believe that the issue of over-
2
oxidation could be avoided by optimizing reaction times (e.g., exchange of anode electrolyte
before complete oxidation) to obtain more chemicals.
We observed a significant decrease in lignin’s characteristic absorption at around 280
nm associated with the etherified hydroxyl groups in the β-O-4 linkage as well as the
1
2
hydroxyl/methoxyl groups linked to the aromatic rings of lignin units (Fig. S10a†). The
results indicate the photoelectrochemical oxidation of lignin, which was also evident from the
decolorization of the lignin solution. Moreover, the oxidation of lignin also showed a clear
dependency on the applied potential (Fig. S10b†); and negligible changes were observed under
dark conditions. In addition, we confirmed that other lignocellulosic components such as
cellulose and xylan exhibited negligible effect on the oxidation of lignosulfonate by BiVO4
1
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-1
even at the same concentration as lignin (1 mg mL ) (Fig. S11†). The results suggest that
BiVO electrode oxidizes lignin (rather than other components) because of electrical activities
4
of lignin originating from their diverse redox-active groups.
+
-
-
The reactive species such as h , e , •OH, H O , and •O produced by the
2
2
2
photoelectrochemical redox process of BiVO are identified as the key active species for the
4
C-C and C-O bond cleavage.34-38 We investigated the effect of triethylamine (TEA, hole
scavenger), sodium persulfate (Na S O , electron scavenger), tert-butyl alcohol (TBA,
2
2
8
hydroxyl radical scavenger) and sodium pyruvate (SP, H O scavenger) on lignin oxidation by
2
2
photoexcited BiVO (Fig. S12†). According to our results, the hole scavenger inhibited the
4
oxidation of lignin, indicating that the lignin oxidation is driven by the photogenerated holes.
The enhanced oxidation of lignin in the presence of electron scavenger also supports the hole-
driven oxidation of lignin, as the electron-hole separation of BiVO could be accelerated by the
4
electron scavengers. Both TBA and SP exerted negligible effect on lignin oxidation, implying
•
that the OH radical and H O did not play a dominant role. In addition, we tested the effect of
2
2
reduced oxygen species that could be generated by the counter electrode (Pt wire) during the
reaction (Fig. S12e†). The oxygen reduction at the cathode did not affect the oxidation of lignin,
which indicate that the superoxide species have negligible effect on the oxidation of lignin.
Thus, we speculate that BiVO facilitates direct transfer of photogenerated holes to lignin,
4
which could lead to the weakening C-C bonds and inducing fragmentation and oxidation of
lignin into organic acids.33, 39 We also observed a negligible effect of dissolved oxygen on the
oxidation of lignin by BiVO . According to our 2D-NMR analysis (Fig. S13†), there was no
4
significant difference in lignin oxidation under both of O and N purged conditions, indicating
2
2
that O is not involved in the photoelectrochemical oxidation of lignin.
2
1
2

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Bias-free photoelectrochemical oxidation of lignin
According to our result (Fig. S10b†), an external bias of over 200 mV (vs. Ag/AgCl, pH 8.2)
was required to drive lignin oxidation by photoexcited BiVO electrode. To achieve the “bias-
4
free” photoelectrochemical oxidation of lignin, we have designed a tandem structure that
consists of a BiVO photoanode and an organometallic perovskite PV. Considering the
4
remarkable band gaps (1.6 eV for perovskite and 2.5 eV for BiVO ), we expected that the
4
potential of electrons generated from the BiVO electrode would be further elevated in the
4
tandem structure where the photovoltaic absorbs the remaining part of the visible light passing
through the photoanode. We chose perovskite PV as a green option due to its superb output
photo-voltage to drive the bias-free photoelectrochemical reaction, as well as its easy synthesis
of solution processing that does not require energy-intensive vacuum deposition process. We
2
fabricated a large-area PV cell with a band gap of ≈1.6 eV (active area: 1 cm ) on the base of
4
0
a triple-cation perovskite absorber. The assembly with PV led to a cathodic shift of 920 mV
in the J-V profile of BiVO (Fig. 3a), which is consistent with the open-circuit voltage of
4
2
8
perovskite (V , ≈0.94 V). The BiVO -perovskite tandem system (BiVO /PV) also exhibited
oc
4
4
an enhanced incident photon-to-current conversion efficiency up to 700 nm due to the
photovoltaic effect of the perovskite solar cell (Fig. S14†).
To examine the effect of lignin concentration on the performance of the tandem cell,
we tested the photocurrent generation from BiVO /PV and bare BiVO (Fig. 3a). Note that the
4
4
increasing amount of lignin did not significantly affect the generation of oxidative photocurrent
from both configurations. Also, lignin solution did not have significant absorption at over 440
nm (Fig. S15†), which would not interfere with the visible-light absorption by the PV. The
results show that the perovskite solar absorber provides additional power with which to achieve
unbiased photoelectrochemical lignin oxidation.
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Lignin-fueled, bias-free regeneration of NADH
We coupled photoelectrochemical lignin oxidation with electrochemical reduction of
+
2
nicotinamide cofactor (NAD ) on a carbon cloth cathode (geometric surface area: 1ꢀcm ). We
2
+
observed this reduction via a mediator {M, [Cp*Rh(bpy)H O] } (Fig. S16†); the cathodic
2
reduction peak of M at around -700 mV (vs. Ag/AgCl, pH 6.5) increased significantly by the
+
catalytic reaction in the presence of NAD (2 mM), which we attributed to the sequential
+
electron transfer from the electrode to M and to NAD . We observed highly efficient NADH
regeneration upon light irradiation to the tandem BiVO /perovskite compartment containing 5
4
-1
mg mL of lignin under no bias (Fig. S17). The results imply that lignin oxidation on a BiVO
4
+
electrode provides most of the electrons required for NAD reduction, rather than through the
slow reaction of water oxidation (i.e., without lignin). In contrast, NADH was not regenerated
in the absence of perovskite PV, indicating that the solar absorber is essential for unbiased
coupling of the reduction and oxidation reactions. Under illumination, the initial rate of NADH
regeneration exhibited a proportional relationship to the lignin concentration, which was
-1
-1
saturated at approximately 2.4 mM h with 5 mg mL lignin (Fig. 3b). Note that lignin
-1
oxidation on the BiVO photoelectrode was also saturated at over 5 mg mL of lignin as well
4
-1
as 0.7 mg mL of guaiacylglycerol-beta-guaiacyl ether, a lignin model dimer, according to the
LSV analysis (Fig. S18 and S19, †). We attribute the saturation behavior to the limited number
of active sites (and thus limited photocurrent generation) on the surface of the BiVO electrode
4
for lignin oxidation at high lignin concentrations.
Lignin-fueled biocatalytic reduction
Building upon the successful regeneration of NADH in the unbiased PEC platform using lignin,
we reduced CO to formate using a NADH-dependent formate dehydrogenase from
2
Thiobacillus sp. (TsFDH) (Fig. S20†). In the lignin-fueled PEC system, the rate of formate
1
4

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-1
synthesis (~0.37 mM h ) was 4.5 times higher than that of the reaction based on water
-1
oxidation (~0.08 mM h ) (Fig. 4a). The negative control results obtained without PV, TsFDH,
+
NAD , M, or CO confirm the critical role of each reaction component in facilitating the
2
photobiocatalytic CO -to-formate conversion. Our cyclic voltammetric analysis of carbon cloth
2
electrode in N and CO purged buffers showed that there was no sign of direct electrochemical
2
2
reduction of CO under the applied potential up to -1.2 V (vs. Ag/AgCl) (Fig. S21†).
2
-1
Lignin concentration (1~5 mg mL ) exhibited a proportional relationship to the rate
of formate formation (Fig. 4b), which is in agreement with Fig. 3b. The results indicate that
the oxidation of lignin is a rate-determining factor for its counterpart reaction of biocatalytic
CO reduction through NADH regeneration. Obviously, because of the NADH-dependent
2
+
nature of TsFDH, increased NAD concentration showed a positive effect on the rate of CO
2
reduction (Fig. S22†). The biocatalyst concentration also had a significant influence on the
overall efficiency of CO reduction; however, the initial rate of formate formation versus the
2
-1
concentrations of TsFDH was saturated with a maximum (~0.35 mM h ) at 10 U TsFDH (Fig.
S23†). This supports our assumption that electron transport by lignin oxidation is a rate-
limiting step that governs the platform’s overall efficiency. We further observed the production
of 5.71 mM of formate after 24 hours of reaction (Fig. S24†). The slow rate of formate
production at the initial reaction stage is attributable to the lag phase of NADH regeneration
+
for reaching the equilibrium ratio of [NAD ]/[NADH] needed to drive TsFDH for CO
2
reduction.21, 41 Previous studies on biocatalytic CO reduction based on PEC platforms (e.g., α-
2
4
2
41
Fe O /BiFeO photocathode, Co-Pi-deposited α-Fe O /ITO, and triple-junction silicon on
2
3
3
2
3
4
3
ITO/H-SiNW ) required additional bias as high as 1.0 V (Table S4†). The lignin-fueled
tandem PEC system achieved a high formate yield using only light; to the best of our
knowledge, this study is the first example of the photoelectrochemical reformation of lignin
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coupled with biocatalytic fuel production from CO and visible light, which suggests an
2
alternative solution for valorizing waste lignin to yield valuable chemicals.
In addition to the biocatalytic CO reduction, we also tested another model reaction,
2
selective amination of α-ketoglutarate into L-glutamate by L-glutamate dehydrogenase (GDH)
(Fig. S25†). According to our results, the lignin-fueled PEC platform facilitated the biocatalytic
reduction by GDH under unbiased conditions, which demonstrates its versatility for the
environment-friendly utilization of lignin and the green-synthesis of value-added chemicals.
Conclusions
In summary, we have demonstrated bias-free, photoelectrochemical oxidation of lignin and
biocatalytic reductions (e.g., CO to formate and α-ketoglutarate to L-glutamate) in a tandem
2
photoanode/PV structure. Lignin oxidation by BiVO photoanode produced small carboxylated
4
organic molecules, which simultaneously supplied electrons to the cathode for NADH
regeneration coupled with redox biotransformations by TsFDH and L-glutamate
dehydrogenase. The tandem structure with perovskite-based PV facilitated unbiased operation
of the PEC system by compensating for the potential gap between lignin oxidation and CO2
reduction through artificial Z-schematic absorption. The BiVO -catalyzed oxidation of lignin
4
-1
-1
(
5 mg mL ) allowed for rapid formate production (0.37 mM h ), at a rate 4.5 times higher than
-1
that of the water oxidation-based reaction (0.08 mM h ). Under no bias and using visible light
as the sole energy source, the lignin-fueled photoelectrochemical platform achieved
approximately 5.71 mM formate production within 24 hours. The current study hints at a new
design for unassisted artificial photosynthesis of value-added chemicals from waste lignin.
Conflicts of interest
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6

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There is no conflict of interest to declare.
Acknowledgements
This work was supported by the National Research Foundation (grant numbers: NRF-2015
R1A3A2066191 and 2017M1A2A 2087630), Republic of Korea.
Notes and References
1
.
C. Xu, R. A. D. Arancon, J. Labidi and R. Luque, Chem. Soc. Rev., 2014, 43, 7485-
500.
7
2
3
4
.
.
.
M. F. Kuehnel and E. Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290-3296.
M. Sasaki and M. Goto, Fuel, 2009, 88, 1656-1664.
L. G. Covinich, D. I. Bengoechea, R. J. Fenoglio and M. C. Area, Am. J. Environ.
Eng., 2014, 4, 56-70.
5
6
.
.
P. Parpot, A. Bettencourt, A. Carvalho and E. Belgsir, J. Appl. Electrochem., 2000,
3
0, 727-731.
R. Tolba, M. Tian, J. Wen, Z.-H. Jiang and A. Chen, J. Electroanal. Chem., 2010,
49, 9-15.
6
7
.
.
W. Jin, R. Tolba, J. Wen, K. Li and A. Chen, Electrochim. Acta, 2013, 107, 611-618.
E. Reichert, R. Wintringer, D. A. Volmer and R. Hempelmann, Phys. Chem. Chem.
Phys., 2012, 14, 5214-5221.
8
9
1
.
P. Cai, H. Fan, S. Cao, J. Qi, S. Zhang and G. Li, Electrochim. Acta, 2018, 264, 128-
1
39.
0.
D. Shao, J. Liang, X. Cui, H. Xu and W. Yan, Chem. Eng. J., 2014, 244, 288-295.
1
7

Green Chemistry
Page 18 of 25
View Article Online
DOI: 10.1039/D0GC01521K
1
1
1
1
1.
2.
3.
4.
M. Zirbes, D. Schmitt, N. Beiser, D. Pitton, T. Hoffmann and S. R. Waldvogel,
ChemElectroChem, 2019, 6, 155-161.
L. Song, X. Zhao, L. Cao, J.-W. Moon, B. Gu and W. Wang, Nanoscale, 2015, 7,
1
6695-16703.
C. A. Gouvêa, F. Wypych, S. G. Moraes, N. Durán and P. Peralta-Zamora,
Chemosphere, 2000, 40, 427-432.
R. G. Marques, A. M. Ferrari-Lima, V. Slusarski-Santana and N. R. C. Fernandes-
Machado, J. Environ. Manage., 2017, 195, 242-248.
1
1
1
5.
6.
7.
J. Chen, W. Liu, Z. Song, H. Wang and Y. Xie, BioEnerg. Res., 2018, 11, 166-173.
J. Kim and C. B. Park, Curr. Opin. Chem. Biol., 2019, 49, 122-129.
S. H. Lee, D. S. Choi, S. K. Kuk and C. B. Park, Angew. Chem. Int. Ed., 2018, 57,
7
958-7985.
A. Y. Ru Ng, B. Boruah, K. F. Chin, J. M. Modak and H. S. Soo, ChemNanoMat,
020, 6, 185-203.
1
8.
2
1
2
9.
0.
X. Liu, X. Duan, W. Wei, S. Wang and B.-J. Ni, Green Chem., 2019, 21, 4266-4289.
F. Hollmann, B. Witholt and A. Schmid, J. Mol. Catal. B: Enzym., 2002, 19-20, 167-
1
76.
2
2
2
1.
2.
3.
H. Choe, J. C. Joo, D. H. Cho, M. H. Kim, S. H. Lee, K. D. Jung and Y. H. Kim,
PLoS One, 2014, 9, e103111.
C. Zhao, J. Huang, L. Yang, F. Yue and F. Lu, Ind. Eng. Chem. Res., 2019, 58, 5707-
5
714.
R. El Hage, N. Brosse, L. Chrusciel, C. Sanchez, P. Sannigrahi and A. Ragauskas,
Polym. Degrad. Stabil., 2009, 94, 1632-1638.
2
2
4.
5.
Y. Li and A. J. Ragauskas, J. Wood Chem. Technol., 2012, 32, 210-224.
M. Antolovich, P. Prenzler, K. Robards and D. Ryan, Analyst, 2000, 125, 989-1009.
1
8

Page 19 of 25
Green Chemistry
View Article Online
DOI: 10.1039/D0GC01521K
2
2
6.
7.
H. Kim, Y. Kim, S. Park, J. Lee and J. Choi, Acta Chromatogr., 2020, 32, 64-67.
Y. W. Lee, P. Boonmongkolras, E. J. Son, J. Kim, S. H. Lee, S. K. Kuk, J. W. Ko, B.
Shin and C. B. Park, Nat. Commun., 2018, 9, 4208.
2
8.
S. K. Kuk, Y. Ham, K. Gopinath, P. Boonmongkolras, Y. Lee, Y. W. Lee, S.
Kondaveeti, C. Ahn, B. Shin, J.-K. Lee, S. Jeon and C. B. Park, Adv. Energy Mater.,
2
019, 9, 1900029.
2
3
9.
0.
T. W. Kim and K.-S. Choi, Science, 2014, 343, 990-994.
R. Rinaldi, R. Jastrzebski, M. T. Clough, J. Ralph, M. Kennema, P. C. A. Bruijnincx
and B. M. Weckhuysen, Angew. Chem. Int. Ed., 2016, 55, 8164-8215.
L. Shao, X. Zhang, F. Chen and F. Xu, J. Anal. Appl. Pyrol., 2017, 128, 27-34.
H. Kasap, D. S. Achilleos, A. Huang and E. Reisner, J. Am. Chem. Soc., 2018, 140,
3
3
1.
2.
1
1604-11607.
3
3
3
3
3.
4.
5.
6.
D. W. Wakerley, M. F. Kuehnel, K. L. Orchard, K. H. Ly, T. E. Rosser and E.
Reisner, Nat. Energy, 2017, 2, 17021.
Y. Nakabayashi, M. Nishikawa, N. Saito, C. Terashima and A. Fujishima, J. Phys.
Chem. C, 2017, 121, 25624-25631.
A. Speltini, M. Sturini, D. Dondi, E. Annovazzi, F. Maraschi, V. Caratto, A. Profumo
and A. Buttafava, Photochem. Photobiol. Sci., 2014, 13, 1410-1419.
X. Shi, S. Siahrostami, G.-L. Li, Y. Zhang, P. Chakthranont, F. Studt, T. F. Jaramillo,
X. Zheng and J. K. Nørskov, Nat. Commun., 2017, 8, 701.
3
3
7.
8.
S.-H. Li, S. Liu, J. C. Colmenares and Y.-J. Xu, Green Chem., 2016, 18, 594-607.
N. Luo, M. Wang, H. Li, J. Zhang, H. Liu and F. Wang, ACS Catalysis, 2016, 6,
7
716-7721.
T. Awatani, K. D. Dobson, A. J. McQuillan, B. Ohtani and K. Uosaki, Chem. lett.,
998, 27, 849-850.
3
9.
1
1
9

Green Chemistry
Page 20 of 25
View Article Online
DOI: 10.1039/D0GC01521K
4
4
4
4
0.
1.
2.
3.
P. Boonmongkolras, D. Kim, E. M. Alhabshi, I. Gereige and B. Shin, RSC Adv., 2018,
8
, 21551-21557.
D. H. Nam, S. K. Kuk, H. Choe, S. Lee, J. W. Ko, E. J. Son, E.-G. Choi, Y. H. Kim
and C. B. Park, Green Chem., 2016, 18, 5989-5993.
S. K. Kuk, R. K. Singh, D. H. Nam, R. Singh, J.-K. Lee and C. B. Park, Angew.
Chem. Int. Ed., 2017, 56, 3827-3832.
E. J. Son, J. W. Ko, S. K. Kuk, H. Choe, S. Lee, J. H. Kim, D. H. Nam, G. M. Ryu, Y.
H. Kim and C. B. Park, ChemComm., 2016, 52, 9723-9726.
2
0

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Scheme 1. Schematic illustration of lignin-fueled photoelectrochemical platform for solar-
driven redox biotransformations. The BiVO photoanode promotes photoelectrochemical
4
oxidation of lignin. The triple-cation perovskite photovoltaic (PV) forms a tandem structure
with the BiVO photoanode, providing supplementary photovoltage to drive reactions under
4
+
zero external bias. The carbon-cloth cathode reduces NAD to NADH for the reductive
conversion of substrates (e.g., CO to formate, α-ketoglutarate to L-glutamate) by redox
2
enzymes (formate dehydrogenase (FDH) and L-glutamate dehydrogenase (GDH)).
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1

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Fig 1. Characterization of BiVO photoanode. (a) Scanning electron microscopy (SEM) and
4
optical photograph images of nanoporous BiVO film. (inset: side-view SEM image of the
4
BiVO deposited on a FTO electrode). (b) UV–vis absorption spectrum of BiVO film (inset:
4
4
a Tauc plot for calculation of the optical bandgap (2.5 eV) of BiVO ). (c) X-ray diffraction
4
(
XRD) patterns of BiVO fit with a scheelite-monoclinic structure (PDF No. 14-0688). Black
4
dots denote the XRD peaks of FTO substrate. (d) Linear-sweep voltammetric (LSV) of the
-1
BiVO photoelectrode with various lignosulfonate concentrations (0 ~ 5 mg mL ) under dark
or light conditions (100ꢀmWꢀcm ). Scan rate: 50 mV s .
4
−
2
−1
2
2

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Fig 2. Changes of the chemical structure of alkali lignin during the photoelectrochemical
1
3
1
oxidation process. 2-D C- H heteronuclear single-quantum correlation (HSQC) nuclear
magnetic resonance analysis (a) before and (b) after 12 hours of reaction. (c) Color coded
structural motifs in lignin.
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Fig 3. Photoelectrochemical regeneration of NADH. (a) J-V plot for photocurrent generation
from BiVO /PV tandem structure and bare BiVO . The changes in linear-sweep voltammetric
4
4
(
LSV) of the BiVO /PV tandem structure and bare BiVO with various lignin (lignosulfonate)
4
4
-1
concentrations (0 ~ 5 mg mL ) were recorded under visible light irradiation. A huge shift in
the onset of oxidation (0.92 V) was induced by PV (100ꢀmWꢀcm , scan rate: 50 mV s ). (b)
−
2
−1
Unbiased light-driven regeneration of NADH in the photoelectrochemical cell under various
+
conditions. Reaction conditions: Cathode electrolyte contained M (0.5 mM) and NAD (2.0
-1
mM) in a phosphate buffer (100 mM, pH 6.5).; Anode electrolyte contained 5 mg mL lignin
in a KHCO buffer (10 mM, pH 8.2).; Geometrical surface area of BiVO , perovskite PV cell,
3
4
2
and carbon cloth were 6, 1, and 2 cm , respectively.
2
4

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Fig 4. Bias-free photoelectrochemical lignin oxidation coupled with biocatalytic CO reduction.
2
(a) A series of control experiments for formate formation in the unbiased tandem system. (b)
Initial rate of formate formation with various lignin concentrations. Reaction conditions:
+
Cathode electrolyte contained M (0.5 mM) and NAD (2.0 mM) in a phosphate buffer (100
-1
mM, pH 6.5).; Anode electrolyte contained 0 ~ 5 mg mL lignin in a KHCO buffer (10 mM,
3
pH 8.2).; Geometrical surface area of BiVO , perovskite PV cell, and carbon cloth were 6, 1,
4
2
and 2 cm , respectively.
2
5