Electrochemical Oxidation of 5-Hydroxymethylfurfural with NiFe

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Electrochemical Oxidation of 5-Hydroxymethylfurfural with NiFe Layered Double Hydroxide (LDH) Nanosheet Catalysts WUJUN LIU, Lianna Dang, Zhuoran Xu, Han-Qing Yu, Song Jin, and George W. Huber ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01017 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ABSTRACT

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Electrochemical oxidation of biomass-derived platform molecules can enable the production of

17

value-added oxygenated commodity chemicals under mild conditions in a distributed fashion

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using renewable electricity; however, very few efficient, robust and inexpensive electrocatalysts

19

are available for such electrochemical oxidation. Here we demonstrate that earth-abundant NiFe

20

layered double hydroxide (LDH) nanosheets grown on carbon fiber paper can efficiently catalyze

21

the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) at the

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anode of an electrochemical cell. A near-quantitative yield of FDCA and 99.4% Faradaic

23

efficiency of HMF conversion under ambient conditions can be achieved in the electrochemical

24

process. HMF has a higher rate of oxidation than water, and can act as an alternative anodic

25

reaction for alkaline H2 evolution in water splitting cells. As the first report on using bimetallic

26

metal hydroxide/oxide catalysts for electrochemical oxidation of HMF, this work opens up

27

opportunities in electrochemical devices to simultaneously produce building-block chemicals

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from biomass-derived molecules and clean H2 fuels under ambient conditions with earth

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abundant materials.

30 31

KEYWORDS: Electrochemical oxidation, HMF, FDCA, NiFe LDH, biomass conversion.

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INTRODUCTION

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5-Hydroxymethylfurfural (HMF) is one of the most widely studied biomass-derived platform

35

molecules.1

36

5-hydroxymethyl-2-furancarboxylic

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2,5-furandicarboxylic acid (FDCA).5-10 FDCA is gaining increasing interest as a monomer to

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synthesize bio-based polymers including polyethylene 2,5-furandicarboxylate (PEF).11-14

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is a renewable polymer that can be used for many applications and is currently used as

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replacement for petroleum derived polyethylene terephthalate (PET).15-16 PEF has a similar

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structure to PET where the FDCA replaces terephthalic acid. PEF has improved barrier

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properties compared to PET including a 19-fold and 10-fold reduction in CO2 and O2

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permeability respectively and has similar mechanical and thermal properties as PET. HMF is

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typically oxidized to FDCA in an alkaline aqueous solution (pH ≥ 13) at elevated temperatures

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(30–130 °C) under high-pressure air or O2 (e.g., 0.3-2.0 MPa), with noble metal-based catalysts

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(e.g., Au, Pt, Ru, and Pd).17-19 For example, Wan et al. reported that Au-Pd supported on carbon

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nanotube (CNT) catalysts can selectively oxidize HMF (25 mmol/L) into FDCA under 0.5 MPa

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of O2 at 100 oC.20 Yi et al. investigated the catalytic oxidation of HMF (100 mmol/L) over a

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commercial Ru/C catalyst using 0.2 MPa of O2 at 120 oC, yielding 85% FDCA after 10 hours.21

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It would be highly desirable to develop improved methods that use milder conditions and less

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expensive catalysts to produce FDCA.

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HMF

can

be

selectively acid

oxidized (HFCA),3

into maleic

2,5-diformylfuran anhydride

(DFF),2

(MA),4

and

PEF

Electrochemical HMF oxidation is a promising alternative approach to conventional


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heterogeneous catalytic aerobic oxidation, as it is usually performed under ambient temperatures

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with the oxidation driven by the potential applied to the anodic electrode, thus avoiding the use

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of O2 or other hazardous chemical oxidants.22 Electrochemical oxidation can be more

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conveniently performed in smaller scale reactors at distributed locations. It will also be

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increasingly cost competitive as the cost of electricity from renewable sources (such as solar and

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wind) continues to decline. During electrochemical HMF oxidation, the surface reactions can be

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tuned by the applied potential. Electrochemical HMF oxidation has been limited to noble metal

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based catalysts (Pt, Au, Ru, and Pd)9-10,

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Chadderdon et al.,9 explored the electrocatalytic oxidation of HMF in alkaline media over carbon

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supported Au and Pd nanoparticles. They showed that HMF (20 mM) can be totally oxidized at a

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potential of 0.9 V (vs RHE) in one hour, but the highest FDCA selectivity was only 83%. In

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another study by Cha and Choi,22 it was found that HMF can be near-quantitatively converted to

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FDCA at a Faradaic efficiency of 100% with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a

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mediator over an Au electrode at a potential of 1.54 V (vs RHE). However, the use of TEMPO

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would increase the downstream separation costs.

22

with a low selectivity to FDCA. For example,

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Recently, earth-abundant metal phosphides and sulfides have been exploited for the

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electrochemical HMF oxidation. For example, Sun and co-workers employed Ni2P, Ni2S3, and

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even metallic Ni as the anodic catalysts for the HMF oxidation, reporting a nearly quantitative

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conversion of HMF to FDCA at a Faradaic efficiency of 98% to 100%.7-8, 23-24 However, metal

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sulfides and phosphides are usually thermodynamically less stable than their corresponding


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under

oxidizing

potentials,

and

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oxides/hydroxides

the

likely

formation

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oxides/hydroxides under the electrocatalytic conditions means that the active species may not in

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fact be the phosphides and sulfides, especially in the strongly oxidative environments in aqueous

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solutions, such as in water oxidation.25-27 Transition metal Ni, Co, and Fe-containing layered

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double hydroxides (LDH) or generally metal oxyhydroxides were reported as competitive water

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oxidation catalysts in alkaline electrolytes compared to Ir and Ru-based oxides, with onset

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overpotentials of less than 300 mV and excellent stability. 28-29 LDHs are synthetically analogues

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to the naturally occurring hydrotalcite minerals, with brucite-like positively-charged mixed metal

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hydroxide layers intercalated with water and charge-balancing anions.30 Their simple synthesis

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allows for their direct growth onto three-dimensional conductive substrates such as carbon fiber

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paper and Ni foams,31-32 and exfoliation of the layers provides further improvements in the water

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oxidation performance.31, 33 Within various binary LDHs, the NiFe material is considered the

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most promising electrocatalysts for water oxidation, although recently ternary compositions have

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shown even lower overpotentials to achieve similar current densities.32,

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catalysts, kinetic barriers to water oxidation persist for this challenging four-electron

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proton-coupled electron transfer reaction and the identity of the active catalytic site(s) remains

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controversial.36-37

34-35

of

metal

Yet with these

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In this work, we investigate and demonstrate that earth-abundant NiFe LDH nanosheets are

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efficient and robust anodic electrocatalysts for the oxidation of HMF to FDCA. A hydrothermal

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method was used to grow NiFe LDH nanosheets on carbon fiber paper as the anode. FDCA was


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directly produced from electrochemical oxidation of HMF at a potential of 1.23 V vs RHE in a

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yield of 98% with a faradic efficiency of 99.4%. Furthermore, since the HMF oxidation is

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kinetically more favorable than water oxidation, it could act as an alternative anodic reaction in

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water splitting cells to enhance the H2 evolution and oxidatively produce high-value organics.38

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EXPERIMENTAL SECTION

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Chemicals and materials

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5-Hydroxymethylfurfural

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2-formyl-5-furancarboxylic acid (FFCA), potassium hydroxide (KOH), nickel (II) nitrate

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hexahydrate (Ni(NO3)26H2O), iron (II) chloride tetrahydrate (FeCl24H2O), triethanolamine

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(TEOA), urea, and ethanol (EtOH) were purchased from Sigma Aldrich and were used as

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received unless otherwise noted. All aqueous solutions were prepared using nanopure deionized

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water (Thermo Scientific Banstead Nanopure) with a resistivity > 18 MΩ cm. Carbon paper

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(TGP-H-060) was purchased from Fuel Cell Earth (Woburn, Massachusetts, USA) and was

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subjected to hydrophilic treatment before use: The carbon paper was O2 plasma cleaned at 150 W

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for 5 minutes for each side, then placed in a preheated 800 °C oven for 5 minutes to remove

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oxidized contaminants. The treated carbon paper no longer floats in water and sinks with gentle

110

agitation.

(HMF),

2,5-furandicarboxylic

acid

(FDCA),

111 112


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Synthesis of the NiFe layered double hydroxide (LDH) nanosheets

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The NiFe LDH nanosheets were grown on the carbon paper through a hydrothermal approach.

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Generally, 14.9 mg (75 µmol) FeCl24H2O was added to a 15 mL centrifuge tube with 22.4 mg

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(150 µmol) neat TEOA. Next, 65.4 mg (225 µmol) Ni(NO3)26H2O was added before adding 15

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mL of water. Lastly, 90.1 mg (1.5 mmol) urea was added to the solution before the solution was

118

mixed thoroughly and transferred to a 23 mL Teflon-lined stainless steel autoclave with a 3 cm x

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1 cm pre-weighed piece of hydrophilic treated carbon paper. The autoclave was placed in a

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preheated 120 ºC oven for 6 h before cooling naturally to room temperature. Upon opening the

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autoclave, the carbon paper was rinsed with nanopure water, EtOH, and dried with a gentle N2

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flow. The mass loadings of the carbon paper electrodes were determined by weighing the

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substrate before growth and after at least 30 min after N2 drying. Powders were collected from

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the remaining solution and washed in EtOH twice before vacuum drying for at least 1 h. For

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comparison, the NiAl LDH, NiGa LDH, and Ni(OH)2 were synthesized through a similar way.

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The NiAl & NiGa LDHs were synthesized without TEOA in aqueous solution from

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Al(NO3)39H2O and Ga(NO3)3xH2O precursors. The Ni(OH)2 nanosheets were synthesized

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through the same method for the synthesis of NiFe LDH, but without adding the Fe precursor.

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Structural characterizations

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Scanning electron microscopy (SEM) images were collected on a LEO SUPRA 55 VP scanning

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electron microscope operating at 1 kV. Energy-dispersive X-ray spectroscopy (EDS) was


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performed on the catalysts as-grown on carbon paper substrates with the SEM described above

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equipped with a Thermo-Fisher EDS detector at 15 kV. Powder X-ray diffraction (PXRD)

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pattern was collected with a Rigaku Rapid II diffractometer equipped with a Mo Kα source

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operating at 50 kV and 50 mA and a total exposure time of 15 min between 2θ = 5° to 45°. The

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sample was ground into a fine powder and packed into a polymer capillary prior to analysis.

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X-ray photoelectron spectroscopy (XPS) was conducted on K-alpha XPS spectrometer (Thermo

139

Scientific) with a micro-focused monochromated Al Kα X-ray source. Samples were analyzed at

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10-7 mbar and room temperature with the flood gun on to avoid sample charging. Spectra were

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taken in the region of C 1s, O 1s, Ni 2p and Fe 2p. The binding energy (BE) values were referred

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to the BE of O 1s peak at 531.0 eV. The peak fitting was performed using Avantage (Thermo

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Scientific) software package.

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Electrochemical oxidation of HMF with NiFe LDH nanosheet electrodes

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The electrochemical HMF oxidation and water oxidation were carried out using a Metrohm

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Autolab electrochemical workstation (Metrohm Autolab B.V., Utrecht, The Netherlands) with a

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H-type electrochemical cell using 1 M KOH as the electrolyte at room temperature. The

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electrochemical cell is configured with a three-electrode system: the as-synthesized NiFe LDH

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nanosheets grown on carbon paper was directly used as the working electrode (anode), a Pt wire

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electrode as the counter electrode (cathode), and a Ag/AgCl electrode as the reference electrode.

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A Nafion 115 membrane obtained from Nara Cell-Tech (USA) was used to separate the anode


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and cathode. The measured potentials vs, Ag/AgCl were converted to a reversible hydrogen

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electrode (RHE) scale according to the Nernst equation:

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ERHE = EAg/AgCl + 0.059 pH + 0.197

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The electrochemical oxygen evolution, HMF oxidation, and hydrogen evolution experiments

157

were conducted in 30 mL of KOH solution (1 M) with and without HMF (from 10 to 100 mM) at

158

a scan rate of 10 mV/s. The cycle performance of the NiFe LDH nanosheets for HMF oxidation

159

was carried out via chronoamperometry at potential of 1.33 V vs RHE in 30 mL of KOH solution

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with 10 mM of HMF for 4 successive cycles. The electrochemical data were obtained and

161

presented without iR- correction.

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The concentration variations of HMF and its oxidation products during the electrochemical

163

reactions were monitored through high performance liquid chromatography (HPLC, Shimadzu

164

Prominence LC-20AD) on aliquots taken from the electrochemical cells with an

165

ultraviolet-visible detector set at 261 nm. Sulfuric acid (H2SO4, 5 mM) was used as the mobile

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phase at the isocratic mode with a constant flow rate of 0.6 mL/min. In each measurement, 100

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µL of the electrolyte solution was withdrawn from the cell during chronoamperometry testing

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and diluted to 1.5 mL with dilute sulfuric acid solution to make the pH below 7.0, then 10 µL of

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the diluted solution was injected directly into a BioRad Aminex 87H column. The identification

170

of the HMF and its various oxidation products was achieved by comparing their retention times

171

in the chromatograms with those of the standard solution, and their concentrations were

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determined from calibration curves by applying standard solutions with known concentrations.


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The conversion of HMF (ηHMF) and yields of its oxidation products (YP) can be calculated with

174

the following equations:

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η

176

YP = CP/C0-HMF × 100%

177

where the C0-HMF and CHMF are the initial HMF concentration and the concentration of HMF at

178

different reaction times, respectively, and Cp is the concentration of HMF oxidation products

179

(FDCA or FFCA) at different reaction times.

= (1-CHMF/C0-HMF) × 100%

HMF

180

The Faradaic efficiency towards HMF conversion can be calculated from the total amount

181

of charge Q (in units of coulombs) passed through the electrochemical cell and the total amount

182

(in units of moles) of HMF conversion N. Q = J × S × t, where J (A/cm2) is the current density at

183

a specific applied potential, S is the electrode area (cm2) and t is the reaction time (seconds).

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Since 6 electrons are required to convert one HMF molecule to FDCA, the Faradaic efficiency

185

can be calculated as follows: Faradaic efficiency = 6F × N(HMF)/Q = 6 F × N(HMF) / (J × S × t),

186

where F is the Faraday constant (F= 96485 C/mol).

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RESULTS AND DISCUSSION

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The NiFe LDH nanosheets were synthesized directly on hydrophilic-treated carbon fiber paper

190

through a facile hydrothermal method. As shown in Figure S1, the growth of NiFe LDH

191

nanosheets includes two main steps: 1) the complexation of Ni2+ and Fe2+ with the weak base

192

triethanolamine (TEOA), which can also help to protect the oxidation of Fe2+ precursor to Fe3+


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over the course of the reaction, and 2) is the nucleation and growth of metal hydroxide

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nanosheets via the reaction with NH3 formed in the hydrolysis of urea. The controlled hydrolysis

195

due to the slow release of NH3 and thus controlled crystal growth under low supersaturation

196

conditions facilitate the formation of ultrathin LDH nanosheets. 31, 36

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The morphology of the electrode material was analyzed with SEM. As shown in Figure 1a,

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the as-obtained electrode material consisted of nanosheets which were vertically aligned on the

199

conducting carbon fibers and fully covered the substrate. Some individual hexagonal plates were

200

observed, consistent with the LDH crystal habit.39-40 EDX elemental mapping (Figure S2, SI)

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confirmed that Ni, Fe, and O are evenly distributed on the carbon fiber paper. XRD pattern of the

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as-synthesized NiFe LDH powder (Figure 1b) shows diffraction peaks that match those of

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reevesite (ICSD #107625), a naturally occurring NiFe LDH mineral.41 Figure S3 displays the

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XPS survey spectrum of the NiFe LDH nanosheets. The deconvoluted Ni 2p XPS spectrum

205

(Figure 1c) displays two spin-orbit doublets (identified as Ni 2p3/2 and 2p1/2) and two shakeup

206

satellites (identified as “Sat.”). The binding energy (BE) at 855.48 and 873.07 eV can be

207

assigned to Ni(II) in NiO, while the BE at 857.79 and 874.95 eV can be attributed to Ni(II) in

208

Ni(OH)2.28, 42 As for the Fe 2p XPS spectrum (Figure 1d), two dominant peaks centered at BE of

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712.20 and 725.00 eV were observed, which can be assigned to Fe2+ 2p3/2 and Fe2+ 2p1/2,

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respectively, while the small peaks at 715.12 and 727.79 eV can be attributed to Fe3+ 2p3/2 and

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Fe3+ 2p1/2, respectively.43

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214 215 216 217 218 219 220

Figure 1. (a) SEM image of the NiFe LDH nanosheets grown on carbon fiber paper; (b) XRD pattern (using Mo Kα source) of the NiFe LDH in comparison with simulated reevesite pattern (ICSD #107625); (c) XPS Ni 2p spectrum and (d) XPS Fe 2p spectrum of the NiFe LDH nanosheets.

221

We evaluated the electrocatalytic HMF oxidation performance of NiFe LDH nanosheets on

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conducting carbon paper directly used as the anodic electrode in a modified H-type

223

electrochemical cell as shown in Figure 2. The first step in HMF oxidation is the conversion of

224

HMF into either 2,5-diformylfuran (DFF) or 5-hydroxymethyl-2-furancarboxylic acid (HMFCA).

225

Both DFF and HMFCA can be further oxidized into 5-formyl-2-furancarboxylic acid (FFCA) 12 ACS Paragon Plus Environment

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and then to FDCA.22 Water oxidation is the main competing reaction.23

227 228 229

Figure 2. Schematic diagram of the electrochemical system used for HMF oxidation with cathode, anode and the overall cell reactions.

230 231

Figure 3a compared the linear sweep voltammetry (LSV) curves of HMF oxidation (10 mM)

232

and water oxidation (no HMF) in 1 M KOH solution with the NiFe LDH electrode and pristine

233

carbon fiber paper. The NiFe LDH shows an onset potential of 1.37 V (vs RHE) towards water

234

oxidation in HMF-free electrolyte and reaches a current density of 20 mA/cm2 at a potential of

235

1.53 V, which is comparable to the performance of similar LDH electrodes.36 In contrast, the

236

onset potential for HMF oxidation is lower at 1.25 V, and a current density of 20 mA/cm2 can be

237

achieved at a potential of 1.32 V. This indicates that HMF oxidation is favored over water

238

oxidation at the lower applied potentials. The pristine carbon fiber has very low activity for both

239

HMF and water oxidation (also see entries 13-14, Table 1). To further probe the HMF oxidation

240

performance of the NiFe LDH, we compared its relative electrochemically active surface area in

241

HMF oxidation and water oxidation using cyclic voltammetry (CV) measurements by extracting 13 ACS Paragon Plus Environment

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the double-layer capacitance. Figure S4a and S4b show the CV curves collected in the

243

non-faradaic region of 0.93-1.0 V vs RHE, in which the current response should be only due to

244

the double layer capacitance. Based on these curves, the capacitance of NiFe LDH for HMF

245

oxidation is 18.4 mF/cm2, six times of that for water oxidation (3.1 mF/cm2) (Figure S4c),

246

indicating that the NiFe LDH displays higher active electrochemical area for HMF oxidation

247

than water oxidation, thus confirming that the NiFe LDH should also be an effective catalyst for

248

HMF oxidation.

249

Tafel analysis of water and HMF oxidation is shown in Figure 3b. The Tafel slope of 75

250

mV/dec for HMF oxidation is much lower than that for water oxidation (143 mV/dec), further

251

confirming that HMF oxidation is faster than water oxidation at these potentials. It should be

252

pointed out that the Tafel slope for water oxidation (143 mV/dec) in this work is higher than

253

commonly reported values (usually 30-65 mV, determined in single three-electrode cells).44-45

254

The reason for the high Tafel slope is that the Nafion membrane used to separate the anode and

255

cathode of the cell (see Figure 2) contains a considerable membrane resistance during the

256

electrochemical process, leading to a larger apparent Tafel slope. To verify this hypothesis, we

257

carrried out a control experiment using the same NiFe LDH catalyst in a single three-electrode

258

cell without a membrane, and the results are shown in Figure S5, display a Tafel slope of 63.2

259

mV/dec, much lower than that in the electrochemical cell with a Nafion membrane (143

260

mV/dec).


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Figure 3. (a) LSV curves of the NiFe LDH nanosheets growth on carbon fiber paper at a scan rate of 10 mV/s in 1 M of KOH with and without 10 mM of HMF; (b) The corresponding Tafel plots.

265 266

For the NiFe LDH, the introduction of Fe could increase the amount of active sites and

267

enhanced the catalytic activity for HMF electrochemical oxidation. We have carried out

268

additional comparison HMF oxidation experiments using Ni(OH)2 nanosheets. As shown in

269

Figure S6, the Ni(OH)2 shows an onset potential of 1.28 V vs RHE for the HMF oxidation,

270

slightly higher than that of NiFe LDH (1.25 V vs RHE). Ni(OH)2 nanosheets also reach a current

271

density of 20 mA/cm2 at 1.41 V, 40 mV higher than that of NiFe LDH nanosheets, indicating that

272

the introduction of Fe could indeed improve the catalytic performance for HMF oxidation.

273

We have also explored other Ni-based bimetallic (NiAl and NiGa) LDH materials, but their

274

electrochemical performance for both HMF and water oxidation was worse than NiFe LDH

275

(Figure S7, also see Table 1). We further probed the differences between these catalysts by

276

studying their electrochemically active surface areas in the non-faradiac potential range. As

277

shown in Figure S8, the double layer capacitance of NiFe LDH for HMF oxidation is 18.4 15 ACS Paragon Plus Environment

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mF/cm2, much higher than those of NiAl (3.4 mF/cm2) and NiGa (4.1 mF/cm2) LDHs. The

279

higher electrochemically active surface area of NiFe LDH toward HMF oxidation suggests that

280

NiFe LDH has more catalytic active sites for the electrochemical oxidation of HMF than NiFe

281

and NiGa. It is interesting that, among the three LDH materials examined (NiFe, NiAl, and NiGa)

282

there seems to be some correlation of the catalytic activities toward OER and HMF oxidation,

283

that is a catalyst with high OER activity usually has a high HMF oxidation activity (see Figure

284

S7). One might imagine that there are some similarity between the OER reaction processes and

285

the elementary process of electrochemical oxidation of HMF, such as the binding of the

286

-OH/hydroxyl group and -OOH group to metal oxide/hydroxide surfaces, thus some correlation

287

between the two catalysts may not be totally surprising. (The oxidation of HMF could also go

288

through H* and CHx* intermediates.) However, there have been so few earth-abundant metal

289

oxide catalysts studied for the HMF electrochemical oxidation reaction, especially virtually there

290

have been no previous studies on bimetallic oxide/hydroxide catalysts (recall that the examples

291

of Ni2P, Ni2S3 and metallic Ni are essentially nickel (hydr)oxides), such presumed trend needs to

292

be further verified by more studies in the future. To our knowledge, this is the first report on

293

using bimetallic LDH materials to enable electrochemical oxidation of HMF, and we hope the

294

excellent catalytic performance and high reaction yield reported will stimulate more research in

295

this class of catalysts for this important reaction. More importantly, future theoretical studies, as

296

well as in situ experimental studies, to examine the catalytic mechanisms of HMF oxidation on

297

metal oxide/hydroxide surfaces and propose potential descriptors to help the research community


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to search for even more efficient catalysts in an effective way.

299

The chronoamperometric tests of water oxidation and HMF oxidation were carried out at

300

constant potentials of 1.23 to 1.43 V vs RHE. As shown in Figure S9a, the current density for the

301

water oxidation at 1.43 V increased in the first 13,000 seconds (3.5 hours), and then did not

302

change, suggesting that the catalytic activity of the NiFe LDH can increase as the reaction

303

proceeds. The current density for HMF oxidation slightly increased in the first few seconds, and

304

then decreased (Figure S9b-d). The concentrations of reactant and product concentrations during

305

the electrochemical reactions were monitored using HPLC (Figure 4a). Besides FDCA and HMF,

306

a small peak identified as FFCA was also observed. However, DFF and HMFCA, the main

307

byproducts in the conventional heterogeneous catalytic aerobic HMF oxidation and noble metal

308

catalyzed electrochemical HMF oxidation,7, 22, 46 were not found in the chromatograms from this

309

work. As the chronoamperometric test progressed, the HMF concentration decreased, and the

310

color of the anodic electrolyte changed from saffron yellow to colorless (Figure 4b). The

311

concentration changes of HMF and its oxidation products with the time of chronoamperometric

312

test are presented in Figure 4c, which shows that oxidation of HMF to FDCA was essentially

313

complete in 90 min. Moreover, the FFCA concentration was always less than 1% throughout the

314

test. The Faradaic efficiency for HMF conversion was 98.6% (entry 3, Table 1). Note that the

315

light yellow color of the starting HMF solution suggests that HMF could polymerize or degrade

316

under alkaline conditions10; but those species can still be oxidized into FDCA under oxidation

317

conditions, leading to a fading of the yellow color as the electrochemical oxidation reaction


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proceeds, as observed in Figure 4b. However, when the applied potential was increased to 1.43 V,

319

the Faradaic efficiency for HMF conversion decreased to 77.2% (entry 4, Table 1) even though

320

the reaction proceeded more rapidly (reaction rate of 1.65 x 10-4 mmol/s at a potential of 1.43 V,

321

compared to 6.67 x 10-5 mmol/s at 1.33 V. This decrease in the Faradaic efficiency is due to the

322

competition from water oxidation (onset at 1.37 V vs RHE). When the applied potential was 1.23

323

V, which is below the onset potential for water oxidation, the Faradaic efficiency was ever higher

324

at 99.4%, but with a slower reaction rate of 8.33 x 10-6 mmol/s (calculated from entry 2, Table 1).

325

These indicate that NiFe LDH is one of the most active and selective catalysts for the

326

electrochemical oxidation of HMF to FDCA compared to the previously reported catalysts (Table

327

S1 in the Supporting Information).

328


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Table 1. Electrochemical oxidation of HMF catalyzed by different anodic catalysts and under different conditions in 1 M KOH.

329 330

Entry

Catalyst

CHMF

E (onset)

(mM)

V vs RHE

E(j=20 mA/cm2)

Potential

Reaction

V vs RHE

applied

time (min)

HMF conversion

FDCA

Faradaic

yield

Efficiency

1

NiFe LDH

0

1.37

1.53

-

-

-

-

-

2

NiFe LDH

10

1.25

1.32

1.23 V

600

99%

98%

99.4%

3

NiFe LDH

10

1.25

1.32

1.33 V

90

98%

98%

98.6%

4

NiFe LDH

10

1.25

1.32

1.43 V

30

98%

97%

77.2%

5

NiFe LDH

50

1.18

1.30

1.33 V

350

92%

90%

98.7%

6

NiFe LDH

100

1.13

1.19

1.33 V

400

91%

90%

90.2%

7

NiAl LDH

0

1.55

1.96

-

-

-

-

8

NiAl LDH

10

1.29

1.45

-

-

-

-

10

NiGa LDH

0

1.55

1.70

-

-

-

-

11

NiGa LDH

10

1.34

1.52

-

-

-

-

12

Ni(OH)2

10

1.28

1.41

-

-

-

-

14

Pristine carbon paper

0

1.58

>2.0

-

-

-

-

15

Pristine carbon paper

10

1.30

1.55

-

-

-

-

-

331 332


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333

334

335 336 337 338 339 340 341 342 343

Figure 4. (a) HPLC chromatogram traces of the various products at different reaction times follow an HMF electrochemical oxidation reaction (mobile phase solution: 1 mM H2SO4, flow rate: 0.6 mL/min); (b) Digital photograph showing the color change of the anodic electrolyte during the electrochemical HMF oxidation process; (c) Concentration changes of HMF and its oxidation products with the time of chronoamperometric test at 1.33 V vs RHE; (d) HMF concentration changes during 4 successive cycles (the reaction rate for each cycle are calculated as 6.67 x10-5, 6.61 x10-5, 6.43 x10-5 and 6.20 x10-5 mmol-HMF/s for the four successive cycles, respectively).

344 345 346

Four successive cycles of chronoamperometry were tested to evaluate the durability of this electrode during HMF oxidation (Figure 4d). The conversion of HMF decreased from 98% to 93%


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347

after 4 cycles. The reaction rate slightly decreased from 6.67 x 10-5 to 6.20 x 10-5 mmol-HMF/s

348

in these four successive cycles. This decrease may be caused by the reduction of the charge

349

density passed through the electrode during the same reaction time interval. The Faradaic

350

efficiencies for each cycle did not change (Figure S10). The electrodes were characterized by

351

XPS and SEM-EDX after the chronoamperometric tests (1.33 V for more than 6 h, 4 cycles). The

352

used NiFe LDH catalyst contains some large aggregates on the carbon fiber paper (Figure S11).

353

EDX elemental mapping shows that the even distribution of Ni and Fe on carbon paper is

354

maintained, although their concentrations are diminished compared to the fresh electrode (Figure

355

S12). The XPS Ni 2p spectrum of the used NiFe LDH catalyst displays two peaks at 855.33 and

356

856.50 eV (Figure 5a), assigned to Ni(OH)2 and NiO, respectively. The shape and position of the

357

peaks of the spent catalyst are essentially the same as those from the fresh catalyst, but the

358

relative content of Ni(OH)2 is higher than the fresh catalyst, confirming that some NiO is

359

converted into Ni(OH)2 after HMF oxidation (Figure 1c). The XPS Fe 2p spectrum (Figure 5b)

360

shows that more Fe3+ is formed after the HMF oxidation, suggesting that some Fe2+ is oxidized

361

into Fe3+ during the HMF oxidation process. For catalytic water oxidation using NiFe LDH, it is

362

believed that Fe3+ has a higher activity than Fe2+, therefore oxidation of Fe2+ to Fe3+ may

363

increase the catalytic activity of the NiFe LDH, which is consistent to the observed results (see

364

Figure S9). The LDH structure of the catalyst and catalytic performance are maintained

365

throughout the HMF oxidation reaction cycles even though there are some changes in the metal

366

oxidation states of the catalyst.


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367 368 369

Figure 5. XPS Ni 2p (a) and Fe 2p (b) spectra of the cycled NiFe-LDH/carbon fiber paper electrode.

370 371

The electrochemical performance of the NiFe LDH nanosheet electrode was also evaluated

372

in more practical higher HMF concentrations. As shown in the LSV curves in Figure 6a, as the

373

HMF concentration increases, both the E(onset) and E(j=20 mA cm-2) decrease (entries 5-6, Table 1).

374

As shown in Figure 6b the HMF concentration as a function of time can be fit to a first order

375

kinetic model with respect to HMF, at all three HMF concentrations. The selectivity to FFCA is

376

relatively higher at the initial stage of the reactions for the HMF concentrations of 50 and 100

377

mM (Figure 6c, d, and Figure S13). As a result, the corresponding Faradaic efficiencies decrease

378

slightly compared to the low concentration HMF oxidation. As the HMF concentration increases

379

beyond 100 mM, crossover of HMF from the anode to the cathode became more significant.

380

Therefore, improved membranes are needed to make this technology practical.


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381 382 383 384 385 386

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Figure 6. (a) LSV curves of the NiFe-LDH/carbon fiber paper catalyst towards the oxidation of HMF at high concentrations (50 and 100 mM) in 1 M KOH; (b) First-order kinetics models of the oxidation of HMF at different concentrations; (c) and (d) Concentration changes of HMF and its oxidation products over the duration of chronoamperometric test (potential 1.33 V vs RHE in 1 M KOH).

387 388

The overall electrochemical process can reduce water to H2 at the cathode while generating

389

FDCA at the anode as shown in Figure 2. In order to couple the anodic HMF oxidation with

390

cathodic H2 evolution, the cathode should have unchanged catalytic performance toward H2

391

evolution in the presence of HMF caused by potential crossover through the Nafion membrane.23

392

Here, we used a Pt wire as a model cathode and Figure S14 shows the cathodic LSV curves with

393

and without 10 mM of HMF in 1 M KOH. The introduction of HMF only slightly suppressed the

394

current density at low potentials, while at a high potential, the current density achieved with


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395

HMF was even higher than that without HMF. The Tafel slopes for the H2 evolution with and

396

without HMF are almost the same (75 and 74 mV/dec, respectively) (Figure S15). Figure S16

397

presents a 13-hour chronoamperometric test at -0.33 V vs RHE in 1 M of KOH with 10 mM of

398

HMF. These results confirm that the cathodic H2 evolution is not significantly influenced by

399

HMF crossover, and therefore, we can conclude that coupling the cathodic H2 evolution with

400

anodic HMF oxidation in an integrated electrochemical cell is feasible. Since the focus of this

401

work is on the electrocatalyst for HMF oxidation we used conventional Pt cathode. However,

402

many highly active earth-abundant electrocatalysts for hydrogen evolution are available, and

403

could be integrated with our NiFe LDH catalys.47

404

405

CONCLUSIONS

406

In summary, NiFe LDH nanosheets have been demonstrated as an efficient and robust

407

catalyst for the direct electrochemical oxidation of HMF to FDCA at ambient pressure and room

408

temperature with high yield and selectivity for the first time. An electrochemical cell based on

409

NiFe LDH nanosheets integrated on carbon fiber paper as the anode and Pt wire as the cathode

410

was constructed to electrochemically produce FDCA from high concentration of HMF (up to 100

411

mM) for multiple cycles while also producing H2. The high Faradaic efficiency achieved for

412

HMF conversion (up to 99.4%) indicates that the HMF oxidation is more kinetically favorable

413

than the water oxidation at low applied potentials. These results show that NiFe LDH, and

414

potentially other earth-abundant water oxidation catalysts,28-29, 48 may also serve as highly active 24 ACS Paragon Plus Environment

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Page 24 of 29

415

electrocatalysts for HMF oxidation. As the first report on using bimetallic metal hydroxide/oxide

416

catalysts for electrochemical oxidation of HMF, this work provides a sustainable and efficient

417

approach for direct anodic conversion of biomass-derived platform chemicals to value-added

418

chemicals in an electrochemical system, with simultaneous H2 production.

419 420

ASSOCIATED CONTENT

421

Supporting Informaion

422

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

423

Table S1 and Figures S1-S16.

424 425

AUTHOR INFORMATION

426

* Corresponding authors

427

Email: [email protected] (G.W.H.); [email protected] (S.J.)

428 429

Notes

430

The authors declare no competing financial interest.

431 432

ACKNOWLEDGMENTS

433

L.D. and S.J. thank the support by NSF Grant 1508558. W.-J. L. Thanks China Scholarship

434

Council for finacial support. L.D. also thanks NSF Graduate Research Fellowship for support. 25 ACS Paragon Plus Environment

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