Solar-assisted co-electrolysis of glycerol and water for concurrent production of formic acid and hydrogen

Article abstract of DOI:10.1039/d1ta02654b

Renewable electricity-driven water splitting provides a pathway to manufacturing hydrogen as a promising alternative to fossil fuels. A typical water electrolysis device is comprised of a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER). Unfortunately, the OER consumes most of the overall electricity supply while generating negligible economic value, which inhibits the large-scale deployment of the water electrolysis technology. Here, we explored alternatives to the OER and demonstrated that electrooxidation of glycerol (a cheap byproduct of biodiesel and soap production) could lower anodic electricity consumption by up to 0.27 V while producing high-value formic acid with 96.2% faradaic efficiency (FE). Further, glycerol electrooxidation was combined with the photoelectrochemical HER to diminish the electricity requirement to 1.15 V, reducing the electricity consumption by ～30% relative to typical water electrolysis. This study suggests that solar-assisted co-electrolysis of high-volume block chemicals and water may be an energy efficient and economically viable strategy to realize the sustainable production of value-added chemicals and hydrogen energy.

Full text of DOI:10.1039/d1ta02654b

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Solar-assisted co-electrolysis of glycerol and water
for concurrent production of formic acid and
hydrogen†
Cite this: DOI: 10.1039/d1ta02654b
Zunjian Ke,ab Nicholas Williams,b Xingxu Yan,c Sabrina Younan,b Dong He,a
a
b
Xianyin Song,e Xiaoqing Pan,cd Xiangheng Xiao
and Jing Gu
*
*
Renewable electricity-driven water splitting provides a pathway to manufacturing hydrogen as a promising
alternative to fossil fuels. A typical water electrolysis device is comprised of a cathodic hydrogen evolution
reaction (HER) and an anodic oxygen evolution reaction (OER). Unfortunately, the OER consumes most of
the overall electricity supply while generating negligible economic value, which inhibits the large-scale
deployment of the water electrolysis technology. Here, we explored alternatives to the OER and
demonstrated that electrooxidation of glycerol (a cheap byproduct of biodiesel and soap production)
could lower anodic electricity consumption by up to 0.27 V while producing high-value formic acid with
96.2% faradaic efficiency (FE). Further, glycerol electrooxidation was combined with the
photoelectrochemical HER to diminish the electricity requirement to 1.15 V, reducing the electricity
consumption by ꢀ30% relative to typical water electrolysis. This study suggests that solar-assisted co-
electrolysis of high-volume block chemicals and water may be an energy efficient and economically
viable strategy to realize the sustainable production of value-added chemicals and hydrogen energy.
Received 30th March 2021
Accepted 29th June 2021
DOI: 10.1039/d1ta02654b
rsc.li/materials-a
This high cost of hydrogen production substantially diminishes
the market share for water electrolysis. Consequently, the cost
Introduction
of hydrogen production via water electrolysis must reduce to
ꢀ$2 per kg to achieve a large-scale application.5,6 Generally,
costs associated with water electrolysis are dictated by operation
expenses and capital input, with the former primarily domi-
nated by electricity consumption.4 Unfortunately, a high overall
electricity input (>1.5 V of the cell voltage) is required for typical
water electrolysis devices (Scheme 1a), which limits the indus-
trial deployment of these devices.7 The culprit behind this
barrier is the thermodynamically sluggish OER, which
consumes the majority of electricity input while producing
negligible economic benets (ꢀ$0.1/kgꢁ1 of oxygen price).8 As
a consequence, exploring alternative strategies that circumvent
sluggish OER thermodynamics while generating high-value
products has the potential to lower energetic requirements
and enable water electrolysis to replace traditional steam
reforming processes. Recently, a few efforts have been reported
that employ lower-energetic anodic reactions (such as electro-
oxidation reactions of ethanol,9 urea,10 hydrazine,11 etc.) in water
electrolysis. While these explorations are interesting, the
oxidation of the reported chemicals may not be sustainable. For
instance, the electrooxidation of urea will contribute to the
emission of carbon dioxide.
The detrimental impact of global fossil fuel consumption on the
climate and environment has instilled a tremendous interest in
developing renewables-powered solutions to manufacturing
clean energy.1 As a crucial carbon-neutral energy carrier and
chemical reaction intermediate, hydrogen has been recognized
as a renewable alternative to fossil fuels.2 At present, more than
100 Mt of hydrogen is manufactured per year. However, 95% is
still produced by energy-intensive steam methane reformation,
and only 4% through electricity-driven water splitting.3,4
Producing hydrogen by steam methane reformation renders the
cost of hydrogen to be ꢀ$1.2–1.5/kgꢁ1, whereas the cost of
hydrogen produced via water electrolysis is more than $4/kgꢁ1
.
aDepartment of Physics, Key Laboratory of Articial Micro- and Nano-structures of
Ministry of Education, Wuhan University, Wuhan 430072, Hubei, China. E-mail:
xxh@whu.edu.cn
bDepartment of Chemistry and Biochemistry, San Diego State University, San Diego,
California 92182-1030, USA. E-mail: jgu@sdsu.edu
cDepartment of Materials Science and Engineering, University of California, Irvine,
Irvine, California 92697, USA
dDepartment of Physics and Astronomy, University of California, Irvine, Irvine,
California 92697, USA
eCollege of Materials Science and Engineering, Hunan University, Changsha 410082,
Hunan, China
Herein, we investigate the electrooxidation of glycerol to
formic acid as a promising substitute for the OER. Theoretically,
the redox potential of glycerol to formic acid is 0.69 V, 44%
lower than the 1.23 V required for the OER (Scheme 1a and b).12
† Electronic supplementary information (ESI) available: Supplementary SEM,
XRD, TEM, XPS characterizations, and related electrochemical data. See DOI:
10.1039/d1ta02654b
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hydrogen. This value, to our understanding, is the lowest elec-
tricity input reported for water electrolysis (Table S2, ESI†). In
addition, the versatility of the N-CoOx catalyst was further
conrmed by its ability to upgrade biomass-derived ethylene
glycol into formic acid with a high FE of 62.3%. Overall, we have
achieved highly selective, energy-efficient, and economically
viable co-production of hydrogen fuel and high-value chemicals
by solar-assisted co-electrolysis of water and cheap biomass
waste (Scheme 1d).
Results and discussion
Material characterization
The nitrogen doped cobalt oxide electrocatalyst (N-CoOx) was
directly deposited on carbon bre paper by one-step facile
atomic layer deposition (ALD). The precursor, a nitrogen-
containing organometallic cobalt complex (bis(N,N0-di-i-
propylacetamidinato) cobalt(II)) (Fig. 1a and S1, ESI†), was
employed as the cobalt and nitrogen source for in situ nitrogen
doping (see ESI†). The morphology of N-CoOx was characterized
by scanning electron microscopy (SEM), in which the visibly
smooth carbon surface conrms the uniform and ultrathin
Scheme
1 Overview of hydrogen production and formic acid
manufacturing. (a) Typical water electrolysis device. (b) Energy-effi-
cient and value-added hydrogen production system in this work. (c)
Industrial formic acid production. (d) Solar-assisted formic acid elec-
trosynthesis in this work.
This indicates that utilizing glycerol electrooxidation as an nature of the deposited catalyst lm (Fig. S2a, ESI†). Atomic
anodic reaction could signicantly lower electricity require- resolution high-angle annular dark-eld (HAADF) scanning
ments. More importantly, glycerol is a cheap by-product of transmission electron microscopy (STEM) was employed to
biodiesel and soap production ($0.24/kgꢁ1),13 whereas its major determine the structure and composition of N-CoOx (Fig. 1b).
oxidation product, formic acid, is widely used in the fuel cell The resulting images clearly show lattice fringes and three
and hydrogen storage industries.14 As a high-value commodity characteristic crystal facets with interplanar spacings equal to
chemical ($1.0 per kg),15,16 manufacturing formic acid by 0.286, 0.463, and 0.241 nm. These crystal facets were indexed to
coupling glycerol electrooxidation with the HER retains high the (220), (111), and (311) planes of Co3O4 with a standard
economic value ($17.25) upon producing 1 kg of hydrogen spinel structure, respectively. The corresponding fast Fourier
(Table S1, ESI†). In comparison, O2 produced by traditional transform (FFT) patterns (Fig. 1b, inset) suggest the presence of
water electrolysis (Scheme 1a) is only worth $0.80 to generate multiple crystalline orientations of N-CoOx. Furthermore, the
the same amount of hydrogen. In this regard, ambient glycerol low magnication HAADF-STEM image and corresponding
electrooxidation will allow formic acid to be generated in a more elemental mappings in Fig. 1c validate the uniform distribution
sustainable manner than the industrial high temperature and of cobalt, oxygen, and nitrogen elements throughout the
pressure methyl formate hydrolysis (Scheme 1c).17 Therefore, sample. These results are also conrmed by energy-dispersive X-
highly efficient and selective electrocatalysts for glycerol
oxidation are desired. However, most glycerol electro-oxidation
catalysts so far are noble metals (like Pt,18 Au,19 Pd20) and their
alloys (like PtRu,21 PtSb,22 PdxBi23). Moreover, the reaction
pathways of glycerol oxidation involve various C1–C3 interme-
diates which can interconvert into many different oxidation
products, thus suffering from low selectivity.24
In this work, we developed an efficient and cost-effective
nitrogen-doped cobalt oxide (N-CoOx) electrocatalyst. The
catalyst is derived from an organometallic cobalt complex and
can achieve a highly selective formic acid synthesis by glycerol
electrooxidation (96.2% FE). The anodic electricity consump-
tion reduced up to 0.27 V relative to the OER, which is compa-
rable with previous glycerol oxidation studies.32,33 In order to
further diminish the electricity requirements, a cathodic pho-
Fig. 1 Structure and chemical states of N-CoOx. (a) Structure of the
bis(N,N0-di-i-propylacetamidinato)
cobalt(II) (Co(iPr-MeAMD)2)
complex for N-CoOx synthesis. (b) and (c) HRTEM image and STEM-
EDX elemental map of N-CoOx (inset: fast Fourier transform pattern).
(d) XPS Co 2p, (e) XPS N 1s, and (f) EPR spectra of N-CoOx and Co3O4.
toelectrochemical HER was coupled with anodic glycerol
oxidation (Scheme 1b). The hybrid system could be driven by
solar energy with an additional electricity supply of 1.15 V to
accomplish concurrent production of formic acid and
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ray spectroscopy of SEM (SEM-EDS) in Fig. S3, ESI.† The 1.58 V at 10 mA cmꢁ2 (Fig. 2a). Addition of 1.0 M glycerol caused
elemental compositions were further quantied by STEM-EDS a negative shi in the onset potential and the potential at 10 mA
(Fig. S4, ESI†), in which the presence of the non- cmꢁ2 to 1.2 V and 1.31 V, respectively. Here, a visibly signicant
stoichiometric ratio of cobalt and oxygen suggests the exis- shi of 0.3 V and 0.27 V occurred for the onset potential and the
tence of multiple types of cobalt oxides in N-CoOx, corre- potential at 10 mA cmꢁ2, respectively. Upon evaluating the
sponding effectively to the FFT patterns in Fig. 1b. X-ray difference in potentials and current densities for N-CoOx before
diffraction (XRD) patterns were collected to identify the crystal and aer adding glycerol, lower energetic requirements are
structure of the catalysts (Fig. S2b, ESI†).
witnessed for each current density when glycerol is present
In these results, only one broad peak, belonging to the (Fig. 2b). This result veries the aptitude of N-CoOx for glycerol
amorphous substrate, was identied. The lack of distinct oxidation. It should be noted that the optimal glycerol
characteristic peaks in the XRD pattern may be attributed to the concentration was determined to be 1.0 M by linear sweep
catalyst's ultrathin nature.25 On the other hand, the sample voltammetry (LSV), where the lowest onset potential was
contains some amorphous regions in the HAADF-STEM image determined by varying glycerol concentrations from 0 to 2.0 M.
(Fig. 1b), which is evidenced by the amorphous ring in FFT Notably, an extra peak at 1.45 V (pointed out by an arrow in
patterns (Fig. 1b), correlating well with the XRD results. The Fig. S7, ESI†) appeared with the addition of 0.1 M glycerol,
surface chemical states of N-CoOx were elucidated by X-ray suggesting the ability of the OER to compete with glycerol
photoelectron spectroscopy (XPS) (Fig. 1d, e and S5, ESI†). oxidation at a higher potential (>1.45 V) while using a lower
The XPS survey spectrum in Fig. S5, ESI† revealed the presence concentration of glycerol.
of Co and O elements in N-CoOx and Co3O4 samples. Besides,
The role of the N dopant in enhancing kinetics for glycerol
an obvious N 1s signal at 399.1 eV in N-CoOx inevitably oxidation is demonstrated by the Tafel and impendence
conrmed the existence of N in N-CoOx. In contrast, the N 1s measurements. The Tafel slope (Fig. 2c) for glycerol oxidation
signal in the Co3O4 control sample is negligible.26 The peak at was determined to be 155 mV decꢁ1, which is lower than that of
398.2 eV in the tted high-resolution N 1s spectra (Fig. 1e) for N- the OER (179 mV decꢁ1). This indicates that N-CoOx achieves
CoOx references the existence of N–Co species, while the faster kinetics for glycerol oxidation relative to that of the OER.
399.9 eV peak identies adsorbed N2 on both N-CoOx and Co3O4 To clarify the inuence of N dopants on N-CoOx catalytic
samples.26,27 To analyse the chemical states of cobalt, the Co 2p activity, the oxidation performances of N-CoOx and Co3O4 were
XPS spectra are provided in Fig. 1d. The Co 2p3/2 peaks for compared (Fig. S8, ESI†). Results show that Co3O4 requires
Co3O4 may be tted to the peaks at 780.4 eV and 781.8 eV and a higher potential (1.43 V) than N-CoOx (1.31 V) to drive the
are assigned to tetrahedral and octahedral Co–O coordination current density of 10 mA cmꢁ2 for the reaction forward. Co3O4
geometries, respectively.28 Notably, the additional peak at also exhibited a higher Tafel slope (173 mV decꢁ1) than N-CoOx
782.9 eV in Co 2p for N-CoOx further conrmed the presence of (155 mV decꢁ1) (Fig. S8a and b, ESI†). Since the Tafel slopes
Co–N bonds in the catalyst.26,29 Here, it was worth noting that indicate the catalytic reaction rate's dependence on applied
Co–Co interactions (around 778.8 eV) were not observed, potentials, these results show that the N-CoOx anode possesses
precluding the presence of cobalt nitride species in N-CoOx.30 much faster glycerol oxidation kinetics than that of Co3O4. This
To further reveal the effects of the N dopant, the defects of conclusion is further supported by the electrochemical
catalysts were characterized by electron paramagnetic reso- impendence spectra (EIS), where N-CoOx showed much lower
nance (EPR) spectroscopy (Fig. 1f). The strong EPR signal electric impedance than Co3O4 (Fig. S8c and Table S3, ESI†).
located at g ¼ 2.004 corresponds to the existence of oxygen Altogether, these results suggest that the incorporation of N
vacancies. In addition, N dopant's potential to induce addi- leads to better electrical contact, lower electric impedance, and
tional oxygen vacancies is indicated by the N-CoOx peak inten- faster charge transfer kinetics for glycerol oxidation.
sity being twice that of Co3O4.26,27,31
Product analysis
Glycerol electrooxidation
1
Glycerol oxidation products were collected and analyzed by H
Electrochemical investigations were conducted to compare the and 13C nuclear magnetic resonance (NMR) spectroscopy
performance of N-CoOx in glycerol oxidation to its performance (Fig. 2d and e). A relatively low potential of 1.35 V was chosen
1
in water oxidation. Compared to Co3O4, N-CoOx required here to limit the competitive OER. The H NMR spectra before
a lower potential (1.58 V) to reach the current density of 10 mA and aer 12 hours of electrolysis identify formic acid as the
cmꢁ2 (Fig. S6a, ESI†). Likewise, the higher activity of N-CoOx main product (Fig. 2d). Negligible changes in current density
was conrmed by its lower Tafel slope (179 mV decꢁ1) with aer 12 hours of electrolysis demonstrate the catalyst's high
respect to Co3O4 (187 mV decꢁ1) (Fig. S6b, ESI†). On average, the stability during extended operation (Fig. S9a, ESI†). LSV curves
OER activities of both catalysts were very close and moderate, before and aer 12 hours of electrolysis exhibit a slight deteri-
indicating that the N dopant did not contribute signicantly to oration in catalytic activity (Fig. S9b, ESI†). This may be attrib-
the OER. Next, systematic elucidation of the electrocatalytic uted to a decrease in glycerol concentration and the possible
activity of N-CoOx towards glycerol oxidation is provided in change in the electrolyte pH.32 To further understand the
Fig. 2. Without glycerol, N-CoOx showed moderate OER activity stability of N-CoOx, post 12 h the LSV in a fresh electrolyte was
by delivering an onset potential of 1.50 V and a potential of compared to the LSV prior to electrolysis, where negligible
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Fig. 2 Glycerol electrooxidation performance. (a) Linear sweep voltammetry (LSV) plots (all LSV plots were collected with a scan rate of 5 mV sꢁ1
in N2-saturated electrolyte with no I–R compensation). (b) Input potentials at various current densities. (c) Tafel slopes for the OER and glycerol
oxidation in 1.0 M KOH solution both with and without 1.0 M glycerol. Purple arrows inset indicate the significant potential decrease at various
1
current densities when 1.0 M glycerol is added. (d) H NMR spectra of the formic acid standard and electrolytes (1.0 M KOH + 1.0 M glycerol)
before and after 12 h of electrolysis at 1.35 V vs. RHE. Due to the higher initial glycerol concentration, the intensity of 1H NMR peaks for glycerol
did not decrease significantly after electrolysis. (e) High-resolution 1H NMR spectra (water peak was removed from 4 to 6 ppm) of the 1.0 M KOH
+ 1.0 M glycerol solution after 12 h of electrolysis (the symbols on the top of the peaks represent each type of proton present). (f) Faradaic
efficiencies of formic acid at various potentials (each potential underwent 12 h of electrolysis). (g) Faradaic efficiencies of formic acid for five
successive electrolysis cycles at 1.35 V vs. RHE. (h) Concentration changes of the reactant (glycerol), main product (formic acid), and repre-
sentative intermediate species (glycolic acid) with electrolysis time.
changes in onset potential and overpotential are noticed respect to the formic acid calibration curve (Fig. S12a and b, ESI†).
(Fig. S10a, ESI†). Further, the intact Co 2p XPS spectrum aer From this analysis, the FE for formic acid production at 1.35 V was
electrolysis conrms the superior stability of N-CoOx toward calculated to be 96.2%, which exceeds most glycerol upgrading
oxidizing glycerol (Fig. S10b, ESI†). The products of glycerol studies reported (Table S2, ESI†).32,33 FEs at varied potentials were
electrolysis were further conrmed by the 13C NMR. As shown in investigated in Fig. 2f, where the FE reaches a maximum (96.2%)
Fig. S11, ESI,† formic acid remained as the main product at 1.35 V and gradually decreases to 58% and 75% at 1.25 V and
identied aer 60 hours of electrolysis, which correlates well 1.5 V, respectively. The FE and potential dependence may be
with 1H NMR ndings. The small amount of carbonate detected explained by the strength of C–C bonds, which are harder to break
in the 13C NMR spectra may be attributed to the high solubility at lower potentials. Unfortunately, at higher potentials, the OER
of atmospheric CO2 in strongly alkaline media. A decrease in competes with glycerol oxidation and diminishes glycerol
glycerol concentration was also detected by 13C NMR, suggest- conversion selectivity. N-CoOx also demonstrated great durability
ing a successful electrically-driven glycerol conversion. In and recyclability by maintaining an average FE of 93.8% aer ve
addition to formic acid, glyceric acid, glycolic acid, lactic acid, cycles of electrolysis (Fig. 2g). Lastly, concentration changes of the
and acetic acid were detected at concentrations much lower reactant (glycerol), intermediate (glycolic acid), and product (for-
than what was seen for formic acid (Fig. 2e). These intermediate mic acid) were monitored during electrolysis (Fig. 2h). A linear
species will be used to unravel the glycerol oxidation mecha- relationship between the reaction time and the concentration of
nism, which is discussed in the mechanism investigation the product generated (or the reactant consumed) was observed,
section.
while the intermediate species demonstrated a nearly constant
To determine the FE of formic acid, 1H NMR peaks of formic concentration throughout the reaction. Moreover, the FE of H2 at
acid in a range of standard concentrations were collected with the cathode is found to be close to 100% in ve successive
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electrolysis cycles (Fig. S13, ESI†). The result indicated that the this route, one glycerol molecule may be completely converted
HER is the only cathodic reaction, which also further indicates into three formic acid molecules through an eight-electron
that replacing the anodic OER with glycerol oxidation will not transfer process. However, very small amounts of lactic acid
affect the cathodic half-reaction.
and acetic acid are observed in the NMR results (Fig. 2e). Thus,
To determine the potential of N-CoOx to oxidize other we hypothesize that there is a minor reaction pathway (Fig. 3). In
biomass-derived compounds, its catalytic activity towards the minor reaction pathway, glyceraldehyde will reversibly react
ethylene glycol (EG) oxidation was investigated (Fig. S14a–c, with OH– to form dihydroxy acetone. Dihydroxy acetone is then
ESI†). An onset potential of 1.25 V and a potential of 1.35 V at 10 converted into 2-hydroxypropenal/pyruvaldehyde aer a dehy-
mA cmꢁ2 were observed (1.0 M EG), which saved 0.23 V of the dration process. Further, 2-hydroxypropenal and pyruvaldehyde
potential (10 mA cmꢁ2) compared to the OER (Fig. S12a, ESI†), will convert to lactic acid aer a Cannizzaro rearrangement. In
thus proving that EG oxidation with N-CoOx reduces energy the last step, the lactic acid will be oxidized to acetic acid and
consumption at the anode. This potential shi was noticed at formic acid via C–C bond breakage.35,36 Similarly, the electro-
other current densities as well (10–40 mA cmꢁ2) (Fig. S14b, chemical oxidation mechanism for ethylene glycol conversion
ESI†). Regarding selectivity, an optimal FE of 62.3% at 1.40 V into formic acid is proposed in Fig. S15.† In this mechanism,
was obtained for formic acid production from EG (Fig. S14c, a primary –OH group from ethylene glycol is oxidized to an
ESI†). From these results, we conclude that N-CoOx exhibits aldehyde via a two-electron process, followed by oxidation into
promising results towards converting glycerol and ethylene carboxyl. Finally, the glycolic acid intermediate will completely
glycol into value-added formic acid. Currently, commercial convert into formic acid via an oxidative cleavage of C–C bonds.
production of formic acid involves reacting methanol with toxic
carbon monoxide at an elevated pressure. This reaction
produces the intermediate species methyl formate, which is
Integrating glycerol electrooxidation with hydrogen
generation
then transformed into formic acid by a two-step hydrolysis
The results reported thus far are based on half-cell systems and
reaction (Fig. 1c). This approach involves intensive energy and
demonstrate how well N-CoOx facilitates the conversion of
cost inputs.17 Instead, our work provides a facile route to
biomass-derived alcohols. Aside from its critical role in
synthesize formic acid by biomass electrolysis at room
oxidizing biomass-derived alcohols, N-CoOx also performs
temperature and atmospheric pressure.
effectively as an HER catalyst under basic conditions. As dis-
played in Fig. S15a, ESI,† N-CoOx achieves an overpotential of
Mechanism investigation
0.265 V at 10 mA cmꢁ2, whereas Co3O4 requires 0.423 V of
The proposed reaction mechanism is provided in Fig. 3. In the overpotential to reach 10 mA cmꢁ2. Likewise, N-CoOx's 147 mV
rst step, a primary hydroxyl group (–OH) on glycerol will be decꢁ1 Tafel slope implies N-CoOx's fast kinetics towards the
oxidized to an aldehyde via two-electron transfer.34 Further, the HER (Fig. S16b, ESI†). To understand the catalyst's overall water
aldehyde will be oxidized to a carboxyl group, forming glyceric splitting performance, N-CoOx was simultaneously used as an
acid, which is supported by the 1H NMR (Fig. 2e). Then, we anode and cathode for water electrolysis in a two-electrode
propose that the formic acid was produced with an oxidative system (Fig. S16a and 18a, ESI†). A high cell voltage close to
cleavage of C–C bonds in glyceric and glycolic acid, where both 1.9 V was required to reach 10 mA cmꢁ2, which signicantly
intermediates are well identied by the NMR (Fig. 2e). Based on contributes to OER's sluggish kinetics. Replacing the OER with
Fig. 3 Proposed reaction pathway of glycerol electro-oxidation on N-CoOx. The solid green and dashed yellow arrows represent the major and
minor reaction routes, respectively. Two-way dashed yellow arrows represent reversible processes. Formic acid was the major product (purple),
while acetic acid and carbonate were minor products. Except for the unstable species that were difficult to detect under basic conditions (dashed
rectangular boxes), NMR was employed to identify all intermediates and products and prove the feasibility of the proposed reaction routes. The
route from glyceraldehyde to lactic acid is believed to undergo intermolecular dehydration and rearrangement under basic conditions, which
does not involve electron transfer. All other routes involve electrochemical oxidation processes with related electron transfer pathways.
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glycerol oxidation dramatically reduces the cell voltage to 1.59 V strategy has the potential to save at least 0.4 V in cell voltage
(10 mA cmꢁ2) (Fig. S17 and 18b, ESI†). O2 gas evolution was not compared to the conventional water splitting congurations,
detected at the anode upon adding 1.0 M glycerol, indicating reducing ꢀ30% electricity consumption.
the catalyst's preference for glycerol oxidation over the OER.
To further reduce the dependence on electricity and promote
renewable energy utilization, a PEC cell equipped with a Pt-
Conclusion
coated silicon nanowire (Si NW-Pt) photocathode was coupled
with the N-CoOx anode. The schematic diagram in Fig. 4a and
digital photos in Fig. 4b and S22, ESI† for the hybrid system
display how the Si NW-Pt photocathode may drive glycerol
oxidation and the HER simultaneously under 1 sun illumination
with a limited external bias. In this work, we utilized our previ-
ously reported method to fabricate Si NW-Pt photocathodes.37
SEM images and corresponding EDX elemental mapping of the Si
NW-Pt photocathode are demonstrated in Fig. S18 and S20, ESI,†
where the silicon nanowires are ꢀ2 mm long and Pt nanoparticles
are uniformly presented across the surface of the silicon nano-
wires. Upon evaluating the HER performance of the Si NW-Pt
photocathode under both basic (1.0 M KOH) and acidic (0.5 M
H2SO4) conditions, the photocathode showed an onset potential
of 0.25 V vs. RHE in both electrolytes (Fig. S21, ESI†). Si NW-Pt
was then coupled with the N-CoOx anode (N-CoOxkSi NW-Pt)
into a two-electrode system for simultaneous glycerol oxidation
and PEC hydrogen evolution. As shown in Fig. 4c, before adding
glycerol, the N-CoOxkSi NW-Pt hybrid system required a cell
voltage of 1.34 V to reach 10 mA cmꢁ2, which is much lower than
the energy required in the N-CoOxkN-CoOx system (1.81 V, no
glycerol). This voltage was further reduced to 1.15 V aer adding
1.0 M glycerol, much lower than that of the N-CoOxkN-CoOx
system (1.59 V, with glycerol). To the best of our knowledge, it is
the rst time that PEC hydrogen evolution has been coupled with
glycerol oxidation, which demonstrates great success in saving
the overall cell voltage (Fig. 4d and Table S2, ESI†). The cell
voltage reported here is at least ꢀ0.2 V less compared with
previous systems (Fig. 4d and Table S2, ESI†). Overall, our
In summary, we demonstrated an inexpensive N-CoOx electro-
catalyst that could efficiently oxidize glycerol (a cheap byproduct
of biodiesel production) and biomass-derived ethylene glycol.
N-CoOx successfully converts glycerol into high-value formic
acid at a low potential of 1.31 V (10 mA cmꢁ2), which is 0.27 V
less than the potential required to drive the OER. Additionally,
selective conversion of glycerol to formic acid achieved FEs up
to 96.2%. The catalyst also exhibited high activity for oxidizing
ethylene glycol (1.35 V at 10 mA cmꢁ2) to formic acid with 62.3%
FE. Coupling the N-CoOx anode with a Si NW-Pt photocathode
in a two-electrode PEC cell further reduced the cell voltage (only
need 1.15 V to produce 10 mA cmꢁ2) required to concurrently
upgrade glycerol to formic acid and generate hydrogen fuel.
These remarkable results revealed that glycerol electrooxidation
would be a promising alternative to the traditional energy
intensive OER process due to low electricity consumption.
Besides, harvesting solar energy further reduced the system's
dependence on electricity. More importantly, utilizing glycerol
electrooxidation to replace the OER will generate signicant
economic prots relative to typical water electrolysis. As
a consequence, solar-assisted co-electrolysis of water and high-
volume chemicals may be an attractive pathway to achieve
sustainable co-production of hydrogen energy and high-value
chemicals due to improved techno-economics. This work
provides a solid foundation to design devices for concurrent
fuel generation and biomass upgradation in the future.
Experimental
Materials
Bis(N,N0-di-i-propylacetamidinato)
cobalt(II)
(C16H34CoN4,
99.99%) was purchased from STREM. Cobalt(II) nitrate hexa-
hydrate (Co (NO3)2$6H2O, 98%) and gallium indium alloy were
purchased from Alfa Aesar. Ammonium chloride (NH4Cl, 99%),
potassium hexachloroplatinate(IV) (K2PtCl6, 98%), glycerol
(C3H8O3, 99%), formic acid (CH2O2, 98%), ethylene glycol
(C2H6O2, 99%), and deuterium oxide (D2O, 99.9%) were
purchased from Sigma-Aldrich. Glycolic acid (C2H4O3, 98%,
TCI) and glyceric acid (C3H6O4, 98%) were purchased from TCI.
The silicon wafer (p-type, boron doping) was obtained from
WAFERPRO. Silver paste was from Ted Pella Inc. Copper wire
was purchased from Fisher Scientic. Toray carbon ber paper
(TGP-H-60) was obtained from Advance Instruments. Deionized
water (DIW) was used in all experiments. All chemicals and
materials were directly used without further purication and
treatment.
Fig. 4 The N-CoOxkSi NW-Pt hybrid system. (a) Schematic diagram
and (b) digital photo of the N-CoOxkSi NW-Pt hybrid system. (c)
Cathodic current–voltage curves of the hybrid photoelectrochemical
system. (d) A comparison of cell voltages in recently reported hybrid
systems.
Synthetic procedures
The N-CoOx was deposited using a GEM Star XT Atomic Layer
Deposition System. In a typical process, a nitrogen-containing
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Journal of Materials Chemistry A
cobalt-based
molecular
compound
(bis(N,N0-di-i- Grand and high-angle angular dark-lled (HAADF)-STEM
propylacetamidinato) cobalt(II)) was used as the cobalt and images were acquired at a convergence semiangle of 22 mrad
nitrogen precursor. Ozone and helium were employed as the and inner and outer collection angles of 83 and 165 mrad,
oxidizer and carrier gas, respectively. The cobalt precursor was respectively. Energy dispersive X-ray spectroscopy (EDS) was
ꢂ
ꢂ
heated to 85 C and the manifold was maintained at 115 C to conducted using JEOL dual EDS detectors and a specic high
avoid condensation of the precursor in the gas lines. The count analytical TEM holder. For TEM sample preparation, the
temperature in the deposition chamber was 150 ꢂC. Nitrogen copper TEM grid was directly put into an ALD chamber for N-
doped CoOx (N-CoOx) was uniformly deposited on carbon ber CoOx deposition.
paper aer 300 ALD cycles.
Co3O4 nanosheets on carbon ber paper were fabricated by Photo-/electro-chemical measurements
the electrodeposition method. Typically, a-Co(OH)2 was rst
electrodeposited on carbon ber paper in 100 ml of 0.02 M
All electrochemical measurements were carried on CHI 600E
and 660E workstations. We initially evaluated the performance
of our samples for the HER, OER, glycerol oxidation, and
ethylene glycol oxidation in a typical three-electrode system
where the graphite rod and Ag/AgCl were used as the counter
and reference electrodes, respectively. Then the N-CoOx anode
and Si NW-Pt photocathode were coupled in a two-electrode
system to investigate hybrid glycerol electrolysis and hydrogen
Co(NO3)2$6H2O solution with 0.1 M NH4Cl at ꢁ2.0 mA for
10 min. Then, Co3O4 was obtained by annealing a-Co (OH)2
nanosheets in air under 250 ꢂC for 2 h. All electrodeposition
experiments were conducted via a two-electrode system where
the carbon ber paper and graphite rod were used as the
working and counter electrodes, respectively.
Si NW-Pt photoelectrode fabrication:
a metal-assisted
evolution. A Naon lm was used to separate the oxidation and
reduction reactions. A steady DC-powered 150 W Xe-arc lamp
(Newport) was used as a simulated sunlight source. A water lter
was used to block the infrared irradiation. The incident light
intensity was calibrated to 100 mW cmꢁ2 (one sun) using a Si
photodiode. All LSV curves were collected at a scan rate of 5 mV
sꢁ1 and without IR compensation. All the electrochemical
measurements were performed in N2-saturated electrolyte. The
potentials were calibrated to the reversible hydrogen electrode
(RHE) scale according to the equation: ERHE ¼ EAg/AgCl + 0.059
pH + 0.197 V.
chemical etching strategy was employed to synthesize silicon
nanowires (Si NW). Typically, the back side of a silicon wafer
was rstly covered by polyimide to avoid being etched, and was
then sonicated in acetone and DI water for 15 min to remove
impurities. Further, the wafer was soaked in 5% HF for 90 s to
etch the silicon oxide layer. Then, the washed clean wafer was
soaked in 10% HF solution with 0.02 M AgNO3 for 60 s to
deposit silver particles on the surface. Subsequently, the wafer
was soaked into a mixture of 10% HF and 30% H2O2
(10 : 1, vol%) for 10 min and then into 35% HNO3 for 15 min to
remove residual Ag. Finally, the wafer was used to fabricate the
photoelectrode according to the following steps: (1) the silicon
backside was cleaned with 5 wt% HF for 30 s; (2) ohmic contact
was formed by coating the Ga–In eutectic on the backside; (3)
the backside was xed on a conductive copper coil with
conductive silver paint; (4) the copper coil tail was passed
through a glass tube for electric contact. (5) The photoelectrode
assembly was encapsulated by coating Loctite 9462 epoxy and
dried at room temperature overnight. For platinum deposition,
the obtained photoelectrode was rstly cleaned in 5 wt% HF for
30 s and then soaked into 1 wt% HF solution with 5 mM K2PtCl6
for 60 s.
Product quantication
A long-term electrolysis experiment was employed to determine
products of glycerol oxidation and ethylene glycol oxidation.
Electrolytes aer electrolysis were collected and analysed using
a nuclear magnetic resonance (NMR) spectrometer. All liquids
to be tested contained 540 ml electrolyte and 60 ml D2O and were
1
1
measured by H and 13C NMR spectra. H NMR and 13C NMR
spectra were recorded on Varian spectrometers at 400 MHz.
Formic acid standard was prepared by dissolving formic acid
(15 mM for H NMR and 60 mM for 13C NMR) in 1.0 M KOH
1
aqueous solution, and 1H and 13C NMR spectra of the standard
were collected under the same conditions as the solution aer
biomass-polyol electrolysis. The gaseous products produced
from the cathode aer electrolysis were collected and analysed
by gas chromatography (GC-2014C, SHIMADZU). According to
the proposed reaction pathway, the conversion from glycerol to
formic acid could be described by the following equation (eqn
(1)).
Material characterization
The X-ray diffraction (XRD) pattern was collected on a PAN-
˚
alytical X' Pert with Cu-Ka radiation (l ¼ 1.5418 A). X-ray
photoelectron spectroscopy (XPS) measurements were per-
formed using the Kratos AXIS Supra equipped with a mono-
chromatic Al Ka X-ray source, running at a power of 300 W, and
operating at 15 kV. All samples were measured with a 300 ꢃ 700
mm2 spot size and operating chamber pressure < 10ꢁ8 torr. XPS
spectra were tted with XPS peak soware to analyse the atomic
compositions and the possible chemical species. The binding
energy scale was calibrated using the C 1s peak at 284.8 eV.
Electron paramagnetic resonance (EPR) measurements were
performed on a Magnettech MS 5000. Scanning transmission
electron microscopy (STEM) was performed using the JEOL
C3H8O3 (glycerol) ꢁ 8eꢁ + 8OHꢁ ꢁ 5H2O ¼ 3CH2O2 (formic
acid)
(1)
Hence, the faradaic efficiency of formic acid for glycerol
oxidation can be determined by eqn (2),
Nðformic acid yieldÞ
FEð%Þ ¼
ꢃ 100%
(2)
Qtotal=ðZ1 ꢃ FÞ
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and the conversion from glycerol to formic acid could be
described by the following equation (eqn (3)).
9 W. Wang, Y. B. Zhu, Q. Wen, Y. Wang, J. Xia, C. Li,
M. W. Chen, Y. Liu, H. Li, H. A. Wu and T. Zhai, Adv.
Mater., 2019, 31, e1900528.
10 B. Zhu, Z. Liang and R. Zou, Small, 2020, 16, e1906133.
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474.
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C2H6O2 (ethylene glycol) ꢁ 6eꢁ + 6OHꢁ ꢁ 4H2O ¼ 2CH2O2 (3)
Similarly, the faradaic efficiency of formic acid for ethylene
glycol oxidation was described by eqn (4),
Nðformic acid yieldÞ
FEð%Þ ¼
ꢃ 100%
(4)
Qtotal=ðZ2 ꢃ FÞ
in which Qtotal is the total charge passed through the electrodes,
Z1 ¼ 8/3 and Z2 ¼ 3 are the numbers of electrons that form
a mole of formic acid, and F is the Faraday constant (96485 C
molꢁ1).
Author contributions
16 Formic Acid Prices, https://www.intratec.us/chemical-
markets/formic-acid-price, accessed.
17 D. A. Bulushev and J. R. H. Ross, ChemSusChem, 2018, 11,
821–836.
18 Z. Zhang, L. Xin and W. Li, Appl. Catal., B, 2012, 119–120, 40–
48.
19 J. Qi, L. Xin, D. J. Chadderdon, Y. Qiu, Y. Jiang, N. Benipal,
C. Liang and W. Li, Appl. Catal., B, 2014, 154–155, 360–368.
J. G. and X. X. lead the project. Z. K. designed and performed
experiments. W. N. assisted with product analysis by NMR. X. Y.
and X. P. conducted the STEM characterization. D. H. and X. S.
helped with mechanism studies for this research. S.Y. helped
with some electrochemical measurements and involved into
writing this paper.
˜
20 M. Simoes, S. Baranton and C. Coutanceau, Appl. Catal., B,
2010, 93, 354–362.
Conflicts of interest
21 H. J. Kim, S. M. Choi, M. H. Seo, S. Green, G. W. Huber and
W. B. Kim, Electrochem. Commun., 2011, 13, 890–893.
22 S. Lee, H. J. Kim, E. J. Lim, Y. Kim, Y. Noh, G. W. Huber and
W. B. Kim, Green Chem., 2016, 18, 2877–2887.
23 A. Zalineeva, A. Serov, M. Padilla, U. Martinez,
K. Artyushkova, S. Baranton, C. Coutanceau and
P. B. Atanassov, J. Am. Chem. Soc., 2014, 136, 3937–3945.
24 M. Simoes, S. Baranton and C. Coutanceau, ChemSusChem,
2012, 5, 2106–2124.
25 S. Wang, Y. Wang, S. L. Zhang, S. Q. Zang and X. W. D. Lou,
Adv. Mater., 2019, 31, e1903404.
26 Q. Yu, C. Liu, X. Li, C. Wang, X. Wang, H. Cao, M. Zhao,
G. Wu, W. Su, T. Ma, J. Zhang, H. Bao, J. Wang, B. Ding,
M. He, Y. Yamauchi and X. S. Zhao, Appl. Catal., B, 2020,
269, 118757.
There are no conicts to declare.
Acknowledgements
Jing Gu acknowledges San Diego State University (SDSU) start-
up funds and NSF award (CEBT-1704992) to support this
research. We gratefully acknowledge the UC Irvine Materials
Research Institute (IMRI) for helping with STEM and XPS
characterization. IMRI was funded in part by the National
Science Foundation Major Research Instrumentation Program
under grant no. CHE-1338173.
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