Efficient H2 Evolution Coupled with Oxidative Refining of Alcohols via A Hierarchically Porous Nickel Bifunctional Electrocatalyst

Article abstract of DOI:10.1021/acscatal.7b00876

Water electrolysis to produce H₂ and O₂ with renewable energy input has been generally viewed as an attractive route to meet future global energy demands. However, the sluggish O₂ evolution reaction usually requires high overpotential and may yield reactive oxygen species (ROS) that can degrade the electrolyzer membrane and hence shorten the device lifetime. In addition, the potential gas crossover may result in an explosive H₂/O₂ mixture and hence safety risks. To address these issues, we herein report a general electrolysis strategy for the simultaneous H₂ production and alcohol oxidative upgrading (e.g., benzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, ethanol, and 5-hydroxymethylfurfural), in which the thermodynamics of the latter is much easier than that of water oxidation. A facile and environmentally friendly template-free electrodeposition was used to obtain a 3D hierarchically porous nickel-based electrocatalyst (hp-Ni) for such an integrated electrolysis, requiring a voltage of ～220 mV smaller than that of water splitting to achieve 50 mA cm^-2 together with robust stability, high Faradaic efficiencies, and no formation of ROS, as well as production of valuable products at both the cathode (H₂) and anode (alcohol oxidation products). More importantly, we demonstrated that these diverse alcohol oxidations over hp-Ni exhibited similar onset potentials which were largely determined by the desirable oxidation potential of hp-Ni, irrespective of the different intrinsic thermodynamics of these alcohol oxidation reactions. This result provides a new direction for the rational design of heterogeneous transition-metal-based electrocatalysts with lower oxidation potential for more highly efficient electrocatalytic alcohol oxidation. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.7b00876

Article
Efficient H2 Evolution Coupled with Oxidative Refining of Alcohols
via A Hierarchically Porous Nickel Bifunctional Electrocatalyst
Bo You, Xuan Liu, Xin Liu, and Yujie Sun
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ACS Catalysis
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Efficient H2 Evolution Coupled with Oxidative Refining of Alcohols
via A Hierarchically Porous Nickel Bifunctional Electrocatalyst
Bo You,†,ǁ Xuan Liu,†,ǁ Xin Liu,†,‡,ǁ and Yujie Sun*,†
†Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
‡Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Chinese Academy of Sci-
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ences, 18 Xinning Road, Xi’ning, Qinghai 810008, China
ABSTRACT: Water electrolysis to produce H2 and O2 with renewable energy input has been generally viewed as an attrac-
tive route to meet the future global energy demands. However, the sluggish O2 evolution reaction usually requires high
overpotential and may yield reactive oxygen species (ROS) that can degrade electrolyzer membrane and hence shorten
the device lifetime. In addition, the potential gas crossover may result in explosive H2/O2 mixture and hence safety risks.
To address these issues, we herein report a general electrolysis strategy for the simultaneous H2 production and alcohol
oxidative upgrading (e.g., benzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, ethanol, and 5-
hydroxymethylfurfural), in which the thermodynamics of the latter is much easier than that of water oxidation. A facile
and environmentally friendly template-free electrodeposition was used to obtain 3D hierarchically porous nickel-based
electrocatalyst for such an integrated electrolysis, requiring a voltage of ~220 mV smaller than that of water splitting to
achieve 50 mA cm-2 together with robust stability, high Faradaic efficiencies, no formation of ROS, as well as producing
valuable products at both cathode (H2) and anode (alcohols oxidation products). More importantly, we demonstrated
that those diverse alcohols oxidation over hp-Ni exhibited similar onset potentials which was largely determined by the
desirable oxidation potential of hp-Ni, irrespective of the different intrinsic thermodynamics of those alcohol oxidation
reactions. This result provided a new direction for rational design of heterogeneous transition-metal-based electrocata-
lysts with lower oxidation potential for higher efficient electrocatalytic alcohol oxidation.
KEYWORDS:
Electrocatalysis,
alcohol
oxidation,
hydrogen
evolution,
water
splitting,
nickel
INTRODUCTION
In addition, traditional water electrolysis produces H2
and O2 simultaneously.20-22 Therefore, potential gas cross-
over may result in the formation of explosive H2/O2 mix-
ture and hence poses safety issues.23-26 Meanwhile, the
coexistence of H2, O2, and catalysts may yield reactive
oxygen species (ROS) which will shorten the lifetime of
the water electrolyzer.24 Furthermore, the sluggish kinet-
ics of OER typically restricts the rate of HER, and the
product of OER, O2, is actually not a highly valuable
chemical.27-31 Within these considerations in mind, people
have been devoted to exploring alternative strategies to
decouple H2 production from OER. For instance, a novel
method of electron-coupled proton buffer (ECPB) has
been introduced to produce H2 and O2 at separate times
during water splitting.23-26 However, the associated requi-
site of noble metals-based catalysts and three-
Hydrogen (H2) generation from water electrolysis driven
by renewable energy offers an attractive option to allevi-
ate the impending global energy crisis and associated en-
vironmental issues due to the consumption of finite fossil
fuels.1-6 Conventional room-temperature water electrolyz-
ers usually perform under either strongly alkaline or acid-
ic conditions with the state-of-the-art noble metal-based
catalysts (e.g., Pt, Ir, and Ru). However, the high price and
scarcity of these noble-metal catalysts undoubtedly pro-
hibit their large-scale implementation and necessitate the
exploration of low-cost alternatives. Recent years have
witnessed the emergence of promising transition metal
(such as Mo, W, Co, Ni, and Fe)-based borides, carbides,
nitrides, phosphides, sulfides, and selenides for the H2
evolution reaction (HER).2,3,7,8 Besides, transition metal-
based oxides, hydroxides, and (oxy)hydroxide have also
been reported with appreciable activities for the O2 evolu-
tion reaction (OER).9-12 In particular, our group reported
the concept of “single bifunctional electrocatalyst for
overall water splitting under alkaline solution” based on
cobalt-phosphorous-derived (Co-P) films.13 This bifunc-
tional electrocatalyst strategy was subsequently extended
to other transition metal derivatives and even metal-free
nanocarbons by us13 and other groups.14-19 However, high
overpotentials are still needed to drive water splitting at
high current densities.
compartment configuration complicates device construc-
tion and potentially increases the cost.
23-26
More im-
portantly, it still needs high potential to catalyse OER,
and the possible decomposition and high cost of ECPB
may limit their large-scale commercialization.23-26 In addi-
tion, most nonprecious OER electrocatalysts cannot sur-
vive strongly acidic environment of the ECPB-containing
electrolytes.32-34
In order to address the above concerns, we reason that
replacing OER with thermodynamically more favourable
alcohol oxidation will be an appealing option. Integrating
decoupled H2 production from water splitting with alco-
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Page 2 of 18
hol oxidation will not only reduce the voltage require- Hydroxymethylfurfural (HMF) and 2,5-furandicarboxylic
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ment to generate H2 but also avoid the H2/O2 mixture
issue. Furthermore, such an integrated electrolyzer will
also exclude the formation of ROS and thus may lead to
longer device lifetime. In fact, alcohol oxidation reactions
play a vital role in converting many petroleum- and bio-
mass-derived feedstocks to value-added chemicals.27,35 For
example, the oxidation products of benzyl alcohol, etha-
nol, and 5-hydroxymethylfurfural (HMF) are primary
building blocks to produce diverse large-scale commodi-
ties, polymers, and pharmaceuticals.35-39 The overall reac-
tions for H2 production paired with alcohol oxidation un-
der alkaline conditions are depicted in Figure 1.
acid (FDCA) were purchased from Alfa Aesa and Chem-
Impex International Inc, respectively. 2,5-Diformylfuran
(DFF) and 2-formyl-5-furancarboxylic acid (FFCA) were
purchased from Ark Pharm, Inc. 5-Hydroxymethyl-2-
furan-carboxylic acid (HMFCA) was purchased from Asta
Tech. Potassium hydroxide was used as received from Alfa
Aesar. Ammonium chloride was purchased from Avantor.
Nickel chloride was purchased from Fisher, Nickel foam
with purity >99.99% was purchased from MTI. The anion
exchange membrane (Fumasep FAA-3-PK-130) was pur-
chased from Fuel Cell Store. All chemicals were used as
received without purification. Water deionized (18
MΩ⋅cm) from a Barnstead E-Pure system was used in all
experiments.
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Synthesis of hp-Ni Electrocatalyst. The hp-Ni bifunc-
tional electrocatalyst was prepared by a facile template-
free cathodic electrodeposition of 3D hierarchically po-
rous Ni microspheres on a nickel foam (hp-Ni). Typically,
the electrodeposition was performed under a standard
two-electrode configuration at room temperature with an
electrolyte consisting of 2.0 M NH4Cl and 0.1 M NiCl2. A
piece of commercial nickel foam with a size of 0.5 cm ×
0.5 cm was employed as the working electrode and a Pt
wire as the auxiliary electrode. The galvanostatic electro-
deposition was carried out at -3.0 A cm-2 for 500 s to ob-
tain hp-Ni samples with a mass loading of ~75 mg cm-2.
Figure 1. Chemical reactions for integrated H2 production
and alcohol oxidation in alkaline media.
Herein, we report a novel and general strategy of using
3D hierarchically porous nickel framework (hp-Ni) as
nonprecious bifunctional electrocatalyst for H2 produc-
tion and oxidative upgrading of various alcohols (e.g.,
benzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl
alcohol, ethanol, and HMF) under ambient conditions.
Since alcohol oxidation occurs at the hp-Ni anode while
HER takes place at the hp-Ni cathode, no O2 is generated
and hence the issues of H2/O2 mixing and ROS no longer
exist. Furthermore, due to the superior electrocatalytic
performance of hp-Ni for alcohol oxidation relative to
OER, the cell voltage of our hp-Ni-based two-electrode
electrolyzer at a benchmark current density (50 mA cm-2)
was reduced by ~220 mV, compared with that of water
electrolysis. Additionally, high Faradaic efficiencies were
obtained for both H2 production and alcohol oxidation,
together with robust durability. Compared with the pre-
vious reports for electrocatalyst synthesis,15,16,29,31 the prep-
aration of hp-Ni in the present work is much more facile
and environmentally friendly which avoids high tempera-
ture treatment (~300 °C) and toxic chemical reagents (e.g.,
sodium hypophosphite monohydrate, sulfur, ammonia
gas, etc). Moreover, the relationship between the elec-
tronic effect of alcohols on their oxidation behaviours
over hp-Ni catalyst was also investigated, which provides
guidance in developing more competent electrocatalysts
for alcohol oxidation coupled with H2 production. Overall,
the low cost and green preparation for bifunctional elec-
trocatalyst, high-value products at both electrodes, and
better energy conversion efficiency render our strategy
particularly attractive for renewable energy conversion
technologies, which can be applied to couple HER with
many other organic oxidation reactions.
Physical Methods. Scanning electron microscopy and
element mapping were performed on a FEI QUANTA FEG
650 (FEI, USA) at the Microscopy Core Facility of Utah
State University. X-ray diffraction patterns were obtained
by a Rigaku MinifexII Desktop X-ray diffractometer. The
X-ray photoelectron spectroscopy (XPS) analyses were
conducted on a Kratos Axis Ultra instrument (Chestnut
Ridge, NY) at the Surface Analysis Laboratory, University
of Utah Nanofab. XPS data were analysed via the CASA
XPS software, and energy corrections on high resolution
scans were calibrated by referencing the C 1s peak of ad-
ventitious carbon to 284.5 eV.
Electrocatalytic Measurements. Electrochemical HER,
OER, and alcohol oxidation measurements were per-
formed with a Gamry Interface 1000 electrochemical
workstation under three-electrode configuration. The as-
prepared hp-Ni was directly used as working electrode, a
Ag/AgCl (sat. KCl) electrode as reference electrode, and a
carbon rod as counter electrode. All the reported poten-
tials were quoted with respect to reversible hydrogen
electrode (RHE) through ERHE = EAg/AgCl + 0.059 × pH +
0.197 V, and overpotential for OER (η) was calculated
from η = ERHE – 1.23 V. The electrochemical HER, OER,
and alcohols oxidation experiments were conducted in 10
mL, 1.0 M KOH solution in the presence or absence of 10
mM organic substrates. H2 and O2-saturated electrolytes
were used for HER and OER measurements, respectively.
For two-electrode electrolysis, the two hp-Ni were em-
ployed as bifunctional catalyst electrodes for both anode
and cathode. All the potential range was scanned at a
scan rate of 2 mV s-1. iR (current times internal resistance)
EXPERIMENTAL SECTION
Chemicals. Benzyl alcohol (BA), benzoic acid, benzalde-
hyde, 4-nitrobenzyl alcohol (NBA) and 4-methylbenzyl
alcohol (MBA) were used as received from Sigma-Aldrich.
Ethanol was used as received from Decon Labs. 5-
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ACS Catalysis
(Figure 2c). These distinct differences clearly confirmed
compensation was employed in all the electrochemical
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measurements to adjust the voltage drop between refer-
ence and working electrodes by the Gamry FrameworkTM
Data Acquisition Software 6.11. The stability tests of hp-Ni
for BA and HMF oxidation were evaluated by chronoam-
perometry at 1.423 V vs. RHE in 10 mL, 1.0 M KOH con-
taining 10 mM corresponding organic substrates for five
consecutive cycles.
the successful electrodeposition of porous Ni
microspheres on Ni foam. Elemental mapping images
(Figure S2) revealed the presence of abundant Ni, plus a
small amount of O due to slight surface oxidation in air.
XPS analysis further confirmed the presence of metallic
Ni and O (Figure S3), in line with the XRD and elemental
mapping results. It is expected that these hierarchically
porous structure with 3D configuration can facilitate
substrate transport and gas diffusion to boost the
utilization efficiency of active sites, which are beneficial
for electrocatalytic applications.22,34,40-44
Product Quantification. To analyse the products of BA
and HMF oxidation quantitatively and calculate the cor-
responding Faradaic efficiencies, 10 μL aliquots of the
electrolyte solution during chronoamperometry testing
were collected periodically from the electrolysis solution
and diluted with 490 μL water, which were then analysed
using HPLC (Shimadzu Prominence LC-2030C system
equipped with an ultraviolet-visible detector and a 4.6
mm× 150 mm Shim-pack GWS 5 μm C18 column). The
wavelengths of detector were set at 230 nm for benzoic
acid, 254 nm for benzyl alcohol and benzaldehyde, and
265 nm for HMF and its corresponding products. An elu-
ent mixture of 5 mM ammonium formate aqueous solu-
tion (A) and methanol (B) was used. The HPLC analysis
was conducted for the BA oxidation products by A/B (v/v:
4/6), while HMF oxidation products by A/B (v/v: 7/3)
within 10 min at a flow rate of 0.5 mL min-1. The qualita-
tive and quantitative analyses of reactants and products
were conducted based on the corresponding calibration
curves by applying standard solutions with known con-
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centrations. The H NMR spectra were obtained on a
Bruker Avance III HD Ascend 500 MHz NMR.
A SRI 8610C gas chromatography with a Molecular
Sieve 13 packed column, a HayesSep D packed column,
and a thermal conductivity detector was used to quantify
the generated H2 during electrolysis. The oven tempera-
ture was set at 80 oC and argon was used as the carrier gas.
Figure 2. (a) XRD pattern of hp-Ni and the standard pattern
of metallic Ni. (b,c) SEM images of hp-Ni at different magni-
fications.
Alcohol oxidation is an important reaction in the
chemical industry with applications ranging from
petroleum chemical refining and biomass utilization to
pharmaceutical and fine chemical synthesis.27,35 The
conventional methods for alcohol oxidation typically
require stoichiometric chemical oxidants and expensive
metal catalysts (such as Au, Pd, and Pt) under harsh
conditions (e.g., high pressure and/or elevated
temperature),37 hence it is of paramount importance to
explore greener and lower-cost alternative methods for
alcohol oxidation. Electrocatalytic oxidation under
ambient conditions is an appealing approach as the
oxidation can be solely performed by electricity without
additional chemical oxidants.28-31,35,39 Herein, we chose
benzyl alcohol (BA) as our first candidate to evaluate the
electrocatalytic activity of hp-Ni for alcohol oxidation.
The target product is benzoic acid. Under alkaline
condition (1.0 M KOH), the oxygen evolution reaction can
potentially compete with organic oxidation reactions.
Therefore, an ideal electrocatalyst should possess high
preference towards BA oxidation rather than oxygen
evolution, which guarantees the minimum Faradaic
RESULTS AND DISCUSSION
The hp-Ni electrocatalyst with 3D open porosity was
prepared
by
a
facile
one-step
self-template
electrodeposition of metallic Ni framework on
commercial Ni foam at -3.0 A cm-2 for 500 s. The
concomitant formation of H2 bubbles during
electrodeposition functioned as templates for the
resulting porosity.22 The successful formation of metallic
Ni for hp-Ni was verified by the corresponding X-ray
diffraction (XRD) pattern (Figure 2a). Scanning electron
microscopy (SEM) image (Figure 2b) indicated the 3D
hierarchically macroporous feature of hp-Ni with pore
sizes of several hundred micrometers, inherited from the
pristine Ni foam (Figure S1a). In addition, there were
abundant smaller macropores with diameters of
approximately 10 μm on the interconnected macropore
walls of hp-Ni (Figure 2b), distinctively different from the
featureless surface of a pristine Ni foam (Figure S1b). A
closer inspection of the smaller macropores at higher
magnification showed 3D porous structure and the
presence of stacked Ni nanospheres with smooth surface
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efficiency loss due to water oxidation. Figure 3a showed was needed for the complete oxidation of BA to benzoic
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the linear sweep voltammetry (LSV) curves of BA
oxidation and water oxidation catalyzed by hp-Ni in 1.0 M
KOH in the presence and absence of 10 mM BA,
respectively. In the absence of BA, the onset potential of
our hp-Ni was ~1.51 V vs. RHE and high current density
beyond 1.55 V vs. RHE was observed, implying the OER
activity of hp-Ni. After adding 10 mM BA, the hp-Ni
exhibited negatively-shifted onset potential (~1.35 V vs.
RHE) and a large current density within 1.40 V vs. RHE,
indicating that the oxidation of BA on hp-Ni was
significantly easier than OER. In stark contrast, the
pristine nickel foam (NF) exhibited much worse activities
for both OER and BA oxidation reaction (Figure S4),
underscoring the positive effect of 3D hierarchically
porous Ni framework deposited on Ni foam for
electrocatalysis. Such an improved catalytic performance
of hp-Ni compared to that of NF is attributed to the
synergetic effect of its unique 3D structure which
facilitates gas diffusion and substrate transportation,
acid. As depicted in Figure 3a, no noticeable oxygen
evolution could occur at 1.423 V, hence a high Faradaic
efficiency for benzyl alcohol (BA) oxidation was
anticipated. The resulting HPLC chromatograms (Figure
S6) undoubtedly demonstrated the increase of benzoic
acid (Figure S6a) and decrease of BA (Figure S6b) over
passed charges, implying the conversion of benzyl alcohol
into benzoic acid (Figure 3b). After passing ~38 C, the
peak of BA nearly disappeared (Figure S6b) while the
benzoic acid peak reached its maximum intensity (Figure
S6a). Eventually, a Faradaic efficiency of 98% was
obtained (See Supporting Information for details of
calculation). Note that the intermediate, benzaldehyde
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(Figure
4a),
was
also
detected
during
the
chronoamperometry experiment but remained at low
concentration (Figure S6c).
The stability of hp-Ni for BA oxidation was also
estimated by repeating the above constant potential
electrolysis using the same hp-Ni catalyst. As exhibited in
Figure 3c, the Faradaic efficiencies of benzoic acid
production for these five trials were calculated to be 96-
98%, unambiguously demonstrating the robust durability
of hp-Ni for BA oxidation.
Figure 3. (a) Linear sweep voltammograms of hp-Ni at 2 mV
s-1 in 1.0 M KOH containing 0 and 10 mM benzyl alcohol (BA).
(b) Conversion or yield (%) dependence of BA and its oxida-
tion products on passed charges during electrochemical oxi-
dation at 1.423 V vs. RHE in 10 mL, 1.0 M KOH with 10 mM
BA (Ph-CHO: benzyl aldehyde; Ph-COOH: benzoic acid). (c)
Faradaic efficiencies of benzoic acid production for con-
trolled potential electrolysis with hp-Ni in 10 mL, 1.0 M KOH
with 10 mM BA at 1.423 V vs. RHE under five successive cy-
cles. (d) LSV curves of hp-Ni at 2 mV s-1 in 1.0 M KOH with 10
mM BA, 4-methylbenzyl alcohol (MBA), or 4-nitrobenzyl
alcohol (NBA).
Figure 4. (a) Pathway of benzyl alcohol (BA) oxidation to
benzoic acid. (b) Chemical structures of MBA and NBA.
As an initial attempt to investigate the electronic effect
on the electrocatalytic activity of hp-Ni for alcohol
oxidation, two derivatives of benzyl alcohol with
electronic withdrawing (4-nitrobenzyl alcohol, NBA) and
donating (4-methylbenzyl alcohol, MBA) substituents on
the benzene ring (Figure 4b) were subjected to similar
electrocatalytic oxidation. As shown in Figure 3d, both
LSV curves of these two new substrates took off at a quite
similar potential compared to that of the parent benzyl
alcohol. This result implies that the catalytic onset
potential of these three alcohols oxidation is primarily
determined by the desirable oxidation state of the
together with higher electrochemically active surface area
which will boost the utilization efficiency of active sites
(Figure S5).
electrocatalyst,
rather
than
their
intrinsic
Next, long-term chronoamperometry utilizing hp-Ni as
the electrocatalyst was carried out at 1.423 V vs. RHE in 10
mL, 1.0 M KOH containing 10 mM BA. The concentration
evolution of BA and its oxidative products (benzyl
aldehyde and benzoic acid) during the electrolysis were
analyzed by HPLC. Herein, a theoretical charge of ~38 C
thermodynamics, distinctive from many molecular
electrocatalysts.6 Therefore, we anticipate that the
optimal solid-state electrocatalysts for alcohol oxidation
should require a lower potential to reach their functional
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In order to demonstrate that such a strategy of HER
oxidation states (high-valence states), which will be
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pursured in our future studies.
coupled with alcohol oxidation is general and can be
extended to upgrade other alcohol compounds, we
further evaluated the performance of hp-Ni for the
4
oxidative
upgrading
of
ethanol
and
5-
5
hydroxymethylfurfural (HMF), both of which have
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attracted increasing attention as representative
28-31,37
compounds for oxidative alcohol upgrading.
As
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depicted in Figure 6, after adding 10 mM ethanol and
HMF, the catalytic onset potentials of hp-Ni both shifted
to ~1.35 V vs. RHE and obvious catalytic current density
rises were viewed within 1.40 V vs. RHE, indicative of
more favourable alcohol oxidation than OER. The
overpotentials to afford a benchmark current density (e.g.,
50 mA cm-2) for both ethanol and HMF oxidation
reactions were at least 200 mV smaller than that of OER
(Figure 6). Chronoamperometry experiments carried out
at 1.423 V for the oxidation of ethanol and HMF further
demonstrated the almost complete conversion to their
corresponding value-added acid products (top of Figure
6). After passing the theoretical amount of charge,
quantitative conversion to the desirable acids were
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obtained according to the corresponding H NMR and
HPLC results (Figures S9 and 7a).
Figure 5. (a, b) SEM images of post-BA hp-Ni at different
magnifications. (c) SEM image and the corresponding
elemental mapping of post-BA hp-Ni. High-resolution XPS
spectra of (d) Ni 2p3/2 and (e) O 1s for the fresh and post-BA
hp-Ni electrocatalysts.
We then employed XRD, SEM, and XPS techniques to
probe the structure and composition details of the hp-Ni
electrocatalyst after the above stability tests (named as
post-BA hp-Ni). Although its SEM images at different
magnifications (Figure 5a and b) and XRD pattern (Figure
S7) suggested the interigty of the whole three
dimensional hierarchical porous morphology and mainly
metallic Ni phase, elemental mapping results (Figure 5c)
indicated that the presence of Ni and a large amount of O
for post-BA hp-Ni. Energy-dispersive X-ray spectroscopy
also revealed the increased O content in the post-BA hp-
Ni relative to that of fresh hp-Ni (Figure S8),
corroborating its surface oxidation during benzyl alcohol
oxidation. Additionally, the high-resolution Ni 2p3/2 XPS
spectra for the post-BA hp-Ni electrocatalyst showed an
decreased peak at 852.6 eV ascribed to metallic Ni, while
an increased peak at 856.0 eV attributed to oxidized Ni
species,22 supporting the partial oxidation of metallic Ni
(Figure 5d). This nickel oxidation was further verified by
the enhanced peak intensity of high-resolution O 1s XPS
spectra (Figure 5e). Collectively, it is most likely that the
true catalytic active sites of benzyl alcohol and other
related alcohols are the high-valent nickel species.9,29
Figure 6. Oxidation of ethanol (a) and HMF (b) to value-
added acids and the corresponding linear sweep
voltammograms of hp-Ni at 2 mV s-1 in 1.0 M KOH in the
presence (red) and absence (black) of 10 mM alcohol
substrates.
It has been acknowledged that HMF, a dehydration
product of C6 carbohydrates from biomass, holds a
pivotal place in biomass refinery, as HMF is a platform
chemical and can be upgraded to a wide variety of
important commodity chemicals (Figure S10), including
2,5-bishydroxymethulfuran, 2,5-dimethylfuran, and 2,5-
furandicarboxylic acid (FDCA).36-39 In particular, FDCA
has been suggested as a substitute of terephthalic acid to
produce polyamides, polyesters, and polyurethanes.36-39
Because of the prominent role of HMF as a biomass-
derived intermediate compound, we also examined the
details of HMF oxidation to FDCA on hp-Ni through a
similar chronoamperometry experiment at 1.423 V vs.
RHE in 10 mL, 1.0 M KOH with 10 mM HMF. In this case,
the theoretical amount of ~58 C charge was calculated for
the complete oxidation of HMF to FDCA. As shown in
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Figure 7b, a high Faradaic efficiency for oxidation of HMF
mV),45 η-MoCx/C (~ 220 mV),46 CoP/CC (> 300 mV),47 and
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was also expected due to the absence of water oxidation
at 1.423 V vs. RHE. The obtained HPLC chromatograms
(Figure 7a) evidently exhibited the decrease of peak for
HMF and rise of peak for FDCA over electrolysis time,
intimating the conversion from HMF to FDCA (Figure 7b).
After passing ~58 C, a Faradaic efficiency of 98% was
obtained for the FDCA production. It is known that there
are two possible paths for oxidation of HMF to FDCA
(Figure S11): one starts from the oxidation of the aldehyde
group of HMF to generate HMFCA and the other
proceeds through the formation of DFF. Subsequently,
both routes yield FFCA prior to the eventual formation of
FDCA. The relatively higher concentration of HMFCA
relative to that of DFF revealed the hp-Ni-mediated HMF
oxidation most likely followed the first-step formation of
HMFCA route, similar to those commonly observed
aerobic oxidations.36 Nevertheless, the DFF route was also
included, since the DFF intermediate was still identified
via HPLC (Figure 7a). Moreover, the high Faradaic
efficiencies (92-98%) of FDCA formation for five
successive electrolysis cycles demonstrated the robust
stability of hp-Ni for HMF oxidation as well (Figure 7c).
other HER catalyst (Table S1) to reach the same current
density. Moreover, hp-Ni exhibited robust long-term
stability, as revealed by its stable overpotential of ~ 230
mV to reach -50 mA cm-2 for a 18 h chronopotentiometry
experiment (Figure S12b).
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Figure 8. (a) Linear sweep voltammograms and (b)
comparison of overpotential at various current densities
utilizing the hp-Ni couple in 1.0 M KOH containing 0 and 10
mM BA. c) Experimental H2 quantity compared with the
theoretical quantity during HER catalyzed by hp-Ni couple.
Given the outstanding performance of hp-Ni for both
alcohol oxidation and H2 evolution as aforementioned, it
is highly anticipated that hp-Ni can function as a
bifunctional electrocatalyst for simultaneous production
of H2 and organic acid in a two-electrode configuration
under alkaline condition. In orde to validate this
hypothesis, we chose benzyl alcohol as the organic
substrate and an anion exchange membrane was used to
separate the two electrodes. As depicted in Figure 8a, our
hp-Ni catalyst couple needed a cell voltage of ~1.69 V to
afford 10 mA cm-2 for water splitting, comparable to or
even better than those of previously reported noble
metal-free bifunctional water splitting catalysts including
NiFe LDH/NF (1.70 V),48 Ni5P4/NF (1.70 V),49 Ni3S2/NF
(>1.70V),50 and others (Table S2) to reach same current
density. More importantly, upon addition of 10 mM
benzyl alcohol, the cell voltages to afford 10, 20, 50, and
100 mA cm-2 were noticeably reduced to 1.50, 1.54, 1.60,
and 1.66 V, respectively (Figure 8b), substantially smaller
than those of pure water electrolysis. In order to quantify
the generated H2 and benzoic acid under this two-
electrode setup, a durable electrolysis was performed at a
cell voltage of 1.50 V. As exhibited in Figure 8c, the
produced H2 measured by gas chromatography (GC)
agreed with the theoretical amounts very well, suggesting
a high Faradaic efficiency of ~100 % for HER. Quantifying
the resulting liquid products by HPLC also implied a
Faradaic efficiency of ~97% for benzoic acid formation. If
Figure 7. (a) HPLC traces of electrocatalytic HMF oxidation
catalyzed by hp-Ni at 1.423 V vs. RHE in 10 mL, 1.0 M KOH
with 10 mM HMF. (b) Conversion of HMF or yield of the
oxidation products during the electrolysis. (c) Faradaic
efficiencies of FDCA production using the same hp-Ni
catalyst for five successive cycles.
We acknolwedge that metallic Ni has been widely used
as a cathode catalyst for H2 production from water
electrolysis under alkaline condition, mainly due to its
low cost and excellent catalytic activity.44 Indeed, our hp-
Ni also exhibited great activity for HER. As shown in
Figure S12a, the LSV curve of hp-Ni showed a small onset
potential and achieved a HER current density of -50 mA
cm-2 at an overpotential of approximately 219 mV in 1.0 M
KOH, comparable to or even better than those of recently
reported nonprecious HER catalysts like β-Mo2C (> 250
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ACS Catalysis
Funded Overseas Study Program and Qinghai Institute of
HMF was selected as the organic substrate, similar low
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energy input and high Faradaic efficiencies for the
production of both H2 and FDCA could also be obtained
(Figure S13), demonstrating the versatility of our new-type
electrolysis.
Salt Lakes, Chinese Academy of Sciences. The NMR
measurements were conducted on a NMR spectrometer
supported by the NSF MRI Award (CHE-1429195).
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In conclusion, we report a general strategy for concur-
rent H2 generation and alcohol oxidation catalyzed by a
low-cost hp-Ni with nearly unity Faradaic efficiencies.
Owing to the more favourable thermodynamics of these
alcohol oxidations than that of OER on hp-Ni, the electro-
lyzer voltage to produce benchmark current densities was
reduced by ~220 mV compared to water splitting electrol-
ysis. It is even more exciting that value-added products
were generated at both electrodes (H2 at cathode and val-
uable organic acids at anode). Our strategy provides an
alternative approach to avoid the issues of H2/O2 mixing
and ROS formation during traditional water electrolysis.
Given the advantages of inexpensive catalyst, high energy
conversion efficiency, great Faradaic efficiency, and ambi-
ent reaction condition (room temperature, atmospheric
pressure, and aqueous solution), our new-type electrolysis
strategy of cathodic H2 production coupled with anodic
alcohol oxidation will inspire researchers to explore many
other oxidative organic upgrading reactions to pair with
HER, maximizing energy conversion efficiency and yield-
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er voltage input. Further studies along this line are un-
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ASSOCIATED CONTENT
Supporting Information. Additional figures, electro-
chemical plots, tables, XRD, XPS, SEM, NMR data, HPLC
chromatograms. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yujie.sun@usu.edu.
Author Contributions
ǁB.Y, X.L, and X.L contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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We acknowledge the support of the National Science
Foundation (CHE-1653978) and the Microscopy Core Fa-
cility at Utah State University. X. L. acknowledges the
financial support from the Chinese Academy of Sciences
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