Engineering of aerogel-based electrocatalysts for oxygen evolution reaction
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1 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113
Received: 31 May 2021
Revised: 9 July 2021
Accepted: 12 July 2021
Engineering of aerogel-based electrocatalysts for oxygen
evolution reaction
Cunyuan Gao Xin Zhang Jinhua Zhan Bin Cai
School of Chemistry and Chemical
Engineering, Shandong University, Jinan, The oxygen evolution reaction (OER) is considered as one of the major road-
China blocks for many renewable electrochemical energy technologies, such as water
Correspondence
splitting, rechargeable metal-air batteries. Thus the development of efficient and
Bin Cai, School of Chemistry and Chem- low-cost OER electrocatalysts has achieved a lot of research and development
ical Engineering, Shandong University, attention. Aerogels, as featured by large surface area, high porosity, and self-
Shanda South Road 27, Jinan 250100,
China. supportability, provide an excellent platform for the design of efficient electrocat-
Email: bin.cai@sdu.edu.cn alysts. This review summarizes the recent progress of the aerogel-based design of
catalysts in tackling the challenges in the field of OER electrocatalysis. The cor-
Funding information
Qilu Young Scholar Start-up Fund relation between catalyst design strategy and catalytic performances is evaluated
to provide a roadmap for the development of aerogel-based OER electrocatalysts.
Thereafter, the current state of our understanding of the application of the aero-
gel concept in OER electrocatalysis is provided, which could serve as a guide to
the development of future aerogel electrocatalysts.
KEYWORDS
aerogel, electrocatalysis, oxygen evolution reaction, porous materials
1 INTRODUCTION is typically needed to achieve the desired current
density.[19–22] This will inevitably increase the production
The continuous consumption of traditional fossil fuels cost of hydrogen production by electrolysis of water,
and environmental problems, such as global warming, air thus hindering the practical application in industry. The
pollution, etc., make the development of new clean and OER mechanism under acidic and alkaline conditions is
renewable energy sources urgent.[1–11] Hydrogen energy debatable. The most widely accepted reaction mechanism
has the advantages of high energy density and carbon-free of OER is through the adsorption and desorption process
energy carriers.[12–15] It is considered promising future of oxygen-containing intermediates, such as equations (1)-
energy, which can be directly obtained by electrochem- (4) or as described in equations (5)-(8), where * represents
ically splitting water. In particular, water splitting can the adsorption site, and OH*, O*, and OOH* represent the
be divided into two reactions: the hydrogen evolution adsorbed intermediates. Among them, the step with the
reaction (HER) at the cathode and the oxygen evolution highest reaction kinetic barrier is the rate-determining step
reaction (OER) at the anode.[16–18] Of these two reactions, (RDS) of the catalyst, which is very important for heteroge-
the OER is more kinetically demanding as it requires the neous catalysis.[22] Therefore, one essential way to develop
transfer of four protons and four electrons to produce high-performance OER catalysts is to reduce the Gibbs
a single oxygen molecule. Thus a large overpotential free energy of the RDS. At present, noble metals such as
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© 2021 The Authors. Electrochemical Science Advances published by Wiley-VCH GmbH
Electrochem. Sci. Adv. 2021;e2100113. wileyonlinelibrary.com/journal/elsa 1 of 14
https://doi.org/10.1002/elsa.202100113
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2 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113
Ru- and Ir-based catalysts exhibited high catalytic activity, 2 SYNTHESIS AND FEATURES OF
but their practical application is hindered by the high AEROGEL STRUCTURE
price and poor stability.[23–33]
Under acidic conditions: Since the invention of aerogel in the 1930s, aerogel has
been extensively explored by widening the building block
H2 O → OH∗ + H+ + e− (1) from inorganic to organic nanomaterials.[52] According to
IUPAC’s definition, the aerogel can be described as a syn-
OH∗ → O∗ + H+ + e− (2) thetic solid with meso- and macropores with diameters
up to a few hundred nanometers and a porosity of more
than 95% of its volume occupied by gas.[53] As a synthetic
H2 O + O∗ → OOH∗ + H+ + e− (3)
solid material, aerogels have outstanding physicochemical
properties, including ultralow density (approaching 1.2 ×
OOH∗ → O2 ∗ + H+ + e− (4) 10−4 g/cm3 ), high continuous porosity, and extremely large
surface areas (up to 1000 m2 /g). Thus aerogels find sig-
Under basic conditions: nificant applications in heterogeneous catalysis and many
other fields such as energy storage, piezoelectrics, ther-
OH → OH∗ + e− (5) moresistors, and sensors. The first-generation aerogels are
typically fabricated based on the condensation of specific
OH + OH∗ → H2 O + O∗ + H+ + e− (6) molecular precursors, thus restricting the modification of
surface properties. Along with the rapid growth of nan-
otechnology, colloidal nanomaterials have been explored
OH + O∗ → OOH∗ + e− (7) as precursors for designing aerogels, thus leading to the
generation of nanocrystal aerogel that bridges the nano
OH + OOH∗ → O2 + H2 O + e− (8) and macro worlds.[49]
In general, the synthesis of aerogel follows the sol-gel
In recent years, the use of 3D aerogel skeletons as process, where a networked monolith (i.e., hydrogel) sepa-
OER catalysts or catalyst supports has attracted wide rates from the solution, followed by the replacement of the
attention because of its unique nano-characteristics, solvent with air while retaining the network structure. The
ideal volume characteristics, and processability.[34–47] essence of the gelation process is to destabilize the colloidal
Conventional aerogel materials are generally obtained sol in a controllable manner and facilitate the intercon-
from molecular precursors through polycondensation nection of the precursor nanomaterials into 3D networks.
reactions.[48–50] Hence their surface properties gener- Such gelation process could be realized via two strategies:
ally cannot be changed, thereby limiting the design of via gelation of preformed nano building blocks (i.e.,
active structure.[51] Using graphene, carbon nanotubes, two-step) or an in situ spontaneous gelation process (i.e.,
nanocrystals, and other catalytically functional nanoma- one-step). The two-step method provides a more versatile
terials as precursors to fabricate aerogels can effectively platform and has been widely used in nanocarbons
solve such problems. This method can effectively improve and noble metals with adjustable morphology and
the catalytic performance and retain the overall charac- structure.[10,54–56] On the other hand, the one-step method
teristics of aerogel materials. Therefore, in terms of OER provides a more straightforward strategy that com-
catalysis, carbon nanomaterial aerogels and metal aerogels bines the formation of precursor units and the bonding
have achieved rapid development. We divide aerogel-based processes.[57–61]
OER electrocatalysts into two main aspects: (a) based on The drying step to produce aerogels is to remove the
graphene, heteroatom-doped graphene, and other carbon solvent of the hydrogels. Under ambient conditions, the
aerogels; (b) based on metal and metal oxide or sulfide drying process will cause a severe shrink of the network
aerogels. Here, we summarize the synthetic strategies of structure because of the high surface tension and capillary
different types of aerogel catalysts and analyze the rela- forces. Therefore, freeze-drying and supercritical drying
tionship between their physical-chemical properties and were generally adopted to prevent this phenomenon (Fig-
the electrocatalytic activity of OER. A better understand- ure 1). Freeze drying removes the solvent through sublima-
ing of structure design, synthesis, and OER electrocatalytic tion, which reduces the capillary force and thus preserves
performance could assist in designing more suitable OER the network structure. However, during the formation of
electrocatalysts. Finally, an outlook on the future develop- solvent crystals, the freezing process could cause a certain
ment and challenges of aerogel-based materials for OER degree of damage to the network structure. The supercriti-
electrocatalysis is provided. cal drying uses CO2 to replace the replace and then remove
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3 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113
dant catalytic active sites, fast electron transfer, and excel-
lent structural stability. In the past decades, significant
progress has been made in the design of nanocarbon-based
aerogel and metal aerogel electrocatalysts. This section
summarizes several key studies in this field, highlighting
the synthetic design concept and the corresponding elec-
trocatalytic performance.
3.1 3D graphene aerogel and other
carbon aerogel
Graphene aerogel has excellent physical and chemical
properties, such as large specific surface area, porous
and layered structure, fast electron/ion transfer, and
good chemical stability, thus considered as a promising
F I G U R E 1 Phase diagram of the liquid phase to be removed support for OER electrocatalysts. Recently, it has been
from the gel network. Simple drying is the direct transition from the extensively studied to immobilize structural units with
liquid to the gaseous state. S: solid; L: liquid; G: gas.[39] Copyright catalytic activity to form composite aerogel electrocatalysts
2017, Wiley-VCH (Table 1).[62–43] Wang et al. used Prussian blue analogs
as precursors to immobilize (Ni,Co)Se2 nanocages on 3D
graphene aerogels ((Ni,Co)Se2 -GA) (Figure 2).[62] The
the solvent at the supercritical state. Such a strategy com- unique nanocage morphology of (Ni, Co)Se2 can provide
pletely diminishes the impact of surface tension during the extensive exposed active sites, and it grows directly on
drying process, and a non-shrinked gel structure could be graphene oxide to further promote the electron transfer
obtained. process. The Brunauer-Emmett-Teller (BET) surface area
of (Ni,Co)Se2 -GA is 123.0 cm2 /g and such high surface
3 PROMOTING OER CATALYSIS UPON area guarantees the exposure of more active sites during
AEROGEL STRUCTURE the OER electrocatalysis. As a result, (Ni,Co)Se2 -GA
exhibited excellent OER activity, requiring only 250 mV
Aerogel structures exhibit unique physical and chemical of overpotential to reach a current density of 10 mA cm−2
properties that benefit OER electrocatalysis, such as abun- under alkaline conditions. In another example, nickel
T A B L E 1 Overview of the OER performances of carbon aerogel electrocatalysts obtained from three-electrode setups. Overpotentials are
derived from the potential at 10 mA/cm2 geo (η10). Since the current density normalized by the geometrical area does not reflect the intrinsic
activity of the catalysts, the surface area of the catalysts and their loading should be taken into account during the evaluation of the
overpotentials
Catalyst Scan
BET loading rate Overpotential Tafel slop
Aerogel materials [m2 /g] Electrolyte [µg/cm2 ] [mV/s] [mV] [mV/dec] Reference
B,N/graphene 227 0.1 M KOH 140 10 370 379.3 [37]
Ni-MnO/graphene 109 0.1 M KOH 250 5 370 67 [43]
NiCo2 S4 /graphene 262.4 1 M KOH 1000 10 275 70 [45]
Co9 S8 /N,S,P-graphene 478 1 M KOH 283 10 343 82 [66]
(Ni,Co)Se2 /graphene 123 1 M KOH 2650 1 370 70 [62]
Bi-CoP/N,P-graphene 143 0.1 M KOH 250 5 370 – [68]
FeNi-P/graphene 274 1 M KOH 318 – 280 43 [63]
Mo4 S16 /Carbon – 1 M KOH 280 – 370 126.78 [39]
Ni/N-Carbon 409.03 1 M KOH – 2 380 89.8 [38]
WSe2 /NiFe-LDH/N,S-graphene 110 1 M KOH 1200 2 250 86 [64]
(Co,Ni)S2 /N-graphene 99.1 1 M KOH 285 0.5 330 47 [65]
Ru/N-graphene 244.8 0.1 M KOH 100 20 390 – [67]
Mini review 4 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113 F I G U R E 2 (A) Schematic illustration. (B) OER polarization curves and (C) the corresponding Tafel plots of (Ni,Co)Se2 -GA, (Ni,Co)Se2 , GA, and RuO2 catalyst loaded on Ni foam obtained in 1 M KOH at 1 mV/s. Copyright 2017, American Chemical Society F I G U R E 3 (A) Schematic illustration. (B) OER polarization curves and (C) the corresponding Tafel plots of FeNi-P/GA, NiHCF/GA, and RuO2 catalyst obtained in O2 -saturated 1 M KOH at 1600 rpm. Copyright 2018, ScienceDirect hexacyanoferrate was anchored on graphene aerogel under alkaline conditions to reach a current density of (termed as NiHCF/GA) followed by heating with a 10 mA/cm2 . Tang et al. combined Ni and MnO for the first phosphorus source, led to the generation of FeNi-P/GA time, and synthesized a 3D graphene aerogel composite composite electrocatalyst with improved OER activity loaded with Ni/MnO (termed as Ni-MnO/rGO) through (Figure 3).[63] The surface area of phosphating samples cal- a simple hydrogel route (Figure 4).[43] The key to the syn- culated by Brunauer Emmett Teller (BET) is as high as 274 thesis step is the formation of poly(vinylalcohol) hydrogel m2 /g. The catalyst only needs an overpotential of 280 mV connected to graphene oxide, which can immobilize
Mini review 5 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113 F I G U R E 4 (A) Schematic illustration. (B) Schematic interaction between PVA and GO. (C) SEM image and (D) TEM image of the Ni-MnO/rGO aerogel. (E) OER polarization curves and (F) the corresponding Tafel plots of the Ni-MnO/rGO aerogel, MnO/rGO aerogel, Ni/rGO aerogel, and RuO2 catalyst obtained in O2 -saturated 0.1 M KOH at 1600 rpm and 5 mV/s. (G) Chronoamperometric responses of the Ni-MnO/rGO aerogel at 1.60 V in O2 -saturated 0.1 M KOH (insert shows OER polarization curves of the Ni-MnO/rGO aerogel before and after the stability testing). Copyright 2018, ScienceDirect Ni/MnO nanoparticles with active sites after pyrolysis. a three-dimensional porous B and N co-doped graphene The specific surface area is 109 m2 /g according to the N2 aerogel through a simple hydrothermal method followed adsorption-desorption analysis. The Ni-MnO/rGO catalyst by a freeze-drying process.[37] The optimal BN-GA exhib- needs an overpotential of 370 mV to drive a current density ited a high electrocatalytic activity close to the commer- of 10 mA/cm2 under alkaline conditions. The Tafel slope cial RuO2 catalyst. The decoration of nanoparticles within of the Ni-MnO/rGO catalyst is only 67 mV/dec, indicating the aerogel framework improves the catalytic performance excellent catalytic kinetics. More importantly, the com- significantly by avoiding the aggregation and detachment posite electrocatalyst exhibited excellent stability, showing of the nanoparticles. It represents a promising method to only 13.8% of the current attenuation, which is inseparable design a high-performance electrocatalyst by dispersing from the characteristics of the aerogel structure. nanocatalysts on hetero-atom dopped graphene aerogels. The use of non-metallic heteroatoms to dope 3D He et al. prepared nitrogen (N), sulfur (S), and phosphorus graphene oxide aerogel will significantly improve the cat- (P) ternary doped 3D graphene aerogels loaded with Co9 S8 alytic activity of the material due to its electron neutral nanoparticles by a simple pyrolysis method (Figure 5).[66] destruction and charge regulation (Table 1).[64–67] Wang The resulting Co9 S8 -graphene aerogel exhibit a specific et al. used NH4 B5 O8 as the source of B and N, and prepared surface area as high as 657 m2 g−1 . At the same time,
Mini review 6 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113 F I G U R E 5 (A) N2 adsorption-desorption isotherms and (B) BJH pore-size distribution of Co9 S8 /NSPG-900. (C) OER polarization curves of Co9 S8 /NSPG-900, Co9 S8 /NSG-900, NSPG-900, and RuO2 catalyst obtained in O2 -saturated 1 M KOH at 1600 rpm and 10 mV/s. (D) The Tafel plots of Co9 S8 /NSPG-900 and RuO2 catalyst. Copyright 2017, American Chemical Society F I G U R E 6 (A) Schematic illustration. (B) OER polarization curves of Bi-CoP/NP-DG, Bi-CoP/NP-G, CoP/NP-G, and RuO2 catalyst obtained in O2 -saturated 0.1 M KOH at 1600 rpm and 5 mV/s. (C) Overpotentials at the chosen current density of 10 mA/cm2 and current densities at the chosen potential of 1.65 V. Copyright 2019, The Royal Society of Chemistry
Mini review 7 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113 F I G U R E 7 (A) Schematic illustration. (B) OER polarization curves and (C) the corresponding Tafel plots of Co-SA@NCA, Mo-SA@NCA, Ni-SA@NCA, Co3 O4 , NiO, MoO3 , NCA, and IrO2 catalyst obtained in 1 M KOH at 2 mV/s. Copyright 2021, ScienceDirect F I G U R E 8 (A) The calculated free-energy diagram of OER for Ni-SA@NCA (NiC2 N2 -o-5). (B) Free-energy diagram of OER for Co-SA@NCA (CoC2 N2 -o-5). (C) The projected density of states on Ni-SA@NCA. Copyright 2021, ScienceDirect
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due to the doping of various elements, graphene possess
a large number of defects and pore structures. As a result,
it exhibits the low overpotential of 343 mV at the current
density of 10 mA cm−2 . Jin et al. prepared N, P co-doped
graphene aerogels loaded with Bi-CoP composite nanopar-
ticles (Figure 6).[68] The graphene aerogel can effectively
prevent the aggregation of nanoparticles. The defects on
the graphene play an essential role in improving the cat-
alytic performance and the stability of the catalyst. The
overpotential of 0.37 V at the benchmarked 10 mA cm−2
is lower than the commercial RuO2 catalysts.
The single-atom catalysis with optimum atom uti-
lization rate has received extensive attention in recent
years.[27,69–75] However, increasing the single-atom load-
ing generally results in single-atom aggregation, thus
reducing the catalytic efficiency. Nitrogen-doped car-
bon aerogels have the function of stabilizing individual
metal-atoms and also play a vital role in regulating the
electronic environment of active centers.[38] Therefore, it F I G U R E 9 Relative density/pore size parameter space for
is a promising method to immobilize single metal atoms porous metallic materials. Copyright 2013 American Chemical
on nitrogen-doped carbon aerogels to improve the cat- Society
alytic activity of OER. Single metal atom immobilized on
nitrogen-doped carbon aerogel (termed as M-SA@NCA,
M = Co, Ni, Mo) was prepared by in-situ pre-crosslinking porous structures (Table 2). The first pure noble metal
and simple pyrolysis method (Figure 7), which uses guar aerogel was reported by the Eychmüller group, which
Glue and melamine as C and N sources, respectively.[38] released the synthetic method for Ag, Au, Pt, and Pd
The resulting Ni-SA@NCA catalyst exhibited excellent aerogels.[54] Compared with other porous metals, metal-
OER activity, with an initial overpotential of 380 mV lic aerogels have a much lower density and higher specific
at a current density of 10 mA/cm2 . In addition, the surface area (Figure 9).[76] On the other hand, the phys-
Ni-SA@NCA has a low Tafel slope (i.e., 89.8 mV/dec), ical characteristics of aerogels are comparable at multi-
indicating an excellent OER reaction kinetics. The elec- ple scales compared with common porous materials. Such
tronic structure of the active sites in such single-atom excellent properties are derived from the heritage of nano-
electrocatalysts dominates the absolute reaction speed and sized features from its nanostructured precursors, which
catalytic activity, and the N atom in graphene oxide has results in gel networks with similar sizes as the initial
an important impact in altering the coordination environ- nanoparticles.
ment of the single-atom sites, thus reducing the reaction To include OER active sites in aerogel structure, Au-Ir
barrier and increasing the reaction kinetics (Figure 8). metallic aerogel with a core-shell structure was synthe-
Although the nanocarbon-based aerogels feature large sized by a simple one-step method (Figure 10).[42] The Au
specific surface area, porous layered structure, and fast core network structure provides high conductivity, and the
electron/ion transfer, they still suffer from the possible cor- Ir shell exhibits high OER catalytic activity, making the
rosion issues under oxidative potentials, resulting in cat- aerogel an excellent OER catalyst candidate. The overpo-
alyst particle detachment, loss of electrical contact, and tential is 245 mV in an alkaline environment, and the Tafel
thus low durability. Unsupported electrocatalysts such as slope is only 36.9 mV/dec. In addition, the Au-Ir core-shell
metallic aerogels provide opportunities to overcome this aerogels show excellent long-term stability. After 12 h of
issue and are potentially less susceptible to other degrada- continuous electrolysis, the overpotential is still less than
tion problems, such as catalyst particle migration, dissolu- 250 mV. And the OER performance in an acidic environ-
tion, and Ostwald ripening. ment also surpasses the commercial Ir-based catalysts. In
another example, Yamauchi et al. prepared a core-shell
Cu@Fe@Ni metal aerogel by a one-step automatic pro-
3.2 Metal aerogel directly used for OER gramming synthesis.[77] The mild reducing agent dimethy-
lamine borane plays a crucial role in the formation of
Metallic materials play an essential role in catalyzing OER, Cu@Fe@Ni core-shell structure. And Fe can migrate into
especially those with a high specific surface area and the Ni shell by means of electro-activation. During the
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9 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113
T A B L E 2 Overview of the OER performances of metal aerogel electrocatalysts obtained from three-electrode setups. Overpotentials are
derived from the potential at 10 mA cm−2 geo (η10 ). Since the current density normalized by the geometrical area does not reflect the intrinsic
activity of the catalysts, the surface area of the catalysts and their loading should be taken into account during the evaluation of the
overpotentials
Catalyst Scan Tafel
BET loading rate Overpotential slope
Aerogel materials [m2 /g] Electrolyte [µg/cm2 ] [mV/s] [mV] [mV/dec] Reference
NiCo2 O4 134 1 M KOH 500 – – – [41]
Au-Ir – 1 M KOH 20 5 245 36.9 [42]
Ir3 Cu 41.7 0.1 M HClO4 – 10 298 47.4 [60]
EA-Cu@Fe@Ni – 1 M KOH 200 5 240 47 [77]
Cu@Fe@Ni 42 1 M KOH 200 5 280 52 [77]
FeCoW oxide 29.8 1 M KOH 210 1 223 37 [78]
FeCo oxide 47.8 1 M KOH 210 1 277 60 [78]
CoW oxide – 1 M KOH 210 1 300 55 [78]
NiFe2 Ox 198 1 M KOH 40 10 356 57 [40]
F I G U R E 1 0 (A) HAADF-STEM images and the corresponding EDX analysis of Au-Ir aerogels. (B) OER polarization curves, (C) the
corresponding Tafel plots, and (D) chronopotentiometric tests of Au aerogel, Au-Ir aerogel, Au-Ir NPs, Ir/C, and IrO2 catalyst obtained in 1 M
KOH at 1600 rpm and 5 mV/s. (E) OER polarization curves, (F) the corresponding Tafel plots, and (G) chronopotentiometric tests in 0.1 M
HClO4 . Scale bars in all figures are 20 nm. Copyright 2020, Nature
Mini review 10 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113 F I G U R E 1 1 (A) Schematic illustration. (B) OER polarization curves and (C) the corresponding Tafel plots of Ir2 Cu, Ir3 Cu, Ir4 Cu aerogel, and IrO2 catalyst obtained in 1 M KOH at 1600 rpm and 10 mV/s. (D) Schematic illustration of reaction paths for OER. (E) Trends in specific activity plotted as a function of the Oad binding energy. Copyright 2018, American Chemical Society electrochemical test, highly active oxyhydroxides were the desorption and adsorption of the key intermediates, formed on the surface of the Cu@Fe@Ni aerogel, which thus rendering them with the highest OER activity. represents the actual active sites. As a result, the multi- In addition to pure metal aerogel materials, transition metallic aerogel needs only 240 mV of overpotential to metal oxides and sulfides have been widely explored to fab- achieve a current density of 10 mA/cm2 . ricate aerogel electrocatalysts due to the low-cost, decent Both the aerogel structure and the chemical com- stability, and high electrocatalytic activity in alkaline con- position of metallic aerogels affect the catalytic OER ditions (Table 2).[45,78] Gash et al. proposed that epoxides activity. Lin et al. prepared Irx Cu aerogels by a simple are possible precursors for metal oxide aerogels with tun- one-step reduction method with tunable compositions able compositions and structures.[79–80] In 2011, Lu et al. (Figure 11).[60] By adjusting the ratio of Ir and Cu, they prepared NiCo2 O4 aerogel in one step by adding propylene found that Ir3 Cu aerogels have the highest OER activity oxide to an ethanol solution containing metal ions (Fig- while the specific surface area reaches 41.7 m2 /g. Under ure 12).[41] Benefit from its high specific surface area and acidic conditions, the Ir3 Cu aerogel catalyst only needs interconnected 3D network structure, NiCo2 O4 aerogel an overpotential of 298 mV to reach a current density of exhibits excellent OER activity under alkaline conditions, 10 mA/cm2 . Its Tafel slope is 47.4 mV/dec, indicating that where an ultra-low overpotential of 184 mV is required to the catalyst has excellent OER kinetics. Through density reach a current density of 100 mA/cm2 . functional theory calculations (Figure 11), the binding In 2004, Mohanan et al. prepared metal sulfide aerogels energy of hydroxyl and oxygen at the active site of the for the first time, opening a new chapter in the preparation Ir3 Cu aerogel determines the OER activity. Among them, of metal sulfide aerogels.[81] In 2007, Bag et al. invented Ir3 Cu metal aerogel exhibited a better balance between a metathesis method to fabricate metal sulfide aerogels,
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F I G U R E 1 2 (A) TEM images at low magnification and (B) high magnification of NiCo-A200. (C) XRD patterns of NiCo2 O4 aerogel,
NiCo-A200, NiCo-A300, and NiCo-NP. (D) OER polarization curves of NiCo-A200, NiCo-A300, and NiCo-NP obtained in 1 M KOH.
Copyright 2011, The Royal Society of Chemistry
which showed promises in the synthesis of binary metal potential in OER electrocatalysis. First, the aerogel cata-
sulfides, as well as ternary and quaternary metal sulfide lysts have an extremely high surface area, guaranteeing
aerogels.[82–90] However, due to the difficulty of fabricat- a sufficient exposure of more active sites. Second, the
ing metal sulfides from epoxides, it remains a significant porous structure of the aerogel structure is also beneficial
challenge to synthesize aerogels with a rich composition to mass and electron transport. Third, aerogel materials
of precursors and microstructures. Zhi et al. chose dl- have remarkable tunability in chemical compositions and
mercaptosuccinic acid as the gel accelerator, which is ben- heteroatom doping. Although there have been a lot of
eficial to the formation of metal sulfide aerogels.[45] It is reports on aerogel-based OER electrocatalysts, some key
very similar to the strategy of preparing metal oxide aero- difficulties and challenges still need to be tackled before
gels by adding epoxides. The obtained NiCo2 S4 aerogel has practical industrial applications. First, aerogel typically
a high surface area of 262.4 m2 /g. For OER catalysis, the has a very low density, which inevitably occupies a large
NiCo2 S4 aerogel catalyst needs an overpotential of 370 mV thickness in the catalyst layer, thereby causing a high
to reach a current density of 100 mA/cm2 . potential drop. Second, composite catalysts with active
nanostructures loaded on nano-carbon aerogels generally
have low utilization of the active sites. Therefore, without
4 CONCLUSIONS AND OUTLOOK causing particle agglomeration, it is essential to increase
the loading of nanoparticles to improve the utilization
We summarized the recent progress on the application rate of catalysts. Third, the mechanism of how aerogel
of aerogel structure in catalyzing OER. Benefiting from structure promotes OER electrocatalysis remains unclear,
the unique physical and chemical properties of aerogel which requires the application of more in situ advanced
structures, such aerogel-based catalysts exhibited great chracterization techniques in this field.Mini review
12 of 14 Electrochemical Science Advances doi.org/10.1002/elsa.202100113
AC K N OW L E D G M E N T S 26. Y. Zhu, W. Zhou, Y. Chen, J. Yu, M. Liu, Z. Shao, Adv. Mater.
This work is financially supported by the Qilu Young 2015, 27, 7150.
Scholar Start-up Fund of Shandong University. 27. S. Sultan, J. N. Tiwari, A. N. Singh, S. Zhumagali, M. Ha, C.
W. Myung, P. Thangavel, K. S. Kim, Adv. Energy Mater. 2019,
ORCID 9, 1900624.
28. C. Liu, J. Qian, Y. Ye, H. Zhou, C.-J. Sun, C. Sheehan, Z. Zhang,
Cunyuan Gao https://orcid.org/0000-0001-9289-2162
G. Wan, Y.-S. Liu, J. Guo, S. Li, H. Shin, S. Hwang, T. B. Gunnoe,
Jinhua Zhan https://orcid.org/0000-0003-0548-8028 W. A. Goddard, S. Zhang, Nat. Catal. 2021, 4, 36.
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Jinhua Zhan received his How to cite this article: C. Gao, X. Zhang, J.
BS degree in chemistry Zhan, B. Cai, Electrochem. Sci. Adv. 2021, e2100113.
from Northeast Normal https://doi.org/10.1002/elsa.202100113
University, and his Ph.D.
degree from University of
Science and Technology of
China under the direction
of Professor Yitai Qian
in 2000. After two years’
postdoctoral experience in
Professor Chung-Yuan Mou’s group at Taiwan Uni-
versity, he joined the National Institute for Material
Science (NIMS) as a Researcher under the direction
of Professor Yoshio Bando from 2003 till 2006. In
2006, he joined the School of Chemistry and Chemical
Engineering as a Professor at Shandong University.
His current research has focused on nanomaterials for
environmental sensing and remediation.
Bin Cai obtained his Ph.D.
from Technische Univer-
sität Dresden under Prof.
Alexander Eychmüller in
2017. He then continued
his career as a postdoc at
Massachusetts Institute
of Technology with Profs.
Yuriy Román and Yang
Shao-Horn and Pacific
Northwest National Laboratory with Drs. Chun-Long
Chen and James De Yoreo. In 2020, he joined the
School of Chemistry and Chemical Engineering as
a professor at Shandong University and his current
research interest focuses on environmental electro-
chemistry and analytical chemistry.You can also read
