Cobalt–Tannin-Framework-Derived Amorphous Co?P/Co?N?C on N, P Co-Doped Porous Carbon with Abundant Active Moieties for Efficient Oxygen Reactions and Water Splitting

Article abstract of DOI:10.1002/cssc.201802909

It remains a tremendous challenge to develop a low-cost, earth-abundant, and efficient catalyst with multifunctional activities for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, a facile and scalable avenue was developed to prepare amorphous Co?P/Co?N?C supported on N, P co-doped porous carbon (Co?P/Co?N?C/NPC) with a large specific surface area (1462.9 m² g^?1) and abundant reactive sites including Co?P, Co?N and NPC. The prepared electrocatalyst exhibits outstanding catalytic performance for HER (η=234 mV at 10 mA cm^?2), OER (η=374 mV at 10 mA cm^?2), and ORR (E_1/2=0.89 V, vs. reversible hydrogen electrode). Benefiting from the excellent HER performance and outstanding OER activity, the Co?P/Co?N?C/NPC delivers a current density of 10 mA cm^?2 for overall water splitting at a cell voltage of 1.59 V, which is comparable with the IrO₂–Pt/C couple electrode.

Full text of DOI:10.1002/cssc.201802909

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Accepted Article
Title: Cobalt-Tannin-Framework Derived Amorphous Co-P/Co-N-C on
N, P co-doped Porous Carbon with Abundant Active Moieties for
Efficient Oxygen Reactions and Water Splitting
Authors: Zexing Wu, Shuai Wang, Haeseong Jang, Jia Wang, Xien Liu,
and Jaephil Cho
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To be cited as: ChemSusChem 10.1002/cssc.201802909
Link to VoR: http://dx.doi.org/10.1002/cssc.201802909
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Cobalt-Tannin-Framework Derived Amorphous Co-P/Co-N-C on N,
P co-doped Porous Carbon with Abundant Active Moieties for
Efficient Oxygen Reactions and Water Splitting
[a]
Shuai Wang, Haeseong Jang, [b] Jia Wang,[a] Zexing Wu,*[a] Xien Liu,*[a] and Jaephil Cho*[b]
∓
∓
Abstract: It is still a tremendous challenge to develop low-cost, earth-
abundant and efficient catalyst with multi-functional activities for
hydrogen evolution reaction (HER), oxygen evolution reaction (OER)
and oxygen reduction reaction (ORR). Herein, a facile and scalable
avenue was developed to prepare amorphous Co-P/Co-N-C
supported on N, P co-doped porous carbon (Co-P/Co-N-C/NPC) with
large specific surface area (1462.9 m2 g-1) and abundant reactive sites
including Co-P, Co-N and NPC. Then the prepared electrocatalyst
exhibits outstanding catalytic performance for HER (η=234 mV@10
mA cm-2), OER (η=374 mV@10 mA cm-2) and ORR (E1/2=0.89 V, vs
RHE). Benefiting from the excellent HER performance and
outstanding OER activity, the Co-P/Co-N-C/NPC delivers a current
density of 10 mA cm-2 for overall water-splitting at a cell voltage of
and insufficient catalytic performance for HER, OER and ORR
limited their large-scale applications and made these precious
metals decreasingly attractive.[7] To this end, it is urgent to
develop low-cost, earth abundant, efficient and excellent stability
non-noble electrocatalyst with trifunctional activity for HER, OER
and ORR to accelerate the development and large-scale
applications of water-splitting and Zn-air batteries.[8]
Among the explored non-precious electrocatalyst, M-N (Fe/Co-
N) based nanomaterials have been considered as appealing
candidates for Pt due to the earth abundant properties and high
catalytic performance in both acid and alkaline electrolytes for
ORR.[9] However, the obtained M-C catalysts possess poor
catalytic activity for OER which result in sluggish charging
1.59 V which is comparable with the noble IrO2 - Pt/C couple electrode. performance in rechargeable metal-air batteries.[10] Relative to M-
N, M-P based compounds present excellent catalytic activity for
both HER and OER due to the P species can trap protons by
acting as a Lewis base for HER and benefiting the formation of
Introduction
peroxide during OER process.[11] Furthermore, both experimental
and theoretical results demonstrated that M-P can also act as
promising active sites for ORR. For instance, Yu et al reported
that Fe-P exhibits excellent catalytic performance for ORR in both
acid and alkaline media.[12] Zhao et al, using density functional
theory (DFT), predicted that Fe- and Co-P4 moieties can activate
oxygen molecular which is benefit for ORR via an ideal four
electrons avenue.[13] Thus, the combination of M-N and M-P in the
prepared catalysts can afford excellent trifunctional activities for
ORR, HER and OER. Besides, the crystal structure is also a
significant parameter to optimize the catalytic activity and
amorphous structures can make themselves own outstanding
performance in energy conversion benefiting of short-range
structural ordering and abundant active sites.[14] Apart from the
above mentioned factors, the introduction of carbon matrix into
the catalysts is also an important strategy to tune the catalytic
performance by lower the resistance, enlarge the specific surface
area and avoid the aggregation of metal species.[15] The in-situ
formed carbon support via metal-ligand interactions could
effectively avert metal species assemble and contact intimately
which can expose abundant active sites and enhance the
durability.[16]
Ever-increasing environmental pollution and demand for
sustainable energy have stimulated to explore eco-friendly and
renewable energy and conversion system, such as hydrogen gas,
fuel cells and metal-air batteries.[1] Hydrogen has attracted
extensive attentions due to the merits of high gravimetric energy
density and environmental-friendly.[2] Relative to the traditional
technology, electrochemical water-splitting, composed by
hydrogen evolution reaction (HER) and oxygen evolution reaction
(OER), is an efficient and pollution-free avenue to generate
hydrogen gas.[3] Apart from hydrogen gas, Zn-air batteries are
considered as the most promising and appealing candidates to
satisfy our requirement for power devices, because of the low-
cost, high energy density, eco-friendly and great safety merits.[4]
For both water-splitting and Zn-air battery, electrocatalysts act as
significant role to enhance the efficiency and reduce the energy
consumption.[5] At present, Pt-group based noble metals present
the best catalytic performance for both oxygen reduction reaction
(ORR) and HER, and IrO2/RuO2 exhibit the outstanding catalytic
activity for OER.[6] However, the high-cost, scarcity, poor durability
In this contribution, a facile and easily scalable approach was
developed to prepare highly active catalyst with abundant active
moieties of amorphous Co-P and Co-N-C supported on N and P
co-doped porous carbon matrix (Co-P/Co-N-C/NPC) for ORR,
HER and OER (Scheme 1). Moreover, the in-situ prepared carbon
support via cobalt-tannin-framework makes the metal species
distributed uniformly and can contact with the carbon support
intimately. Impressively, the prepared Co-P/Co-N-C/NPC
presents excellent electrocatalytic for ORR, HER and OER. It also
exhibits comparable catalytic performance with noble metals in
[a]
Dr. S. Wang, J. Wang, Dr. Z. X. Wu, Prof. X. E. Liu
State Key Laboratory Base of Eco-chemical Engineering, College of
Chemistry and Molecular Engineering
Qingdao University of Science & Technology, Qingdao, 266042
(P.R. China)
E-mail: splswzx@qust.edu.cn; liuxien@qust.edu.cn
H. Jang, Prof. J. Cho
Department of Energy Engineering School of Energy and Chemical
Engineering
[b]
Ulsan National Institute of Science and Technology (UNIST), Ulsan,
689-798, Korea
E-mail: jpcho@unist.ac.kr
∓
Equally contribution
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full water-splitting which also possesses excellent long-term
stability.
of carbon matrix. TEM was further conducted to investigate the
microstructure of the prepared Co-P/Co-N-C/NPC (Figure 1h)
which no nanoparticles can be observed on the porous carbon.
N2 adsorption/desorption results in Figure 1i demonstrate the
obtained catalyst with large specific surface area (1462.9 m2 g-1)
which is in line with the SEM and TEM results. The elemental
mappings from Figure S4 further demonstrated the distribution of
C, Co, N, and P in the obtained catalyst.
High-resolution XPS spectra of C, N, P and Co were further
analyzed to determine the chemical valence in the prepared Co-
P/Co-N-C/NPC. For C 1s in Figure 2a, the peaks located at 284.5
eV, 285.4 eV and 286.6 eV are ascribed to C-C, C-P and C-N,
respectively, indicating N and P doped into the carbon matrix
successfully.[18] The peaks at 398.2 eV, 399.8 eV, 401.1 eV and
402.2 eV in N 1s are ascribed to pyridinic-N, pyrrolic-N, graphitic-
N and oxidized-N, respectively.[19] Graphitic-N and pyridinic-N, act
as active sites for ORR, possess lone-pair electrons which can
coordinate with metals to form M-N species.[20] As shown in Figure
2c, the spectrum of P 2p confirmed the existence of Co-P (129.8
eV), oxidized P (131.9 eV), P-C (132.7 eV) and Co-O-P (133.9
eV),[15b, 21] indicating the existence of Co-P and P doped into the
carbon matrix successfully. Moreover, in the Co 2p range, the
peaks located at 781.6 eV and 796.5 eV are ascribed to ionic state
Co with shakeup satellites (785.8 eV and 802.2 eV). Thus, the
prepared Co-P/Co-N-C/NPC exhibits Co-N, Co-P and NPC active
moieties which acts as significant role during catalytic process.
Scheme 1. Schematic illustration for the fabrication of Co-P/Co-N-C/NPC.
Results and Discussion
Coordination effects act as important role to form gel and the FT-
IR results (Figure S1) demonstrated the interaction between TA
and Co2+ due to the blue shift of -OH which can efficient avert the
aggregation of metal species. The morphology and nanostructure
of the prepared electrocatalysts were measured scanning
electron microscopy (SEM). As shown in Figure 1a, the obtained
Co-P/Co-N-C/NPC exhibits abundant pores relative to without
acid and NH3 treatment (Figure S2a). The porous structure in the
obtained catalyst can expose abundant active sites and benefit
the release of produced gas during HER and OER due to the
adhered bubbles would hinder the diffusion of electrolyte and
electron transfer which degrade the catalytic activity.[17] Elements
of C, N, Co and P contained in the prepared Co-P/Co-N-C/NPC
before and after acid/NH3 treatment (Figure 1b-e and Figure S2b-
e). X-ray photoelectron spectroscopy (XPS) survey spectrum in
Figure 1f also confirmed the elements in the obtained catalysts.
The X-Ray diffraction (XRD, Figure S3a) of the obtained Co-P/Co-
N-C/NPC was mainly composed by amorphous structure. Two
typical peaks, G and D band, were observed in the obtained Ram-
an spectra (Figure S3b) which also demonstrated the existence
Figure 2. High-resolution XPS spectra of C 1s (a), N 1s (b), P 2p (c) and Co 2p
(d) in the prepared Co-P/Co-N-C/NPC.
The electrocatalytic performance and reaction kinetics for ORR
of the prepared catalysts were conducted in 0.1 M KOH first using
linear sweeping voltammograms (LSVs). An excellent
electrocatalyst for ORR should possess positive half-wave
potential (E1/2) and ideal four-electron reaction pathway.[22] We
first investigated the effects of pyrolysis temperature on the
catalytic activities (Figure S5) and the catalyst prepared at 800 oC
delivers the largest E1/2 and limiting current density, thus all the
samples are prepared at this condition. Figure 3a shows the ring
and disc current density collected using a RRDE at 1600 rpm with
Figure 1. SEM image of Co-P/Co-N-C/NPC (a) and corresponding elemental
mappings of C (b), N (c), Co (d) and P (e). (f) XPS survey spectrum of Co-P/Co-
N-C/NPC. (g) TEM (g) and S-TEM (h) images of Co-P/Co-N-C/NPC. (i) N2
adsorption/desorption isotherm of Co-P/Co-N-C/NPC.
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a sweeping rate of 5 mV s-1 of NPC, Co-P/PC, Co-N/NC, Co-
P/Co-N-C/NPC and Pt/C in O2 saturated electrolyte. As shown in
Figure 3a and 3c, Co-P/Co-N-C/NPC exhibits the highest half-
wave potential (0.89 V) relative to NPC (0.82 V), Co-N (0.82 V)
and Co-P (0.68 V) which is even better than Pt/C (0.83 V) and
comparable with the reported values (Table S1), indicating that
the coexistence of cobalt species and heteroatoms act as
significant role on enhancing the catalytic performance.[7c, 23] The
ring current of Co-P/Co-N-C/NPC is negligible compare with disk
current, indicating the low yield of H2O2 during ORR process. The
electron transfer number and H2O2 yield were obtained via
equations 3 and 4 (see the experimental section). It can be
observed in Figure 3b that the H2O2 yields are lower than 10%
and the electron transfer numbers nearly 4 in the potential range
of 0.3-0.8 V except Co-P/PC, demonstrating the prepared catalyst
mainly through 4 electrons for ORR. Koutecky-Levich (K-L)
equations (see the experimental section) also can be used to
calculate the electron transfer numbers of the prepared catalysts.
As shown in Figure 3d and Figure S 6a, 7a and 8a, the current
density increased with the increasing of rotating speed due to the
accelerating mass transport at the electrode surface. From the
slope of K-L plot (inset in Figure 3d and Figure S6b, 7b), except
Figure S8b, whose electron transfer number is 3.04, the electron
transfer numbers of Co-P/Co-N-C/NPC is around 4 at various
potentials, indicating a predominantly four electrons reaction
avenue to generate water as the dominate product which is in line
with the RRDE results.
Except the catalytic performance for ORR, the resistance of
methanol crossover and long-term durability are also significant
parameters for the prepared catalysts in the practical applications.
The methanol tolerance of the obtained Co-P/Co-N-C/NPC was
measured in O2 saturated 0.1 M KOH with a rotating speed of
1600 rpm. It can be observed that Pt/C exhibits stark current
decline after the addition of methanol at 300 s. On contrast, the
obtained Co-P/Co-N-C/NPC presents negligible current loss at
the same condition, suggesting its superior methanol resistance.
The long-term stability of the prepared Co-P/Co-N-C/NPC was
measured using current time (i-t) chronoamperometric strategy in
O2 saturated 0.1 M KOH at a constant potential of 0.7 V. It can be
clearly observed that the obtained catalyst exhibits negligible
current loss for 24 h relative to Pt/C. As above discussed, the
prepared Co-P/Co-N-C/NPC possesses excellent catalytic
performance via four electrons pathway, outstanding methanol
tolerance and predominant long-term stability in alkaline medium.
Apart from alkaline electrolyte, we also measured the
electrocatalytic performance for ORR in O2 saturated 0.5 M
H2SO4, due to the proton exchange membrane fuel cells (PEMFC)
operated at acid electrolyte which own high energy conversion
efficiency, eco-friendly and low operating temperature.[24]
However, most of the non-precious catalysts present poorer
catalytic activity in acid medium relative to alkaline electrolyte due
to the low reaction kinetics. As shown in Figure 4a, the prepared
Co-P/Co-N-C/NPC presents the most positive E1/2 (0.7 V) relative
to PC, Co-P/PC and Co-N/NC except the reference of Pt/C,
indicating the best catalytic performance for ORR in 0.5 M H2SO4.
The reaction pathway was calculated via K-L plots and the
obtained electron transfer number is 3.99, demonstrating the
prepared Co-P/Co-N-C/NPC goes through an ideal four electrons
Figure 4. (a) LSVs of Co-P/Co-N-C/NPC, Co-N/NC, Co-P/PC, NPC and Pt/C in
O2 saturated 0.5 M H2SO4 with a scanning rate of 5 mV s-1 at a rotating speed
of 1600 rpm. (b) LSVs of Co-P/Co-N-C/NPC with different rotating speed and
K-L plots at various potentials (inset). (c) Methanol durability test of Co-P/Co-N-
C/NPC and Pt/C in O2 saturated acid electrolyte with a rotating speed of 1600
rpm. (d) I-t curves of Co-P/Co-N-C/NPC and Pt/C with a rotating speed of 1600
rpm in O2 saturated electrolyte.
Figure 3. RRDE voltammograms for Co-P/Co-N-C/NPC, Co-N/NC, Co-P/PC,
NPC, Pt/C (a) and corresponding electron transfer number and H2O2 yield (b) in
0.1 M KOH. (c) Half-wave potential of the prepared catalysts. (d) LSVs of Co-
P/Co-N-C/NPC at different rotation speeds of 400, 800, 1200 and 1600 rpm and
K-L plots at different potentials (inset). (d) Methanol resistance measurement of
Co-P/Co-N-C/NPC and Pt/C. (e) I-t curves of Pt/C and Co-P/Co-N-C/NPC in O2
saturated electrolyte with a rotating speed of 1600 rpm.
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