Journal of the American Chemical Society
Article
study concluded that the electron charge density is highly
distributed over the Co−Cu MOF structure,29−31 thus
providing more active sites for ORR electrocatalysis than Pt/
C. The density of states for the Co−Cu MOF is 11.4
electrons/eV near the Fermi level, which mainly consists of p-
orbital (8.54 electrons/eV) and d-orbital electrons (2.5
electrons/eV). For Pt/C, there are only 1.9 and 0.7
electrons/eV at the Fermi level,32 respectively (Figure 6c,d).
The PDOS study of the Co and Cu atoms in the MOF
skeleton revealed a remarkably high electron density of states.
Such electron density decreases the oxygen adsorption energy
states of the ORR intermediate catalytic species leading to
superior catalytic performances for the BTC-Co-O-Cu-BTA
MOF.
Gibbs free energy change (ΔG) at 298.15 K of the proposed
adsorption energy of O2 on Co sites resulted in a formation
energy of −2.45 eV for *OOH as shown in Table 1. Also, the
̇
small ΔG1 of 0.433 eV facilitates a fast O2/OH exchange to
̇
produce (O2) radical species that are further protonated by
H2O molecule to *OOH with ΔG2 of 1.23 eV along with the
oxidation of Co2+ to form a Co3+-OOH intermediate. Finally,
the O−O bond is broken and the original state of Co3+− OH
is recovered with free energies of −0.17 (ΔG3) and 0.28 (ΔG4)
eV, respectively.
CONCLUSIONS
■
We synthesized a new class of bimetallic Co−Cu MOFs
following a one-pot low-temperature hydrothermal strategy.
The catalytic properties of the nonprecious bimetallic MOF
were finely tailored by varying the Co/Cu molar ratios during
the synthetic reaction. The optimized Co/Cu (1:1) MOF
electronic structure formed by well-dispersed O-bridged
bimetallic clusters, which are interconnected into a highly
porous network, led to an unique and very efficient ORR
catalyst. The BTC-Co-O-Cu-BTA surpassed the ORR catalytic
activity of Pt/C in terms of onset potential, half-wave potential,
and electrochemical stability, thus performing as the best
nonprecious ORR catalyst reported to date. The BTC-Co-O-
Cu-BTA MOF also acted as an excellent cathode alternative to
Pt/C in zinc-air batteries. The impressive catalytic perform-
ance was rationalized in terms of the effective interatomic Co−
Cu electron transfer processes, which produce highly active
ORR catalytic sites. This work provides not only a new path to
engineer a low-cost and efficient replacement for Pt/C for
oxygen catalysis but also unveils the underlying details of the
ORR mechanism on bimetallic organic frameworks.
Another interesting aspect is how Cu combines with Co in
the BTC-Co-O-Cu-BTA framework to yield outstanding
electrocatalytic activities.33−35 After the formation of an
octahedral metal−oxygen structure, the negative charge is
then uniformly distributed in a spherical shape around the Co
and Cu metal ions, splitting the d orbitals into two new states
with different energies (Figure 6e). XPS analysis (Figure 2a,b)
shows that the Cu2+ 2p3/2 (932.1 eV for Cu-BTABTCs) and
Co2+ 2p3/2 (781.7 eV for Co-BTABTCs) peaks shift to higher
and lower binding energies (Cu 2p3/2 and Co 2p3/2 of BTC-
Co-O-Cu-BTA are 934.1 and 797.5 eV), respectively. This
phenomenon indicates an electron transfer process from Cu2+
to Co2+ through the bridged oxygen (Figure 6f).36,37 The
electronic (valence) configuration of Co2+ is 3d7 in a high-spin
state, so Co2+ has unpaired electrons in the t2g d-orbitals, which
can interact with O2−(bridged) via π-donation, while the π-
symmetry (t2g) d-orbitals of Cu2+ are completely occupied,38
which greatly facilitates the electron transfer in the Co−O−Cu
junction. The significant repulsion between the O2− and Cu2+
and the attractive Co−O interactions can induce the
interatomic electron transfer. Therefore, the change in the
both Co and Cu -O bond increases and decreases, respectively,
upon the bimetallic electronic coupling in the MOF network,
which gives rise to a high electron localization around the Co
centers, thus providing ultra-active catalytic sites for ORR
electrocatalysis. To verify this conclusion, we performed
density functional theory calculations to fully investigate the
catalytic mechanism. Initially, adsorption energy calculations
were carried out to elucidate the potential active sites for
oxygen evolution. The adsorption energy of an adsorbate is
calculated as Eads = EMOF+adsorbate − EMOF − Eadsorbate where
METHODS
■
Chemicals. Copper chloride tetrahydrate and cobalt chloride
(99.99% AR, grade) were purchased from Sigma-Aldrich, and
benzene-1,3,5-tricarboxylic acid, 1,2,4,5-tetraaminobenzene tetrahy-
drochloride (BTA), Nafion solution (5 weight%), and sodium
hydroxide (NaOH 99.98%) were purchased from Sigma-Aldrich. N,
N-dimethylformamide (DMF) was bought from Fisher Scientific. All
compounds and chemicals were used directly without further
purification. Ultrapure distilled water used in the measurements was
obtained from a Millipore System (Millipore Q).
Synthesis of the Bimetallic MOFs. DMF (15 mL), ethanol (1
mL), and distilled water (5 mL) were mixed in a 100 mL thin
cylindrical tube; 0.5 mmol of BTA and BTC was added to the
solution under ultrasonication. Subsequently, 0.2 mmol of CoCl2 and
the same amount of CuCl2·6H2O were added. The solution was then
magnetically stirred for 10 min to obtain a uniform colloidal
suspension. Afterward, the colloidal suspension was continuously
ultrasonicated for 10 h. Finally, the mixture was transferred into a 45
EMOF+adsorbate, EMOF, and Eadsorbate are the total energies of the
MOF molecule model and adsorbate, MOF, and a gas phase
adsorbate, respectively. As seen in Table 1, O2 adsorbates
exhibit less negative Eads on Co sites than on Cu sites,
indicating a stable exothermic reaction with Co sites being the
most probable active sites for the ORR, in excellent agreement
with the calculated electronic properties of the MOF. The
◦
mL Teflon vessel at 160 C for 24 h in an airtight environment. The
material was then separated and cleaned in an acidic solution to
remove extra unreacted oxides.39−41
Density Functional Calculations. For a novel CoCuBTABTC
MOF structure, it was essential to build the molecular structure of the
material of its building blocks which are formed by 1,3,5- benzene
tricarboxylic acid (BTC), 1,2,4,5-benzenetetramine (BTA), cobalt
metal (Co), and copper metal (Cu).42−44 Initially, experimental X-ray
diffraction pattern (XRD) was used to disclose the unit cell of the
crystal structure by the reflex plus code as implemented in the
materials studio 7.0 software. Then thermogravimetric (TGA) and
FTIR analysis were employed to sketch the molecular structure and
identify bonding environment of the MOF material. Finally, the
geometry optimization calculations were performed on the modeled
Table 1. Calculated Adsorption Energies of OH and O2 and
Formation Energies of OOH and O on Co and Cu Sites
Adsorption Energy
(EV)
Formation Energy
(EV)
(B) Species
(A) Adsorbates
on Co
on Cu
on Co
on Cu
OH
O2
−1.125
−0.394
−1.129
−0.192
OOH
O
−2.45
0.36
−2.17
0.62
4070
J. Am. Chem. Soc. 2021, 143, 4064−4073
Sanad, Mohamed Fathi
