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He, Tianwei, Gao, Guoping, Kou, Liangzhi, Will, Geoffrey, & Du, Aijun(2017)Endohedral metallofullerenes (M@C60) as efficient catalysts for highly ac-tive hydrogen evolution reaction.Journal of Catalysis, 354, pp. 231-235.
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https://doi.org/10.1016/j.jcat.2017.08.025
Endohedral metallofullerenes (M@C60) as efficient
catalysts for highly active hydrogen evolution reaction
Tianwei He, Guoping Gao, Liangzhi Kou, Geoffrey Will and Aijun Du*
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland
University of Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia
ABSTRACT
The cage structure of C60 fullerenes with encapsulated metal atoms, i.e. endohedral fullerenes
(M@C60) possess unique electronic properties with novel applications. By using density
functional theory (DFT), we for the first time predict endohedral M@C60 (M=Na, K, Rb, Cs, Sc, Ti,
Mn, Fe) fullerenes from 20 possible candidates as promising high performance hydrogen
evolution reaction (HER) catalysts. For the pristine C60, the Gibbs free energy is too positive to
prevent the adsorption of H‐atoms on surface carbon atoms. However, when a metal atom is
embedded in the C60 cage, the H‐atom binding free energy on M@C60 can be optimized to
ideal value for HER (ΔGH=0). The catalytic active site is non‐metal C‐atom and the HER
performance of M@C60 are even better than those of the state‐of‐the‐art Pt and MoS2 catalysts.
The excellent catalytic activities are attributed to the charge transfer between the metal atom
and C60 cage. Since the endohedral fullerenes can be easily realized in experiment, our findings
highlight a new class of low‐cost and efficient HER catalyst for experimental validation studies
toward hydrogen production.
Keywords: Endohedral fullerenes, Hydrogen evolution reaction, Density functional theory,
Charge transfer
INTRODUCTION
Hydrogen with its high energy density and eco‐friendly production possibilities becomes an
attractive clean energy carrier which can be used as an alternative to hydrocarbon in fuel
cells.[1‐3] Evolving hydrogen from water splitting using electricity generated from solar or wind
power is regarded as a promising and sustainable strategy[4]. However, the cathodic hydrogen
evolution reaction (HER) is strongly uphill with large overpotential that limits the practical
applications of water splitting.[5] Among a wide variety of available catalysts, noble Pt are the
most dominant catalyst due to their high exchange current density and small Tafel slope.[6]
However, the high cost of Pt prohibits its commercial application for sustainable hydrogen
productions. Hence, the exploration of non‐precious even metal‐free catalysts with high HER
activities and stabilities is highly desirable for the future’s water splitting.[7‐10]
Over the past decade, various non‐noble metal catalysts 2D MoS2/WS2,[11‐15] g‐C3N4[16, 17]and
MXenes[18‐20] have been explored as appropriate catalysts for HER. However, the active sites
of these catalysts are limited at edge11‐13, graphene support14 and surface functionalization15‐16
which may extremely hinder their practical applications. Carbon‐based materials are widely
used to design different kinds of catalysts for energy conversion due to their tuneable molecular
structures, superior conductivity, abundance and strong tolerance to acid/alkaline
environments.[21‐23] Carbon is abundant element in nature and can exhibit multiple forms of
low‐dimensional allotropic structures including 0D fullerenes,[24] 1D carbon nanotubes,[25] and
2D graphene.[26] All of them have been isolated in the laboratory and extensively studied for
developing renewable and green energy technologies, e.g solar cells, fuel cells, and lithium ion
batteries.[27‐29] Among them, buckminsterfullerene (C60) represented one of technologically
relevant composites due to the cage structure with large hollow interior.[24] C60 consists of 20
carbon hexagons and 12 carbon pentagons with a bond along each polygon edge (Figure 1) to
satisfy the isolated pentagon rule. Endohedral metallofullerenes (M@C60), which a metal atom
was trapped in the spherical carbon cage, have attracted great attention in terms of
fundamental scientific curiosity as well as practical considerations.[30‐32] M@C60 (M= Sc, Y, La,
Ca, Sr, Ba, Li, Kr, Ce and La) endohedral fullerenes and its applications have been widely
investigated both theoretically and experimentally.[33‐37] However, to the best of our
knowledge, the investigation of the catalytic performance of M@C60 for HER has not hitherto
attracted any attention.
In this work, we report the calculations of M@C60 (M=Na, K, Rb, Cs, Sc, Ti, Mn and Fe) as novel
catalysts electrocatalytic HER application due to their electronic properties. The H‐atom binding
Gibbs free energy on the M@C60 catalysts are very close to zero which is comparable and even
better than the existing well‐explored catalysts for HER, such as nanostructured MoS2 materials
and the state‐of‐the‐art Pt catalysts. Our DFT calculations on the interaction between the metal
atom and C60 further provide clear evidence that the electron transfer from metal atom to C60
has a significant impact on HER performance. As M@C60 have been experimentally
synthesized[31, 38‐43], our findings highlight a new class of interesting HER catalysts (M@C60)
that can serve as a possible alternatives to noble metals catalysts for realistic applications.
Figure 1. Model for the metal encapsulated in C60 (M@C60) and an H adsorbed on the M@C60. Colour code:
brown, C; pink, metal atom; green, H.
COMPUTATIONAL DETAILS
Density functional theory (DFT) as implemented in the Vienna Ab‐initio Simulation Package
(VASP) code were employed to perform all the calculations.[44, 45] We use the generalized
gradient approximation[46] in the form of the Perdew−Burke−Ernzerhof func onal[47] to
described exchange‐correlation interactions. Blöchl’s all‐electron, frozen‐core projector
augmented wave (PAW) method[48] was used to represent nuclei and core electrons. In all
calculations, the van der Waals interaction was described by using the empirical correction in
Grimme’s scheme, i.e., DFT+D3[49]. The electron wave functions were expanded using the
plane waves with a cut off energy of 500 eV. The geometries were optimized until the energy
and the force were converged to 0.001 eV/Å and 10−6 eV, respectively. Since some metals are
magnetic atoms, spin polarization was considered throughout the calculations. For the k‐point
sampling, we used a single Γ point mesh 1×1×1 in reciprocal space during geometry optimization.
The standard hydrogen electrode (USHE) was theoretically defined in solution [pH=0, p (H2) = 1
bar].
We can describe the overall HER pathway as following equation under standard conditions:
→12
, ∆G 0 (1)
Eq. (1) include an initial state , an intermediate adsorbed H*, and the final product,
½ H2 (g). The total energy of H+ (aq) +e‐ is equal to ½ H2 (g). The free energy of the adsorption
atomic hydrogen (∆G ∗) is calculated as:
∆G ∗ ∆E ∆E T∆S (2)
∆E represents the differential hydrogen adsorption energy and can be described by:
∆E E ∗ E∗ 12 E (3)
where * denotes the catalyst. ∗, E∗and represents total energies of catalyst plus one H
adsorbed hydrogen atoms, the total energies of catalyst without adsorbed hydrogen atoms and
H2 gas, respectively. ΔEZPE is the difference corresponding to the zero point energy between the
adsorbed state and the gas phase. The contributions from the catalysts to both ΔEZPE and ΔSH
are small and can be neglected. Therefore, ΔEZPE is obtained by:[50]
∆E E 12E (4)
where, E is the zero‐point energy of one adsorbed atomic hydrogens on the catalyst without
the contribution of the catalyst. is the zero‐point energy of H2 in the gas phase.
S is the entropy of H2 gas at the standard condition.[51] The ∆ can be obtained by:
∆S ≅ 12 S (5)
The calculated vibrational frequency for H2 gas is 4390 cm‐1, the vibrational frequency of H
adsorbed on M@C60 is 2810 cm‐1, which is not sensitive to metal atom. Therefore the overall
corrections are taken as:
∆G ∗=∆E +0.24 eV (6)
In the volcano‐shaped diagram, the theoretical exchange current i0 are calculated using the
average Gibbs free‐energy of hydrogen adsorption (∆G ∗) on catalysts. The exchange current is
based on the Norskov’s assumption[52] (see the reference for details). If the ∆G ∗ ≤0, the
following expressing for the exchange current at pH=0 is obtained by:
k
11 exp ∆G ∗/
(7)
If the ∆G ∗ >0, the exchange current is obtained by:
k
11 exp ∆G ∗/
(8)
where k0 is the rate constant.
RESULTS AND DISCUSSION
As shown in Figure 1, the M@C60 structures were built by adding one metal atom inside C60
cage. Experimentally, a set of alkali and transition metal atoms have been encapsulated
previously reference needed. Then H atom was added onto different positions of M@C60 and
the adsorption geometry was fully optimized. The most energetically stable site is found to be
the adsorption of H‐atom on the top of surface C‐atoms. The H‐atoms binding free energy ( GH)
is calculated as a good descriptor for evaluating HER activity.[53] A lower GH indicates strong
adsorption on the catalyst, while a higher GH will lead to weak H‐binding. The best HER activity
is obtained when GH is close to 0. For the pristine C60, the GH (=0.44 eV) is quite positive,
which represent a weak interaction between adsorbed H and C60, manifesting poor HER
reaction kinetics. However, when metal atoms encapsulated into the spherical carbon cage, the
M@C60 demonstrated significantly improved activity for HER. Around twenty metal atoms
including Li, Na, K, Rb, Cs, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb and Ag are
encapsulated into C60 and the GH of each species is calculated.
Figure 2. The calculated free‐energy diagram of HER under standard conditions for M@C60 (M=Na, K, Rb, Cs, Sc, Ti,
Mn, Fe) catalysts.
Among all the M@C60 catalysts we studied, the values of GH for M@C60 (M=Na, K, Rb, Cs, Sc,
Ti, Mn, Fe) are very close to zero (see Figure 2), suggesting very high HER performance. Other
M@C60 catalysts that display too large and too small Gibbs free energies for the adsorption of
atomic hydrogen are not presented here. As shown in Figure 2, the value of GH for the pristine
C60 is highly unstable. However, it can be significantly reduced to close to zero (‐0.1 eV < GH <
0.1 eV) for M@C60 catalysts (M=Na, K, Rb, Cs, Sc, Ti, Mn, Fe). The HER performances are
comparable to that of the state‐of‐the‐ art Pt and even better than that of MoS2 catalyst.[54] In
Figure 3, we plotted a volcano curve to compare the HER performance of each M@C60
(including pristine C60). The Gibbs free energies of hydrogen adsorption ( GH) on M@C60 are
the used to obtain the theoretical exchange current (i0) based on eq. 7‐8. The HER performance
of M@C60 can be evaluated by its i0 position and GH values relative to the volcano peak (a
position closer to the peak indicates higher catalytic activity).[52] We can see that M@C60
catalysts with negative and positive GH values are located around the left and right legs of the
volcano curve. Catalysts with the GH values close to zero are near the peak of the volcano
curve. M@C60 (M= Li, Ca, V, Cr, Co, Ni, Cu, Sr, Y, Zr, Nb and Ag) catalysts exhibit either too
negative or too positive GH which are not good to release or adsorb H during the hydrogen
evolution reaction. All alkali metal atoms except for Li encapsulated in C60 cage are very
suitable for HER, in particular Rb@C60, with the GH being very close to the ideal value ( GH=0).
When comparing the geometry of Li@C60 with other M@C60 (M= Na, K, Rb), there is a
noticeable off‐centre shift for Li@C60, which may be attributed to the small atomic radius of the
Li‐atom leading to the structural relocation of Li atom from the central position.
Figure 3. Volcano curve of exchange current (i0) as a function of the Gibbs free energy of hydrogen adsorption ( GH)
for various metal atoms encapsulated in C60.
As shown above, the embedded atom has significant impact on the adsorption of atomic
hydrogen on C60. This is attributed to the electron–acceptor characteristics of C60 which can
easily gain electron density from the atom inside the cage.[33] To further understand this effect,
we take Rb@C60 as an example to calculate the charge density difference ( ρ(r)) for three
representative systems, Rb@C60, C60‐H and Rb@C60‐H based on the following equations:
∆ρ r @ @ (9)
∆ρ′ r @ (10)
Where @ represents the charge density of the system Rb‐C60 with an adsorbed H
atom, @ refers to the charge density of the Rb‐C60 system without H adsorbed,
is the charge density C60, and are the charge density calculated for the
atom of H and Rb at the same coordinates as those in the Rb@C60 and C60 system, respectively.
The final result of the charge density difference for Rb@C60, C60‐H and Rb@C60‐H are
presented in Figure 4.
Figure 4. Charge density difference ( ρ(r)) plot for (a) Rb@C60 (b) C60‐H and (c) Rb@C60‐H as defined in equations
(9) and (10). Yellow and cyan iso‐surface represents electron accumulation and electron depletion; the iso‐surface
value is 0.018 e Å‐3.
As shown in Figure 4a, the embedded Rb atom has led to an increase in electronic charge
density on the inner cage surface at a loss of electron charge density surrounding the metal
atom. There is 0.72 electron transferred from Rb to C60 based on the Bader analysis. When
compared C60‐H and Rb@C60‐H (see Figure 4b and 4c), there is significant charge density
redistribution in the presence of Rb doping, possibly due to the embedded Rb atom driving
strong charge transfer across the whole structure when H is adsorbed. Clearly the dopant atom
ionizes the carbon cage, leading to strong H‐adsorption on M@C60, i.e. the enhanced HER
activity.
CONCLUSIONS
The HER activity of C60 fullerene and a series of endohedral fullerenes M@C60 (M= Li, Na, K, Rb,
Cs, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb and Ag) have been investigated using DFT
calculations. The value of GH for pristine C60 is too positive ( GH=0.44 eV) and leads to tH
bonding too weakly to surface carbon atoms. When a metal atom was encapsulated in C60, the
C atom becomes catalytic active site. The GH can be optimized to the ideal value ( GH = 0)
when M atom (M= Na, K, Rb, Cs, Sc, Ti, Mn, Fe) was embedded in the C60 cage. The values we
obtained are comparable with or even better than those of the state‐of‐the‐art Pt and well‐
studied MoS2 catalysts. The electronic structure analysis indicated that the charge transfer from
metal atom to C60 modified the charge distribution of the C60 cage which enhances the
adsorption of H‐atom on M@C60 (M= Na, K, Rb, Cs, Sc, Ti, Mn, Fe). The endohedral fullerene
has been realized in experiment and our findings highlight a new class of low‐cost and efficient
HER catalysts for experimental verification in the near future.
ACKNOWLEDGEMENTS
We acknowledge generous grants of high‐performance computer time from computing facility
at Queensland University of Technology and Australian National Facility. A. D. greatly
appreciates the Australian Research Council QEII Fellowship (DP110101239) and financial
support of the Australian Research Council under Discovery Project (DP130102420 and
DP170103598).
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