Al13@Pt42 Core-Shell Cluster for Oxygen Reduction Reaction

To increase Pt utilization for oxygen reduction reaction (ORR) in fuel cells, reducing particle sizes of Pt is a valid way. However, poisoning or surface oxidation limits the smallest size of Pt particles at 2.6 nm with a low utility of 20%. Here, using density functional theory calculations, we develop a core-shell Al13@Pt42 cluster as a catalyst for ORR. Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activates the Pt atoms at the edge sites, enabling its high utility up to 70%. Valuably, the adsorption energy of O is located at the optimal range with 0.0–0.4 eV weaker than Pt(111), while OH-poisoning does not observed. Moreover, ORR comes from O2 dissociation mechanism where the rate-limiting step is located at OH formation from O and H with a barrier of 0.59 eV, comparable with 0.50 eV of OH formation from O and H2O on Pt(111).

To increase Pt utilization for oxygen reduction reaction (ORR) in fuel cells, reducing particle sizes of Pt is a valid way. However, poisoning or surface oxidation limits the smallest size of Pt particles at 2.6 nm with a low utility of 20%. Here, using density functional theory calculations, we develop a core-shell Al 13 @Pt 42 cluster as a catalyst for ORR. Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activates the Pt atoms at the edge sites, enabling its high utility up to 70%. Valuably, the adsorption energy of O is located at the optimal range with 0.0-0.4 eV weaker than Pt(111), while OH-poisoning does not observed. Moreover, ORR comes from O 2 dissociation mechanism where the rate-limiting step is located at OH formation from O and H with a barrier of 0.59 eV, comparable with 0.50 eV of OH formation from O and H 2 O on Pt(111). P roton exchange membrane fuel cells (PEMFCs) are promising candidates for mobile and transport applications due to their high energy density, zero emissions, relatively low operating temperature, and minimal corrosion problems 1 . Pt nanoparticles supported on carbon are commonly used as catalysts of the cathode for the oxygen reduction reaction (ORR) 2 . It is observed that the ORR activity highly depends on the size of nanoparticles, where Pt nanoparticles with diameters (D) of 2-5 nm are regarded as the best 3,4 . This is because the percentage of atoms on the active Pt(111) facets over the total number of atoms n, denoting as R (111) , reaches the maximum 2,5,6 . If the particle takes an (111)-enclosed icosahedral shape as shown in Figure 1, in order to maximize R (111) , the corresponding D (D c ) is 2.6 nm with n 5 561, which brings out R (111) 5 20%. Further reducing D could increase the surface/volume ratio, and the percentages of edge and vertex sites (R e and R v ) on the particle surfaces unfortunately increase where R e becomes predominant below D c . Those Pt atoms at the low-coordinated sites are adverse for ORR due to the strong binding of O-containing intermediates 4,[7][8][9][10] . Thus, activating edge and vertex sites is the main challenge to miniaturize D.
As adsorption energies of all intermediates of ORR are related to the O adsorption energy [E ads (O)] on (111) surfaces of transition metals, the activity is proposed to be a function of E ads (O) 2,[11][12][13] . It has demonstrated that E ads (O) of a catalyst with the best activity for ORR should be 0.0-0.4 eV weaker than that of Pt(111) 2,9,13 . Furthermore, a volcano activity curve based on the adsorption energy of OH [E ads (OH)] is present where E ads (OH) is 0.0-0.2 eV weaker than that of Pt(111), due to the scaling relationship between E ads (O) and E ads (OH) 2,6,12,14 . Note that the coordination numbers of the vertex atoms and the edge atoms of nanoparticles differ from that of (111) surface of nine. Thus, the scaling relationship between E ads (O) and E ads (OH) may be changed and E ads (O) and E ads (OH) must be both inquired separately.
On the other hand, numerous experimental and theoretical studies have been carried out to study the kinetics of ORR mechanisms 2,[15][16][17][18] 18 . It is doubted that whether ORR mechanism is changed due to the presence of the low-coordinated atoms at nano size. Thus, in order to fully understand the catalysis, the kinetics of ORR mechanisms needs to be further explored.
Alloying is a general technique to improve the ORR activity and stability of catalysts 13,14,[19][20][21] . Present works have been mainly concentrated on Pt-based alloys consisting of Pt and the late TM elements in the 3d series, typically Pt 3 Fe, Pt 3 Co and Pt 3 Ni. The alloys show better catalysis activity than Pt alone 22 . However, the severe degradation of catalysis and stability of these alloys during the voltage cycling in acids as a consequence of the continuous dissolution of TM atoms are present 22,23 . This can be understood by their negligible heat of formation 13,24 . Following the suggestion of Greeley et al., strong binding between Pt and other alloying elements is needed to improve the stability of any new alloy systems 13 . Since the formation energy of Pt 3 Al is much more negative than Pt-based alloys with late TM elements 24 , Pt 3 Al should be a good substitute of the above Pt-based alloys. It must be admitted that Ptbased alloys with early TM elements show good activity and stability, such as Pt 3 Sc and Pt 3 Y 13,22 . However, these works are still located at the transition metals and the activity enhancement is resulted from the d-d interaction to modify the d band of Pt surface atoms 13,22 . Thus, from the electronic aspect, the p-d interaction of Al-Pt systems would provide an attempt to look beyond the Pt-TM systems and explore novel catalysts.
Recent DFT calculations show that a core-shell structure plays an important role in increasing the stability of Pt-based nanoalloys, such as Co 13 @Pt 42 and Rh 13 @Pt 42 25,26 where Pt shell benefits for the stability of the catalysts under the electrochemical environment 21,23,[27][28][29] . Thus, we here develop a new core-shell Al 13 @Pt 42 cluster as ORR catalyst, whose surface is assembled with the twelve vertex atoms Pt v and the thirty edge atoms Pt e . In addition, Al 13 cluster with icosahedral symmetry have been shown to exhibit enhanced stability compared with other isomers 30,31 . What is more, the ligand stabilized Al nanoparticles with the size range of 1.5 and 4 nm have been synthesized 32,33 . It is noteworthy that although Al@ Pt core-shell nanoparticles have not been synthesized to date, Al@Cu and Al@Co ones are fabricated via a displacement reaction 34,35 . Although these particles had large particle sizes about 5 mm, the utilized experimental technique could also be applied to fabricate Al@Pt core-shell nanostructures as long as the size of Al nanoparticles is small, which has been be synthesized without difficulties 32,33 . Note that although Al could be easily oxidized it has been easily avoided by an inert atmosphere 33 . At last, it is emphasized that our work offers only a theoretical prediction and we hope this new Al@Pt cluster will be picked up by experimentalists for empirical verification.
In light of our calculation by using Density Functional Theory (DFT), Al@Pt cluster possesses good stability due to the covalent bonding between Al 13 core and Pt 42 shell. Also, E ads (O) is located at the optimal range while E ads (OH) on Pt e is 0.30 eV weaker than Pt(111). Furthermore, rate-limiting step (RDS) of the ORR reaction though O 2 dissociation mechanism is located at OH formation from O and H (E a 5 0.59 eV). This barrier is comparable with 0.50 eV of Pt(111) 18 . Thus, alloying with Al effectively activates Pt e atoms and lets the utility of Pt reach 70% (30 edge Pt e atoms from the total 42 Pt atoms). Figure 2(a) shows a core-shell Al 13 @Pt 42 cluster (n 5 55) with an icosahedral structure where 13 Al atoms form an icosahedral core and all Pt atoms are located on the shell. All Pt atoms are all lowcoordinated, which consist of 6-coordination vertex atoms (Pt v ) and 8-coordination edge atoms (Pt e ). The Al 13 @Pt 42 possesses a high symmetry and stability (the mean binding energy E b 5 24.8 eV/ atom compared with 24.75 eV/atom of Pt 55 according to our calculation). To understand physically the interaction between Al core and Pt shell of Al 13 @Pt 42 , partial density of states (PDOS) is shown in Figure 2(c). Compared with Pt 55 , the d band of Pt 42 shell on Al 13 @ Pt 42 is moved away from the Fermi energy E F . That is, the d band center changes from 21.99 of Pt 55 to 22.54 eV of Al 13 @Pt 42 . Furthermore, the d band of alloy cluster is clearly more discrete. The d band of Pt 42 shell is concentrated in between 0 to 26.8 eV. For Al 13 core, the p band has the same trend with the d band of Pt 42 shell, which denotes the strong orbital hybridization. It is obvious that the d orbitals (at 21.7, 22.6, 23.4, 24.5, 25.5 and 26.3 eV) interact with the p orbitals (at 21.8, 2.6, 23.4, 24.9 and 26.3 eV) and weak p-d hybridization is present at 28.1 eV below E F . On the other hand, the main of s band is located below 26.8 eV. Compared with p-d hybridization, the s-d interaction is weak, appearing at 25.1, 26.4, 27.4 and 28.1 eV below E F . Therefore, the enhancement in stability is dominated by hybridization between Pt-5d band and Al-3p band. To confirm this interaction, the electron density difference Dr calculated is presented in Figure 2(d). Obviously, electrons are accumulated between Pt and Al atoms, which are compatible with the observation of the corresponding PDOS and demonstrates the partial formation of the covalent Pt-Al bonds.

Results
In order to further confirm the stability of Al 13 @Pt 42 , we consider the stability of Fe 13 @Pt 42 , Co 13 @Pt 42 and Ni 13 @Pt 42 for a compar- www.nature.com/scientificreports ison purpose as the three alloying elements have been well studied [36][37][38] . Table 1 lists the core-shell interaction energy E cs , which could interpret enhanced phenomenon 13,26,39 , and the dissolution potential U diss (M 13 @Pt 42 ) of the Pt 42 shell in M 13 @Pt 42 icosahedral clusters (M 5 Al, Fe, Co, Ni). It is found that due to the alloying, U diss (M 13 @Pt 42 ) of clusters are enhanced compared with that of Pt 55 . That is, the stronger E cs makes the higher dissolution resistance, which is similar to the relationship between the alloy formation energy and the ORR stability of the Pt 3 M bulk 13,39 . The corresponding order is Al 13 @Pt 42 . Fe 13 @Pt 42 . Co 13 @Pt 42 . Ni 13 @Pt 42 . Pt 55 . Thus, we expect that the stability of Al 13 @Pt 42 acted as ORR catalysts is well.
E ads (O) and E ads (OH) on Al 13 @Pt 42 are firstly examined. For comparison purpose, these values on Pt 55 and Pt(111) are also calculated. According to previous studies, we focused on the adsorption of O on hollow sites and OH on atop sites as the adsorption sites shown in Figure 2(b) 13 11,12 .
Although there is serious OH-poisoning at T1 site, OH-poisoning at T2 site is absent. Thus, OH adsorption on T2 site can easily be removed, and the recovery of T2 site for the next ORR cycle could take place. In light of viewpoint of OH-poisoning, it is likely that the only edge atoms (Pt e ) of Al 13 @Pt 42 are effective for ORR.
The above results are supported by the relationship between electronic structures and atomic ones of Al 13 @Pt 42 . It is known that surface atoms with larger coordination number have a lower d band center and weaker adsorption ability 43 . For Al 13 @Pt 42 , the effective coordination number (N eff ) is proposed to show the effect of the Al alloying 44 . We carried out a simple linear regression analysis to correlate E ads (O) and E ads (OH) adsorbed on T1 and T2 sites with N eff of Pt atom, N eff 5 N Pt 1 XN Al , where subscripts show the correspond-ing elements, X is the effect coefficient of one Al atom corresponding one Pt atom for N eff , which is obtained by fitting technique. By using this technique, X 5 2.5 is obtained. That is, N eff values of Pt v and Pt e on Al 13 @Pt 42 are 7.5 and 11. The average N eff value of Pt 42 shell consisting of Pt v and Pt e is 10, being larger than 7.4 of Pt 55 and 9 of Pt(111). The above atomic structural analysis corresponds to the fact that the d band center moves towards the lower-energy range from 21.99 eV of Pt 55 to 22.54 eV of Al 13 @Pt 42 , which effectively illustrates that the presence of Al reduces the adsorption ability of the low-coordinated Pt atoms, as shown in Figure 2 That is, the electrostatic attraction appears for O adsorption on Pt(111). It is plausible that the weaker E ads (O) of Al 13 @Pt 42 is just due to this electrostatic repulsion between the electronegative O adatom and the Pt atoms 45,46 . In order to demonstrate the effect of the negative charges on E ads (O), we artificially add electrons Q add on Pt(111) and then calculate the corresponding E ads (O) 47 . In Figure 3(c), for O adsorption on Pt(111), the Q(Pt) sign is changed from positive to negative and Q(O) is more negative when Q add is increased. Namely, the interaction between O and Pt(111) is changed from electrostatic attraction to electrostatic repulsion. As shown in Figure 3(d), E ads (O) is weakened as Q add is increased. Furthermore, from the d-PDOS of the Pt(111) with different Q add values shown in Figure 3(e), there is little change of the d band. Thus, the electrostatic repulsion indeed reduces the E ads (O). Since OH adsorption has a similar case of O adsorption, we do not show the corresponding results here. In summary, both electronic and atomic structures of Al 13 @Pt 42 support its high poisoning resistance for ORR.
In order to characterize ORR catalyzed on the Al 13 @Pt 42 , the distinct reaction paths are considered to determine transition states and activation energies or energy barrier (E a ) using nudged elastic band theory (NEB) for all elemental reaction steps involved in ORR in Figure S1 and the corresponding data are listed in Table 3. Firstly, the O 2 dissociation mechanism, including O 2 dissociation, OH formation, and H 2 O formation, is considered. The results are shown in Figure 4 and Table 3. For O 2 dissociation, the E a 5 0.13 eV. Figure  S2 illustrates the spin-polarized partial density of states (PDOS)   18 . It has been demonstrated that the large component of this E a comes from O diffusion from hollow site to a bridge site, which is consistent with our results 15 . On Al 13 @Pt 42 , O is easier to diffuse with 0.44 eV diffusion barrier due to the lower E ads (O) value compared with that of 0.62 or 0.66 eV on Pt(111) 15,16 . That is the reason why E a value of Al 13 @Pt 42 is smaller than that of Pt(111) for OH formation from O and H. Thus, the path for OH formation becomes feasible on Al 13 @ Pt 42 . For H 2 O formation, E a 5 0.31 eV, which is comparable with Pt(111) 18 . Similar with Pt(111), the disappearance of the OH diffusion makes E a for H 2 O formation lower than that for OH formation 16 . The last step is removal of the adsorbed H 2 O and recovery the surface active site. Once H 2 O is formed, it needs to overcome 0.41 eV for desorption.  Discussion Therein, as shown in Figure 4, the rate-limiting step (RDS) of O 2 dissociation mechanism is located at OH formation from O and H with E a 5 0.59 eV. On the other hand, for OOH associative mechanism, RDS is located at OOH formation with E a 5 0.81 eV and E r 5 20.37 eV. Since E a value of RDS of O 2 dissociation mechanism is lower than that of OOH associative mechanism, the former is more effective. In addition, we have excluded the two-electron reduction to H 2 O 2 since H 2 O 2 spontaneous dissociates into OH on Al 13 @Pt 42 , which is consistent with experimental results on Pt and other Pt alloys 49 . When we observe the corresponding data of Pt(111) in Table 3, RDS is located at OH formation from O and H 2 O with E a 5 0.50 eV 18,50 . E a value for OH formation on Al 13 @Pt 42 is comparable with that on Pt(111). It is well known that when E a , 0.75 eV, there is room temperature activity 51 . As results, Al 13 @Pt 42 can effectively catalyze ORR at room temperature.
In summary, the core-shell Al 13 @Pt 42 cluster is a good ORR candidate for the fuel cell application and possesses at least four superi-orities listed below: (1) Excellent cluster stability due to the formation of the Al-Pt covalent bonds; (2) A better activity than Pt(111) due to the optimal O adsorption energy; (3) The maximal Pt atomic utilization of 70% due to the utility of the anti-poisoning edge Pt atoms with consideration of OH adsorption; (4) OH formation with E a 5 0.59 eV (being comparable with Pt(111) of 0.50 eV) as the RDS with O 2 dissociation mechanism.

Methods
Most calculations are performed within the DFT framework as implemented in DMol 3 code 52,53 . The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional is employed to describe exchange and correlation effects 54 . The All Electron Relativistic (AER) core treat method is implemented for relativistic effects, which explicitly includes all electrons and introduces some relativistic effects into the core 55 . In this work, the double numerical atomic orbital augmented by a polarization p-function (DNP) is chosen as the basis set 52   electronic convergence. The spin-unrestricted method is used for all calculations. The convergence tolerance of energy is 1.0 3 10 25 Ha, maximum force is 0.002 Ha/Å , and maximum displacement is 0.005 Å in Dmol 3 . Note that the DNP basis set is the most accurate for our studied systems in Dmol 3 code when Pt element is included in any considered system and is comparable to the Gaussian 6-31(d) basis 56 while DNP results have shown excellent consistency with experiments in literatures 57 .
It is known that the cell size effect for the calculations of 38 atomic clusters is negligible when the size is large than 25 Å 58 . In our case, we consider the calculations of 55 atomic clusters. Thus, we have tested cubic boxes with sizes of 25 Å and 30 Å . The results are shown in Table S1. It is found from It is well known that there is the convergence failure of magnetic systems in DMol 3 code. To compare the stability among the different M 13 @Pt 42 clusters, the core-shell interaction energy E cs and the dissolution potentials of Pt 42 shell U diss (M 13 @Pt 42 ) are calculated in CASTEP code with ultrasoft pseudopotentials 60 . The PBE is employed to describe exchange and correlation effects 54 . The use of a plane-wave kinetic energy cutoff of 400 eV is shown to give excellent convergence of total energies. The convergence tolerance of energy is 1.0 3 10 25 eV/atom, maximum force is 0.05 eV/Å , and maximum displacement is 0.005 Å in CASTEP. The 0.2 eV smearing is adopted for calculations.
To analyze the structural stability of alloy clusters with different numbers of Al atoms, the average binding energy of the cluster E b is adopted, where E cluster , E Pt and E Al are the total energies of Pt 55 or Al 13 @Pt 42 clusters, Pt atom, and Al atom, respectively. N Pt and N Al denote the numbers of Pt and Al atoms. The core-shell interaction energy E cs is calculated as following, where E(M 13 @Pt 42 ), E(Pt 42 ) and E(M 13 ) are the total energies of M 13 @Pt 42 clusters, Pt 42 shell and M 13 core, respectively. Following the idea of Noh et al. 26 , we define the dissolution potential of M 13 @Pt 42 cluster as the lowest potential at which the Pt-skin layer dissolves into acidic solution. Specifically, we considered the electrochemical reaction of M 13 @Pt 42 cluster of eq.
where n shell is the number of Pt atoms in the M 13 @Pt 42 (n shell 5 42 where E species , E catalyst and E sys are the total energy of an isolated adsorbate species, the catalyst [Pt(111), Pt 55 and Al 13 @Pt 42 ] and the adsorption system, respectively. E ads , 0 corresponds to an exothermic adsorption process.