Magnetism Enhanced Surface Bonding of O$_{2}$ on CoPt

TL;DR

This study uses density functional theory to show that magnetic properties of CoPt enhance oxygen molecule bonding, offering insights into designing more efficient catalysts for fuel cells.

Contribution

It reveals how magnetism in CoPt influences oxygen reduction reactions and demonstrates tuning of adsorption energies by altering Pt layer thickness.

Findings

Spin-polarized Co-d states enhance O2 surface bonding.

Magnetic exchange splitting affects oxygen adsorption.

Pt layer thickness modulates adsorption and dissociation energies.

Abstract

For large-scale deployment and use of polymer electrolyte fuel cells, high-performance electrocatalysts with low platinum consumption are desirable. One promising strategy to meet this demand is to explore alternative materials that retain catalytic efficiency while introducing new mechanisms for performance enhacement. In this study, we investigate a ferromagnetic CoPt as a candidate material to accelerate oxygen reduction reactions. By using density functional theory calculations, we find the spin-polarized Co-$d$ states to enhance O$_2$ surface bonding due to local exchange splitting of Co-$d$ carriers at the Fermi level. Furthermore, O and O$_2$ adsorption and dissociation energies are found to be tuned by varying the thickness of the Pt layers. Our study gives insight into the role magnetism plays in the oxygen reduction reaction process and how magnetic ions may aid in the design of new advanced catalysts.

Tables

Table 1. Table 1: Adsorption and dissociation energy values for the various configurations as shown in Fig.  2 .

Energy	n Layers	Configuration	Vertical	Horizontal
$E_{\text{ADS}}$	1 Layer	With Moment	-1.845487	-4.911013
		Without Moment	-1.907118	-6.357935
	3 Layers	With Moment	-2.050356	-5.045840
		Without Moment	-2.086599	-6.044806
	All Layers	With Moment	-2.088957	-5.101708
		Without Moment	-2.088957	-5.101708
$E_{\text{DISS}}$	1 Layer	With Moment	7.527343	3.504383
		Without Moment	7.468168	3.017351
	3 Layers	With Moment	7.324932	3.369556
		Without Moment	7.288688	3.330481
	All Layers	With Moment	7.286330	3.313688
		Without Moment	7.286330	3.313688

Equations

$$\Delta E=E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}+\mathrm{O}}-E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}}-E_{\mathrm{O}},$$

$$\Delta E_{\mathrm{ADS}}=E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}+{\mathrm{O}}_2}-E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}}-E_{{\mathrm{O}}_2},$$

$$\Delta E_{\mathrm{DISS}}=E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}+{\mathrm{O}}_2}-E_{\mathrm{CoP}{\mathrm{t}}_{\mathrm{n}}}-2E_{\mathrm{O}}.$$

$${ϵ}_d=\frac{\int ϵg_d(ϵ)dϵ}{\int g_d(ϵ)dϵ};,$$

Full text

I Introduction

The oxygen reduction reaction (ORR) process is critical in the development of energy conversion technologies such as fuel cells zhang2008pem and metal-air batteries cheng2012metal . Controlling the reduction process on the cathode presents significant challenges due to the slow kinetics involved in activating O2, breaking the O-O bond, and removing oxides. These obstacles place high demands on the catalyst wang2019review ; nie2015recent . Currently, platinum (Pt) based materials are the most effective catalysts for accelerating sluggish ORR kinetics. Given the high economic cost of Pt, finding alternative catalysts that reduce Pt usage or fully substitute it without compromising performance is essential debe2012electrocatalyst ; shao2016recent ; setzler2016activity ; yu2012review .

One emerging approach to tune catalytic reactions involving paramagnetic species, such as O2 ren2021spin ; okada2003effect ; abel2021ferromagnetic ; kicinski2019enhancement ; vensaus2024enhancement ; zhang2020recent ; wang2016effect ; feng2023recent , is through the introduction of magnetic elements. The application of an external magnetic field to paramagnetic catalysts has been shown to enhance ORR activity, improving electron transfer efficiency through the alignment of unpaired spinsokada2003effect . Notable examples include Co3O4 nanofiber compositeszeng2018magnetic and Fe/N/S-Co-doped carbon gelskicinski2019enhancement , with studies reporting substantial performance gains under moderate magnetic fields garces2019direct ; yan2021direct . It is expected that the adsorption (dissociation) energies and ORR activity bhattacharjee2016improved in general may be tuned by modifying the local electronic structure of a catalyst’s active sites via the magnetic ions in close proximity. A common framework for predicting catalytic activity on transition metal surfaces is the d-band center model, introduced by Hammer and Nørskov hammer2000theoretical that relates the d-band center position to adsorbent binding strength. However, it becomes insufficient when spin-polarized d-states are considered. Spin exchange between the adsorbent and the surface add complexity that the d-band model does not fully capture, especially in systems with magnetic ions.

Spin polarization can either enhance or diminish the ORR process, depending on the magnetic and electronic properties of the catalysts  bhattacharjee2016improved ; wang2016effect . To better understand this interplay, it is crucial to study materials where magnetism significantly influences catalytic activity. CoPt is one such material, known for its hard ferromagnetic properties and magnetocrystalline anisotropy ariake2005magnetic ; klemmer1995magnetic ; hu2014structural ; li2020structural . Beyond its magnetic characteristics, CoPt also exhibits strong catalytic activity in ORR xia2021high ; pan2022ordered ; guo2013fept ; li2019hard . These combined properties make CoPt an ideal candidate for exploring how magnetism can be harnessed to enhance ORR performance.

In this article, we perform a systematic first-principles study to investigate the adsorption and dissociation processes of O2 on the CoPt (001) surface. Our results show that the intrinsic magnetic moment of Co enhances the binding strength. When considering the Pt layer thickness, we observe that a thin layer of Pt, which places Co atoms closer to the surface, significantly amplifies the surface binding effects. Finally, we explain the underlying mechanism for the enhanced binding energy by refining the d-band center model to incorporate effects of spin exchange. Consequently, CoPt emerges as a promising material for surface-enhanced O2 trapping, though it exhibits slower ORR kinetics compared to Pt due to the magnetism-induced increase in binding strength. Nonetheless, understanding how magnetism modifies adsorption opens the door to rationally tuning spin-related interactions. These include magnetic anisotropy, external fields, alloying or strain engineering to strike a balance between strong binding and efficient reaction kinetics.

I.1 Methods

Electronic structure calculations were carried out with density function theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) kresse1996efficient . We used the pseudopotential projector augmented-wave method with an energy cutoff of 700 eV for the plane-wave basis set kresse1993ab . Exchange-correlation effects were treated using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation density functional kresse1999ultrasoft ; perdew1996generalized . In order to simulate a surface, we used a $2\times 2\times 7$ slab for the (001) surface with a vacuum thickness of 20 Å. We have chosen sufficiently large super cells to avoid interactions between the O atoms (O2 molecules) in the neighboring periodic images. The Brillouin zone integration was performed using a $16\times 16\times 16$ ($8\times 8\times 1$) $\Gamma$-centered Monkhorst Pack k-point grid for the bulk (surface) calculations. Spin-orbit coupling effects were included self-consistently.

II Results

To investigate how magnetism affects the ORR process, we concentrate on understanding the binding of O (O2) to various surface sites on CoPt, as well as their adsorption and dissociation energies. Our study specifically examines L10-CoPt due its high structural stability, performance, and durability as an ORR catalyst pan2022ordered ; li2019hard ; abel2021ferromagnetic . Bulk CoPt crystallizes in the tetragonal L10 structure as shown in Fig. 1 (a), where the gray, blue, and red spheres denote Pt, Co, and O atoms, respectively. We used the experimental unit cell with lattice parameters $a=3.8$ Å  and $c/a=0.972$ in all calculations. DFT calculations were performed with the magnetic moments aligned along different crystalline axes: in-plane [100] and [110] directions, and along the out-of-plane [001] axis. In agreement with previous reports liu2016first , we find a self-consistent magnetic moment of 1.959 $\mu_{B}$ for Co, and a 0.441 $\mu_{B}$ for Pt, with moments along the [001] (easy axis) direction yielding the lowest energy configuration.

A key descriptor for catalytic activity is the binding energy between the catalyst and the adsorbent medford2015sabatier at the various surface sites. Here, the [001] surface admits three unique sites including the top, bridge, and hollow sites, see the inset in Fig. 1 (b). The binding energy of oxygen adsorbed on CoPt is defined as:

[TABLE]

where $n$ denotes the number of surface Pt layers, E${}_{\mathrm{CoPt_{n}+O}}$ is the total energy of the combined CoPt$+$O system, E${}_{\mathrm{CoPt_{n}}}$ is the total energy of bare CoPt surface, and EO is the energy of an isolated oxygen atom.

Figure 1 (b) presents the binding energy as a function of O height from the slab surface for various surface binding sites on the $L1_{0}-$CoPt (001) surface. The highly coordinated hollow site (red line) is the most energetically favorable, exhibiting a binding energy of $-4.69$ eV at a distance of $\approx 1.2$ Å from the surface. The bridge site (orange line) is slightly higher in energy by 10 meV, suggesting possible mixing between these sites at room temperature. The top site, on the other hand, displays a significantly reduced binding energy of $-3.12$ eV, for which we anticipate accidental perturbations will drive O to migrate to the bridge and hollow sites. Notably, by setting the spin polarization to zero on the Co atomic sites in the calculations, we find the binding energy to reduce by 70 meV (black line). That is, the presence of magnetism appears to strengthen the CoPt-O bond.

To further elucidate the role that Co plays in the ORR process, we examine the adsorption and dissociation of O2 on the CoPt surface with increasing surface layers of Pt. Initially, O2 binds to the surface with an energy of

[TABLE]

similar to atomic oxygen, as illustrated in Fig. 1(c). Then, following the adsorption of O2 the energy necessary to dissociate the O2 molecule is

[TABLE]

The balance between these two energies dictates the kinetics of the ORR process.

Figure 2 compares adsorption and dissociation energies for various surface Pt thicknesses, Co polarizations, and molecular orientations of O2 for a (001) surface of CoPt. When Co is not spin polarized, increasing the number of Pt surface layers, monotonically decreases (increases) EADS (EDISS) to the pure Pt slab in accord with previously reported values gross2003unified ; xue2018dissociative ; li2016first ; yang2010density ; norskov2004origin ; xu2004adsorption ; kitchin2004modification . However, when Co is allowed to be polarized, the trend inverts. That is, local exchange interactions between Co and O2 induce a systematic decrease in EADS (EDISS); while when the Co layers go deeper away from the surface, the energies EADS and EDISS approach to the pure Pt case. Overall the presence of magnetic Co atoms near the surface enhances the binding of O2 to the surface of CoPt by $\approx$ 0.1 eV per O. Moreover, the orientation of the O2 molecule has a marked effect on the adsorption and dissociation energies, yielding significantly reduced values when O2 is horizontally oriented in almost all cases. This effect has also been reported for L10-FePt lu2020regulation . We also found our results to be insensitive to the orientation of the Co magnetic moments, e.g. in or out of the plane.

The enhanced surface bonding in the presence of magnetism can be understood by examining the chemical bond effect between the CoPt surface and O2. Bond formation between a transition metal and an adsorbent is often described by the d-band model hammer2000theoretical . The position and width of the d-band relative to the Fermi level influences the strength of the chemical bond, and therefore, the catalytic activity of the surface. However, while the d-band model provides valuable insights for non-magnetic systems, it is inadequate to fully capture the catalytic activity of magnetically polarized surfaces hammer2000theoretical ; bhattacharjee2016improved . Additionally, understanding how bond formation emerges as a function of adsorbent distance from the surface can further clarify the nature of the chemical bond norskov2011density ; hammer2000theoretical .

Figure  3 (a) and (b) compares the atomic site resolved projected density of states for Co-$d$ and Pt-$d$, and Co-$d$ and O-$p$ orbitals, respectively, as a function of O height from the L10-CoPt (001) surface, where the vertical dashed lines indicate the d-band center:

[TABLE]

where $g_{d}(\epsilon)$ is the Co-$d$ or Pt-$d$ partial density of states. The Co d-band center is closer to the Fermi level by $\sim$1 eV for almost all O (O2) distances from the surface, indicating Co is predominantly facilitating O (O2) binding to the surface. Consequently, when Co is not spin polarized the d-band center moves to the left, as indicated by the black arrow in Fig. 3 (a) and (b), suggesting weaker Co-O interactions, and thus weaker O surface binding CoPt. On comparing the Co-d and O-p density of states (Fig. 3 (b)), they are found to overlap and evolve together as the O atom is brought closer to the surface, thereby suggesting strong hybridization between these states. In particular, when the O atom is far from the surface, the O-p states are highly localized, but when oxygen is brought in proximity to the surface, the density of states becomes broader and splits, forming bonding and antibonding states.

To rationalize the effect of magnetism on chemical bonding at the surface, we extend the common d-band picture bhattacharjee2016improved ; hammer2000theoretical as follows. An O adsorbent far away from the metal surface, i.e., 3 - 4 Å, produces sharply peaked states below the Fermi level (Fig. 3 (c)). As the adsorbent approaches the surface its wave function starts to overlap and hybridize with that of the metallic surface. This broadens, shifts, and splits the adsorbent states into bonding and antibonding pairs (Fig. 3 (d)). The strength of the hybridization is gauged by the amount of d-states available for bonding at the Fermi level, typically quantified by the proximity of the d-band center to the Fermi energy norskov2004origin ; nilsson2005electronic ; greeley2002electronic . If exchange splitting is present, as is the case for ferromagnetic CoPt, the d-states are split, with the majority spin states pushed below the Fermi level rendering them inactive, whereas the minority spin states are shifted toward higher energies, thereby making more states available at the Fermi level for bonding and giving rise to stronger surface-adsorbent hybridization (Fig. 3 (e)). These minority-spin states near the Fermi level can accept electron density from the adsorbate, enhancing the orbital overlap and leading to stronger chemical bonding.

The activity of the oxygen reduction reaction (ORR) is highly sensitive to the d-orbital configuration and charge transfer properties of the metallic surface kicinski2019enhancement . To elucidate these interactions, we analyze the projected density of states (PDOS) and identify the key atomic orbitals involved in bonding. Specifically, the d${}_{z}^{2}$-orbitals of Co and Pt are found to be the primary contributors to the bonding between the adsorbent and the surface, while the $d_{xy}$- and $d_{yz}$-orbitals of Pt play comparatively minor roles. Furthermore, significant charge transfer is observed between the pz-states of oxygen and the Pt surface layers, consistent with the behavior expected of efficient catalysts such as Pt. However, when the Pt layer is reduced in thickness, cobalt assumes a dominant role in the bonding interaction. Due to the exchange splitting of its d-states, the extent of charge transfer is diminished, which adversely affects the catalytic efficiency and leads to surface poisoning (i.e. partial or total deactivation of a catalyst caused by the very strong interaction of some reaction specifies with the active sites on the catalyst surfacewandelt2018encyclopedia ).

In addition to the subtle balance between d-band center and magnetic exchange splitting, surface-adsorbent interactions can be further enhanced when the adsorbent has an appreciable magnetic dipole bhattacharjee2016improved . O is paramagnetic and, in light of its two unpaired electrons, the electrons can align with an external magnetic field, causing them to be attracted to the field okada2003effect . Experimentally, the inclusion of an external magnetic field can, in principle, facilitate and increase oxygen transfer in the ORR process wang2016effect . However, if the applied external field is lower than the saturation magnetization of a ferromagnetic catalyst, then performance can be degraded by reduced oxygen transfer rates. As a result, the inclusion of magnetism in the ORR process is a delicate process that depends sensitively on multiple competing factors that dictate catalyst performance. In systems like CoPt, where magnetism leads to overly strong binding, one strategy may be to modulate the magnetic exchange interaction through strain to weaken the binding just enough to optimize reaction kinetics. Alternatively, local field enhancement or selective surface engineering (adding several layers of Pt) could be used to spatially confine magnetic effects without universally increasing adsorption strength.

III Conclusion

In summary, we have demonstrated that magnetism plays a critical role in influencing the ORR process of O2 with L10-CoPt. Specifically, the presence of spin polarized cobalt atoms near the surface enhances bonding between O2 and the CoPt, tipping the scales towards slower ORR kinetics. Our findings suggest that catalytic performance can be tuned by adjusting the magnetic and structural environment, such as increasing Pt overlayer thickness. This insight can guide the design of future catalysts leveraging magnetic properties to improve ORR efficiency and reduce reliance on platinum-based materials.

Acknowledgements

Acknowledgements.

We acknowledge helpful discussions with Xiaojing Wang, Shengzhi Zhang, and Vivien Zapf. The work at Los Alamos National Laboratory was carried out under the auspices of the US Department of Energy (DOE) National Nuclear Security Administration under Contract No. 89233218CNA000001. It was supported by the LANL LDRD Program, and in part by the Center for Integrated Nanotechnologies, a DOE BES user facility, in partnership with the LANL Institutional Computing Program for computational resources. Additional computations were performed at the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02-05CH11231 using NERSC award ERCAP0020494. K.J.A. and E.M. have been supported by the Robert A. Welch Foundation under grant No. C-2114.

Appendix A Adsorption and Dissociation Energies