Elucidating the Adsorption and Co-adsorption of Potassium and Oxygen on (110) MnO 2 , TiO 2 and VO 2 Surfaces

. In metal air battery, oxygen reacts with lithium ions on the cathode side of the cell which makes it much lighter than conventional cathodes used in Li-ion batteries. Density functional theory (DFT) study is employed in order to investigate the surfaces of, (Rutile) R-MnO 2 , TiO 2 and VO 2 (MO 2 ), which act as catalysts in metal-air batteries. Adsorption and co-adsorption of metal K and oxygen on (110) β-MO 2 surface is investigated, which is important in the discharging and charging of K-air batteries. Only five values of (gamma) are possible due to the size of the supercell and assuming that oxygen atoms occupy bulk-like positions around the surface metal atoms. The manganyl, titanyl and vanadyl terminated surface are not the only surfaces that can be formed with Γ= +2, oxygen can be adsorbed also as peroxo species (O 2 ) 2-, with less electron transfer from the surface vanadium atoms to the adatoms than in the case of manganyl, titanyl or vanadyl formation. MnO 2 promotes formation of KO 2 for all configurations whereas TiO 2 partially promote nucleation of KO 2 whereas VO 2 surfaces form very stable KO 2 clusters, thus VO 2 is not a good catalyst for the formation of KO 2 . The fundamental challenge that limits the use of metal air battery technology, however, is the ability to find a catalyst that will promote the formation and decomposition of discharge products during the charging and discharging cycle, i.e. oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).


Introduction
The development of high energy density power sources has been sparked by increased energy demand as a result of rising living standards and population. Even if Li-ion battery technology has improved, it has not kept up with the development of portable devices, resulting in a so-called "power gap" that is expected to widen in the future years. Metal air batteries are garnering a lot of attention as an alternative because of their capacity to produce high theoretical specific energies, which are nearly 6-10 times that of Li-ion batteries [1,2,3].
The fundamental challenge that limits the use of metal air battery technology, however, is the ability to find a catalyst that will promote the formation and decomposition of discharge products during the charging and discharging cycle, i.e. oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Previous studies used ab-initio thermodynamics to investigate the stable rutile MO2 (M=Mn, Ti &V) surfaces and their redox behaviour. The estimated stability of the (110), (100), and (101) stoichiometric surfaces follows the trends previously obtained for rutile TiO2 and rutile-type SnO2: (110) > (100) > (101) [16]. Generally, the stoichiometric surfaces are expected to be observed under environmentally favourable conditions, which are consistent with experiments that reported a manganese valence of 4.0 at the β-MnO2 surface [4].
The nucleation and growth of KO2 in K-O2 batteries influence its size and morphology, which play an important role in passivating the conducting electrode surfaces and impeding the transport of oxygen through the porous oxygen electrode. After the assembly of K-O2 batteries, oxygen molecules dissolve and diffuse into the nonaqueous electrolyte until oxygen concentration reaches equilibrium. Upon discharging, dissolved oxygen is reduced to O2anions on the carbon cathode surface, at which the O2-and K+ combine to generate the initial KO2 nuclei [5].
In contrast with LiO2 and NaO2, KO2 is thermodynamically stable and commercially available. The oxygen reduction and oxidation potential gap in the electrolyte with K + is much smaller than that in the electrolyte with Li + . This implies that a K−O2 battery may operate at much lower overpotentials and thus has a higher energy efficiency than a Li−O2 battery [2].
In the present work, the density functional theory (DFT) approach is employed in order to investigate the surfaces of, (Rutile) R-MnO2, TiO2 and VO2 (MO2), which act as catalysts in metal-air batteries. Adsorption and co-adsorption of metal K and oxygen on (110) β-MO2 surface is investigated, which is important in the discharging and charging of K -air batteries.

Computational Methods
Periodic density functional theory (DFT) computations were carried out using the Vienna Ab initio Simulation Package (VASP) code [6,7] in the form of the Perdew, Burke, and Ernzerhof (PBE) exchange correlation functional [8] in the generalized gradient approximation (GGA). A cutoff kinetic energy of 600 eV was utilized to determine the number of planewaves, and the Monkhorst-Pack Brillouin zone sampling approach with 6x6x9 and 6x6x1 k-points mesh for the bulk and surface structures, respectively, was used. We adopt Liechtenstein's non-simplified rotationally invariant Hubbard correction with effective Coulomb parameter set U = 2.8 eV and exchange parameter J = 1.2 eV, and U = 4.6 eV and exchange parameter J = 0.0 eV [9,10,11]. The VO 2 calculations were done without the Hubbard correction and were not spin polarized.
[12]  We first look at the stability of the (110) surface by doing periodic calculations in a slab with a stoichiometric composition, thicknesses of 14 Å (depending on the oxidation state), and vacuum gaps approximately14 Å as shown in Fig 1. The two surfaces-of each slabs are symmetrically equivalent, and this symmetry is maintained throughout the calculations, preventing the creation of electric dipole moments that can occur with asymmetric slabs. With variances of around +0.8% and -3.1% for lattice parameters a and c, respectively, and 1.6 percent in the cell volume for the MO2, the lattice parameters were in good agreement with the experimental results.

Oxygen Adsorption On (110) Metal Oxides
We compute the adsorption energies of several stoichiometries in order to explore the redox characteristics of the MO2 (110) surface. Only differences in oxygen content are taken into account (the number of M atoms is fixed). If we keep to bulk-like oxygen sites, there are five potential values of: Γ= 0 (stoichiometric surface), Γ= 1 and 2, Γ= 3 and 4. Total oxidation is the process of adding a thick coating of oxygen ions to previously unsaturated M sites, generating titanyl, vanadyl, and manganyl-like terminations. Fig 2 depicts the "monoperoxo" and "bridging-peroxo" modes of O2 adsorption.
Oxygen adsorption energies on MnO2 has been discussed previously in detail [11] together with those on VO 2 (110) surface [12]. Adsorption of an oxygen atom on a 5-fold coordinated Ti site (Γ= 1) yielded adsorption energy of 2.41eV. The configuration with Г= 2 has higher electron transfer from surface Ti atom to the adatom compared to other configurations. The calculated adsorption energy in this configuration is 0.69eV, implying that the surface oxidation was endothermic and thus thermodynamically unfavorable. We then adsorbed oxygen as a bridging-peroxo unit (O2 2-) shared by two Ti surface cations, which requires the least amount of charge transfer per Ti cation of any oxidation method. The mono-nuclear configuration has an adsorption energy of 0.070 eV and the bridging configuration has an adsorption energy of 0.37eV; these values indicate that the processes are endothermic, implying a non-spontaneous process [13]. The mono-nuclear configuration is the most energetically stable configuration in which oxygen molecules are adsorbed in various orientations.

Potassium Adsorption on (110) MO2 Surfaces
It is important to understand the interaction of potassium (K) and where it prefers to adsorb itself on the (110) MO2 surface. Consequently, K was randomly adsorbed on the surfaces of MnO2, TiO2 and VO2; the system was allowed to relax using the parameters stated in the methodology which yielded the structures in  Where bbp (potassium adsorbed between two bridging and one in-plane oxygen atoms), bpp (potassium adsorbed between two in-plane and one bridging oxygen atoms) and bb (potassium adsorbed between two bridging oxygen atoms. The calculated adsorption energies for potassium adsorbed on MnO2 surface generally show the spontaneous reaction, which is obvious due to the negative adsorption energies, which are exothermic reactions. The most preferred position of K is (-1.88eV) where it was adsorbed between two bridging and one in-place oxygen atoms; where K is triply coordinated to the surface oxygen atoms. This is followed by the adsorption energy of -1.82eV which is also a coordination of K three oxygen atoms (bpp). The least stable adsorption energy is associated to the least oxygen coordination to potassium atom, which is when it was on top of two bridging oxygen atoms. When K was adsorbed on TiO2 surface, it showed that the highly oxygen coordinated region is more stable with bbp was more negative than bpp and bb configuration with their adsorption energies of -1.78 eV, -1.62 eV and -1.63 eV respectively.Contrary to the other metal oxides, when K was adsorbed on VO2 surface, potassium showed a more negative adsorption energy when it was placed on top of two bridging oxygen atoms i.e bb which yielded -3.24 eV. Followed by the adsorption on top on two bridging and one in-plane oxygen atoms with adsorption energy of -3.01 eV and the least stable adsorption energy is that of bpp configuration with -2.88 eV.

Adsorption Of Oxygen At The K/MO2 (110) Surface
We took into account the oxygen adsorption on the K/MO2 (110) surface (two O atoms per surface cell, = 2). We additionally considered two K atoms per surface cell, assuming that they both occupy the most stable bbp sites, in order to explore the stability of K-O-O-K species, which are known to be significant in the K-air battery. This structure corresponds to complete coverage of the bbp sites at the surface, i.e., a monolayer of K adatoms, as there are only two bbp sites at each surface in our simulation cell. As depicted in table 2, the resulting configurations yielded are dissociated, dissociated', peroxo on M/K, and superoxide. The first configuration with an oxygen atom on the "bulklike" positions on top of each of the M cations, but with additional bonds formed with the K adatoms which resulted in the dissociated configuration yielded the adsorption energy of -2.21 eV/O, -2.04eV/O and -4.21eV/O for MnO2, TiO2 and VO2 respectively. The second configuration also yielded the dissociated' cluster which has different adsorption energies, this is due to the initial adsorption sites of oxygen atoms which were placed in the form of peroxo on K which dissociated after relaxation and gave -2.01 eV/O , -2.62 eV/O and -3.99 eV/O for the metal oxides (MnO2, TiO2 and VO2 sequentially. For the superoxide configuration the yielded adsorption energies as follows (-1.12 eV/O, -2.23 eV/O and -4.21 eV/O) in the order described above. The final cluster that was calculated was the peroxo on M/K whereby oxygen atoms were placed between M and K and the adsorption energies gave 370, 02001 (2022) https://doi.org/10.1051/matecconf/202237002001 MATEC Web of Conferences 2022 RAPDASA-RobMech-PRASA-CoSAAMI Conference the similar trend with those of dissociated', superoxide and peroxo on M/K where by MnO2 was the least stable, followed by TiO2 and VO2 is the most stable adsorption energy.
Parallel to the study of Li/Na-air batteries, K-air batteries have emerged as an alternative, based on the substitution of lithium by potassium, despite their lower theoretical energy density, can exhibit better reversibility and much lower overpotentials than lithium-based cells [14]. During the discharge process of a battery, molecular oxygen is reduced in the cathode in the presence of K cations and electrons, resulting in the formation of potassium superoxide (KO2) particles: The formation energy of the KO2 was reported to be -2.25ev/O [15] which is the reference material that is expected to be yielded. If the clusters calculated gives energy than is lower that the formation energy of KO2 it then implies that the clusters will stick to the surface and as such, formation of the KO 2 is not supported.

Conclusion
DFT calculations under PBEsol+U(+J) and PBEsol were successfully validated for the adsorption and co-adsorption of K and O on MO2. Potassium adsorbs strongly on the highly coordinated oxygen atom for MnO2 and TiO2 whereas in VO2, it prefers to adsorb on the bridging oxygen. MnO2 promotes formation of KO2 for all configurations whereas TiO2 partially promote nucleation of KO2 and VO2 surfaces form very stable KO2 clusters, thus VO2 is not a good catalyst for the formation of KO2. This is the same observation that has been reported when Li and O are adsorbed on MO2 surface.