Electrochemical reduction of CO2 in useful products using renewable electricity is recognized as a potential solution for reducing atmospheric levels of CO2. Single-atom catalysts (SACs) have recently shown great potential for the electrochemical conversion of CO2 to valuable products. Herein, we report, by means of density functional theory (DFT) calculations and computational hydrogen electrode (CHE) model, a descriptor-based design principle to explore the potential of 3d/4d and 5d transition metal (TM) single atoms (SAs) embedded gallium nitride (GaN) monolayers as electrocatalysts for CO2.
Based on this analysis, we easily identified 15 TM-SACs capable of activating the inert CO2 molecule to form a carbon dioxide radical anion (CO2•–). Our mechanistic analysis and descriptor-based screening approach predicted the Mn/Rh/Os/Ir-SAC to be the most promising CO2RR electrocatalyst. Remarkably, the proposed lowest free energy pathway analysis indicated that the Rh-SAC has the best activity in HCOOH formation with the limiting potential (UL) of -0.42 V, while the Ir-SAC has a better catalytic activity shows for the production of CH4 with a UL of -0.48 V and a selectivity of 97% compared to the competing HER process.
Projected density of states (PDOS) and crystal orbital Hamiltonian population states (COHP) plotted for Os-SAC and (d) Ir-SAC with *CO intermediate adsorbed state…….……….31 Figure 2.11 Free energy profiles for CO2 reduction to CH3OH on Os-SAC at zero and applied potential via different reduction pathways (P1 to P Figure 2.12 Free energy profiles of CO2 reduction to CH3OH on Ir-SAC at zero and applied potential via different reduction pathways (P1 to P Figure 2.13 Free energy) profiles of CO2 -reduction to CH4 on Os-SAC at zero and applied potential via different reduction pathways (P1 to P. CO2•– Carbon dioxide radical anion CO2RR CO2 reduction reaction NRR Nitrogen reduction reaction OER Oxygen evolution reaction ORR Oxygen reduction reaction HER Hydrogen evolution reaction GaN Gallium nitride.
Introduction
- Background
- Electrochemical reduction of CO 2
- Density Functional Theory
- Computational hydrogen electrode (CHE)
At high applied potential, the competitive hydrogen evolution reaction (HER) could easily dominate and reduce the overall CO2RR efficiency.22,23 Despite the ultimate technological viability of CO2RR, this is still thwarted by high overpotentials caused by the chemical inertia of the CO2 -molecule. leading to low selectivity for the desired product. Density Functional Theory (DFT) is perhaps the most widely used method for calculating electronic structure.25, 26 The modern approach to DFT evolved from the works of Hohenberg, Kohn and Sham.27, 28 Theorem developed in 1964 by Hohenberg and Kohn proved that there was an exact solution for the ground state of the many-body system based solely on the electron density. 𝐸ks= ∑ 𝑓𝑛 𝑛⟨𝜓𝑛|𝑇̂|𝜓𝑛⟩+ ∫ ∫ 𝜌(r)𝜌(r‖r−r′‖′)ⅆr ⅆr′+ 𝐸xc(𝑛) (1 .1) where the first term represents the kinetic energy of the Kohn-Sham states |𝜓𝑛⟩ weighted by their occupancy numbers fn, while the second term is Coulomb energy, which accounts for the interactions with the electron density and ionic nuclei. The functional exchange correlation (XC) describes all the effects of particle interactions that single-particle wave functions cannot.
An advanced approach known as generalized gradient approximation (GGA) uses both the local density and the gradient of the density at each point. Comparison of the most uphill free energy changes can predict the best electrocatalysts with lower boundary potentials. The standard hydrogen electrode (SHE) is one of the most commonly used reference electrodes.29 Platinum electrodes are often used in experiments because of their fast kinetics and ability to quickly reach equilibrium for hydrogen redox reactions.
Atomic-scale calculations of the electrochemical interface, using DFT and CHE, enable thermodynamically efficient screening over a wide range of materials and active sites. This method allows for a consistent evaluation of the free energies of the species involved in an electrochemical reaction network.
Computational Methods
Spin-polarized density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP)86 within the projector augmented wave (PAW) formalism. expand the orbitals of the valence electrons. We constructed a 5×5×1 supercell to evaluate the free energies of different adsorbates involved in CO2RR. Ernzerhof method (RPBE)88 to describe the exchange-correlation interaction, while the Tkatchenko-Scheffler (TS) method is used to calculate van der Waals (vdW) interactions.89 We added a vacuum gap of 20 Å in the z direction for avoid any spurious interactions between periodic images.
TM-SAC substrates were modeled by inserting a single TM atom into the supercell containing either a Ga-SV vacancy (Ga-SV), N-vacancy (N-SV), or a double vacancy (Ga/N-DV) ( see Figure 2.1). For geometry optimization and vibrational frequency calculations of these substrates, Brillouin zone sampling used a k-point grid, however, a denser 7×7×1 k-point grid was chosen for electronic structure calculations. To calculate the center of the d band, we determined the total d-PDOS and obtained the average energy of the d states.
The convergence criteria for the energy and forces on each atom were set to 10-5 eV and 0.01 eV Å-1, respectively. To account for the solvation effect, the implicit model with a dielectric constant of 78.4 (as water) was used to consider the realistic electrolyte environment as implemented in VASPsol. 91 We used the bond image push elastic band (CI-NEB) method to estimate reaction pathways and kinetic energy barriers.92. 𝛥𝐸ads = 𝐸surf−ads− 𝐸surf− 𝐸ads (2.2) where Esurf-ads and Esurf represent the energy of the complex adsorption surface and the pure surface, respectively, while Eads denotes the energy of the adsorbate.
The calculated zero-point energy change and entropy contribution for gaseous molecules and various reaction intermediates are given in Tables 2.5 and 2.6 respectively. ΔGU = neU, where n is the number of electrons transferred from electrode during the reaction process and U is the applied electrode potential. ΔGpH is the free energy correction for pH, ΔGpH = kBT × ln 10 × pH, where kB is the Boltzmann constant and pH is set to zero.
The distance from the TM to the nearest neighbor surface coordination atoms (dTM-Ga/N) are highlighted for each TM-SAC defect site.
Results and Discussion
- Structure and Stability
- Activation of CO 2
- CO 2 RR Mechanistic Analysis
- CO 2 RR via Six and Eight-Electron Pathways
- Kinetics of CO 2 RR via the optimized pathway
- Selectivity Evaluation of CO 2 RR Catalysts
As illustrated in Figure 2.3(b) and (c) the calculated values of the Estab and Udiss descriptors of all considered TM-SACs fulfill both stability criteria (Estab <0 and ❑diss> 0), suggesting stability high thermodynamic and electrochemical performance of these catalysts. Adsorption of CO2 on the catalytic surface and subsequent activation are believed to be necessary for efficient CO2RR, although the inertness of the CO2 molecule makes this step challenging. The binding of CO2 is governed by the orientation of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with respect to the surface.
The possible adsorption conformations that can be formed upon activation of CO2 on any surface are shown in Figure 2.5. In this study, we investigated the adsorption and activation of CO2 by calculating the Eads of the proposed catalytic surfaces and analyzed the optimized geometries of the CO2-adsorbed systems. Although chemisorption of CO2 is important for its catalytic conversion into useful products (hydrocarbons and alcohols).
The bond lengths of C=O and the bond angles of O=C=O (φOCO) are noted to be ∼1.18 and ∼180°, respectively, which is consistent with the geometric parameters of a gas-phase molecule of CO2. The chemisorbed systems exhibit different types of CO2 adsorption configurations either by sharing a single metal active site in the mono- and bi-dentate manner or the surface Ga atom-stabilized configuration through Ga-O coordination (e.g. Ru, Rh , Re and Os-SACs) . Adsorption energy of CO2 (𝐸ads[∗CO2]), angle between O=C=O (𝜑𝑂𝐶𝑂), distance from transition metal to carbon atom of adsorbed CO2 (dTM-C), distance between oxygen and carbon atoms (dO-C), and the corresponding configurations of CO2•–species.
Carbon dioxide radical anion (CO2•–) (a highly reactive intermediate) is formed as a result of charge transfer from the substrate to the CO2 molecule, resulting in geometrical changes in the linear structure of CO2. Considering the activation of CO2 as an obligatory step for its electrocatalytic reduction, we removed TM-SACs (TM = Sc, Fe, Co, Ni, Cu, Zn, Y, Pd, Ag, Cd, Pt, Au) from further studies because of their failure to absorb and activate the CO2 molecule. The calculated Gibb's free energies of SA integrated Ti/V/Cr/Mn/Zr/Nb/Mo/Ru/Rh/Hf/Ta/W/Re showed that they favor the reaction *CO2 + H+ + e- → *OCHO and stabilize the *OCHO intermediate, while Os and Ir-SAC follow the path of *CO2 + H+ + e- → *COOH.
Free energy profiles for the corresponding SACs for HCOOH and CO formation are shown in Figures 2.8-2.9 and Figure 2.10, respectively. However, these surfaces were further analyzed as *CO is considered a crucial intermediate while considering the deep reduction of CO2. As shown in Figure 2.10(c) and (d), the bonding populations (red) are below the Fermi level with filled states, while the antibonding populations (blue) are with the partially filled states because the energy levels are aligned across Fermi. level.
The insets of Figure 2.10(c) and (d) show the charge transfer of -0.64 e– (Os-SAC) and -0.35 e– (Ir-SAC) from surface to adsorbed *CO intermediate, indicating that this support potential are candidates for carrying out deep reductions of CO2. The first few steps are the same as those in the protonation of CO2 to CH3OH ie. *CO protonates to *COH with a high-energy barrier that eventually becomes the PLS for the first two pathways (R1 and R2). The optimized structures of the initial (IS), final (FS) and transition state for the PLSs are shown in Figures 2.17 and 2.18.
The optimized H-adsorbed configurations and Δ𝐺∗H values for promising CO2RR catalysts are shown in Figure 2.20 (a).
Conclusion
All praise is due to Allah, the Creator and Sustainer of the universe, for showering His blessings throughout my research work to complete the research successfully, and may His peace and blessings be upon our Holy Prophet, the eternal torch of guidance, knowledge, love and peace for all mankind. I especially appreciate and thank Muhammad Umer for guiding me through every step of the research - from planning to completion. I dedicate this work to my parents, sisters and brother for their endless love, support and encouragement throughout my life.
