(307g) Rational Design of Atomically Dispersed Single Atom Catalysts for Electrochemical Reactions

Single-atom catalysts (SACs) have been considered one of the promising electrocatalysts in the field of heterogeneous catalysis research. It offers an atomically dispersed single site with full atom utilization and high catalytic selectivity and activity toward numerous chemical reactions. In my presentation, I will address the challenges behind the designing of highly efficient SACs for various electrochemical reactions such as hydrogen evolution reaction (HER), CO₂ reduction reaction (CO₂RR), and Oxygen reduction or evolution reaction (ORR/OER). In the first part, I will discuss the HER activity correlation with the SACs electronic properties for a series of transition metals (TM) supported on nitrogen-doped graphene with experimental validation. Next, I will talk about newly develop constant potential-based calculations to predict reaction mechanism and kinetics and its application towards CO2RR on different nitrogen coordinated Ni-SACs. Finally, I will present the effect of spin-crossover on ORR/OER activity under operating conditions for different 3d TM-based SACs.

The stability is often an issue for metal/metal-oxide based SACs due to the low conductivity and instability in a harsh electrolytic environment (strong acid and base). Graphene addresses all the issues as it provides a large surface area, high electrical conductivity, good stability, and high dispersion for electrocatalysts. Although SACs show promising activity towards various electrochemical reactions, the proper choice of nonprecious transition metals as SACs and the origin of activity remains a bottleneck for designing highly efficient electrocatalysts for various electrochemical reactions. Besides predicting activity, the synthesis of SACs is always challenging due to the high surface energy of atomically dispersed sites causing aggregations. Herein, we report an activity correlation as a function of electronic properties, to clarify the origin of reactivity for a series of transition metals supported on nitrogen-doped graphene as SACs for HER by a combination of density functional theory calculations and electrochemical measurements. We found that Co-SAC exhibits the highest electrochemical activity at 0.13 eV among 17 studied transition metals as a single site. Electronic configuration studies show that the antibonding state orbital is neither empty nor filled in the case of Co-SAC is the main reason for its ideal hydrogen adsorption energy. To confirm our theoretical prediction, we synthesized three kinds of single atom (Co, Ni, and W) catalysts using graphene oxide (GO) based wet chemistry method followed by carbonization at high temperature. Finally, the electrochemical measurement shows that Co-SAC exhibits superior hydrogen evolution activity over Ni-SAC and W-SAC, confirming the theoretical calculation. This systematic study gives a fundamental understanding of the design of highly efficient SACs for HER.[1]

Later, we develop potential based macroscopic theory called the grand canonical potential kinetics (GCP-K) and apply it to calculate reaction mechanisms and rates. Quantum mechanics based (QM) calculations, such as density functional theory (DFT), are always used a fixed number of electrons to perform electrochemical activity prediction.[2] Hence, it is essential to modify the fixed electron based QM calculation to more extent potential dependent calculation in order to appropriately account for electrochemical conditions at a specified applied voltage occurs in experimental activity measurements. Here we report the application of GCP-K to predict the reaction mechanism and rates for CO₂RR over Ni-SACs for the various nitrogen coordination with Ni such as Ni-N₂C₂, Ni-N₃C₁, and Ni-N₄ sites embedded in graphene. We find that Ni-N₂C₂ leads to the lowest onset potential of -0.84 V (vs RHE) to achieve 10 mA cm^−2 current density, leading to a Tafel slope of 52 mV dec^−1 and a turn-over frequency (TOF) of 3903 h^−1 per Ni site at neutral (pH 7) electrolyte conditions, showing best agreement with various experimental observations at lower overpotentials. However, we also predict the onset potential for 10 mA cm^−2 current density of -0.92 V for Ni-N₃C₁ and -1.03 V for Ni-N₄ (which exhibits the highest saturation current for high applied potentials). [3]

Recently, it has been found that spin transition in a single metal site plays a significant role in predicting electrochemical activities, which is often ignored during theoretical calculations. The exact structure and proper spin state of the catalytic active site in SACs remain mysterious to facilitate the design of optimized, next-generation single atom-based catalysts. Here, we studied spin stability and activity for different planar and non-planar M-N₄ based SACs geometry. We found as synthesized Ni²⁺-SAC highly stable at high spin ( state due to the transition of square planar geometry (D_4h symmetry) of Ni-N₄ moiety into distorted tetrahedral geometry (D_2d symmetry). The quantum mechanics calculation shows the isotropic coupling constant (A_iso) value for the non-planar Ni-N₄ tetrahedrally distorted structure of dihedral angle between planes 153° to 123° very close to experimentally measured value of 20±5 MHz via ⁶¹Ni Mossbauer Spectroscopy. [4] Later, we studied spin transition for Co, Mn and Fe-based single-atom catalysts for oxygen evolution and reduction reaction as a function of applied potential. We found that the square planar of M-N₄ geometry broke its symmetry and converted to highly stable high spin (HS) square-pyramidal (C4v symmetry) when adsorbate like O*, OH* strongly binds on single site as a function of applied potential. The activity for various intermediate changes across the applied potential as their spin state changes.

References

[1] Md Delowar Hossain et al Rational Design of Graphene‐Supported Single Atom Catalysts for Hydrogen Evolution Reaction; Advanced Energy Materials, 1803689, 2019.

[2] Y. Huang et al Reaction Mechanism for the Hydrogen Evolution Reaction on the Basal Plane Sulfur Vacancy Site of MoS₂Using Grand Canonical Potential Kinetics, J. Am. Chem. Soc. 2018, 140, 48, 16773–16782

[3] Md Delowar Hossain, et al Reaction Mechanism and Kinetics for CO2 Reduction on Nickel Single Atom Catalysts from Quantum Mechanics; Nature Communications 11 (1), 1-14, 2020.

[4] David M. Koshy, Md Delowar Hossain et al. Investigation of the structure of atomically dispersed NiN_x sites in Ni, N-doped carbon electrocatalysts by ⁶¹Ni Mössbauer Spectroscopy and Simulations, 2022 (Under review)