Electrocatalytic CO2 Reduction Empowered by 2D Hexagonal Transition Metal Borides

Abstract

Electrocatalysis holds immense promise for producing high-value chemicals and fuels through the carbon dioxide reduction reaction (CO₂RR), advancing global sustainability and carbon neutrality. However, conventional electrocatalysts based on transition metals are often limited by significant overpotentials. Since the discovery of the first hexagonal MAB (h-MAB) phase, Ti₂InB₂, and its 2D derivative in 2019, 2D hexagonal transition metal borides (h-MBenes) have emerged as promising candidates for various electrochemical applications. This study presents the first theoretical investigation into the CO₂RR catalytic properties of pristine h-MBenes (h-MB) and their ─O (h-MBO) and ─OH (h-MBOH) terminated counterparts, focusing on metals such as Sc, Ti, V, Zr, Nb, Hf, and Ta. These results reveal while h-MB and h-MBO exhibit poor catalytic performance due to overly strong or weak interactions with CO₂, h-MBOH shows great promise. Notably, ScBOH, TiBOH, and ZrBOH display exceptionally low limiting potentials (U_L) of -0.46, -0.53, and -0.64 V, respectively. These findings uncover the unique role of ─OH in tuning the electronic properties of h-MBenes, thereby optimizing intermediate adsorption, which prevents excessive binding and enhances catalytic efficiency. This research offers valuable insights into the potential of h-MBenes as highly efficient CO₂RR catalysts, underscoring their versatility and significant prospects for electrochemical applications.

Keywords: electrochemical CO2 reduction reaction; first principles; functional groups; h‐MBenes.

Figure 1

Schematic diagram illustrating the workflow of the study. It outlines the steps of structure construction, screening of eCO₂RR catalytic properties, and the design of functionalized h‐MBs catalysts. In the left panel, blue, green, and brown spheres represent the metal (M) atoms, boron (B) atoms, and functional groups (T), respectively.

Figure 2

The eCO₂RR catalytic activity of 2D h‐MBs. a) Gibbs free energy diagrams for the eCO₂RR pathway on the ScB surface. b) Limiting potential and potential‐determined steps for eCO₂RR on h‐MBs. In panel (a), the blue and red lines indicate favorable and unfavorable CO₂ reduction pathways, respectively.

Figure 3

Stability of h‐ScBT _x (T = O or OH, 0 ≤ x ≤ 1). Calculated surface Pourbaix diagram (left panel) and illustration of surface functional groups (right panel) for h‐ScBT _x (T = O or OH, 0 ≤ x ≤ 1).

Figure 4

eCO₂RR catalytic activity of bare (h‐MBs) and ─OH functionalized 2D h‐MBenes (h‐MBOHs). a) Calculated Gibbs free energy diagram illustrating the most feasible pathway for CO₂ reduction to C₁ products on h‐MBOHs. b) Comparison of the limiting potentials for eCO₂RR on h‐MBs versus h‐MBOHs surfaces. c) Relationship between the Gibbs free energy change for the first hydrogenation step of eCO₂RR and HER on h‐MBs and h‐MBOHs surfaces. d) Difference in limiting potential between eCO₂RR and HER on h‐MBOHs, with the vertical dashed line indicating U _L(eCO₂RR) = −0.74 V and the horizontal dashed line representing U _L(eCO₂RR) – U _L(HER) = −0.71 V.

Figure 5

Analysis of the effect of ─O and ─OH functional groups on the eCO₂RR catalytic performance of ScB h‐MB. a) Projected density of states of ScB, ScBO, and ScBOH. b) Schematic representation of the orbital interactions between the catalysts ScB, ScBO, and ScBOH with different d‐band centers E _d and the adsorbate CO₂. c) Mechanism illustration of the relationship between d‐band center E _d and catalytic activity. d) Volcano‐shaped relationship between U _L and E _d.

Figure 6

Functional group stability and microscopic kinetics analysis for the eCO₂RR performance of ScBOH h‐MBene. a) Calculated applied potential for h‐MBenes with various surface conditions. b) Reaction pathway for eCO₂RR on the ScBOH surface with black numbers indicating the calculated kinetic barriers for the four elementary steps leading to the formation of C₁ products. c) Calculated turnover frequency (TOF) for HCOOH, CH₃OH, and CH₄.