  The development of high-activity and low-price cathodic catalysts to
facilitate the electrochemical sluggish O$_2$ reduction reaction (ORR) is very
important to achieve the commercial application of fuel cells. Here, we have
investigated the electrocatalytic activity of two-dimensional single-layer
Nb-doped zirconium diselenide (2D Nb-ZrSe$_2$) towards ORR by employing the
dispersion corrected Density Functional Theory (DFT-D) method. Through our
study, we computed structural properties, electronic properties, and energetics
of the 2D Nb-ZrSe$_2$ and ORR intermediates to analyze the electrocatalytic
performance of the 2D Nb-ZrSe$_2$. The electronic properties calculations
depict that the 2D monolayer ZrSe$_2$ has a large band gap of 1.48 eV, which is
not favorable for the ORR mechanism. After the doping of Nb, the electronic
band gap vanishes and 2D Nb-ZrSe$_2$ acts as a conductor. We studied both the
dissociative and associative pathways through which the ORR can proceed to
reduce the oxygen molecule (O$_2$). Our results show that the more favorable
path for O$_2$ reduction on the surface of the 2D Nb-ZrSe$_2$ is the 4e$^-$
associative path. The detailed ORR mechanisms (both associated and
dissociative) have been explored by computing the changes of Gibbs free energy
({\Delta}G). All the ORR reaction intermediate steps are thermodynamically
stable and energetically favorable. The free energy profile for the associative
path shows the downhill behavior of the free energy vs. the reaction steps,
suggesting that all ORR intermediate structures are catalytically active for
the 4e$^-$ associative path and a high 4e$^-$ reduction pathway selectivity.
Therefore, 2D Nb-ZrSe$_2$ is a promising catalyst for the ORR which can be used
as an alternative ORR catalyst compared with expensive platinum (Pt).
