Potential relationship of spin magnetic moment with thermal conductivity and catalytic performance in Fe–Co bimetallic catalysts: a machine-learning interatomic potential and density functional theory study

The thermal transport properties of catalysts are important for the stable operation of proton exchange membrane fuel cells. In this study, density functional theory and moment tensor potentials are used to solve the Boltzmann transport equation and investigate the thermal transport properties of various designed configurations of Fe-Co bimetallic catalysts, which are the most promising non-noble metal catalysts for the oxygen reduction reaction (ORR). It is found that the velocity of the phonon group in the <i>y</i> direction is always higher than that in the <i>x</i> direction, which leads to the anisotropic thermal conductivity of these catalysts. The uniformity of the thermal conductivity exhibited by bimetallic catalysts is better than that of single-metal atom catalysts. In the designed configurations, G-FeCoN₆-3 is found to show a high thermal conductivity value (55.57-376.98 W m^-1 K^-1), which is even higher than that of G-FeN₄ (35.66-132.75 W m^-1 K^-1). The analysis of phonon transport properties shows that the difference in thermal conductivity is mainly due to the difference in the phonon lifetime. The results indicate that thermal conductivity is governed by low-frequency phonons and the size effects are intensified in bimetallic catalysts. It is revealed from results on electronic structures that the low thermal conductivity may be related to the existence of band gaps in valence bands. Furthermore, these structures exhibit superior electrical conductivities with values of 0.98-2.2 × 10⁴ Ω^-1 m^-1. Additionally, through the results of the maximum electron thermal conductivity of these structures, it is revealed that the thermal conductivity of these catalysts is mainly dominated by the lattice thermal conductivity. Finally, a potential relationship of spin magnetic moment with the thermal conductivity and catalytic performance is revealed.