*2.1. Sample Preparation*

Low crystalline MoO3/carbon composite microspheres, in which MoO3 nanocrystals were distributed homogeneously in the amorphous C matrix, were directly prepared by a one-step spray pyrolysis. The spray pyrolysis system used in this study is shown in Figure S1. In brief, droplets were generated by a 1.7-MHz ultrasonic spray generator that consisted of six vibrators, and the droplets were carried to a quartz tube reactor (length = 1200 mm, diameter = 50 mm) by a flow of N2 (flow rate = 5 L min−1). The reactor temperature was fixed at 900 ◦C. The spray solution was prepared by dissolving 0.1 M of MoO3 (98%, Sigma Aldrich, St. Louis, MO, USA), 12 g L−<sup>1</sup> of polyvinylpyrrolidone (PVP, Mw 40,000, Daejung Chemicals and Metals, Siheung, Korea), and 0.02 M of sucrose in distilled water. Subsequently, an appropriate amount of hydrogen peroxide (30% H2O2, Sigma-Aldrich) was added to the above solution to obtain a clear spray solution. For the bare sample, MoO3 powders without any carbon content were also prepared by spray pyrolysis. For this, the spray solution was prepared by dissolving 0.1 M of MoO3 without a carbon precursor in H2O2 contained in distilled water. Subsequently, the spray pyrolysis was carried out with the prepared solution at a temperature of 900 ◦C by a flow of air (flow rate = 5 L min−1).
