Solution plasma synthesis of birnessite-type MnO₂/carbon composites: support-dependent surface chemistry and alkaline oxygen reduction performance

Here, CNT-supported and Ketjen Black (KB)-supported composites were synthesized by a solution plasma process (SPP) for 10 min without external heating. XRD and SEM indicate the formation of poorly crystalline birnessite-type oxide on both supports, with support- and precursor-dependent deposition characteristics. The supports also differ in surface area, surface chemistry, and pretreatment; consequently, this comparison does not isolate morphology as an independent variable. XPS indicates differences in the Mn spectral response, whereas the direction of interfacial charge transfer and the identity of a low-binding-energy C 1s contribution remain unresolved. In O₂-saturated 0.1 M KOH, KB-M1 exhibited the highest onset potential among the synthesized composites, 0.873 V vs. RHE, and a geometric current density of -4.21 mA cm^-2 at 0.464 V. CNT-M0.5 showed the highest onset potential within the CNT series, 0.804 V. CNT-M0.5 and KB-M1 retained 93.1% and 90.8% of their initial currents, respectively, after 20 000 s of chronoamperometry. The results demonstrate a short SPP route and support-dependent alkaline ORR behavior, while highlighting the need for matched controls and interface-specific measurements to resolve the underlying mechanisms.