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Summary A water‐based iron oxide/cobalt/reduced graphene oxide (Fe2O3/Co/rGO) precursor was spray‐coated via supersonic spraying onto a flexible substrate to fabricate supercapacitor electrodes. The inclusion of Co in Fe2O3 increased the number of electrochemical reaction sites and enhanced the local electron mobility, which in turn improved the overall energy storage capability of the electrodes. The Co salt concentration was varied to determine the optimal electrochemical performance. Supersonic spraying was utilized to coat rGO sheets, on which the bimetallic CoFe2O3 particles were evenly distributed. The uniform coating of rGO and its excellent electrical conductivity increased the electron mobility of the electrode and provided shorter ionic diffusion pathways. Furthermore, the catastrophic impact of supersonic spraying led to the exfoliation of the rGO sheets, which increased the surface area and enhanced the adherence of the CoFe2O3 particles to the substrate. This rGO exfoliation also enabled the accommodation of the CoFe2O3 particles between the exfoliated graphene sheets, thereby enhancing electron transport throughout the electrode. The optimal sample exhibited a specific capacitance of 1.11 F·cm−2 at a current rate of 2 mA·cm−2, energy density of 0.62 mWh·cm−2 at a power density of 8 mW·cm−2, and capacitance retention of 89% after 10,000 charge‐discharge cycles.

Spray cooling has been used to quench metal slabs during casting, cool nuclear reactors, suppress accidental fires, and remove heat from high-power density electronics. In particular, the miniaturization of electronic devices inevitably results in an increased power density or heat flux on the microelectronics surfaces and poses a threat of a thermal shutdown of such devices when cooling is insufficient. Surface nanotexturing effectively augments additional liquid-to-substrate surface area, thereby increasing cooling capability, as well as an effective heat transfer coefficient. In spray cooling, surface dynamic wettability also affects drop impact dynamics and subsequent coolant evaporation on a hot surface. Herein, we introduced various nanotextured surfaces and affected dynamic wettability using the so-called thorny-devil nanofibers, nickel nanocones, Teflon and titania nanoparticles, and zinc nanowires. The effect of these different nanoscale architectures on drop impact phenomena and subsequent evaporative cooling was investigated. These nanotextured surfaces were fabricated using various deposition methods, including electrospinning, electroplating, supersonic spraying, aerosol deposition, and chemical bath deposition. We found that the surface with greater dynamic wettability related to the hydrodynamic focusing considerably improved the heat removal capability by furthering the Leidenfrost limit and facilitating drop spreading. In particular, the thorny-devil nanofiber surface yielded the highest heat flux at all ranges of the Reynolds and Weber numbers. Spray cooling on a model electronic kit also confirmed that the thorny-devil nanofibers were most effective in cooling the surface of the model kit during multiple cycles of water spraying.