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Unmanned aerial vehicles (UAVs) at low altitude flight may significantly degrade their performance and the safety under wind disturbances and incorrect operations. This paper presents a robust control strategy for UAVs to achieve good performance of low altitude flight and disturbance rejection. First, a novel second-order hexacopter dynamics is established and the position tracking is translated to the altitude and the rotational angle tracking problem. An integrated control scheme is created to deal with the challenges faced by hexacopter at low altitude flight, in which the influence of near-ground threshold distance and the desired roll, pitch, and yaw are analyzed. Moreover, an improved flying altitude planner and an attitude planner for low altitude conditions are designed respectively to avoid the overturning risk due to the big reaction torque and external disturbances. Second, a sliding-mode-based altitude tracking controller and an attitude tracking controller are designed to reduce the tracking errors and improve the robustness of the system. Finally, the proposed control scheme is tested on simulation and experiment platforms of multi-rotor UAV to show the feasibility and accurate trajectory tracking at low altitude flight. INDEX TERMS Modeling, tracking control, low altitude, hexacopter, robustness. YUQING CHEN received the master's and Ph.D. degrees in control theory and control engineering from the

Electrocatalytic reduction of carbon dioxide (CO2) to formate is an effective solution to address the continuous increase in CO2 in the atmosphere. Here, we report a vanadium-doped (V-doped) bismuth oxide (Bi2O3) electrocatalyst synthesized using a facile one-step hydrothermal method for highly efficient electrochemical reduction of CO2 to formate. The doping of V can tune the intrinsic crystal and electronic structure of Bi2O3, that is, causing partial amorphization in the Bi2O3 nanosheet and decreasing electron density around Bi active sites. The partial amorphous region can provide more reactive sites; meanwhile, the electron-deficient environment around Bi enhances the adsorption of CO2. The synergistic crystal and electronic structure modulation in the V-doped Bi2O3 provides excellent electrocatalytic CO2RR performance with a high formate selectivity of 94.2% and a high partial current density of 45.03 mA cm–2 at −1.1 V (vs RHE).