## 7 Current Challenges--Future Trends  
A challenge the industry faces is the production of hydrogen fuel cell-powered UAVs for commercial usage. Commercial UAVs driven by hydrogen fuel cells are still in the experimental and development stages and are not yet widely available. However, several businesses are striving to create fuel cell-powered UAVs for a range of uses, including surveillance, inspection, and mapping. One illustration is the \"Alpha 800\" UAV created by UAVOS Inc., ([[https://www.uavos.com/](https://www.uavos.com/)]([https://www.uavos.com/](https://www.uavos.com/)), Redwood City, CA, USA) which uses a hydrogen fuel cell and has a maximum flight time of 2.5 h [70]. Another example is Horizon Unmanned Systems' (HUS) \"Hygopter\", a multi-rotor UAV with ultra-light fuel cells that convert hydrogen in the aircraft frame into electricity to power the rotors. According to citehorizon, the technology will transform existing 20-30 min multi-rotor operations into flights lasting several hours, allowing for cheaper/faster aerial surveys and making drone delivery more realistic [71]. The aerospace industry has shifted its focus towards UAVs, as they strive to improve their operational efficiency, power management, and application versatility in the future. This way, air vehicles can become more effective and adaptable to various use cases. The aerospace industry is exploring new power-source solutions, one of which is the integration of lithium-sulfur batteries (Li-S). These batteries offer several advantages, including cost-effectiveness due to the use of low-cost materials such as sulfur as opposed to nickel or cobalt. Additionally, Li-S batteries have a higher power density with a theoretical energy of 2700 Wh/kg [72]. This has been demonstrated by the successful use of Li-S batteries in a UAV by the chemical department of LG in South Korea, which completed a stable high-altitude flight test. Furthermore, Li-S batteries have been integrated with a solar cell-powered drone and have proven to be effective in a broad-wing UAV [73]. Dalhousie University and Tesla's battery partner conducted research on advanced battery technologies with a focus on cells with higher energy density (Figure 7). They highlighted that such cells hold promise for not just electric cars but also drones and electric planes. The stability of their cycles remains a significant hurdle for these cells. Although the target was to attain 50 rotations in the past year, the utilization of a refined electrolyte has the potential to prolong longevity up to 200 rotations. Nevertheless, this advancement might not meet the commercialization standards, as the present industry demands 800 to 1000 rotations [74]. The cell has a gravimetric energy density of 360 Wh/kg and a volumetric energy density of 1000 Wh/kg [74]. When utilizing drones to enhance network connectivity and IoT applications, safeguarding security and privacy is of utmost importance. Hostile user attacks on drones can compromise the confidentiality, privacy, and integrity of data. To ensure the protection of drones, advanced technologies such as distributed ledger and wireless physical layer security must be thoroughly examined and evaluated. Along with security, forthcoming research must also strive to strike a balance between the level of security offered by these technologies and the necessity to sustain top-notch service and reliability. Energy consumption is a critical challenge faced by swarm UAVs. A swarm of UAVs may impede the successful completion of missions due to their size, which increases with battery capacity. Therefore, for the successful deployment of swarm UAVs, it is essential to find means of reducing the size and improving energy efficiency. The battery capacity of a swarm of UAVs is crucial to the successful completion of their missions. The size of the UAVs also increases with the battery capacity. A review by [PERSON] et al. [75], discusses the use of autonomous battery maintenance mechatronics systems as a potential solution to this problem.