The internet of things (IoT) is a distributed heterogeneous network of lightweight nodes with very minimal power and storage. The IoT energy system for smart applications such as smart grid, smart building, and smart transportations depends on the IoT architecture, determining the high or lowenergy consumption levels. Most of the IoT objects are power-driven by batteries with short life spans that require replacement. The replacement phase is tedious; hence this paper comprehensively discussed the IoT energy system, energy resources, and energy storage as these three elements are crucial to enable energy efficiency for the IoT applications. In comparison to the batterypowered solutions, the scavenging of infinite quantities of energy makes the IoT systems robust. Therefore, this paper further elaborates on understanding the current situation in terms of renewable energy harvesting for these lowenergy systems. The IoT energy storage highlighted in this paper includes fuel cell, lithium battery, and supercapacitor technology. This paper also provides the findings for IoT energy system challenges and open issues in management and storage in terms of bidirectional, continuity, autonomy, fluctuation, conversion, consumption, integration, multifunction, and stability. This paper will assist researchers in more quality yet practical energy usage and savings, better IoT system architecture, and smart application sustainability.

Fuel cells efficiently turn chemicals in fuel into electricity by chemical reaction and have been described as among the most recent advances in the upcoming cleaner energy sector. In recent times, fuel cells are being used in medicine, including experimental studies and current and potential goods, having numerous benefits over previous batteries, such as the convenience of recharging, eco-sustainable character, and high safety. This article highlights the up-to-date development of this energy system focusing on biofuel cells in implantable medical devices (IMDs) that use microbes, enzymes, and noble metals as catalysts. Furthermore, a diversity of fuel cell applications on the vitro medical kit (including alcohol tester, wound treatment instrument, blood glucose meter) was also described. The integration of fuel cells into implementable medical devices is at an initial phase of research, but this technology's possibility and potency is a reward. Obviously, after successfully integrating fuel cells into the patient's psyche, civilization will move throughout an innovative diagnostic transition.