IntroductionEnergy (also known as energy resources, including coal, oil, natural gas, water, wind, sunlight, etc.) is the basic ingredient for human survival and as the driving force of social development plays a very important role in human society. The great development in the energy utilization pattern has made huge changes in human society. The use of fire was the first step to civilization, which enhanced the ability of humans to conquer nature. With the invention of the steam engine, the new era of steam power came into being which was not available for ancient humans. Industrialization and economic growth were supported by fossil fuels during the past century. Nowadays, fastest developments in the history of humans are taking place in industry, agriculture, high-tech industry, and so on. Development of the economy and the increasing population make the demand for energy increase rapidly. But the nonrenewable fossil energy reserves are becoming more and more scarce. Obviously, they may run out very soon. The energy crisis has been deepening because of the critical shortage of resources and low efficiency in their exploitation and utilization, as well as severe pollution that has brought about serious environmental problem such as the greenhouse effect, ozone depletion, acid rain, water pollution, and so on. The increasing energy depletion and environmental pollution problems constitute one of the biggest challenges of this century.At present, most of the energy in biomass is wasted directly. In the United States, it takes 15 GW of electricity to deal with the organic wastewater, which accounts for 3% of the total output power [1]. But there is about 1.5 × 10 3 GW of potential energy contained in wastewater [2]. As the world's largest developing country, China accounts for a large proportion of energy production and consumption. Along with the accelerating process of industrialization, wastewater discharges have also increased dramatically in China. Rivers, lakes, and groundwater are seriously polluted, which greatly affect people's health. The exploitation of clean and green renewable energy is urgently required.At present, the main countries in the world are actively developing various kinds of new energy sources, such as solar energy, hydropower, wind energy, biomass Graphene-based Energy Devices, First Edition. Edited by A. Rashid bin Mohd Yusoff.
