A large amount of low-grade waste heat from industry is usually discarded. In order to recover the huge amount of waste heat for a sustainable society, extensive efforts have been made to develop high-performance and low-cost thermoelectric generators based on the Seebeck effect of solid semiconductors. In addition, liquid-based heat-to-electricity systems have been developed and demonstrated a good potential for practical applications. This Review aims to provide an overview of the newly developed liquid-based technologies for waste heat recovery, including thermo-electrochemical cells (TECs), thermally regenerative electrochemical cycles (TRECs), and thermo-osmotic energy conversion (TOEC) systems. The working principles of these technologies will be introduced, and the key factors affecting their performance will be discussed. A perspective to discuss the current challenges and application potential of each system and future research directions will be provided.