Low-grade heat (<100 °C) is abundant but mostly wasted because its utilization requires efficient energy harvesting systems with low cost and high efficiency. The thermally regenerative electrochemical cycle is a promising strategy to harvest low-grade heat, which exploits the dependence of electrochemical potential on temperature. In each cycle between low and high temperature, the electrochemical cell is charged at a lower voltage and discharged at a higher voltage, therefore converting heat to electricity. The temperature coefficient (α) of the system is a key parameter to determine the conversion efficiency, and the α value of the full cell will be maximized if those of the cathode and the anode have opposite signs. Previously, most studies have worked on electrode materials with negative α. In this study, we focused on lithium manganese oxide (LMO), a widely used lithium-ion battery cathode material, showing a positive α of 0.62 mV K −1 and stable performance in an aqueous electrolyte. We demonstrate an electrochemical system consisting of an LMO cathode and a copper hexacyanoferrate anode in the Li + and K + hybrid electrolyte for low-grade heat harvesting. The α of the full cell is 1.061 mV K −1 and the heat-to-electricity conversion efficiency can reach 1.8% in the temperature range of 10−40 °C. The relative efficiency reaches 18.8% of the theoretical Carnot limit, which is enhanced to 27% with the assumption of 50% heat recuperation. This work may open new opportunities for studies on the electrode materials with positive α and hybrid electrolyte systems with both a positive-α and a negative-α material for efficient low-grade heat harvesting.

Kinetic energy is an ideal energy source for powering wearable devices or internet of things (IoTs) because of its abundant availability. Currently, most kinetic energy harvesting systems are based on friction or deformation, which require high-frequency motion or high material durability for sustainable energy harvesting. Here, we introduce selective ion sweeping in a hybrid cell consisting of an ionadsorbing activated carbon and an ion-hosting Prussian blue analogue nanoparticle for electrochemical kinetic energy harvesting. The flow of electrolyte induced by kinetic motion of the cell causes ion sweeping only on the surface of the supercapacitor and induces current flow between the supercapacitor and the battery electrode. This method exhibits 24.9 μW cm −2 as maximum power of system with 34 Ω load in half-cell test, which is several thousand times smaller than the load used in conventional methods. In a long-term test with full cell, this method supplies a continuous current flow ∼6 μA cm −2 at the flow of 40 mL min −1 for 500 cycles without performance decay. The prototype of the hybrid cell demonstrated kinetic energy harvesting from bare hand press with the various flow speeds from 0.41 to 1.39 cm s −1 as well as walking, running, and door closing, which are representative examples of low-frequency kinetic motions in daily life. We believe that the simple structure of the hybrid cell will enable power supply to various applications from miniaturized systems (e.g., IoTs and wearables) to large-scale systems (e.g., ocean wave energy harvesting).

The growth of renewable energy generation has necessitated electric energy storage (EES) systems. Batteries are the most promising small-scale EES system, given their portability, quick response time, and high compatibility with electrical devices. We present a thermally assisted K/Zn ion hybrid battery for high roundtrip efficiency. The battery contains a copper hexacyanoferrate (CuHCFe) cathode and a zinc metal anode with a discharging voltage of 1.75 V in a K/Zn ion hybrid aqueous electrolyte. The battery’s roundtrip efficiency improved by 2.1–4.5% when cycled between 10 and 30 °C. Its discharging voltage and temperature coefficient improved dramatically when the K ion was replaced with a Rb ion. The proposed thermally assisted battery cycle can be used with various conventional batteries to reduce EES energy loss during charge/discharge cycles.