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This invention encompasses a charging-free electrochemical cell for harvesting low-grade heat. The cell consists of a Fe(CN)63-/4- positive electrode and a solid Prussian blue nanoparticle negative electrode. The cell is first discharged at room temperature. When the entire cell is heated, the voltage decreases to less than zero due to the negative thermogalvanic coefficient of the full cell. In the subsequent reverse process at high temperature, which is spontaneous, the cell is discharged again as the absolute value of voltage decreases. Thus, the two discharge processes at different temperatures, fueled by low grade heat, can be cycled without charging and can provide an alternating current to a load.