For many applications, the nodes in a wireless sensor network must be cubic-millimeter sized and have a long lifetime. The amount of energy that can be stored in a sensor node is determined by its size. A cubic-millimeter-sized node with a battery as the sole power source has a severely limited lifetime. Alternatively, energy harvesting devices may be used to recharge the battery or to directly power the sensor and communication components, thus allowing for small nodes with unlimited lifetime. Small-scale energy harvesting devices based on thermoelectric, vibration, and radiofrequency power conversion have been considered for this purpose. An alternative type of thermal energy harvesting, based on thermogalvanic cells, accomplishes both energy harvesting and energy storage in the same device. This multi-functionality is an important space-saving advantage because it eliminates the need for an interface between the energy harvester and the battery. A thermogalvanic cell (or non-isothermal cell) is an electrochemical cell in which the two electrodes are at different temperatures. In symmetric thermogalvanic cells (those with compositionally identical electrodes), the temperature gradient produces a proportional voltage output. The voltage per degree temperature difference in thermogalvanic cells is typically ~1 mV/K or higher, which is four to five times higher than that of the best thermoelectric materials. Unlike the thermoelectric Seebeck effect, thermogalvanic voltage is closely related to the partial molar entropy of the electrode reaction and the thermal diffusion potential of the electrolyte. A thermogalvanic cell with symmetric, single-phase intercalation electrodes undergoes a charge-discharge cycle when supplied with oscillating heat flow. Partial-molar entropies and thermogalvanic measurements for a range of electrode-electrolyte combinations will be presented in order to identify materials that provide the best performance in such cells.