Fuel cells for selectively reacting a feedstock material with or without generating electricity, and associated systems and methods are disclosed. A fuel cell system in accordance with a particular embodiment includes a first electrode positioned in a first region, a second electrode positioned in a second region, an ion-transport medium between the first and second regions, and an electrical circuit connected between the first and second electrodes. The system is operable in a first mode to react the feedstock material by a non-electricity-generating reaction to produce a product and in a second mode to react the feedstock material by an electricity-generating reaction to produce electricity. A controller receives an input (e.g., corresponding to a change in demand for electricity) and causes the system to switch between operating in the first mode and operating in the second mode in response to the input.