A concentration cell(1-3) is composed of two equivalent half-cells made of the same material but differing in the concentration of reactants. As these concentrations equilibrate, the increase in entropy is converted into a flow of electricity with the voltage output determined by the Nernst equation and proportional to the logarithm of the concentration ratios. However, as diffusion constantly strives to erase all concentration gradients(4-6), concentration cells produce only moderate voltages (typically tens of millivolts at room temperature(7,8)) over relatively short times(9) and, consequently, such devices have not been regarded as promising for energy storage(10-12). Here, we report a concentration cell that produces significantly higher voltages (similar to 0.5 V) for over 100 h. The key to our design is that the citric acid molecules involved in the electrode reactions are tethered onto magnetic nanoparticles, and a sharp gradient (10(7)-10(11) anode/cathode concentration ratio) is maintained at one of the electrodes by a permanent magnet external to the cell. Our cell does not result in corrosion of the electrodes, produces no harmful by-products, and can be regenerated by recoating used nanoparticles with fresh citric acid. We show that a series of such centimetre-sized cells produces enough electricity to power small electronic devices (timers and calculators) for several tens of hours. Our results illustrate how redox-active molecules that are, in themselves, non-magnetic can be effectively concentrated by magnetic fields to produce electrical energy(13-15).