Abstract

Due to the irreversible mixing of fresh and salty water an enormous free-energy dissipation of about 2kJ per liter river water takes place in river mouths, equivalent to a waterfall of 200m. This source of energy, which is renewable and emission-free, is currently untapped but could globally account for several percent of the energy demand (and for much more locally in river deltas). Harvesting this so-called ‘blue energy’ has become possible in recent years due to the development of nanostructured devices, either based on ion-selective membranes or on nanoporous solid-state electrodes that can form supercapacitors with internal surfaces of the order of a km2/kg. In this talk we will mostly focus on cyclic charging and discharging processes of these supercapacitors immersed in salty and fresh water, respectively, as constructed by Brogioli and coworkers [1,2]. We will perform a thermodynamic analysis of this cycle, mapping the ion flow and electric voltage-charge work of this ‘blue engine’ onto the heat flow and the mechanical pressure-volume work of Stirling’s heat engine [3]. Our mapping naturally leads to the prediction of the most efficient ‘blue cycle’ as an analogue of the Carnot cycle, which harvests the full 2kJ/liter fully reversibly. Interestingly, running the Carnot-like cycle backward is the basis for the thermodynamically cheapest desalination process, where brackish water is separated into fresh and salty water at the expense of a minimum energy input [3]. Microscopically, on the nanometer scale, these devices are governed by electric double layers at the electrode-electrolyte interface, where ionic packing, hydration, and polarisation affect the voltage-charge relation and the capacitance [4,5], which we will briefly discuss. Finally, we will focus on some recent and ongoing work on maximum power conditions (involving non-equilibrium (dis-)charging processes on the RC-time scale [6]), temperature effects (involving cold sea water and warm fresh water for more efficient blue-energy harvesting using industrial waste-heat [7]), and generalisations to run these devices on other chemical gradients (such as CO2 in clean air and in combustion gases [8]).

[1] D. Brogioli, Phys. Rev. Lett. 103, 058501 (2009).

[2] D. Brogioli, R. Zhao, P.M. Biesheuvel, Energy Environ. Sci. 4, 772 (2011).

[3] N. Boon and R. van Roij, Mol. Phys. 109, 1229 (2011).

[4] M.M. Hatlo, R. van Roij, and L. Lue, Europhys. Lett. 97, 28010 (2012).

[5] R. van Roij, in “Electrostatics of soft and disordered matter”, Pan Stanford Publishing, Singapore, 2012, eds D.S. Dean, J. Dobnikar, A. Naji, and R. Podgornik; cond-mat 1211.1269.

[6] M. Kooiman, master thesis, Utrecht University (2012).

[7] M. Janssen, A. Härtel, and R. van Roij, cond-mat 1405.5830.

[8] H.V.M. Hamelers et al., Environ. Sci. Tech. Lett. 1, 31 (2014).