Capacitive technology for energy extraction from chemical potential differences

This thesis introduces the principle of Capacitive energy extraction based on Donnan Potential (CDP) to exploit salinity gradients. It also shows the fundamental characterization and improvements of CDP. An alternative application of this technology aimed at thermal gradients was tested. Chapter 2 introduces the principle and initial tests. The entropy increase of mixing two solutions of different salt concentrations can be harnessed to generate electrical energy. Worldwide, the potential of this resource, the controlled mixing of river and seawater, is enormous, but existing conversion technologies are still complex and expensive. Here we present a small-scale device that directly generates electrical power from the sequential flow of fresh and saline water, without the need of auxiliary processes or converters. The device consists of a sandwich of porous “supercapacitor” electrodes, ion-exchange membranes, and a spacer and can be further miniaturized or scaled-out. Our results demonstrate that alternating the flow of saline and fresh water through a capacitive cell allows direct autogeneration of voltage and current and consequently leads to power generation. Theoretical calculations aid in providing directions for further optimization of the properties of membranes and electrodes. In Chapter 3, traditional electrochemical techniques (galvanostatic charge–discharge and cyclic voltammetry) were used to investigate intrinsic properties of this open system. This study demonstrates the feasibility to characterize the capacitive behavior of the cell in low concentration (0.5 M). Presence of membranes, as well as the possibility of having the electrolyte flowing through the cell was investigated. In the studied cell, the presence of membranes showed a limitation by the anion exchange membrane at low current densities but no effect at high current densities. The flow rate did not influence the capacitance of the system either. Chapter 4uses again a stack of eight cells coupled in parallel to investigate the viability of this technology. An average power density of 0.055W/m2was obtained during the peak of the different cycles, though reasonable optimization suggests an expectation of 0.26W/m2at 6.2 A/m2. It was found that 83 ± 8% of the theoretical driving potential was obtained during the operating process. By studying the polarization curves during charging and discharging cycles, it was found that optimizing the feed fluid flow is currently among the most beneficial paths to make CDP a viable salinity difference power source. Another parallel route for increasing the efficiency is lowering the internal ohmic resistances of the cell by design modifications. A modification is proposed in Chapter 5, approaching the electrodes geometry that has a relevant impact on internal resistance and overall performance in CDP. In this work, we present the first effort to use wire-shaped electrodes and its suitability for improving CDP. Analytical evaluation and electrical measurements confirm a strong nonlinear decrease in internal resistance for distances between electrodes smaller than 3 mm. We also demonstrate that we get more power per material invested when compared to traditional flat plate designs. These findings show the advantages of this design for further development of CDP into a mature technology. Alternatively, in Chapter 6, we present a new principle for producing electricity from low temperature differences by using an affordable assembly combining ion exchange membranes and supercapacitor carbon electrodes. Our proposed design involves two isolated salty solutions, with equal concentration but different temperatures. The operation consists of an alternately and cyclic exposure of the electrodes to these electrolytes. This difference in temperature generates a thermomembrane potential that acts as a driving force for ionic adsorption/desorption cycles on the electrodes. Our simple system is interesting for exploiting the potential of low temperature waste heat. When two volumes with equal concentration have different temperatures, it is possible to immerse a pair of electrodes (anode and cathode) into the low temperature one and have ion adsorption. An electric current is then generated in the external circuit to achieve electro neutrality. After saturation, the same electrodes are immersed in the high T volume and then ions desorb from the electrodes and are released to the volume, leading to a reverse electric current in the external circuit compared to the first step. These experiments prove the principle and the direct dependence of the temperature gradient for energy extraction. Finally, Chapter 7discusses the internal energy losses identified and faced throughout this thesis. We summarize the solutions encountered for the major contributions hindering the CDP performance and give suggestions to further develop the technology.