Electrochemical Energy Generation from Natural and Synthetic Salinity Gradients using Reverse Electrodialysis and Capacitive Mixing

Open Access
Author:
Hatzell, Marta Catherine
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 09, 2014
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Adrianus C Van Duin, Committee Member
  • Chao Yang Wang, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • Salinity Gradient Energy
  • Reverse Electrodialysis
  • Capacitive Mixing
  • Electrochemical Conversion
  • Hydrogen
  • Capacitor
Abstract:
Salinity gradient energy (SGE) technologies are emerging systems designed to recover energy from engineered and natural mixing processes. Two electricity producing SGE systems are reverse electrodialysis (RED) and capacitive mixing (CapMix). RED captures mixing energy using a series of ion exchange membranes that drive electrochemical reactions at redox electrodes. CapMix utilizes polarizable electrodes to store charge in the surfaces electric double layer (EDL). Energy generation can then occur when the EDL is expanded and compressed in different concentration solutions. The use of themolytic salt solutions (e.g. ammonium bicarbonate–AmB) within a RED system is promising, as AmB can be regenerated using low-grade waste–heat (e.g. 40-60oC). One disadvantage to using AmB is the potential for gas bubbles (CO2, NH3) to form within the stack. Accumulation of bubbles can impede ion migration, and reduce system performance. The management and minimization of gaseous bubbles in RED flow fields is an important operational issue, and has not previously been addressed within RED literature. Flow field design with and without spacers in a RED stack was analyzed to determine how fluid flow and geometry effected the accumulation and removal of bubbles. In addition, the performance changes, in terms of power and resistance were measured in the presence of bubbles. Gaseous bubble accumulation was minimized using short vertically aligned channels, which resulted in a reduction in the amount of the membrane area which was restricted due to bubbles from ~20% to 7%. The stack power density improved by 12% when all gaseous bubbles were removed from the cell. AmB-RED systems can potentially produce hydrogen or electrical energy through altering the cathodic reaction. With a kinetically favorable cathodic reaction (oxygen reduction reaction), the projected electrical energy generated by a single pass AmB–RED system approached 78 Wh per m–3 (low concentrate). However, when RED was operated with the less kinetically favorable reaction (hydrogen evolution reaction), and hydrogen gas was harvested, the energy recovered increased by as much ~1.5 times to 118 Wh m–3 (low concentrate). Indirect hydrogen production through coupling an RED stack with an external electrolysis system was only projected to achieve 35 Wh m–3 (low concentrate) or a third of that produced through direct hydrogen generation. The flexibility of the RED architecture allows for the potential for simultaneous hydrogen and electricity production, whereas competing technologies such as PRO and CapMix only produce electricity. Several approaches to generate electrical power using CapMix have recently been developed, but power densities have remained low. By immersing the capacitive electrodes in ionic fields generated by exoelectrogenic microorganisms in bioelectrochemical reactors, it was shown that energy capture using synthetic river and seawater could be increased ~65 times, and power generation ~46 times, when compared to controls (no ionic fields). Favorable electrochemical reactions due to microbial oxidation of organic matter, coupled to oxygen reduction at the cathode, created this ionic flow field that enabled more effective passive charging of the capacitive electrodes, and higher energy capture. This ionic-based approach is not limited to the use of river water-seawater solutions. Forced charging of the capacitive electrodes, using energy generated by the bioelectrochemical system and a thermolytic solution, further increased the maximum power density to 7 W m–2 (capacitive electrode). The amount of salinity gradient energy that can be obtained through capacitive–mixing based on double layer expansion (CDLE) also depends on the extent that the materials electric double layer (EDL) expands in a low concentration electrolyte (e.g. river water). I show here that the individual electrode rise potential, which is a measure of the EDL expansion process, significantly (P = 10–5) depends on the concentration of strong acid surface functional groups. Electrodes with a low concentration of strong acid functional groups (0.05 mmol g–1) resulted in a positive–potential–rise of dU+/– = +59 ± 4 mV (dUcell = 16 ± 0.7 mV) in synthetic river water, whereas activated carbons with high concentrations of strong acid groups (0.36 mmol g–1) produced a negative-potential-rise of dU+/– = –31 ± 5 mV (dUcell ~–11 ± 1 mV). Dissimilar electrodes, which coupled a negative electrode with a high concentration of strong acid groups with positive electrode with a low concentrations of strong acid groups, produced a whole cell potential rise which was 5.7 times greater than produced with similar electrodes (from 15 ± 0.2 to 89 ± 3 mV). Therefore, tuning the surface chemistry of known materials can be conducted through a variety of methods (oxidation, ammonia treatment, etc.) to more optimally extract energy through CapMix processes.