Organic Molecules for High Performance Aqueous Redox Flow Battery and Electrochemically-Mediated Carbon Capture

Abstract

CO2 emission primarily from fossil fuel combustion is causing climate change at an alarming rate. Consequently, there are increasing efforts worldwide to reduce societal reliance on fossil fuel-based energy and to switch to virtually emission-free power sources such as nuclear, solar, wind and geothermal. In addition, given that the global rate of transition to low-carbon energies is presently too slow to avoid the 2 degree Celsius global warming threshold, carbon capture approaches are urgently required to deal with the problem of rising air CO2 level.

Despite the rapid decrease in the price of renewable energy sources such as wind and solar, the intermittent nature limits their integration in to the grid unless proper energy storage system is available. Aqueous organic redox flow batteries (AORFB), which enables decoupled energy and power, may offer the solution to the problem. Excess power from renewable sources can charge AORFB by driving the electrons uphill from posolyte to negolyte; the reverse downhill reaction will release energy and powers the grid when demand is high. The use of organic molecules enables easy structural modification that gives high cell voltage, long lifetime and high capacity.

Chapter 1 presents a highly stable phosphonate-functionalized viologen (BPP-Vi) as the redox-active material in a negolyte for AORFB operating at nearly neutral pH. The negative charges of the phosphonate substiuents enable low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery with 1 M BPP-Vi negolyte demonstrates an extremely low capacity fade rate of 0.016% per day as well as increased cell voltage and power density compared to previous viologen negolytes. The chapter also explores the adverse effect of overcharging on viologen decomposition.

Chapter 2 introduces a water-miscible anthraquinone with polyethylene glycol (PEG)-based solubilizing groups for AORFB negolyte. A series of PEG-substituted anthraquinones (PEGAQs) are carefully screened and one of its isomers, AQ-1,8-3E-OH, which has high electrochemical reversibility and is completely miscible in water of any pH, is chosen for cell level studies. A negolyte containing 1.5 M AQ-1,8-3E-OH, when paired with a ferrocyanide-based positive electrolyte across an inexpensive, non-fluorinated permselective polymer membrane at pH 7, exhibits an open-circuit potential of 1.0 V, a high volumetric capacity of 80.4 Ah/L, and an energy density of 25.2 Wh/L.

The use of organic molecules in a flow cell also brings new opportunities in electrochemically mediated carbon capture. Compared to traditional temperature- and pressure-swing based carbon capture methods, electrochemically mediated methods can exploit the cheap intermittent renewable electricity and perform at mild operating conditions of ambient temperature and pressure. Chapter 3 describes a thermodynamic analysis of the energetic cost of CO2 separation from flue gas (0.1 bar CO2(g)) and air (400 ppm CO2) using a pH swing created by electrochemical redox reactions involving proton-coupled electron transfer from molecular species in aqueous electrolyte-based flow cell. In this scheme, electrochemical reduction of these molecules results in the formation of alkaline solution, into which CO2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in the acidification of the solution, triggering the release of pure CO2 gas. A sulfonated phenazine, DSPZ, is chosen as the proton carrier for the experimental cycles. The ideal cycle work of 34 kJ/molCO2is compared with the experimental cycle work of 48.9 kJ/molCO2, obtained at an equal inlet and exit CO2 partial pressure of 0.47 bar at the low-current-density limit.

Chapter 4 continues to explore the pH-swing based carbon capture in a flow cell with DSPZ as the proton carrier. The measured electrical work of separating CO2 from a binary mixture with N2, at CO2 inlet partial pressures ranging from 0.1 to 0.5 bar, and releasing to a pure CO2 exit stream at 1.0 bar, is measured for electrical current densities of 20 to 150 mA/cm2. The work for separating CO2 from a 0.1 bar inlet and concentrating into a 1 bar exit is 61.3 kJ/molCO2 at a current density of 20 mA/cm2. Depending on the initial composition of the electrolyte, the molar cycle work for capture from 0.4 mbar extrapolates to 121-237 kJ/molCO2 at 20 mA/cm. Recognizing the oxygen-sensitivity of reduced DSPZ and many other redox molecules, this chapter also introduces an electrochemical rebalancing method that extends cell lifetime by recovering the initial electrolyte composition after it is perturbed by side reactions. The implications of these results for future low-energy electrochemical carbon capture devices are discussed.