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Chapter 6. Electrochemical Impedance of Single Flow Cells Full cell EIS studies were carried out in a capacity-unbalanced configuration (see Figure 3.7 (b)). The cell hardware, electrodes, and membrane were assembled as described for the cell cyc- ling experiments in Chapter 5. In addition to the two types of carbon papers used for cell cycling experiments, EIS studies were conducted with 1186HCBA carbon cloth electrodes (see Table A.3 in Appendix A for the supplier information). Impedance spectra on both full and symmetric cells were recorded either on a Gamry Reference 3000 potentiostat, a BioLogic VSP-300 multichannel potentiostat, or a BioLogic SP-300 potentiostat. Un- less otherwise stated, impedance spectra were recorded using potentiostatic EIS (PEIS) at OCV with an amplitude of 10mV and 12 points per decade. The frequency range is given specifically for each experiment. Furthermore, and unless otherwise stated, the inductance was subtracted from the presented impedance spectra, as it is an instrumental artefact not related to the electrochemistry of the investigated systems. Electrolyte solutions were prepared as described in Section 5.1 and were purged with argon for 30 min before use. For experiments with ferri-/ferrocyanide, the 50/50 % mixtures were obtained by mixing equal molar amounts of the two potassium salts. The studied quinones were only commercially available in the oxidised state, so the 50/50 % mixtures were prepared by fully charging a solution of known volume and concentration against an excess of ferrocyanide and then mixing the charged quinone solution with an equal volume of uncharged solution. 6.1.1 Modifications to the Setup The setup used for EIS studies went through several iterations of modifications, but always consisted of the cell hardware, one or two electrolyte containers, a pump, tubing, and a tray in which the setup was placed to contain possible leaks. The first major modification was introducing pulsation dampeners between the pump and the cell inlets. Recording high-quality impedance at low frequencies requires the system to be highly stable, which also applies to the flow of electrolyte through the cell. The pulsation dampeners were thus introduced to smoothen the pulses intrinsically generated by the used peristaltic pump. For most of the presented experiments, it consisted of a single 100 mL glass bottle fitted with a 4-port HPLC cap. The tight fit creates an overpressure in the bottle when electrolyte is pumped into the bottle, which forces the liquid out with a steady flow. For the remainder of the experiments, a KNF FPD 06 flow pulsation dampener was placed between each of the cell inlets and the pump. The second major modification was introducing temperature control of the cell assembly. It was observed during several experiments conducted at room temperature that the series resistance fluctuated over the course of a day, which led to the hypothesis that fluctuations in temperature was the cause. The cell hardware was heated by inserting a heating cartridge into each end plate and the temperature was monitored by a K-type thermocouple inserted into one of the end plates. During the stay at Harvard University, a CSC32 benchtop controller from Omega Engineering was used to control the voltage of the heating cartridges and close the feedback loop with the thermocouple. While working at DTU, the voltage to the heating cartridges was provided by a BK Precision 1687B DC power supply, whereas the thermocouple feedback loop was closed by a home-built PID control unit. 88PDF Image | Organic Redox Flow Batteries 2023
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