Metal Chemistry in Electrocoagulation and Redox Flow Battery Technologies
Sacrificial iron or aluminum electrodes are used in electrocoagulation water treatment processes. The process involves simultaneous metal dissolution and hydrogen evolution, leading to formation of metal hydroxide coagulants. Electrode fouling / passivation leads to operational challenges, and polarity reversal is typically used to mitigate these detrimental processes. Recent studies on the impact of solution chemistry and operating conditions on the fouling and electrode processes will be presented1,2. Visualization of the pH boundary layer and gas-solid-liquid interactions in the electrode boundary layer (see Figure 1) have provided new insights into the electrocoagulation process.
In redox flow batteries (RFBs), redox active metal species dissolved in liquid electrolytes are used to store energy. Metal solutions are pumped through the battery during charge and discharge. RFBs are being developed and commercialized for energy storage on electricity networks, to integrate intermittent renewable energy and to improve network performance. Widely used metals used in RFB systems include zinc, vanadium, iron and chromium. Performance of RFBs depends upon the metal chemistry, including the redox potential, solubility, electrochemical activity, and speciation. The effect of metal impurities in the electrolyte and approaches being used to develop new RFB systems with improved performance and lower cost will be discussed.
Figure 1. Operando visualization of pH distribution and phases close to an aluminum cathode during electrocoagulation, obtained by laser scanning fluorescence microscopy. In synthetic process water (a), with a low buffering capacity, the high pH at the electrode mitigates aluminum hydroxide fouling, while for field samples with high buffering capacity, the pH at the electrode is lower, leading to precipitation close to the electrode surface and associated fouling.
- Fuladpanjeh-Hojaghan, Elsutohy, Trifkovic, Roberts(2019) In-Operando Mapping of pH Distribution in Electrochemical Processes. Chem. 131, 16971 –16975.
- Chow, Ingelsson, Roberts, Pham (2021) How does periodic polarity reversal affect the faradaic efficiency and electrode fouling during iron electrocoagulation? Water Research 203, 117497.