Decoupling Capacitance from Desalination Performance in Polyaniline-Modified Carbon Electrodes with Different Porosity for Capacitive Deionization

Carbon electrodes for capacitive deionization (CDI) are usually ranked by capacitance measured under conditions different from those of desalination. Six electrodes—a microporous activated carbon from waste coffee grounds, a micro-mesoporous templated carbon from magnesium citrate, their 1:1 mixture, and each with 5 wt% polyaniline (PANI)—were characterized electrochemically in 1 M H2SO4 and in the NaCl feed at the CDI cell voltage, and tested in batch-mode CDI at +1.6 V. For the three pristine electrodes, capacitance ranked the initial salt adsorption capacity (SAC) correctly in both electrolytes and by every technique, and their 1:1 mixture showed no synergy under the conditions tested. PANI raised the capacitance by 16–18% in acid, but in the feed its contribution varied between the carbons, was partly irreversible and did not follow the SAC, which fell for the coffee-waste carbon and rose for the templated one. No electrochemical measurement anticipated durability: all electrodes retained 95–100% of their capacitance over 10,000 cycles in acid, whereas the pristine ones lost most of their SAC within 100 CDI cycles, and PANI raised the mean SAC retention of the templated carbon from 12% to 76%. Capacitance is thus a useful first screen for purely capacitive carbons, but not for electrodes with a pseudocapacitive component or for predicting service life.