This study proposes a thermodynamic machine that operates between acid and basic reservoirs in four stages. Two of these stages are buffered isothermal steps. The other two stages constitute an open system and allow the passage of acid and base. The machine consists of a neutralization pseudocapacitor that, after a full cycle, carries out work generated from partial change in entropy associated with a change in the hydrogen potential after the neutralization process. Thermodynamic formalism is presented under reversible stages. This presentation enables determination of the maximum efficiency, related to the difference between the hydrogen potential of the acid reservoir and of the resulting solution after neutralization in the machine. Hence, the hydrogen potential scale can be defined as a function of the efficiency of the reversible acid−base machine regardless of the electrochemical cell composition. Electroactive thin films formed from phosphomolybdic acid and poly(3,4ethylenedioxythiophene) have been investigated as proof of concept in electrolytic solutions at several pH values; their efficiency was close to the efficiency predicted by the thermodynamic approach. Therefore, this model allows one to estimate the maximum energy harvesting of neutralization pseudocapacitors and financial return for the treatment of acid wastewater, contributing to sustainable growth.

We have developed an electrochemical system that performs electrical work due to changes in alkaline ion and proton activities associated with acidic solution neutralization. This system can be used to treat wastewater, contributing to sustainable growth. The system includes an electrochemical machine that operates between an acidic and a basic reservoir to produce work in cycles comprising four stages: two isothermal ionic insertion/de-insertion steps and two steps involving acid and base injection. On the basis of the mixing free energy associated with the reaction free energy, we have developed the thermodynamic formalism by considering reversible electrochemical processes to determine the maximum work performed by this acid-base machine and the efficiency. Electrochemical methods in the time and frequency domains helped in investigating the kinetics of sodium ions and proton insertion in host matrices consisting of copper hexacyanoferrate and phosphomolybdic acid, respectively, to improve our understanding of the factors underlying dissipation as a function of pH and pNa. The full cell composed of these insertion electrodes was used as a proof of concept. It performed a maximum work of 26.4 kJ per mol of electro-inserted ion from HCl solution neutralization with the addition of NaOH, to simulate acidic wastewater treatment in a profitable and sustainable way.

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