Thermodynamic Energy Efficiency of Electrochemical Systems Performing Simultaneous Water Desalination and Electricity Generation

Abstract

An emerging class of electrochemical systems utilize redox-active chemicals as input to simultaneously desalinate water and produce electricity within a single cell. This contrasts with traditional desalination technologies, such as reverse osmosis, electrodialysis and capacitive deionization, which consume net electricity during desalination. The underlying reason is that traditional desalination technologies perform solely a separation process, while such chemical-energy driven systems perform a combined process consisting of a spontaneous chemical reaction and a separation. Thermodynamic energy efficiency (TEE) of traditional technologies is defined as the ratio of the minimum energy to drive the separation process reversibly to the energy needed to perform the separation in practice. However, such a definition is not appropriate for systems which co-generate electricity and desalinated water. We propose that for these latter systems, TEE should be defined as the device electricity output divided by the maximum available energy. We develop a theoretical framework predicting the maximum available energy yielded by a combined redox reaction-desalination process. We utilize our framework to explore various input redox chemistries, and predict a maximum energy output of up to ∼25.74 kWh per m3 of desalinated seawater. We further introduce and experimentally characterize a desalination fuel cell driven by a hydrogen-oxygen redox couple, and apply our model to provide a first-time quantification of its TEE.