With continued growth of marine exploration and underwater missions, there is a need to design new diving technologies. Currently, dive time is limited by scuba tank weight and volume. The use of an electrolysis system, powered by a battery, for driving chemical reactions could be introduced to produce a steady supply of oxygen during dives. However, batteries have a limited capacity, are heavy, and are detrimental to the environment when disposed. Developing an electrochemical technology that utilizes resources abundantly available to a diver (salt water) to drive the oxygen evolution reaction would expand the duration and sustainability of ocean dives. Reverse electrodialysis (RED) is a multi-membrane system that aims to convert salinity gradient energy into electrical energy. Redox mediators are typically used to convert ionic mixing into electricity at electrodes placed adjacent to a stack of membranes. However, replacing redox mediators with water splitting electrodes can enable the sustainable production of fuel (H₂) and/or oxygen (O₂). We aim to examine if a RED oxygen generation system could meet oxygen requirements for scuba diving. Evolving oxygen from water would enable the development of artificial gills, allowing for an inexhaustible supply of oxygen thus displacing the need to carry oxygen tanks. Here we propose a system where instead of an air tank, a diver carries a RED cell, a 2 L volume of dilute solution, and a cylinder of other compressed gases for breathing (Fig. 1a). A RED cell is comprised of a stack of alternating anion and cation exchange membranes separated by channels of high and low concentration solutions (HC and LC) (Fig. 1b). A stack is made up of repeating units, which consist of a pair of selective membranes and a pair of solution channels, between the anode and cathode chambers. Salt ions in solution migrate due to the concentration gradient and two redox based electrodes convert this ionic flux into electric flux. We show here that the oxygen evolution activation overpotentials are a significant fraction of internal resistance (77%) in a RED system with a small number of cell pairs (N=5). However, this resistance is nearly negligible (3.2%) as the number of cell pairs increases (N=500). We further compare the RED systems to four different battery-electrolysis systems (Li-ion, Ni-MH, Ni-Cd, and lead acid) to contextualize the performance of the RED oxygen generation system with competing technologies. For large (N=100+) systems and long (10 hr) dives, RED is comparable in size to a battery-electrolysis system. With 500 membrane pairs, a RED powered diving system is 10.8 L and weighs 29 kg. Typical 12 L scuba tanks weigh 16 kg, and the size of a Li-ion battery powered electrolysis system for a 1 hour dive is, 2.2 L and 3.5 kg (19.2 L and 32.6 kg for a 10 hour dive).

Figure 1. (a) Configuration of a RED cell for scuba divers using the surrounding seawater, a fixed dilute volume, and a fixed volume of additional gases for breathing carried by the diver. (b) Schematic of RED cell. Oxygen and hydrogen are produced at the anode and cathode by redox reactions.