Desalination Fuel Cells: Producing Clean Energy and Water

Abstract

Desalination has evolved into a viable alternative to fresh water supply, increasing water availability and decreasing scarcity1. Reverse osmosis (RO) is the most-widely used technology today for desalination, and requires significant electrical energy investment, about 4 kWh/m3 of treated water, when desalinating sea water2. In contrast to such conventional desalination systems which utilize energy, we will here dicsuss desalination fuel cells (DFCs), an emerging electrochemical desalination technology proposed by our group3. DFC’s utilize hydrogen gas to simultaneously desalinate water and produce electricity from a single cell. Thus, water can be desalinated without any external electrical supply required. The desalination fuel cell is based on continuous energy conversion from chemical to electrical, and thus is not cyclic as with capacitive deionization4.As with an ED cell, our cell consists of one anion and one cation exchange membrane which sandwich a desalination channel fed with feedwater. Unlike an ED cell, on the opposite side of the anion exchange membrane is a hydrogen anode and anolyte, while an oxygen cathode and catholyte are placed opposite to the CEM. During operation, the reductant present in the anolyte (hydrogen) and oxidant present in the catholyte (oxygen) react spontaneously at the anode and cathode surfaces, respectively, providing an electric current between the anode and cathode which can be delivered to a load. The half-reactions also give rise to a spontaneous ionic current through the cell, which drives ion removal from the desalination channel (Figures a,b).The cell was characterized by running it in two modes, with either near-neutral pH in all channels (H2|O2) (Figure a) or with a pH-gradient mode (H2+B|O2+A) (Figure b), which allowed for deep insight into cell performance and detailed characterizations (Figures c-f)5. The results show that our prototype can desalinate water effectively while generating electricity, it was also found that operation in H2+B|O2+A mode enabled improved DFC performance, higher OCV, and produced electricity of up to 10 kWh/m3 (Figure g)5. A detailed voltage breakdown, elucidating key sources of loss in the cell was also demonstrated adding quasi-reference electrodes in all flow channels of the cell. It was shown that voltage loss across ion exchange membranes was generally insignificant, but the cathode is generally the component associated with the largest voltage loss, largely due to Nernstian losses exacerbated by likely chloride poisoning of the cathode catalyst (Figure i)6. Chloride poisoning was studied in-situ, by flowing different catholytes through the cell, and ex-situ using an RRDE. We further synthesized and optimized custom, non-precious metal-based Fe/N/C catalyst for desalination fuel cell cathodes, and showed nearly equal catalytic performance to that of the Pt/C commercial cathode (Figure h)7.