No abstract

Clean technologies, which utilize or generate clean energy rather than fossil fuel-based energy, are under intense development to aid in addressing climate change. Current water desalination technologies are a growing user of fossil fuelderived electricity. A recently developed technology, termed the desalination fuel cell (DFC), can address this issue by instead using hydrogen gas to drive both feedwater desalination and green electricity generation simultaneously in a single cell. The main bottleneck is the use of Pt-based catalysts, which leads to high device costs and catalyst surface poisoning due to chloride ions (Cl − ) present in the feedwater. We here propose and demonstrate the first use of non-platinum group metal (non-PGM) catalysts toward DFCs. We synthesized a Fe/N/C based catalyst which demonstrated effective and Cl − tolerant oxygen reduction reaction ex situ and while used as a DFC cathode. The synthesis temperature and the metal concentrations were optimized using rotating disk electrode measurements, with an onset potential of up to 0.84 V vs RHE, on par with that of commercial Pt/C catalysts in a Cl − environment. When using the optimized Fe/N/C catalyst as a cathode in a prototype DFC, open circuit voltage was significantly improved relative to Pt/C, and measured cell voltage and desalination performance versus current density were nearly equivalent. Overall, these results show that non-PGM catalysts maintain or improve cell performance while significantly reducing cell costs, improving greatly the outlook for this nascent technology.

The desalination fuel cell (DFC) is a nascent technology for co-production of clean electricity and water from a single cell, driven by the hydrogen-oxygen redox couple. Previously, DFCs employed a single cation and anion exchange membrane pair, and demonstrated desalination of feedwaters of 30 g/L NaCl while generating up to 8.6 kWh per m3 of desalinated water. Here, we propose and demonstrate a scaling strategy for DFCs, by adding additional alternating anion and cation exchange membrane pairs. Such a strategy is supported by previous measurements showing small potential loss across membranes in DFCs relative to that at the electrodes. We characterize and compare three cell configurations, including single, double or triple membrane pairs. We show that adding membrane pairs maintains the desalination performance of our DFC, while only slightly reducing cell power density. In addition, adding membrane pairs enables increased flexibility in the choice of anolyte and catholyte solutions, as these are no longer the primary brine channels. We show that optimizing the anolyte and catholyte enables a record power density of up to 10 mW/cm2 during desalination. We further develop and implement expressions for quantification of the thermodynamic energy efficiency of a multiple membrane pair DFC.

We will present a nascent technology which desalinates water and produces net electricity simultaneously from a single electrochemical cell, driven by the hydrogen/oxygen redox couple [1]. The cell combines hardware of PEM fuel cells, alkaline fuel cells and electrodialysis cells, and thus we term this device a "desalination fuel cell" [2]. We will describe both the operating principle and lab-scale cell results, as well as our development of the fundamental thermodynamics to predict the maximum available electricity production from our cell during its combined chemical reaction-separation process [2]. Our recent advances will be described, including the development of chloride-tolerant non-platinum group metal ORR catalysts [3], achievement of >95% thermodynamic energy efficiency [4], and establishment of system scaling rules. This technology promises to extend the concept of the hydrogen economy to water purification, and we will discuss the outlook on this technology and potential application areas. References: [1] Suss ME, Zhang Y, Atlas I, Gendel Y, Ruck EB, Presser V. Emerging, hydrogen-driven electrochemical water purification. Electrochemistry Communications. 2022 [2] Atlas I, Khalla SA, Suss ME. Thermodynamic energy efficiency of electrochemical systems performing simultaneous water desalination and electricity generation. Journal of The Electrochemical Society. 2020. [3] Asokan A, Abu-Khalla S, Abdalla S, Suss ME. Chloride-Tolerant, Inexpensive Fe/N/C Catalysts for Desalination Fuel Cell Cathodes. ACS Applied Energy Materials. 2022. [4] Abu Khalla S, Atlas I, Litster S, Suss ME. Desalination Fuel Cells with High Thermodynamic Energy Efficiency. Environmental science & technology. 2021. Figure 1: Schematic of a desalination fuel cell, which utilizes chemical energy to desalinate water and produce electricity simultaneously. The cell is driven by the hydrogen-oxygen redox couple. Figure 1

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. References: Kummu, M. et al. The world’s road to water scarcity: Shortage and stress in the 20th century and pathways towards sustainability. Rep. 6, 1–16 (2016). Malaeb, L. & Ayoub, G. M. Reverse osmosis technology for water treatment: State of the art review. Desalination 267, 1–8 (2011). Atlas, I., Abu Khalla, S. & Suss, M. E. Thermodynamic Energy Efficiency of Electrochemical Systems Performing Simultaneous Water Desalination and Electricity Generation. Electrochem. Soc. 167, 134517 (2020). Porada, S., Zhao, R., Van Der Wal, A., Presser, V. & Biesheuvel, P. M. Review on the science and technology of water desalination by capacitive deionization. Mater. Sci. 58, 1388–1442 (2013). Abu Khalla, S., Atlas, I. & Suss, M. E. Desalination fuel cells with high thermodynamic energy efficiency. Environmental Science & Technology. Accepted. Abdalla, S., Abu Khalla, S. & Suss, M. E. Voltage loss breakdown in desalination fuel cells. Electrochemistry Communications 107136 (2021). Asokan, A., Abu-Khalla, S., Abdalla, S. & Suss., M. E. Chloride-tolerant, inexpensive Fe/N/C catalysts exceed platinum catalysts for desalination fuel cell cathodes. ACS Applied Energy Materials. Submitted. Figure 1