Short communicationA novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells
Highlights
► A phosphoric acid-based fuel cell liquid electrolyte for T > 200 °C without pressure. ► An unhumidified electrolyte of H3PO4, saturated with a new silicophosphoric acid. ► Polarization curves at 226 °C with OCV >0.95 V, and current densities to 1.0 A cm−2. ► Fuel cell of exceptional current efficiency – burns almost all the available fuel.
Introduction
The phosphoric acid fuel cell is known as one of most researched, and commercially most advanced types of fuel cell [1], [2]. Initially developed as a liquid electrolyte device, prone to leaking and ionic shorts, it is now being refined, by incorporation of H3PO4 in polymeric materials [3] such as polybenzimidazole, into a very promising and cost-competitive alternative [4] to the Nafion fuel cell [5], [6]. An advantage it holds over its Nafion rival is the higher temperature range of operation permitted by the electrolyte. It can be used up to 160 °C before water loss, to form pyrophosphoric acid, leads to reduced conductivity and poorer performance. The range may be extended by elaborate humidification and pressurization provisions, which of course also increase the expense and failure-probability of the system. The water management problem remains a barrier to the full commercialization of this type of fuel cell. Any modification of the electrolyte that could extend its operating temperature range and reduce its dependence on the presence of some water, without reducing its other favorable characteristics would be of great interest.
Here we describe an inexpensive, easily synthesized, modification of H3PO4 (presumed to be a binary solution with a new more complex acid) that extends the temperature range of the phosphoric acid fuel cell to 250 °C, while improving its operating fuel efficiency considerably. To our knowledge, the current efficiency (= i/itheoretical, i = current) obtained with the electrolyte we will describe, is higher than that of any other type of fuel cell operating at 1 atm. pressure. Its voltage efficiency is also comparable to (somewhat better than) that of the reported phosphoric acid-based fuel cells. We describe only the first stage of this development, viz., the liquid electrolyte and its fuel cell performance, leaving the important second stage – development of a membrane based on proper incorporation of the liquid into a robust polymer host – to future reports.
Section snippets
Chemistry
The additive is a derivative of phosphoric acid in which phosphoric acid is combined with silicon to form a silicophosphoric acid which probably has multiple molecular forms – yet to be fully resolved. Initially it was thought to be a substantial component of the electrolyte, but later work suggests that is a minor but temperature-dependent component that stabilizes the phosphoric acid against loss of more than a small amount of water. Its possible molecular form will be discussed briefly
Conductivity
The presence of SiPOH and an equilibrium content of H4P2O7 in the phosphoric acid, leads to an increase in electrolyte viscosity. Nevertheless, above 150 °C the electrolyte conductivity, determined using a twin electrode dip-type cell of cell constant 1.83 cm−1, is superior to that of pure H3PO4 as shown in Fig. 1. This is presumably due to a superior “free” proton contribution at high temperatures. The conductivity is reversible up to 285 °C. Tested at a constant 250 °C, the conductivity remained
Discussion
Phosphorus–silicon–oxygen combinations incorporating H3PO4 have been used previously to produce proton-conducting electrolytes for fuel cell applications. Matsuda et al. [17] have described phosphosilicate gel powders and H3PO4-impregnated porous silica gel powders that have moderately high conductivity and stability at elevated temperatures, but they are made by sol-gel processes and must have chemical constitutions that are rather different from the anhydrous systems of the present work. A
Acknowledgments
We appreciate the support of this research by the DOD Army Research Office under grant no. W911NF0710423. We are grateful to Dr. Wei Huang for preliminary NMR studies that will become the subject of a more detailed future paper on the structures of the subject materials.
References (21)
J. Membr. Sci.
(2001)- et al.
J. Power Sources
(2001) - et al.
Solid State Ionics
(2003) - et al.
J. Power Sources
(2010) - et al.
Solid State Ionics
(2002) - et al.
J. Non-Cryst. Solids
(2002) - et al.
Solid State Nucl. Magn. Reson.
(2005) - et al.
Ann. Rev. Energy Environ.
(1999) - et al.
Electrochim. Acta
(2010) - et al.
J. Am. Chem. Soc.
(2006)
Cited by (18)
A study of double functions and load matching of a phosphoric acid fuel cell/heat-driven refrigerator hybrid system
2016, EnergyCitation Excerpt :Phosphoric acid fuel cells (PAFCs) operating at moderate temperatures [1–6] have been considered as one of the most advanced technologies because they have simple construction and no special requirements for the high temperature properties of materials.
A flexible all-inorganic fuel cell membrane with conductivity above Nafion, and durable operation at 150 °c
2016, Journal of Power SourcesCitation Excerpt :In the present contribution we provide conductivity data for this new material and, more importantly, show how the new material can serve as the membrane in simple H2/O2 fuel cells that can produce stable currents in excess of one amp/cm2 at temperatures up to at least 150 °C [24]. When the stable milky suspension that is the liquid electrolyte of ref. [23], is centrifuged for a long period, a thick paste, described as SiPOH, from which most of the unaltered phosphoric acid has been separated, is obtained. The exact composition of SiPOH is still unknown.
Maximum power output and load matching of a phosphoric acid fuel cell-thermoelectric generator hybrid system
2015, Journal of Power SourcesCitation Excerpt :Among various fuel cells, the phosphoric acid fuel cell (PAFC) has been regarded as one of the most advanced technologies [1–9] because it has the relatively low operation temperature and simple construction.
Speciation and Proton Conductivity of Phosphoric Acid Confined in Mesoporous Silica
2022, ACS Applied Materials and InterfacesInvestigating the role of GeO<inf>2</inf>in enhancing the thermal stability and proton mobility of proton-conducting phosphate glasses
2021, Journal of Materials Chemistry A