A novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells

doi:10.1016/j.jpowsour.2013.03.003

Journal of Power Sources

Volume 237, 1 September 2013, Pages 47-51

https://doi.org/10.1016/j.jpowsour.2013.03.003 Get rights and content

Abstract

We build on the success of phosphoric acid as a fuel cell electrolyte, by designing a variant of the molecular acid that provides increased temperature range without sacrifice of high temperature conductivity or open circuit voltage. This is achieved by introduction of a hybrid component, based on silicon coordination of phosphate groups, which prevents decomposition or water loss to 250 °C, while enhancing free proton motion. We report conductivity studies to 285 °C and full H₂/O₂cell polarization curves to 226 °C with careful monitoring of fuel consumption. The current efficiency we report (current density per unit of fuel supplied per sec) is as high as the highest on record. A power density of 184 mW cm⁻² is achieved at 226 °C with hydrogen flow rate of 4.1 ml min⁻¹.

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/i_theoretical, 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)

K.D. Kreuer
J. Membr. Sci.
(2001)
C. Yang et al.
J. Power Sources
(2001)
S.-H. Kwak et al.
Solid State Ionics
(2003)
S.-Y. Lee et al.
J. Power Sources
(2010)
A. Matsuda et al.
Solid State Ionics
(2002)
H. Niida et al.
J. Non-Cryst. Solids
(2002)
C. Lejeune et al.
Solid State Nucl. Magn. Reson.
(2005)
S. Srinivasan et al.
Ann. Rev. Energy Environ.
(1999)
M.-Q. Li et al.
Electrochim. Acta
(2010)
N. Asano et al.
J. Am. Chem. Soc.
(2006)

There are more references available in the full text version of this article.

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Short communicationA novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells

Abstract

Highlights

Introduction

Section snippets

Chemistry

Conductivity

Discussion

Acknowledgments

J. Membr. Sci.

J. Power Sources

Solid State Ionics

J. Power Sources

Solid State Ionics

J. Non-Cryst. Solids

Solid State Nucl. Magn. Reson.

Ann. Rev. Energy Environ.

Electrochim. Acta

J. Am. Chem. Soc.

Short communication
A novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells