Journal of Power Sources 159 (2006) 399–404 Short communication Organic esters of phosphoric acid as electrolytes for a protonic photoelectrochromic window Maciej Siekierski a,∗ , Jacek Lipkowski b , Marcin Ciosek a , Anna Lasota b , Ewa Zygadlo-Monikowska b , Wladyslaw Wieczorek a , Zbigniew Florja´ nczyk b a Warsaw University of Technology, Faculty of Chemistry, Polymer Ionics Research Group ul. Noakowskiego 3, 00-664 Warsaw, Poland b Warsaw University of Technology, Faculty of Chemistry, Department of Polymer Chemistry and Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland Available online 27 April 2006 Abstract The paper deals with phosphoric acid organic esters such as commercially available diphenylphosphate and custom made cyclic phosphoric ester of oligo(propylene oxide). The N,N-dimethylformamide and propylene carbonate solutions of these compounds were tested for potential application as electrolytes in a protonic electrochromic cell with tungsten trioxide as an active compound. As a reference, a solution of phosphoric acid of identical concentration was used. Cyclic chronovoltamperometry studies were used to test both the stability window of the electrolytes on a platinum electrode and the stability of indium tin oxide and tungsten trioxide electrodes against the electrolyte solution. The proton intercalation process was also studied. The experiments were performed both for dry solutions (vacuum drying of compounds) and for ones, which were on purpose contaminated with water. Additionally, the dc conductivity of the studied systems was measured for the whole possible operational temperature range. Generally, it was observed that the ester solutions give a better photochromic response of the WO 3 electrode but are more aggressive to its surface. This observation is valid especially for the water-contaminated systems. © 2006 Elsevier B.V. All rights reserved. Keywords: Electrochromic cell; Cyclic voltammometry; Phosphoorganic esters; Electrochemical stabilty 1. Introduction The photoelectrochromic phenomenon is widely known for transition metal oxides (WO 3 , MoO 3 , NiO 2 ) for a long period of time [1,2]. The optical properties are modulated when ions are inserted into (intercalation) or extracted from (deinterca- lation) an oxide film upon the application of a voltage of the order of few volts between the outer electrodes. Anhydrous sys- tems based on lithium ion intercalation have been widely tested [3,4] and introduced into practical applications (auto-dimming rear windows for cars, smart windows [5]). To overcome the disadvantages of lithium electrolytes, a protonic system can be used [2]. The proton mobility in the electrolyte is much higher in comparison with that of the lithium cation. Addi- tionally, the intercalation/deintercalation processes of protons into and out of the oxide layer are quicker than in the case of lithium ions. The potential range needed for an effective device operation is in this case narrower, which additionally ∗ Corresponding author. Tel.: +48 601 26 26 00; fax: +48 22 628 27 41. E-mail address: alex@soliton.ch.pw.edu.pl (M. Siekierski). leads to the diminishing of the danger of the electrode deteriora- tion in the case of moisture intake by the cell. Some designs of laminated devices incorporating a proton polymeric electrolyte were presented in [6,7]. Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS), branched polyethyleneimine (BPEI), poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB) and others were used in combination with different oxide materials. An electrochemical and optical response of the WO 3 |PEO,H 3 PO 4 |ITO (Indium Tin Oxide) was studied in [8], to determine the intercalation mechanism in this system. The authors use a model of superficial diffusion assuming homoge- neous diffusion of absorbed hydrogen in the electrode material to explain the obtained data. This model is limited to short time scale processes only. A similar system plasticized by the addition of MeCN was studied in [9]. The electrochemical redox behav- iors and electrochromic characteristics of WO 3 and Prussian blue have been examined in two- and three-electrode systems with polymeric gels as electrolyte [10]. They were based on copolymers of 2-hydroxyethylmethacrylate and neopentylgly- coldimethacrylate with lithium perchlorate and a solvent mixture of propylene carbonate–butylene carbonate. With the conductiv- ity of the material equal to 10 -3 S cm -1 at 303 K, a reversible 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.03.011