Oxygen Reduction and Hydrogen Production at the Liquid/Liquid interfaces

Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE) interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and cobalt porphyrin catalysts like cobalt porphine (CoP) cobalt 2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-aminophenylporphyrin (CoAP) at the interface. The reaction is electrocatalytic as its rate depends on the applied Galvani potential difference between the two phases. We also report herein hydrogen evolution by direct proton reduction with DMFc (Decametylferrocene) at a soft interface between water and DCE. The soft interface between two immiscible electrolyte solutions (ITIES) is formed between two liquid solvents of a low mutual miscibility, such as water and 1,2dichloroethane (DCE), each containing an electrolyte. Electrochemical polarization of ITIES allows studies of electron transfer and ion transfer reactions, as well as the adsorption phenomena. With this advantage, an ITIES has been considered to be a suitable model for investigation of heterogeneous reactions occurring in biological systems, which are in most cases ion-coupled electron transfer reactions such as the proton-coupled oxygen (O2) reduction. Recently, we have studied Proton-Coupled Oxygen Reduction at liquid-liquid interfaces catalyzed by cobalt porphine [1] and cobalt 2,8,13,17-tetraethyl3,7,12,18-tetramethyl-5-p-aminophenylporphyrin (CoAP). The reaction proceeds as a proton coupled electron transfer process (PCET), with protons supplied by the aqueous phase and electrons provided by Fc (ferrocene) in DCE as shown in Figure 1. We also present a heterogeneous hydrogen evolution reaction at a soft interface, formed between an aqueous acidic solution and an immiscible organic solvent, 1,2-dichloroethane (DCE), containing DMFc as an electron donor [2]. The reaction proceeds by assisted proton transfer by DMFc across the water–DCE interface with subsequent proton reduction in DCE. The interface essentially acts a proton pump, allowing hydrogen evolution by directly using the aqueous proton. Figure 1: Interfacial PCET mechanism This work was supported by EPFL, the Swiss National Science Foundation (FNRS 200020116588), CNRS, Grant Agency of the Czech Republic (No. 203/07/1257), and European Cost Action D36/007/06 and CNRS. I.H. and M.E. also gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) under the 2212-PhD Scholarship Program. [email protected] [1] Hatay, I., Su, B., Li, F., Agudelo, M.A.M, Khoury, T., Gros, C. P Barbe,J-M., Ersoz, M., Samec, Z., and Girault H. H. 2009 Proton Coupled Oxygen Reduction at Liquid-Liquid Interfaces Catalyzed by Cobalt Porphine. Journal of the American Chemical Society 131: 13453– 13459 [2] Hatay, I., Su, B., Li, F., Partovi-Nia, R., Vrubel, H., Hu, X., Ersoz, M., and Girault H.H., 2009 Hydrogen Evolution at Liquid–Liquid Interfaces. Angewandte Chemie International Edition 48: 1 -5 Oral Presentation, Theme L : Nanotechnology for Energy 6th Nanoscience and Nanotechnology Conference, zmir, 2010 182 Hydrothermal preparation and electrochemical properties of Smand Gd, codoped ceria-based electrolytes for intermediate temperature-solid oxide fuel cells Sibel Dikmen, Hasan Aslanbay, Erdal Dikmen Department of Chemistry, Suleyman Demirel University, Isparta 32260, Turkey Department of Physics, Suleyman Demirel University, Isparta 32260, Turkey AbstractThe structure, ionic and electronic conductivities of Ce0.8Sm0.2-xMxO2(for M: Gd, and La, x = 0-0.1) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (23-64 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400C, significantly lower temperature, compared to that at 1600-1650C required for ceria solid electrolytes prepared by solid state techniques. The maximum conductivity, 700oC 6.50 10 Scm, Ea = 0.59 eV, is found at x = 0.1 for Gdcodoping. The electrolytic domain boundary (EDB) of Ce0.8Sm0.1La0.1O2has been found to be lower than that of singly doped samples. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes. The structure, ionic and electronic conductivities of Ce0.8Sm0.2-xMxO2(for M: Gd, and La, x = 0-0.1) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (23-64 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400C, significantly lower temperature, compared to that at 1600-1650C required for ceria solid electrolytes prepared by solid state techniques. The maximum conductivity, 700oC 6.50 10 Scm, Ea = 0.59 eV, is found at x = 0.1 for Gdcodoping. The electrolytic domain boundary (EDB) of Ce0.8Sm0.1La0.1O2has been found to be lower than that of singly doped samples. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes. Fuel cells are electrochemical devices that directly convert the chemical energy of a fuel into electrical energy in a highly clean, cheap and efficient way [1]. Electrolytes used for fuel cells are usually the main components determining the performance of the cell. A typical solid oxide fuel cell electrolyte, 8mol% yttria-stabilized zirconia (YSZ), having thermal and mechanical strength both toward anode reduction and cathode oxidation requires to operate at high temperatures (800–1000 C) to provide high level of ionic conductivity. This limits the range of materials used for interconnection, electrodes and sealing due to the corrosion of metallic components [2]. Some singly doped-electrolytes, such as Ce1 xGdxO2 (GDC), Ce1 xSmxO2 (SDC), Ce1 xYxO2 (YDC), etc., show high oxide ion conductivity at intermediate temperatures (500–700 C) [3–5].Substitution of the Ce cations by a lowervalent metal ion (e.g., M) in the lattice results in the oxygen vacancy formation and increases the ionic conductivity. In this research, with the aim to develop new ceria-based electrolyte materials with improved electrochemical properties, Sm and Gd co-doped ceria materials were prepared for the first time hydrothermally. Similar to the previously reported systems [6–7], the electrical conductivity of Ce0.8Sm0.2 xGdxO2 increases systematically with increasing gadolinium substitution and reaches a maximum for the composition Ce0.8Sm0.1Gd0.1O2 , ( 700 C 6.50×10 2 Scm ) Fig. 1) Fig.1 Arrhenius plots of the ionic conductivity of Ce0.8Gd0.2 xSmxO2 solid solutions The ceria-based electrolytes easily develop n-type electronic conduction when exposed to the reducing atmosphere of the fuel cell anode which decreases the fuel cell efficiency. It is therefore important to make efforts towards the reduction of electronic conductivity. The dependence of total conductivities of Ce0.8Sm0.2 xGdxO2 as a function of oxygen partial pressure has been shown in Fig. 2. As can be seen, the total electrical conductivity ( t) is predominantly ionic and remains constant at moderate PO2 , whereas at low PO2 , the total electrical conductivity increases as PO2 decreases and is predominantly electronic.. Fig.2 Oxygen partial pressure dependence of the total conductivity of Ce0.8Gd0.2 xSmxO2 solid solutions at 973 K. The data are fitted with t = i +kPO2 1/4 . From these results we can conclude that co-doping with Sm and Gd can lead to an improvement of the stability of ceriabased electrolytes at intermediate temperatures. This study was supported by TUB TAK under the Grant No: 106T536. *Corresponding author: [email protected] [1] S. Dikmen, Journal of Alloys and Compounds, 491 , 106 (2010) [2] H. Inaba, H. Tagawa, Solid State Ion., 83, 1 (1996) [3] S.W. Zha, C.R. Xia, G.Y. Meng, J. Power Sources, 115, 44 (2003) [4] D.J. Kim, J. Am. Ceram. Soc., 72 (8), 1415 (1989). [5] S.J. Hong, A.V. Virkar, J. Am. Ceram. Soc., 78 (2) (1995) 433–439. [6] S. Dikmen, P. Shuk, M. Greenblatt, Solid State Ion., 126, 89 (1999). [17] S. Dikmen, P. Shuk, M. Greenblatt, H. Gocmez, Solid State Sci., 4, 585 (2002) -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 10 12 14 16 18 20 22 24 x = 0 0.05 0.1 0.15 0.2