Design of materials for solid oxide fuel cells cathodes and oxygen separation membranes based on fundamental studies of their oxygen mobility and surface reactivity

. Design of materials for solid oxide fuel cells cathodes and oxygen separation membranes and studying their oxygen transport characteristics are important problems of modern hydrogen energy. In the current work, fundamentals of such materials design based on characterization of their oxygen mobility by oxygen isotope exchange with C 18 O 2 and 18 O 2 in flow and closed reactors for samples of Ruddlesden – Popper-type oxides Ln 2-x Ca x NiO 4+δ , perovskite-fluorite nanocomposites PrNi 0.5 Co 0.5 O 3-δ – Ce 0.9 Y 0.1 O 2-δ , etc. are presented. Fast oxygen transport was demonstrated for PNC – YDC ( D O ~10 -8 cm 2 /s at 700°C) nanocomposites due to domination of the fast diffusion channel involving oxygen of the fluorite phase with incorporated Pr cations and developed perovskite-fluorite interfaces. For LnCNO materials a high oxygen mobility ( D O ~10 -7 cm 2 /s at 700°C) provided by the cooperative mechanism of its migration was demonstrated. Depending on Ca dopant content and Ln cation nature, in some cases 1–2 additional channels of the slow diffusion appear due to decreasing the interstitial oxygen content and increasing the energy barrier for oxygen jumps due to cationic size effect. Optimized by the chemical composition and nanodomain structure materials of these types demonstrated a high performance as SOFC cathodes and functional layers in asymmetric supported oxygen separation membranes.


Introduction
Design of materials with mixed oxygen ion and electron conductivity (MIECs) for solid oxide fuel cells (SOFC) cathodes and catalytic membrane reactors for oxygen separation based on their oxygen transport characteristics is important problem of modern hydrogen energy [1][2][3][4][5]. Along with the microstructure features, the electrode performance correlates with the electrode material oxygen self-diffusion coefficient and surface exchange constant values [6,7]. For oxygen separation membranes' materials a high mixed ionic-electronic conductivity is required to obtain a high oxygen flux across the membrane providing efficient performance in catalytic reactions of fuels selective oxidation into syngas [4,5].
Conventional perovskite materials such as Sr-doped La manganites, ferrites-nickelates, ferrites-cobaltites have problems in their thermomechanical and chemical compatibility with electrolyte materials as well as chemical stability such as carbonization in CO2-rich atmospheres, which limit their application [1][2][3][4][5]. Undoped perovskites with a larger size of A-cation (e.g. Pr nickelates-cobaltites) are promising materials for SOFC and membrane applications due to stability to carbonization, compatibility with electrolytes (such as doped ceria) and a high oxygen mobility and surface reactivity [4,5,[8][9][10]. Ruddlesden -Popper phases (RP) are promising due to their moderate thermal expansion coefficients and a high mixed ionic-electronic conductivity. A high oxygen mobility of these materials (oxygen tracer diffusion coefficient ~10 -7 cm 2 /s at 700°C) is provided by the cooperative mechanism of its migration involving both regular and highly mobile interstitial oxygen of perovskite and rock salt layers, respectively [5,[11][12][13][14][15][16][17].
In the current work, fundamentals of such materials design based on characterization of their oxygen mobility and surface reactivity by the temperature-programmed and isothermal oxygen isotope exchange with C 18 O2 and 18 O2 in flow and closed reactors with detailed mathematical modeling for powdered samples of Ruddlesden -Popper-type oxides Ln2-xCaxNiO4+δ (Ln = La, x = 0-0.3; Ln = Pr, x = 0-0.6; Ln = Nd, x = 0-0.5) and perovskite-fluorite nanocomposites PrNi0.5Co0.5O3-δ -Ce0.9Y0.1O2-δ (PNC -YDC) are presented. Performance studies of symmetric cells' electrodes, SOFC cathodes and functional layers of asymmetric supported oxygen separation membranes based on the materials involved are reviewed.
Oxygen mobility and surface reactivity of materials were studied by oxygen isotope exchange with C 18 O2 and 18 O2 in temperature-programmed (TPIE) and isothermal modes using flow and closed reactors. Gas composition was monitored by SRS QMS200 (for closed reactor experiments) and UGA-200 (Stanford Research Systems, USA) (for flow reactor experiments) mass spectrometers. Responses of 18 O atoms fraction (α) and 16 O 18 O (or C 16 O 18 O) molecules fraction (f16-18) in the gas phase were analyzed using mathematical model [5,18,19]: The initial and boundary conditions are: Here αg, αS and αbulk are atomic fractions of 18 O in the gas phase, on the surface and in the bulk of samples, respectively; Ng, Ns and Nbulk are amounts of oxygen atoms in the gas phase, on the surface and in the bulk of samples respectively; f16-18 is the molecular fraction of 16 O 18 O (in the case of exchange with O2) or С 16 O 18 O (in the case of exchange with CO2); R 0 , R 1 , R 2 are rates of exchange going by different types of mechanism according to Muzykantov classification; R  = 0.5R 1 +R 2 = k * Nbulk/Vsample is the total rate of heteroexchange, k * is the surface exchange constant, Vsample is the total volume of sample; D * is the oxygen tracer diffusion coefficient; h is characteristic particle size of the sample, η is dimensionless parameter of the distance from the particle surface; t is time; O(τ) is operator, which depends on the mass transport mode in the reactor: 16 18  is the contact time in the reactor,  is dimensionless sample length. The exchange rates and tracer diffusion coefficients are considered to have dependence on temperature according to Arrhenius law: are the exchange rate and tracer diffusion coefficient at the reference temperature Tref; ER(i) and ED are their effective activation energies [5,18,19].
Asymmetric supported membranes (Fig. 1) for oxygen separation were built using Ni/Al foam substrates.

Structure features
According to XRD data, LnNCO materials were single phase oxides with Ruddlesden -Popper structure. The crystal structure was orthorhombic (O) or tetragonal (T) with space group Fmmm, I4/mmm or Bbcm depending on Ca doping level and, thus, interstitial oxygen content. Structure details and δ values are summarized in Table 1. The XRD pattern of the PNC -YDC nanocomposite sintered at 1100°C is presented in Fig. 2. The unit cell parameters are a = 5.3750 Å, b = 7.6040 Å, c = 5.3960 Å for perovskite phase and a = 5.4030 Å for fluorite phase. The average X-ray particle size is > 100 nm. The redistribution of cations between phases takes place via incorporation of Pr cations into doped ceria phase, which disorders both phases. The average compositions of perovskite and fluorite phases are Pr0.8Ni0.5Co0.5O3-δ and Ce0.65Pr0.25Y0.1O2-δ [4,5,[8][9][10].

Oxygen mobility and surface reactivity
Typical TPIE curves obtained for PNC -YDC nanocomposite are given in Fig. 3(a). According to TPIE data modelling, two channel of oxygen diffusion related to perovskite (slow) and fluorite phases and interfaces (fast) were revealed; the oxygen tracer diffusion coefficient D * is ~10 -8 cm 2 /s at 700°C for fast diffusion channel (Fig. 3(b)). Thus, fast oxygen transport was demonstrated for PNC -YDC with domination of the fast diffusion channel involving ~70% of bulk oxygen. These transport properties are provided by structural features (fluorite phase with incorporated Pr cations having a high oxygen mobility due to Pr 3+/4+ chains) and developed perovskite-fluorite interfaces [4,5,[8][9][10]. For LnCNO materials a high oxygen mobility (D * ~10 -7 cm 2 /s at 700°C) was demonstrated. This is provided by the cooperative mechanism of its migration involving both interstitial and regular oxygen [12,13]. Doping with Ca decreases oxygen mobility due to decreasing the interstitial oxygen content as charge compensation and increasing the energy barrier for oxygen jumps between the interstitial and apical positions in octahedral layers due to cationic size effect. Depending on Ca dopant content and Ln cation nature, in some cases 1-2 additional channels of the slow diffusion appear [14][15][16][17]24]. TPIE curves for the sample with 3 diffusion channels are given in Fig. 4. Comparison of D * values depending on the dopant content and Ln cation nature is shown in Fig. 5.

Performance of symmetric and solid oxide fuel cells
The studies performed for the symmetrical cells with LnCNO functional layers have shown that the influence of Ca doping reveals itself differently in dependence on the nature of lanthanide [14,15,17,20,21]. However, in all cases there was no direct interrelation of the electrode activity with either the electrical properties or the interstitial oxygen content. In the case of La-based materials Ca doping drastically increased the polarization resistance of the electrodes (at 700°C, 0.73 and 4.01 Ω cm 2 for x = 0.0 and 0.3, respectively), while conductivity increased (at 700°C, 58 and 93 S/cm). In the case of Pr-based materials the lowest polarization resistance was obtained at x = 0.1 (0.17 Ω cm 2 ) while the maximal conductivity was reached at x = 0.5 (145 S/cm). In the case of Nd-based materials the lowest polarization resistance was obtained at x = 0.4 (0.37 Ω cm 2 ), at the same time the maximal conductivity was reached at x = 0.3 (133 S/cm). We have revealed correlation of the polarization resistance of the electrodes with characteristics of oxygen transfer in the electrode material (self-diffusion coefficient, surface exchange constant) (Fig. 6). It was found that the electrochemical characteristics of the materials studied are inversely proportional to the product of the self-diffusion coefficient of oxygen and surface exchange constant, which corresponds to the theoretical model of the relationship between the diffusion impedance and the kinetic parameters of the electrode materials [6,7]. Fig. 6. Correlation between polarization resistance (Rη) and oxygen transport characteristics [14][15][16][17]24].

Performance of asymmetric supported membranes for oxygen separation
In CH4 partial oxidation and oxi-dry reforming, carbonization of membrane surface was negligible. Syngas selectivity in CH4 partial oxidation increases with CH4 inlet fraction and temperature (Fig. 7). Increase of syngas yield and decrease of CO2 content were observed in experiments with high feed rates. These performance characteristics are possible due to a high oxygen permeability (permeation flux up to 7-10 ml O2/(cm 2 min) at 950°C), which is provided by a high oxygen mobility of PNC-YDC nanocomposite [5,22,23].

Conclusions
Oxygen transport properties of novel materials for SOFC cathodes and oxygen separation membranes (Ruddlesden -Popper-type oxides, perovskite-fluorite nanocomposites) and their relationship with structural features were reviewed. Optimized by the chemical composition and nanodomain structure materials of these types demonstrated a high performance as SOFC cathodes and functional layers in asymmetric supported oxygen separation membranes used for transformation of biofuels into syngas.