Cobalt-free Cathode Materials Research Articles

Novel, cobalt-free, and highly active Sr2Fe1.5Mo0.5−xSnxO6−δ cathode materials for intermediate temperature solid oxide fuel cells

One of the technical hurdles to commercialization of intermediate temperature solid oxide fuel cells (IT-SOFCs) is the requirement of highly efficient cathode materials. Herein, we report the evaluation of Sr2Fe1.5Mo0.5−xSnxO6−δ (x = 0, 0.1, 0.3 and 0.5, abbreviated as SFM, SFMS1, SFMS3, and SFS) oxides as cobalt-free cathode materials of IT-SOFCs. XPS analysis demonstrates the presence of variable valences among Fe, Mo and Sn elements, suggesting a small polaron hopping mechanism for electronic conduction. First principle calculations reveal that SFMS3 provides the lowest average formation energy of oxygen vacancy (EvacO*) among these perovskites. The relatively low area specific resistances are obtained with SFMS3 electrode based on La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolytes, indicating its high activity for oxygen reduction reaction. Power density of the single cell using SFMS3 cathode as high as 618 mW cm−2 at 800 °C is achieved, and operation lasts for 200 h without obvious degradation. The encouraging results promise SFMS3 as an alternative cathode material for IT-SOFCs.

Ca and In co-doped BaFeO3−δ as a cobalt-free cathode material for intermediate-temperature solid oxide fuel cells

We report Ba0·95Ca0·05Fe0·95In0·05O3−δ (BCFI), a novel cobalt-free perovskite, as a promising cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). We synthesize this new material, and systematically characterize its lattice structure, thermal stability, chemical composition, electrical conductivity, and oxygen reduction reaction (ORR) activity. The cubic phase of BaFeO3−δ is stabilized by light isovalent and lower-valence substitution, i.e., 5% Ca2+ in the Ba2+ site and 5% In3+ in the Fe3+/Fe4+ site, in contrast with the typical approach of substituting elements of higher valence. Without resorting to co-doping strategy, the phase of BaFe0·95In0·05O3−δ (BFI) is rhombohedral, while Ba0·95Ca0·05FeO3−δ (BCF) is a mixture of the cubic phase together with BaFe2O4 impurities. The structure of BCFI is cubic from room temperature up to 900 °C with a moderate thermal expansion coefficient of 23.2 × 10−6 K−1. Thanks to the large oxygen vacancy concentration and fast oxygen mobility, BCFI exhibits a favorable ORR activity, i.e., we observe a polarization resistance as small as 0.038 Ω cm2 at 700 °C. The significantly enhanced performance, compared with BFI and BCF, is attributed to the presence of the cubic phase and the large oxygen vacancies brought by the isovalent substitution in the A-site and lower-valence doping in the B-site.

Improving the cycling stability of Li2MnO3 by surface treatment

An electrochemically stable and cobalt-free cathode material is prepared by impregnating Li2MnO3 with nickel nitrate and subsequent calcination at 650 °C. X-ray diffraction and scanning electron microscopy reveal a phase pure material with a homogenous distribution of nickel over the particle surface. Structural changes related to the treatment before and after cycling as well as electrochemical performance are investigated. The surface modified samples exhibit improved cycling stability and rate capability. In particular, the material with a nickel content of 5.7 wt% shows 173 mAh g−1 at rate C/10 and 104 mAh g−1 at rate 3C. Furthermore, a constant capacity of 155 mAh g−1 is obtained at rate C/5. Derivative plots of galvanostatic curves in combination with Raman spectroscopy depict a reduced transition from a layered to spinel-like structure compared with pristine Li2MnO3, while ICP measurements reveal a drastically decreased dissolution of manganese.

Performance of cobalt-free double-perovskite NdBaFe2−xMnxO5+δ cathode materials for proton-conducting IT-SOFC

A novel series of cobalt-free cathode materials, NdBaFe2−xMnxO5+δ (0.0⩽x⩽0.3), are prepared by a citric acid–nitrate process. X-ray diffraction (XRD) analysis indicates that all the samples are double-perovskite phases with cubic structure. The conductivity dependence of the cathode materials on temperature (300–800°C) and oxygen partial pressure (1–10−10atm) is investigated. Among the tested samples, NdBaFe1.9Mn0.1O5+δ (NBFM10) exhibits the highest conductivity of 114Scm−1 in air at 550°C. The H2/air fuel cell with the NBFM10–BZCY composite cathode and NiO–BZCY composite anode as well as BaZr0.1Ce0.7Y0.2O3−α (BZCY) electrolyte membrane (ca. 30μm thickness) was assembled and tested at 500–700°C. The peak power density of the cell reaches 453mWcm−2, and the interfacial polarization resistance Rp is only 0.06Ωcm2 under open circuit conditions, at 700°C.

Li Storage Properties of (1-x-y)Li[Li1/3Mn2/3]O2-xLiFeO2-yLiNiO2 Solid Solution Cathode Materials

Uniform solid solution of (1-x-y)Li[Li1/3Mn2/3]O2-xLiFeO2-yLiNiO2 (x+y≤0.5) were prepared by citric acid assisted sol-gel process including three-step of “preparation of wet gel—formation of spray-dried gel—annealing at high temperature”, and the effects of composition on the Li-storage properties of as-prepared samples were investigated by XRD, XPS and charge-discharge tests. The results indicate that all samples prepared at 650 °C ~800 °C show pure hexagonal phase (or monoclinic phase). Obvious cooperative effects are observed for the samples with suitable amount of composited LiNiO2 and LiFeO2 (x+y≈0.3~0.4), which can present high capacity of >210 mAhg-1, good structural stability and cycling performance. In view of capacity, cycling performance and cost, the samples with x+y≈0.3~0.4 and x/y=1.6~3.0 are considered to be more promising new cobalt-free cathode materials with high capacity of more than 240 mAhg-1.

Application of thermodynamic activity models to the equilibrium potential for lithium intercalation process of a cobalt-free cathode material with a layered structure

An electrode composed of a cobalt-free cathode material with a layered structure – lithium nickel manganese oxide (LiNi0.7Mn0.3O2) – is charged and the data are interpreted with thermodynamic activity models. Equilibrium potentials of the cathode material are measured with galvanostatic intermittent titration technique (GITT) and compared with the estimated results by Nernst equation and thermodynamic activity models. No phase transition is observed in the equilibrium potential curve of the cathode material. Non-random Two Liquids (NRTL) and Universal Quasi-chemical (UNIQUAC) model are used to calculate the activity coefficients of the components in the cathode material. The equilibrium potentials estimated by both the models show good agreement with the measured data. UNIQUAC with 5 adjustable parameters is better than NRTL with 4 parameters in fitting the experimental data. Excess Gibbs free energy and activity coefficients calculated by UNIQUAC are lower than those by NRTL indicating that non-ideality in the phase behavior of LiNi0.7Mn0.3O2 is estimated higher by UNIQUAC than NRTL.

Preparation and electrochemical performance of cobalt-free cathode material Ba0.5Sr0.5Fe0.9Nb0.1O3−δ for intermediate-temperature solid oxide fuel cells

Perovskite oxide Ba0.5Sr0.5Fe0.9Nb0.1O3−δ(BSFN) as a cobalt-free cathode for intermediate-temperature solid oxide fuel cells(IT-SOFCs) on the Ce0.8Sm0.2O1.9(SDC) and La0.9Sr0.1Ga0.8Mg0.2O3−δ(LSGM) electrolytes was prepared and investigated. The single phase BSFN oxide with a cubic perovskite structure and relatively high electrical conductivities was obtained after sintering at 1250 °C for 10 h in air. The BSFN cathode exhibited excellent chemical stability on the SDC and LSGM electrolytes at temperatures below 950 °C. The area specific resistance of the BSFN cathode on the SDC and LSGM electrolytes were 0.024 and 0.021 Ω·cm2 at 800 °C, respectively. The maximum power densities of the single cell with BSFN cathode in 300 μm-thick SDC and LSGM electrolytes achieved 414 and 516 mW/cm2 at 800 °C, respectively. These results show that the BSFN material is a promising cobalt-free cathode candidate to be used in IT-SOFCs. A combination of the BSFN cathode and LSGM electrolyte is preferred owing to its excellent electrochemical performance.

Novel cobalt-free cathode material (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O4+δ for intermediate-temperature solid oxide fuel cells

The Ruddlesden–Popper phase, (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O4+δ (NLNCA), has been investigated as an alternative cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Its electrical conductivity is 51–60Scm−1 over the temperature range of 650–800°C. The average thermal-expansion coefficient (TEC) value of NLNCA is 13.2×10−6K−1. The area specific resistance (ASR) of the NLNCA cathode measured on a Sm0.2Ce0.8O1.9 (SDC) electrolyte reaches a relatively low value of 0.048Ωcm2 at 800°C. A maximum power density of 521mWcm−2 is obtained at 800°C for single-cell with thickness of ~300µm SDC electrolyte.

Microwave-irradiation synthesis of Li1.3NixCoyMn1−x−yO2.4 cathode materials for lithium ion batteries

Li1.3NixCoyMn1−x−yO2.4 (typically Li1.3Ni1/6Co1/6Mn4/6O2.4, Li1.3Ni1/5Co1/10Mn7/10O2.4 and Li1.3Ni1/4Mn3/4O2.4) cathode materials have been synthesized by microwave irradiation at 800°C in a rapid calcination period of 1.5h. The influence of transition metal ratios on the structure and electrochemical performance of the samples have been carefully investigated by powder X-ray diffraction (XRD) together with Rietveld refinement, inductive coupled plasma (ICP), scanning electron morphology (SEM), galvanostatically charge/discharge test, cyclic voltammetry (CV) in conjunction with electrochemical impedance spectroscopy (EIS) measurements. The as-prepared Li1.3Ni1/6Co1/6Mn4/6O2.4, Li1.3Ni1/5Co1/10Mn7/10O2.4 and Li1.3Ni1/4Mn3/4O2.4 materials can deliver discharge capacities of 291, 287 and 217mAhg−1, respectively, at a charge/discharge current density of 12.5mAg−1 in the voltage range of 2.5-4.7V. These cathode materials have a relatively small initial irreversible capacity loss (around 50mAhg−1), comparing with the commonly reported values (70–100mAhg−1). When the charge/discharge current density increases to 50mAg−1, they have initial discharage capacities of 267, 236 and 166mAhg−1, which are 92%, 82% and 76% of the corresponding capacity values at 12.5mAg−1. The Li1.3Ni1/6Co1/6Mn4/6O2.4 cathode exhibits the largest discharge capacity value and best rate capability due to the highest hexagonal ordering, the lowest cation mixing, content of spinel impurity phase, and charge transfer resistance. The cobalt-free cathode material, Li1.3Ni1/4Mn3/4O2.4, shows better cycling performance than the cobalt-contained counterparts.

Synthesis and characterization of aluminum-doped perovskites as cathode materials for intermediate temperature solid oxide fuel cells

(Ba0.5Sr0.5)(Fe1-xAlx)O3-δ (BSFAx, x = 0–0.2) oxides have been synthesized as novel cobalt-free cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) using a sol-gel method. The BSFAx (x = 0–0.2) materials have been characterized by X-ray diffraction and scanning electron microscopy. The electrical conductivities and electrochemical properties of the prepared samples have also been measured. At 800 °C, the conductivity drops from 15 S cm−1 to 5 S cm−1 when the doping level of aluminum is increased to 20%. The aluminum-doping concentration has important impacts on the electrochemical properties of BSFAx materials. The BSFA0.09 cathode shows a significantly lower polarization resistance (0.26 Ω cm2) and cathodic overpotential value (55 mV at the current density of 0.1 A cm−2) at 800 °C. Furthermore, an anode-supported single cell with BSFA0.09 cathode has been fabricated and operated at a temperature range from 650 to 800 °C with humidified hydrogen (∼3vol% H2O) as the fuel and the static air as the oxidant. A maximum power density of 676 mWcm−2 has been achieved at 800 °C for the single cell. We believe that BSFA0.09 is a promising cathode material for future IT-SOFCs application.

A novel cobalt-free cathode material for proton-conducting solid oxide fuel cells

Developing a tailored cathode is of great importance for the improvement of proton-conducting solid oxide fuel cell (SOFC-H) performance. In this work, a novel cobalt-free cathode BaCe0.40Sm0.20Fe0.40O3−δ was designed for use in a SOFC-H. It was composed of homogeneously distributed BaCe1−x(Sm/Fe)xO3−δ and BaFe1−y(Sm/Ce)yO3−δ, which were synthesized by a simple in situ method, eliminating the separate synthesis and the mechanical mixing processes for the conventional composite materials. The BaCe0.40Sm0.20Fe0.40O3−δ cathode exhibited protonic, oxygen-ionic, and electronic conduction simultaneously in wet air, expanding the triple phase boundaries to the whole cathode. The symmetrical cell tests with BaCe0.40Sm0.20Fe0.40O3−δ as electrodes showed that the diffusion of O−ad and reduction of O−TPB were the rate limiting steps in wet air. The power density of the anode-supported single cell with Ni–BaCe0.80Sm0.20O3−δ (580 μm) anode, BaCe0.80Sm0.20O3−δ (70 μm) electrolyte and BaCe0.40Sm0.20Fe0.40O3−δ (53 μm) cathode was 194.0, 169.2, and 137.1 mW cm−2 at 750, 710, and 650 °C, respectively. These results are encouraging considering the cobalt-free nature and rather low electrical conductivity of the cathode material. The BaCe0.40Sm0.20Fe0.40O3−δ material demonstrated excellent catalytic activity towards the reactions on the cathode. Accordingly, the BaCe0.40Sm0.20Fe0.40O3−δ material can be a promising cathode for SOFC-H.

A high performance BaZr 0.1Ce 0.7Y 0.2O 3- δ-based solid oxide fuel cell with a cobalt-free Ba 0.5Sr 0.5FeO 3- δ–Ce 0.8Sm 0.2O 2- δ composite cathode

A cobalt-free Ba 0.5Sr 0.5FeO 3- δ –Ce 0.8Sm 0.2O 2- δ (BSF–SDC) composite is employed as a cathode for an anode-supported proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr 0.1Ce 0.7Y 0.2O 3- δ (BZCY) as the electrolyte. The chemical compatibility between BSF and SDC is evaluated. The XRD results show that BSF is chemically compatible with SDC after co-fired at 1000 °C for 6 h. A single cell with a 20-μm-thick BZCY electrolyte membrane exhibits excellent power densities as high as 792 and 696 mW cm −2 at 750 and 700 °C, respectively. To the best of our knowledge, this is the highest performance reported in literature up to now for BZCY-based single cells with cobalt-free cathode materials. Extremely low polarization resistances of 0.030 and 0.044 Ωcm 2 are achieved at 750 and 700 °C respectively. The excellent performance implies that the cobalt-free BSF–SDC composite is a promising alternative cathode for H-SOFCs. Resistances of the tested cell are investigated under open circuit conditions at different operating temperatures by impedance spectroscopy.

Investigation of cobalt-free cathode material Sm 0.5Sr 0.5Fe 0.8Cu 0.2O 3− δ for intermediate temperature solid oxide fuel cell

A cobalt-free cubic perovskite oxide Sm 0.5Sr 0.5Fe 0.8Cu 0.2O 3−δ (SSFCu) was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion coefficient (TEC) of SSFCu was close to that of Sm 0.2Ce 0.8O 1.9(SDC) electrolyte and the electrical conductivity of SSFCu sample reached 72–82 S cm −1 in the commonly operated temperatures of IT-SOFCs (400–600 °C). Symmetrical electrochemical cell with the configuration of SSFCu/SDC/SSFCu was applied for the impedance study and area specific resistance (ASR) of SSFCu cathode material was as low as 0.085 Ω cm 2 at 700 °C. Laboratory-sized tri-layer cells of NiO-SDC/SDC/SSFCu were operated from 450 to 700 °C with humidified hydrogen (∼3% H 2O) as fuel and the static air as oxidant. A maximum power density of 808 mW cm 2 was obtained at 700 °C for the single cell.

A novel cobalt-free layered GdBaFe 2O 5+ δ cathode for proton conducting solid oxide fuel cells

While cobalt-containing perovskite-type cathode materials facilitate the activation of oxygen reduction, they also suffer from problems like poor chemical stability in CO 2 and high thermal expansion coefficients. In this research, a cobalt-free layered GdBaFe 2O 5+ δ (GBF) perovskite was developed as a cathode material for protonic ceramic membrane fuel cells (PCMFCs) based on proton conducting electrolyte of stable BaZr 0.1Ce 0.7Y 0.2O 3− δ (BZCY7). The button cells of Ni–BZCY7|BZCY7|GBF were fabricated and characterized using complex impedance technique from 600 to 700 °C. An open-circuit potential of 1.007 V, maximum power density of 417 mW cm −2, and a low electrode polarization resistance of 0.18 Ω cm 2 were achieved at 700 °C. The results indicate that layered GBF perovskite is a good candidate for cobalt-free cathode material, while the developed Ni–BZCY7|BZCY7|GBF cell is a promising functional material system for solid oxide fuel cells.

Novel cobalt-free cathode materials BaCe xFe 1− xO 3−δ for proton-conducting solid oxide fuel cells

A series of cobalt-free and low cost BaCe x Fe 1− x O 3−δ ( x = 0.15, 0.50, 0.85) materials are successful synthesized and used as the cathode materials for proton-conducting solid oxide fuel cells (SOFCs). The single cell, consisting of a BaZr 0.1Ce 0.7Y 0.2O 3−δ (BZCY7)-NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane and a BaCe x Fe 1− x O 3−δ cathode layer, is assembled and tested from 600 to 700 °C with humidified hydrogen (∼3% H 2O) as the fuel and the static air as the oxidant. Within all the cathode materials above, the cathode BaCe 0.5Fe 0.5O 3−δ shows the highest cell performance which could obtain an open-circuit potential of 0.99 V and a maximum power density of 395 mW cm −2 at 700 °C. The results indicate that the Fe-doped barium cerates can be promising cathodes for proton-conducting SOFCs.