Comparative study of oxygen reduction reaction on RuxMySez (M = Cr, Mo, W) electrocatalysts for polymer exchange membrane fuel cell

Electrochemical evaluation of the Ru_xM_ySe_z (M = Cr, Mo, W) type electrocatalysts towards the oxygen reduction reaction (ORR) is presented. The electrocatalysts were synthesized by reacting the corresponding transition metal carbonyl compounds and elemental selenium in 1,6-hexanediol under refluxing conditions for 3 h. The powder electrocatalysts were characterized by scanning electron microscopy (SEM), and X-ray diffraction (XRD). Results indicate the formation of agglomerates of crystalline particles with nanometric size embedded in an amorphous phase. The particle size decreased according to the following trend: Ru_xCr_ySe_z > Ru_xW_ySe_z > Ru_xMo_ySe_z. Electrochemical studies were performed by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Kinetic parameters exhibited Tafel slopes of 120 mV dec⁻¹; exchange current density of around 1 × 10⁻⁵ mA cm⁻² and apparent activation energies between 40 and 55 kJ mol⁻¹. A four-electron reduction was found in all three cases. The catalytic activity towards the ORR decreases according to the following trend: Ru_xMo_ySe_z > Ru_xW_ySe_z > Ru_xCr_ySe_z. However this trend was not maintained when the materials were tested as cathode electrodes in a single polymer exchange membrane fuel cell, PEMFC. The Ru_xW_ySe_z electrocatalyst showed poor activity compared to Ru_xMo_ySe_z and Ru_xCr_ySe_z which were considered suitable candidates to be used as cathode in PEMFCs.     

Microwave assisted synthesis of high performance Ru85Se15/MWCNTs cathode catalysts for PEM fuel cell applications

The Ru₈₅Se₁₅ nanoparticles supported on commercial Vulcan XC-72R or multi-walled carbon nanotubes (MWCNTs) were synthesized by microwave assisted polyol method with different solution pH. The Ru₈₅Se₁₅ nanoparticles on citric acid (CA)-treated supports prepared at pH = 7 exhibited the most uniform particle distribution, higher degree of graphitization on supports, and four-electron ORR mechanism. Using lower loading of 0.138 mg Ru cm⁻², the maximum power densities (P_max) for the Ru₈₅Se₁₅/CA-MWCNTs and Ru₈₅Se₁₅/CA-XC72R were 380 mW cm⁻² at 1430 mA cm⁻² and 336 mW cm⁻² at 1230 mA cm⁻², respectively, with oxygen, while 166 mW cm⁻² at 710 mA cm⁻² and 126 mW cm⁻² at 510 mA cm⁻², respectively, with air. The P_max of 103 mW cm⁻² with air for the Ru₈₅Se₁₅/CA-MWCNTs could be retained (38% loss), while 46 mW cm⁻² for Ru₈₅Se₁₅/CA-XC72R (64% loss) upon 6000 cycles. The four-electron ORR mechanism and highly graphitized MWCNTs might be responsible for the high performance and durability of Ru₈₅Se₁₅/CA-MWCNTs.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Oxygen reduction reaction increased tolerance and fuel cell performance of Pt and RuxSey onto oxide-carbon composites

Pt and Ru_xSe_y nanoparticles were selectively deposited onto oxide sites of oxide-carbon composite substrates using the photo-deposition process and compared to conventional carbon support materials. The oxide was essentially anatase phase. Cyclic voltammetry and rotating disk electrode measurements for the oxygen reduction reaction (ORR) in formic acid containing-electrolyte showed a tolerance improvement for ORR of Pt supported on composite substrates. This positive substrate effect on platinum, turned out not to be favorable for Ru_xSe_y catalyst centers. On the other hand, the methanol tolerance for ORR was increased for both nanostructured materials supported on the oxide-carbon composite. Single H₂/O₂ fuel cell results were in agreement with half-cell electrochemical measurements on Pt, showing an improvement of the power density when using the oxide-carbon as substrate for the cathode. The composites were evaluated as cathode catalysts of an HCOOH laminar-flow fuel cell (LFFC) in which commercial Pd/C was used as an anode catalyst. The cathodes with Ru_xSe_y and Pt supported on TiO₂/C improved the specific power density by 15% and 24%, respectively, with respect to carbon as support.     

Preparation and characterization of Pr1−xSrxFeO3 cathode material for intermediate temperature solid oxide fuel cells

A kind of cathode material of Pr_1−xSr_x FeO₃ (x = 0–0.5) for intermediate temperature solid oxide fuel cells (IT-SOFCs) was prepared by the coprecipitation method. Crystal structure, thermal expansion, electrical conductivity and electrochemical performance of the Pr_1−xSr_xFeO₃ perovskite oxide cathodes were studied by different methods. The results revealed that Pr_l−xSr_xFeO₃ exhibited similar orthorhombic structure from x = 0.1 to 0.3 and took cubic structure when x = 0.4–0.5. The unit cell volume decreased and the thermal expansion coefficient (TEC) of the materials increased as the strontium content increased. When 0 < x ≤ 0.3, the samples exhibited good thermal expansion compatibility with YSZ electrolyte. The electrical conductivity increased with the increasing of doped strontium content. When x = 0.3–0.5, the electrical conductivities were higher than 100 S cm⁻¹. The conductivity of Pr_0.8Sr_0.2FeO₃ was 78 S cm⁻¹ at 800 °C. Compared with the La_0.8Sr_0.2MnO₃ cathode, Pr_0.8Sr_0.2FeO₃ showed higher polarization current density and lower polarization resistance (0.2038 Ω cm²). The value of I₀ for Pr_0.8Sr_0.2FeO₃ at 800 °C is 123.6 mA cm⁻². It is higher than that of La_0.8Sr_0.2MnO₃. Therefore, Pr_1−xSr_xFeO₃ can be considered as a candidate cathode material for IT-SOFCs.     

A-site deficient Ba1−xCo0.7Fe0.2Ni0.1O3−δ cathode for intermediate temperature SOFC

A-site cation-deficient Ba_1−xCo_0.7Fe_0.2Nb_0.1O_3−δ (B_1−xCFN, x = 0.00-0.15) oxides are synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The reactivity between B_1−xCFN and gadolinia doped ceria (GDC) is observed at different temperature, and no second phase is detected under 1050 °C. The increasing in A-site cation deficiency results in a steady decrease in cathode polarization resistance. Among the various B_1−xCFN oxides test, GDC based anode supported cells with B_0.9CFN cathode possess the smallest interfacial polarization resistance (R_p). The R_p is as low as 0.283 and 0.046 Ω cm² at 500 and 600 °C, respectively. The anode supported cell with B_0.9CFN provides maximum power densities of 1062 and 1139 mW cm⁻² at 600 and 650 °C, respectively. The results suggest that B_0.9CFN is a great potential cathode material for IT-SOFCs.     

La substituted Sr2MnO4 as a possible cathode material in SOFC

Sr_2−xLa_xMnO_4+δ (x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr_2−xLa_xMnO_4+δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm⁻¹ was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr_1.4La_0.6MnO_4+δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm² at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr_1.4La_0.6MnO_4+δ electrode.     

Electrochemical behavior of Li3−xM′xV2−yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries

Composites of monoclinic Li_3−xM′_xV_2−yM″_2y(PO₄)₃ (M′ = K, M″ = Sc, Mg + Ti) with carbon were synthesized by solid-state reaction using oxalic acid or 6% H₂/Ar gas mixture as reducing agents at sintering temperature of 850 °C. The samples were characterized by X-ray diffraction (XRD), voltammetry and electrochemical galvanostatic cycling. The capacity of Li₃V₂(PO₄)₃ synthesized using hydrogen as the reducing agent was 127 mA h g⁻¹ and decreased to 120 mA h g⁻¹ after 20 charge-discharge cycles. The substitution of lithium and vanadium for other ions did not result in the improvement of the electrochemical characteristics of the samples.     

The influence of compositional change of 0.3Li2MnO3·0.7LiMn1−xNiyCo0.1O2 (0.2 ≤ x ≤ 0.5, y = x − 0.1) cathode materials prepared by co-precipitation

Cathode materials prepared by a co-precipitation are 0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (0.2 ≤ x ≤ 0.4) cathode materials with a layered-spinel structure. In the voltage range of 2.0-4.6 V, the cathodes show more than one redox reaction peak during its cyclic voltammogram. The Li/0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (x = 0.3, y = 0.2) cell shows the initial discharge capacity of about 200 mAh g⁻¹. However, when x = 0.2 and y = 0.1, the cell exhibits a rapid decrease in discharge capacity and poor cycle life.     

SmBaCo2O5+x double-perovskite structure cathode material for intermediate-temperature solid-oxide fuel cells

SmBaCo₂O_5+x (SBCO), an oxide with double-perovskite structure, has been developed as a novel cathode material for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). The electrical conductivity of an SBCO sample reaches 815–434 S cm⁻¹ in the temperature range 500–800 °C. XRD results show that an SBCO cathode is chemically compatible with the intermediate-temperature electrolyte materials Sm_0.2Ce_0.8O_1.9 (SDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM). The polarization resistances of an SBCO cathode on SDC and LSGM electrolytes are 0.098 and 0.054 Ω cm² at 750 °C, respectively. The maximum power densities of a single cell with an SBCO cathode on SDC and LSGM electrolytes reach 641 and 777 mW cm⁻² at 800 °C, respectively. The results of this study demonstrate that the double-perovskite structure oxide SBCO is a very promising cathode material for use in IT-SOFCs.     

Carbon supported Ru1−xFexSey electrocatalysts of pyrite structure for oxygen reduction reaction

Structure, activity, and stability of the Ru_1−xFe_xSe_y/C (x = 0.0–0.46, y = 0.4–1.9) catalysts, synthesized via hydrogen annealing of carbonyl precursors, are analyzed using X-ray diffraction, transmission electron microscopy, rotating (ring) disk electrode voltammetry, and cyclic voltammetry. Hydrogen annealing at 400 °C enables nucleation of the pyrite phase, which strengthens the catalyst stability for oxygen reduction reaction (ORR) in the acidic electrolyte. Evaluation on the stable Ru_1−xFe_xSe_y/C catalysts indicates that the ORR activity increases with increasing iron content and Se content. Substitution of base metal Fe enhances the activity and reduces the material cost, but it also increases the H₂O₂ yield, which is measured less than 3.0% between 0.7 and 0.9 V in contrast to 1.0% of the catalyst without iron. The high activity of Ru_0.54Fe_0.46Se_1.9/C decays more rapidly than that of RuSe_2.0/C in the stability test, probably because Fe and Se are preferentially leached in the potential cycling up to 1.2 V. Still the two catalysts with pyrite structure exhibit much higher durability than the cluster-type catalyst of RuSe_cluster/C (Ru:Se = 1:0.3) that was not annealed.     

High-performance PrBaCo2O5+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cell

PrBaCo₂O_5+δ-Ce_0.8Sm_0.2O_1.9 (PBCO-SDC) composite material are prepared and characterized as cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction result proves that there are no obvious reaction between the PBCO and SDC after calcination at 1100 °C for 3 h. AC impedance spectra based on SDC electrolyte measured at intermediate temperatures shows that the addition of SDC to PBCO improved remarkably the electrochemical performance of a PBCO cathode, and that a PBCO-30SDC cathode exhibits the best electrochemical performance in the PBCO-xSDC system. The total interfacial resistances R_p is the smallest when the content of SDC is 30 wt%, where the value is 0.035 Ω cm² at 750 °C, 0.072 Ω cm² at 700 °C, and 0.148 Ω cm² at 650 °C, much lower than the corresponding interfacial resistance for pure PBCO. The maximum power density of an anode-supported single cell with PBCO-30SDC cathode, Ni-SDC anode, and dense thin SDC/LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ)/SDC tri-layer electrolyte are 364, 521 and 741 mW cm⁻² at 700, 750 and 800 °C, respectively.     

Fabrication, electrochemical characterization and thermal cycling of anode supported microtubular solid oxide fuel cells

This work describes the manufacture and electrochemical characterization of anode supported microtubular SOFC's (solid oxide fuel cells). The cells consist of a Ni-YSZ anode tube of 400 μm wall-thickness and 2.4 mm inner diameter, a YSZ electrolyte of 15-20 μm thickness and a LSM-YSZ cathode. The microtubular anode supporting tubes were prepared by cold isostatic pressing. The deposition of thin layers of electrolyte and cathode are made by spray coating and dip coating respectively. The cells were electrochemically characterized with polarization curves and complex impedance measurements using 5% H₂/95% Ar and 100% of H₂, humidified at 3% as reactant gas in the anodic compartment and air in the cathodic one at temperatures between 750 and 900 °C. The complex impedance measurements show an overall resistance from 1 to 0.42 Ω cm² at temperatures between 750 and 900 °C with polarization of 200 mA cm⁻². The I-V measurements show maximum power densities of 0.3-0.7 W cm⁻² in the same temperature interval, using pure H₂ humidified at 3%. Deterioration in the cathode performance for thin cathodes and high sintering temperatures was observed. They were associated to manganese losses. The cell performance did not present considerable degradation at least after 20 fast shut-down and heating thermal cycles.     

Nearly carbon-free printable CIGS thin films for solar cell applications

In order to fabricate low cost and printable CuIn_xGa_1−xSe_yS_2−y (CIGS) thin film solar cells, a precursor solution based method was developed. Particularly, in this method, nearly carbon-free CIGS film was obtained by applying a three-step heat treatment process: the first for the elimination of carbon residue by air annealing, the second for the formation of CIGS alloy by sulfurization, and the third for grain growth and densification in the CIGS film by selenization. The film also revealed very large grains with a low degree of porosity, similar to those produced by the vacuum based method. A solar cell device with this film showed current-voltage characteristics of J_sc=21.02 mA/cm², V_oc=451 mV, FF=47.3%, and η=4.48% at standard conditions.     

Co-doped Sr2FeNbO6 as cathode materials for intermediate-temperature solid oxide fuel cells

Sr₂Fe_1−xCo_xNbO₆ (0.1 ≤ x ≤ 0.9) (SFCN) oxides with perovskite structure have been developed as the cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). These materials are synthesized via solid-state reaction and characterized by XRD, SEM, electrical conductivity, AC impedance spectroscopy and DC polarization measurements. The reactivity tests show that the Sr₂Fe_1−xCo_xNbO₆ electrodes are chemically compatible with the Zr_0.85Y_0.15O_1.925 (YSZ) and Ce_1.9Gd_0.1O_1.95 (CGO) electrolytes at 1200 °C, and the electrode forms a good contact with the electrolyte after sintering at 1200 °C for 12 h. The total electrical conductivity that has a considerable effect on the electrode properties is determined in a temperature range from 200 °C to 800 °C. The highest conductivity of 5.7 S cm⁻¹ is found for Sr₂Fe_0.1Co_0.9NbO₆ at 800 °C in air. The electrochemical performances of these cathode materials are studied using impedance spectroscopy at various temperatures and oxygen partial pressures. Two different kinds of reaction rate-limiting steps exist on the Sr₂Fe_0.1Co_0.9NbO₆ electrode, depending on the temperature. The Sr₂Fe_0.1Co_0.9NbO₆ electrode on CGO electrolyte exhibits a polarization resistance of 0.74 Ω cm² at 750 °C in air, which indicates that the Sr₂Fe_0.1Co_0.9NbO₆ electrode is a promising cathode material for IT-SOFCs.     

Structural and transport properties of LixNi1−y−zCoyMnzO2 cathode materials

In this work structural and transport properties of layered LiNi_1−y−zCo_yMn_zO₂ (y = 0.25, 0.35, 0.5 and z = 0.1) cathode materials are presented. In the considered group of oxides, LiNi_1−y−zCo_yMn_zO₂, there is no clear correlation between electrical conductivity and the a parameter (M-M distance in the octahedra layers). A non-monotonic modification of electrical properties of Li_xNi_0.65Co_0.25Mn_0.1O₂ cathode materials is observed upon lithium deintercalation.     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.     

An amorphous LiCo1/3Mn1/3Ni1/3O2 thin film deposited on NASICON-type electrolyte for all-solid-state Li-ion batteries

Amorphous LiCo_1/3Mn_1/3Ni_1/3O₂ thin films were deposited on the NASICON-type Li-ion conducting glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 130 °C. The amorphous films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiCo_1/3Mn_1/3Ni_1/3O₂/Au all-solid-state cells were fabricated to investigate the electrochemical performance of the amorphous films. It was found that the low-temperature deposited amorphous cathode film shows a high discharge voltage and a high discharge capacity of around 130 mAh g⁻¹.     

Phase stability and conductivity of Ba1−ySryCe1−xYxO3−δ solid oxide fuel cell electrolyte

The structure, phase stability, and electrical properties of BaCe_1−xY_xO_3−δ (x = 0-0.4) in humidity air and CO₂ atmosphere are investigated. XRD results indicate that the BaCe_0.9Y_0.1O_3−δ sample has a symmetric cubic structure, and its phase changes to tetragonal as the Y³⁺ doping amount increases to 20 mol%. The conductivity of BaCe_1−xY_xO_3−δ increases with temperature, and it depends on the amount of yttrium doping and the atmosphere. BaCe_0.8Y_0.2O_3−δ exhibits the highest conductivity of 0.026 S cm⁻¹ at 750 °C. The activation energy for conductivity depends on yttrium doping amount and temperature. The conductivity of BaCe_0.8Y_0.2O_3−δ is 0.025 S cm⁻¹ in CO₂ atmosphere at 750 °C which is 3.8% lower than that in air due to reactions with CO₂ and BaCO₃ and the CeO₂ impure phases formed. The structure of BaCe_0.8Y_0.2O_3−δ is unstable in water and decomposes to Ba(OH)₂ and CeO₂ phases. It is found that the activation energy of samples in CO₂ atmosphere is higher than that of sample in air. Sr-doped Ba_1−ySr_yCe_0.8Y_0.2O_3−δ (y = 0-0.2) is prepared to improve the phase stability of BaCe_0.8Y_0.2O_3−δ in water. The conductivity of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is 0.023 S cm⁻¹ at 750 °C which was 11% lower than that of BaCe_0.8Y_0.2O_3−δ, however, the phase stability of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is much better than that of BaCe_0.8Y_0.2O_3−δ in water.     

Electrochemical properties of doped lithium titanate compounds and their performance in lithium rechargeable batteries

Several substituted titanates of formula Li_4−xMg_xTi_5−xV_xO₁₂ (0 ≤ x ≤ 1) were synthesized (and investigated) as anode materials in rechargeable lithium batteries. Five samples labeled as S1–S5 were calcined (fired) at 900 °C for 10 h in air, and slowly cooled to room temperature in a tube furnace. The structural properties of the synthesized products have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transmission infrared (FTIR). XRD explained that the crystal structures of all samples were monoclinic while S1 and S3 were hexagonal. The morphology of the crystal of S1 was spherical while the other samples were prismatic in shape. SEM investigations explained that S4 had larger grain size diameter of 15–16 μm in comparison with the other samples. S4 sample had the highest conductivity 2.452 × 10⁻⁴ S cm⁻¹. At a voltage plateau located at about 1.55 V (vs. Li ⁺), S4 cell exhibited an initial specific discharge capacity of 198 mAh g⁻¹. The results of cyclic voltammetry for Li_4−xMg_xTi_5−xV_xO₁₂ showed that the electrochemical reaction was based on Ti⁴⁺/Ti³⁺ redox couple at potential range from 1.5 to 1.7 V. There is a pair of reversible redox peaks corresponding to the process of Li⁺ intercalation and de-intercalation in the Li–Ti–O oxides.