Electrochemical performances of spinel oxides as cathodes for intermediate temperature solid oxide fuel cells

The spinel-type oxides of (Mn, Co, Cu)₃O₄ prepared via a citric–EDTA acid process were investigated as candidate cathodes of intermediate temperature solid oxide fuel cells (IT-SOFCs). (Mn, Co)₃O₄ spinel oxide shows a phase transition from tetragonal to cubic when the doping amount of cobalt element increases. Their electric conductivities increase with the cobalt content and are enough high for them used as cathodes of IT-SOFCs. A fuel cell with (Mn, Co)₃O₄ spinel cathode was successfully evaluated based on YSZ electrolyte. (Mn, Co)₃O₄ spinel cathodes show good electrochemical activities, demonstrating the feasibility of the spinel oxide being a cathode of IT-SOFC. As copper doped into (Mn, Co)₃O₄ spinel, the P_peak for Cu_0.5MnCo_1.5O₄ cathode rise to 343, 474 and 506 mW cm⁻² at 700, 750 and 800 °C, respectively. The results reveal that the spinel-type oxides are promising cathodes for IT-SOFCs, especially for Cu_0.5MnCo_1.5O₄.     

Electrochemical study of lithiated transition metal oxide composite as symmetrical electrode for low temperature ceramic fuel cells

In this work, Lithiated NiCuZnO_x (LNCZO) composite is synthesized and evaluated as a potential symmetrical electrode for ceria-carbonate composite electrolyte based low temperature ceramic fuel cells. Its crystal structures, the hydrogen oxidation/oxygen reduction electrochemical activities and fuel cell performances are systematically examined on the symmetrical cell configuration. Nano crystallite particles in the form of composite are observed for these oxides. The LNCZO shows relatively high catalytic activities for hydrogen oxidation and oxygen reduction reaction according to the electrochemical impedance spectroscopy measurements. A remarkable low oxygen reduction activation energy of 42 kJ mol⁻¹ is obtained on the LNCZO/ceria-carbonate composite, demonstrating excellent electro-catalytic activity. Especially, the catalytic activity can be further improved in the presence of water in the cathode chamber. The results show that the lithiated transition metal oxide composite is a promising symmetrical electrode for ceria-carbonate electrolyte and composite approach might a probable solution to develop super-performance electrodes for reduced temperature ceramic fuel cells.     

In situ activation with Mo of Ni–Co alloys for hydrogen evolution reaction

In this study Ni–Co alloys have been activated during hydrogen electrochemical production by adding Mo ions into the alkaline electrolyte. After dissolving different amounts of sodium molybdate in the Na(OH) electrolyte, Ni–Co alloys were used as cathodes for hydrogen evolution reaction. Afterwards a comparison between hydrogen overvoltage measured on Ni–Co alloys with and without in situ activation has been made. The in situ activation clearly shows an improvement of electrocatalytic properties of Ni–Co alloys for hydrogen evolution reaction. Depending on the alloy the best conditions are reached with different amounts of sodium molybdate in the electrolyte. The values of exchange current density for Ni–Co alloys without Mo, are an average of about 4.1 10⁻⁶ A/cm², while by using in situ activation, these values are about 3.5·10⁻⁴ A/cm². Therefore, exchange current density presents a value nearly one hundred-fold higher when molybdate ions are present in solution. Moreover, two Tafel slope values have been determined for HER on Ni–Co alloys with and without Mo in situ activation. Those Tafel slope values are different, so as their range of both overvoltage and current density, probably highlighting a different kinetic mechanism.     

Promoted electrocatalytic performance of palladium nanoparticles using doped-NiO supporting materials toward ethanol electro-oxidation in alkaline media

Electrocatalyst support materials play significant role in the performance, durability and commercialization of fuel cells. This research work describes the preparation of metal oxide (nickel oxide (NiO), cobalt (Co) and copper (Cu) doped NiO) support materials on meshed titanium (Ti) substrate via a simple electro-deposition method for their application as novel support material for palladium (Pd) catalyst. Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) techniques are employed to study morphology, composition and structure of fabricated electrodes, respectively. The electrocatalytic performance of fabricated electrodes toward ethanol oxidation reaction (EOR) in alkaline solution is examined by the cyclic voltammetry (CV), chronoamperometry (CA) techniques. Low peak potential (−0.2 V), increased peak current density (62.54 mA cm⁻²) and large electrochemical active surface area (16.02 m² g⁻¹) were remarkable properties of Pd/Cu–NiO/Ti electrode. The results of other electrochemical measurements, CO-striping voltammetry, long-term stability and electrochemical impedance spectroscopy (EIS) revealed that the Pd/Cu–NiO/Ti electrode has the privileged electrocatalytic performance for EOR relative to other prepared electrodes. Accordingly, the Pd/Cu–NiO/Ti can be considered as a hopeful electrode for ethanol electro-oxidation reaction in DEFCs.     

Possible use of non-flammable phosphonate ethers as pure electrolyte solvent for lithium batteries

Dimethyl methyl phosphonate (DMMP) was selected and tested as a non-flammable solvent for primary and secondary lithium batteries, because of its non-flammability, good solvency of lithium salts and appropriate liquidus properties. Experimental results demonstrated that DMMP can solvate considerable amount of commonly used lithium salts to form non-flammable and Li⁺-conducting electrolyte, which has very wide electrochemical window (>5 V vs. Li) and excellent electrochemical compatibility with metallic lithium anode and oxide cathodes. Primary Li–MnO₂ cells using DMMP-based electrolyte showed almost the same discharge performances as those using organic carbonate electrolytes, and also, Li–LiMn₂O₄ cells using DMMP electrolyte exhibited greatly improved cycleability and dischargeability, suggesting a feasible application of this new electrolyte for constructing high performance and non-flammable lithium batteries.     

Electrochemical characterization of cobalt-encapsulated nickel as cathodes for MCFC

The stability of the NiO cathodes in molten carbonate fuel cell (MCFC) has been improved through microencapsulation of the NiO cathode with nanostructured Co. Cobalt was deposited on the NiO cathode using an electroless deposition process. The electrochemical oxidation behavior of the Co-coated electrodes is similar to that of the bare NiO cathode. The cobalt-coated electrodes have a lower solubility in the molten carbonate melt when compared to bare nickel oxide electrodes in the presence of cathode gas. The solubility decreased more than 50% due to microencapsulation with cobalt. The thermal oxidation rate was also lower in case of the cobalt-encapsulated electrode. Impedance data from the modified electrode indicate that the oxygen reduction reaction depended inversely on the CO₂ and directly on the oxygen partial pressures respectively suggesting a similar reaction mechanism to that of nickel oxide. The results indicated that cobalt-encapsulated NiO is a viable solution in the development of alternate cathodes for MCFC applications.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

High performance Ni–Fe alloy supported SOFCs fabricated by low cost tape casting-screen printing-cofiring process

Novel Ni–Fe alloy supported solid oxide fuel cells, with Ni cermet as functional anode, La_0.8Sr_0.2MnO₃ coated Ba_0.5Sr_0.5Co_0.2Fe_0.8O₃ as cathode and Gd-doped Ce₂O₃ as electrolyte, are successfully fabricated by the cost effective method of tape casting-screen printing-cofiring. The Ni–Fe porous substrate is obtained by reduction (in H₂ at 650 °C for 2 h) of sintered NiO-10 wt% Fe₂O₃ consisting of NiO and NiFe₂O₄. The cell is subjected to evaluation in the aspects of electrochemical performance and redox capability at temperatures between 500 and 650 °C. The result shows a peak power density of 1.04 W cm⁻² at 650 °C. Furthermore, the metal support cell exhibits excellent tolerance to redox cycles. Five redox recycles for cells are operated at 600 °C, which shows no significant degradation in open circuit voltage and power density.     

Cathodic hydrogen evolution in acidic solutions using electrodeposited nano-crystalline Ni–Co cathodes

Nano-crystalline Ni and Ni–Co electrodes were prepared by electrodeposition on copper substrates. The obtained materials were characterized morphologically and chemically by XRD and scanning electron microscopy, SEM, coupled with EDX analysis. The incorporation of Co into the Ni matrix causes surface modification, which catalyzes the hydrogen evolution reaction, HER. The electro-catalytic performance of the prepared electrode layers was studied by means of polarization techniques and electrochemical impedance spectroscopy, EIS, in acidic solutions. The Results reveal a decrease in the hydrogen overpotential by increasing the Co content up to ≈50 at% in the deposited cathode layer. The Nyquist impedance plots of the different investigated materials at different potentials in the hydrogen evolution region showed a single semicircle, which means that a single time constant is controlling the HER. Ni–Co deposits with ≈50 at% Co contents show the highest rate of hydrogen evolution as a consequence of the synergetic combination of Ni and Co. The increase of the Co content more than ≈50 at% was accompanied by a decrease in the rate of HER. The low hydrogen over-potential and high hydrogen adsorption on the Ni-50 at% Co is attributed to the synergetic effects of Co and Ni together.     

Study of NiO cathode modified by ZnO additive for MCFC

The preparation and subsequent oxidation of nickel cathodes modified by impregnation with zinc oxide (ZnO) were evaluated by surface and bulk analysis. The electrochemical behaviors of ZnO impregnated NiO cathodes was also evaluated in a molten 62 mol% Li₂CO₃ + 38 mol% K₂CO₃ eutectic at 650 °C by electrochemical impedance spectroscopy (EIS) as a function of ZnO content and immersion time. The ZnO impregnated nickel cathodes showed the similar porosity, pore size distribution and morphology to the reference nickel cathode. The stability tests of ZnO impregnated NiO cathodes showed that the ZnO additive could dramatically reduce the solubility of NiO in a eutectic carbonate mixture under the standard cathode gas condition. The impedance spectra for cathode materials show important variations during the 100 h of immersion. The incorporation of lithium in its structure and the low dissolution of nickel oxide and zinc oxide are responsible of these changes. After that, the structure reaches a stable state. The cathode material having 2 mol% of ZnO showed a very low dissolution and a good catalytic efficiency close to the NiO value. We thought that 2 mol% ZnO/NiO materials would be able to adapt as alternative cathode materials for MCFCs.     

Potential low-temperature application and hybrid-ionic conducting property of ceria-carbonate composite electrolytes for solid oxide fuel cells

Ceria-carbonate composite materials have been widely investigated as candidate electrolytes for solid oxide fuel cells operated at 300-600 °C. However, fundamental studies on the composite electrolytes are still in the early stages and intensive research is demanded to advance their applications. In this study, the crystallite structure, microstructure, chemical activity, thermal expansion behavior and electrochemical properties of the samaria doped ceria-carbonate (SCC) composite have been investigated. Single cells using the SCC composite electrolyte and Ni-based electrodes were assembled and their electrochemical performances were studied. The SCC composite electrolyte exhibits good chemical compatibility and thermal-matching with Ni-based electrodes. Peak power density up to 916 mW cm⁻² was achieved at 550 °C, which was attributed to high electrochemical activity of both electrolyte and electrode materials. A stable discharge plateau was obtained under a current density of 1.5 A cm⁻² at 550 °C for 120 min. In addition, the ionic conducting property of the SCC composite electrolyte was investigated using electrochemical impedance spectroscopy technique. It was found that the hybrid-ionic conduction improves the total ionic conductivity and fuel cell performance. These results highlight potential low-temperature application of ceria-carbonate composite electrolytes for solid oxide fuel cells.     

Electrochemical behavior of Mg–Li,Mg–Li–Al and Mg–Li–Al–Ce in sodium chloride solution

Mg–Li, Mg–Li–Al and Mg–Li–Al–Ce alloys were prepared and their electrochemical behavior in 0.7 M NaCl solutions was investigated by means of potentiodynamic polarization, potentiostatic current–time and electrochemical impedance spectroscopy measurements as well as by scanning electron microscopy examination. The effect of gallium oxide as an electrolyte additive on the potentiostatic discharge performance of these magnesium alloys was studied. The discharge activities and utilization efficiencies of these alloys increase in the order: Mg–Li < Mg–Li–Al < Mg–Li–Al–Ce, both in the absence and presence of Ga₂O₃. These alloys are more active than commercial magnesium alloy AZ31. The addition of Ga₂O₃ into NaCl electrolyte solution improved the discharging currents of the alloys by more than 4%, and enhanced the utilization efficiencies of the alloys by more than 6%. It also shortened the transition time for the discharge current to reach to a steady value. Electrochemical impedance spectroscopy measurements showed that the polarization resistance of the alloys decreases in the following order: Mg–Li > Mg–Li–Al > Mg–Li–Al–Ce. Mg–Li–Al–Ce exhibited the best performance in term of activity, utilization efficiency and activation time.     

Pr2NiO4–Ag composite cathode for low temperature solid oxide fuel cells with ceria-carbonate composite electrolyte

Pr₂NiO₄–Ag composite was synthesized and evaluated as cathode component for low temperature solid oxide fuel cells based on ceria-carbonate composite electrolyte. X-ray diffraction analysis reveals that the formation of a single phase K₂NiF₄–type structure occurs at 1000 °C and Pr₂NiO₄–Ag composite shows chemically compatible with the composite electrolyte. Symmetrical cells impedance measurements prove that Ag displays acceptable electrocatalytic activity toward oxygen reduction reaction at the temperature range of 500–600 °C. Single cells with Ag active component electrodes present better electrochemical performances than those of Ag-free cells. An improved maximum power density of 695 mW cm⁻² was achieved at 600 °C using Pr₂NiO₄–Ag composite cathode, with humidified hydrogen as fuel and air as the oxidant. Preliminary results suggest that Pr₂NiO₄–Ag composite could be adopted as an alternative cathode for low temperature solid oxide fuel cells.     

A direct synthesis of nickel nanoclusters embedded on multicomponent mesoporous metal oxides and their catalytic properties for glycerol steam reforming to hydrogen

Nickel nanoclusters embedded in multicomponent mesoporous metal oxides (Ni–MMOs) are obtained at various support compositions by a single-step synthesis of Ni ion incorporated mesoporous metal oxides (NiO–MMOs) followed by selective reduction of the NiO to Ni metal clusters. The resultant Ni–MMOs catalysts displayed enhanced Ni dispersion with well-developed mesopore structures at various support composition, exhibiting superior catalytic properties when compared to a siliceous SBA-16-supported Ni catalyst prepared by a conventional impregnation method. Glycerol steam reforming conducted at 873 K on 1Ni–2Al₂O₂–2ZrO₂ and 1Ni–2SiO₂–2ZrO₂ catalysts exhibited considerably higher glycerol conversions over the 10 wt%-Ni/SBA-16 catalyst with similar Ni loading amount. This was primarily due to the enhanced Ni dispersion resulting from the direct synthesis process. The multicomponent mesoporous supports also significantly affect product selectivity, favoring higher hydrogen concentration in the product stream. The water–gas shift reaction appears to be positively affected by the 2Al₂O₂–2ZrO₂ and 2SiO₂–2ZrO₂ multicomponent metal oxide matrices, which facilitated the conversion of the CO produced by the glycerol reforming further to additional hydrogen. Direct single-step synthesis of Ni–MMO catalysts was effective in enhancing the dispersion of Ni nanoclusters, as well as variation of the support components of the mesoporous catalyst systems.     

Electrocatalytic activity of crystalline Ni–Co–M (M = Cr,Mn, Cu) alloys on the oxygen evolution reaction in an alkaline environment

Ternary Ni₆₀Co₃₀M₁₀ (M = Cr, Mn, Cu) crystalline alloys have been characterized by means of microstructural and electrochemical techniques in view of their possible applications as electrocatalytic materials for oxygen evolution reaction (OER). The electrochemical efficiency of the electrodes has been studied on the basis of electrochemical data obtained from steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques in 1 M NaOH solution at 298 K. The results were compared with those obtained on a Ni₆₀Co₄₀ commercial alloy. The overall experimental data indicate that alloying Ni–Co with Cr, Mn and Cu leads to an increase of electrocatalytic activity in oxygen evolution with respect to the Ni–Co alloy. High catalytic efficiencies were achieved on Ni₆₀Co₃₀Mn₁₀ and Ni₆₀Co₃₀Cr₁₀ electrodes, the latter being the best electrocatalyst for the OER.     

Performances of SmBaCoCuO5+δ–Ce0.9Gd0.1O1.95 composite cathodes for intermediate-temperature solid oxide fuel cells

SmBaCoCuO_5+δ–xCe_0.9Gd_0.1O_1.95 (SBCCO–xGDC, x = 10, 30, 50, 60, wt%) composite cathodes have been investigated for their potential utilization in intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion behavior shows that the thermal expansion coefficient (TEC) values of SBCCO cathode decrease with GDC addition. The TEC of SBCCO–50GDC cathode is 13.1 × 10⁻⁶ K⁻¹ from 30 to 850 °C in air. By means of DC polarization and AC impedance spectroscopy, the electrochemical performance of SBCCO–xGDC composite cathodes on GDC electrolyte is examined. Results indicate that the proper addition of GDC could improve the performance of SBCCO cathode. The optimum content of GDC in the composite cathodes is 50 wt% with the polarization resistance (R_p) of 0.040 Ω cm² at 800 °C. An electrolyte-supported single-cell configuration of SBCCO–50GDC/GDC/Ni–GDC attains a maximum power density of 628 mW cm⁻² at 800 °C. Preliminary results indicate that SBCCO–50GDC is especially promising as a cathode for IT-SOFCs.     

Microstructural effects on the electrical and mechanical properties of Ni–YSZ cermet for SOFC anode

The electrical and mechanical properties of Ni–YSZ cermet as the anode support of solid oxide fuel cell (SOFC) are determined by the metallic and ceramic components, respectively. We used YSZ and NiO commercial powders of the average particle size from 1 to 10 μm to fabricate Ni–YSZ cermets with different microstructures. The porosity of the cermets was also modified by the amount of carbon black addition. The distribution of each phase of cermets was analyzed with scanning electron microscopy combined with energy dispersive spectroscopy. The electrical conductivity and fracture strength of the Ni–YSZ cermets were investigated and interpreted in a view of percolation phenomena. The finer particles, either NiO or YSZ, were interlinked well by sintering and the electrical and mechanical properties of Ni–YSZ cermets were enhanced by the percolation of Ni and YSZ, respectively.     

Investigation of La2NiO4+δ-based cathodes for SDC–carbonate composite electrolyte intermediate temperature fuel cells

Lanthanum nickelate based oxides, including La₂NiO_4+δ (LN), La₂Ni_0.8Co_0.2O_4+δ (LNC82) and La₂Ni_0.8Fe_0.2O_4+δ (LNF82), were investigated as cathodes for intermediate temperature fuel cells with samaria doped ceria (SDC)–carbonate composite electrolytes. These oxides were synthesized by glycine–nitrate process and characterized by XRD and SEM, showing that all samples annealed at 800 °C for 2 h exhibit a K₂NiF₄ phase and a foam-like structure. The electrochemical properties of these cathodes were evaluated by fabricating and testing fuel cells with two kinds of composite electrolytes, SDC-20 wt.% (0.53Li/0.47Na)₂CO₃ and SDC-30 wt.% (0.67Li/0.33Na)₂CO₃, referred to as SDC(53L47N)20 and SDC(67L33N)30, respectively. Among these three cathodes, LNC82 shows the best cell performances at 500–600 °C. Moreover, fuel cells with SDC(67L33N)30 composite electrolyte present much higher power output than those with SDC(53L47N)20 composite electrolyte. It reveals that cobalt doping greatly enhances the electrochemical property of lanthanum nickelate, and such cathodes are more compatible with the SDC(67L33N)30 composite electrolyte.     

Electrodeposition of nickel–copper alloys to use as a cathode for hydrogen evolution in an alkaline media

The electrocatalytic activity of nickel–copper (Ni–Cu) alloy coated electrodes for the hydrogen evolution reaction (HER) in an alkaline media was studied. The Ni–Cu alloys were electrodeposited on a Cu substrate by direct current (DC) and pulse current (PC) electrodeposition in a fixed plating bath. A wide alloy composition range (6–81 mol% Ni) was achieved by controlling the applied current density between 5 and 300 mA cm⁻². It was found that the electrocatalytic activity for the HER depended on the composition of the Ni–Cu alloys, where electrodes having low Ni content gave high electrocatalytic activities. DC electrodeposition resulted in better electrocatalytic performances than PC. Pulse plating parameters other than the magnitude of the applied current density did not substantially influence the electrocatalytic performance of the Ni–Cu alloy electrodes. Ni content was found to have a stronger effect on the electrocatalytic activity for the HER than the deposit morphology.     

Development of Fe2O3–TiO2 mixed oxide incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

Considering the electronic parameters and chemical characteristics, a synergistic catalytic effect of Fe₂O₃ along with TiO₂ could be achieved for electrochemical reactions if both the oxides are produced in a mixed oxide form. The present study explored the mixed oxide composite viz; Fe₂O₃–TiO₂, synthesized via thermal decomposition method, to increase the catalytic efficiency of Ni–P electrodes, the well known catalytic electrodes for hydrogen evolution reaction in alkaline medium. The incorporation of the Fe₂O₃–TiO₂ mixed oxide into Ni–P matrix substantially reduced overpotential during hydrogen evolution reaction (HER) in 32% NaOH solution. A significant improvement on the electrochemical activity of the Ni–P coated electrodes was achieved as evidenced from the results of Tafel and impedance studies. The incorporation of Fe₂O₃–TiO₂ mixed oxide composite into the Ni–P matrix has improved both metallurgical and electrochemical characteristics and hence its amount of incorporation should be optimum. The electrodes exhibited high stability under dynamic experimental conditions. The role of the composite and the possible mechanism are discussed in this paper.