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.     

Electrochemical properties of Ti–Ni–H powders prepared by milling titanium hydride and nickel

Mechanical alloying has been carried out to synthesize a hydrogen storage alloy by milling titanium hydride and nickel. The structure and electrochemical properties such as discharge capacity, charge-transfer, and hydrogen diffusion of the milled powders were investigated. The results of X-ray diffraction showed that an amorphous phase was formed after ball milling. The electrode potentials of the milled powders were −0.989, −0.878 and −0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40, and 60 h, respectively. The Ti–Ni–H powders milled for 60 h had a maximum discharge capacity of 102.2 mAh/g at a discharge current density of 60 mA/g. The results of the linear polarization showed that the exchange current density decreased as the hydrogen concentration within the powders decreased. The electrochemical impedance spectroscopy (EIS) demonstrated the same consequence and presented that the hydrogen diffusion decreased by decreasing the hydrogen concentration.     

Sm and Sr co-doped ceria prepared by citrate–nitrate auto-combustion method

A few compositions in the system, Ce_{1 − x − y}Sm_xSr_yO_1.90 have been prepared by citrate–nitrate auto-combustion method. X-ray diffraction data show that all the compositions are solid solution having cubic fluorite structure. Density of the samples sintered at 1350 °C has been found to be more than 95% of the theoretical value. Surface morphology has been studied by scanning electron microscope. AC impedance spectroscopy measurements have been carried out to study the grains, grain boundaries and total ionic conductivity of the samples in the temperature range 200–600 °C. The composition, Ce_0.82Sm_0.16Sr_0.02O_1.90 shows the maximum conductivity i.e. 2.67 × 10⁻² S-cm⁻¹ at 600 °C among all the compositions investigated. This is about two times higher than that of Ce_0.80Sm_0.20O_1.90.     

One-step prepared prussian blue/porous carbon composite derives highly efficient Fe–N–C catalyst for oxygen reduction

Preparation of high-efficiency oxygen reduction reaction (ORR) catalysts with abundant and inexpensive biomass materials have been a hot research topic. We use nitrogen-rich lentinus edodes and potassium ferrate (K₂FeO₄) to simultaneously activate the carbon material and prepare prussian blue (PB), and a porous carbon composite (PB/C) containing PB is synthesized. Finally, using ammonium chloride (NH₄Cl) as a nitrogen source to further synthesize a Fe–N–C catalyst (PB/CN₁T₈₀₀) containing a trace amount of Fe for ORR. Results show that the prepared PB/CN₁T₈₀₀ catalyst forms a coral-like structure, which mainly contains mesopores and possesses a large specific surface area of approximately 1582 m² g⁻¹. Moreover, the onset potential of PB/CN₁T₈₀₀ is 0.95 V, and the half-wave potential is 0.83 V, which are consistent with those of commercial Pt/C. Thus the PB/CN₁T₈₀₀ material is an ORR catalyst with excellent performance. This work provides a basis for simple and efficient conversion of rich biomass into PB/porous carbon composites to prepare highly efficient catalysts.     

Biomass-derived CO2 rich syngas conversion to higher hydrocarbon via Fischer-Tropsch process over Fe–Co bimetallic catalyst

Biomass-derived syngas (CO₂ + CO + H₂) has emerged as a potential non-fossil fuel source to yield transportation fuel via Fischer Tropsch Synthesis (FTS) reaction. Thus, the present study demonstrates the conversion of CO₂ containing syngas into fuel range hydrocarbon via Fischer Tropsch Synthesis over Fe–Co bimetallic catalyst. The experimental tests were carried out in a fixed bed continuous reactor to investigate the effect of CO₂ on CO/CO₂ conversion. Accordingly, obtained data were validated by FTS kinetic model for a plug flow reactor. It was found that the unique combination of Fe and Co bimetallic catalyst facilitates both FTS and water gas shift (WGS) reaction simultaneously that helps to convert CO₂ along with CO. It was also observed that the presence of iron in the catalyst helps in conversion of CO₂ into hydrocarbons, only when a particular concentration of CO₂ in syngas is reached, i.e., critical ratio R_C (CO₂/CO + CO₂) due to the occurrence of reverse water gas reaction (RWGS) which varies with the temperature and the feed gas composition (H₂/CO/CO₂ molar ratio). At 240 °C and hydrogen deficient condition, the critical ratio was measured to be 0.74 whereas for hydrogen balanced condition, it was measured 0.6. The kinetic model developed in the present study predicted trends for % CO conversion, % carbon conversion, and % CO₂ conversion which is applicable for a wide range of critical ratio R_C (CO₂/(CO + CO₂) = 0 to 1). The model also predicted that a positive conversion of CO₂ could be achieved at lower CO₂ concentration by increasing the reaction temperature. At 260 °C and 280 °C, the value of Rc were 0.31 and 0.18 were measured.     

Electrocatalytic activity for the hydrogen evolution of the electrodeposited Co–Ni–Mo,Co–Ni and Co–Mo alloy coatings

The electrocatalytic activity for the HER of the ternary Co–Ni–Mo and the binary Co–Ni and Co–Mo alloy coatings is investigated in 1 M KOH solution. The surface morphology and the structure of the studied coatings is characterized by SEM and XRD analysis. The electrocatalytic activity for the HER is evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, cathodic polarization and chronopotentiometry techniques. XRD analysis reveals that all studied coatings are composed of the Co hcp structure. However, alloy deposits with Mo is characterized by more nanocrystalline structure. Electrochemical experiments reveal superior electrocatalytic activity of coatings with Mo in comparison to Co–Ni alloy. This is the results of larger real surface area of Co–Mo and Co–Ni–Mo alloys, which is confirmed by the higher surface roughness factors (R_f) calculated based on the EIS results. The ternary alloy coating is characterized by the highest R_f parameter and the highest catalytic activity for the HER.     

Effect of synthesis method on the oxygen reduction performance of Co–N–C catalyst

In recent years, Co, N co-doped carbon (Co–N–C) materials as oxygen reduction reaction (ORR) catalysts have attracted great attention because of their good ORR stability as well as decent activity. Co-doped zeolitic imidazolate framework-8 (Co@ZIF-8) as the precursor for synthesizing Co–N–C has attracted great interest recently. Co@ZIF-8 synthesis method may affect the properties of the as-synthesized Co@ZIF-8 precursors, which will surely affect the properties and ORR performance of Co@ZIF-8-derived Co–N–C catalysts. Herein, three methods, viz. room-temperature stirring method, reflux method, and hydrothermal method, were used to synthesize Co@ZIF-8 precursors. Physical characterization shows that the synthesis method has a great influence on the textural properties, composition, and graphitization degree of the as-synthesized Co–N–C catalysts. Electrochemical characterization shows that Co–N–C-R synthesized with reflux method exhibits an onset potential (E_onset) of 0.905 V, a half-wave potential (E_1/2) of 0.792 V and a limiting current density (J_L) of 5.85 mA cm^?2 in acidic media, which are higher than those of Co–N–C–S (E_onset = 0.870 V, E_1/2 = 0.770 V, J_L = 4.71 mA cm^?2) and Co–N–C–H (E_onset = 0.892 V, E_1/2 = 0.785 V, J_L = 4.68 mA cm^?2) synthesized with room-temperature stirring method and hydrothermal method, respectively. The better ORR activity observed on Co–N–C-R can be attributed to its larger graphitization degree and larger mesopore volume. Catalytic stability test shows that Co–N–C-R has negligible activity loss after 5000 potential cycles. This work demonstrates that reflux method is a more suitable method for synthesizing Co–N–C catalysts for ORR.     

High selective and efficient Fe2–N6 sites for CO2 electroreduction: A theoretical investigation

Developing high efficient and cheap electrocatalysts for carbon dioxide reduction reaction (CO₂RR) is the key to achieve CO₂ transformation into clean energy. Herein, a series of transition metal dimer and nitrogen codoped graphene (M₂N₆-Gra, M = Cr–Cu) acting as electrocatalysts for CO₂RR are investigated based on the density functional method. For M₂N₆-Gra (M = Cr, Mn), the selectivity is poor and CO poisoning is serious. Fe₂N₆-Gra is the best CO₂RR catalyst due to the good selectivity and catalytic activity. The calculated overpotential is very small, i.e., 0.03 V for COOH channel, 0.05 V for HCOO channel. Hydrogen evolution reaction is also refrained on the Fe₂N₆-Gra surface, which further supports its high catalytic performance. For M₂N₆-Gra (M = Co, Ni, Cu), the catalytic activity is poor due to large overpotentials. These results indicate that if designed carefully, the transition metal dimer and nitrogen codoped graphene would be good candidate for the high efficient and selective CO₂RR catalyst.     

Hydrogen storage properties of Mg–TM–La (TM = Ti,Fe, Ni) ternary composite powders prepared through arc plasma method

For the first time, Mg based Mg–Transition metal (TM) –La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg–TM–La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La₂O₃ introduced during passivation. Based on the Pressure–Composition–Temperature measurements, the hydrogenation enthalpies of Mg are determined to be −68.7 kJ/mol H₂ for Mg–Ti–La powder, −72.9 kJ/mol H₂ for Mg–Fe–La powder and −82.1 kJ/mol H₂ for Mg–Ni–La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg–TM–La composites when compared to those in the binary Mg–TM or Mg–RE composites. This is especially true for the Mg–Ni–La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg₂Ni and La₂O₃. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage.     

Methanation of CO2 on bulk Co–Fe catalysts

The efficiency of CO₂ methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO₂ methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO₂ molecule on the ensemble and detailed reaction mechanism of CO₂→CH₄ conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H₂O molecule was considered as the rate-limiting step.     

Electrochemical performance of crystalline Ni–Co–Mo–Fe electrodes obtained by mechanical alloying on the oxygen evolution reaction

Electro-active _Co30Ni70${\mathrm{Co}}_{30}{\mathrm{Ni}}_{70}$, _Co30Mo70${\mathrm{Co}}_{30}{\mathrm{Mo}}_{70}$, _Ni30Mo70${\mathrm{Ni}}_{30}{\mathrm{Mo}}_{70}$, _Co10Ni20Mo70${\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70}$, _Fe10Co30Ni60${\mathrm{Fe}}_{10}{\mathrm{Co}}_{30}{\mathrm{Ni}}_{60}$ and _Co10Fe30Ni60${\mathrm{Co}}_{10}{\mathrm{Fe}}_{30}{\mathrm{Ni}}_{60}$ wt% alloys have been prepared by mechanical alloying. The electrocatalytic behavior of obtained electrodes has been evaluated in 30 wt% KOH aqueous solutions as a function of different temperatures (298, 323 and 343 K) and overvoltages (η=200$\eta =200$, 300, 400 mV). The effect of Fe contamination during mechanical milling was also analyzed. The electrode performance was studied by cyclic voltammetry, ac–impedance and steady-state polarization techniques. Appreciable current values for oxygen evolution reactions (OER) were measured at Ni–Co–Mo–Fe electrodes produced by this technique. It was also found that the electrochemically formed Co and Fe oxides and/or hydroxides did not show an activity for OER as good as reported on hydrogen evolution reaction (HER). Tafel plots of preoxidized at 1 cycle and prolonged cycled (50 cycles) for _Co30Ni70${\mathrm{Co}}_{30}{\mathrm{Ni}}_{70}$, _Co30Mo70${\mathrm{Co}}_{30}{\mathrm{Mo}}_{70}$, _Ni30Mo70${\mathrm{Ni}}_{30}{\mathrm{Mo}}_{70}$, _Co10Ni20Mo70${\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70}$, _Fe10Co30Ni60${\mathrm{Fe}}_{10}{\mathrm{Co}}_{30}{\mathrm{Ni}}_{60}$ and _Co10Fe30Ni60${\mathrm{Co}}_{10}{\mathrm{Fe}}_{30}{\mathrm{Ni}}_{60}$ crystalline electrodes were very different, which might be related to changes in the surface enrichment of one or two of the alloy constituents. The Mo electrocatalytic effects seem to be more important on the OER at higher temperatures than those showed previously for HER. Molybdenum containing crystalline Co and Ni powders (_Co10Ni20Mo70)$({\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70})$ showed the best catalytic behavior for the OER.     

Facile electrodeposited amorphous Co–Mo–Fe electrocatalysts for oxygen evolution reaction

A new type of superior activity and highly cost-effective amorphous electrocatalyst Co–Mo–Fe on nickel foam (NF) supports is prepared by facile one-step rapid electrodeposition. The amorphous electrocatalyst Co–Mo–Fe/NF shows excellent oxygen evolution reaction (OER) performance, with a small overpotential of 218 mV at 10 mA cm^?2 current density in 1 M KOH. It only needs overpotential of 252 mV at 50 mA cm^?2 current density in 1 M KOH, and the Tafel slope is 45 mV dec^?1. The results show that the doping of Fe significantly improves the oxygen evolution capacity of the Co–Mo–Fe system. The synergistic effect of the three metals and the doping of the third metal iron make the oxygen evolution active sites of the whole system increase significantly. This provides a feasible direction for the oxygen evolution reaction of cobalt transition metal.     

Electrochemical hydrogen storage property of Co–S alloy prepared by ball-milling method

A series of Co–S alloys were synthesized by means of ball milling of Co and S powders at different hours and investigated as the negative material for Ni/MH batteries. The structures and surface configuration of the alloys were characterized by XRD and TEM. The electrochemical measurements demonstrated that the Co–S particles showed excellent electrochemical reversibility and considerably high charge–discharge capacity. Among the alloys, the Co–S alloy milled 20 h showed relatively high discharge capacity and excellent cycling stability at discharge current density 25 mA/g. Its highest discharge capacity was about 350 mAh/g and remained 300 mAh/g after 100 cycles, the capacity retention rate was about 86%. The hydrogen storage mechanism was studied by XRD and TPD measurements.     

Iron-doped cobalt nitride nanoparticles (Fe–Co3N): An efficient electrocatalyst for water oxidation

The exploration of efficient, low-cost and earth-abundant oxygen-evolution reaction (OER) electrocatalysts and the understanding of the intrinsic mechanism are important to advance the clean energy conversion technique based on electrochemical water oxidation. In this work, Fe-doping Co₃N catalysts were successfully synthesized by a simple nitridation reaction of the Co_3-xFe_xO₄ precursor. This material exhibited a low overpotential of 294 mV at a current density of 10 mA cm^?2, and a small Tafel slope of 49 mV dec^?1 in 1 M KOH solution, superior to the performance of Co₃N and IrO₂. As revealed by the spectroscopic and electrochemical analyses, the enhanced OER performance mainly originates from the electronic modulation induced by the incorporation of Fe into Co₃N, benefitting the formation of CoOOH as active surface species and thus facilitating the OER process. These findings also demonstrate the introduction of heterogeneous element is a simple and effective strategy to regulate the OER property of the cobalt nitrides (Co₃N) catalysts.     

Ti–Fe based alloys prepared by ball milling for electrochemical hydrogen storage

In this study, the Ti_1.04Fe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_0.06 composite was prepared by using vacuum induction melting under inert atmosphere. Then, the specimen was milled with 5 wt% Ni powders for 10–40 h to realize the general improvements in hydrogenation performance. The phase component was determined and the morphology and microscopic structure were observed using XRD, SEM and HRTEM, respectively. The electrochemical properties of the alloys were studied. The results showed that the as-milled specimens got the maximal discharge capacity without any activation. It reached 305 mAh/g for the 30 h milling specimen, which was better than the other specimens. Besides, ball milling can enhance the electrochemical cyclic stability of the experimental alloys. The capacity retention rate (S₁₀₀) increased from 57.6 to 70.2% after 100 charging and discharging cycles with increasing milling duration from 10 to 40 h. The high rate discharge ability of the 30 h milling specimen had the maximal value of 92.8%.     

A metal-organic framework-derived Fe–N–C electrocatalyst with highly dispersed Fe–Nx towards oxygen reduction reaction

Metal and nitrogen co-doped catalysts have been promising alternatives to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR) over the past few decades. Herein, we have synthesized an efficient Fe–N–C catalyst by the co-calcination of NH₂-MIL-101@PDA and melamine. The best Fe–N–C shows a positive half-wave potential of 0.844 V, which is 14 mV higher than that of Pt/C catalyst, as well as superior methanol resistance and long-term durability in alkaline electrolyte. In addition, Fe–N–C also exhibits pronounced catalytic activity and a direct 4e⁻ reaction pathway in acid electrolyte. We ascribed the excellent ORR performance of Fe–N–C to its crumpled structure, large specific surface area (584.6 m² g⁻¹) and high content of Fe-Nx sites (1.22 at. %). This study provides a simple way for the fabrication of excellent PGM-free ORR catalysts.     

Spherical mesoporous Fe–N–C catalyst for the air cathode of membrane-less direct formate fuel cells

Fe–N–C catalysts with excellent performance regarding the oxygen reduction reaction (ORR) have aroused enormous interest in direct-formate fuel cells (DFFCs). However, their limited mass transfer ability, insufficient ORR active sites, and complex fabrication processes remain significant obstacles to the widespread application of Fe–N–C catalysts. Herein, we propose a simple hydrothermal-annealing method with agarose powders to synthesize a uniform spherical Fe–N–C catalyst (∼3 μm) with well-developed mesopores (Fe/rG@C/H-Agar-900). The resultant Fe/rG@C/H-Agar-900 catalyst possesses rich oxygen-containing functional groups and enhanced interconnected pores, which can significantly boost the content of catalytic sites and facilitate mass transport, resulting in a high content of active sites. In the meantime, the mesopore content of Fe/rG@C/H-Agar-900, which can facilitate the formation of the three-phase gas/electrolyte/catalyst interfaces, was optimized by varying the annealing temperature. As a result, the Fe/rG@C/H-Agar-900 demonstrates a half-wave potential of 0.91 V vs. RHE, nearly four-electron pathway selectivity, excellent durability, and excellent formate tolerance for ORR. Furthermore, when used as the air cathode in membrane-less DFFCs, the Fe/rG@C/H-Agar-900-based device exhibits a remarkable peak power density of 24.5 mW cm⁻², significantly outperforming the 20 wt% commercial Pt/C. This research facilitates the synthesis of an advanced Fe–N–C catalyst and promotes the practical development of membrane-less DFFCs.     

Novel cake-like Fe–N–C hybrid for H2 activation

The activation and utilization of hydrogen energy is an effective method to solve the global energy crisis and environmental pollution. Herein, biomass-derived Fe–N–C catalysts for H₂ activation were synthesized via the imitation of sponge cake baking. The sample pyrolyzed at 500 °C (Fe–N–C-500) presented the well-defined cake-like architecture with uniform distribution of Fe₃O₄ nanoparticles (NPs). Doping N species dispersed around metallic NPs in high density. Fe–N–C-500 exhibited excellent performance in the catalytic hydrogenation of nitrobenzene. The activity of Fe–N–C-500 depended on Fe₃O₄ NPs and pyridinic N, rather than Fe–N. The typical core-shell structure deemed vital for H₂ activation in previous reports was not necessary. Notably, water could significantly promote the H₂ activation, which might establish the communication between hydrogen molecules adsorbed on Fe₃O₄ NPs and doping N species through hydrogen bonds. Moreover, low temperature pyrolytic Fe–N–C-500 exhibited excellent stability and provided a promising potential for selective hydrogenation of nitroarenes or alkyne by regulating the reaction condition. This work provides an innovative approach to construct heterogeneous catalysts for H₂ activation.     

Preliminary process analysis for the closed cycle operation of the iodine–sulfur thermochemical hydrogen production process

In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H₂SO₄ phase and HI_x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I₂ molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I₂ solution from the HI_x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I₂ solution from the distillation column decreased the flow rate of I₂ solution fed to the distillation column from the separator. The variation of I₂ molar fraction in the H₂SO₄ phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I₂ solubility, 2-liquid phase disappearance and SO₂ reaction amount.     

Catalytic activity of electrodeposited ternary Co–Ni–Rh thin films for water splitting process

The influence of the deposition parameters on the composition and structure of Co–Ni–Rh ternary alloys was studied. The catalytic activity of the coatings for the hydrogen evolution process was investigated in 6 M KOH electrolyte. The thin films were deposited from baths containing a mixture of Co²⁺, Ni²⁺, and Rh³⁺ chloride complexes. A wide range of alloy compositions were achieved by applying different deposition potentials from −0.5 to – 0.9 V vs SCE. The obtained coatings were examined by energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) techniques. The surface morphology and chemical composition were also characterized with scanning electron microscopy (SEM) combined with EDX. The hydrogen evolution activity of some selected electrodes were examined in 6 M KOH using current-potential curve and electrochemical impedance spectroscopy (EIS) techniques. The SEM results showed that the surface morphology of the electrodes can be tailored by modification of the deposition potential. The higher exchange current densities were observed in catalytic measurements for the ternary alloys, which confirms their better catalytic activity in the water-splitting process.