CH4 Fuels Research Articles

Selective Photocatalytic CO2 Reduction Through Plasmonic Z‐Scheme Ag‐Bi2O3‐ZnO Heterostructures

AbstractCombination of plasmonic noble metal with semiconductors offers a promising and potential solar energy harvesting and conversion route. Herein, plasmonic Z‐scheme Ag‐Bi2O3‐ZnO (AgBZ) heterostructure is designed and synthesized via solution combustion and photo‐deposition method. Metallic Ag nanoparticles (NPs) were deposited over Bi2O3‐ZnO heterostructure; thus, they exhibit strong visible light absorption owing to surface plasmon resonance (SPR). The photocatalytic efficiency of the synthesized catalysts is investigated by the photocatalytic CO2 reduction to fuels under simulated solar light irradiation. The present system has shown an outstanding photocatalytic CO2 reduction and excellent selectivity of CO. The evolved rate of CO fuel, the amount of CO produced per unit time, over optimized Ag‐Bi2O3‐ZnO (3AgBZ) heterostructures was (96.74 μmol g−1/h) which was approximately 12 times larger than that of Bi2O3‐ZnO (0AgBZ) and 49‐fold than the quantity of evolved CO fuels over pure ZnO photocatalyst. The percentage of evolved CO to CH4 fuels over optimized Ag‐Bi2O3‐ZnO (3AgBZ) heterostructures was about 50 : 1compared to that of 2 : 3 over Bi2O3‐ZnO (0AgBZ). The great improvement and the high selectivity would be ascribed to the synergistic effect of metallic surface plasmon resonance and the Z‐scheme charges transfer system. This work would open a window to construct a single photocatalyst with different charge separation systems and excellent CO2 reduction selectivity. Finally, the mechanism for the enhanced photocatalytic performance by the synergistic effect was described and discussed.

Operational condition and furnace geometry for premixed C3H8/Air MILD combustion of high thermal-intensity and low emissions

MILD combustion is usually operated at relatively low thermal intensity to achieve uniform temperature field and low pollutant emissions, further understanding of its emission characteristics at high thermal intensity condition is required to widen its application. In the present numerical study of premixed C3H8/air MILD combustion, different operational conditions (equivalence ratio Φ and thermal input Pinput) and furnace geometries (side wall angle α and aspect ratio AR) are considered to reveal their correlations with pollutant emissions including CO, NOx and unburned hydrocarbon (UHC) at thermal intensities up to 1.06 MW/m3. By adopting reactor network calculation with detailed combustion chemistry, it is found that AR plays a notable role in controlling CO, NOx and UHC emissions, while NOx emission is insensitive to the variation of side wall angle. Both the CO and UHC emissions are related to the location of the internal recirculating vortex. Moreover, following the parametric study and NOx formation analysis, the optimized furnace geometry and operating condition of AR = 5, α = 0° and Φ = 0.6 are obtained to significantly reduce the pollutant emissions by 90 % for both C3H8 and CH4 fuels. The present investigation offers valuable insights into the high thermal intensity MILD combustion.

Conceptual design and assessment of Integrated capture and methanation of CO2 from flue gas using chemical-looping scheme of dual function materials

Integrated Carbon Capture and Methanation (ICCM) with renewable hydrogen is considered a promising Power-to-Gas technology. Although ICCM is expected to reduced CO2 emission, the potentials of net CO2 reduction and economic feasibility are still unrevealed by careful assessments. This study developed three ICCM processes at industrial scale, and performed an assessment in terms of energy conversion efficiency, comprehensive energy consumption, net CO2 emission, and levelized cost of CO2 conversion. Special attentions were paid to investigate the effects from electricity and hydrogen sources, and plant configuration on the technical, environmental, and economic feasibilities. This work also identified the opportunities and challenges as operation pressure, SNG price, and carbon tax are consideredthe primary factors. A comparison was made between ICCM processes and the reference process that combines CO2 capture with aqueous solutions and CO2 catalytic hydrogenation for CH4 fuels. 89.0 % of energy conversion efficiency could be achieved with ICCM processes, much higher than that of reference process. When ICCM processes are driven by renewable energy, the best comprehensive energy consumption is estimated to be –20.68 MJ/kg CO2; negative CO2 emissions could be easily achieved for ICCM processes, and the best net CO2 emissions is –1.57 kg CO2/kg CO2 when powered by wind energy, demonstrating the technical and environmental feasibility as ICCM process is coupled with renewable energy. The levelized costs of CO2 conversion are in the range of 0.98–2.0 US$/kg CO2. The economic feasibility of ICCM process is primarily limited by H2 production cost. But the levelized costs of CO2 conversion decreases rapidly with H2 production cost, and the ICCM processes could become profitable if the cost of H2 production from renewable energy drops to the same level of coal-based hydrogen (<1.4 US$/kg H2). It is possible for European Union to deploy ICCM plants to bring economic revenues because of its high SNG price and carbon tax.

Strong Metal-Support Interactions Stabilize Pt Nanocatalysts for Ceramic Fuel Cells

Nano-sized metal particles are widely used in various chemical/electrochemical fields due to their excellent catalytic activity, but they still suffer from deactivation by sintering, and this leads to serious issues regarding the price and lifespan of catalysts. In this study, we demonstrate that the introduction of nanoscale amorphous TiOx via atomic layer deposition can significantly improve the dispersion and durability of supported Pt nanocatalysts. An ultrathin TiOx layer (< 1 nm) deposited on a conductive Pr0.5Ba0.5MnO3 electrode, with a deposition time of less than 1 min, provides nucleation sites for the Pt nanocrystals impregnated thereon and promotes the migration of TiOx to the Pt surface, creating a metal-oxide interface. As a result, when applied to ceramic fuel cells, the TiOx undercoat achieves remarkable power density increases of over 100% and 400%, respectively, for wet (3% H2O) H2 and CH4 fuels without degradation at 700 °C for 120 h. These observations provide an innovative direction for the design of supported metal catalysts for high-temperature applications.

Performance of red mud oxygen carriers in chemical-looping hydrogen production using different components of plastic waste pyrolytic gas

This study proposed plastic waste-fueled Chemical-looping Hydrogen Production (CLHP) based on the concept of three-reactor chemical-looping process for the purposes of sustainable treatment of plastic wastes while producing high purity H2 and separating CO2. Oxygen carrier prepared from red mud wastes (Bauxite residues) was suggested to be circulating materials to improve combustion efficiency and H2 yield. The feasibility of using pelletized oxygen carrier prepared from red mud wastes was experimentally investigated with major components (H2, CH4 and C2H4) of plastics pyrolysis gases as fuels. The red mud oxygen carrier composed of around 38 wt% Fe2O3 and 50 wt% inert materials, and CuO-promoted oxygen carriers composed of 8.0 wt% CuO and 92 wt% red mud wastes were used. The two oxygen carriers were examined by parallel experiments, focusing on the aspects of oxygen transport, stability, hydrogen productivity and carbon deposition. Temperature and different components of plastic pyrolysis gases had significant impacts on oxygen transport behaviors; reduction tests using CH4 or C2H4 fuels suggested that the operation temperature of Fuel Reactor should be higher than 800 °C for deep reduction and high H2 productivity. CuO addition significantly promoted oxygen carrier's reactivity and oxygen transport capacity with various fuel components. The cyclic CLHP experiments, performed over 10 redox cycles with C2H4 as fuels, showed that there was only a limited degradation in oxygen carriers caused by carbon deposition and element migration. The developed oxygen carriers can meet the requirements of long-term operation for CLHP process. Carbon deposition on the oxygen carriers derived from hydrocarbon fuel cracking seemed to be inevitable, which experimentally determined in the range of 0.01–0.043 g g−1 oxygen carriers as C2H4 was used as fuels. However, the reactivity of these carbon deposits was extremely low; their initial steam-gasification temperatures were around 750 °C, much higher than the temperature used for water splitting reaction. This provides the potential opportunities for an industrial CLHP process to produce high purity H2 (>97%) despite of carbon deposits presents in Fuel Reactor.

Electrochemical performances of LiNiCoMn bifunctional electrode for low temperature solid oxide fuel cells

Lithium transition metal oxides LiNi0.83Co0.11Mn0.06O2 (NCM-83) and LiNi0.8Co0.1Mn0.1O2 (NCM-811) are prepared and acted as cathodes and bifunctional electrodes for low temperature solid oxide fuel cells with H2 and CH4 fuels. The Ni anode-supported cell with NCM-83 cathode exhibits maximum power density (Pmax) of 0.72 W cm−2 with H2 fuel at 600 °C. The symmetric cell with NCM-83 electrodes shows high Pmax of 0.465 W cm−2 with H2 fuel and 0.354 W cm−2 with CH4 fuel at 600 °C. And the Pmax of the cell with NCM-811 as anode and NCM-83 as cathode is 0.204W cm−2 with H2 fuel at 600 °C. The oxygen vacancies in NCM materials are conducive to the rapid oxygen ion conduction of the cathode, and in the anodic reduction atmosphere, the NCM materials will generate Ni/Co active particles in situ, proving the NCM materials can be advanced bifunctional electrode materials for hydrogen oxidation reaction and oxygen reduction reaction at low temperature.

Influence of nonthermal termolecular reactions on the optimization and uncertainty quantification of a hydrogen combustion model

Nonthermal termolecular reactions were shown to have significant impacts on combustion predictions of H2, CH4, and other fuels. But they are not considered in earlier model development efforts, many of which involve tuning or optimization. While adding new chemistry to optimized models is not recommended, we need to quantify its influence. The impact on model optimization has been examined on the H2 sub-mechanism of the Foundational Fuel Chemistry Model Version 1.0. The current work compared the optimization using the model as is, and with termolecular chemistry fully implemented, with new termolecular reactions and corrections to the stabilization reactions. The models optimized with and without termolecular chemistry have comparable performance and both could reconcile all the experimental targets considered. The two models show similar predictions under most target conditions, which can be explained by the compensation of kinetic parameters within their uncertainty ranges. Extrapolation of the model to conditions outside the target list shows larger discrepancy. While models with proper inclusion of nonthermal reactions should in general give more reliable predictions, it is expected that the difference can be minimized with a proper target list and optimized models can be used with attention on conditions very sensitive to termolecular chemistry. The conclusions provided guidance on model optimization with termolecular chemistry in consideration.

NOx formation from ammonia, and its effects on oxy-combustion of hydrocarbon fuels under supercritical-CO2 conditions

NOx formation from ammonia impurities and its effects on fuel oxidation is not well understood for high-pressure supercritical CO2 oxy-combustion conditions. This effect is investigated computationally and experimentally in the present work. A chemical kinetic analysis revealed that the reaction between NH radical and CO2 plays an important role in determining the rate of NOx formation at these conditions. It was also found that a significant reduction in the NOx formation from ammonia oxidation occurs with sCO2 oxy-combustion at 300 atm when compared to traditional gas turbine conditions. A chemical reactor network simulation of realistic sCO2 cycle conditions confirmed the reduced NOx emissions. Monte Carlo simulations used to study the sensitivity of model input variables on the emissions showed that CO2 cooling impacts the CO emissions while most of the NOx are generated in the flame zone. Uncertainty analysis showed that the reaction between NH and CO2 to form HNO radical is an important contributor to the model uncertainty under sCO2 oxy-combustion conditions.The presence of combustion-generated NOx in the recycled-CO2 can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NOx on ignition, high-pressure shock tube ignition delay time experiments were performed at conditions relevant to sCO2 oxy-combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition under supercritical conditions using CO2 as the bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes ignition at these conditions. The effect is more pronounced for CH4 compared to syngas. Nitric oxide acts as a chemical catalyst to promote ignition by increasing the combustion radical pool. The catalytic cycle involves the conversion of NO to NO2 which also contributes to CH4 oxidation by H-atom abstraction to generate CH3 radical.

Evaluation of Tubular SOFC’s Performance at Elevated Pressure for Highly-Efficient Clean Power Generation

Growing global energy needs and demand over imminent reductions of harmful greenhouse gas emissions, call for development of highly efficient, clean and reliable power generation from both renewable and fossil resources. A hybrid solid oxide fuel cell gas turbine (SOFC-GT) system is a promising candidate for fulfilling such need of a low emission, highly efficient power generation. This system can be applied to stationary, transportation, aviation, and maritime power generation applications. This study focuses on understanding operational characteristics of the pressurized SOFC as a key component for the successful design and construction of a highly efficient SOFC-GT hybrid system.Pressurization of an SOFC can cause improvement on electrode kinetics, stack lifetime and round-trip efficiency by cell integration with downstream components such as gas turbines and catalytic reactors (for electrolysis) [1-4]. However, literature data from experimental studies on the electrochemical behavior of the SOFC as a function of operating pressure is relatively scarce. This is presumably due to the complexity involved in the construction and operation of pressurized system. To address such research challenge, a pressurized SOFC test rig as shown in Fig. 1 has been constructed and commissioned at Tennessee Technological University (TTU) to allow tubular SOFCs to be characterized under a maximum operating pressure of 60 psia (~4 bar) and maximum operating temperature of 825oC. A detailed system set-up shown in Fig. 2. The power generation capacity for the test rig can be scaled up to 2.4 kW. For this study, performance of single anode supported SOFC tubes with H2 and CH4 fuels were examined by varying pressure from 1 bara to 4 bara and temperature from 675oC to 775oC, with fuel utilization held constant at 50% and an oxygen-to-carbon ratio (O/C) of 0.6 for the methane operation. The testing cells were gratuitously supplied by Special Power Sources (SPS) LLC, USA, with cathode/electrolyte outside-layers supported on the anode inner-layer as a 14”-long tube. Silver ink and wires are used for cell current collection from active surface area of ⁓100 cm2 . The connection fixture can be seen in Fig. 1. V-I curves of tubular SOFCs in Fig. 3 measured from step-wise electronic load profile indicate total performance gain of 13% as H2 pressure increases from 1 bara to 4 bara, with highest peak power density obtained at 0.61 Wcm-2. As comparison Fig.4 and 5 demonstrate pressurization suppressed gas diffusion limit for tubular SOFC at higher current density in CH4 fuel, leading to 44 % increase of peak power density to 0.51 Wcm-2 at 4 bara.Electrochemical impedance spectroscopy (EIS) studies were performed using a Solartron 1260/PAR 263 electrochemical testing system in 4-point connection measurement to understand effects of pressurization on the SOFC performance. SOFC impedance spectra as Nyquist and Bode plots in Fig.6 (b) and (d) indicate the tubular SOFC performance is mainly limited by anode mass transports, which are greatly improved by pressurization. From non-linear, least-square cell impedance fitting and previous findings on impedance of similar larger cells, an equivalent-circuit model (ECM) in Fig. 6(a) has been identified to best represent impedance responses from the most rate-limiting processes of tubular SOFCs. The Warburg component (Ws) in the ECM with resonance frequency around 10Hz typically corresponds to gas diffusion in porous electrode of tubular SOFC [3-5], while paralleled R-CPE component at <1Hz resonance frequency can be attributed to gas-conversion impedance (GCI) due to gas phases concentration gradient along fuel-flow channel resulting from reactant conversion [5,6]. Experimentally observed changes of such impedances with fuel compositions, flow rates and tubular SOFC configuration are generally in agreement with theoretical assumptions and other reports [4-6]. Overall, results of this work demonstrate pressurization can effectively boost anode-supported SOFCs’ performance by overcoming fuel-side mass-transport limit associated with practical cell configuration and testing geometry. The experimental performance data collected can then be used to fit key constants and estimate parameters for numerical modeling of the SOFC system performance at elevated pressures.References can be seen in the figure document. Figure 1

Improving the Catalytic Activity of (Ba0.95La0.05)(Fe0.8Zn0.2)O3−δ Protonic Ceramic Fuel Cell Cathodes by Cation Substitution

(Ba0.95La0.05)(Fe0.8Zn0.2)O3-δ (BLFZ) is a good candidate for protonic ceramic fuel cell cathodes owing to its high proton concentration and decent electronic conductivity. However, the poor catalytic activity of BLFZ limits the electrochemical reaction on the cathode surface, resulting in poor fuel cell performance in hydrocarbon fuels. In this study, to enhance the catalytic activity of BLFZ, Fe cations were substituted with cobalt (Co) cations to synthesize (Ba0.95La0.05)(Co x Fe0.8-x Zn0.2)O3−δ (BLCFZ) (x = 0–0.8). A single-phase perovskite structure was obtained when the mol% of Co (x) was 0.2 and 0.4, while secondary phases were observed at x = 0.6 and 0.8. The cathode performance was improved by doping Co into BLFZ. The polarization resistances of the fuel cell with BLCFZ (x = 0.2) in H2 and CH4 fuels at 600 °C were smaller than those of the fuel cell with BLFZ (x = 0) and the power density in H2 fuel at 600 °C increased from 0.7 to 0.9 Wcm−2. Equivalent circuit models were used to analyze the contribution of Co in improving the catalytic activity of the cathode.

Bi2WO6‐based Z-scheme photocatalysts: Principles, mechanisms and photocatalytic applications

The development of novel photocatalysts for efficient utilization of solar energy is highly essential for the most critical humanitarian challenges, i.e., energy and water crises as well as environmental pollution. Bismuth tungstate (Bi2WO6), an outstanding Aurivillius phase perovskite, has attracted intensive attention as a visible-light-responsive photocatalyst because of its non-toxicity, low cost, and outstanding physicochemical characteristics, i.e., nonlinear dielectric susceptibility, ferroelectric piezoelectricity, pyroelectricity, catalytic behavior, modifiable morphology, strong oxidation power, and good photochemical stability. However, the photocatalytic activity of bare Bi2WO6 is restricted because of the inherent drawbacks such as poor light-harvesting efficiency, weak reduction potential, relatively low specific surface area, the fast recombination rate of photoinduced charge carriers, and thus poor quantum yields of photocatalytic reactions. Moreover, the impossibility of simultaneous strong redox ability (demanding wide bandgap) and high light-harvesting efficiency (requiring narrow bandgap) is considered as a big challenge for the practical application of Bi2WO6. Undeniably, the construction of Z-scheme photocatalytic systems is recommended strategy to overcome the above-mentioned disadvantages because of the efficient spatial separation of photogenerated charge carriers and the boosting the redox performance. This review summarizes the principles and recent developments on Z-scheme photocatalytic systems with special emphasis on the Bi2WO6-based photocatalysts, including the types, photocatalytic mechanisms and practical applications. Moreover, major differences between type-II heterojunction and Z-scheme photocatalyst have also been discussed. Additionally, the significant role of unique structures (e.g., core-shell and 2D/2D) for the improvement of photocatalytic activity of Z-scheme photocatalyst has been presented. Indeed, Bi2WO6-based Z-scheme photocatalysts have exhibited superior photocatalytic activity for various applications. For example, they show high photocatalytic activity towards water/wastewater treatment (removal of organic and inorganic pollutants, as well as microorganisms), air purification (decomposition of volatile organic compounds and inorganic matters), “green” energy conversion (e.g., generation of H2 and CH4 fuels under solar irradiation), and organic synthesis. It is thought that this remarkable activity of Bi2WO6-based Z-scheme photocatalysts might be attributed to the efficient solar light harvesting, separation and further transfer of charge carriers and strong redox ability. To the best of our knowledge, the present paper is the first attempt to summarize the Bi2WO6-based Z-scheme photocatalytic reactions, providing important insights and up-to-date information for the scientific community to fully explore the potential of Bi2WO6-based photocatalysts for renewable environmental remediation, energy conversion, and chemical synthesis.

The decreasing effect of ammonia enrichment on the combustion emission of hydrogen, methane, and propane fuels

In the present study, non-premixed combustion and NOx emission of H2, NH3, C3H8, and CH4 fuels have been studied in a combustion test unit under lean mixture conditions (λ = 4) at 8.6 kW thermal capacity. Furthermore, the combustion and NOx emission of the H2, C3H8, and CH4 fuels have been investigated for various NH3 enrichment ratios (5, 10, 20, and 50%) and excess air coefficients (λ = 1.1, 2, 3, and 4) at the same thermal capacity. The obtained results have been compared for each fuel. Numerical simulation results show that H2 emits intense energy through the reaction zone despite the lowest fuel consumption in mass, among others, due to its high calorific value. Therefore, it has a higher flame temperature than others. At the same time, C3H8 has the lowest flame temperature. Besides, NH3 has the shortest flame length among others, while C3H8 has the most extended flame form. The highest level of NOx is released from the NH3 flame in the combustion chamber, while the lowest NOx is released from the CH4. However, the lowest NOx emission at the combustion chamber exit is obtained in NH3 combustion, while the highest NOx emission is obtained with H2 combustion. It results from the shortest flame length of NH3, short residence time, and backward NOx reduction to N2 for NH3. As for H2, high flame temperature and relatively long flame, and high residence time of the products trigger NOx formation and keep the NOx level high. On the other hand, excess air coefficient from 1.1 to 2 increases NOx for H2, CH4, and NH3 due to their large flame diameters, unlike propane. Then, NOx emission levels decrease sharply as the excess air coefficient increases to 4 for each fuel. NH3 fuel also emits minimum NOx in other excess air coefficients at the exit, while H2 emits too much emission. With NH3 enrichment, the NOx emissions of H2, CH4, and C3H8 fuels at the combustion chamber exit decrease gradually almost every excess air coefficient apart from λ = 1.1. As a general conclusion, like renewable fuels, H2 appears to be a source of pollution in terms of NOx emissions in combustion applications. In contrast, NH3 appears to be a relatively modest fuel with a low NOx level. In addition, the high amount of NOx emission released from H2 and other fuels during the combustion can be remarkably reduced by NH3 enrichment with an excess air combustion.

Low Nickel, Ceria Zirconia-Based Micro-Tubular Solid Oxide Fuel Cell: A Study of Composition and Oxidation Using Hydrogen and Methane Fuel

The study examines the effect of using low nickel (Ni) with high ceria (CeO2) anode content towards the oxidation of H2 and CH4 fuel by evaluating the activation energy of the ohmic process and charge transfer process. Using a micro-tubular solid oxide fuel cell (MT-SOFC), the anodes are made up of 50% YSZ with varying NiO:CeO2 percentages from 0% NiO, 50% CeO2 to 50% NiO, 0% CeO2. The performance is measured based on maximum power density (MPD), electrochemical impedance spectroscopy (EIS) and activation energy, Ea of the ohmic (Rohm) and charge transfer (Rct) processes. We found that by lowering the Ni content to lower than 50% NiO, anode conductivity will drop by 7-fold. An anode containing 37.5% NiO, 12.5% CeO2 yield MPD of 41.1 and 2.9 mW cm−2 when tested on H2 and CH4 fuels thus have the lowest Ni content without an abrupt negative effect on the MPD and EIS. The significant effect of conductivity drops on MPD and EIS are observed to occur at 25% NiO, 25% CeO2 and lower NiO content. However, anode content of 25% NiO, 25% CeO2 has the lowest Ea for Rct (29.74 kJ mol−1) for operation in CH4, making it the best anode composition to oxidize CH4. As a conclusion, an anode containing 25% NiO:25% CeO2:50% YSZ and 37.5% NiO:12.5% CeO2:50% YSZ shows promising results in becoming the low Ni anode for coking-tolerant SOFC.

Efficient CO2 reduction with H2O via photothermal chemical reaction based on Au-MgO dual catalytic site on TiO2

Using solar energy for the conversion of H2O and CO2 into H2, CO, CH4, and other solar fuels have attracted great interest worldwide. However, the greatest challenge is how to use both the light energy and the heat energy to improve the utilization efficiency of solar energy. Photo-thermal chemical reaction has been used for conversing H2O and CO2 into H2, CO, CH4, and other solar fuels and improved by Au and MgO co-loaded TiO2(AuMgTi). The average yields of CO, H2, and CH4 on AuMgTi were estimated to 45.495 umol/g, 45.072 umol/g, and 6.624 umol/g, respectively. Also, the selectivities of carbon-containing products increased from 29 % to 53.6 % when compared to P25. MgO on the surface of AuMgTi can enhance the chemisorption of CO2 and initiated the CO2 reduction reaction to converse CO2 into CO and CH4. Au can improve the utilization of visible light through the Localized Surface Plasmon Resonance (LSPR) effect, and reduce the recombination rate of photogenerated carriers due to the existence of the Schottky barrier. Moreover, Au might act as a catalytically active center on the surface to assist in the formation of the intermediate groups and facilitate the reaction. This study demonstrated that the AuMgTi catalyst can effectively utilize full spectrum solar energy for CO2 reduction by photo-thermal chemical reaction.

Effects of oxygen reduction and hydration on performance of protonic ceramic fuel cells in hydrocarbon fuels

The performance of composite cathodes with different cathode materials, such as (Ba0.95La0.05) (Fe0.8Zn0.2)O3-δ (BLFZ), (Ba0.5Sr0.5) (Co0.8Fe0.2)O3-δ (BSCF), and (La0.8Sr0.2)CoO3-δ (LSC), are investigated to find the effects of the oxygen reduction reaction (ORR) and hydration on the fuel cell performance of protonic ceramic fuel cells. BLFZ, which exhibits the highest degree of hydration among these three materials, demonstrates the best electrode performance in hydrogen fuel, although BLFZ shows poor ORR activity compared with BSCF and LSC. When LSC is added into the BLFZ composite cathode to enhance the ORR activity, the electrode performance in H2 fuel improves slightly, but that in CH4 fuel improves significantly, thereby resulting in an increase in the power density from 0.48 to 0.72 W/cm2 in CH4 fuel at 600 °C. By analyzing the relaxation time distribution, the contributions of the ORR and hydration reaction at the cathode to the performance of H2 and CH4 fuels are investigated separately. It is demonstrated that the ORR activity and hydration reaction can be optimized simultaneously by infiltrating the ORR catalysts into cathode materials with high hydration.

Effects of Various Fuels on Combustion and Emission Characteristics of a Four-Stroke Dual-Fuel Marine Engine

A numerical study was carried out to investigate the effects of methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and dimethyl ether (DME) on the combustion and emission characteristics of a four-stroke gas-diesel dual-fuel (DF) marine engine at full load. Three-dimensional simulations of the combustion process and emission formation inside the engine cylinder in the diesel and DF modes were performed using the AVL FIRE R2018a simulation software to analyze the in-cylinder pressure, temperature, and emission characteristics. The simulation results agreed well with the measured values reported in the engine shop test technical data. The simulation results showed reductions in the in-cylinder peak pressure and temperatures, as well as the emission formations, in the DF modes in comparison to the diesel mode. The DF mode could significantly reduce nitric oxide (NO) emissions (up to 96.225%) of DME compared to the diesel mode. Meanwhile, C3H8 and CH4 fuels effectively reduced the soot (up to 82.78%) and carbon dioxide (CO2) emissions (by 21.33%), respectively, compared to the diesel mode. However, the results also showed longer ignition delay times of the combustion processes when the engine operated in the DF mode, particularly in the DME-diesel mode. The combustion and emission characteristics of the engine were also analyzed when varying the injection timing; the results showed that applying the injection timing adjustment method could further address NO emission problems but led to a decrease in the engine power. Therefore, it is necessary to consider the benefits and disadvantages of adopting the injection timing adjustment strategy to address certain engine emission problems. This study successfully analyzed the benefits of using various gas fuels as alternative fuels and the injection timing adjustment method in DF marine engines to meet the International Maritime Organization (IMO) emission regulations without the use of any emission after-treatment devices.

Triple-component composite cathode for performance optimization of protonic ceramic fuel cells

The cathode of a protonic ceramic fuel cell must be able to facilitate ion and electron transfer, while simultaneously possessing a high catalytic activity for steam generation and the dissociation of gas-phase molecules. In this study, the performance of a cathode for protonic ceramic fuel cells is optimized by employing a triple-component composite cathode design, which integrates proton conductors, mixed electronic–ionic conductors, and a catalytic layer. Additionally, two other composite cathodes are fabricated for comparison. Owing to its higher electrical conductivity but lower catalytic activity, the composite cathode with protonic ceramic and (Ba0.95La0.05) (Fe0.8Zn0.2)O3-δ (BLFZ) exhibits lower ohmic resistance but poor catalytic activity compared to the composite cathode with protonic ceramic and Ba(Co0.4Fe0.4Zr0.1Y0.1)O3-δ (BCFZY). The triple-component cathode is fabricated by infiltrating BCFZY into a composite cathode composed of BLFZ and protonic ceramic, and both the ohmic and non-ohmic resistances of the cathode are optimized in CH4 and H2 fuels. In particular, the performance of CH4 fuel is significantly improved by adopting a triple-component cathode. These results suggest a possible contribution of the oxygen reduction reaction at the cathode to the reformation of CH4 at the anode.

Anodic Performance of BaO-Added Ni/SDC for Solid Oxide Fuel Cell Fed With Dry CH4

SOFCs fed with dry H2 and CH4 fuels were examined using 20 wt% Ni/SDC and 0.2 wt% BaO-added 20 wt% Ni/SDC [Ni(BaO)/SDC] anodes. The i–v characteristics of the cells in H2 and CH4 resulted in a higher output produced by CH4 fuel compared to that produced by H2 fuel in both anodes. In both fuels, better anode characteristics were obtained for Ni(BaO)/SDC. Consequently, the anodic performance was in the order of Ni(BaO)/SDC in CH4 &amp;gt; Ni/SDC in CH4 &amp;gt; Ni(BaO)/SDC in H2 &amp;gt; Ni/SDC in H2. A significant carbon deposition was observed in the Ni/SDC anode in CH4, but the carbon deposition observed in Ni(BaO)/SDC was less. From the DC electrical resistance measurement of the anode films, a remarkable decrease in resistance was observed in Ni/SDC due to the carbon deposition after CH4 exposure. The resistance of Ni(BaO)/SDC was higher than that of Ni/SDC and did not change even after CH4 exposure because of the less carbon deposit. The high dispersibility of Ni particles was confirmed in both anodes and was particularly remarkable in Ni(BaO)/SDC. The highest anodic performance in Ni(BaO)/SDC was attributed to the high Ni dispersibility which might promote CH4 decomposition by producing less carbon deposit. It was speculated that the higher cell output in CH4 than that in H2 is due to the locally high concentration of H2 and/or CO gas on the anode surface by the promotion of CH4 decomposition.

Potassium/oxygen co-doped polymeric carbon nitride for enhanced photocatalytic CO2 reduction

Polymeric carbon nitride (CN) is a cheap and visible-light photocatalyst for potentially real application of CO2 photoreduction into solar fuels. However, CN prepared by conventional one-step method always shows serious charge recombination and low charge mobility. Herein, we use urea as the starting material with K2S2O8 to synthesize defect-modified ultrathin CN nanosheets. The as-prepared samples exhibit much higher photocatalytic CO2-reduction efficiency than the pristine CN with the production of CO and CH4 fuels. The enhanced photocatalytic activity of modified samples is ascribed to the co-doping of K and O atoms into the CN frameworks, which promotes electron transfer from the bulk to the interface and narrows the band gap to provide a broad range of visible-light response. This work provides a simple but effective strategy to prepare defect-modified polymeric carbon nitride for various photocatalytic applications.

A step-by-step synergistic stripping approach toward ultra-thin porous g-C3N4 nanosheets with high conduction band position for photocatalystic CO2 reduction

The pristine g-C3N4 (BCN) with a low conversion efficiency of CO2 exits with small specific surface area, weak CO2 adsorption and severe recombination of photo-generated charges. The stripping of few-layer g-C3N4 represents excellent photocatalytic performance, which attracts extensive attention in photocatalytic CO2 reduction. In the present study, the ultra-thin porous g-C3N4 (THCN) with high specific surface area and high position of conduction band was prepared using step-by-step synergistic exfoliation. Further, we treated it with HCl-assisted hydrothermal stripping and successive thermal stripping/etching in air. Our results showed that the THCN exhibited the best CO2 conversion efficiency from CO2 to CH4 and CO fuels, compared with g-C3N4 (HCN) prepared by HCl-assisted hydrothermal stripping and g-C3N4 (TCN) prepared by thermal stripping/etching in air. Further, the excellent photocatalytic performance for CO2 reduction was mainly attributed to its high specific surface area and rich pores, excellent separation and utilization efficiency of photo-generated carriers, and upper position of conduction band. Due to its wide band gap and high specific surface area, the THCN also showed significantly better degradation for Rhodamine B than BCN, HCN and TCN. Nonetheless, using a simple two-step stripping strategy, we prepared and obtained an ultra-thin porous g-C3N4 nanosheets with a high specific surface area for CO2 conversion to CH4 and CO fuels. This ultimately provided a reference for preparation of other two-dimensional ultra-thin materials for CO2 reduction.