Free Catalyst Research Articles

Elucidating Synergistic Effects of Different Metal Ratios in Bimetallic Fe/Co-N-C Catalysts for Oxygen Reduction Reaction

Lowering or eliminating the noble-metal content in oxygen reduction fuel cell catalysts could propel the large-scale introduction of commercial fuel cell systems. Several noble-metal free catalysts are already under investigation with the metal-nitrogen-carbon (Me-N-C) system being one of the most promising. In this study, a systematic approach to investigate the influence of metal ratios in bimetallic Me-N-C fuel cells oxygen reduction reaction (ORR) catalysts has been taken. Different catalysts with varying ratios of Fe and Co have been synthesized and characterized both physically and electrochemically in terms of activity, selectivity and stability with the addition of an accelerated stress test (AST). The catalysts show different electrochemical properties depending on the metal ratio such as a high electrochemical mass activity with increasing Fe ratio. Properties do not change linearly with the metal ratio, with a Fe/Co ratio of 5:3 showing a higher mass activity with simultaneous higher stability. Selectivity indicators plateau for catalysts with a Co content of 50% metal ratio and less, showing the same values as a monometallic Co catalyst. These findings indicate a deeper relationship between the ratio of different metals and physical and electrochemical properties in bimetallic Me-N-C catalysts.

Production of dual functional carbon material from biomass treated with NaOH for supercapacitor and catalyst

AbstractIn this research, tea factory waste (TFW), an organic biomass source, was used as a metal free basic catalyst for the production of hydrogen from NaBH4 by methanolysis. In the preparation of the catalysts, tea factory wastes taken from the tea factory and released during tea production were used. For the production of H2 from NaBH4 by methanolysis, TFW was pre‐treated with different base materials (KOH, NH4Cl, and NaOH). Tests have been made on different mixing ratios, combustion temperatures, and durations for catalyst optimization by selecting the base with the highest hydrogen yield. Hydrogen production efficiency was recorded as 2832.2 and 6560.8 mL min−1 g−1 at 30°C and 60°C, respectively. The activation energy of the catalyst was defined as 18.07 kJ mol−1. Characterization of TFW catalyst was examined by SEM‐EDX, XRD, and FT‐IR analysis. When the obtained data were evaluated and some studies available in the literature were considered, it was concluded that tea factory wastes, which are a biomass resource, can be used in catalyst production. The capacitance value of the electrode was computed as 8 to 12.9 F g−1.

Reduced formation of peroxide and radical species stabilises iron-based hybrid catalysts in polymer electrolyte membrane fuel cells

The incorporation of Pt into an iron-nitrogen-carbon (FeNC) catalyst for the oxygen reduction reaction (ORR) was recently shown to enhance catalyst stability without Pt directly contributing to the ORR activity. However, the mechanistic origin of this stabilisation remained obscure. It is established herein with rotating ring disc experiments that the side product, H2O2, which is known to damage FeNC catalysts, is suppressed by the presence of Pt. The formation of reactive oxygen species is additionally inhibited, independent of intrinsic H2O2 formation, as determined by electron paramagnetic resonance. Transmission electron microscopy identifies an oxidised Fe-rich layer covering the Pt particles, thus explaining the inactivity of the latter towards the ORR. These insights develop understanding of FeNC degradation mechanisms during ORR catalysis, and crucially establish the required properties of a precious metal free protective catalyst to improve FeNC stability in acidic media.

Extended Enolates: Versatile Intermediates for Asymmetric C-H Functionalization via Noncovalent Catalysis.

Catalyst‐controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site‐ and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate‐catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field.

Recyclable Catalysts for Alkyne Functionalization.

In this review, we present an assessment of recent advances in alkyne functionalization reactions, classified according to different classes of recyclable catalysts. In this work, we have incorporated and reviewed the activity and selectivity of recyclable catalytic systems such as polysiloxane-encapsulated novel metal nanoparticle-based catalysts, silica–copper-supported nanocatalysts, graphitic carbon-supported nanocatalysts, metal organic framework (MOF) catalysts, porous organic framework (POP) catalysts, bio-material-supported catalysts, and metal/solvent free recyclable catalysts. In addition, several alkyne functionalization reactions have been elucidated to demonstrate the success and efficiency of recyclable catalysts. In addition, this review also provides the fundamental knowledge required for utilization of green catalysts, which can combine the advantageous features of both homogeneous (catalyst modulation) and heterogeneous (catalyst recycling) catalysis.

A Liquid–Vapor Two-Phase Model of Direct Methanol Fuel Cells With Platinum Group Metal-Free Cathode Catalyst

Abstract This study has developed a two-dimensional, two-phase transport model to investigate the transport characteristics in direct methanol fuel cells (DMFCs) using platinum group metal (PGM)-free cathode catalysts. The model considered anisotropic properties of the gas diffusion layer (GDL) caused by current collector’s mechanical compression, the interfacial mass transfer of water and methanol between liquid and vapor, and unique properties of the cathode PGM-free catalyst layer. Results showed that the liquid methanol solution from the anode could provide sufficient water to hydrate the proton exchange membrane (PEM), and the relative humidity of the cathode air did not impact the membrane hydration. Fully hydrating the cathode air may deteriorate the fuel cell performance, especially when the operating temperature is close to 100 °C because the exponential increase of the saturated water pressure with temperature decreased the partial pressure of oxygen. The optimized operating temperature increased with the increase of air pressure and was about 80 °C at 1.5 atm cathode pressure. To achieve the US Department of Energy’s performance target of 300 mW/cm2 peak power density, catalytic activities of both the anode and cathode catalysts need to be improved by one order of magnitude compared with the state-of-the-art commercial catalysts.

Atomically Dispersed Single Metal Site Catalysts for Oxygen Reduction in Acidic Media

Proton exchange membrane fuel cells (PEMFCs) are leading candidates in utilizing clean energy resources for application in transportation, stationary, and portable devices. In PEMFCs, cathode catalysts are crucial for overall performance and durability due to kinetically slow oxygen reduction reactions (ORR). Because platinum (Pt), a state-of-the-art ORR catalyst, is rare and expensive, the development of high-performance platinum metal group (PGM)-free catalysts is highly desirable for future fuel cell technologies. Among various PGM-free catalyst formulations, metal and nitrogen co-doped carbon (M-N-C, M: Fe, Co, or Mn) catalysts have exhibited encouraging activity and stability in acidic media for the ORR, which possess great potential to replace Pt in the future. Therefore, based on our extensive experience in the field of ORR catalysis, the talk will comprehensively summarize the basic principles in the design and synthesis of M-N-C catalysts for durable, inexpensive, and high-performance PEMFCs with an emphasis on Co- and Mn-N-C catalysts to avoid Fenton reactions between Fe2+ and H2O2, which can generate free radicals and lead to the degradation of catalysts, ionomers, and membranes in PEMFCs. Furthermore, template-free 3D hydrocarbon frameworks as attractive precursors to advanced M-N-C catalysts will be discussed to enhance intrinsic ORR activities in acidic media significantly. Also, the long-term performance durability of M-N-C cathodes will be discussed extensively to provide potential solutions to enhance catalyst stability in PEMFCs. Finally, this talk will provide an overall perspective on the progress, challenges, and solutions of PGM-free catalysts for future PEMFC technologies.

Enabling Efficient Water Splitting with Advanced Materials Designed for High pH Membrane Interface

This research focuses on durable, high-performance materials and interfaces for advanced water splitting, enabling a clear pathway for achieving <$2/KgH2 (on scale) via anion exchange membrane (AEM)-based electrolysis. We aim to advance this goal via an improved fundamental understanding of both hydrogen and oxygen evolution reactions (HER/OER) leading to platinum group metal (PGM)-free catalyst materials. Here, we use NiFeCo and NiMo as OER and HER catalysts, respectively, in a full alkaline electrolysis cell. A thermally stable, multi-cation AEM is used to operate the cell at elevated temperatures thereby lowering the operating potential by increasing the kinetics of the respective catalytic reactions. The transition metal catalysts, paired with this novel AEM, have been used to achieve an operating potential of 1.79 V at 1 A/cm2. This potential is 300 mV lower than similar electrolysis cells equipped with PGM-based catalysts. We have also demonstrated the efficacy of the less caustic potassium carbonate solution as a replacement for potassium hydroxide as an electrolyte. By using a carbonate-based electrolyte, the hydroxide ion required for the anodic reaction is continuously replenished by re-establishing an equilibrium with water. This consideration, coupled with the reduced alkalinity of the carbonate solution, yields an electrolysis cell capable of sustained operation with minimal increase in operating potential.

(Invited) Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) have shown high oxygen reduction reaction activity in alkaline media. In order to reduce the overpotential losses in anion-exchange membrane fuel cells (AEMFCs), PGM-free catalysts need to have a high site density to compensate for their which will enable reaching high cell current and power densities. Herein, we will present our work on the synthesis, characterization, and utilization of heat-treated iron porphyrin aerogels as cathode catalysts in AEMFCs. The heat treatment effect was thoroughly studied using several techniques, and the best performing aerogel was showing an excellent performance in AEMFCs, reaching a peak power density of 580 mW cm-2 and a limiting current density of as high as 2.0 A cm-2, which can be considered as the state-of-the-art for PGM-free based AEMFCs.

Impact of the Cathode Layer Printing Process on the Performance of MEA Integrating PGM Free Catalyst

In this work, platinum group metal (PGM) free-based cathode active layers were prepared using different printing techniques. The membrane electrode assemblies (MEAs) integrate a PGM free catalyst based on Fe, N and C atoms at the cathode side. Scanning electron microscopy (SEM) images of MEA cross sections showed the strong impact of the fabrication process on the cathode structure, the porosity and the ionomer repartition. The MEAs were characterized in a 25 cm2 single cell using cyclic voltammetry under H2/N2. The performance of the MEAs and the double layer capacity of the cathodes were also shown to be linked to the process used. The comparison of the electrochemical accessible surface of the catalyst and of its surface area (SBET) led to the determination of a utilization factor. The coated membrane (CCM) made using the decal transfer process gives the best performances.

Noble metal free catalyst with High activity and stability for catalytic wet air oxidation of N, N-dimethylformamide

In this study, we propose a simple strategy to prepare a noble-metal free catalyst, i.e., CeO2 supported on multi-wall carbon nanotube (CNTs). CNTs are treated by acid before the loading of CeO2, which is vital to the performance of catalysts. CeO2 on treated CNTs (CNTs-M) presents much higher TOC conversion and stability when compared with CeO2 on raw CNTs (CNTs-Raw). Kinetic study indicates that acid treatment greatly decreases the activation of the DMF oxidation. To investigate these phenomena, detailed characterizations are performed. We found that the acid treatment generated surface functional groups on CNTs which leads to the strong interaction between CNTs and CeO2. As a result, CeO2 on CNTs-M obtained better dispersion and higher amount of surface oxygen, which accounts for its superior performance.

Efficient hydrothermal growth of high-performance MoS2/pyramid-Si photocathodes by surface hydrophilicity engineering

Compared with traditional precious metal catalysts, MoS2 is regarded as the promising hydrogen evolution reaction catalyst for Si photocathodes, because of its higher photochemical stability and lower hydrogen adsorption free energy. However, the layers of MoS2 are connected by van der Waals force, and the interaction force between the layers of MoS2 and Si is weak. Therefore, the homogeneous MoS2 nanosheets directly synthesized on Si by the hydrothermal method remain challenging. Herein, we report a facile strategy for the fast and efficient growth of vertically standing MoS2 on pyramid Si via surface hydrophilicity tailoring by inserting a thin TiO2 layer. Thanks to the improved light trapping and catalytic kinetics, the optimal MoS2/TiO2/Si shows a short-circuit photocurrent density of 9.79 mA/cm2 and an onset potential of 0.31 V vs reversible hydrogen electrode under 100 mW/cm2 Xe-lamp illumination. By comparing with different oxide buffer layers, surface hydrophilicity is found to be essential for promoting the homogenous nucleation and growth of vertically standing MoS2 by the hydrothermal method. These results not only provide valuable insight into the growth kinetics of transition metal chalcogenides in aqueous solution but also deliver an efficient route for assembling noble-metal free catalysts on silicon-based photoelectrodes.

An Efficient and Functional Fe₃O₄/Co₃O₄ Composite for Oxygen Evolution Reaction.

The design of efficient, stable, durable and noble metal free electro catalysts for oxygen evolution reaction (OER) are of immediate need, but very challenging task. In this study, iron induction into cobalt oxide (Co₃O₄) has resulted composite structure by wet chemical method. The iron impurity has brought an electronic disorder into Fe₃O₄/cobalt oxide composite thereby efficient oxygen evolution reaction is demonstrated. An addition of iron content into composite resulted the alternation of morphology from Nano rods to clusters of nanoparticles. The successive addition of iron into composite system reduced the onset potential of OER as compared to the pristine cobalt oxide. A Tafel slope of 80 mVdec-1 indicates the favorable oxygen evolution reaction kinetics on the sample 4. An over-potential of 370 mV is required to reach a 10 mAcm-2 current density which is acceptable for a nonprecious catalyst. The catalyst is highly durable and stable for 30 hours. Electrochemical impedance spectroscopy further provided a deeper insight on charge transfer resistance and sample 4 has low charge transfer resistance that supported the OER polarization curves. The sample 4 has more electrochemical active surface area of 393.5 cm². These obtained results are exciting and highlighting the importance of composite structure and leave a huge space for the future investigations on composite materials for energy related applications.

Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion‐Exchange Membrane Fuel Cells

AbstractPlatinum group metal (PGM)‐free catalysts for oxygen reduction reaction have shown high oxygen reduction reaction activity in alkaline media. In order to further increase the power density of anion‐exchange membrane fuel cells (AEMFCs), PGM‐free catalysts need to have a high site density to reach high current densities. Herein, synthesis, characterization, and utilization of heat‐treated iron porphyrin aerogels are reported as cathode catalysts in AEMFCs. The heat treatment effect is thoroughly studied and characterized using several techniques, and the best performing aerogel is studied in AEMFC, showing excellent performance, reaching a peak power density of 580 mW cm−2 and a limiting current density of as high as 2.0 A cm−2, which can be considered the state‐of‐the‐art for PGM‐free based AEMFCs.

Aqueous/solid state Zn-air batteries based on N doped graphdiyne as efficient metal-free bifunctional catalyst

Abstract The undesirable electrocatalytic activity, durability and cost of cathode catalyst are main obstacles of rechargeable zinc air batteries (ZABs). Here, the stable operated aqueous ZABs and solid state ZABs are fabricated firstly with N-doped graphdiyne (GDY) as efficient metal free bifunctional catalyst. Benefiting from the uneven charge density caused by the coexistence of sp-, sp2- hybridized C atoms and pyridinic N, the abundant reaction interface and mass transmission channels provided by plentiful pores, N-doped GDY shows high catalytic performance. The as-prepared catalyst endows aqueous ZAB with high open circuit voltage of 1.54 V, lower polarization level than commercial Pt/C or IrO2, and especially long-time stability for 300 h (450 cycles). Density functional theory calculations results also confirm the better catalytic activity of pyridinic N than graphite N and amino-N in GDY nanostructure, since it renders the adjacent C atoms positively charged most effectively. This study provides a new kind of effective and ultra-stable metal free bifunctional catalyst applied for ZABs, especially the solid-state ZAB with excellent flexibility and variable shape for wearable electronics.

Promoting Atomically Dispersed MnN4 Sites via Sulfur Doping for Oxygen Reduction: Unveiling Intrinsic Activity and Degradation in Fuel Cells.

Carbon supported and nitrogen coordinated single Mn site (Mn-N-C) catalysts are the most desirable platinum group metal (PGM)-free cathode catalysts for proton-exchange membrane fuel cells (PEMFCs) due to their insignificant Fenton reactions (vs. Fe), earth abundances (vs. Co), and encouraging activity and stability. However, current Mn-N-C catalysts suffer from high overpotential due to low intrinsic activity and less dense MnN4 sites. Herein, we present a sulfur-doped Mn-N-C catalyst (Mn-N-C-S) synthesized through an effective adsorption-pyrolysis process. Using electron microscopy and X-ray absorption spectroscopy (XAS) techniques, we verify the uniform dispersion of MnN4 sites and confirm the effect of S doping on the Mn-N coordination. The Mn-N-C-S catalyst exhibits a favorable oxygen reduction reaction (ORR) activity in acidic media relative to the S-free Mn-N-C catalyst. The corresponding membrane electrode assembly (MEA) generates enhanced performance with a peak power density of 500 mW cm-2 under a realistic H2/air environment. The constant voltage tests of fuel cells confirm the much-enhanced stability of the Mn-N-C-S catalyst compared to the Fe-N-C and Fe-N-C-S catalysts. The electron microscopy and Fourier transform XAS analyses provide insights into catalyst degradation associated with Mn oxidation and agglomeration. The theoretical calculation elucidates that the promoted ORR activity is mainly attributed to the spatial effect stemmed from the repulsive interaction between the ORR intermediates and adjacent S dopants.

Laboratory-Scale Research of Non-Catalyzed Supercritical Alcohol Process for Continuous Biodiesel Production

This work investigates the non-catalyzed supercritical methanol (SCM) process for continuous biodiesel production. The lab-scale setup was designed and used for biodiesel production in the temperature range of 520–650 K and 83–380 bar with an oil-to-methanol molar ratio ranging from 1:5 to 1:45. The experiments were performed in the coiled plug flow tubular reactor. The volumetric flow rate of the methanol/oil ranged from 0.1–10 mL/min. This work examines a new reactor technology involving preheating and pre-mixing of the methanol/oil mixture to reduce setup cost and increase biodiesel yield under the same reaction conditions. Work performed showed that FAME’s yield increased rapidly with temperature and pressure above the methanol critical points (i.e., 513 K and 79.5 bar). The best methyl-ester yield using this reaction technology was 91% at 590 K temperature and 351 bars with an oil-to-methanol ratio of 39 and a 15-min residence time. Furthermore, the kinetics of the free catalyst transesterification process was studied in supercritical methanol under different reaction conditions.

Boron-rich boron nitride nanomaterials as efficient metal-free catalysts for converting CO2 into valuable fuel

Searching for low-cost and metal free catalysts for efficient reduction of CO2 is urgent because metal free catalysts have many advantages, such as high stability and environmental friendly. Based on the comprehensive density functional theory (DFT) calculations of CO2 conversion on boron nitride nanomaterials, the results indicate that the pristine boron nitride nanosheet (BNNS) and boron nitride nanotube (BNNT) cannot efficiently activate CO2, while boron-rich BNNS and BNNT can efficiently convert CO2 to CH4 with very low limiting potentials of −0.27 V and −0.23 V, respectively. In addition, the studies of Mulliken charge analysis of these catalysts also indicate that the two boron-rich structures prefer to donating electrons to CO2 and result in the efficient activation of the C = O double bonds, which promote the activation of CO2 and the following conversion. Moreover, the better performance of CO2 reduction on the B-rich BNNT indicates that the curvature of the surface can further improve its catalytic activity.

Introducing a novel technique for measuring hydrogen crossover in membrane-based electrochemical cells

Hydrogen crossover that is the unwanted hydrogen permeation across the membrane driven by the difference of gas concentrations causes a critical concern of safety and efficiency for electrochemical cells, such as fuel cells and electrolyzer cells. Although the hydrogen crossover measurement in fuel cells that employ platinum based catalysts is simple and widely used in laboratory settings, it is questionable to apply existing limiting current method to water electrolyzer cells and alkaline exchange membrane (AEM) systems, which is due to the typical catalyst materials used and membrane properties, respectively. In this work, we demonstrate the operation of a compact and low-cost method of measuring hydrogen crossover that works for both AEM and proton exchange membrane (PEM) systems. The method entails a tandem configuration that utilizes an upstream crossover cell with a downstream cell in hydrogen pump configuration to measure the crossover in the cell of interest. We have successfully measured the hydrogen crossover with different membranes at various differential pressures. The developed method can be applied to catalyst-free membranes (both PEM and AEM) as well as PGM free catalyst containing cells. It will be a promising technique for measuring hydrogen crossover in-situ for a real operating membraned-based electrochemical cell or stack.

Composite of bentonite and cyclodextrin as an efficient catalyst for promoting chemical transformations in aqueous media

Combining the encapsulating capability of cyclodextrin and instinctive features of bentonite clay, a versatile metal free catalyst has been developed that could promote various chemical reactions such as Knoevenagel condensation, synthesis of xanthan and octahydroquinazolinones in aqueous media under ultrasonic irradiation. To prepare the catalyst, bentonite was Cl-functionalized and then reacted with isatin and guanidine successively to furnish amino functionalized bentonite. The latter then reacted with tosylated cyclodextrin. The resultant catalytic composite was characterized via XRD, SEM, EDS, BET, elemental mapping analysis, TGA and FTIR. The catalytic activity tests approved excellent activity of the catalyst as well as broad substrate scope. Notably, the catalyst could be simply recovered and reused for several reaction runs. Moreover, the activity of the composite was superior to that of its components.