Free Catalyst Research Articles

Performance and Durability of Mn-Based Platinum Group-Metal Free Catalyst in Membrane Electrode Assemblies

Proton exchange membrane fuel cells (PEMFCs) have drawn increasing attention in automotive and residential applications. To significantly reduce the cost of the PEMFC stacks, current platinum-group metal (PGM) catalysts must be replaced by PGM-free catalysts. Although Fe-based PGM-free catalysts (Fe-N-C) have exhibited promisingly high activity for the oxygen reduction reaction (ORR)1, they suffer from insufficient stability. Additionally, a major issue associated with Fe-containing catalysts is that dissolved iron ions can facilitate Fenton’s reaction thus forming a strong radical known to degrade the ionomer and membrane2.Mn-based catalysts have recently been developed, aiming to improve the catalyst durability and mitigate the Fenton’s reaction3. However, the catalyst stability and integration into membrane electrode assemblies (MEAs) have yet to be studied. In this work, density function theory (DFT) has been applied to study the active sites and their degradation mechanism. Computational fluid dynamic (CFD) modeling was also applied to study water, oxygen and proton transport in the electrode. Electrode fabrication optimizations were conducted based on the guidance from the CFD results. The optimizations include ionomer content, electrode porosity and MEA fabrication approaches It has been found that Mn-based catalysts showed improved MEA durability compared to Fe-based catalysts due to the higher carbon graphitization degree, reduced de-metallization and mitigation of peroxide radical formation. However, there is still a significant performance degradation compared to PGM-based MEAs. X-ray absorption has been applied to examine the catalyst and corresponding MEAs before and after durability. The degradation is primarily attributed to the oxidation and agglomeration of active sites. Acknowledgement: The project is financially supported by the Department of Energy’s HFTO under the Grant DE-EE0008075. Reference:1. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science, 332, 443, 2011.2. F Coms, H Xu, T. McCallum, C. Mittelsteadt, ECS Trans. 64, 389, 2014.3. Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nature Catalysis 2018, 1 (12), 935-945.

Flexible and disposable gold nanoparticles-N-doped carbon-modified electrochemical sensor for simultaneous detection of dopamine and uric acid

Catalytic and electrocatalytic applications of supported metal nanoparticles are hindered due to an aggregation of metal nanoparticles and catalytic leaching under harsh operations. Hence, stable and leaching free catalysts with high surface area are extremely desirable but also challenging. Here we report a gold nanoparticles-hosted mesoporous nitrogen doped carbon matrix, which is prepared using bovine serum albumin (BSA) through calcination. BSA plays three roles in this process as a reducing agent, capping agent and carbon precursor, hence the protocol exhibits economic and sustainable. Gold nanoparticles at N-doped BSA carbon (AuNPs@NBSAC)-modified three-electrode strip-based flexible sensor system has been developed, which displayed effective, sensitive and selective for simultaneous detection of uric acid (UA) and dopamine (DA). The AuNPs@NBSAC-modified sensor showed an excellent response toward DA with a linear response throughout the concentration range from 1 to 50 μM and a detection limit of 0.05 μM. It also exhibited an excellent response toward UA, with a wide detection range from 5 to 200 μM as well as a detection limit of 0.1 μM. The findings suggest that the AuNPs@NBSAC nanohybrid reveals promising applications and can be considered as potential electrode materials for development of electrochemical biosensors.

2H-MoS2 on Mo2CTx MXene Nanohybrid for Efficient and Durable Electrocatalytic Hydrogen Evolution.

The development of highly efficient and durable earth-abundant hydrogen evolution reaction (HER) catalysts is crucial for the extensive implementation of the hydrogen economy. Members of the 2D MXenes family, particularly Mo2CTx, have recently been identified as promising HER catalysts. However, their inherent oxidative instability in air and aqueous electrolyte solutions is hindering their widespread use. Herein, we present a simple and scalable method to circumvent adventitious oxidation in Mo2CTx MXenes via in situ sulfidation to form a Mo2CTx/2H-MoS2 nanohybrid. The intimate epitaxial coupling at the Mo2CTx/2H-MoS2 nanohybrid interface afforded superior HER activities, requiring only 119 or 182 mV overpotential to yield -10 or -100 mA cm-2geom current densities, respectively. Density functional theory calculations reveal strongest interfacial adhesion was found within the nanohybrid structure as compared to the physisorbed nanohybrid, and the possibility to tune the HER overpotential through manipulating the extent of MXene sulfidation. Critically, the presence of 2H-MoS2 suppresses further oxidation of the MXene layer, enabling the nanohybrid to sustain industrially relevant current densities of over -450 mA cm-2geom with exceptional durability. Less than 30 mV overpotential degradation was observed after 10 continuous days of electrolysis at a fixed -10 mA cm-2geom current density or 100,000 successive cyclic voltammetry cycles. The exceptional HER durability of the Mo2CTx/2H-MoS2 nanohybrid presents a major step forward to realize practical implementation of MXenes as noble metal free catalysts for broad-based applications in water splitting and energy conversion.

Fabrication of a metal free catalyst for chemical reactions through decoration of chitosan with ionic liquid terminated dendritic moiety

In attempt to develop a biocompatible metal-free catalyst, a dendritic moiety was grown on chitosan through successive reactions with 2,4,6-trichloro-1,3,5-triazine and ethylenediamine. Subsequently, the terminal functional groups of the dendron were decorated with 1-methylimidazolium chloride. The catalyst was characterized with SEM, EDS, TGA, FTIR, XRD and mapping analysis. Then, the catalytic activity of the resultant composite was scrutinized for catalyzing Knoevenagel condensation and synthesis of xanthene derivatives in aqueous media under mild reaction condition. The results confirmed high activity of the catalyst, superior to ionic liquid free counterpart and bare chitosan. This observation was ascribed to the instinct catalytic activity of ionic liquid. Moreover, using control catalysts, it was confirmed that the presence of the dendritic moiety that could increase the content of ionic liquid on the backbone of the catalyst enhanced the catalytic activity.

N-p Heterojunction of TiO2-NiO core-shell structure for efficient hydrogen generation and lignin photoreforming

Hydrogen evolution from biomass photoreforming has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution, as it could generate sustainable H2 and value-added bioproducts simultaneously. Combining p-type semiconductors with n-type semiconductors to form n-p heterojunction is an effective strategy to improve the photocatalytic quantum efficiency by enhancing the separation of photogenerated electrons and holes, which could greatly facilitate the realization of such biomass photorefinery concept. However, the incompact contact between the n-type and p-type semiconductors often induces the aggregation of photogenerated electrons and holes. In this work, we design and synthesize an ultrafine n-p heterojunction TiO2-NiO core-shell structure to overcome the incompact contact in the n-p interface. When the n-p heterojunction photocatalysts are evaluated for photocatalytic water splitting and biomass lignin photoreforming respectively, the as-fabricated TiO2-NiO nanocomposite with 3.25% NiO demonstrates the highest hydrogen generation of 23.5 mmol h−1 g−1 from water splitting and H2 (0.45 mmol h−1 g−1) and CH4 (0.03 mmol h−1 g−1) cogeneration with reasonable amount of fatty acids (palmitic acid and stearic acid) production from lignin photoreforming. The excellent photocatalytic activity is ascribed to the synergistic effects of high crystallinity of TiO2 ultrafine nanoparticles, core-shell structure and n-p heterojunction with NiO nanoclusters. This present work demonstrates a simple and efficient method to fabricate ultrafine n-p heterojunction core-shell structure for noble-metal free catalyst for both water splitting and biomass photoreforming.

Carbon and Nitrogen Composition for Non-Precious Metal Catalyst to Physical Characterization and Electrochemical Properties

This paper investigates the physical properties and electrochemical properties of the innovative non-precious metal catalyst using different carbon types. The cathode catalyst for PEMFC (Proton Exchange Membrane Fuel Cell) is an important part of fuel cell because the reaction of the cathode is three times lower than the anode. Otherwise, the high cost of Pt/C catalyst for cathode needs to be replaced using low-cost material. Therefore, this research fabricated Pt free catalyst. FeCl3.6H2O was used as a metal precursor. Urea and PVP as a nitrogen (N) source were mixed with carbon. The variations of carbon are Graphite (Gt), Charcoal Active (CA), and Calcined Petroleum Coke (CPC). As prepared catalysts, were noted as Fe/N-Gt, Fe/N-CA, and Fe/N-CPC. Catalysts without nitrogen addition also were synthesized such as Fe-Gt, Fe-CA, and Fe-CPC for comparison. The electrochemical properties can be evaluated form Cycle Voltammograms (CV) curve. Graphite supported catalyst has anodic and cathodic peak otherwise has the lowest capacity. It means that the redox reaction occurs during CV measurement for Fe/N-Gt and Fe-Gt catalyst. Nitrogen addition of graphite supported catalyst has a higher current density than Fe-Gt catalyst. The morphology of the catalyst was identified by Scanning Electron Microscope (SEM). Different particle shape for carbon types can be observed by SEM image of obtained catalyst. Energy Dispersive X-Ray EDX to identify the chemical composition. Nitrogen-doped carbon caused the formation of Fe2O3 and it was determined by X-ray diffraction (XRD).

Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts

Atomically dispersed and nitrogen coordinated single metal sites (M-N-C, M=Fe, Co, Ni, Mn) are the popular platinum group-metal (PGM)-free catalysts for many electrochemical reactions. Traditional wet-chemistry catalyst synthesis often requires complex procedures with unsatisfied reproducibility and scalability. Here, we report a facile chemical vapor deposition (CVD) strategy to synthesize the promising M-N-C catalysts. The deposition of gaseous 2-methylimidazole onto M-doped ZnO substrates, followed by an in situ thermal activation, effectively generated single metal sites well dispersed into porous carbon. In particular, an optimal CVD-derived Fe-N-C catalyst exclusively contains atomically dispersed FeN4 sites with increased Fe loading relative to other catalysts from wet-chemistry synthesis. The catalyst exhibited outstanding oxygen-reduction activity in acidic electrolytes, which was further studied in proton-exchange membrane fuel cells with encouraging performance.

Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts

AbstractAtomically dispersed and nitrogen coordinated single metal sites (M‐N‐C, M=Fe, Co, Ni, Mn) are the popular platinum group‐metal (PGM)‐free catalysts for many electrochemical reactions. Traditional wet‐chemistry catalyst synthesis often requires complex procedures with unsatisfied reproducibility and scalability. Here, we report a facile chemical vapor deposition (CVD) strategy to synthesize the promising M‐N‐C catalysts. The deposition of gaseous 2‐methylimidazole onto M‐doped ZnO substrates, followed by an in situ thermal activation, effectively generated single metal sites well dispersed into porous carbon. In particular, an optimal CVD‐derived Fe‐N‐C catalyst exclusively contains atomically dispersed FeN4 sites with increased Fe loading relative to other catalysts from wet‐chemistry synthesis. The catalyst exhibited outstanding oxygen‐reduction activity in acidic electrolytes, which was further studied in proton‐exchange membrane fuel cells with encouraging performance.

Atomically Dispersed MnN4 Catalysts via Environmentally Benign Aqueous Synthesis for Oxygen Reduction: Mechanistic Understanding of Activity and Stability Improvements

Development of platinum group metal (PGM)-free and iron-free catalysts for the kinetically sluggish oxygen reduction reaction (ORR) is crucial for proton-exchange membrane fuel cells. A major chall...

Water management strategies for PGM-free catalyst layers for polymer electrolyte fuel cells

Platinum group metal–free (PGM-free) catalysts are promising candidates to catalyze the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs). Because of their low activity, larger loadings are used resulting in thicker catalyst layers. Transport, particularly water management, thereby becomes a more prominent performance factor. Currently, very few works attempted to understand water management in PGM-free catalyst layers, mainly because of other challenges that had to be overcome first, such as enhancing their activity and durability. The field has also been active in a hypothesis discussion of micropores flooding that led to the belief that poor stability of the PEFC performance is linked to active sites flooding within the micropores. We present here an overview of recent advances in understanding water management in the PGM-free catalyst layer for oxygen reduction reaction in PEFCs and provide an opinion on design guidance in optimizing catalyst layers to avoid flooding.

Methods for assessment and measurement of the active site density in platinum group metal–free oxygen reduction reaction catalysts

The recent rise in the activity of platinum group metal–free oxygen reduction reaction catalysts for polymer electrolyte fuel cells, made them very interesting alternative for Pt-based catalysts. Currently, the application of this family of catalysts in fuel cells is impeded by their relatively low durability and stability. The lack of simple methods to follow the specific degradation of these catalysts in situ and in operando fuel cells inhibits durability and stability studies, where the key catalyst performance descriptor is the turnover frequency, for which the electrochemical site density needs to be accurately measured. Herein, the most commonly available techniques for measuring the active site density were surveyed, along with their advantages and disadvantages, leading to the conclusion that only electrochemical techniques such as square wave voltammetry and Fourier transform alternating current voltammetry can be used as standard measurements.

Design, Construction, and Characterization of a New Regioisomer and Diastereomer Material Based on the Spirooxindole Scaffold Incorporating a Sulphone Function

The 1,3-dipolar cycloaddition reaction is one of the most rapid, and efficient protocols to access, and construct highly divergent heterocycle chiral auxiliaries. Free catalyst synthesis of spirooxindole scaffold incorporating sulphone moiety via one pot–three component reaction of 6-chloro-isatin, L-proline, and the phenylvinylsulphone as dienophile is presented. The new regioisomer and diastereomer was isolated by precipitation without the tedious purification step, and then characterized via NMR and single crystal X-ray diffraction analysis. Using Hirshfeld analysis, the analysis of molecular packing was performed. It depended mainly on strong O…H and N…H interactions, and weak H…H, C…H, and Cl…H interactions as well. DFT calculations were used to optimize the experimental X-ray structure, which was found well matched with the calculated one. Reactivity descriptors based on the energies of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals were calculated. Additionally, the donor–acceptor interactions which stabilized the system via σ–σ*, π→π*, n→σ* and n→π* electron delocalization processes were also computed using NBO calculations. The net interaction energies are 49.96, 235.38, 179.66 and 107.06 kJ/mol, respectively. Additionally, the calculated NMR chemical shifts correlated well with the experimental data (R2=0.99).

Exploration of Stable and Phosphine Ligand Free Heterogeneous Catalyst for Ethylene Hydroformylation

The development of highly active,stable and phosphine ligand free heterogeneous catalyst for hydroformylation reaction remains an open challenge.Herein,we developed highly active,stable and phosphine ligand free heterogeneous catalyst for ethylene hydroformylation reaction by replacing single strongly coordinating phosphine ligand with multiple weakly coordinating oxygen functional groups on the activated carbon surface.Applying of the typical Rh/C-3 catalyst in hydroformylation of ethylene affords unprecedented turnover frequencies(TOFs)up to 57889/h.Besides,the Rh/C-3 catalyst exhibited substantially improved stability in the hydroformylation of ethylene in a fixed-bed reactor(at least 2500 h).For the first time,detailed characterizations revealed that lactone group(-CO2-)on the surface of activated carbon might play a crucial role to gain this catalyst with remarkable activity and stability,which facilitate the homogeneous dispersion and stability of Rh nanoparticles.This work provides a general guidance for the design of highly active,stable and phosphine ligand free heterogeneous catalysts for hydroformylation of olefins.

Voltammetric Detection of Nitrite Anions Employing Imidazole Functionalized Reduced Graphene Oxide as an Electrocatalyst

AbstractA kind of metal free catalyst with excellent electrocatalytic activity toward nitrite anions oxidation was synthesized through functionalization of graphene oxide (GO) with imidazole aromatic rings. Imidazole rings linked to graphene oxide nanosheets via 3‐chloropropyl trimethoxysilane (CPTMS) mediators following simultaneous thermally reduction of GO during the functionalization, to form rGO−Im for modification of gold electrode (GE). The sensing platform characterized by SEM, XRD, IR and Raman methods. The resulting rGO−Im‐GE sensor, responded sensitively to nitrite concentrations within a wide linear range (1 to 1000 μM) and indicated a relative low detection limit (LOD=0.28 μM, S/N=3) without interfering with common ions existing in aqueous media. Furthermore acceptable recoveries were obtained when applying the proposed sensor to tap water samples.

Construction of Co3O4/Fe2O3 nanosheets on nickel foam as efficient electrocatalyst for the oxygen evolution reaction

The development of earth–abundant and high–performance electrocatalysts for oxygen evolution reaction (OER) is a very important part for electrochemical water splitting to generate cleanliness and renewable hydrogen. The preparation of heterogeneous nanostructure composite is an effective method to further improve the performance of noble–metal–free catalysts. Herein, heterogeneous nanostructure of Co3O4/Fe2O3@NF electrocatalyst for OER was fabricated by a hydrothermal method and the subsequent oxidation process. The as–prepared Co3O4/Fe2O3@NF presents a unique thin–wall cell structure. Benefiting from the thin–wall cell structure and the synergistic effect between Co3O4 and Fe2O3, as well as the three–dimensionally (3D) conductive feature of the substrate of nickel foam (NF), the Co3O4/Fe2O3@NF electrode demonstrated excellent electrocatalytic activity for OER in alkaline solution. The overpotential is 254 mV at 10 mA cm−2 current density, the Tafel slope is only 33 mV dec−1, and the electrode also retained good stability of 24 h at the current density of 20 mA cm−2. Compared with pristine Co3O4/Fe2O3 (η10 = 493 mV), Co3O4@NF (η10 = 350 mV) and Fe2O3@NF (η10 = 378 mV), the Co3O4/Fe2O3@NF electrode displayed obviously enhanced electrocatalytic performance.

Phosphate tolerance of nitrogen-coordinated-iron-carbon (FeNC) catalysts for oxygen reduction reaction: A size-related hindrance effect

The greatest challenges facing the large – scale commercialization of the intermediate temperature phosphoric acid fuel cell (PAFC) technology are: (i) the high cost of Pt catalyst that is required to overcome the slow kinetics of oxygen reduction reaction (ORR) on the PAFC cathode, and (ii) deactivation of the Pt catalyst by exposure to the phosphoric acid electrolyte. Inexpensive carbon-based materials, particularly iron-nitrogen-coordinated carbon supported catalysts (FeNC), are promising cathodes for PAFCs. The higher ORR activation energy and phosphate poisoning resistance of FeNC present a clear advantage over the Pt-based commercial catalysts that are easily susceptible to deactivation after exposure to phosphate anions. While the immunity of FeNC catalysts to phosphate anion poisoning has been reported previously, this work explores the counter-intuitive and conditional nature of this poisoning resistance based on the properties of the carbon support used in the preparation of the FeNC catalyst. A combination of electrochemical measurements with characterization experiments performed using X-ray photoelectron spectroscopy, infrared spectroscopy and in-situ X-ray absorption spectroscopy is used to explore the phenomenon of phosphate poisoning on FeNC catalysts synthesized from two different carbon supports. Results from these studies reveal a “size-related hindrance effect” wherein the ORR active Fe centers in FeNC remain inaccessible to larger anions such as phosphate when the Fe sites are present deeper inside the pores of a carbon support with high porosity. However, FeNC catalyst is poisoned by phosphate anions when synthesized using a carbon support with low porosity. This work helps gain insights into the nature of active sites in FeNC catalysts that will be useful in designing precious-metal free ORR catalysts for intermediate temperature fuel cell technologies.

The influence of Ba addition on thermal stability and catalytic activity of Cu-based mixed oxide

Catalytic reduction of NO by CO was studied over La2-xBaxCuO4 (x = 0.0 or 0.4) mixed oxides prepared by coprecipitation method. The catalysts were characterized by X ray diffraction (XRD), O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR) and X-ray absorption near edge structure (XANES). NO + CO reaction was carried out in a fixed-bed flow reactor, from room temperature to 500 °C, at atmospheric pressure. Both fresh catalysts were active for the NO + CO reaction but when were submitted to thermal aging, the barium free catalyst significantly lost its activity. The partial substitution of La by Ba enhanced the thermal stability and although the activity of the catalyst has decreased, it remains high, approaching 100 % at 500 °C. These results can be associated with reducible copper species observed by XANES in the thermal aged catalyst containing Ba.

Fabrication of rGO/CdS@2H, 1T, amorphous MoS2 heterostructure for enhanced photocatalytic and electrocatalytic activity

The synthesis of high efficiency noble metal free catalysts is an important target for H2 production by water-splitting. In this work, rGO/CdS@MoS2 heterostructure with two catalytic paths was successfully synthesized and as the first applied the heterostructure in the field of electrocatalysis. The MoS2 structure is adjusted by controlling hydrothermal process. Moreover, the effects of structure and loading amount of 2H–MoS2, 1T-MoS2 and amorphous MoS2 (A-MoS2) on catalytic performance were also studied. The catalytic activity of rGO/CdS@MoS2 heterostructure has been improved obviously. Compared with 2H–MoS2, the distortion of 1T-MoS2 and the defect of A-MoS2 make it have more unsaturated S, so rGO/CdS@1T-MoS2 and rGO/CdS@A-MoS2 have better catalytic activity. For photocatalytic H2 evolution, loading MoS2 and rGO on catalysts changes the energy band structure, promotes the separation of electron-holes and provides a large number of active sites. Among them, the visible light photocatalytic H2 production rate of rGO/CdS@1T-MoS2 with 0.1 mol of 1T-MoS2 (CT0.1-G1) is 18.26 mmol/g/h. During the electrocatalytic H2 evolution, introducing MoS2 and rGO improves electronic structure and increases active sites. rGO/CdS@1T-MoS2 with 0.5 mol of 1T-MoS2 (CT0.5-G1) shows the low overpotential (312 mV) and Tafel slopes (85 mV/dec).

Protection and Isolation of Bioorthogonal Metal Catalysts by Using Monolayer-Coated Nanozymes.

We demonstrate here the protection of biorthogonal transition metal catalysts (TMCs) in biological environments by using self-assembled monolayers on gold nanoparticles (AuNPs). Encapsulation of TMCs in this hydrophobic environment preserves catalytic activity in presence of pH conditions and complex biological media that would deactivate free catalyst. Significantly, the protection affords by these nanozymes extends to isolation of the catalyst active site, as demonstrated by the independence of rate over a wide pH range, in strong contrast to the behavior of the free catalyst.

Insight into the hydrogen oxidation electrocatalytic performance enhancement on Ni via oxophilic regulation of MoO2

Abstract Exploring platinum-group-metal (PGM) free electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline media is essential to the progress of anion exchange membrane fuel cells (AEMFCs). In this work, a Ni/MoO2 heterostructure catalyst with comparable HOR activity in alkaline electrolyte with PGM catalyst was prepared by a simple hydrothermal-reduction method. Remarkably, the Ni/MoO2 presents a mass kinetic current density of 38.5 mA mgNi−1 at the overpotential of 50 mV, which is higher than that of the best PGM free HOR catalyst reported by far. Moreover, the HOR performance of Ni/MoO2 under 100 ppm CO shows negligible fading, together with the superior durability, render it significant potential for application in AEMFCs. A particular mechanistic study indicates that the excellent HOR performance is ascribed to the accelerated Volmer step by the incorporation of MoO2. The function of MoO2 was further confirmed by CO striping experiment on Pt/C-MoO2 that MoO2 can facilitated OH adsorption thus accelerate the HOR process. On account of the high performance and low cost, the Ni/MoO2 electrocatalyst encourages the establishment of high performance PGM free catalyst and shows significant potential for application in AEMFCs.