Free Catalyst Research Articles

Computational investigation of the oxygen reduction reaction on the edges of differently-sized, shaped and terminated graphene nanoclusters

Graphene nanoclusters (GNCs), part of the graphene family of nanomaterials, are attracting interest as promising Pt free catalysts for the Oxygen Reduction Reaction (ORR) at the cathode of the fuel cells. This investigation focuses on the theoretical prediction of their possible edge activity towards ORR determined by their shape, size and termination type. In this framework, using density functional theory (DFT) we examined the zigzag (zz) and armchair terminated (ac) triangular (T), rhombohedral (R) and hexagonal (H) GNCs of various sizes, ranging from C13 to C114. The theoretical onset overpotential (ηth,onset) indicate that several GNC edges could be active for ORR. The model indicates that the zigzag edges of all triangular GNCs (C13–C46) as well as of the largest investigated rhombohedral shapes (C48, C70) are active for ORR. A correlation between the band gaps, spin densities and p-band centers of the edge carbon atoms and the adsorption energies of the ORR intermediate moieties (HOO*/O*/HO*) are discussed and which are directly related to their activity. One trend is related to the decrease of the adsorption energies with the decrease of the band gap and accordingly an increase of their ORR activities. The case studies, show that the explicit water model developed by using machine learning potentials and combined with DFT, stabilizes the ORR intermediates stronger than the implicit one, thus increasing slightly the onset overpotential. Overall, this computational study enhances our fundamental understanding of the edges of GNC for their potential application in electrocatalysis without doping.

Advancing Sustainable Climate Solutions: Overcoming Challenges in Designing Carbon-Based Metal-Free Catalysts for Electrochemical CO2 Reduction

This review thoroughly inspects the challenges and limitations inherit in the system design of carbon-based metal free catalysts for the electrochemical reduction of CO2. The study sits at the interaction of the urgent need for the sustainable energy transformation and the pursuit of efficient carbon capture technologies, and provides insight into the complex barriers that impede the seamless development of these catalysts. The urgent global need for sustainable energy solutions and strategies to curb CO2 emissions highlight the urgency of this exploration. In this framework, this review focuses specifically on carbon-based metal-free catalysts as key elements towards achieving sustainable electrochemical CO2 reduction. Against this delicate background, the review highlights complexities that researchers. By leveraging insights from recent research, this review not only documents current challenges but also serves as a dynamic repository of evolving knowledge. Each identified barrier is highlighted, revealing the complex nuances that define the carbon-based catalyst research landscape. As research progresses the varied behavior of these challenges will be outlined and inspire collective intellectual exploration to innovate solutions that advance the field towards an impactful and viable future.

DNA Adjuvant Hydrogel-Optimized Enzymatic Cascade Reaction for Tumor Chemodynamic-Immunotherapy.

Chemodynamic therapy (CDT) shows immense potential in cancer treatment as it not only directly kills tumor cells but also induces anti-tumor immune responses. However, the efficacy of CDT is hampered by challenges in targeting CDT catalysts specifically to tumors using nanomaterials, along with the limitations of low H2 O2 levels and short catalyst duration within the tumor microenvironment. In this study, DNA adjuvant hydrogel to arrange a glucose oxidase-ferrocene cascade for continuously generating reactive oxygen species (ROS) from glucose in situ for tumor CDT combined with immunotherapy is employed. By precisely tuning the catalyst spacing with DNA double helix, ROS production efficiency is elevated by up to nine times compared to free catalysts, resulting in stronger immunogenetic cell death. Upon intratumoral injection, the DNA hydrogel system elicited potent anti-tumor immune responses, thereby effectively inhibiting established tumors and rejecting re-challenged tumors. This work offers a novel platform for integrated CDT and immunotherapy in cancer treatment.

Catalytic role of the enol ether intermediate in the intramolecular Stetter reaction: a computational perspective.

The intramolecular Stetter reaction catalyzed by a carbene is investigated by density functional theory (DFT) calculations and kinetic simulations. Catalyst 1 first reacts with aldehyde 2 to give the primary adduct (PA). The PA undergoes the intramolecular oxa-Michael reaction to irreversibly generate enol ether intermediate 9. The conversion of the enol ether to the Breslow intermediate (BI) requires the assistance of a base such as the PA. The next step involves formation of a carbon-carbon bond through the Michael addition, and expulsion of the catalyst generates the Stetter product 7. Calculations show that the catalytic cycle is composed of two irreversible processes: the first one involves the exergonic formation of the enol ether intermediate, while the second one is the conversion of the enol ether to the final product. Kinetic simulations using initial concentrations of [1]0 = 0.005 M and [2]0 = 0.025 M demonstrate that under a steady-state condition, 35% of the catalyst rests on the state of the enol ether (0.0018 M). The catalyst resting state therefore consists of the unbound form (the free catalyst) and its bound form (the enol ether species). According to variable time normalization analysis, the reaction exhibits a second-order dependence (first order in catalyst and first order in substrate), which agrees with experiments. The oxa-Michael reaction to form the enol ether is identified to be turnover limiting in the intramolecular Stetter reaction, which rationalizes the observed electronic effect of the Michael acceptor on the reactivity, as well as the measured isotope effect with respect to the aldehydic proton/deuteron. The base that participates in the BI formation has a significant effect on the build-up of the resting state 9 and the active catalyst concentration. In addition, the thermodynamic stability of the enol ether is found to depend on the tether length between the aromatic aldehyde and the Michael acceptor, as well as the chemical nature of the carbene catalyst. The favorability for the oxa-Michael reaction is therefore suggested to govern the reactivity of the intramolecular Stetter transformation.

Guanidine functionalized porous SiO2 as heterogeneous catalysts for microwave depolymerization of PET and PLA

Chemical recycling is an important strategy to tackle the growing global problem of plastic waste pollution. Covalent attachment of oganocatalysts to porous SiO2 creates metal free heterogeneous catalysts for open and closed loop recycling of PET and PLA.

P-block doped semi-metallic xenes as highly selective and efficient transition-metal free single atom catalysts for electrochemical CO reduction

Electronic structure of boron-doped silicene is optimized for CORR and selective towards methanol with an overpotential of less than 0.1 V.

Treasure-bowl style bifunctional site in cerium-tungsten hetero-clusters for superior solar-driven hydrogen production.

Electrochemical water splitting powered by renewable energy sources hold potential for clean hydrogen production. However, there is still persistent challenges such as low solar-to-hydrogen conversion efficiency and sluggish oxygen evolution reactions. Here, we address the poor kinetics by studying and strengthening the coupling between Ce and W, and concurrently establishing Ce-W bi-atomic clusters on P,N-doped carbon (WN/WC-CeO2-x@PNC) with a "treasure-bowl" style. The bifunctional active sites are established using a novel and effective self-sacrificial strategy involving in situ induced defect formation. In addition, by altering the coupling of the W(d)-N(p) and W(d)-Ce(f) orbitals in the WN/WC-CeO2-x supramolecular clusters, we are able to disrupt the linear relationship between the binding energies of reaction intermediates, a key to obtain high catalytic performance for transition metals. Through the confinement of the WN/WC-CeO2-x composite hetero-clusters within the sub-nanometre spaces of hollow nano-bowl-shaped carbon reactors, a stable and efficient hydrogen production via water electrolysis could be achieved. When assembled together with a solar GaAs triple junction solar cell, a solar-to-hydrogen conversion efficiency of 18.92% in alkaline media could be realized. We show that the key to establish noble metal free catalysts with high efficiency lies in the fine-tuning of the metal-metal interface, forming regions with near optimal adsorption energies for the reaction intermediates participating in water electrolysis.

Orbital and free energy landscape expedition towards the unexplored catalytic realm of aromatically modified FLPs for CO2 sequestration.

The emergence of frustrated Lewis pairs (FLPs) has created a whole new dimension in the development of metal free catalysts for CO2 sequestration. Efforts have been made to enhance the catalytic activity of the FLPs. The aromatic modulation of the catalytic sites has been successfully demonstrated to enhance the activity towards CO2. Although various aromatically modified geminal FLPs have been investigated for CO2 capture, the catalytic space of these FLPs has not been fully resolved yet. Thus, to fulfil the knowledge gap in the understanding of the catalytic behaviour and to extend the concept of aromatically enhanced FLPs, in the present study all the possible combinations of aromatic and antiaromatic modulations of the acidic and basic sites have been proposed and examined using density functional theory based orbital analysis. Further to verify the results obtained from the orbital analysis and to fully explore the catalytic space of the proposed systems, free energy landscapes have been examined using metadynamics simulations. The detailed intrinsic bond orbital (IBO) and principal interacting orbital (PIO) analyses capture crucial details of the reactions. Furthermore, evolution of anisotropy of induced current density (AICD) along the reaction justifies the effect of aromatic/antiaromatic modulation on the catalytic sites. The results show that highly asynchronous mechanisms have been found due to the aromatic/antiaromatic modulations. The simultaneous favourable aromatic/antiaromatic modification on the acidic and basic sites may greatly reduce the CO2 activation barrier. The enhancement of the acidic character of the B atom in the intramolecular FLPs (IFLPs) leads to a thermodynamically more feasible reaction with stable CO2 adducts. The acidic site has been found to play a major role in controlling the kinetics and thermodynamics of the reaction. This study provides valuable insights into the catalytic realm of the aromatically modified FLPs, which can be utilized to design more efficient and specific next-generation FLPs.

Synthesis of Polyaromatic Based Heat Transfer Fluid via Suzuki Coupling Using Ligand Free Pd Catalyst in Environmentally Benign Solvent

Synthesis of Polyaromatic Based Heat Transfer Fluid via Suzuki Coupling Using Ligand Free Pd Catalyst in Environmentally Benign Solvent

(Invited) Durable and Highly Active Fe-N-C Catalysts for PEM Fuel Cells

Atomically dispersed and nitrogen-coordinated single iron sites (FeN4) embedded in carbon (Fe-N-C) catalysts are the most promising platinum group metal (PGM)-free catalysts. However, they have yet to match their Pt counterparts for oxygen reduction reaction (ORR) activity and stability in proton exchange membrane fuel cells (PEMFCs). Here, we developed viable Fe-N-C catalysts, which, for the first time, demonstrated competitive activity to that of Pt/C catalysts and dramatically enhanced stability and durability under practical PEMFC operating conditions. The most active Fe-N-C catalyst achieved a record half-wave potential (E1/2 = 0.915 V vs. RHE at 0.6 mgcatcm-2) and an ORR mass activity of 10.5 mA mgcat at 0.9 V in (RDE) tests, exceeding a Pt/C baseline catalyst (60 µgPt cm-2) by 40 mV in acidic electrolytes. This compelling activity of the Fe-N-C catalyst in aqueous acids on rotating disk electrode (RDE) was successfully transferred to fuel cell membrane electrode assembly (MEAs), generating an initial current density of 44.2 mA cm-2 exceeding the U.S. DOE 2025 target (i.e., 44 mA cm-2) at 0.9 VIR-free under O2. Under practical hydrogen-air conditions, record 151 mA cm-2 at 0.8 V and peak power density of 601 mW cm-2 were achieved. Importantly, we discovered that depositing nitrogen-carbon species on the catalyst surface via chemical vapor deposition (CVD) dramatically enhanced catalyst stability, evidenced by performance durability after accelerated stress tests (30 000 square-wave voltage cycles under H2/air) and long-term steady-state life tests (> 300 hours at 0.67 V). Innovative identical location-scanning transmission electron microscopy (IL-STEM) experiments confirmed that the CVD process leads to deposition of nitrogen-doped carbon onto the catalyst surfaces. Along with theoretical modeling, a reconstruction of the carbon structure adjacent to FeN4 sites leads to increased robustness against demetallation and carbon oxidation. This work opens new avenues for developing earth-abundant iron-based catalysts with extraordinary activity and stability, thus competing with Pt and addressing the cost barrier of current PEMFCs.

Exploring a Facile Synthetic Procedure of Iron-Based Platinum Group Metal-Free Electrocatalyst for Polymer Electrolyte Fuel Cells

The transportation sector, mainly powered by fossil fuels, is responsible for approximately 14% of all greenhouse gas emissions1. Polymer electrolyte fuel cells (PEFCs) are promising candidates for sustainable transport as they only emit water, can be refilled fast, and enable long ranges with a single refill. Their current costs and dependence on scarce materials challenge their broad implementation in heavy-duty vehicles and maritime transportation 2. Another challenge is the catalyst degradation during operation, therefore the goal of at least 35,000 lifetime hours for heavy-duty transportation is ambitious as existing fuel cell vehicles have been engineered for ca. 5,000 hours3. The state-of-the-art catalyst layers are based on platinum nanoparticles supported on carbon and global efforts have been made to develop high-activity catalysts with reduced platinum contents, including platinum alloys such as Pt-Ni4 and Pt-Co5, alternative supports such as nanotubes6, or a combination aforementioned methods. While platinum-based materials remain the most active for the oxygen reduction reaction, their elevated cost and instability during operation motivate the exploration of alternative materials.Platinum group metal (PGM)-free catalysts have gained prominence in the PEFCs field, where Fe-N-C based catalysts have shown high oxygen reduction reaction activity in acidic media7, approaching that of platinum-based catalyst layers, yet their practical application is hindered by insufficient stability under operating conditions8. Inspired by recent efforts7,8, our goal is to develop stable, active, and low-cost catalyst layers using earth-abundant materials. In this poster presentation, I will discuss my efforts in the synthesis of Fe-N-C catalysts by the incorporation of iron in a calcinated nitrogen-doped zeolitic imidazolate framework (ZIF). ZIFs are excellent supports due to their high porosity of approximately 60% and a surface area of 1600 m2/g, leading to a high density of active sites9. After the incorporation of iron, post-treatments are applied including leaching and the utilization of secondary nitrogen precursors, where efforts are made to develop non-hazardous activation approaches. The incorporation of secondary nitrogen precursors such as cyanamide have been shown to promote the formation of metal-N-C bonds and increase the performance towards the oxygen reduction reaction10,11. We employ microscopic, spectroscopic and electrochemical techniques, such as the rotating disc electrode, to correlate the catalyst composition with electrochemical performance and durability. With this work, we hope to gain an understanding of composition-structure relationships to enhance the performance of iron-based platinum-free catalysts for PEFCs. Acknowledgements SH2IPDRIVE has received funding from the Ministry of Economic Affairs and Climate Policy, RDM regulation, carried out by the Netherlands Enterprise Agency. References W.F. Lamb et al., 2021, Environmental Research Letters vol. 16.S.T. Thompson et al., 2018, Solid State Ion 319, 68–76.D.A. Cullen et al., 2021, Nature Energy vol. 6 462–474.S.I. Choi et al., 2013, Nano letters 13(7), 3420-3425.P.R. Kumar et al., 2018, Surfaces and Interfaces 12,116-123.D. Schonvogel et al., 2017, Journal of the Electrochemical Society 164(9), F995.A. Mehmood et al., 2022, Nature Catalysis 5(4), 311-323.X. Wan, and J. Shui, 2022, ACS Energy Letters 7(5), 1696-1705.J.C.Tan et al, 2010, Proceedings of the National Academy of Sciences 107(22), 9938-9943.X. Wu et al., 2022, Journal of The Electrochemical Society 169.1, 016507.N.D. Leonard et al., 2018, ACS Catalysis 8(3), 1640-1647.

Fe-N-C Catalysts Synthesized on Conductive Non-Carbon Support Materials for Improved PEMFC Durability

Global energy demand will be continuously rising in the foreseeable future. To mitigate the results of climate change it is necessary to reduce the emittance of green house gasses. One very promising approach is to switch to hydrogen as energy carrier and use proton membrane fuel cells (PEMFCs) as the primary energy generation technology. Here, hydrogen and oxygen/air are used to directly generate electrical energy with only water as byproduct. Both reactions need to happen efficiently at the same time. The oxygen reduction reaction (ORR), however, is a very sluggish four-electron-reaction and needs efficient catalysts.The state-of-the-art catalyst is platinum but that is a very scarce and expensive metal. Hence, it is important to switch to precious-metal-group (PGM) free catalysts. In recent years, bio inspired iron-based Fe-N-C catalysts have been shown to approach ORR activities that are competitive with those of platinum, even though they need a higher catalyst loading than Pt to reach the desired current densities.1-3 Those catalysts feature single Fe atoms surrounded by four N atoms in graphitic carbon. The drawback of those materials is their lower stability compared to PGM materials. In a nutshell, the carbon matrix oxidizes during operation in PEMFCs, changing the morphology of the catalytic center and rendering it less active, or inactive, depending on the carbon oxidation extent. Demetallation phenomena are also possible, leaving a N4 cavity without metal cation center. It was shown recently by Wu et al.4 that it is possible to raise durability by adding an additional carbon layer on the catalyst. One alternative approach to reduce those degradations is to interface Fe-N4 sites with a carbon-free support. The targeted support material must be immune to corrosion in acid and withstand the typical ORR electrochemical potentials. It also has to be electrically conductive to facilitate electron transfers, both locally and on long scale.In this work, conductive ceramic metal oxide materials were investigated as possible alternative catalyst supports. Those materials are resistant to acid and can withstand prolonged potentials at operating conditions in PEMFCs. The conductivity is usually raised by doping, since many of those ceramics are semi-conductors. However, during the synthesis of the Fe-N-C catalysts, high temperatures are generally applied. Such treatments can be detrimental to the conductivity of the ceramics due to metal aggregation (in case of doped materials) and may also lead to morphological changes. A robust method to synthesize Fe-N-C catalysts is by mixing the separate Fe, N and C precursors and using optimized thermal treatment at high temperatures. They can be further pyrolyzed with ammonia to introduce defects and basicity into the structure, which leads to higher catalytic activity but even lower stability in acid medium. In this study, the effect of the pyrolysis conditions on conductive oxide materials was investigated with various characterization methods, e.g. x-ray diffraction, scanning electron microscopy and N2 sorption isotherms. This gives insights into the changes that happen during the thermal treatment and can be used to find optimal synthetic conditions to keep most desired parameters of the ceramics preserved or only minimally lowered. Several different approaches were used to combine Fe-N-C catalysts with conductive ceramic metal oxide materials. The resulting materials were characterized and tested with rotating disc electrode techniques and in PEM fuel cells for their ORR-activity and durability.(1) F. Jaouen et al., Nature Catalysis, 2022, 5, 311–323(2) H. A. Gasteiger et al., Science, 2009, 324, 48−49(3) L. Elbaz et al., ACS Applied Energy Materials 2022, 5 (7), 7997-8003(4) G. Wu et al., Nature Energy, 2022, 7, 652–663

Structure and Activity-Durability Tradeoff of Coated Fe-N-C Catalysts for the Oxygen Reduction Reaction

Fe-based catalysts stand out for oxygen reduction reactions among the platinum group metal (PGM)-free catalysts for reducing the reliance on expensive and scarce platinum. With the endeavor of a PGM-free community, the state-of-the-art Fe-N-C catalysts have demonstrated encouraging initial activity in Proton Exchange Membrane Fuel Cell (PEMFC) 1-2. However, their long-term stability and durability remain a challenge, both in acidic liquid electrolyte cells and in single-cell PEMFCs3. Recently, a novel approach has demonstrated improved durability of Fe-N-C catalysts during ORR, which consisted of coating a pre-existing Fe-N-C with a thin carbon overlayer obtained via chemical vapor deposition of a metal-organic framework (ZIF-8), at 1100 °C4. However, the underlying mechanism for enhanced durability remains unclear. One of the keys to improved stability lies in the specific structure of the catalysts, including the coordination of the ORR-active iron atoms and the structure of the nitrogen-doped carbon matrix, especially the structure located at the top surface of catalysts. The CVD conditions that affect the structure of FeNC must be optimized, to achieve the optimal activity-durability trade-off.This presentation will report on the structure, activity, and durability trends of a suite of FeNC catalysts coated at different CVD conditions. The key parameters have been investigated, such as deposition temperature, overlayer thickness, and iron speciation on their ORR activity and durability. The coated Fe-N-C catalysts have been evaluated for their activity/durability (in oxygenated environments) both in liquid electrolyte cells and PEMFCs. The coated Fe-N-C demonstrated inferior initial activity and improved durability (Figure 1). The correlation between the structural information derived from multiple techniques (XRD, Mossbauer spectroscopy, TEM, Raman spectroscopy, N2 sorption, etc) and their electrochemical behaviors will be discussed.References Jiao, L.; Li, J.; Richard, L. L.; Sun, Q.; Stracensky, T.; Liu, E.; Sougrati, M. T.; Zhao, Z.; Yang, F.; Zhong, S.; Xu, H.; Mukerjee, S.; Huang, Y.; Cullen, D. A.; Park, J. H.; Ferrandon, M.; Myers, D. J.; Jaouen, F.; Jia, Q., Nature Materials 2021, 20 (10), 1385-1391.Mehmood, A.; Gong, M.; Jaouen, F.; Roy, A.; Zitolo, A.; Khan, A.; Sougrati, M.-T.; Primbs, M.; Bonastre, A. M.; Fongalland, D.; Drazic, G.; Strasser, P.; Kucernak, A., Nature Catalysis 2022, 5 (4), 311-323.Martinez, U.; Komini Babu, S.; Holby, E. F.; Zelenay, P., Current Opinion in Electrochemistry 2018, 9, 224-232.Liu, S.; Li, C.; Zachman, M. J.; Zeng, Y.; Yu, H.; Li, B.; Wang, M.; Braaten, J.; Liu, J.; Meyer, H. M.; Lucero, M.; Kropf, A. J.; Alp, E. E.; Gong, Q.; Shi, Q.; Feng, Z.; Xu, H.; Wang, G.; Myers, D. J.; Xie, J.; Cullen, D. A.; Litster, S.; Wu, G., Nature Energy 2022, 7 (7), 652-663. Figure 1

Non-PGM Cathode Electrocatalysts for PEM Fuel Cells

Polymer electrolyte membrane fuel cells, PEMFCs, are considered the most ideal next generation energy conversion device, able to provide power to a broad range of applications; automotive, combined heat and power, portable electronic devices. Pt-based electrocatalysts are regarded as ideal for promoting the desired redox reactions of a fuel cell, especially the sluggish oxygen reduction at the cathode, ORR. High expense and scarcity of Pt are the greatest impediments to successful commercialization of this technology. The replacement of unsustainable noble-metal catalysts with platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) is essential for material wise and market value viable PEMFCs.Among the different attempts towards non-PGM catalysts, incorporating atomically dispersed transition metal atoms (Fe, Mn, etc.) and heteroatoms (N, S, etc.) into carbon frameworks is the most promising [1,2]. Large specific surface area, high intrinsic ORR electrocatalytic activity, efficient porosity and sufficient stability in acid media and/or in the actual working environment are the least requirements to consider when designing the catalysts. So far, despite the tremendous efforts, many drawbacks still persist that include poor understanding of the nature of active centres and their control over the synthesis, as well as fast decay of the structure under operation.In this work, we will present a rationally designed strategy to prepare hybrid composite catalysts by developing Fe–N–C materials onto different 2D and 3D templates [3,4] with differentiated surface chemistry (silicon and heteroatom-doped carbon structures). Despite structural and physicochemical characterization, the study includes extensive electrochemical characterization in terms of activity and durability in relative to the application conditions. An example is depicted in Figure 1 for a single atom Fe-N-C catalyst synthesized using porous silica nanotubes as the sacrificial template resulting in a hierarchically porous structure with a surface area of 370 m2/g. Electrochemical study revealed a half-wave potential of 0.72 V and an onset potential of 0.88 V with high stability in acidic environment.Very importantly, the different findings are correlated to one another, thereby establishing a composition to function relationship to provide a pathway for the integrated engineering of novel structures with improved performance. Acknowledgments Funding from the Operational Program “Competitiveness, Entrepreneurship & Innovation” (EPAnEK), NSRF 2014-2020 through the project Solar2HyP Τ2ΕΔΚ-01877 References [1] L. Osmieri, et al., Current Opinion in Electrochemistry 25 (2020) 100627.[2] T. Asset, et al., Joule 4 (2019) 33-44.[3] Jin-Cheng Li et al., Asia Materials 10 (2018) e461[4] Nan Zhang et al., Energy Environ. Sci. 13 (2020) 111-118 Figure 1

Non-Precious Hydrogen Oxidation Catalysts for Anion Exchange Membrane Fuel Cells

Green hydrogen as an energy carrier has the potential to dramatically reduce our greenhouse gas emissions and meet the net-zero emissions targets set by the Paris Agreement countries by 2050. This hydrogen can be produced by electrolysis using water and renewable electricity, and be converted into electricity on demand in a fuel cell. Proton exchange membrane fuel cells (PEMFC) are already commercialized, but due to their acidic environment, these devices require precious metal catalysts, in particular platinum, which is an obstacle to their large-scale deployment. In contrast, anion exchange membrane fuel cells (AEMFCs) operate at high pH, facilitating the use of cost-effective and sustainable precious metal free catalysts.To catalyze the hydrogen oxidation reaction (HOR) in alkaline media, nickel has shown interesting performances to replace precious metal catalysts at the anode of AEMFCs [1]. In order to improve the intrinsic activity of nickel-based materials and reach the performance of precious metals catalysts, two approaches are studied in the literature: optimization of intrinsic properties by alloying Ni with other earth abundant elements, or core@shell nanostructuration of Ni by a protective carbon shell. We are focused on this second approach through the mechanochemical synthesis of a nickel-based Metal-Organic Framework (MOF) from a Ni2+ salt and BTC (1,3,5-benzenetricarboxylic acid), followed by pyrolysis under different atmospheres (Figure 1.a). As already shown in the literature [2], there is a significant effect of the pyrolysis atmospheres (NH3, H2, N2, in various ratios) on the electrochemical performance of the catalysts (Figure 1.b). In this study, we employed ex situ and operando studies approaches to investigate the influence of these different atmospheres on the nanoparticles structuration and to relate them to the electrochemical performances obtained.Our Ni-based anode materials were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption, and electrochemical techniques, including AEMFC tests. Furthermore, the transformation of the MOF into an active catalyst was followed with in-situ X-ray absorption spectroscopy.[1] Zhao, J. Chen, W. Sun, H. Pan, «Non-Platinum Group Metal Electrocatalysts toward Efficient Hydrogen Oxidation Reaction», Adv. Funct. Mater., vol. 31, no 20, 2021, p. 2010633.[2] Ni et al., « An efficient nickel hydrogen oxidation catalyst for hydroxide exchange membrane fuel cells », Nat. Mater., vol. 21, no 7, 2022, p. 804. Figure 1

Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity.

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs free energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results. The molecular recognition by the catalyst is approximately 3 kJ mol-1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol-1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions, the self-association of both the catalyst and the substrate in toluene was found to play an important role and thus needs to be taken into account in reaction screening.

Functional graphitic carbon nitride/hydroxyapatite heterojunction for robust formaldehyde removal at ambient temperature

Ambient-temperature catalytic removal of formaldehyde (HCHO) hazard has been currently reckoned as a practicable and promising approach for abating indoor HCHO which severely haunts the human’s health. Catalyst with high-effectiveness and low-cost is a pivotal parameter determining the reliability and availability of this strategy. Herein, bifunctional graphitic carbon nitride/hydroxyapatite heterostructures (g-C3N4/HAP) were developed for indoor HCHO removal at ambient temperature. The optimal catalyst (g-C3N4/HAP-20) displayed enhanced HCHO removal efficiency of 36.6% and conversion (into CO2) efficiency of 28.4% within 2 h, compared with g-C3N4 (18.0% and 24.0%) and HAP (36.2% and ∼ 0%). Indoor fluorescent light accelerated HCHO elimination and its subsequent decomposition into CO2 over g-C3N4/HAP-20, delivering a HCHO removal efficiency of 82.7% and conversion efficiency of 100% within 2 h. Various characterizations and theoretical calculation reveal that g-C3N4/HAP possessed superoxide radicals and unpair electrons which were responsible for HCHO removal at ambient temperature; and a S-scheme heterostructure remarkably benefited the charge transfer and the photogenerated carriers’ separation, and consequently upgraded HCHO catalytic oxidation under indoor fluorescent-light illumination. This work offers a potent way to construct noble-metal free advanced catalysts with multi-functionality for indoor pollutant purification.

Air activation enhances the porosity and N,O synergistic effect towards an efficient metal free carbon cathode for Li-O2 battery

The achievement of Li-O2 batteries with high energy density and extended cycle life is often hindered by the disorderly growth of solid discharge product Li2O2 on the cathode and its sluggish decomposition reaction kinetics. Herein, under the guidance of DFT simulations, the N,O co-doped carbon was identified as a potential efficient metal free cathode catalyst to remit these problem in Li-O2 batteries. Then, a spherical shell-like N,O co-doped carbon was prepared through the direct pyrolysis of guanosine 5′-monophosphate disodium salt (5′-GMP, 2Na). After a further activation process in air, the carbon shells exhibit a finer porous surface with greatly enhanced specific surface area and surface N,O contents. As a result, the morphology of Li2O2 changed from large toroidal to film by using the activated carbon as a cathode catalyst due to its tailored surface structure with improved N and O synergistic effect. What's more, the film-like Li2O2 with Li vacancy can not only form an increased interfacial contact and interaction with the active sites on carbon, but also be conducive to the transfer of Li+/electrons during the decomposition of Li2O2, thus accelerating the electrochemical performances of the battery.

Bypassing Formation of Oxide Intermediate via Chemical Vapor Deposition for the Synthesis of an Mn-N-C Catalyst with Improved ORR Activity.

A significant barrier to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of the platinum-based oxygen reduction reaction (ORR) cathode electrocatalysts. One viable solution is to replace platinum with a platinum-group metal (PGM) free catalyst with comparable activity and durability. However, PGM-free catalyst development is burdened by a lack of understanding of the active site formation mechanism during the requisite high-temperature synthesis step, thus making rational catalyst design challenging. Herein we demonstrate in-temperature X-ray absorption spectroscopy (XAS) to unravel the mechanism of site evolution during pyrolysis for a manganese-based catalyst. We show the transformation from an initial state of manganese oxides (MnOx) at room temperature, to the emergence of manganese-nitrogen (MnN4) site beginning at 750 °C, with its continued evolution up to the maximum temperature of 1000 °C. The competition between the MnOx and MnN4 is identified as the primary factor governing the formation of MnN4 sites during pyrolysis. This knowledge led us to use a chemical vapor deposition (CVD) method to produce MnN4 sites to bypass the evolution route involving the MnOx intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by the pyrolysis of a mixture of Mn, N, and C precursors.

Effect of Fe–N–Cs as Catalytic Active Support for Platinum towards ORR in Acidic Environment

Metal-nitrogen-carbon (M–N–C) compounds such as Fe–N–Cs are currently the most promising platinum group metal free catalysts for oxygen reduction in acidic environment. Regarding the overriding goal of reducing PEMFC production costs by reducing the platinum content, the use of Fe–N–Cs as catalytic active support for low Pt amounts is investigated in this study. Activity and stability of Pt in different contents on a commercial Fe–N–C is compared to Pt on a typical carbon black. Pt nanoparticles are well-distributed on both support substrate classes. Although the electrochemical surface and mass activity of Pt is lower on Fe–N–C compared to carbon black, the Fe–N–C has a contribution to total ORR activity depending on the Pt/Fe–N–C ratio, which is quantified. In the low Pt content case of 1 wt%, the ORR activity is increased by factor of two in presence of Fe–N–C. This boosting effect on ORR activity is important for future strategies to lower the Pt content in PEMFCs.