Kinetics Of Oxidation Reaction Research Articles

Transient simulation of conjugate heat transfer of kerosene with thermal oxidative and surface coking reactions at a supercritical pressure

The regenerative cooling technology in aerospace applications often involves supercritical-pressure heat transfer of a hydrocarbon fuel, in which thermal oxidative reactions of the fuel would lead to surface coke accumulation on the cooling channel wall. In this paper, the long-term transient conjugate heat transfer of kerosene has been numerically investigated in a circular mini tube at a supercritical pressure of 3 MPa. The modified chemical mechanism and kinetics of thermal oxidative and surface coking reactions of kerosene are proposed, which, with an additional surface reaction, are capable of treating large coke deposition in regions with sharp temperature gradients. Transient surface coke accumulation and its effect on heat transfer are handled by a dynamic mesh technique. A series of numerical studies are conducted to elucidate the influences of surface heat flux, dissolved oxygen mass fraction, inlet fuel mass flow rate, and coke layer thermal conductivity on the supercritical-pressure heat transfer and thermal oxidative reactions. The effects of different reaction pathways on surface coke accumulation are analyzed comprehensively, and it is revealed that the oxygen-consuming surface coke deposition is initiated only at the high fuel temperature at around 590 K. The present numerical results would enhance fundamental understanding of the long-term transient heat transfer process of a hydrocarbon fuel at supercritical pressures in the regenerative cooling applications.

Analysis of the Effect of Operating Conditions on the Performance of a Direct Ammonia Fuel Cell Using Multiphysics Modelling

Abstract Understanding the physical and chemical basis of device operation is important for their development. While hydrogen fuel cells are a widely studied topic, direct ammonia fuel cells (DAFCs) are a smaller field with fewer studies. Although the theoretical voltage of a DAFC is approximately equal to that of a hydrogen fuel cell, the slow kinetics of the ammonia oxidation reaction (AOR) hamper cell performance. Therefore, development of anode catalysts is especially needed for practical viability of the DAFCs. To study DAFC operation, specifically interactions between reaction kinetics and different transport phenomena, we developed a one-dimensional model of a DAFC and performed a sensitivity analysis for several parameters related to the cell operating conditions (e.g., temperature, relative humidity) and properties (e.g., catalyst loading). As expected, temperature and relative humidity were very important for cell power. However, while faster reaction kinetics improved the cell performance, simply increasing the catalyst loading did not always produce a comparable enhancement. These and other observations about the relative importance of the operating parameters should help to prioritize and guide future development of and research on DAFCs. Further studies are needed to understand and optimize e.g. humidity management in different scenarios.

Exploring Disposal Efficiency of COD, NH3-N, and Turbidity in Diluted Real Leachate Using Ti/PdO-CoO Electrodes in an Electrooxidation Cell: An Investigation into Oxidation Mechanism and Reaction Kinetics Studies

In this study, Pd and Co metal oxides, electrochemically deposited on a titanium (Ti) substrate, were utilized to remove chemical oxygen demand (COD), NH3-N, and turbidity from diluted Bingöl leachate. The plating bath was prepared with 7 mM palladium chloride (PdCl2) and 1 mM cobalt chloride (CoCl2), along with 1.68 M NH4Cl, and 0.16 M H3BO3. In the electrooxidation (EO) cell, the anode consisted of a Ti/PdO-CoO electrode, while the cathode was a stainless-steel electrode. The Ti/PdO-CoO electrode demonstrated an actual functional life of 96 h, as determined through accelerated life testing. X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy examinations revealed that the surface of the Ti substrate was coated with PdO and CoO. Turbidity, NH3-N, and COD were electrooxidized indirectly due to the high chloride content (718 mg l−1). In the presence of 10 mM NaCl, the highest removal efficiency for NH3-N, COD, and turbidity was 60.5%, 64.9%, and 96.5%, respectively. The removal of COD, NH3-N, and turbidity fit pseudo-second-order (PFO) kinetics (R2 0.97–0.99). For a COD efficiency of 60.5% at 25 mA cm−2, the corresponding energy consumption, unit energy consumption, and electrode cost were 56.25 kWh m−3, 200.89 kWh/kg-COD, and 2.37 ($/kg-COD), respectively.

Tuning electrochemical water activation over NiIr single-atom alloy aerogels for stable electrochemiluminescence

The anodic oxygen evolution reaction is a well-acknowledged side reaction in traditional aqueous electrochemiluminescence (ECL) systems due to the generation and surface aggregation of oxygen at the electrode, which detrimentally impacts the stability and efficiency of ECL emission. However, the effect of reactive oxygen species generated during water oxidation on ECL luminophores has been largely overlooked. Taking the typical luminol emitter as an example, herein, we employed NiIr single-atom alloy aerogels possessing efficient water oxidation activity as a prototype co-reaction accelerator to elucidate the relationship between ECL behavior and water oxidation reaction kinetics for the first time. By regulating the concentration of hydroxide ions in the electrolyte, the electrochemical oxidation processes of both luminol and water are finely tuned. When the concentration of hydroxide ions in electrolyte is low, the kinetics of water oxidation is attenuated, which limits the generation of oxygen, effectively mitigates the influence of oxygen accumulation on the ECL strength, and offers a novel perspective for harnessing side reactions in ECL systems. Finally, a sensitive and stable sensor for antioxidant detection was constructed and applied to the practical sample detection.

Revealing interfacial charge redistribution of homologous Ru-RuS2 heterostructure toward robust hydrogen oxidation reaction

Precisely tailoring the surface electronic structures of electrocatalysts for optimal hydrogen binding energy and hydroxide binding energy is vital to improve the sluggish kinetics of hydrogen oxidation reaction (HOR). Herein, we employ a partial desulfurization strategy to construct a homologous Ru-RuS2 heterostructure anchored on hollow mesoporous carbon nanospheres (Ru-RuS2@C). The disparate work functions of the heterostructure contribute to the spontaneous formation of a unique built-in electric field, accelerating charge transfer and boosting conductivity of electrocatalyst. Consequently, Ru-RuS2@C exhibits robust HOR electrocatalytic activity, achieving an exchange current density and mass activity as high as 3.56 mA cm−2 and 2.13 mA μgRu-1, respectively, exceeding those of state-of-the-art Pt/C and most contemporary Ru-based HOR electrocatalysts. Surprisingly, Ru-RuS2@C can tolerate 1000 ppm of CO that lacks in Pt/C. Comprehensive analysis reveals that the directional electron transfer across Ru-RuS2 heterointerface induces local charge redistribution in interfacial region, which optimizes and balances the adsorption energies of H and OH species, as well as lowers the energy barrier for water formation, thereby promoting the HOR performance.

Modulating the d-Band Center of Palladium via Ethylene Glycol Modification: Accelerating Had Desorption for Enhanced Formate Electrooxidation.

This study addresses the critical challenge in alkaline direct formate fuel cells (DFFCs) of slow formate oxidation reaction (FOR) kinetics as a result of strong hydrogen intermediate (Had) adsorption on Pd catalysts. We developed WO3-supported Pd nanoparticles (EG-Pd/WO3) via an organic reduction method using ethylene glycol (EG), aiming to modulate the d-band center of Pd and alter Had adsorption dynamics. Cyclic voltammetry demonstrated significantly improved Had desorption kinetics in EG-Pd/WO3 catalysts. Density functional theory (DFT) calculations revealed that the presence of EG reduces the d-band center of Pd, leading to weaker Pd-H bonds and enhanced Had desorption during the FOR. This research provides a new approach to optimize catalyst efficiency in DFFCs, highlighting the potential for more effective and sustainable energy solutions through advanced material engineering.

High‐Performance Anion Exchange Membrane Fuel Cells Enabled by Nitrogen Configuration Optimization in Graphene‐Coated Nickel for Enhanced Hydrogen Oxidation

Anion exchange membrane fuel cell (AEMFC) technology is attracting intensive attention, due to its great potential by using non‐precious‐metal catalysts (NPMCs) in the cathode and cheap bipolar plate materials in alkaline media. However, in such case, the kinetics of hydrogen oxidation reaction (HOR) in the anode is two orders of magnitude sluggish than that of acidic electrolytes, which is recognized as the grand challenge in this field. Herein, we report the rationally designed Ni nanoparticles encapsulated by N‐doped graphene layers (Ni@NG) using a facile pyrolysis strategy. Based on the density functional theory calculations and electrochemical performance analysis, it is witnessed that the rich Pyridinic‐N within the graphene shell optimizes the binding energy of the intermediates, thus enabling the fundamentally enhanced activity for HOR with robust stability. As a proof of concept, the resultant Ni@NG sample as the anode with a low loading (1.8 mg cm−2) in AEMFCs delivers a high peak power density of 500 mW cm−2, outperforming all of those of NPMC‐based analogs ever reported.

Simultaneously Regulating charge separation and proton supply-demand in polyphenol amine for hydrogen peroxide photosynthesis

In the H2O2 photosynthesis, apart from the charge separation, the balance of proton supply and demand was also required between the kinetically faster oxygen reduction reaction (ORR) (μs ∼ ms) and the sluggish kinetics of water oxidation reaction (WOR) (s). Faced by this challenge, the article designed the heterojunction photocatalyst formed by in-situ polymerization of polyphenol amine on the surface of CdS under alkaline conditions. As a result, the synthesized CdS-Poly(catechol-hexamethylenediamine) (PCHA) produced 6.1 mM H2O2 after 8 h of visible light irradiation, which is 29 times that of CdS. The experimental results evidenced that the coating of PCHA increased the charge separation and mobility, inhibited the photocorrosion of CdS, and thus enhanced the stability and photoactivity. Moreover, the catechol units of PCHA could function as a proton supplier and regulate local protons concentrations around the catalytic active sites to balance the proton supply–demand in the ORR and WOR at different reaction time scales.

Symmetry-Broken Ru Nanoparticles with Parasitic Ru-Co Dual-Single Atoms Overcome the Volmer Step of Alkaline Hydrogen Oxidation.

Efficient dual-single-atom catalysts are crucial for enhancing atomic efficiency and promoting the commercialization of fuel cells, but addressing the sluggish kinetics of hydrogen oxidation reaction (HOR) in alkaline media and the facile dual-single-atom site generation remains formidable challenges. Here, we break the local symmetry of ultra-small ruthenium (Ru) nanoparticles by embedding cobalt (Co) single atoms, which results in the release of Ru single atoms from Ru nanoparticles on reduced graphene oxide (Co1 Ru1,n /rGO). In situ operando spectroscopy and theoretical calculations reveal that the oxygen-affine Co atom disrupts the symmetry of ultra-small Ru nanoparticles, resulting in parasitic Ru and Co dual-single-atom within Ru nanoparticles. The interaction between Ru single atoms and nanoparticles forms effective active centers. The parasitism of Co atoms modulates the adsorption of OH intermediates on Ru active sites, accelerating HOR kinetics through faster formation of *H2 O. As anticipated, Co1 Ru1,n /rGO exhibits ultrahigh mass activity (7.68 A mgRu -1 ) at 50 mV and exchange current density (0.68 mA cm-2 ), which are 6 and 7 times higher than those of Ru/rGO, respectively. Notably, it also displays exceptional durability surpassing that of commercial Pt catalysts. This investigation provides valuable insights into hybrid multi-single-atom and metal nanoparticle catalysis.

Symmetry‐Broken Ru Nanoparticles with Parasitic Ru‐Co Dual‐Single Atoms Overcome the Volmer Step of Alkaline Hydrogen Oxidation

AbstractEfficient dual‐single‐atom catalysts are crucial for enhancing atomic efficiency and promoting the commercialization of fuel cells, but addressing the sluggish kinetics of hydrogen oxidation reaction (HOR) in alkaline media and the facile dual‐single‐atom site generation remains formidable challenges. Here, we break the local symmetry of ultra‐small ruthenium (Ru) nanoparticles by embedding cobalt (Co) single atoms, which results in the release of Ru single atoms from Ru nanoparticles on reduced graphene oxide (Co1Ru1,n/rGO). In situ operando spectroscopy and theoretical calculations reveal that the oxygen‐affine Co atom disrupts the symmetry of ultra‐small Ru nanoparticles, resulting in parasitic Ru and Co dual‐single‐atom within Ru nanoparticles. The interaction between Ru single atoms and nanoparticles forms effective active centers. The parasitism of Co atoms modulates the adsorption of OH intermediates on Ru active sites, accelerating HOR kinetics through faster formation of *H2O. As anticipated, Co1Ru1,n/rGO exhibits ultrahigh mass activity (7.68 A mgRu−1) at 50 mV and exchange current density (0.68 mA cm−2), which are 6 and 7 times higher than those of Ru/rGO, respectively. Notably, it also displays exceptional durability surpassing that of commercial Pt catalysts. This investigation provides valuable insights into hybrid multi‐single‐atom and metal nanoparticle catalysis.

Fe–Ni–F electrocatalyst for enhancing reaction kinetics of water oxidation

Highly active and low-cost oxygen evolution reaction (OER) catalytic electrodes are extremely essential for exploration of green hydrogen via water splitting. Herein, an advanced Fe–Ni–F electrocatalyst is fabricated by a facile annealing strategy using ammonium fluoride, of which the structure feature is unveiled by XRD, FESEM, TEM, EDS, BET, and XPS measurements. The as-prepared Fe–Ni–F addresses a low overpotential of 277 mV and a small Tafel slope of 49 mV dec−1 at a current density of 10 mA cm−2, significantly outperforming other control samples as well as the state-of-the-art RuO2. The advanced nature of our Fe–Ni–F catalyst could also be further evidenced from the robust stability in KOH alkaline solution, showing as 5.41% degradation after 24 h continuous working. Upon analysis, it suggests that the decent catalytic activity should be attributed to the formed bimetallic (oxy)hydroxides because of the introduction of fluoride and the synergistic effect of iron and nickel towards oxygen generation. This work represents the potential of Fe- and/or Ni-based fluoride as efficient catalyst for low-energy consumption oxygen generation.

Unraveling the atmospheric oxidation mechanism and kinetics of naphthalene: Insights from theoretical exploration

Naphthalene, the most abundant polycyclic aromatic hydrocarbon in the atmosphere, significantly influences OH consumption and secondary organic aerosol (SOA) formation. Naphthoquinone (NQ) is a significant contributor to ring-retaining SOA from naphthalene degradation, impacting the redox properties and toxicity of ambient particles. However, inconsistencies persist regarding concentrations of its isomers, 1,2-NQ and 1,4-NQ. In present work, our theoretical investigation into naphthalene's reaction with OH and subsequent oxygenation unveils their role in SOA formation. The reaction kinetics of initial OH and subsequent O2 oxidation was extensively studied using high-level quantum chemical methods (DLPNO-CCSD(T)/aug-ccpVQZ//M052x-D3/6–311++G(d,p)) combined with RRKM/master equation simulations. The reactions mainly proceed through electrophilic addition and abstraction from the aromatic ring. The total rate coefficient of naphthalene + OH at 300 K and 1 atm from our calculation (7.2 × 10−12 cm3 molecule−1 s−1) agrees well with previous measurements (∼1 × 10−11 cm3 molecule−1 s−1). The computed branching ratios facilitate accurate product yield determination. The largest yield of 1-hydroxynaphthalen-1-yl radical (add1) producing the major precursor of RO2 is computed to be 93.8 % in the ambient environment. Our calculated total rate coefficient (5.2 × 10−16 cm3 molecule−1 s−1) for add1 + O2 closely matches that of limited experimental data (8.0 × 10−16 cm3 molecule−1 s−1). Peroxy radicals (RO2) generated from add1 + O2 include 4-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-4OOadd-cis/trans, 66.0 %/17.5 %), 2-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-2OOadd-cis/trans, 10.3 %/6.3 %). Regarding the debated predominance of 1,4-NQ (corresponding to the parent RO2, i.e., add1-4OOadd-cis/trans) and 1,2-NQ (corresponding to the parent RO2, i.e., add1-2OOadd-cis/trans) in the atmosphere, our findings substantiate the dominance of 1,4-NQ. This study also indicates potential weakening of 1,4-NQ's dominance due to competition from decomposition reactions of add1-4OOadd-cis/trans and add1-2OOadd-cis/trans. Precise reaction kinetics data are essential for characterizing SOA transformation derived from naphthalene and assessing their climatic impacts within modeling frameworks.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting.

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

Modified photoanode by in situ growth of covalent organic frameworks on BiVO4 for oxygen evolution reaction.

The development of efficient oxygen evolution reaction (OER) catalysts is of great significance because the water oxidation reaction at the photoanode is the rate-determining step in photoelectrocatalytic (PEC) water splitting. Herein, two hybrid photoanodes named BiVO4/COF-Azo and BiVO4/COF-Ben were prepared by in situ solvothermal growth on a modified BiVO4 photoanode. Characterization results revealed that the Azo and Ben COFs could match with BiVO4 well to form heterojunctions, which could effectively enhance the separation efficiency of photogenerated carriers. Also, the smaller impedance of the composite photoanodes and faster kinetics of the water oxidation reaction promoted the charge transmission and enhanced the reaction efficiency of the surface-reaching holes, respectively. As a result, the composite photoanodes exhibited a larger photocurrent and more negative onset potential compared to the pristine BiVO4. This work not only provides a new strategy to construct efficient hybrid photoanodes, but also expands the applications of COFs.

Oxidation pathways and kinetics of the 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) reaction with Cl-atoms and subsequent aerial degradation of its product radicals in the presence of NO.

To give a comprehensive account of the environmental acceptability of 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) in the troposphere, we have examined the oxidation reaction pathways and kinetics of CF2CF-CH2F initiated by Cl-atoms using the second-order Møller-Plesset perturbation (MP2) theory along with the 6-31+G(d,p) basis set. We also performed single-point energy calculations to further refine the energies at the CCSD(T) level along with the basis sets 6-31+G(d,p) and 6-311++G(d,p). The estimation of the relative energies and thermodynamic parameters of the CF2CF-CH2F + Cl reaction clearly shows that Cl-atom addition reaction pathways are more dominant compared to H-abstraction reaction pathways. The value of the rate coefficient for each reaction channel is calculated using the conventional transition state theory (TST) over the temperature range of 200-1000 K at 1 atm. The estimated overall rate coefficients for the title reaction are found to be 1.10 × 10-12, 1.21 × 10-10, and 1.13 × 10-8 cm3 per molecule per s via the respective calculation methods viz. MP2/6-31+G(d,p), CCSD(T)//MP2/6-31+G(d,p), and CCSD(T)/6-311++G(d,p)//MP2/6-31+G(d,p), at 298.15 K. Moreover, the calculated rate coefficients and percentage branching ratio values suggest that the Cl-atom addition reaction at the β-carbon atom is more preferable to that of the α-carbon addition to CF2CF-CH2F. Based on the rate coefficient values calculated by the three different methods, the atmospheric lifetime for the title reaction at 298.15 K is estimated. The radiative efficiency (RE) and Global Warming Potential (GWP) results of the title molecule show that its GWP would be negligible. Further, we have explored the degradation of its product radicals in the presence of O2 and NO. From the degradation results, we have found that CF2(Cl)COF, FCOCH2F, FCFO and FCOCl are formed as stable end products along with various radicals such as ˙CF2Cl and ˙CH2F. Therefore, these findings of kinetic and mechanistic data can be applied to the development and implementation of a novel CFC replacement.

Alleviating the competitive adsorption of hydrogen and hydroxyl intermediates on Ru by d–p orbital hybridization for hydrogen electrooxidation

Promoted hydrogen oxidation reaction kinetics in alkaline electrolytes and enlarged effective potential range were achieved simultaneously by alleviating the competitive adsorption of hydrogen and hydroxyl intermediates.

(Invited) Understanding PEMFC by Advanced Electron Microscopy

A thorough knowledge of the atomic structure and composition of catalyst nanoparticles is paramount to the development of advanced materials for proton exchange membrane fuel cells (PEMFC), one of the most promising energy conversion devices for automotive and stationary applications. Pt and Pt-based alloys nanoparticles (NPs) are currently used as the catalyst to promote the kinetics of the hydrogen oxidation and oxygen reduction reactions in the anode and cathode of the fuel cell, respectively. Yet, the durability of the catalysts remains a major issue for their commercialization. In addition, it is essential to characterize the ionomer distribution and coverage over the catalyst NPs and the carbon support, as the ionomer/catalyst interface plays a critical role in the ORR and MEA fuel cell performance.Ideally, voltage cycling of the membrane electrode assemblies would be performed in-situ within a TEM, but these experiments are highly challenging, since separate gas phase reactants need to be present simultaneously, in particular hydrogen and oxygen Instead, a very close approximation to a true in-situ experiment is done via potential cycling in a half-cell in the presence of liquid electrolyte, as opposed to solid electrolyte in a single cell. This allows us to observe the exact same location before and after cycling through a technique called identical location TEM/STEM (IL-TEM/STEM). Under these conditions, various electron microscopy techniques, such as bright-field TEM and HAADF STEM coupled with EDS and EELS can be performed. To better acquire and understand TEM/STEM-based experimental images, in particular the selection of appropriate operating conditions, and interpretation/validation of the results, it is critical to perform multislice TEM/STEM image simulations to calculate the interaction of an electron beam with a material system.In this talk, the focus is to understand the behavior of Pt and Pt-alloy NPs during the various stages of fuel cell cycling. For this purpose a set-up was developed to simulate the effect of voltage cycling on the cathode side of the fuel cell. In this set up, catalyst NPs supported on carbon nanotubes and amorphous carbon were observed by advanced transmission electron microscopy, before and after cycling. In particular, we studied Pt and Pt-alloy NPs and carbon support degradation mechanisms as a function of cycling using for the first time a combination of 2D IL TEM and 3D IL TEM. This new approach reveals many unpredictable features of morphology changes/behavior of Pt NPs and carbon support, after experiencing the long durability test. These new findings provide an insight which great surpasses what have been observed using solely 2D IL TEM.The experiments show particle migration in conjunction with carbon corrosion during the initial cycles, whereas the appearance of single atoms and atomic clusters on the surface of the carbon support appear after additional voltage cycling as a result of surface dissolution of NPs. For the case of alloyed NPs, the experiments show a heterogeneous deposition of Pt on the NPs. Moreover, we demonstrate a novel approach to distinguish the ionomer phase from the carbon support in PEMFCs, by employing the carbon signal in STEM-EELS, using two types of detectors. In this fashion, not only the ionomer, but also the carbon support, can be mapped in high spatial resolution, despite some damage caused by the electron beam.

Electrochemical Impedance Analysis of Direct Ammonia Fuel Cell Operation and Clamping Effects

In addition to being an essential chemical for fertilizers, ammonia can also be used as a hydrogen carrier or directly as a fuel. Compared to hydrogen, the main advantages are that ammonia can be liquefied in mild conditions and the technology for ammonia shipping and pipelines already exists. Compared to carbon-based synthetic fuels, no carbon dioxide is produced by the oxidation, only water and nitrogen. The theoretical voltage of a direct ammonia fuel cell (DAFC) is 1.17 V, nearly equal to the 1.23 V of hydrogen fuel cells (HFC) but a central drawback is the slow kinetics of the ammonia oxidation reaction (AOR). The AOR is even slower than the oxygen reduction reaction (ORR) at the cathode, which in turn is the main voltage loss component in HFCs. Consequently, due to the AOR kinetics, DAFCs typically require significantly higher platinum group metal (PGM) catalyst loadings than HFCs for the same power output.To improve the performance of our aqueous-feed DAFC, especially using non-PGM catalysts, we carried out comparisons of catalysts and anion exchange membranes (AEM), and supplemented the comparison of power, voltage, and current parameters with electrochemical impedance spectroscopy (EIS). Additionally, we also compared measurements in two different test cells to try to identify the cause of measurement repeatability issues that we had when using one of the cells.Initially, we assumed that the reproducibility of catalyst deposition (or lack of it) and AEM or catalyst degradation in the ammonia-containing electrolyte had caused our repeatability problems. However, following a preliminary analysis of measurement statistics, we consider problems with the cell itself a more likely cause. The maximum power of the cell depended almost inversely on the series resistance (R S, i.e., the high-frequency resistance/real-axis intercept of the EIS spectrum), and the current at given voltage varied more than the differences in the resistance alone could explain when most measurement parameters were not varied (e.g., ammonia and hydroxide concentrations, liquid and gas flow rates). The general inverse dependence on R S remained also when measuring the same electrodes in different cells, and the performance did not necessarily degrade over time so, clearly, degradation and differences between electrodes could not explain everything. When testing different screw torques and gaskets in our AEM electrolyser, we had noticed that the voltage differences were often high compared to the R S differences, and it is known that clamping torque can affect the operation of fuel cells and electrolysers significantly. Nevertheless, more detailed analysis is ongoing to determine possible other contributing factors, to optimize the use of the cell(s) in measurements, and to guide future work.This research was done under the TELEGRAM project. This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under grant agreement No 101006941. The project started on the 1st of November 2020 with a duration of 42 months. The authors acknowledge support from the Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A). Figure 1

Enhancing Direct Methanol Oxidation with Pt-Pd Alloy Nanoparticles Synthesized By Gas Diffusion Electrocrystallization (GDEx)

Direct methanol fuel cells (DMFCs) are a promising sustainable energy solution due to their high energy density, low-temperature operation, and portability. However, sluggish methanol oxidation reaction (MOR) kinetics hinders their successful commercialization.1,2 Pt is a highly active MOR catalyst due to its ability to adsorb and decompose methanol on its surface. However, when pure Pt is used, the adsorption of oxidation intermediaries, i.e., CO, occurs, blocking catalytically-active sites and reducing the catalytic efficiency.2 A promising strategy to improve the catalytic activity and CO tolerance of Pt is by alloying. Among several candidates, Pd stands out due to its identical crystal structure and similar lattice constant to Pt, resulting in single-phase bimetallic materials with strong coupling and improved catalytic performance. In addition, the active surface area is enhanced, and stability is improved by using Pt-Pd alloys.3 We recently reported an electrochemical method for synthesizing platinum group (PGMs, i.e., Pt, Pd, Rh) nanoparticles (NPs) using Gas-Diffusion Electrocrystallization, best known as the GDEx process.4 In GDEx, CO2 and water are reduced using a gas diffusion electrode, producing CO and H2, which in turn reduce PGMs ions in solution, forming metal nanoclusters of 10 nm–40 nm composed of even smaller nanocrystals of 2 nm–4 nm. In this work, we used GDEx to synthesize Pt-Pd alloy NPs with different metal ratios (Pt100, Pt75-Pd25, Pt50-Pd50, Pt25-Pd75 and Pd100). The Pt-Pd alloy NPs were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy. The size distribution of the synthesized nanomaterials was between 15 nm–30 nm, increasing in size with increasing Pd content. The catalytic activity of the Pt-Pd alloy NPs toward methanol oxidation was measured in 0.1 M HClO4. The electrochemical surface area (ECSA), measured by the Cu underpotential deposition method,5 was 37.1 ± 2.1, 36.1 ± 2.7, 42.5 ± 1.7 and 56.1 ± 2.9 m2 gPt -1 for Pt100, Pt75-Pd25, Pt50-Pd50, and Pt25-Pd75 respectively. The mass activity (MA, fig.1a), defined by the current density per unit mass of Pt loading at the forward peak, was 452 ± 10, 345 ± 16, 365 ± 21 and 189 ± 15 for Pt100, Pt75-Pd25, Pt50-Pd50, Pt25-Pd75. Even though the MA for Pt75-Pd25 and Pt50-Pd50 was lower than for Pt100, their onset potential was smaller (0.503 VRHE for Pt75-Pd25 and 0.488 VRHE for Pt50-Pd50) than for Pt100 (0.530 VRHE) indicating that the MOR is more favourable on those alloys. Besides, during the accelerated stability test (ADT, Fig.1b), after 1000 cycles, Pt75-Pd25 and Pt50-Pd50 showed higher stability and resistance towards CO poisoning than Pt100 by holding 80% and 70% of their peak current density. In comparison, Pt100 only kept ∼60% of its peak current density under the same conditions.In summary, the GDEx process allows the synthesis of Pt-Pd alloy NPs with a similar or superior catalytic performance towards MOR than pure Pt NPs, i.e., similar MA but with a better tolerance towards CO poisoning. The selectivity of the GDEx process for PGMs allows the synthesis of Pt-Pd catalysts for DMFCs not only from synthetic solutions but also from complex matrixes that contains a mixture of both PGMs (i.e., leachates from spent automotive catalytic converters).This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements No. 958302 (PEACOC project) and No. 101091715 (FIREFLY project). Yuda A, Ashok A, Kumar A. Catalysis Reviews. 2022;64(1):126-228.Ren X, Lv Q, Liu L, Liu B, Wang Y, Liu A, Wu G. Sustain. Energy Fuels. 2020;4(1):15-30.Yousaf AB, Imran M, Uwitonze N, Zeb A, et al. J . Phy s. Chem . C. 2017;121(4):2069-79.Martinez-Mora O, Pozo G, Leon-Fernandez LF, Fransar J, Dominguez-Benetton X., RSC Sustain. 2023; DOI: 10.1039/D3SU00046J.Green CL, Kucernak A. J. Phys. Chem. B. 2002;106(5):1036-47. Figure 1

Oxidation differences on Si- versus C-terminated surfaces of SiC during planarization in the fabrication of high-power, high-frequency semiconductor device

Silicon carbide (SiC) wafers have attracted attention as a material for advanced power semiconductor device applications due to their high bandgap and stability at high temperatures and voltages. However, the inherent chemical and mechanical stability of SiC poses significant challenges in the chemical mechanical planarization (CMP) process, an essential step in reducing defects and improving surface flatness. SiC exhibits different mechanical and chemical properties depending on SiC terminal faces, affecting SiC oxidation behavior during the CMP process. Here, we investigate the process of oxide layer formation during the CMP process and how it relates to the SiC terminal faces. The results show that under the same conditions, the C-terminated face (C-face) exhibits higher oxidation reaction kinetics than the Si-terminated face (Si-face), forming an oxide layer of finer particles. Due to the different oxidation kinetic tendencies, the oxide layer formed on the C-face has a higher friction coefficient and more defects than the oxide layer formed on the Si-face. This results in a higher removal rate during CMP for the C-face than the Si-face. Furthermore, by controlling the physicochemical properties of the oxide film, high removal rates can be achieved by friction with the pad alone, without the need for nanoparticle abrasives.