Mechanism Of Hydrogen Oxidation Research Articles

Towards a uniform description of recombiners performance by a consistent CFD approach with the use of a detailed mechanism of hydrogen oxidation

For a consistent CFD substantiation of the recombiner performance, a detailed mechanism of hydrogen and oxygen recombination is used. The detailed mechanism of chemical kinetics (multi-step recombination reaction) makes it possible to claim universality, both in the numerical justification of the recombiner performance and in the justification of the flameless recombination threshold and makes it possible to justify the method for optimizing the recombiner to improve its characteristics. The models developed based on this approach were applied to both flat and cylindrical catalytic elements, which are used in FR and RVK recombiners, respectively. As part of the numerical studies, the detailed recombination mechanism was verified, namely the temperature distribution along the catalytic elements was compared and the performance of catalytic elements was compared as well. Good agreement was obtained between the calculated and experimental data. The approach considers not only the mechanism of surface recombination of hydrogen and oxygen on platinum, but also the mechanism of recombination in the gas phase. This makes it possible to calculate the onset of intense combustion outside the catalytic plates, which is a sign of volumetric ignition of the hydrogen-air environment. The concentrations at which such ignition is possible were obtained at different contents of water vapor in the medium. Thus, the proposed approach and the created models make it possible to fully describe the performance of recombiners of distinct designs without the use of additional experimental data, which is extremely necessary when justifying the hydrogen explosion safety of nuclear power plants.

Re-Structuring of Interfacial Water into Strongly Hydrogen Bonded “Ice-like” Structures as the Unifying Descriptor for Improving Sluggish HOR/HER in Alkaline Electrolyte

The 2 orders of magnitude loss of Platinum Group Metal (PGM) activity toward Hydrogen Oxidation and Reduction reactions (HOR/HER) has hindered implementation of alkaline based electrolyzers and fuel cells and demonstrated a significant knowledge gap in our fundamental understanding of electrochemical interfaces.1 Beneath the seemingly simple reaction lies a potentially convoluted mechanism, with various researchers assigning the activity loss to shifts in electrode potential of zero free charge,2 changed binding energies of adsorbates and reactive intermediates,3,4 and the re-arrangement of interfacial water.5 Recently, Monteiro et al. showed an activity dependence on interfacial cation concentration and identity, suggesting that charge dense cations are able to better stabilize the OH- produced from the unfavorable but necessary water splitting in the HER direction near the negatively charged surface.6,7 In this work, we challenge this notion by demonstrating that competition for water molecules between Outer Helmholtz Plane (OHP) cations and the interfacial water structure is responsible for the activity loss. Through the use of traditional Rotating Disk Electrode, in situ Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) and judicious choice of crown ether additives to hinder the water-interfacial cation interaction, we show a positive correlation between the strength and number of interfacial water-water hydrogen bonds and platinum’s HOR/HER activity.The typical double layer structure in an aqueous electrochemical system consists of specifically adsorbed species in the Inner Helmholtz Plane (IHP), the first layer of non-adsorbates at the OHP, and water hydrating the charged species. ATR-SEIRAS has previously been used to analyze the structure of these hydration shells on a CO covered Pt surface, demonstrating hydrating waters interacting mainly with alkali metal cations through the increase of a 3600 cm-1 ν(O-H) stretch and weak interactions with surrounding interfacial water.8 Through chelation with crown ether, these hydration shells convert from strongly to weakly cation interacting, favoring instead hydrogen bonding with other water molecules, shown with increasing absorption bands at lower wavenumbers (3400 cm-1). This transition is accompanied by an increase in 3000 cm-1 on the CO-free Pt surface, previously assigned to strongly hydrogen bonded “ice-like” interfacial water.9 We further show a similar increase in the 3000 cm-1 band for other kinetic enhancing additives recently reported in the literature, supporting the notion that increasing water-water hydrogen bonding is the unifying descriptor for promoting a rigid interfacial “ice-like” structure and improving PGM HOR/HER activity. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Zhao, K. et al. Enhancing Hydrogen Oxidation and Evolution Kinetics by Tuning the Interfacial Hydrogen-Bonding Environment on Functionalized Platinum Surfaces. Angew. Chemie - Int. Ed. 61, (2022).Monteiro, M. C. O., Goyal, A., Moerland, P. & Koper, M. T. M. Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media. ACS Catal. 11, 14328–14335 (2021).Goyal, A. & Koper, M. T. M. The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media. Angew. Chemie - Int. Ed. 60, 13452–13462 (2021).Yamakata, A. & Osawa, M. Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations. J. Electroanal. Chem. 800, 19–24 (2017).Osawa, M., Tsushima, M., Mogami, H., Samjeské, G. & Yamakata, A. Structure of water at the electrified platinum-water interface: A study by surface-enhanced infrared absorption spectroscopy. J. Phys. Chem. C 112, 4248–4256 (2008). Figure 1

The parallel pathways of hydrogen oxidation reaction on high active decorated Ni–YSZ electrode in electrochemical cell with GDC protective layer

In this study, we considered the electrochemical behavior of a symmetrical electrochemical cell with a highly conductive supporting La0.88Sr0.12Ga0.82Mg0.18O2.85 electrolyte, a protective Ce0.82Gd0.18O2–δ layer and highly active Ni + Zr0.84Y0.16O1.92 electrodes in H2 + H2O + Ar gas mixtures at constant partial pressure of H2, H2O and constant ratio pH2/pH2O. It is shown that the presence of a protective Ce0.82Gd0.18O2–δ layer makes it possible to significantly increase the electrode activity by about 7 times at 800 °C and about 24 times at 700 °C. After the introduction of highly dispersed particles of cerium oxide into the electrode by the method of solution impregnation, the polarization resistance of electrode was about 0.03 Ω cm2 at 800 °C. It is shown that in the case of decorated electrodes, the mechanism of hydrogen oxidation includes two parallel pathways. One is similar to the hydrogen oxidation route for conventional nickel–ceramic electrodes, and the second includes slow stages of oxygen ions transport both in ceria particles and in the protective Ce0.82Gd0.18O2–δ layer. It is shown for the first time that with an increase in oxygen partial pressure the ratio of the rates of these pathways changes.

Evidence for Caffeine’s Positive Impact on HOR/HER Activity through Water Rearrangement As Opposed to Direct Influence on the Interfacial Electric Field

The puzzling pH dependence of HOR/HER leading to the approximately 2 orders of magnitude loss in platinum group metal (PGM) activity in base has hindered both the practical design of electrochemical devices as well as the fundamental understanding of electrode/electrolyte interfaces.1 Explanations for this effect have ranged from the presence of adsorbates specific to alkaline conditions,2 shifts in the electrode potential of zero free charge (pzfc) and subsequent strengthening of the interfacial electric field,3 the orientation of interfacial water molecules, and changes in the binding energy of reaction intermediates.4–6 Recently, several innovative in situ techniques have been used to probe these theories, ranging from Fourier Transform Infrared Spectroscopy (FTIR) to X-Ray Absorption Spectroscopy (XAS), as well as novel computational studies to isolate the contributions of surface adsorbed hydroxide and water.2–4,7 Additionally, the identification of caffeine as a “double-layer dopant” capable of improving HOR/HER activity 5-fold on Pt(111) has provided the field with a model system with which to systematically test these theories.8 Despite this, an overarching understanding of the mechanism and caffeine’s promoting role is still unclear, and a more rigorous analysis of the electrode surface is necessary.The electrochemical interface is traditionally viewed with a double layer model, with specific electrochemical adsorbates existing in the Inner Helmholtz Plane (IHP), the first layer of non-adsorbates at the Outer Helmholtz Plane (OHP), and water in abundance. One leading theory used to understand the activity loss from acid to alkaline environments is the decrease in surface potential relative to the pzfc in base resulting in stronger electric fields which restrict the approach of reactive intermediates to the IHP/OHP. Caffeine is suspected of reducing the IEF strength through decreasing the pzfc to relevant HOR/HER potentials, creating an acidic like environment. In this work, we challenge this notion by using SEIRAS with CO as a probe molecule to directly measure the interfacial field strength in 0.1 M KOH solutions with the model caffeine system showing HOR/HER enhancement, as well as different concentrations of various organic species with no promoting kinetic impact. FTIR studies using CO as a probe molecule to measure stark tuning rates have long been used to directly measure the interfacial electric field strength, dictated by the majority species in the OHP.9,10 The low ST rate of 22 cm-1V-1 found for all of these species in our experiments, regardless of their impact on HOR/HER kinetics, implies reductions in IEF are not solely responsible for the KOH-caffeine system’s increased activity. Rather, caffeine induces changes to the interfacial water structure as evidenced by differences in water’s ν(O-H) stretching modes, allowing for more facile kinetics, the main parameter responsible for the “apparent pH dependence” of PGM HOR/HER. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).Liu, E. et al. Interfacial water shuffling the intermediates of hydrogen oxidation and evolution reactions in aqueous media. Energy Environ. Sci. 13, 3064–3074 (2020).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Rebollar, L. et al. On the relationship between potential of zero charge and solvent dynamics in the reversible hydrogen electrode. J. Catal. 398, 161–170 (2021).Cheng, T., Wang, L., Merinov, B. V. & Goddard, W. A. Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH. J. Am. Chem. Soc. 140, 7787–7790 (2018).Intikhab, S. et al. Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis. ACS Catal. 10, 6798–6802 (2020).Lambert, D. K. Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models. J. Chem. Phys. 89, 3847–3860 (1988).Anderson, M. R., Blackwood, D. & Pons, S. The behavior of the infrared spectrum of carbon monoxide adsorbed at platinum electrodes from non-aqueous solvents. J. Electroanal. Chem. 256, 387–395 (1988). Figure 1

How Hydrogen Evolution and Oxidation Reactions Proceed at the Platinum-Electrolyte Interface in Aqueous Solution

The hydrogen evolution and oxidation reactions (HER/HOR) are the most fundamental reactions in electrochemistry. They are also the electrochemical reactions taking place in practical devices such as H2 fuel cells and electrolyzers for the conversion between hydrogen fuel and electricity. Despite their fundamental and practical importance, the HER/HOR kinetics has remained not explicitly clear even for the platinum (Pt) electrode that has been mostly studied. This is manifested by the lack of consensus on why the HER/HOR rate of Pt is about two orders of magnitude slower in alkaline (pH = 0) than in acid (pH = 13),1 a long-standing puzzle in electrochemistry. Clear understanding of electrochemical reaction kinetics requires clear understanding of the charge transfer and chemical transformation at the electrode-electrolyte/membrane interface (EEI), which is however hampered by the lack of means to explore the complex electrochemical processes at the EEI in situ/operando.Herein, by employing a set of N-methylimidazoles as the HER/HOR promotors and interface probes, we gave a reasonable explanation on how the HER/HOR of Pt proceeds within the EEI in aqueous solution.2 A combination of in situ spectroscopic characterization on the EEI showed that the N-methylimidazoles promoted the HER/HOR of Pt in alkaline solution by reorientating interfacial water against the interfacial electric field. We accordingly proposed that the HER/HOR in acidic and alkaline solution proceeds via diffusion of proton and hydroxide, respectively, through the H-bond network of interfacial water by the Grotthuss mechanism (Figure 1). We further demonstrated that adding 1,2-dimethylimidazole into the alkaline solution fed into the cathode of an anion exchange membrane electrolyzer led to an 40% performance improvement. The diffusion roles of interfacial water unraveled here may apply to many other electrochemical processes in aqueous solution. Acknowledgements This work was supported by the Office of Naval Research (ONR) grant N000141712608. This research used beamline 7-BM (QAS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory Contract DE-SC0012704. Beamline operations were supported in part by the Synchrotron Catalysis Consortium Grant DE-SC0012335. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255-2260 (2014).Sun et al. Mechanistic insights into pH-dependent hydrogen electrocatalysis by flipping interfacial water. Under revision by Nat. Energy. Figure 1

H/D isotopic exchange and electrochemical kinetics of hydrogen oxidation on Ni-cermets with oxygen-ionic and protonic electrolytes

Ni–Zr0.82Y0.18O1.91 (YSZ) and Ni–La0.90Sr0.10ScO2.95 (LSS) cermets for solid oxide and protonic ceramic electrochemical cells were studied by means of H/D isotope exchange with the gas phase equilibration method for the first time. The experiments were carried out at 2 mbar of dry hydrogen in the temperature range of 400–800 ᵒC. The three parallel channels of hydrogen surface exchange were found to exist in both Ni-YSZ and Ni-LSS cermets. The mechanism of hydrogen isotope exchange for Ni-YSZ was found to be temperature independent, while, in the case of Ni-LSS, the mechanism changed with temperature and was more complicated at lower temperatures (400–600 °C). It was revealed that hydrogen spillover was the rate-determining step for both cermets. The electrochemical kinetics of hydrogen oxidation were studied on symmetric cells with a YSZ supported electrolyte and Ni-YSZ electrodes and with an LSS supported electrolyte and Ni-LSS electrodes in a wet (3 vol % H 2O) hydrogen atmosphere at 1 atm. The mechanism of hydrogen oxidation on the Ni-LSS electrode differs from that of the Ni-YSZ electrode. It changes with temperature and is also more complicated at a lower temperature range.

Evidence for the Lack of Caffeine Specific Adsorption and Its Impact on Water Structure to Increase HOR/HER Activity on Pt

As the cornerstone for much of our electrochemical knowledge, the inexplicable 2 orders of magnitude decrease in the hydrogen oxidation and evolution reactions (HOR/HER) in moving from pH 1 to 14 on precious metals continues to undermine our understanding of and ability to better design electrochemical systems.1 Several theories have been proposed for this anomalous change, from electric field effects, the influence of cations, co-adsorption of H and OH, and the orientation of interfacial water, driving innovations in in situ electrochemical techniques, such as potential changes with Fourier Transform Infrared Spectroscopy (FTIR) or X-ray Absorption Spectroscopy (XAS).2–6 Recently, it was found that including small concentrations of caffeine in pH 13 was able to drastically increase the HOR/HER catalytic activity of platinum through what was postulated as surface specific adsorption.7 In this work, we challenge this notion through an in situ surface sensitive technique, termed Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) while simultaneously analyzing the underlying theory of water reorganization. Due to SEIRAS’ s surface sensitivity, we can distinguish between specific electrochemical surface adsorption and molecules merely located in the outer Helmholtz plane (OHP) through stark tuning, where species located in the inner Helmholtz plane (IHP) experience the strongest electric fields, and demonstrate a potential dependence of their vibrational frequency.8 Further, trends in caffeine induced water structure changes support our hypothesis that surface bound water layer changes observed in our in situ spectra are responsible for the “apparent pH dependence” of HOR/HER.The electrochemical interface is traditionally viewed as a double layer model, with specific electrochemical adsorbates existing in the Inner Helmholtz Plane and the first layer of non-adsorbates at the Outer Helmholtz Plane. ATR-SEIRAS enhanced surface sensitivity allows for strong signals to be obtained within 5 nm of the surface, selectively capturing both the IHP and OHP with decreasing strength into the diffuse layer and bulk electrolyte. In situ surface sensitive IR shows strong peaks corresponding to caffeine on addition of 10-4 M to basic electrolyte, but a lack of stark tuning suggests no specific adsorption of caffeine on the Pt surface. Furthermore, SEIRAS spectra reveal subtle changes to the interfacial water with addition of caffeine, namely losses of certain types of interfacial water and changes in the water-water hydrogen bonding network. These spectra compared with similar samples collected in traditional 0.1 M HClO4 electrolyte point toward the “apparent pH dependence” being a function of pH induced changes to the interfacial water structure. 1. Durst, J., Siebel, A., Simon, C., Hasche, f., Herranz, J., & Gasteiger, H. A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014). Chen, X., McCrum, I. T., Schwarz, K. A., Janik, M. J. & Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 56, 15025–15029 (2017).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).Liu, E. Jiao, L., Li, J., Stracensky, T., Sun, Q., Mukerjee, S., & Jia, Q. Interfacial water shuffling the intermediates of hydrogen oxidation and evolution reactions in aqueous media. Energy Environ. Sci. 13, 3064–3074 (2020).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Intikhab, S. et al. Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis. ACS Catal. 10, 6798–6802 (2020).Lambert, D. K. Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models. J. Chem. Phys. 89, 3847–3860 (1988). Figure 1

Inhibition of hydrogen–oxygen/air gaseous detonations using CF3I, H2O, and CO2

The ignition of hydrogen-air mixtures can cause intense explosions. To ensure the safe utilization of hydrogen fuel and prevent catastrophic explosion accidents, the inhibiting effect of flame inhibitors must be understood since they have important consequences for hazard analyses. In the present study, the detonability of hydrogen mixtures diluted with CF3I, H2O, CO2, and N2 is investigated using numerical computations. The inhibiting effect of chemically active (CF3I, H2O, and CO2) and chemically inert (N2) diluents on the chemical length and time scales of a ZND detonation structure was studied using a detailed chemical kinetic mechanism for hydrogen oxidation. The inhibition efficiency of a halogenated compound, CF3I, towards quenching of hydrogen-air/oxygen gaseous detonations was also studied. As an additive, CF3I was found to promote detonation at lower concentrations. However, at higher concentrations, CF3I suppresses gaseous detonations by acting as a radical-scavenging agent. In the present study, a given retardant's effectiveness for suppressing a given detonation wave was evaluated for CF3I, H2O, CO2, and N2. The combined suppression effects of CF3I, H2O, and CO2 were also investigated. The efficacy of a composite inhibitor towards the quenching of a detonation wave was also evaluated and compared.

Hydrogen oxidation kinetics on a redox stable electrode for reversible solid-state electrochemical devices: The critical influence of hydrogen dissociation on the electrode surface

The results of detailed studies of the kinetics of hydrogen oxidation on a Sr2Fe1.5Mo0.5O6-δ (SFM) electrode in contact with La0.85Sr0.15Ga0.85Mg0.15O3 electrolyte are presented for the first time. For a detailed understanding of the mechanism of the electrode reaction, studies were carried out in H2+H2O+Ar gas mixtures at a constant partial pressure of hydrogen, water and at a constant hydrogen/water ratio. Also, to understand the pathways of the electrode reaction, the results of the comparison of the mechanism of hydrogen oxidation on the SFM electrode with different organisation of current probes to the studied electrode are presented, as well as a comparison of the kinetics of electrode reaction on the SFM electrode and the SFM electrode decorated with nickel particles. It is shown that the rate of hydrogen oxidation on the SFM electrode is determined by the low rate of oxygen heterogeneous exchange, as well as by the slow dissociation of hydrogen on the SFM surface. At 800 °C, the rates of these processes are comparable; with temperature decrease the rate of hydrogen oxidation is mainly limited by hydrogen dissociation. Decorating the SFM electrode with nickel particles significantly increases the rate of hydrogen dissociation and increases the activity of the electrode more than two-fold, up to a polarisation resistance of about 0.04 Ω cm2 at 800 °C.

Measurements of the laminar flame speed of premixed, hydrogen-air-argon stagnation flames

As the foundation for detailed hierarchical combustion chemistry models, the accuracy of the hydrogen oxidation mechanism is paramount to achieve truly predictive models. The recent introduction of new chemically termolecular reactions to this system suggests that our understanding of this fundamental chemistry could have initially been plagued with structural inaccuracies that have been carried along in the development of larger thermochemical mechanisms. Consequently, there is a need for additional independent experimental data in various experimental setups to assess the accuracy of model predictions. An atmospheric jet-wall stagnation-flame configuration is used in this work to measure the reactivity of premixed, lean-to-rich hydrogen-air flames diluted with argon. Mixtures with estimated adiabatic flame temperatures of ∼1800K are obtained by argon dilution to maintain the oxygen-nitrogen ratio while allowing for stable, laminar flames over the desired conditions. The accuracy of various thermochemical mechanisms available in the literature is assessed with direct comparisons of the velocity profiles in the stagnation configuration obtained through Particle Tracking Velocimetry to the solution of the quasi–1D flame simulation. Unstretched laminar flame speeds (SLo) are subsequently inferred using a direct comparison technique between the experiments and the simulations at multiple stretch values, similarly to a non-linear extrapolation to unstretched conditions. Significant discrepancies are observed between model predictions and experimental measurements, which supports the current efforts in model development. The inaccuracies in kinetic rates of the hydrogen oxidation model must first be resolved to avoid optimization and validation errors in the hierarchical development of increasingly complex hydrocarbon mechanisms.

QM/MM study of the binding of H2 to MoCu CO dehydrogenase: development and applications of improved H2 van der Waals parameters

The MoCu CO dehydrogenase enzyme not only transforms CO into CO2 but it can also oxidise H2. Even if its hydrogenase activity has been known for decades, a debate is ongoing on the most plausible mode for the binding of H2 to the enzyme active site and the hydrogen oxidation mechanism. In the present work, we provide a new perspective on the MoCu-CODH hydrogenase activity by improving the in silico description of the enzyme. Energy refinement—by means of the BigQM approach—was performed on the intermediates involved in the dihydrogen oxidation catalysis reported in our previously published work (Rovaletti, et al. “Theoretical Insights into the Aerobic Hydrogenase Activity of Molybdenum–Copper CO Dehydrogenase.” Inorganics 7 (2019) 135). A suboptimal description of the H2–HN(backbone) interaction was observed when the van der Waals parameters described in previous literature for H2 were employed. Therefore, a new set of van der Waals parameters is developed here in order to better describe the hydrogen–backbone interaction. They give rise to improved binding modes of H2 in the active site of MoCu CO dehydrogenase. Implications of the resulting outcomes for a better understanding of hydrogen oxidation catalysis mechanisms are proposed and discussed.

Effect of Varying Carbon Supports on Pt Based HOR Electrocatalysts for Alkaline Media Synthesized By CVD

The synthesis of electrocatalysts for the chemical reactions of hydrogen systems (e.g. fuel cells and electrolyzer systems) continues to be a significant area of research in the renewable energy community. The kinetics of hydrogen reactions decrease by two orders of magnitude in alkaline environments compared to acidic, which has rendered significant challenges in the development as catalysts of alkaline fuel cell systems[1,2]. Chemical vapor deposition (CVD) is a conventional synthesis process applied to manufacture electrocatalysts[3,4]. In this study, a modified form known as “Poor Man’s” CVD (PMCVD) is utilized, to further its impact in electrochemical catalyst development. This work assesses the deposition mechanism as well as its ability to develop tuneable catalysts for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) for alkaline media.In our last efforts, we showed Pt based catalysts synthesized using the PMCVD method exhibited higher activities for the HOR/HER in alkaline media than commercially available catalysts[5]. The performance was attributed to the tunability of crystallite size by varying the temperature and pressure of the PMCVD process. In an effort to further understand the deposition mechanism of this processes, we investigated Pt catalysts on various carbon supports. In doing so, Pt was deposited on three supports: XC-72, Acetylene Black, and Graphitized Nanoplatelets with 30% Pt by weight. The carbons varied in both surface areas and degree of graphitization. This work will summarize the HOR/HER activities of these three Pt/C catalysts in alkaline media. It will also help elucidate the impact of the deposition mechanism as a function of each support based on their material properties.[1] Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energ. Environ. Sci. 7, 2255–2260 (2014).[2] Sheng, W. et al. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes.” Electrochem. Soc. 2010 volume 157, issue 11, B1529-B1536[3] Garcia, Vargas, et al., “Chemical Vapor Deposition of Iridium, platinum, Rhodium, and Palladium.” Materials Transactions, Vol. 44, No. 9 (2003) pp. 1717 to 1728[4] Serp, Philppe, et. al., “Chemical Vapor Deposition Methods for the Controlled Preparation of Supported Catalytic Materials.” Chem. Rev. 2002, 102, 3085−3128[5] Williams, S. et. al. “Synthesis and Characterization of Electrocatalysts for the Hydrogen Evolution Reaction in Alkaline Environment.” ECS Fall 2019. Figure 1

Understanding Electric and Non-Electric Field Effects on Electrochemical Double Layer Restructuring for the Model Platinum HOR

Recent progresses in hydroxide exchange membrane fuel cells (HEMFCs) have led to renewed interest in the strong pH effect on hydrogen oxidation reaction (HOR) rate on precious metals: two orders of magnitude decrease when pH is changed from 0 to 14.1 Various theories have been proposed to explain this, ranging from adsorbed cation effects, changes in the electric field strength, to pH induced changes to the apparent hydrogen binding energy.2-4 One leading argument is the increase of pH leads to stronger electric field and thus more rigid ordering of water at the electrode/electrolyte interface, resulting in greater resistance to proton/hydroxide transfer through the double layer. This increased energetic barrier due to reorganization results in the decreased activity.5 In this work, we examine this theory by using a combination of traditional electochemical and spectroscopic techniques to highlight the importance of both electric field and non-electric field effects on reaction rates. Further we use this distinction to understand how each factor contributes to water structure re-arrangement on the surface and in the double layer, which we hypothesize to be the pivotal factor in the aforementioned activity decrease from acid to base.The electrochemical double layer is traditionally described by specifically adsorbed species (inner helmholtz plane, IHP), the next layer of hydrated ions (outer Helmholtz plane, OHP) and further layers (diffuse region) protruding and mixing into the bulk electrolyte. It has been shown that the stark tuning rate, which is a function of electric field strength, is dictated by hydrated cation size, and provides a metric to measure the thickness of the electrochemical double layer.6-8 By introducing crown ether to chelate cations, the stark tuning rate, and thus the electric field, can be decreased up to a 0.5:1 ratio of crown ether to cation. Beyond this point, no change in stark tuning rate and electric field were observed. However, further decreases in HOR activity are observed beyond this threshold. This decreasing activity at a constant field strength provides strong evidence for mixed electric and non-electric field effects. We hypothesize that cations and ether molecules impact the electric field strength to a degree, but also interact with surface water species. Spectroscopic and reactivity data demonstrates that both chelating and linear ethers, while having drastically different impacts on the strength of the electric field, result in similar activity decreases. Additionally, both types of ether demonstrate removal of the same types of interfacial water, suggesting that their influence on the surface water structure rather than their effect on the electric field strength determines the HOR activity.(1) Durst, J.; Siebel, A.; Simon, C.; Hasché, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7 (7), 2255–2260. https://doi.org/10.1039/c4ee00440j.(2) Chen, X.; McCrum, I. T.; Schwarz, K. A.; Janik, M. J.; Koper, M. T. M. Co-Adsorption of Cations as the Cause of the Apparent PH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 2017, 56 (47), 15025–15029. https://doi.org/10.1002/anie.201709455.(3) Yang, X.; Nash, J.; Oliveira, N. J.; Yan, Y.; Xu, B. Understanding the PH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 2019, 58, 17718–17723. https://doi.org/10.1002/ange.201909697.(4) Zheng, J.; Nash, J.; Xu, B.; Yan, Y. Perspective—Towards Establishing Apparent Hydrogen Binding Energy as the Descriptor for Hydrogen Oxidation/Evolution Reactions. J. Electrochem. Soc. 2018, 165 (2), H27–H29. https://doi.org/10.1149/2.0881802jes.(5) Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Interfacial Water Reorganization as a PH-Dependent Descriptor of the Hydrogen Evolution Rate on Platinum Electrodes. Nat. Energy 2017, 2 (4), 1–7. https://doi.org/10.1038/nenergy.2017.31.(6) Ringe, S.; Clark, E. L.; Resasco, J.; Walton, A.; Seger, B.; Bell, A. T.; Chan, K. Understanding Cation Effects in Electrochemical CO 2 Reduction. Energy Environ. Sci. 2019. https://doi.org/10.1039/C9EE01341E.(7) Dunwell, M.; Wang, J.; Yan, Y.; Xu, B. Surface Enhanced Spectroscopic Investigations of Adsorption of Cations on Electrochemical Interfaces †. Phys. Chem. Chem. Phys 2017, 19, 971. https://doi.org/10.1039/c6cp07207k.(8) Waegele, M. M.; Gunathunge, C. M.; Li, J.; Li, X. How Cations Affect the Electric Double Layer and the Rates and Selectivity of Electrocatalytic Processes. J. Chem. Phys. 2019, 151 (16), 160902. https://doi.org/10.1063/1.5124878. Figure 1

Research Progress of Hydrogen Oxidation and Hydrogen Evolution Reaction Mechanism in Alkaline Media

Research Progress of Hydrogen Oxidation and Hydrogen Evolution Reaction Mechanism in Alkaline Media

Effects of hydrogen enrichment of methane on diffusion flame structure and emissions in a back-pressure combustion chamber

In the present study, the effects of hydrogen enrichment of methane are investigated numerically from the diffusion flame structure and emissions aspect. Fluent code is utilised as the simulation tool. In the first part of the study, four experiments were conducted using natural gas as fuel. A non-premixed burner and a back-pressure boiler were utilised as the experimental setup. The natural gas fuel consumption rate was changed between 22 Nm3/h and 51 Nm3/h. After the experimental studies, the numerical simulations were performed. The non-premixed combustion model with the steady laminar flamelet model (SFM) approach was used for the calculations. The methane-air extinction mechanism was utilised for the calculation of the chemical species. The numerical results were verified with the experimental results in terms of the flue gas emissions and flue gas temperature values. In the second part of the study, four different hydrogen-enriched methane combustion cases were simulated using the same methane-air extinction mechanism, which included the hydrogen oxidation mechanism as a sub mechanism. The same energy input (432 kW) was supplied into the boiler for all the studied cases. The obtained results show that the hydrogen addition to methane significantly change the diffusion flame structure in the combustion chamber. The hydrogen-enriched flames become broader and shorter with respect to the pure methane flame. This provides better mixing of the reactants and combustion products in the flame regions due to the use of a back-pressure boiler. In this way, the maximum flame temperature values and thermal NO emissions are reduced in the combustion chamber, when the hydrogen addition ratio is less than 15% by mass. The maximum temperature value is calculated as 2030 K for the case with 15% hydrogen addition ratio by mass, while it is 2050 K for the case without hydrogen enrichment. Therefore, it is determined that the hydrogen-enriched methane combustion in a back-pressure combustion chamber has the potential of reducing both the carbon and thermal NO emissions.

Synthesis and Characterization of Electrocatalysts for the Hydrogen Evolution and Hydrogen Oxidation Reactions in Alkaline Environment

The development of catalysts for the electrochemical processes of hydrogen systems (e.g. fuel cells and electrolyzer systems) continues to be a significant area of research for renewable energy technologies. One significant challenge has been the development of hydrogen catalysts that can be utilized in alkaline environments as the kinetics of hydrogen reactions decrease by two orders of magnitude in alkaline environments compared to acidic[1,2]. Chemical vapor deposition (CVD) is a conventional tool used in the synthesis of such catalysts[3,4]. This presentation discusses the extending work being done using a variant CVD process known as “Poor Man’s” CVD (PMCVD), to further its impact in electrochemical catalyst application[5]. PMCVD is utilizes an inexpensive vacuum oven to sublime commercially available metal-organic precursors; lowering synthesis cost while providing increased control of particle size and distribution. This work describes the work done to assess the PMCVD mechanisms as well as its use to develop catalysts for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) in alkaline environments. Preliminary studies have shown that PMCVD can develop catalysts with highly dispersed nanoparticles on various supports. As depicted in Figure 1, PMCVD synthesized Pt/C and PtRu/C catalysts have shown to have improved kinetics over commercial developed catalysts exhibiting approximately a 40% and 45% increase in average exchange current densities respectively when compared to Tanaka Kikinzoku catalysts of the same loading. An extended function of the PMVCD process is the ability to synthesize alloyed catalysts in a single deposition process. By understanding the parametric effects of temperature and pressure on the deposition process with respect to particle size and distribution, we can optimize the PMCVD process to develop highly active catalysts that can be used in alkaline hydrogen systems. [1] Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energ. Environ. Sci. 7, 2255–2260 (2014). [2]Sheng, W. et al. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. . Electrochem. Soc. 2010 volume 157, issue 11, B1529-B1536 [3] Garcia, Vargas, et al. “Chemical Vapor Deposition of Iridium, platinum, Rhodium, and Palladium.” Materials Transactions, Vol. 44, No. 9 (2003) pp. 1717-1728 [4] Serp, Philppe, et. al. “Chemical Vapor Deposition Methods for the Controlled Preparation of Supported Catalytic Materials.” Chem. Rev. 2002, 102, 3085−3128 [5] Papandrew, A. et al. Advanced Electrodes for Solid Acid Fuel Cells by Platinum Deposition on CsH2PO4. Chem. Mater. 2011, 23, 7, 1659-1667 Figure 1

Development and automation of algorithm for determining basis of nonlinear parameter functions of kinetic constants

Mathematical modeling of catalytic processes is necessary for the complete and accurate description, as well as for controlling the quality and physicochemical studied of catalysts. In the paper, theoretical issues of industrial catalysis are discussed. The work is devoted to theoretical graph analysis of informativity of kinetic parameters of the model of a complex chemical reaction. The aim is the development and automation of algorithm for determining basis of nonlinear parameter functions in solving inverse problems of chemical kinetics in order to define the number and form of independent combinations of rate constants of elementary stages. A program package for analysis of informativity of kinetic parameters of the mathematical model of a complex catalytic reaction is developed and described. The obtained functional relations between the kinetic parameters can be useful for experimentalists in physicochemical interpretation and analysis of mechanisms of chemical reactions. In other words, the proposed method allows independent combinations of kinetic constants to be distinguished that results in shortening the number of the model parameters and, as a consequence, enhance the accuracy of the mathematical model. The mechanism of hydrogen oxidation over a platinum catalyst is given as an example of the use of the software.

Comparison of electrochemical hydrogen oxidation on different metal/ceramic model anodes and mechanistic implications

Micro-patterned metal/ceramic model thin film electrodes were used to investigate the hydrogen oxidation mechanism on various metal/ion conductor combinations such as Ni/yttria-stabilized zirconia (YSZ), Pt/YSZ, Ni/scandia-stabilized zirconia, Ni/titania/YSZ. For all electrode types besides those with a continuous titania interlayer a triple phase boundary (TPB) and an area pathway could be separated. Moreover, activation energies as well as hydrogen and water reaction orders were determined for both pathways and compared with previously published data on Ni/YSZ. It was found that when varying the electronically conducting phases on YSZ the activation energy of the TPB path remains virtually the same. However, changing the ion conductor significantly influenced the activation energy, indicating a rate-limiting step on the electrolyte. However, the stark difference in water reaction order of Ni/YSZ and Pt/YSZ electrodes shows that the metal phase is also actively participating in the reaction chain. It is suggested that the electrolyte surface is responsible for the rate-limiting electrochemical redox reaction while the metal interacts with the gas phase molecules and thus supplies the reactants. For samples with a thin mixed conducting titania layer on YSZ, the electrochemical behavior drastically changed, which can be explained by an extension of the reaction zone away from the TPB.

Hydrogen Oxidation Reaction on Metal@Pt Core-Shell Catalysts in Alkaline Electrolyte

Hydrogen oxidation reaction (HOR) as the key reaction in hydrogen-oxygen fuel cell has been widely investigated, and ultra low loading Pt (≤0.05 mg cm–2) is required for HOR in proton exchange membrane fuel cells.(1, 2) However, the HOR kinetics becomes sluggish in base, which is at least two orders of magnitude slower than that in acid.(3) The sluggish HOR kinetics in alkaline electrolyte hinders the development of alkaline exchange membrane fuel cell, which is more commercially viable because of the potential to forego noble metal catalysts and less serious corrosion.(4) To obtain the catalysts with high HOR activity in alkaline environment, it is essential to understand the reason for this puzzling difference of HOR kinetics in acid and base. The understanding of the sluggish hydrogen oxidation reaction (HOR) kinetics in alkaline electrolyte is crucial for designing high performance electrocatalysts. Here, we report a clear and convincing result on this problem by investigating the relationship between the HOR kinetics and Pt-adsorbate energetics. Using electrochemical analysis for well-modified Pt surfaces with distinct Pt-adsorbate interactions, we establish a clear trend in activity for HOR in alkaline electrolyte, that is, the activity changes in the order Au@Pt < Pt < Pd@Pt < Ru@Pt (Figure 1a). A decisive role of Pt－Had energetics in the HOR kinetics on Pt surfaces is determined, while no favorable effects of Pt－OHad energetics in the HOR kinetics were found (Figure 1b). The in-situ XANES analysis demonstrated a higher Had coverage on the pure Pt surface than that on the Pt surface with Ru core due to more strongly bonded Had on pure Pt surface (Figure 1c,d). These findings provide us a novel aspect to understand the HOR kinetics on Pt surfaces and also provides fundamental insights into the influence of the Pt－Had interaction on the HOR mechanism. Figure Caption Figure 1. (a) HOR polarization curves normalized to the limiting current density. (b) Trend in the exchange current density for HOR in alkaline solution as a function of the Hupd coverage. Error bars stand for the variation of at least two sets of experimental repeats. (c) Magnified XANES from Pt L 3 edge of commercial Pt/C in 0.1 M KOH solution at different potentials. (d) Comparison of the relative changes in the intensity of absorption peaks for Ru@Pt/C and commercial Pt/C shown in the XANES spectra. References M. K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells. Nature 486, 43-51 (2012).H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B: Environ. 56, 9-35 (2005).J. Durst et al., New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255-2260 (2014).J. R. Varcoe, R. C. T. Slade, Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 5, 187-200 (2005). Figure 1

Ignition kinetics of a homogeneous hydrogen/air mixture using a transient hot jet

The ignition characteristics of a homogenous hydrogen/air mixture using a hot transient jet generated by the combustion of syngas (H2/CO) with varying CO concentrations from 33% to 95% in a pre-chamber is numerically investigated with particular attention to the chemical kinetics. Detailed reaction mechanism for hydrogen and syngas mixture oxidation with 15 species and 41 reactions is employed. The hot jet ignition delay time is determined by the onset of OH∗ radicals and found to increase with increasing CO molar fractions in the pre-chamber fuel, and this increase is more profound for high CO content. The radicals that formed in the main chamber are examined separately from the radicals within the hot jet. Their temporal evolutions reveal that O and OH radicals in the jet play a crucial role in abstraction of H atoms form H2/air mixture in the main chamber, which initiates ignition. Further analysis of the H2O2 rate of change identifies two ignition regimes. For high temperature (T > 1000 K) hot jets, ignition is caused by the chain branching reaction H+O2↔O+OH directly, resulting in short ignition delay times (0.14, 0.19, 0.26 ms). For low temperature (T < 1000 K) hot jets, ignition is dominated by the accumulation and decomposition of H2O2, resulting in long ignition delay times (0.4, 0.67, 1.26 ms). By separating the thermal and chemical effects of the hot jet, it is found that the thermal effects are dominant but composition of the hot jet has little effect on the ignition characteristics.