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Li–O2 batteries have received much attention for next-generation energy storage devices due to their high specific energy. However, Li–O2 batteries still face several challenges including low energy efficiency and poor cycle life, which are mainly caused by the low stability of electrolytes and cathodes towards aggressive reduced oxygen species, e.g., O2− intermediate and Li2O2. It has been reported that water can be used as an effective additive in aprotic Li–O2 batteries to increase the discharge capacity and to alleviate parasitic reactions by solvating and trapping the highly aggressive O2− intermediate. In this study, Mn-MOF-74 nanoparticles directly grown on carbon nanotubes (Mn-MOF-74@CNTs) via a facile additive-mediated synthesis are proposed as catalytic cathode materials for Li–O2 batteries to be operated in humid oxygen environment to generate less-reactive discharge product LiOH compared to Li2O2. Due to the formation of LiOH by the nano-architectured Mn-MOF-74@CNTs hybrid catalyst, Mn-MOF-74@CNTs-based oxygen cathode exhibits less side reactions during battery operation and much-enhanced cycling performance in humid oxygen containing 200 ppm moisture than those of conventional carbon cathodes (Ketjenblack and CNTs) in both dry and humid oxygen where Li2O2 was formed as discharge products. Furthermore, a series of controlled experiments and thermodynamic analysis are conducted to investigate the formation mechanism of LiOH. Based on the results, we report that the formation pathway of LiOH is a chemically-catalytic process via a chemical conversion of Li2O2 occurring at Mn2+/Mn3+ metal centers in Mn-MOF-74@CNTs hybrid, instead of an electrocatalytic process via a direct four-electron reduction of oxygen.

In this study, we approach several common problems with the Ni-catalyzed methane steam reformation reaction (MSR) using a two-pronged approach combining density functional theory (DFT) calculations with experimental work. Specifically, we look at the deactivation of a Ni catalyst due to coke formation, its high operating temperature requirements, and the high steam to methane (H2O/CH4) ratio needed for proper MSR operation. A DFT-based microkinetic model was developed in the presence and absence of electric fields, and the results were compared with experimental results. The microkinetic model shows that, under various electric fields, the most favorable MSR mechanism changed slightly. It also shows that the presence of a positive electric field decreases the surface coverage of carbon, increases the water coverage, accelerates the rate-limiting step of the C–H bond cleavage in methane, and increases the desorption rates of the syngas product (CO + H2) during MSR. Consequently, for a given methane convers...

This work demonstrates the benefits of applying an external electric field to the methane steam reforming reaction (MSR) in order to tune the catalytic activity of Ni. Through combined DFT calculations and experimental work, we present evidence for the usefulness of an electric field in improving the efficiency of current MSR processes—namely by reducing coke formation and lowering the overall temperature requirements. We focus on the influence of an electric field on (i) the MSR mechanisms, (ii) the rate-limiting step of the most favorable MSR mechanism, (iii) the methanol synthesis reaction during the MSR reaction, and (iv) the formation of coke. Our computational results show that an electric field can change the most favorable MSR mechanism as well as alter the values of the rate constants and equilibrium constants at certain temperatures and, hence, significantly affect the kinetic properties of the overall MSR reaction. Both computational and experimental results also suggest that a positive electri...

By changing the electrical bias imposed on a Ni catalyst attached to an external circuit, the selectivity of catalytic formic acid decomposition is shown to change—favoring CO2/H2 production under negative bias and CO/H2O production under positive bias. A method for estimating the strength of externally generated surface electric fields by measuring this selectivity change is presented and used to approximate field strengths on the order of 0.20 V/nm. A COMSOL model of the catalyst was created which indicated that the presence of Ni particles increased field strength and uniformity across the catalyst. Comparing this model to SEM imaging of the catalyst verified that the field strengths are highest on the surface of the catalyst particles, pore edges, and microscopic defects. The methods developed herein may be useful to future reaction engineers seeking to incorporate applied electric fields into process designs.

Mo2C/graphene nanostructures were used to investigate the nature of gas-phase formic acid decomposition into either CO/H2O or CO2/H2 products. The experimental data show that the Mo2C/graphene can facilitate both decarboxylation and dehydration pathways for the formic acid decomposition reaction. Its selectivity is strongly influenced by the reaction temperature where the decarboxylation predominates at a low temperature (e.g., ≤ 280 °C) and the dehydration predominates at a high temperature (e.g., ≥ 370 °C). These experimental data are compared to Monte Carlo simulations. It was found that the decarboxylation pathway for the production of CO/H2O can be simulated and explained by an Eley-Rideal type mechanism that involves interaction of gas-phase HCOOH with surface H*. Furthermore, the dehydration pathway for the production of CO2/H2 can be simulated and explained by a Langmuir-Hinshelwood type mechanism that involves unimolecular decomposition of surface HCO*O* to form CO2 and H*.

To better understand the water/Ni electronic interaction relevant to many industrial catalytic processes, we performed a comprehensive investigation of water dehydrogenation–formation on Ni surfaces. Three perspectives were considered: interactions of water (i) over two distinct Ni surfaces, (ii) in the presence of a pre-adsorbed O atom, and (iii) under the influence of different electric fields. The theoretical results showed that Ni(2 1 1) lowered the overall reaction energies of water dehydrogenation–formation reactions as compared with Ni(1 1 1). A pre-adsorbed O atom largely accelerated the water dehydrogenation reaction on Ni(2 1 1). A negative electric field significantly promoted water dehydrogenation processes, while a positive field greatly enhanced water formation processes. Experimental evidence of water consumption and Ni surface compositions verifies these qualitative findings during methane steam reforming under different applied field conditions. Overall, an applied electric field can play a critical role in catalytic reaction dynamics, which is important when designing water-based catalytic processes.

Electric fields were applied to an unsupported nickel catalyst during methane steam reforming with a low steam-to-methane ratio of 2:1. As expected, catalysts operated in standard mode (without an applied field) experienced high levels of coking. Coke formation was almost completely suppressed by the application of a positive electric field. This is explained using density functional theory computations which indicate that positive electric fields (1) increase the affinity for water adsorption on the surface, thereby increasing the amount of oxygen available to remove carbon, (2) stabilize individual carbon fragments, thus reducing their tendency to polymerize, and (3) increase the oxygen storage capabilities of the catalyst by inducing sub-surface oxygen formation and bulk oxidation.

Owing to their five times higher energy density (~3500 Wh/kg) than that of lithium-ion batteries, lithium–oxygen (Li–O2 or Li­–air) batteries have been considered as one of the most promising next-generation energy storage devices.1 However, Li–O2 batteries still suffer from several challenges that hinder their practical applications, including low energy efficiency and short cycle life. It has been well-documented that such challenges are mainly caused by the parasitic reactions between cell components (e.g., cathodes and electrolytes) and aggressively reactive oxygen species (e.g., singlet O2 and superoxide radicals), which result in the repaid passivation and failure of Li–O2 batteries.2,3 Therefore, the cycling performance of Li–O2 batteries can be significantly improved by suppressing those parasitic reactions. Metal–organic frameworks (MOFs), an emerging type of crystalline microporous materials, have been widely studied as cathodes, separators, and electrolytes for high-energy Li batteries due to their large surface area, high porosity, and chemically unsaturated metal sites.4 For example, our group recently reported a MOF/CNT cathode for Li–O2 batteries with enhanced cycling performance via the formation and decomposition of less-reactive LiOH compared to Li2O2.5 Besides, MOF-modified separators have also been reported as molecular/ionic sieves to mitigate the shuttling effect of polysulfides in lithium–sulfur batteries and redox mediators in Li–O2 batteries.6,7 Herein, we present our mechanistic study on the enhanced performance of MOF-based Li–O2 batteries. Operando characterization techniques including Raman and mass spectrometry and ex-situ quantitative techniques including NMR and titration were carried out to comprehensively understand the reaction mechanism and the reversibility of Li­–O2 chemistry. The enhanced battery performance and reaction mechanism will be discussed in this presentation. Reference (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li–O2 and Li–S batteries with high energy storage. Nat. Mater. 2012, 11, 19-29. (2) Mahne, N.; Fontaine, O.; Thotiyl, M. O.; Wilkening, M.; Freunberger, S. A. Mechanism and Performance of Lithium–Oxygen Batteries–a Perspective. Chem. Sci. 2017, 8, 6716-6729. (3) Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G. Advances in Understanding Mechanisms Underpinning Lithium–Air Batteries. Nat. Energy 2016, 1, 16128. (4) Zhang, X.; Dong, P.; Song, M.-K. Metal–Organic Frameworks for High-Energy Lithium Batteries with Enhanced Safety: Recent Progress and Future Perspectives. Batteries & Supercaps 2019, https://doi.org/10.1002/batt.201900012. (5) Zhang, X.; Dong, P.; Lee, J.-I.; Gray, J. T.; Cha, Y.-H.; Ha, S.; Song, M.-K. Enhanced Cycling Performance of Rechargeable Li–O2 Batteries via LiOH Formation and Decomposition Using High-Performance MOF-74@CNTs Hybrid Catalysts. Energy Storage Mater. 2019, 17, 167-177. (6) Bai, S.; Liu, X.; Zhu, K.; Wu, S.; Zhou, H. Metal–organic framework-based separator for lithium–sulfur batteries. Nat. Energy 2016, 1, 16094. (7) Qiao, Y.; He, Y.; Wu, S.; Jiang, K.; Li, X.; Guo, S.; He, P.; Zhou, H. MOF-Based Separator in an Li–O2 Battery: An Effective Strategy to Restrain the Shuttling of Dual Redox Mediators. ACS Energy Lett. 2018, 3, 463-468.