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DOE OSTI2025

Highly Selective Electrolytic Reduction of CO 2 to Ethylene

We investigate the reduction of CO 2 to ethylene across buffered anolyte pH values 4 to 14 using a copper–phosphorus (Cu–P) electrocatalyst in a zero-gap membrane electrode assembly. Electrochemical CO 2 reduction using alkaline electrolytes typically shows limited carbon efficiencies and single-pass efficiencies, while acidic conditions typically favor the hydrogen evolution reaction. Results from this work show that weakly phosphate-buffered acidic anolytes (pH 6) maximize ethylene production with a 73% FE at 300 mA cm –2 and 51% FE at 500 mA cm –2 , including a 51% single-pass CO 2 conversion efficiency for over 400 h of continuous operation. We propose a mechanism based on pH-dependent CO coverage that controls the selectivity at the *HCCOH intermediate. Low CO coverage at pH 6 favors hydroxide elimination to *CCH, yielding ethylene (98% of C 2 products), while high coverage at pH 14 promotes hydrogenation to ethanol (44% of C 2 ). The HER mechanism transitions from H 2 O-mediated at pH 14 to phosphate-mediated (H 2 PO 4 – /HPO 4 2– ) at weakly acidic pH, minimizing HER competition at pH 6. This mechanistic understanding enables controlled C 2 product selectivity through manipulation of the CO coverage and local proton activity.

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NASA NTRS1974

Development of sputtered techniques for thrust chambers, task 1

Filler materials proposed for use in the sputter fabrication regeneratively cooled thrust chambers were evaluated. Low melting castable alloys, CERROBEND. CERROCAST, and CERROTRU, slurry applied SERMETEL 481 and flame-sprayed aluminum were investigated as filler materials. Sputter deposition from a cylindrical cathode inverted magnestron was used to apply an OFHC copper closeout layer to filled OFHC copper ribbed-wall cylindrical substrates. The sputtered closeout layer structure was evaluated with respect to filler material contamination, predeposition machining and finishing operations, and deposition parameters. The application of aluminum by flame-spraying resulted in excessiver filler porosity. Though the outgassing from this porosity was found to be detrimental to the closeout layer structure, bond strengths in excess of 10,500 psi were achieved. Removal of the aluminum from the grooves was readily accomplished by leaching in a 7.0 molar solution of sodium hydroxide at 353 K. Of the other filler materials evaluated, CERROTRU was found to be the most suitable material with respect to completely filling the ribbed-wall cylinders and vacuum system compatibility. However, bond contamination resulted in low closeout layer bond strength with the CERROTRU filler. CERROBEND, CERROCAST, and SERMETEL 481 were found to be unacceptable as filler materials.

Mullaly, J. R.↗

DOE OSTI2025

From Micro-environments to Macroscopic Effects: How the Alkaline Hydrogen Evolution Reaction Drives Cu Cathodic Corrosion

Cathodic corrosion of copper (Cu) has posed a significant challenge for over a century, impeding various technological progresses such as electrochemical conversion of CO 2 (eCO 2 RR) into fuels and other value-added carbon products. Here, in this study, employing a combined Density Functional Theory (DFT) and kinetic Monte Carlo (kMC) simulation approach, we delve into the atomistic level mechanism driving this phenomenon in Cu. Our hypothesis posits the pivotal role of alkaline hydrogen evolution reaction (HER) in facilitating cathodic corrosion in Cu. We rigorously develop a pH-dependent hydroxide (OH) adsorption mechanism and calculate the equilibrium OH coverage (𝜃 OH ) at varying pH levels, the thermodynamic stability of subsurface oxygen (O sub ), as well as the Cu-vacancy mediated diffusion of subsurface oxygens (O sub ). Through comprehensive analysis, we establish correlation among various microenvironments, including oxygen diffusion in subsurface layers, pH-dependent OH adsorption, and Cu dissolution into the electrolyte as (Cu-OH) complexes. Furthermore, our investigation explores the correlation between surface coordination environment of active sites and cathodic corrosion of Cu. Finally, by integrating DFT-derived thermodynamic data into a kMC model, we successfully predict the formation of experimentally observed corrosion pits on Cu-surfaces. This combined approach not only advances our fundamental understanding of Cu cathodic corrosion but also offers insights crucial for developing effective corrosion mitigation strategies.

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DOE OSTI2023

Insertion reactions and structural studies of [(NHC)CuH] 2 with nitrogen-based substrates

The stoichiometric and catalytic reactions of Cu-H dimers supported by N-heterocyclic carbenes (NHCs) have mainly focused on the insertions of aldehydes, ketones, CO 2 , and unsaturated hydrocarbons. Here we investigated the stoichiometric reactions of dimeric [(NHC)CuH] 2 (NHC = IPr*, 6Dipp) with unsaturated nitrogen-based substrates of PhN=NPh, N 3 Ad, pyrazine, and N 2 O. The spectroscopic and structural chacterizations of the resulting monomeric, two-coordinate Cu(I) complexes containing diphenyl hydrazido, triazenido, pyrazinyl, and hydroxide ligands are discussed. The hydrazido complex of (IPr*)Cu(NPh-NHPh) and triazenido complex of (IPr*)Cu(HNNNAd) are resistant to N—N bond cleavage and loss of N 2 , respectively, to form the corresponding amido complexes even when heated at 80 °C over several hours.

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DOE OSTI2023

Ammonia Electrosynthesis from Nitrate Using a Ruthenium–Copper Cocatalyst System: A Full Concentration Range Study

Electrochemical synthesis of ammonia via the nitrate reduction reaction (NO3RR) has been intensively researched as an alternative to the traditional Haber–Bosch process. Most research focuses on the low concentration range representative of the nitrate level in wastewater, leaving the high concentration range, which exists in nuclear and fertilizer wastes, unexplored. The use of a concentrated electrolyte (≥1 M) for higher rate production is hampered by poor hydrogen transfer kinetics. Herein, we demonstrate that a cocatalytic system of Ru/Cu 2 O catalyst enables NO3RR at 10.0 A in 1 M nitrate electrolyte in a 16 cm 2 flow electrolyzer, with 100% faradaic efficiency toward ammonia. Detailed mechanistic studies by deuterium labeling and operando Fourier transform infrared (FTIR) spectroscopy allow us to probe the hydrogen transfer rate and intermediate species on Ru/Cu 2 O. Ab initio molecular dynamics (AIMD) simulations reveal that adsorbed hydroxide on Ru nanoparticles increases the density of the hydrogen-bonded water network near the Cu 2 O surface, which promotes the hydrogen transfer rate. Finally, our work highlights the importance of engineering synergistic interactions in cocatalysts for addressing the kinetic bottleneck in electrosynthesis.

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DOE OSTI2026

Mechanism of Catechol Oxidation by the Coupled Bicupric Active Site of Tyrosinase: Completion of the Oxygenase/Oxidase Reaction Cycle

Tyrosinase contains a coupled binuclear copper (CBC) active site, which in its bicuprous form (deoxy-Ty) binds O 2 to form a side-on peroxide [Cu(II) 2 O 2 ] 2+ intermediate (oxy-Ty) that performs the regioselective monooxygenation of monophenols to catechols and their subsequent 2e − oxidation to quinones. Previously, we used spectroscopic, kinetic, and computational methods to elucidate the mechanism of the initial steps in the monooxygenation reaction. Herein, we investigated the final step in catecholate formation and provide experimental and computational results elucidating the catechol oxidation reaction, formally a proton-coupled 2e − process. We employed single-turnover stoppedflow absorption to observe the elusive catecholate-Cu(II) 2 OH intermediate via the anaerobic reaction of resting Cu(II) 2 OH Ty (met-Ty) with the catecholate substrate. This intermediate was cryo-trapped and characterized by electron paramagnetic resonance and X-ray absorption spectroscopies. These experimental results were correlated to quantum mechanics/molecular mechanics (QM/MM) and QM calculations to describe the complete catalytic cycle of tyrosinase, revealing that the 2e − oxidation of catechol to quinone involves two steps: (i) coupled 1H + /1e − transfer from the bound monoanionic catecholate to the bridged hydroxide of the met-Ty active site, followed by (ii) the second 1e − transfer. The latter is calculated to be the rate-limiting step in catechol oxidation, confirmed by experimental solvent kinetic isotope effect studies. Our computational data suggest that quinone release from the protein provides the necessary driving force for this reaction. These results establish a detailed description of this oxidase cycle of the tyrosinase mechanism and more broadly provide molecular-level insights into the diverse reactivity of CBC sites in biology.

QM/MM modeling↗

DOE OSTI2024

Evaluation of lithium as a tritium storage medium for betavoltaics

Lithium foils were demonstrated to absorb surrogate protium for tritium-powered betavoltaics. 20μm thick lithium foils were hole-punched from a ribbon of electrodeposited lithium on copper foil. The lithium foils were loaded with hydrogen in a custom Sievert apparatus where the pressure drop showed full hydriding at a hydrogen pressure of 2 bar and at all loading temperatures above the lithium melting point at 190, 200, 225, 250, and 300. Lithium hydride formation was confirmed with Raman spectroscopy after hydrogen loading. The kinetics of experimental hydride formation was compared to the diffusion-limited Mintz–Bloch model. While the Mintz–Bloch model showed good fit with the experimental loadings, the model overpredicted the loading kinetics starting at 250 °C and at higher temperatures. In conclusion, The overprediction was either caused by lithium hydride outgassing due to some reduction with some residual lithium hydroxide created from brief air exposure when sealing the lithium in the reactor or a transition from diffusion-limited hydride growth to surface or metal–hydride interface-limited hydride growth.

25 ENERGY STORAGE↗

DOE OSTI2024

Increasing Al-Pair Abundance in SSZ-13 Zeolite via Zeolite Synthesis in the Presence of Alkaline Earth Metal Hydroxide Produces Hydrothermally Stable Co-, Cu- and Pd-SSZ-13 Materials

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis gel during the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH 4 -form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of an increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO 4 – motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements and show their utility for NOx pollutant adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and, thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with an increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH) 2 does not lose its PNA capacity after extremely harsh aging at 850 and 900 °C (10 h in 10% H 2 O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 °C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 °C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications.

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