Showing papers by "Cao-Thang Dinh published in 2022"

N-Heterocyclic Carbene-Stabilized Hydrido Au24 Nanoclusters: Synthesis, Structure, and Electrocatalytic Reduction of CO2.

Abstract: Atomically precise hydrido gold nanoclusters are extremely rare but interesting due to their potential applications in catalysis. By optimization of molecular precursors, we have prepared an unprecedented N-heterocyclic carbene-stabilized hydrido gold nanocluster, [Au24(NHC)14Cl2H3]3+. This cluster comprises a dimer of two Au12 kernels, each adopting an icosahedral shape with one missing vertex. The two kernels are joined through triangular faces, which are capped with a total of three hydrides. The hydrides are detected by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy, with density functional theory calculations supporting their position bridging the six uncoordinated gold sites. The reactivity of this Au24H3 cluster in the electrocatalytic reduction of CO2 is demonstrated and benchmarked against related catalysts.

Bipolar membrane electrolyzers enable high single-pass CO2 electroreduction to multicarbon products

Abstract: In alkaline and neutral MEA CO2 electrolyzers, CO2 rapidly converts to (bi)carbonate, imposing a significant energy penalty arising from separating CO2 from the anode gas outlets. Here we report a CO2 electrolyzer uses a bipolar membrane (BPM) to convert (bi)carbonate back to CO2, preventing crossover; and that surpasses the single-pass utilization (SPU) limit (25% for multi-carbon products, C2+) suffered by previous neutral-media electrolyzers. We employ a stationary unbuffered catholyte layer between BPM and cathode to promote C2+ products while ensuring that (bi)carbonate is converted back, in situ, to CO2 near the cathode. We develop a model that enables the design of the catholyte layer, finding that limiting the diffusion path length of reverted CO2 to ~10 μm balances the CO2 diffusion flux with the regeneration rate. We report a single-pass CO2 utilization of 78%, which lowers the energy associated with downstream separation of CO2 by 10× compared with past systems.

Catalyst Regeneration via Chemical Oxidation Enables Long-Term Electrochemical Carbon Dioxide Reduction.

Abstract: Electrochemical CO2 reduction (ECR) with industrially relevant current densities, high product selectivity, and long-term stability has been a long-sought goal. Unfortunately, copper (Cu) catalysts for producing valuable multicarbon (C2+) products undergo structural and morphological changes under ECR conditions, especially at high current densities, resulting in a rapid decrease in product selectivity. Herein, we report a catalyst regeneration strategy, one that employs an electrolysis method comprising alternating "on" and "off" operating regimes, to increase the operating stability of a Cu catalyst. We find that it increases operating lifetime many times, maintaining ethylene selectivity ≥40% for at least 200 h of electrolysis in neutral pH media at a current density of 150 mA cm-2 using a flow cell. We also demonstrate ECR to ethylene at a current density of 1 A cm-2 with ethylene selectivity ≥40% using a three-dimensional Cu gas diffusion electrode, finding that this system under these conditions is rendered stable for greater than 36 h. This work illustrates that Cu-based catalysts, once they have entered into the state conventionally considered to possess degraded catalytic activity, may be recovered to deliver high C2+ selectivity. We present evidence that the combination of short periods of electrolysis, which minimizes the morphological changes during "on" segments, with the progressive chemical oxidation of Cu atoms on the catalyst surface during "off" segments, united with the added effects of washing the accumulated salt and decreasing the catholyte temperature prolong together the catalyst's operating lifetime.

Ga doping disrupts C-C coupling and promotes methane electroproduction on CuAl catalysts

Abstract: The electrochemical CO2 reduction reaction (CO2RR) provides a route to store intermittent electricity in the form of fuels like methane. We reasoned that disrupting C-C coupling while maintaining high ∗CO coverage could enhance methane selectivity and suppress the hydrogen evolution reaction (HER). We studied the effect of doping CuAl, a material at the top of the CO2RR activity and selectivity volcano plot, with elements having low ∗CO binding energies: Au, Zn, and Ga. Encouraged by initial improvements in selectivity to methane, we optimized the Ga content and showed that the presence of uniformly dispersed Ga is crucial in CO2RR-to-methane performance enhancement. We rule out porosity and roughness and conclude that the presence of Ga in the doped catalysts enables high methane selectivity. The Ga-doped CuAl catalysts achieve a methane Faradaic efficiency (FE) of 53% by suppressing HER to 23% in neutral electrolyte at −1.4 V versus reversible hydrogen electrode.

In situ regeneration of copper catalysts for long-term electrochemical CO2 reduction to multiple carbon products

Abstract: The valorization of carbon dioxide (CO2) via electrochemical CO2 reduction (ECR) has attracted great interest as a pragmatic approach to tackle greenhouse gas emissions. Multiple carbon (C2+) products, such as...

Coordination Polymer Electrocatalysts Enable Efficient CO‐to‐Acetate Conversion

Abstract: Upgrading carbon dioxide/monoxide to multi‐carbon C2+ products using renewable electricity offers one route to more sustainable fuel and chemical production. One of the most appealing products is acetate, the profitable electrosynthesis of which demands a catalyst with higher efficiency. Here, a coordination polymer (CP) catalyst is reported that consists of Cu(I) and benzimidazole units linked via Cu(I)‐imidazole coordination bonds, which enables selective reduction of CO to acetate with a 61% Faradaic efficiency at −0.59 volts versus the reversible hydrogen electrode at a current density of 400 mA cm−2 in flow cells. The catalyst is integrated in a cation exchange membrane‐based membrane electrode assembly that enables stable acetate electrosynthesis for 190 h, while achieving direct collection of concentrated acetate (3.3 molar) from the cathodic liquid stream, an average single‐pass utilization of 50% toward CO‐to‐acetate conversion, and an average acetate full‐cell energy efficiency of 15% at a current density of 250 mA cm−2.

Strategies for decarbonizing natural gas with electrosynthesized methane

Abstract: Natural gas supplies nearly a quarter of the world’s energy and is growing faster than any other energy source. One pathway to reduce the CO2 emission intensity of natural gas without transitioning end-use infrastructure is to synthesize a natural gas substitute from CO2 and renewable energy via electrochemical CO2 reduction. To improve the economic viability of electrogas, this work examines the possibility of using electrolyzer products without downstream separation. We quantify the electrolyzer performance needed to replicate the key heating value, safety, and emissions characteristics of natural gas. We find that, except in the case of unrealistically high device performance, directly synthesized electrogas is unable to reproduce all necessary properties of natural gas. We discover, however, a range of safe and low-emitting electrogas compositions likely achievable with current technology that can be blended with natural gas to reduce its CO2 intensity while retaining sufficient heating value.

Zn-Based Catalysts for Selective and Stable Electrochemical CO2 Reduction at High Current Densities

Abstract: Practical electrochemical carbon dioxide (CO2) reduction requires the development of selective and stable catalysts based on low-cost and Earth-abundant materials. In this work, we develop catalysts for CO2 conversion to CO based on ZnO with various morphologies, including nanoparticles, nanorods, nanosheets, and random shapes. We found that ZnO nanorods exhibit the highest CO2 to CO efficiency, with a high CO Faradaic efficiency (FE) of over 80% in a current density range of 50–160 mA cm2 in both flow-cell and membrane electrode assembly (MEA) reactors. We found that the CO selectivity of ZnO-based catalysts slowly decreased over time at high current densities because of the depletion of the ZnO phase. We have developed an in-situ regeneration strategy for catalysts that involves periodic oxidations of the catalysts during electrochemical CO2 reduction. Using this approach, we have demonstrated the conversion of CO2 to CO with a stable CO FE of above 80% for 100 h at a current density of 160 mA cm–2.