We report the use of polymer N -heterocyclic carbenes (NHCs) to control the microenvironment surrounding metal nanocatalysts, thereby enhancing their catalytic performance in CO 2 electroreduction. Three polymer NHC ligands were designed with different hydrophobicity: hydrophilic poly(ethylene oxide) (PEO–NHC), hydrophobic polystyrene (PS–NHC), and amphiphilic block copolymer (BCP) (PEO- b -PS–NHC). All three polymer NHCs exhibited enhanced reactivity of gold nanoparticles (AuNPs) during CO 2 electroreduction by suppressing proton reduction. Notably, the incorporation of hydrophobic PS segments in both PS–NHC and PEO- b -PS–NHC led to a twofold increase in the partial current density for CO formation, as compared to the hydrophilic PEO–NHC. While polymer ligands did not hinder ion diffusion, their hydrophobicity altered the localized hydrogen bonding structures of water. This was confirmed experimentally and theoretically through attenuated total reflectance surface-enhanced infrared absorption spectroscopy and molecular dynamics simulation, demonstrating improved CO 2 diffusion and subsequent reduction in the presence of hydrophobic polymers. Furthermore, NHCs exhibited reasonable stability under reductive conditions, preserving the structural integrity of AuNPs, unlike thiol-ended polymers. The combination of NHC binding motifs with hydrophobic polymers provides valuable insights into controlling the microenvironment of metal nanocatalysts, offering a bioinspired strategy for the design of artificial metalloenzymes.
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The Conventional Gas Diffusion Electrode May Not Be Resistant to Flooding during CO 2 /CO Reduction
The electrochemical CO 2 or CO reduction to chemicals and fuels using renewable energy is a promising way to reduce anthropogenic carbon emissions. The gas diffusion electrode (GDE) design enables low-carbon manufacturing of target products at a current density (e.g., 500 mA cm −2 ) relevant to industrial requirements. However, the long-term stability of the GDE is restricted by poor water management and flooding, resulting in a significant hydrogen evolution reaction (HER) within almost an hour. The optimization of water management in the GDE demands a thorough understanding of the role of the gas diffusion layer (GDL) and the catalyst layer (CL) distinctively. Herein, the hydrophobicity of the GDL and CL is independently adjusted to investigate their influence on gas transport efficiency and water management. The gas transport efficiency is more enhanced with the increase in hydrophobicity of the GDL than the CL. Direct visualization of water distribution by optical microscope and micro-computed tomography demonstrates that the water flow pattern transfers from the stable displacement to capillary fingering as GDL hydrophobicity increases. Unfortunately, only increasing the hydrophobicity is not sufficient to prevent flooding. A revolutionary change in the design of the GDE structure is essential to maintain the long-term stability of CO 2 /CO reduction.
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- Award ID(s):
- 2033343
- NSF-PAR ID:
- 10378497
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- Volume:
- 169
- Issue:
- 10
- ISSN:
- 0013-4651
- Page Range / eLocation ID:
- 104506
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Direct conversion of CO2into carbon‐neutral fuels or industrial chemicals holds a great promise for renewable energy storage and mitigation of greenhouse gas emission. However, experimentally finding an electrocatalyst for specific final products with high efficiency and high selectivity poses serious challenges due to multiple electron transfer, complicated intermediates, and numerous reaction pathways in electrocatalytic CO2reduction. Here, an intrinsic descriptor that correlates the catalytic activity with the topological, bonding, and electronic structures of catalytic centers on M–N–C based single‐atom catalysts is discovered. The “volcano”‐shaped relationships between the descriptor and catalytic activity are established from which the best single‐atom catalysts for CO2reduction are found. Moreover, the reaction mechanisms, intermediates, reaction pathways, and final products can also be distinguished by this new descriptor. The descriptor can also be used to predict the activity of the single‐atom catalysts for electrochemical reactions such as hydrogen evolution, oxygen reduction and evolution reactions in fuel cells and water‐splitting. These predictions are confirmed by the experimental results for onset potential and Faraday efficiency. The design principles derived from the descriptors open a door for rational design and rapid screening of highly efficient electrocatalysts for CO2conversion as well as other electrochemical energy systems.
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CO 2 reduction reaction (CO 2 RR) is a promising technique for mitigating global warming and storing renewable energy if it can be obtained with a highly selective, efficient, and durable electrocatalyst. Here, we report CO 2 RR catalyzed by Au nanoparticles (NPs) stabilized by pyridines and pyrimidines (e.g., 2-mercaptopyridine (2Mpy), 4-mercaptopyridine (4Mpy), and 2-mercaptopyrimidine (2Mpym)) on a nanostructured carbon-doped TiO 2 nanowire (NanoCOT) electrode, which has been previously reported by our team for electrocatalytic water oxidation. An online gas chromatography (GC) set-up with improved gaseous product sensitivity with real-time pressure monitoring is used to quantify CO and hydrogen products from the Au NP-modified NanoCOT electrode. High CO selectivity is observed at Au-2Mpy coated NanoCOT electrode. CO 2 reduction products are not observed at bare NanoCOT suggesting CO 2 is reduced at the Au nanoparticle sites of the electrode. Moreover, CH 3 OH is not detected at the Au-Mpy/Mpym NPs during rotating ring disk electrode (RRDE) analysis which implies pyridine attached to the Au NPs has no catalytic effects on CO 2 RR as claimed by others in the literature. A durable complete H-cell using a NanoCOT anode and Au NP-NanoCOT cathode electrodes is assembled for complete water splitting, CO 2 RR, and stability test.more » « less
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Abstract The electrochemical CO2reduction reaction (CO2RR) is a promising approach to achieving sustainable electrical‐to‐chemical energy conversion and storage while decarbonizing the emission‐heavy industry. The carbon‐supported, nitrogen‐coordinated, and atomically dispersed metal sites are effective catalysts for CO generation due to their high activity, selectivity, and earth abundance. Here, we discuss progress, challenges, and opportunities for designing and engineering atomic metal catalysts from single to dual metal sites. Engineering single metal sites using a nitrogen‐doped carbon model was highlighted to exclusively study the effect of carbon particle sizes, metal contents, and M−N bond structures in the form of MN4moieties on catalytic activity and selectivity. The structure‐property correlation was analyzed by combining experimental results with theoretical calculations to uncover the CO2to CO conversion mechanisms. Furthermore, dual‐metal site catalysts, inheriting the merits of single‐metal sites, have emerged as a new frontier due to their potentially enhanced catalytic properties. Designing optimal dual metal site catalysts could offer additional sites to alter the surface adsorption to CO2and various intermediates, thus breaking the scaling relationship limitation and activity‐stability trade‐off. The CO2RR electrolysis in flow reactors was discussed to provide insights into the electrolyzer design with improved CO2utilization, reaction kinetics, and mass transport.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1149/1945-7111/ac9b96
