In electrocatalysis, mechanistic analysis of reaction rate data often relies on the linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to 10 electrochemical cells, where each cell can have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We used this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms that are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.
\nElectrochemical reactions can access a significant range of driving forces under operationally mild conditions and are thus envisioned to play a key role in decarbonizing chemical manufacturing. However, many reactions with well-established thermochemical precedents remain difficult to achieve electrochemically. For example, hydroformylation (thermo-HFN) is an industrially important reaction that couples olefins and carbon monoxide (CO) to make aldehydes. However, the electrochemical analogue of hydroformylation (electro-HFN), which uses protons and electrons instead of hydrogen gas, represents a complex C–C bond-forming reaction that is difficult to achieve at heterogeneous electrocatalysts. In this work, we import Rh-based thermo-HFN catalysts onto electrode surfaces to unlock electro-HFN reactivity. At mild conditions of room temperature and 5 bar CO, we achieve Faradaic efficiencies of up to 15% and turnover frequencies of up to 0.7 h–1. This electro-HFN rate is an order of magnitude greater than the corresponding thermo-HFN rate at the same catalyst, temperature, and pressure. Reaction kinetics and operando X-ray absorption spectroscopy provide evidence for an electro-HFN mechanism that involves distinct elementary steps relative to thermo-HFN. This work demonstrates a step-by-step experimental strategy for electrifying a well-studied thermochemical reaction to unveil a new electrocatalyst for a complex and underexplored electrochemical reaction.
\nThe lithium-mediated nitrogen reduction reaction (LiNRR) produces ammonia in ambient conditions. This electrochemical pathway is dependent on a catalytic solid–electrolyte interphase—a nanoscale passivation layer formed from reductive electrolyte decomposition on the surface of lithium metal. The catalytic solid–electrolyte interphase is a unique nanostructured environment that exists on reactive metal surfaces and intimately influences product selectivity. Here we explore recent progress made in the field of lithium-mediated nitrogen reduction to ammonia, especially in light of growing knowledge about the nature of the catalytic solid–electrolyte interphase. We systematically analyse the observed chemical species and reactions that occur within the solid–electrolyte interphase. We also summarize key developments in kinetic and transport models, as well as highlight the cathodic and complementary anodic reactions. Trends in ammonia selectivities and rates with varying electrolyte compositions, cell designs and operating conditions are extracted and used to articulate a path forward for continued development of lithium-mediated nitrogen reduction to ammonia.
\nDirect electrochemical propylene epoxidation by means of water-oxidation intermediates presents a sustainable alternative to existing routes that involve hazardous chlorine or peroxide reagents. We report an oxidized palladium-platinum alloy catalyst (PdPtO\u2093/C), which reaches a Faradaic efficiency of 66 ± 5% toward propylene epoxidation at 50 milliamperes per square centimeter at ambient temperature and pressure. Embedding platinum into the palladium oxide crystal structure stabilized oxidized platinum species, resulting in improved catalyst performance. The reaction kinetics suggest that epoxidation on PdPtO\u2093/C proceeds through electrophilic attack by metal-bound peroxo intermediates. This work demonstrates an effective strategy for selective electrochemical oxygen-atom transfer from water, without mediators, for diverse oxygenation reactions.
", "doi": "10.1126/science.adh4355", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "2024-01-05", "series_number": "6678", "volume": "383", "issue": "6678", "pages": "49-55" }, { "id": "authors:qs0g8-1jw44", "collection": "authors", "collection_id": "qs0g8-1jw44", "cite_using_url": "https://authors.library.caltech.edu/records/qs0g8-1jw44", "type": "article", "title": "Beyond lithium for sustainable ammonia synthesis", "author": [ { "family_name": "Yusov", "given_name": "Michael A.", "orcid": "0000-0002-0744-0469", "clpid": "Yusov-Michael-A" }, { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" } ], "abstract": "Using an electrochemical continuous flow cell, nitrogen reduction to ammonia is rigorously demonstrated through a calcium-mediated approach.
", "doi": "10.1038/s41563-023-01747-2", "issn": "1476-1122", "publisher": "Nature Publishing Group", "publication": "Nature Materials", "publication_date": "2023-12-18" }, { "id": "authors:fs031-ps690", "collection": "authors", "collection_id": "fs031-ps690", "cite_using_url": "https://authors.library.caltech.edu/records/fs031-ps690", "type": "article", "title": "Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO\u2082 reduction", "author": [ { "family_name": "Corpus", "given_name": "Kaitlin Rae M.", "orcid": "0000-0002-3846-7576", "clpid": "Corpus-Kaitlin-Rae-M" }, { "family_name": "Bui", "given_name": "Justin C.", "orcid": "0000-0003-4525-957X", "clpid": "Bui-Justin-C" }, { "family_name": "Limaye", "given_name": "Aditya M.", "orcid": "0000-0003-0639-4154", "clpid": "Limaye-Aditya-M" }, { "family_name": "Pant", "given_name": "Lalit M.", "orcid": "0000-0002-0432-3902", "clpid": "Pant-Lalit-M" }, { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" }, { "family_name": "Weber", "given_name": "Adam Z.", "orcid": "0000-0002-7749-1624", "clpid": "Weber-Adam-Z" }, { "family_name": "Bell", "given_name": "Alexis T.", "orcid": "0000-0002-5738-4645", "clpid": "Bell-Alexis-T" } ], "abstract": "In electrocatalysis, the rate of a reaction as a function of applied potential is governed by the Tafel equation, which depends on two parameters: the Tafel slope and the exchange current density (i0). However, current methods to determine these parameters involve subjective removal of data due to the convoluted effects of mass transfer and competitive surface or bulk reactions, resulting in unquantifiable uncertainty. To overcome this challenge, we couple covariance matrix adaptation with a continuum model of CO2 reduction (CO2R) that explicitly deconvolutes non-kinetic effects to extract kinetic parameters associated with 26 literature datasets of CO2R over Ag and Sn catalysts. The fitted kinetic parameters do not converge to a unique set of values, and the Tafel slope and i0 possess an apparent correlation, which we suggest is a consequence of variations in catalyst preparation methods. This work facilitates rigorous benchmarking of electrocatalysts in systems where mass transfer is relevant.
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