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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Design and Synthesis of Highly-Dispersed WO 3 Catalyst with Highly Effective NH 3 –SCR Activity for NO x Abatement

The development of high-efficiency catalysts, especially for selective catalytic reduction of NO x with NH 3 (NH 3 –SCR) catalysts, with outstanding low-temperature catalytic reactivity and impressive lifetime remains a great challenge in the field of eliminating vehicle engine exhausts. In this work, we present a novel gas-bubbling-assisted dispersed hybrid nanocrystal-assisted hydrothermal deposition (GBD-HNHD) strategy to fabricate a W@CeZrAlO x catalyst with highly dispersed WO 3 nanocrystals (10 wt %) (W@CZA-HNHD). Compared with the W/CeZrAlO x catalyst prepared with the impregnation method (W@CZA-IM), W@CZA-HNHD not only exhibits highly effective low-temperature deNO x activity (the light-off temperature being lowered 45 °C) but also shows an exceptional stability (100 h time-on-stream testing with seven measurement cycles) and long lifetime (compares well with 135,000 miles). This work not only provides a new route to prepare an advanced catalyst with high loading isolated active sites but also lays a foundation for optimizing the utilization efficiency of the catalytically active species in the practical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Atomically Dispersed Metal Catalysts for Oxygen Reduction

The unprecedented oxygen reduction reaction (ORR) activity of atomically dispersed and nitrogen-coordinated metal site (e.g., Fe, Co, or Mn) catalysts makes them promising low-cost candidates to replace platinum group metal (PGM) catalysts in proton exchange membrane fuel cells (PEMFCs). This Perspective focuses on emerging developments in mechanistic understanding, innovative synthetic concept, and performance improvement of these high-risk, high-reward PGM-free catalysts. Starting from theoretical and computational analysis of active sites and ORR pathways, here we provide a concise overview of recent progress in catalyst synthesis, characterization, and catalytic performance with an aim to elucidate structure–property correlations. We also examine remaining challenges and future directions to realize atomically dispersed metal catalysts with sufficient activity and stability for viable PEMFC applications.

08 HYDROGEN↗
DOE OSTI2018

SAPO-34/5A Zeolite Bead Catalysts for Furan Production from Xylose and Glucose

SAPO-34 zeolite crystals were grown on zeolite 5A beads, characterized, and then utilized to produce furfural from xylose and 5-hydroxymethylfurfural (HMF) from glucose. The SAPO-34/5A bead catalysts resulted in moderate furfural and HMF yields of 45% from xylose and 20% from glucose (463 K; 3 h) and were easier to recover than the SAPO-34 powder catalyst. At 463 K, the SAPO-34/5A beads were more selective than 0.02 M sulfuric acid for producing HMF and, unlike the sulfuric acid system, no levulinic acid was formed. The SAPO-34/5A bead catalysts had no significant loss in activity after three rounds of recycle when water washed or heated overnight between reactions; however, the heat-treated beads did show signs of thermal stress after the second reuse. The SAPO-34/5A bead catalysts show promise for dehydration reactions to produce furfural and HMF from xylose and glucose, respectively, and adapting the catalyst and the support bead could lead to even higher selectivities and yields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Chemical Recycling of Polybutadiene Rubber with Tailored Depolymerization Enabled by Microencapsulated Metathesis Catalysts

The effective management of plastic waste streams to prevent plastic land and water pollution is a growing problem that is also one of the most important challenges in polymer science today. Polymer materials that are stable over their lifetime and can also be cheaply recycled or repurposed as desired could more easily be diverted from waste streams. However, this is difficult for most commodity plastics. It is especially difficult to conceive this with intractable, cross-linked polymers such as rubbers. In this work, we explore the utility of microencapsulated Grubbs’ catalysts for the in-situ depolymerization and reprocessing of polybutadiene (PB) rubber. Second-generation Hoveyda-Grubbs catalyst (HG2) contained within glassy thermoplastic microspheres can be dispersed in PB rubber below the microsphere’s glass transition temperature (T g ) without adverse depolymerization, evidenced by rubber with and without these microspheres obtaining similar shear storage moduli of ≈16 and ≈28 kPa, respectively. The thermoplastic’s T g can be used to tune the depolymerization temperature, via release of HG2 into the rubber matrix. For example, using poly(lactic acid) (PLA) vs polysulfone results in an 85 and 162 °C depolymerization temperature, respectively. Liquefaction of rubber to a mixture of small molecules and oligomers is demonstrated using a 0.01 mol % catalyst loading using PLA as the encapsulant. Furthermore, at that same catalyst loading, depolymerization occurs to a greater extent in comparison to two ex-situ approaches, including a conventional solvent-assisted method, where it occurs at roughly twice the extent at each given catalyst loading. In addition, depolymerization of the microsphere-loaded rubbers was demonstrated for samples stored under nitrogen for 23 days. Lastly, we show that the depolymerized products can be reprocessed back into solid rubber with a shear storage modulus of ≈32 kPa. Thus, we envision that this approach could be used to recycle and reuse cross-linked rubbers at the end of their product lifetime.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Selective Hydrogenation of Biomass-Derived 2(5H)-Furanone to γ-Butyrolactone over Ni-Based Bimetallic Catalysts

2(5H)-Furanone was selectively hydrogenated to γ-butyrolactone (GBL) over Ni-based bimetallic catalysts, Ni-M/SiO2 (M = Fe, Co, Cu, and Zn). Catalyst characterization showed that the metal dispersion and reducibility were changed after introducing a second metal, while the physical morphology was nearly unchanged. The most active and selective bimetallic catalyst was found to be Ni–Fe/SiO2. The reaction rate constant over the 4 wt %Ni–1 wt %Fe/SiO2 bimetallic catalyst was 0.06 min–1, doubling that over 5 wt %Ni/SiO2, and the selectivity to GBL increased slightly from 86.1 to 89.5%. The Fe/(Ni+Fe) weight ratio was changed to optimize the promoting effect of Fe. The Ni–Fe bimetallic effect was further investigated using temperature-programmed desorption experiments on single crystal model surfaces with different Fe/Ni(111) structures, revealing that the enhanced catalyst activity was mainly contributed by the formation of the Ni–Fe surface alloy.

bio-inspired↗
DOE OSTI2018

Reduction of Propionic Acid over a Pd-Promoted ReO x /SiO 2 Catalyst Probed by X-ray Absorption Spectroscopy and Transient Kinetic Analysis

A Pd-promoted Re/SiO 2 catalyst was prepared by sequential impregnation and compared to monometallic Pd/SiO 2 and Re/SiO 2 . All samples were characterized by electron microscopy, H 2 and CO chemisorption, H 2 temperature-programmed reduction, and in situ X-ray absorption spectroscopy at the Re L III and Pd K-edges. The samples were also tested in the reduction of propionic acid to 1-propanol and propionaldehyde at 433 K in 0.1–0.2 MPa H 2 . Whereas monometallic Pd was inactive for carboxylic acid reduction, monometallic Re catalyzed aldehyde formation but only after high-temperature prereduction that produced metallic Re. When Pd was present with Re in a bimetallic catalyst, Pd facilitated the reduction of Re in H 2 to ~+4 oxidation state at modest temperatures, producing an active catalyst for the conversion of propionic acid to 1-propanol. Under the conditions of this study, the orders of reaction in propionic acid and H 2 were approximately zero and one, respectively. Transient kinetic analysis of the carboxylic acid reduction to alcohols revealed that at least 50% of the Re in the bimetallic catalyst participated in the catalytic reaction. In conclusion, the Pd is proposed to enhance the catalytic activity of the bimetallic catalyst by spilling over hydrogen that can partially reduce Re and react with surface intermediates.

1-Propanol↗
DOE OSTI2020

In Situ Catalytic Fast Pyrolysis using Red Mud Catalyst: Impact of catalytic fast pyrolysis temperature and biomass feedstocks

Catalytic fast pyrolysis (CFP) has been considered as a very promising approach for converting lignocellulosic biomass into higher-quality bio-oils followed by hydrotreating to produce fuelrange products. A reactive, robust, and low-cost catalyst is required to drive the CFP process. Red mud, a side-product produced during the refining of bauxite to alumina, appears to be an effective catalyst for in situ CFP of biomass. In this paper, we report the impact of CFP reaction temperature on the conversion of a pinyon juniper feedstock to bio-oils using red mud as the catalyst and then to fuel-range hydrocarbons by hydrotreating of the produced bio-oil. The yield and quality of the CFP bio-oil produced and the yield and quality of hydrotreated final products were determined. When the CFP process temperature was lowered from 450 to 400ºC, the bio-oil yield increased with minimal differences in the oxygen content, hydrogen-to-carbon ratio, water content, etc. In addition, CFP bio-oils at both temperatures were processed in a single-stage continuous hydrotreater without reactor plugging during the testing period (i.e., ~100 hours on stream). The yield of CFP bio-oil produced at 400ºC was lower than the yield at 450ºC. However, the overall yield, from biomass to hydrocarbon fuel, was still higher for CFP processing at 400ºC than for processing at 450ºC. This indicates the potential for enhancing overall carbon efficiency, including CFP processing and hydrotreating, by employing red mud catalysts for biomass pyrolysis and tuning the CFP parameters. Detailed analysis of bio-oil and hydrotreated products showed a lower content of lignin-derived species in both samples. Furthermore, CFP processing of three different biomass feedstocks corroborated red mud catalyst development for producing improved quality bio-oil and, when combined with hydrotreating, for the production of fuel range hydrocarbons.

Santosa, Daniel M.↗
DOE OSTI2015

Structural and Electrocatalytic Properties of PtIrCo/C Catalysts for Oxygen Reduction Reaction

This paper describes the results of an investigation of the synthesis of PtIrCo nanoparticles (2-3 nm) for electrocatalytic oxygen reduction reaction. The carbon-supported PtIrCo catalysts (PtIrCo/C) were thermally treated at temperatures ranging from 400 to 900 C. The size, composition, and atomic-scale structures of the PtIrCo/C catalysts were characterized for establishing their correlation with the electrocatalytic activity toward oxygen reduction reaction. The specific activity was found to increase by a factor of 3-5 for the PtIrCo/C catalysts in comparison with Pt/C catalysts. A correlation was identified between the specific activity and the nanoparticle's fcc-type lattice parameter. The specific activity increases whereas the fcc-type lattice parameter decreases with the thermal treatment temperature. This correlation was further substantiated by analyzing the interatomic spatial parameters in the trimetallic nanoparticles based on X-ray absorption fine structure spectroscopic and high-energy XRD experiments. Implications of these findings, along with the durability of the catalysts, to the design of active electrocatalysts were also discussed.

Loukrakpam, Rameshwori↗
DOE OSTI2013

A Cobalt-based Catalyst for CO2 Hydrogenation Under Ambient Conditions

Due to the continually rising levels of CO2 in the atmosphere, research into conversion of CO2 into fuels using carbon-neutral energy is currently an important topic in catalysis. Recent research on molecular catalysts has led to improved rates of CO2 conversion to formate, but unfortunately the resulting catalysts are based on precious metals such as iridium, ruthenium and rhodium and require high temperatures and high pressures for catalytic reactivity. Using established thermodynamic properties, a cobalt-based catalyst system has been designed for the catalytic production of formate from CO2 and H2, even at room temperature and one atmosphere of pressure. Using Co(dmpe)2H (dmpe is bis(dimethylphosphino)ethane) as a catalyst in tetrahydrofuran, room temperature turnover frequencies of 3,400 h-1 at 1 atm of 1:1 CO2:H2 and 74,000 h-1 at 20 atm were obtained. These results highlight the value of basic thermodynamic properties in the rational design of catalysts. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

abundant metals↗
DOE OSTI2024

Interfacial Electrochemistry of Catalyst-Coordinated Graphene Nanoribbons

The immobilization of molecular electrocatalysts on conductive electrodes is an appealing strategy for enhancing their overall activity relative to those of analogous molecular compounds. Here, in this study, we report on the interfacial electrochemistry of self-assembled two-dimensional nanosheets of graphene nanoribbons (GNR-2DNS) and analogs containing a Rh-based hydrogen evolution reaction (HER) catalyst (RhGNR-2DNS) immobilized on conductive electrodes. Proton-coupled electron transfer (PCET) taking place at N-centers of the nanoribbons was utilized as an indirect reporter of the interfacial electric fields experienced by the monolayer nanosheet located within the electric double layer. The experimental Pourbaix diagrams were compared with a theoretical model, which derives the experimental Pourbaix slopes as a function of parameter f, a fraction of the interfacial potential drop experienced by the redox-active group. Interestingly, our study revealed that GNR-2DNS was strongly coupled to glassy carbon electrodes (f = 1), while RhGNR-2DNS was not (f = 0.15). We further investigated the HER mechanism by RhGNR-2DNS using electrochemical and X-ray absorption spectroelectrochemical methods and compared it to homogeneous molecular model compounds. RhGNR-2DNS was found to be an active HER electrocatalyst over a broader set of aqueous pH conditions than its molecular analogs. We find that the improved HER performance in the immobilized catalyst arises due to two factors. First, redox-active bipyrimidine-based ligands were shown to dramatically alter the activity of Rh sites by increasing the electron density at the active Rh center and providing RhGNR-2DNS with improved catalysis. Second, catalyst immobilization was found to prevent catalyst aggregation that was found to occur for the molecular analog in the basic pH. Overall, this study provides valuable insights into the mechanism by which catalyst immobilization can affect the overall electrocatalytic performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Mechanistic Insights into Molecular Copper Hydride Catalysis: the Kinetic Stability of CuH Monomers toward Aggregation is a Critical Parameter for Catalyst Performance

The activity of molecular copper hydride (CuH) complexes towards the selective insertion of unsaturated hydrocarbons under mild conditions has contributed significantly to versatile methodologies for upgrading these feedstocks. However, these catalysts are particularly susceptible to deleterious aggregation, leading to the depletion of active CuH species. Little is known about the mechanisms of CuH aggregation, how it influences overall catalyst performance, and how it can be controlled. We address these challenges with mechanistic studies on a model reaction of unactivated alkene hydroboration catalyzed by (IPr*CPh 3 )CuH (LCuH). Here, we report a comprehensive mechanistic investigation of this system, identifying an aggregation pathway that continuously depletes catalytically active LCuH to form inactive CuH clusters during turnover. Deactivation of LCuH is controlled primarily by the competition between the kinetics of the initial LCuH dimerization step and that of alkene insertion. We therefore propose that a more comprehensive understanding of CuH catalyst performance must account for the kinetics of the initial LCuH dimerization step, revising a previously explored thermodynamic understanding of CuH aggregation, where the concentration of active species is controlled by equilibria established between CuH dimers and monomers. With a series of (NHC)CuH congeners (NHC = N-heterocyclic carbene), we demonstrate that ostensibly minor structural modifications to the ligand peripheries can drastically affect the LCuH dimerization kinetics, while maintaining reactivity towards on–cycle alkene insertion. We employed a computational approach based on molecular dynamics simulations to provide an in-depth understanding of how specific structural ligand modifications can substantially increase the kinetic stability of monomeric CuH catalysts. Our combined experimental and computational studies suggest strategies for rational ligand design that can be broadly applied to molecular catalyst systems that are susceptible to deactivation via aggregation pathways.

Ryan, David E. [Pacific Northwest National Laborat↗
DOE OSTI2015

Silica-Supported Cationic Gold(I) Complexes as Heterogeneous Catalysts for Regio- and Enantioselective Lactonization Reactions

An effective approach for the synthesis of heterogeneous gold catalysts has been developed. These catalysts were easily assembled from readily available silica materials and gold complexes. The heterogeneous catalysts reflected superior reactivity in various reactions where protodeauration is the rate-limiting step. Dramatic enhancement in regio- and enantioselectivity was observed when compared to the homogeneous unsupported gold catalyst. The catalysts are easily recovered and recycled up to 11 times without loss of enantioselectivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Deciphering Catalyst–Support Interaction via Doping for Highly Active and Durable Oxygen Evolution Catalysis

The design of oxygen evolution reaction (OER) electrocatalysts demands a delicate balance between activity and stability. Here, in this study, we present a rational design approach that leverages catalyst-support interactions to enhance both the intrinsic activity and durability of Ir-based catalysts. Our study reveals that while Mo doping energetically promotes the formation of high-valent Ir species, enhancing intrinsic catalytic activity, it also leads to a reduction in electrical conductivity. These findings emphasize that supporting doping can introduce both beneficial and limiting effects, highlighting the need for a carefully balanced design strategy to optimize the overall OER performance. Simultaneously, in situ analytical techniques and comparative evaluation reveal the crucial role of oxide supports in stabilizing the catalyst. These findings highlight the pivotal role of interface engineering in maintaining catalyst integrity and the need for support materials that balance dopant-driven electronic promotion with structural and electrochemical robustness. These interconnected degradation pathways highlight the need to move beyond a catalyst-centric view and instead adopt a system-level understanding of the stability. Our approach offers a strong foundation for the rational design and evaluation of high-performance OER electrocatalysts for electrochemical energy applications.

Kim, Jinyeop [Korea Advanced Inst. Science and Tec↗
DOE OSTI2026

In Situ Formed Pt–Ga Hetero Duo-Atomic Catalyst for Efficient Hydrogen Storage in N-Heterocycles

Efficient catalysts for the dehydrogenation and hydrogenation of liquid organic hydrogen carriers (LOHCs) are essential for advancing hydrogen storage and transportation. Conventional nanoparticle catalysts suffer from low metal utilization, while single-atom catalysts (SAC) are limited by isolated active sites. Here, in this work, we present a hetero duo-atomic catalyst, Pt 1 –Ga 1 /CeO 2 DAC, which exhibits exceptional activity, selectivity, stability, and recyclability for N-heterocycle hydrogen storage. Ga plays a critical role in C–H bond activation, acting as a mediator in catalytic bond-breaking and formation. Compared with Pt 1 /CeO 2 SAC, Pt 1 –Ga 1 /CeO 2 DAC enhances metal utilization while overcoming SAC limitations for large substrates. This work establishes a promising strategy for designing highly efficient catalysts for LOHC applications.

Chen, Luning [Univ. of California, Los Angeles, CA↗
DOE OSTI2017

Why Does Alkylation of the N–H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?

Molecular metal/NH bifunctional Noyori-type catalysts are remarkable in that they are among the most efficient artificial catalysts developed to date for the hydrogenation of carbonyl functionalities (loadings up to ~10 –5 mol %). In addition, these catalysts typically exhibit high C$=$O/C$=$C chemo- and enantioselectivities. This unique set of properties is traditionally associated with the operation of an unconventional mechanism for homogeneous catalysts in which the chelating ligand plays a key role in facilitating the catalytic reaction and enabling the aforementioned selectivities by delivering/accepting a proton (H + ) via its N–H bond cleavage/formation. A recently revised mechanism of the Noyori hydrogenation reaction (Dub, P. A. et al. J. Am. Chem. Soc. 2014, 136, 3505) suggests that the N–H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N–H···O hydrogen-bonding interactions (HBIs). Here, the present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N–H functionality within M/NH bifunctional Noyori-type catalysts leads to detrimental catalytic activity. Finally, the purpose of this work is to demonstrate that decreasing the strength of this HBI, ultimately to the limit of its complete absence, are conditions under which the same alkylation may lead to beneficial catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Systematic Catalyst Variation for Improved Stereoselective Epoxide Polymerization: Subtle Modifications Resulting in Superior Efficiency

Isotactic poly(propylene oxide) (iPPO) is a semicrystalline polyether that has emerged as a high-strength, photodegradable material for marine applications. To improve the accessibility of i PPO, catalysts with higher activity and selectivity are required. Using rational catalyst design informed by computational insights, we developed a flexibly tethered, bimetallic chromium catalyst exhibiting high enantioselectivity ( k rel ∼ 100) and unprecedented activity (TOF ∼ 50,000 h −1 ) for propylene oxide (PO) polymerization. Mechanistic studies reveal that high enantioselectivity originates from increased steric bulk at the ortho position of the salicylimine moiety, which increases steric repulsion between the alkoxide chain end and the ligand in the disfavored transition state. Furthermore, introducing geminal dimethyl groups that rigidify the flexible tether between the two ligand moieties significantly enhances catalyst activity by destabilizing the resting state during polymerization. The catalyst remains active at loadings as low as 0.5 ppm, enabling the synthesis of colorless, tough i PPO.

Ren, Bai-Hao [Cornell Univ., Ithaca, NY (United St↗
DOE OSTI2017

Ultrafast Charge Transfer between Light Absorber and Co 3 O 4 Water Oxidation Catalyst across Molecular Wires Embedded in Silica Membrane

Here, the mechanism of visible light-induced hole transfer from a molecular light absorber, in the form of a free-base porphyrin, coupled to a Co 3 O 4 nanoparticle catalyst for water oxidation by a molecular wire (p-oligo(phenylenevinylene) featuring three aryl units) is investigated by transient absorption spectroscopy. The wires are covalently anchored on the Co 3 O 4 surface and embedded in a dense, yet ultrathin (2 nm), silica layer that separates light absorber and catalyst. The porphyrin is electrostatically adsorbed on the silica surface, and aqueous colloidal solutions of the core–shell particles are used for transient optical measurements. Pulsed optical excitation of the porphyrin results in rapid injection of the photogenerated hole onto the molecular wire and concurrent formation of reduced light absorber in less than 1 picosecond (ps). Ultrafast charge separation was monitored by transient absorption of the wire radical cation, which is given by bands in the 500 to 600 nm region and at 1130 nm, while formation of reduced porphyrin was characterized by absorption at 700 nm. Forward transfer of the hole to Co 3 O 4 catalyst proceeds in 255 ± 23 ps. Ultrafast transfer of positive charge from the molecular assembly to a metal oxide nanoparticle catalyst for water oxidation is unprecedented. Holes on Co 3 O 4 recombined with electrons of the reduced sensitizer with biphasic kinetics on a much longer time scale of ten to several hundred nanoseconds. Finally, the unusually efficient hole transfer coupling of a molecular light absorber with an Earth-abundant metal oxide catalyst by silica-embedded p-oligo(phenylenevinylene) offers an approach for integrated artificial photosystems featuring product separation on the nanoscale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Understanding and Design of Bidirectional and Reversible Catalysts of Multielectron, Multistep Reactions

The catalytic properties of the enzymes that transform H2 set very high standards for chemists who aim at designing synthetic electrocatalysts of H2 oxidation and production. Of particular relevance is the observation that some hydrogenases wired to electrodes function in either direction of the reaction depending on the driving force that is applied, and at a significant rate as a result of even a very small deviation from equilibrium. We call the former property "bidirectionality", and the latter "reversibility". Bidirectional and reversible catalysts of H2 oxidation and production have only recently been synthesized. Here we introduce the kinetic modelling of bidirectional two-electron redox reactions in the case of molecular catalysts and enzymes that are either attached to an electrode or diffusing in solution in the vicinity of an electrode. Our theoretical findings are illustrated by previous results obtained with hydrogenases and Nickel diphosphine molecular catalysts. We emphasize that trying to discuss bidirectionality and reversibility in relation to a single redox potential leads to an impasse: the catalyst undergoes two redox transitions, and two catalytic potentials must be defined, which may depart from the two potentials measured in the absence of catalysis. The difference between the two catalytic potentials defines the reversibility; the difference between their average value and the open circuit potential defines the directionality. We aim at giving rules that help discriminate between distinct mechanisms ("ECEC", "EECC" etc.) based on the voltammetric responses, and we discuss the design principles of bidirectionality and reversibility in terms of kinetics. In particular we conclude that neither bidirectionality nor reversibility requires that the catalytic energy landscape be flat. However, sinks and high energy intermediates may negatively impact the catalytic rates. The "efficiency" of a bidirectional catalyst appears to be defined by various figures of merit (rates, directionality, reversibility) that need to be recognized before they can be optimized simultaneously. E.S.W. was supported by the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (USDOE), Office of Science, Office of Basic Energy Sciences (BES). The work discussed here was originally supported for WJS by the Office of Science Early Career Research Program through the USDOE, with current funding from the USDOE, BES, Chemical Sciences, Geosciences, and Biosciences program.

Fourmond, Vincent↗