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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Atomically Dispersed CuN x Sites from Thermal Activation of Boron Imidazolate Cages for Electrocatalytic Methane Generation

Atomically dispersed metal sites (ADMSs) have been recognized as promising candidates for electrochemical conversion. Among a diverse range of molecular precursors for ADMS synthesis, framework materials are particularly interesting due to their high degree of tunability and control over the primary coordination sphere of the metal ions. In this work, we demonstrate that a copper boron imidazolate cage, BIF-29(Cu), is a convenient precursor for a competent catalyst with isolated Cu sites coordinated by N donors for carbon dioxide electroreduction (CO 2 RR). Although BIF-29(Cu) exhibited moderate methane selectivity over hydrogen evolution reaction (HER), the methane selectivity is significantly enhanced by 2 times (55% CH 4 at –1.25 V vs RHE) after mild thermal activation. Extensive characterization methods indicate the transformation of crystalline BIF-29(Cu) into an amorphous carbonaceous material comprising isolated CuN x sites. Moreover, in situ X-ray absorbance spectroscopy indicates stable CuN x sites that are reduced during CO 2 RR. This work encourages the discovery of single-site electrocatalytic systems through a rational selection of molecular precursor and calcination parameters for promoting product selectivity.

10 SYNTHETIC FUELS↗
DOE OSTI2022

Glassy Carbon Substrate Oxidation Effects on Electrode Stability for Oxygen Evolution Reaction Catalysis Stability Benchmarking

Employing benchmarking metrics to capture the activity and stability of electrocatalysts for the oxygen evolution reaction (OER) in acid is a critical practice that enables meaningful comparison of catalyst material candidates reported throughout the literature. In this work, we find that ubiquitously used glassy carbon electrode substrates oxidize under typical OER operating conditions, forming a pacified, electrically insulating, and oxygen-rich surface layer that causes drastic loss of current density over the course of extended chronoamperometric stability tests at an anodic potential of 1.7 VRHE. We show that the experimentally observed stability of glassy carbon-based electrodes is approximately two orders of magnitude lower than that expected solely from dissolution-based catalyst intrinsic stability of Ir-based catalysts. We additionally find that glassy carbon-based electrode stability measured by chronoamperometric holds is greatly impacted by catalyst loading, with high catalyst loadings improving the stability of the overall electrode via a protective effect on the glassy carbon substrate. Altogether, our investigation highlights that glassy carbon is not electrochemically inert under OER conditions on the timescale of common stability tests, which can cause electrodes to exhibit performance losses that do not reflect the intrinsic stability of the actual catalyst material being investigated. In light of our findings, we underscore the usefulness of metrics, such as the S-number, to reflect intrinsic catalyst material stability.

25 ENERGY STORAGE↗
DOE OSTI2022

Combined Rotating Disk Slurry Electrodeposition–Spontaneous Galvanic Displacement for Pt-M (M = Co, Ni, and Cu) Catalyst Synthesis for the Oxygen Reduction Reaction in Alkaline Media

Here, platinum-modified first-row transition-metal catalyst (M = Ni, Co, and Cu) nanoparticles on a carbon black Vulcan XC-72R (CBV) support were synthesized using rotating disk slurry electrodeposition (RoDSE) followed by spontaneous galvanic displacement (SGD) reactions. These RoDSE–SGD catalysts were evaluated for the oxygen reduction reaction (ORR) in 0.1 M KOH using rotating disk electrode techniques. The Ni and Co RoDSEs were done by using an electrochemical applied potential of -0.75 V versus the RHE and for Cu, -0.80 V versus the RHE using a CBV slurry solution containing 0.1 M KClO 4 . These metal nanoparticles on CBV (M/CBV) catalysts were modified with a Pt precursor via a spontaneous galvanic displacement (SGD) reaction, producing a Pt-M/CBV material to catalyze the ORR in an alkaline medium. High-resolution scanning transmission electron microscopy (HR-STEM) analysis indicates that the PtM/CBV samples include M clusters and Pt single atoms. The ORR characterization measurements were done under a controlled temperature (25.0 °C) and with a mass loading of 100 μg/cm 2 on a glassy carbon (GC) rotating disk electrode at 1600 rpm. The PtCo/CBV showed the highest ORR mass activity of 0.741 A/mgPt at 0.90 V versus the reversible hydrogen electrode (RHE) compared with commercial Pt/CBV. The M/CBV RoDSE catalysts were also tested for the oxygen evolution reaction (OER), and Ni/CBV provided the lowest overpotential of 450 mV at 10 mA/cm 2 disk in 0.1 M KOH.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2026

Evaluating Ru-RuO 2 @BN as a Bifunctional Electrocatalyst for the Nitrogen Reduction Reaction and the Hydrogen Evolution Reaction

Electrochemical approaches toward clean energy production have been the focus of significant attention. Here, the nitrogen (N 2 ) reduction reaction (NRR) and the hydrogen (H 2 ) evolution reaction (HER) offer a promising method for producing NH 3 and H 2 , respectively. Nevertheless, practical obstacles that must be overcome in creating optimal catalysts are the sluggish kinetics and low selectivity of NRR and HER. Herein, we report on the synthesis of a Ru-RuO 2 -decorated boron nitride (BN) catalyst that shows excellent activity toward NRR. A rate of NH3 formation (V NH3 ) of 16.8 μg h -1 mg -1 and a corresponding Faradaic efficiency (FE) of 52.9% were noted at a potential of −0.5 V in 0.1 M HCl. However, the HER activity of Ru-RuO 2 @BN was found to be highly suppressed in 0.1 M HCl and did not yield a reasonable overpotential value. Thus, the capability of this material toward NRR suggests its viability as a promising catalyst for clean NH 3 production.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2026

Elucidating the Structural and Electronic Effects of Ni and Mn Cationic Incorporation on CoOOH for Efficient Benzyl Alcohol Electrooxidation

Transition-metal oxyhydroxides such as CoOOH are promising low-cost electrocatalysts for the selective electrooxidation of organic molecules, yet the influence of ubiquitous transition-metal impurities on their performance and durability remains poorly understood. Here, we experimentally probed the individual and synergistic electrochemical and structural effects of Ni and Mn incorporations into model CoOOH electrocatalysts toward an efficient benzyl alcohol oxidation reaction (BAOR). Comprehensive electrochemical, microscopic, and spectroscopic analyses reveal that Ni incorporation enhances charge-transfer kinetics and overall activity through the formation of catalytically active Ni 3+ sites, whereas Mn exhibited a more complex but interesting role. At the early stages of operation, Mn 4+ acts as a stabilizing surface layer that mitigates catalyst degradation but partially blocks Co sites before they undergo gradual leaching. The concurrent incorporation of both Ni and Mn yields a trimetallic 2NMC@NF electrocatalyst that integrates the activity benefits of Ni with the stability conferred by Mn, achieving 92.9% benzyl alcohol conversion and 91.4% Faradaic efficiency after 24 h at 1.5 V vs RHE. These findings elucidate how trace Ni and Mn impurities, often introduced from electrolytes or external sources, can modulate the lattice and electronic structure of CoOOH, offering a design strategy for enhancing both activity and long-term stability in electrocatalytic organic oxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Aluminum Metal–Organic Framework Triggers Carbon Dioxide Reduction Activity

Confinement of metal centers is a powerful tool to manipulate reactivity and tune selectivity in chemical transformations. While aluminum as a foil is inactive for carbon dioxide reduction and shows high selectivity for the hydrogen evolution reaction, here we show that aluminum confined in a metal-organic framework (MOF), MIL-53(Al), suppresses hydrogen evolution reaction activity and enhances carbon dioxide reduction. This aluminum MOF can produce up to 40% faradaic efficiency for carbon monoxide and formic acid. Here, this study demonstrates that the unique reaction environment created by the MOF enables changes in reaction selectivity and can impart atypical catalytic capabilities to metals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu 3– x P Nanocrystals

N-heterocyclic carbenes (NHCs) are an important class of ligands capable of making strong carbon-metal bonds. Recently, there has been a growing interest in the study of carbene-ligated nanocrystals, primarily coinage metal nano-crystals, which have found application as catalysts for numerous reactions. The general ability of NHC ligands to positively affect the catalytic properties of other types of nanocrystal catalysts remains unknown. Furthermore, we present the first carbene-stabilized Cu 3–x P nanocrystals. Inquiries into the mechanism of formation of NHC-ligated Cu 3-x P nanocrystals suggest that crystalline Cu 3–x P forms directly as a result of a high-temperature metathesis reaction be-tween a tris(trimethylsilyl)phosphine precursor and an NHC-CuBr precursor, the latter of which behaves as a source of both carbene ligand and Cu + . To study the effect of the NHC surface ligands on catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu 3–x P nanocrystals and found they possess superior activity to analogous oleylamine-ligated Cu 3–x P nanocrystals. Density functional theory calculations suggest that the NHC ligands minimize unfavorable electrostatic interactions between the copper phosphide surface and H + during the first step of the hydrogen evolution reaction, which contributes to the superior performance of NHC-ligated Cu 3–x P catalysts as compared to oleylamine-ligated Cu 3–x P catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Two-Dimensional Conductive Ni-HAB as a Catalyst for the Electrochemical Oxygen Reduction Reaction

Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal–organic framework consisting of M-N 4 units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability. We show that crystallinity has a positive impact on conductivity and demonstrate that this improved crystallinity/conductivity improves the catalytic performance of our model system. Additionally, density functional theory simulations were performed to probe the origin of M-HAB’s catalytic activity, and they suggest that M-HAB’s organic linker acts as the active site with the role of the metal being to modulate the linker sites’ binding strength.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Improved Rate for the Oxygen Reduction Reaction in a Sulfuric Acid Electrolyte using a Pt(111) Surface Modified with Melamine

The feasible commercialization of alkaline, phosphoric acid and polymer electrolyte membrane fuel cells depends on the development of oxygen reduction reaction (ORR) electrocatalysts with improved activity, stability, and selectivity. The rational design of surfaces to ensure these improved ORR catalytic requirements relies on the so-called "descriptors" (e.g., the role of covalent and noncovalent interactions on platinum surface active sites for ORR). Here, we demonstrate that through the molecular adsorption of melamine onto the Pt(111) surface [Pt(111)-M ad ], the activity can be improved by a factor of 20 compared to bare Pt(111) for the ORR in a strongly adsorbing sulfuric acid solution. Additionally, the M ad moieties act as "surface-blocking bodies," selectively hindering the adsorption of (bi)sulfate anions (well-known poisoning spectator of the Pt(111) active sites) while the ORR is unhindered. This modified surface is further demonstrated to exhibit improved chemical stability relative to Pt(111) patterned with cyanide species (CN ad ), previously shown by our group to have a similar ORR activity increase compared to bare Pt(111) in a sulfuric acid electrolyte, with Pt(111)-M ad retaining a greater than ninefold higher ORR activity relative to bare Pt(111) after extensive potential cycling as compared to a greater than threefold higher activity retained on a CN ad -covered Pt(111) surface. We suggest that the higher stability of the Pt(111)-M ad interface stems from melamine's ability to form intermolecular hydrogen bonds, which effectively turns the melamine molecules into larger macromolecular entities with multiple anchoring sites and thus more difficult to remove.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Improving and Understanding the Hydrogen Evolving Activity of a Cobalt Dithiolene Metal–Organic Framework

Despite the promising previous reports on the development of electrocatalytic dithiolene-based metal–organic frameworks (MOFs) for the hydrogen evolution reaction (HER), these materials often display poor reproducibility of the HER performance because of their poor bulk properties upon integration with electrode materials. In this work, we demonstrate an in-depth investigation of the electrocatalytic HER activity of a cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) MOF. To enhance the durability and charge transport properties of the constructed CoTHT/electrode architecture, CoTHT is deposited as an ink composite (1) composed of Nafion and carbon black. We leverage here the well-established use of catalyst inks in the literature to increase adhesion of the catalyst to the electrode surface and to improve the overall electrical conductivity of the integrated catalyst/electrode. The utilization of the composite 1 leads to a significant improvement in the overpotential (η) to reach a current density of 10 mA/cm 2 (η = 143 mV) compared to prior reports, resulting in the most active MOF-based electrocatalyst for the HER that contains only earth-abundant elements. Extensive density functional theory (DFT) calculations were applied to understand the structure of CoTHT and the mechanistic pathways of the HER. The computational results suggest that an AB stacking geometry is energetically favorable, where one layer is slipped by 1.6 Å relative to the neighboring one along the a and b vectors. Additionally, the DFT calculations indicate that the catalytic cycle likely involves a Volmer discharge step to generate a cobalt hydride, followed by a Heyrovsky step to form a cobalt–H 2 intermediate, and finally the dissociation of H 2 .

DFT calculations↗
DOE OSTI2021

3D Printed Nickel–Molybdenum-Based Electrocatalysts for Hydrogen Evolution at Low Overpotentials in a Flow-Through Configuration

Three-dimensional (3D) printed, hierarchically porous nickel molybdenum (NiMo) electrocatalysts were synthesized and evaluated in a flow-through configuration for the hydrogen evolution reaction (HER) in 1.0 M KOH(aq) in a simple electrochemical H-cell. 3D NiMo electrodes possess hierarchically porous structures because of the resol-based aerogel precursor, which generates superporous carbon aerogel as a catalyst support. Relative to a traditional planar electrode configuration, the flow-through configuration allowed efficient removal of the hydrogen bubbles from the catalyst surface, especially at high operating current densities, and significantly decreased the overpotentials required for HER. An analytical model that accounted for the electrokinetics of HER as well as the mass transport with or without the flow-through configuration was developed to quantitatively evaluate voltage losses associated with kinetic overpotentials and ohmic resistance due to bubble formation in the porous electrodes. Furthermore, the chemical composition, electrochemical surface area (ECSA), and roughness factor (RF) were also systematically studied to assess the electrocatalytic performance of the 3D printed, hierarchically porous NiMo electrodes. An ECSA of 25163 cm2 was obtained with the highly porous structures, and an average overpotential of 45 mV at 10 mA cm–2 was achieved over 24 h by using the flow-through configuration. The flow-through configuration evaluated in the simple H-cell achieved high electrochemical accessible surface areas for electrochemical reactions and provided useful information for adaption of the porous electrodes in flow cells.

08 HYDROGEN↗
DOE OSTI2021

Cu[Ni(2,3-pyrazinedithiolate) 2 ] Metal–Organic Framework for Electrocatalytic Hydrogen Evolution

The application of metal–organic frameworks (MOFs) as electrocatalysts for small molecule activation has been an emerging topic of research. Previous studies have suggested that two-dimensional (2D) dithiolene-based MOFs are among the most active for the hydrogen evolution reaction (HER). Here, a three-dimensional (3D) dithiolene-based MOF, Cu[Ni(2,3-pyrazinedithiolate) 2 ] (1), is evaluated as an electrocatalyst for the HER. In pH 1.3 aqueous electrolyte solution, 1 exhibits a catalytic onset at –0.43 V vs the reversible hydrogen electrode (RHE), an overpotential (η 10 mA/cm 2 ) of 0.53 V to reach a current density of 10 mA/cm 2 , and a Tafel slope of 69.0 mV/dec. Interestingly, under controlled potential electrolysis, 1 undergoes an activation process that results in a more active catalyst with a 200 mV reduction in the catalytic onset and η 10 mA/cm 2 . It is proposed that the activation process is a result of the cleavage of Cu–N bonds in the presence of protons and electrons. Furthermore, this hypothesis is supported by various experimental studies and density functional theory calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Application of Molecular Catalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

Here, the development of precious group metal-free (PGM-free) catalysts for the oxygen reduction reaction is considered as the main thrust for the cost reduction of fuel cell technologies and their mass production. Within the PGM-free category, molecular catalysts offer an advantage over other heat-treated PGM-free catalysts owing to their well-defined structure, which enables further design of more active, selective, and durable catalysts. Even though non-heat-treated molecular catalysts with exceptional performance have been reported in the past, they were rarely tested in a fuel cell. Herein, we report on a molecular catalyst under alkaline conditions: fluorinated iron phthalocyanine (FeFPc) supported on cheap and commercially available high-surface area carbon-BP2000 (FeFPc@BP2000). It exhibits the highest activity ever reported for molecular catalysts under alkaline conditions in half-cells and fuel cells.

25 ENERGY STORAGE↗
DOE OSTI2022

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers-such as pinholes, cracks, or particle inclusions-may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiOx) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions-the hydrogen evolution and Fe(III) reduction reactions-can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiOxoverlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Lastly, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

25 ENERGY STORAGE↗
DOE OSTI2023

Robust Copper-Based Nanosponge Architecture Decorated by Ruthenium with Enhanced Electrocatalytic Performance for Ambient Nitrogen Reduction to Ammonia

Electrochemical conversion of nitrogen to green ammonia is an attractive alternative to the Haber–Bosch process. However, it is currently bottlenecked by the lack of highly efficient electrocatalysts to drive the sluggish nitrogen reduction reaction (N2RR). In this work, we strategically design a cost-effective bimetallic Ru–Cu mixture catalyst in a nanosponge (NS) architecture via a rapid and facile method. The porous NS mixture catalysts exhibit a large electrochemical active surface area and enhanced specific activity arising from the charge redistribution for improved activation and adsorption of the activated nitrogen species. Benefiting from the synergistic effect of the Cu constituent on morphology decoration and thermodynamic suppression of the competing hydrogen evolution reaction, the optimized Ru 0.15 Cu 0.85 NS catalyst presents an impressive N 2 RR performance with an ammonia yield rate of 26.25 μg h –1 mg cat. –1 (corresponding to 10.5 μg h –1 cm –2 ) and Faradic efficiency of 4.39% as well as superior stability in alkaline medium, which was superior to that of monometallic Ru and Cu nanostructures. Additionally, this work develops a new bimetallic combination of Ru and Cu, which promotes the strategy to design efficient electrocatalysts for electrochemical ammonia production under ambient conditions.

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

Assessment of the Degradation Mechanisms of Cu Electrodes during the CO 2 Reduction Reaction

Catalyst degradation and product selectivity changes are two of the key challenges in the electrochemical reduction of CO 2 on copper electrodes. Yet, these aspects are often overlooked. Here, we combine in situ X-ray spectroscopy, in situ electron microscopy, and ex situ characterization techniques to follow the long-term evolution of the catalyst morphology, electronic structure, surface composition, activity, and product selectivity of Cu nanosized crystals during the CO 2 reduction reaction. We found no changes in the electronic structure of the electrode under cathodic potentiostatic control over time, nor was there any build-up of contaminants. In contrast, the electrode morphology is modified by prolonged CO 2 electroreduction, which transforms the initially faceted Cu particles into a rough/rounded structure. In conjunction with these morphological changes, the current increases and the selectivity changes from value-added hydrocarbons to less valuable side reaction products, i.e. , hydrogen and CO. Hence, our results suggest that the stabilization of a faceted Cu morphology is pivotal for ensuring optimal long-term performance in the selective reduction of CO 2 into hydrocarbons and oxygenated products.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗