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DOE OSTI2025

Role of Metal Cocatalysts in the Photocatalytic Production of Hydrogen from Water Revisited

The use of photocatalysts to promote the production of molecular hydrogen from water, following the so-called water splitting reaction, continues to be a promising route for the green production of fuels. The molecular basis of this photocatalysis is the photoexcitation of electrons from the valence band of semiconductors to their conduction band, from which they can be transferred to chemical reactants, protons in the case of water, to promote a reduction reaction. The mechanism by which such a process takes place has been studied extensively using titanium oxide, a simple material that fulfills most requirements for water splitting. However, photocatalysis with TiO 2 tends to be highly inefficient; a cocatalyst, commonly a late transition metal (Au, Pt) in nanoparticle form, needs to be added to facilitate the production of H 2 . The metal is widely believed to help with the scavenging of the excited electrons from the conduction band of the semiconductor in order to prevent their recombination with the accompanying hole formed in the valence band, a step that cancels the initial photon absorption and competes with the photolytic chemical reduction. Here we review and analyze the molecular basis for that mechanism and argue for an alternative explanation, that the role of the metal is to help with the recombination of the atomic hydrogen atoms produced by proton reduction on the semiconductor surface instead. First, we summarize what is known about the electronic structure of these photocatalysts and how the electronic levels need to line up for the reduction of protons in water to be feasible. Next, we review the current understanding of the dynamics of the steps associated with the absorption of photons, the de-excitation via electron–hole pair recombination and fluorescence decay, and the electronic transitions that lead to proton reduction, and contrast those with the rates of the chemical steps required to produce molecular hydrogen. The following section addresses the changes introduced by the addition of the metal cocatalyst, comparatively evaluating its role as either an electron scavenger or a promoter of the recombination of hydrogen atoms. A discussion of the viable chemical mechanisms for the latter pathway is included. Finally, we briefly mention other associated aspects of this photocatalysis, including the possible promotion of H 2 production with visible light via resonant excitation of the surface plasmon of Au nanoparticles, the use of single-metal (Au, Pt) atom catalysts and of yolk–shell nanostructures, and the reduction of organic molecules. We end with a brief personal perspective on the possible generality of the concepts introduced in this Critical Review.

08 HYDROGEN↗

DOE OSTI2021

On the Use of Aqueous Metal-Aqua p K a Values as a Descriptor of Lewis Acidity

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in non-aqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31 P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pK a values of the corresponding metal-aqua species, [M(H 2 O) m ] n+ , measured in H 2 O vs. the 31 P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD 2 Cl 2 ) display tightly co-linear relationships, suggesting similar behavior for these ions in water, d 3 -MeCN, and CD 2 Cl 2 . This collinearity reinforces the utility of the common approach of using the aqueous pK a values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d 3 -MeCN suggest 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Altogether, these data suggest that both aqueous pK a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Expanding the Transuranic Metal–Organic Framework Portfolio: The Optical Properties of Americium(III) MOF-76

Reported here is the synthesis, crystal structure, and solid-state characterization of a new americium containing metal–organic framework (MOF), [Am(C 9 H 3 O 6 )(H 2 O)], MOF-76(Am). This material is constructed from Am 3+ metal centers and 1,3,5-tricarboxylic acid (BTC) ligands, forming a porous three-dimensional framework that is isostructural with several known trivalent lanthanide (Ln) analogs (e.g., Ce, Nd, and Sm–Lu). The Am 3+ ions have seven coordinates and assume a distorted, capped trigonal prismatic geometry with C1 symmetry. The Am 3+ –O bonds were studied via infrared spectroscopy and compared to several MOF-76(Ln) analogs, where Ln = Nd 3+ , Eu 3+ , Tb 3+ , and Ho 3+ . The results show that the strength of the ligand carboxylate stretching and bending modes increase with Nd 3+ < Eu 3+ < Am 3+ < Tb 3+ < Ho 3+ , suggesting the metal–oxygen bonds are predominantly ionic. Optical absorbance spectroscopy measurements reveal strong f–f transitions; some exhibit pronounced crystal field splitting. The photoluminescence spectrum contains weak Am 3+ -based emission that is achieved through direct and indirect metal center excitation. The weak emissive behavior is somewhat surprising given that ligand-to-metal resonance energy transfer is efficient in the isoelectronic Eu 3+ (4f 6 ) and related Tb 3+ (4f 8 ) analogs. The optical properties were explored further within a series of heterometallic MOF-76(Tb 1–x Am x ) (x = 0.8, 0.2, and 0.1) samples, and the results reveal enhanced Am 3+ photoluminescence.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Unraveling Electronic Trends in O* and OH* Surface Adsorption in the MO 2 Transition-Metal Oxide Series

Understanding the bond strength of O* and OH* intermediates to metal-oxide surfaces is key to predicting the catalytic activity in oxygen-based electrochemistry. Here, we uncover highly non-linear trends in O* and OH* adsorption energies across the 3d, 4d, and 5d series of MO 2 transition-metal (TM) oxide surfaces computed within Hubbard- U corrected density functional theory (DFT + U ). Investigating the electronic structure with crystal orbital Hamiltonian populations (COHP) of the relevant metal–oxygen bonds reveals that the spin-dependent coupling strength between metal-d and oxygen-2p atomic orbitals together with the extent of filling of bonding and anti-bonding orbitals are the primary contributors to the adsorption energy. Importantly, we show that the integrated COHP obtained purely from bulk calculations is a highly accurate descriptor for surface adsorption energetics that captures trends across the group 5–12 TM oxide series within 0.19–0.36 eV. Our results suggest a pathway to prediction of adsorption energies for an arbitrary metal–ligand catalyst system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Doping Induced Magnetic and Electronic Phase Transitions in the Ferrimagnetic Half-Metallic Mn 4 Al 11 Compound

The future of spintronics and semiconductor applications demands materials with tailored electronic and magnetic properties. Here, this study uses density functional theory to investigate the electronic structure of the half-metallic compound Mn 4 Al 11 under uniaxial strain and in its Ge-substituted derivatives. Strain analysis shows that although the half-metallic band gap collapses under strain beyond −2%, the ferrimagnetic character remains stable. Ge substitution at six inequivalent Al sites in Mn 4 Al 11 results in varying degrees of metallicity and magnetic properties. Substitution at Al═ (000) induces a metal-to-insulator transition with an indirect semiconducting gap of 0.14 eV. Bonding and hybridization analyses reveal that local Mn–Al interactions due to Ge substitution significantly modify the local electronic structure, causing both electronic and magnetic phase transitions. This work highlights the effectiveness of substitutional doping in tuning half-metallicity and magnetic properties in inorganic solids, enabling the design of materials for future technological applications.

alloys↗

DOE OSTI2023

Biocompatible Co–P Metallic Glasses with Superior Degradation Tolerance in Physiological Environments

Metallic glasses represent a class of metallic alloys with a fully amorphous structure and attractive properties, making them promising in bioimplant applications. Here, the degradation tolerance of biocompatible cobalt–phosphorus (Co–P) metallic glasses was studied in a simulated physiological environment. The metallic glasses were synthesized in the form of coatings through a facile electrodeposition approach. This method utilizes their outstanding surface characteristics and bypasses the size limitations usually associated with their bulk counterparts. Further, the Co–P alloys showed exceptional tribological response with ~14% lower coefficient of friction and 2 orders of magnitude lesser wear rate compared to SS316 stainless steel. In addition, the Co–P alloys showed a 3 times higher hardness and 4 times higher hardness/modulus ratio compared to SS316, indicating better elastic recovery under dynamic shear stresses that are common in load-bearing bioimplants. The Co–P metallic glasses exhibited excellent hemocompatibility and cytocompatibility in terms of lower platelet adhesion, spreading, and aggregation, a hemolysis ratio lower than 1%, and enhanced surface wettability, suggesting a superlative performance in bioimplant applications.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Investigation of Reactive Silver Ink Formula for Reduced Silver Consumption in Silicon Heterojunction Metallization

Silver is the most expensive non-silicon component in photovoltaic cells. This is particularly salient for silicon heterojunction (SHJ) cells, which rely on large quantities of low-temperature silver pastes (LT-SP). SHJ cells would benefit greatly from an industrially scalable metallization process that simultaneously offers low silver consumption, low finger resistivities (<20 μΩ·cm), and low processing temperatures. Printed reactive silver inks (RSI) are an innovative candidate to this end. Furthermore, this work furthers the research on RSI metallization by investigating the impact of ink formula on properties pertinent to SHJ cells, including electrical properties, line width, ink splatter, silver consumption, cell performance, and adhesion. We introduce a scalable, high-throughput flexible needle contact printing approach for metallization that solves many of the issues associated with drop-on-demand printing. The printed silver fingers are characterized using electrical measurements and top-down and cross-sectional microscopy. The best performing ink, consisting of silver acetate, ethylamine, and formic acid, achieved silver fingers with total resistivities of 3.1 μΩ·cm and contact resistivities of 3.2 mΩ·cm 2 when printed at 61 °C. This ink metallized a full-sized 156 mm × 156 mm SHJ cell. This is the first reported data for RSI metallization of a full-sized SHJ cell and shows how an optimized RSI can achieve similar performances to LT-SP while consuming 80–90% less silver.

14 SOLAR ENERGY↗

DOE OSTI2022

Investigating the Influence of Hexanuclear Clusters in Isostructural Metal–Organic Frameworks on Toxic Gas Adsorption

The efficient capture of toxic gases, such as ammonia (NH 3 ) and sulfur dioxide (SO 2 ), can protect the general population and mitigate widespread air pollution. Metal–organic frameworks (MOFs) comprise a tunable class of adsorbents with high surface areas that can meet this challenge by selectively capturing these gases at low concentrations. In this work, we explored how modifying the metal ions in the node of an isostructural MOF series from a transition metal to a lanthanide or actinide influences the electronic environment of the node-based active site. Next, we investigated the adsorption properties of each MOF toward the relatively basic NH 3 and relatively acidic SO 2 gases. Within the NU-907 family of MOFs, we found that Zr 6 -NU-907 exhibits the best uptake toward NH 3 at low pressures, while Th 6 -NU-907 demonstrates the best low-pressure performance for SO 2 adsorption. Here, tracking the infrared (IR) stretching frequency of the node-based μ 3 -OH groups provides insights into the electronegativity of the metal ion and suggests that the most electronegative metal ion (Zr) affords the node with the best NH 3 uptake at low pressures. In contrast, the Th 6 node contains additional coordinated water groups relative to the other M 6 nodes, which appears to yield the MOF with the greatest affinity for SO 2 uptake that occurs predominately through reversible physisorption interactions. Finally, in situ NH3 IR spectroscopic studies indicate that both NH 4 + and Lewis-bound NH 3 species form during adsorption. Combined, these results suggest that tuning the electronic properties and structure of the node-based active site in an MOF presents a viable strategy to change the affinity of an MOF toward toxic gases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Adapting UFF4MOF for Heterometallic Rare-Earth Metal–Organic Frameworks

Heterometallic metal–organic frameworks based on rare-earth metals (RE-MOFs) have potential in a number of applications where energy transfer between nearby metal atoms is required. This observation implies that it is important to understand the level of local mixing that is achieved between metals of different types during synthesis of RE-MOFs. Density functional theory calculations can give quantitative information on the relative energy of different configurations of RE-MOFs, but these calculations cannot be applied to the full range of medium- and long-range orderings that are possible in heterometallic materials. This limitation can be overcome using force field (FF)-based calculations if appropriate FFs are available. In this work, we show that an existing generic FF for MOFs, UFF4MOF, does not accurately predict energies of mixing in heterometallic Nd/Yb MOFs and introduce a modified FF to address this shortcoming. The resulting FF is used to explore metal orderings in large simulation volumes for a Nd/Yb MOF, illustrating the complexities that can arise in the structure of heterometallic RE-MOFs.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Theory and Simulation of Metal–Insulator–Semiconductor (MIS) Photoelectrodes

A metal–insulator–semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H 2 by proton reduction. The metal catalyzes the generation of H 2 using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface. Understanding how the insulator layer determines the photovoltage and what properties lead to high photovoltages is critical to the development of MIS structures for solar-to-chemical energy conversion. Herein, we present a continuum model for charge-carrier transport from the semiconductor to the metal with an emphasis on mechanisms of charge transport across the insulator. The polarization curves and photovoltages predicted by this model for a Pt/HfO 2 /p-Si MIS structure at different HfO 2 thicknesses agree well with experimentally measured data. The simulations reveal how insulator properties (i.e., thickness and band structure) affect band bending near the semiconductor/insulator interface and how tuning them can lead to operation closer to the maximally attainable photovoltage, the flat-band potential. This phenomenon is understood by considering the change in tunneling resistance with insulator properties. The model shows that the best MIS performance is attained with highly symmetric semiconductor/insulator band offsets (e.g., BeO, MgO, SiO 2 , HfO 2 , or ZrO 2 deposited on Si) and a low to moderate insulator thickness (e.g., between 0.8 and 1.5 nm). Beyond 1.5 nm, the density of filled interfacial trap sites is high and significantly limits the photovoltage and the solar-to-chemical conversion rate. These conclusions are true for photocathodes and photoanodes. This understanding provides critical insight into the phenomena enhancing and limiting photoelectrode performance and how this phenomenon is influenced by insulator properties. The study gives guidance toward the development of next-generation insulators for MIS structures that achieve high performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Implications of Electron Transport Layer and Back Metal Contact Variations in Tin–Lead Perovskite Solar Cells Assessed by Spectroscopic Ellipsometry and External Quantum Efficiency

The structural and optical properties of hybrid organic–inorganic metal halide perovskite solar cells are measured by spectroscopic ellipsometry to reveal an optically distinct interfacial layer among the back contact metal, charge transport, and absorber layers. Understanding how this interfacial layer impacts performance is essential for developing higher performing solar cells. Here, this interfacial layer is modeled by Bruggeman effective medium approximations (EMAs) to contain perovskite, C 60 , BCP, and metal. External quantum efficiency (EQE) simulations that consider scattering, electronic losses, and the formation of nonparallel interfaces are created with input derived from ellipsometry structural-optical models and compared with experimental EQE to estimate optical losses. This nonplanar interface causes optical losses in short circuit current density (JSC) of up to 1.2 mA cm –2 . A study of glass/C 60 /SnO 2 /Ag or Cu and glass/C 60 /BCP/Ag film stacks shows that C 60 and BCP mix, but replacing BCP with SnO 2 can prevent mixing between the ETLs to prevent contact between C 60 and back contact metal and enable the formation of a planar interface between ETLs and back contact metals.

14 SOLAR ENERGY↗

DOE OSTI2024

Boosting Thermoelectric Performance in Nanocrystalline Ternary Skutterudite Thin Films through Metallic CoTe 2 Integration

Metal–semiconductor nanocomposites have emerged as a viable strategy for concurrently tailoring both thermal and electronic transport properties of established thermoelectric materials, ultimately achieving synergistic performance. In this investigation, a series of nanocomposite thin films were synthesized, embedding metallic cobalt telluride (CoTe 2 ) nanophase within the nanocrystalline ternary skutterudite (Co(Ge 1.22 Sb 0.22 )Te 1.58 or CGST) matrix. Our approach harnessed composition fluctuation-induced phase separation and in situ growth during thermal annealing to seamlessly integrate the metallic phase. The distinctive band structures of both materials have developed an ohmic-type contact characteristic at the interface, which raised carrier density considerably yet negligibly affected the mobility counterpart, leading to a substantial improvement in electrical conductivity. The intricate balance in transport properties is further influenced by the metallic CoTe 2 phase’s role in diminishing lattice thermal conductivity. The presence of the metallic phase instigates enhanced phonon scattering at the interface boundaries. Consequently, a 2-fold enhancement in the thermoelectric figure of merit (zT ~ 1.30) is attained with CGST-7 wt. % CoTe 2 nanocomposite film at 655 K compared to that of pristine CGST.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Chemical Vapor Transformation of Lithium Metal: Mechanism and Enhanced Stability

Constructing a stable solid electrolyte interface (SEI) with high productivity and scalability is essential for the practical application of thin Li metal anodes. Here, in this work, we report a chemical vapor transformation (CVT) strategy in which Li metal is exposed to trimethylaluminum (TMA), inducing a rapid and spontaneous surface reaction that forms a robust, multilayered SEI. In situ quartz crystal microbalance (QCM) and quadrupole mass spectrometry (QMS), combined with ex situ X-ray photoelectron spectroscopy (XPS), UV Raman spectroscopy, and density functional theory (DFT) calculations, reveal that TMA removes the native passivation layer, reacts with Li metal, and drives a coupled bulk-surface transformation involving Li–Al interdiffusion. The resulting SEI exhibits a chemically graded structure consisting of an inner Li–Al alloy and an outer amorphous carbon layer formed via demethylation and ligand-exchange pathways. This modified surface exhibits significantly enhanced stability compared to that of bare Li in electrochemical cycling using liquid and solid-state electrolytes. This work unveils a unique surface-mediated bulk transformation mechanism for lithium metal and establishes CVT as a scalable and fundamentally distinct approach for the interfacial engineering of reactive metals.

Li metal↗

DOE OSTI2020

Why Conventional Design Rules for C–H Activation Fail for Open-Shell Transition-Metal Catalysts

The design of selective and active C–H activation catalysts for direct methane-to-methanol conversion is challenging. Bioinspired complexes that form high-valent metal–oxo intermediates capable of hydrogen abstraction and rebound hydroxylation are promising candidates. This promise has made them a target for computational high-throughput screening, typically simplified through the use of linear free energy relationships (LFERs). However, their mid-row transition-metal centers have numerous accessible spin and oxidation states that increase the combinatorial scale of design efforts. Here, we carry out a computational design screen of over 2500 mid-row 3d transition-metal complexes with four metals in numerous spin and oxidation states. We demonstrate the importance of spin/oxidation state in dictating design principles, limiting the generalization of strategies derived for widely studied high-spin Fe(II) catalysts to other metals or spin/oxidation states. Combined assessment of the effect of ligand-field tuning on reaction step energetics and on the identity of the ground state allows us to propose refined design strategies for spin-allowed methane-to-methanol catalysis. We observe weak coupling of energetics and design principles between reaction steps (e.g., oxo formation vs methanol release), meaning that LFERs do not generalize across our larger catalyst set. To rationalize relative reactivity in known catalysts, we instead compute independent reaction energies and propose strategies for further improvements in catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Prediction of O and OH Adsorption on Transition Metal Oxide Surfaces from Bulk Descriptors

In the search for stable and active catalysts, density functional theory and machine learning (ML) based models can accelerate the screening of materials. While stability is conveniently addressed on the bulk level of computation, the modelling of catalytic activity requires expensive surface simulations. Here, in this work, we develop models for the surface adsorption energy of O and OH intermediates across a consistent and extensive dataset of pure transition metal oxide surfaces. We show that adsorption energies across metal oxidation states of +2 to +6 are well captured from the metal-oxygen bond strength extracted from the bulk level calculation. Specifically, we calculate the integrated crystal orbital Hamiltonian population (ICOHP) of the metal-oxygen bond in the bulk oxide and employ a simple normalization scheme to obtain a strong correlation with adsorption energetics. By combining our ICOHP descriptor with non DFT features in a Gaussian Process regression (GPR) model, we achieve high model accuracy with mean absolute errors of 0.166 and 0.219 eV for OH and O adsorption, respectively. By targeting the O-OH adsorption energy difference with our GPR model, we predict the the oxygen evolution reaction (OER) activity from bulk descriptors only. Furthermore, we utilize the strong correlation between the COHP and metal oxygen bond lengths to rapidly predict adsorption energetics and catalytic activity from the optimized bulk geometry. Our approach can enable an efficient search for active catalysts by eliminating the need for surface calculations in the initial screening phase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Ca Metal Batteries and Sn Anode Alloying: Resolving Misconceptions in Ca–Sn Alloy Formation

Ca metal anode suffers from surface passivation and struggles to effectively plate and strip in conventional Ca electrolytes, making the development of alloy anode for calcium metal battery essential. This work systematically and carefully evaluates the electrochemical formation and feasibility of the Sn anode and its possibility of alloying with Ca using a combination of cyclic voltammetry, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and synchrotron transmission X-ray microscopy (TXM). Electrochemical measurements and SEM/EDS analyses indicate apparent Ca alloy formation. However, detailed XRD and TXM characterization reveal substantial lithium-ion involvement when lithium metal is employed as the reference electrode, even in electrolytes with high Ca salt concentrations. These findings highlight a significant risk of misinterpretation: Although limited Ca–Sn alloying may occur, the faster kinetics of Li + can dominate the alloying process, leading to a Li-driven reactions at the Sn anode. In contrast, cells constructed with Ca metal as both reference and counter electrodes exhibit distinct redox features only during the initial cycles, followed by rapid cell failure. The high oxidation currents induce severe Ca passivation, occurring nearly concurrently with Sn dealloying. Overall, this study provides critical insight into the practical limitations and common experimental artifacts associated with Ca metal battery systems and underscores the necessity for rigorous cell design and cautious interpretation when evaluating alloy chemistry in multivalent Ca-based batteries.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Water Interplays during Dysprosium Electrodeposition in Pyrrolidinium Ionic Liquid: Deconvoluting the Pros and Cons for Rare Earth Metallization

The electrochemical production of rare earth metals (REMs) in ionic liquids (ILs) has received much attention as a promising, sustainable and replacement to the molten salt electrolysis. Water additives have been recently suggested as a strategy to overcome the limitations of the ionic liquid process, but without complete assessment of its overall viability for the REMs production. In this regard, a full investigation of water impact on dysprosium (Dy) electrodeposition in pyrrolidinium triflate (BMPyOTf) ionic liquid was carried out. Water introduction was revealed to involve an interplay of implications on the electrodeposition process, including coordination, speciation, reduction pathways, interfacial dynamics, nucleation and metal stability and purity. In highly dry conditions, the reduction occurs at very negative potentials (-3.3V) in a consecutive pathway, resulting in negligible metal electrodeposition (low rate and efficiency) at the electrode surface. Initial water introduction (<30% molar fraction) leads to partitioning of the Dy complex between water and IL coordinated speciation, giving rise to an additional wave at more positive potentials (-2.4V). Probing and quantification of the heterogeneous Dy speciation by spectroscopic analyses enabled the uncovering of the reduction mechanism and accurate evaluation of the mass transport properties. While lowering the reduction thermodynamics, water addition also improved the nucleation, deposition rate and faradaic efficiency. In spite of these benefits, stripping voltammetric analysis also indicated substantial chemical reactivity of the deposited Dy metal with water additives and/or electrolyte components, under extended timescales. Surface characterization of the obtained product confirmed limited Dy metal stability (mostly oxidized/fluorinated) and purity (~60%) in the studied system. Moreover, high water introduction (>30% molar fraction) was shown to trigger a fast hydrogen evolution reaction (HER), downgrading the robustness of system efficiency. We report the overall impact of water additives seems to engender both promoting and mitigating effects on the Dy electrodeposition process, making the case for more innovative strategies to be sought.

36 MATERIALS SCIENCE↗

DOE OSTI2020

In Situ Phosphatizing of Triphenylphosphine Encapsulated within Metal–Organic Frameworks to Design Atomic Co 1 –P 1 N 3 Interfacial Structure for Promoting Catalytic Performance

The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal–N x (MN x ) structure, the replacement of N atoms by some other atoms with relatively weak electronegativity has been regarded as a promising strategy for optimizing the coordination environment of an active metal center and promoting its catalytic performance, which is still a challenge. Herein, we proposed a new synthetic strategy of an in situ phosphatizing of triphenylphosphine encapsulated within metal–organic frameworks for designing atomic Co 1 –P 1 N 3 interfacial structure, where a cobalt single atom is costabilized by one P atom and three N atoms (denoted as Co-SA/P-in situ). In the acidic media, the Co-SA/P-in situ catalyst with Co 1 –P 1 N 3 interfacial structure exhibits excellent activity and durability for the hydrogen evolution reaction (HER) with a low overpotential of 98 mV at 10 mA cm –2 and a small Tafel slope of 47 mV dec –1 , which are greatly superior to those of catalyst with Co 1 –N 4 interfacial structure. We discover that the bond-length-extended high-valence Co 1 –P 1 N 3 atomic interface structure plays a crucial role in boosting the HER performance, which is supported by in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculation. With this work, we hope to promote the development of high performance metal single-atom catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗