Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Guidelines”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Flexible MOFs as Pressure-Tunable Filters for Hydrocarbon Separation

Metal–organic frameworks (MOFs) hold significant promise for separating gas mixtures, especially hydrocarbons. While the main focus in the field is the development of new adsorbents for specific separations of binary mixtures, a preferable situation would be using a single framework for separating various species. This has been achieved in some flexible MOFs, where ternary mixtures can be separated at different temperatures. Here, we propose a simple yet fascinating way of utilizing the flexibility of MOFs to create tunable filters as a function of external pressure. We thus replace the more costly temperature-driven tunability with a cost-efficient external-pressure tunability. As a proof-of-concept, we select the CaMOF flexible framework for C6 hydrocarbon separation, however, our results are applicable to other flexible framework and gas molecules. Furthermore, our findings provide mechanistic insight and guidelines to engineer separation filters by designing flexible pores with critical sizes that can be effectively manipulated by external pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Vacancy-Dependent Diffusion Mechanism in Oxygen-Defective SrFeO 3 Perovskite Materials: First-Principles Density Functional Theory and Experimental Approach

Understanding oxygen diffusion at the atomic scale in SrFeO 3−δ perovskites is crucial for developing oxygen storage materials with optimal performance. Such materials are required to have high stability, corrosion resistance, and acceptable oxygen storage capacity at moderate operating temperatures and pressures. Here, in this study, we used first-principles density functional theory and thermogravimetric analysis to study the vacancy-dependent oxygen diffusion in oxygen-deficient SrFeO 3−δ (δ = 0, 0.065, 0.125, 0.25, 0.5) perovskites. The electronic structures, including the partial- and spin-resolved density of states, for different SrFeO 3−δ phases were calculated and compared with available experimental and theoretical results. By mapping the migration pathways, we investigated diffusion mechanisms and calculated the energy barriers for oxygen diffusion in cubic, orthorhombic, and brownmillerite phases of SrFeO 3−δ perovskites. Using the calculated energy barriers, we deduced the diffusion time scales and diffusion coefficients within SrFeO 3−δ . A diffusion coefficient on the order of 10 –8 m 2 /s was obtained for SrFeO 2.875 . We experimentally investigated the roles of temperature and oxygen partial pressures on the redox kinetics and deduced the kinetics rate and diffusion density, which agreed well with the calculated values for the density of diffusing oxygen vacancy in the lattice. Our results showed that the energy barrier tends to reduce at higher oxygen concentrations. Our results serve as an important guideline for designing oxygen storage materials with optimal redox kinetics.

chemical looping with oxygen uncoupling (CLOU)↗
DOE OSTI2025

Dial It Down: The Effect of Strongly Interacting Adsorbates on the BiAg 2 Rashba Surface State

Organic semiconductors interfaced with spin–orbit coupled materials offer a rich playground for fundamental studies of controlling spin dynamics in spintronic devices. The adsorbate–surface interactions at such interfaces play a key role in determining the valence electronic and spin structure and consequently, the device physics as well. Here we show that strong adsorbate–surface alloy interaction leads to weakening of the electronic coupling between the surface alloy atoms and quenches the spin–orbit coupled surface state, demonstrated for the case of the strong organic electron acceptor 2,7-dinitropyrene-4,5,9,10-tetrone (NO 2 –PyT, C 16 H 4 N 2 O 8 ) on the Rashba spin–orbit coupled surface alloy BiAg 2 /Ag(111). Furthermore, our findings demonstrate an important challenge associated with using molecular adsorbates to tailor the spin texture in BiAg 2 /Ag(111), and our work provides guidelines to consider while designing interfacial systems to engineer the spin texture in Rashba surface alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Structural Origins of High MoO 3 Solubility in Peraluminous Borosilicate Glasses

Molybdenum (Mo) imposes strict loading limits in conventional borosilicate nuclear waste glasses due to the tendency of tetrahedral molybdate [MoO 4 ] 2− species to phase-separate and crystallize as alkali molybdates. Here, we demonstrate an unprecedented 13.96 wt % (7.51 mol %) MoO 3 solubility in peraluminous sodium aluminoborosilicate glasses a ∼15× increase over their peralkaline counterparts. Using Raman spectroscopy, multinuclear and dipolarcorrelation magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), and scanning transmission electron microscopy (STEM)-energy dispersive spectroscopy (EDS), we reveal that Nadeficient, low optical basicity conditions stabilize octahedral MoO 6 units, which polymerize into molybdite-like Mo−O clusters dispersed within the glass matrix. These Mo-rich clusters suppress the formation of depolymerized [MoO 4 ] 2− environments typically responsible for Na 2 MoO 4 precipitation and instead promote the formation of Na 2 Mo 2 O 7 as the saturation phase. Concurrently, Mo solubility drives the conversion of AlO 4 − to higher-coordination AlO 5 species, liberating Na + that is subsequently sequestered in molybdate-rich domains. The combined evolution of Mo coordination, modifier redistribution, and network depolymerization provides a mechanistic basis for the markedly enhanced Mo solubility in peraluminous compositions. These findings establish new structural guidelines for designing aluminoborosilicate waste forms with substantially greater capacity to incorporate Mo-rich nuclear waste streams.

Amorphous materials↗
DOE OSTI2026

When Do Band Gap Calculations Agree with Experiments in Monolayer-Protected Cu 14 and Au 20 Atomically Precise Nanoclusters? A (TD)-DFT Comparison of HOMO–LUMO, Fundamental, Optical, and Electrochemical Energy Gaps

In view of the tremendous progress in atomically precise metal nanoclusters where electrochemical and optical energetics are routinely supported by computations to establish structure−function correlations, we explore the relationship between different protocols for measuring and computing band gaps of two distinct organic ligand-protected nanoclusters: [Cu 14 H 10 (MBN) 3 (PPh 3 ) 8 ] + and Au 20 (TBBT) 16 . Through UV/visible spectroscopy and differential pulse voltammetry, we measure optical and electrochemical band gaps in those systems. We then compare these experimentally determined gaps to HOMO−LUMO gaps, fundamental gaps, vertical excitation energies, and E o ox − E o red potentials computed using different density functional theory (DFT) or time-dependent DFT (TDDFT) methods. Specifically, in both copper and gold nanoclusters, we test the effect of truncating inert ligands from the model and compare density functionals with varying degrees of Hartree−Fock (HF) exchange from 0 to 50%, range-separated hybrids with a varying long-range tuning parameter, different correlation functionals, basis sets, and (equilibrium and nonequilibrium) continuum solvation models. Despite having different frontier orbital characters (the copper nanocluster has a metal-to-ligand charge transfer character while the gold nanocluster has metal-centered frontier orbitals), both nanoclusters display a similar sensitivity of the HOMO−LUMO gap to the HF exchange that is partially mitigated when computing the fundamental, optical, and electrochemical gaps. Other factors, such as the nature of the correlation functional, basis set, and geometry relaxation, have a considerably smaller effect on computed band gaps in these systems. Overall, this work provides guidelines for factors of varied importance for correlating computed and experimental band gap values.

Chemical calculations↗
DOE OSTI2021

Revealing Au13 as Elementary Clusters During the Early Formation of Au Nanocrystals

Understanding the formation mechanism of nanocrystals in solution is very fundamental to the development of materials science. For a metal nanocrystal, the cluster mediated formation mechanism is still poorly understood. Particularly, identifying what types of clusters are dominant and how they evolve into a nanocrystal in the early nucleation stage remains a great challenge. Here, using liquid-cell transmission electron microscopy, we directly observe the formation of ultra-small Au clusters (~0.84 nm) in the presence of PAA-Na. These clusters, which correspond to the size of Au13 cluster, coalesce to form nanocrystals. Our molecular dynamics simulations suggest Au13 in an aqueous environment have higher stability when compared to other cluster sizes and provide atomistic details of growth by cluster coalescence. Collectively, our demonstration of Au13 as the dominant species with elaboration of their coalescence kinetics sheds light on nonclassical nanocrystal formation mechanisms and offers useful guidelines for designing innovative pathways for synthesis of nanomaterials.

Au13 clusters, cluster coalescence, liquid-cell TE↗
DOE OSTI2025

Formation of Larger Solvation Shells in a LiFSI Salt Solution for Enhanced Li + Transport

Electrodes and electrolytes in Li-ion batteries (LIBs) are crucial for their electrochemical performance, and fabricating these materials to achieve the desired properties for improved efficiency is challenging. A recent study published in Nature ( Nature 2024, 627, 101−107) outlined guidelines for designing an electrolyte using fluoroacetonitrile (FACN), a small-molecule solvent with low solvation energy. This characteristic enables the formation of an anion-rich inorganic interphase in a 1.3 M lithium bis(fluorosulfonyl)imide (LiFSI) solution, facilitating higher ionic conductivity, even at low temperatures. The formation of a ligand channel was postulated and demonstrated in the simulation due to the attraction of small solvent molecules from the secondary solvation shell to the Li + ion in the primary solvation shell, thereby enhancing Li + transport. While that study primarily focused on Li + ion behavior in LIBs, the behavior of FACN in solutions with different lithium salts and what makes LiFSI in FACN outperform other systems remain to be explored. In conclusion, using an experimental technique sensitive to FACN molecules, here we directly observe formation of larger solvation shells in a LiFSI solution compared to the Li salts with different anions, consistent with the postulated ligand-channel-mechanism and potentially paving the way to achieving enhanced ionic conductivity and energy density during LIB operation.

Batteries↗
DOE OSTI2024

The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins

Closed-loop recycling via an efficient chemical process can help alleviate the global plastic waste crisis. However, conventional depolymerization methods for polyolefins, which compose more than 50% of plastics, demand high temperatures and pressures, employ precious noble metals, and yield complex mixtures of products limited to single-use fuels or oils. Superacidic sulfated zirconia (SZrO) with Hammet Acidity Functions (H 0 ) = -12 (i.e., stronger than 100% H 2 SO 4 ) is an industrially deployed heterogeneous catalyst capable of activating hydrocarbons under mild conditions and is shown to decompose polyolefins at ambient pressure and temperatures near 200 °C. Additionally, confinement of active sites in porous supports is known to radically increase selectivity, coking and sintering resistance, and acid site activity, presenting a possible approach to low-energy polyolefin depolymerization. However, a critical examination of the literature on SZrO led us to a surprising conclusion: despite 40 years of catalytic study, engineering, and industrial use, the surface chemistry of SZrO is poorly understood. Ostensibly spurred by SZrO's impressive catalytic activity, the application-driven study of SZrO has resulted in deleterious ambiguity in requisite synthetic conditions and insufficient characterization of acidity, porosity, and active site structure. Here, this ambiguity has produced significant knowledge gaps surrounding the synthesis, structure, and mechanisms of hydrocarbon activation for optimized SZrO, stunting the potential of this catalyst in olefin cracking and other industrially relevant reactions, such as isomerization, esterification, and alkylation. Toward mitigating these long extant issues, we herein identify and highlight these current shortcomings and knowledge gaps, propose explicit guidelines for characterization of and reporting on characterization of solid acidity, and discuss the potential of pore-confined superacids in the efficient and selective depolymerization of polyolefins.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Self-Assembled Bolaamphiphile-Based Organic Nanotubes as Efficient Cu(II) Ion Adsorbents

Self-assembled organic nanotubes (ONTs) have been actively examined for various applications such as chemical separations and catalysis owing to their well-defined tubular nanostructures with distinct chemical environments at the wall and internal/external surfaces. Adsorption of heavy metal ions onto ONTs plays an essential role in many of these applications, but it has rarely been assessed quantitatively. Herein, we investigated interactions between Cu 2+ and single-/quadruple-wall bolaamphiphile-based ONTs having inner carboxyl groups with different inner diameters, COOH-ONT 10nm and COOH-ONT 20nm . We first examined the effects of Cu 2+ on their nanotubular structures using SAXS, STEM, and AFM. COOH-ONT 10nm was stable in aqueous Cu 2+ solution in contrast to COOH-ONT 20nm owing to the presence of polyglycine-II-type hydrogen bonding networks within its wall. Subsequently, we studied the Cu 2+ adsorption behavior of COOH-ONT 10nm by monitoring the concentration of unbound Cu 2+ using linear sweep anodic stripping voltammetry. The Cu 2+ adsorption was quick, attributable to efficient Cu 2+ partitioning through the open ends of the ONT, followed by fast Cu 2+ diffusion in the uniform, relatively large nanochannel. More importantly, the Cu 2+ adsorption capacity and affinity of COOH-ONT 10nm were measured at different pH using the Langmuir adsorption model. The adsorption capacity was similar at the pH range examined, showing the participation of approximately 25% of the inner carboxyl groups in the adsorption. The adsorption affinity increased with pH, indicating the essential role of the deprotonated carboxyl groups in the Cu 2+ adsorption. Most interestingly, the Langmuir adsorption constant was significantly higher than those of previously reported synthetic adsorbents and planar monolayer based on carboxyl binding sites. The high Cu 2+ affinity of the ONT was attributable to the highly dense binding sites on the well-defined nanoscale concave structure of the inner channel. Furthermore, these results provide a valuable guideline to designing self-assembled nanomaterials for efficient chemical separations, detection, and catalysis.

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

Leveraging Sequential Doping of Semiconducting Polymers to Enable Functionally Graded Materials for Organic Thermoelectrics

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric devices for thermal energy management. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material. While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. Herein, we introduce how molecular doping of semiconducting polymers enables spatial compositional control of thin-film FGMs. Specifically, we utilize sequential vapor doping of poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) to fabricate the simplest form of FGMs – double- segmented thin films. The two thin-film segments are of equal length (7.5 mm), but each set to different doping levels. Our study focuses on understanding the thermoelectric properties (Seebeck coefficient, α, and electronic conductivity, σ) and structural properties (through X-ray scattering, UV-vis-NIR spectroscopy, and Raman spectroscopy) within and across the two segments. We observe the presence of a small diffuse interfacial region of 0.5 – 1 mm between the two segments where the doping level and transport properties vary continuously. Despite the diffuse interface, the measured effective Seebeck coefficient (αeff) across the two segments is simply the average of α within each segment. Importantly, this experimental result is consistent with reported mathematical models describing αeff in graded thermoelectric materials. Finally, our results demonstrate the facile fabrication and characterization of functionally graded organic thermoelectric materials, providing guidelines for further development on more complex FGMs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hot-Carrier Extraction in Nanowire-Nanoantenna Photovoltaic Devices

Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley–Queisser limit was demonstrated in hot-carrier devices based on InAs–InP–InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this study, we combine plasmonic nanoantennas with InAs–InP–InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon-energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of nonthermalized hot electrons when the photoexcited electron energy is above the barrier and by photothermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5–1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures.

14 SOLAR ENERGY↗
DOE OSTI2020

Improving Interface Stability of Si Anodes by Mg Coating in Li-Ion Batteries

Silicon (Si) is a promising anode material for high-energy-density lithium-ion batteries (LIBs), but its short calendar life and poor cycling performance prevent its large-scale adoption. Introducing magnesium (Mg) salt into the electrolyte has been recently shown to form a ternary Li–Mg–Si Zintl phase upon lithiation of Si and improve the cycling performance. However, the ternary Zintl phase formation mechanism and its impact on the solid electrolyte interphase (SEI) are not yet well understood. In this work, we demonstrate the formation of a ternary Li–Mg–Si Zintl phase by Mg coating of the Si anode, where Mg diffuses into the Si film upon deposition and intermixes further during the lithiation process. The presence of the Zintl phase improves the interface stability, alters the nature of the SEI, and enhances the cycling performance of the Si anode. This study provides insights into the formation mechanism of the ternary Zintl phase and guidelines for the future design of Si anodes.

25 ENERGY STORAGE↗
DOE OSTI2024

First-Principles Study on the Role of Cu and Cl-Based Dopants in NiO

Utilization of wide band gap oxide-based materials in thin-film solar energy technologies has increased in recent years. Among the numerous candidate oxide materials, NiO has shown many desirable optoelectronic properties that are applicable to thin-film PV technologies such as cadmium telluride PV. However, one critical factor requiring further investigation is the p-type doping behavior of NiO, specifically when the cadmium telluride solar cell undergoes conventional processes such as copper doping and chlorine-based activation treatment. The previous literature has shown a large degree of variability in hole concentrations in NiO when copper is used as the primary dopant. This study uses first-principles computational modeling based on density functional theory coupled with defect equilibria calculations to quantitatively explore the role of copper and chlorine-based extrinsic dopants in the p-type doping activity of NiO. The study reveals the importance of extrinsic dopants and their binding interactions with nickel vacancies to effectively p-dope NiO. It is suggested that both the formation of V Ni + Cu Ni and V Ni + Cl O defect pairs under a supersaturated state of NiO are potential mechanisms for increasing hole densities. On the other hand, the production of 2Cu Ni severely limits the effectiveness of p-doping in NiO, even in the presence of the aforementioned defect pairs. Furthermore, the study provides a guideline for experimentalists interested in using copper or chlorine species to understand how to controllably p-dope NiO during thin-film synthesis.

chlorine↗
DOE OSTI2020

Optimizing Ammonia Separation via Reactive Absorption for Sustainable Ammonia Synthesis

Metal halide salts such as magnesium chloride have been shown to be promising candidates for ammonia storage materials for energy storage and agriculture applications due to their ability to incorporate several moles of ammonia per mole of salt. Ammonia exiting a synthesis reactor can be separated from nitrogen and hydrogen by absorption into magnesium chloride. Such an absorption can be more complete and hotter than separation via ammonia condensation, the current standard in the Haber–Bosch process. Here, we discuss the optimal conditions for the cyclic uptake and release of ammonia from the supported magnesium chloride absorbents. An automated system was designed for measuring the nonequilibrium working capacity of the absorbent, as well as the impact of important operating conditions such as absorption and desorption temperature, pressure, and desorption time. Measurements of absorption and desorption kinetics provide insight into the mechanisms involved. The temperatures and pressures during absorption and desorption were designed to use minimal energy input to maximize the uptake and release of ammonia within a reasonable amount of time. In a laboratory-scale bed, absorption has a small unused bed length, so it is largely independent of mass transfer; it is dominated by how fast ammonia is fed to the bed. Yet, desorption is restricted both by the speed of heating the bed and by diffusion out of the absorbent. These measurements provide guidelines for ammonia separations and cycling sorbent materials on a larger scale.

10 SYNTHETIC FUELS↗
DOE OSTI2020

Hydrogen-Bonding Interactions in Hybrid Aqueous/Nonaqueous Electrolytes Enable Low-Cost and Long-Lifespan Sodium-Ion Storage

Although “water-in-salt” electrolytes have opened a new pathway to expand the electrochemical stability window of aqueous electrolytes, the electrode instability and irreversible proton co-insertion caused by aqueous media still hinder the practical application, even when using exotic fluorinated salts. In this study, an accessible hybrid electrolyte class based on common sodium salts is proposed, and crucially an ethanol-rich media is introduced to achieve highly stable Na-ion electrochemistry. Here, ethanol exerts a strong hydrogen-bonding effect on water, simultaneously expanding the electrochemical stability window of the hybridized electrolyte to 2.5 V, restricting degradation activities, reducing transition metal dissolution from the cathode material, and improving electrolyte–electrode wettability. The binary ethanol–water solvent enables the impressive cycling of sodium-ion batteries based on perchlorate, chloride, and acetate electrolyte salts. Notably, a Na 0.44 MnO 2 electrode exhibits both high capacity (81 mAh g –1 ) and a remarkably long cycle life >1000 cycles at 100 mA g –1 (a capacity decay rate per cycle of 0.024%) in a 1 M sodium acetate system. The Na 0.44 MnO 2 /Zn full cells also show excellent cycling stability and rate capability in a wide temperature range. The gained understanding of the hydrogen-bonding interactions in the hybridized electrolyte can provide new battery chemistry guidelines in designing promising candidates for developing low-cost and long-lifespan batteries based on other (Li + , K + , Zn 2+ , Mg 2+ , and Al 3+ ) systems.

25 ENERGY STORAGE↗
DOE OSTI2022

Understanding the Role of SEI Layer in Low-Temperature Performance of Lithium-Ion Batteries

Low-temperature electrolytes (LTEs) have been considered as one of the most challenging aspects for the wide adoption of lithium-ion batteries (LIBs) since the SOA electrolytes cannot sufficiently support the redox reactions at LT resulting in dramatic performance degradation. Although many attempts have been taken by employing various noncarbonate solvent electrolytes, there was a lack of fundamental understanding of the limiting factors for low-temperature operations (e.g., -20 to -40 degrees C). Here, the crucial role of the solid-electrolyte-interface (SEI) in LIB performance at low temperature using a butyronitrile (BN)-based electrolyte was demonstrated. These results suggested that an additive formed SEI with low resistance and low charge transfer dictates the LT performance in terms of capacity and cycle life, presenting a useful guideline in designing new electrolytes to address the LT issue.

25 ENERGY STORAGE↗
DOE OSTI2023

Heuristics for Molten-Salt Synthesis of Single-Crystalline Ultrahigh-Nickel Layered Oxide Cathodes

In pursuit of Li-ion batteries with higher energy density, ultrahigh-nickel layered oxides are a leading candidate for next-generation cathode materials. Single-crystalline morphology offers a neat solution to the poor stability of ultrahigh-Ni cathodes; a lower active surface area mitigates electrolyte decomposition at high voltages, and the elimination of grain boundaries improves mechanical resilience and increases volumetric energy density. However, single-crystal cathodes possess their own challenges, several of which originate from synthesis at elevated temperatures meant to induce grain growth. Molten-salt synthesis is an alternative method for obtaining single crystals, accelerating grain growth through the presence of a molten flux without the need for increased temperature. Herein, we offer heuristic guidelines for molten-salt synthesis, discussing key factors for designing reaction mixtures and the necessary exploratory research for novel molten salt/cathode systems. Here, the influence of different salts and synthesis conditions on the morphology and properties of single-crystal LiNiO 2 is presented. It is found that oxidative salts, such as Li 2 O 2 and LiNO 3 , are crucial to supplementing dissolution of gaseous oxygen into the molten phase. Through these discussions, this work aims to provide a set of overarching principles for obtaining higher-quality single-crystal layered oxide cathodes and engender more rigorous and impactful investigation into their fundamental nature and applications.

25 ENERGY STORAGE↗
DOE OSTI2024

Insights into the Electrochemical Oxidation and Reduction of Nickel Oxide Surfaces

Surface oxidation/reduction processes, driven by varying electrochemical potentials, can substantially impact catalyst effectiveness and, consequently, electrolyzer performance. Here, this study combines theoretical and experimental approaches to explore the surface redox behavior of nickel oxides, which are cost-effective and efficient catalysts for many electrochemical reactions. Surface Pourbaix diagrams for three different phases of nickel oxides, i.e., nickel hydroxide (Ni(OH) 2 ), nickel oxyhydroxide (NiOOH), and nickel dioxide (NiO 2 ), were constructed using density functional theory-based simulations. Various experimental methods, including cyclic voltammetry, in situ Raman spectroscopy, and electrochemical titration, were employed to probe the surface redox processes of nickel oxide thin films. Our findings indicate that the ABAB stacking sequence of Ni(OH) 2 lacks stability under oxidizing conditions to host the surface oxidation (deprotonation) events, while the AABBCC stacking sequence of NiOOH is energetically favorable due to the presence of interlayer hydrogen bonding. Rapid charge transfer facilitated by interlayer hydrogen bonding accounts for the higher reactivity of partially oxidized/reduced NiOOH (001) surfaces compared to Ni(OH) 2 (001) and NiO 2 (001) surfaces with the same stoichiometry, where interlayer hydrogen bonding is absent. Insights presented in this work can offer guidelines for optimizing operational conditions and tailoring the surface structures and oxidation states of nickel oxides to enhance performance in applications such as electrocatalysis and supercapacitors.

36 MATERIALS SCIENCE↗