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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Enabling Highly Stable Li–O 2 Batteries with Full Discharge–Charge Capability: The Porous Binder- and Carbon-Free IrNi Nanosheet Cathode

Rechargeable aprotic Li-O 2 batteries have drawn much attention in light of their ultrahigh theoretical energy density. However, the sluggish kinetics of oxygen cathode reactions lead to huge energy loss, while the unfavorable side reactions associated with the widely used carbon and binder account for fast battery decay during cycles. In this work, hierarchically holey IrNi alloy nanosheets are facilely designed on Ni foam substrate as a freestanding, binder- and carbon-free cathode to address these challenges. The IrNi nanosheets consist of numerous interconnected nanoparticles, forming abundant nanoholes, benefiting active-site exposure. Moreover, the meso-/macropores pave channels for the transfer of oxygen and lithium ions and provide enough accommodation for the insoluble Li 2 O 2 product. The corresponding Li-O 2 batteries demonstrate low polarization and excellent cycle stability with full discharge/charge capability (up to 292 cycles at a current density of 0.2 mA cm -2 within a voltage range from 2.3 to 4.3 V). The long-term cycling performance is principally attributed to the exclusive catalytic stability of the holey IrNi nanosheets and its good compatibility with dimethyl sulfoxide-based electrolyte. Our research provides novel insights into the design of robust and freestanding porous cathodes for high energy density Li-O 2 batteries.

25 ENERGY STORAGE↗
DOE OSTI2022

Early Failure of Lithium–Sulfur Batteries at Practical Conditions: Crosstalk between Sulfur Cathode and Lithium Anode

Lithium–sulfur (Li–S) batteries are one of the most promising next-generation energy storage technologies due to their high theoretical energy and low cost. However, Li–S cells with practically high energy still suffer from a very limited cycle life with reasons which remain unclear. Here, through cell study under practical conditions, it is proved that an internal short circuit (ISC) is a root cause of early cell failure and is ascribed to the crosstalk between the S cathode and Li anode. The cathode topography affects S reactions through influencing the local resistance and electrolyte distribution, particularly under lean electrolyte conditions. The inhomogeneous reactions of S cathodes are easily mirrored by the Li anodes, resulting in exaggerated localized Li plating/stripping, Li filament formation, and eventually cell ISC. Manipulating cathode topography is proven effective to extend the cell cycle life under practical conditions. The findings of this work shed new light on the electrode design for extending cycle life of high-energy Li–S cells, which are also applicable for other rechargeable Li or metal batteries.

25 ENERGY STORAGE↗
NASA NTRS2010

Fuel-Cell-Powered Vehicle with Hybrid Power Management

Figure 1 depicts a hybrid electric utility vehicle that is powered by hydrogenburning proton-exchange-membrane (PEM) fuel cells operating in conjunction with a metal hydride hydrogen-storage unit. Unlike conventional hybrid electric vehicles, this vehicle utilizes ultracapacitors, rather than batteries, for storing electric energy. This vehicle is a product of continuing efforts to develop the technological discipline known as hybrid power management (HPM), which is oriented toward integration of diverse electric energy-generating, energy-storing, and energy- consuming devices in optimal configurations. Instances of HPM were reported in five prior NASA Tech Briefs articles, though not explicitly labeled as HPM in the first three articles: "Ultracapacitors Store Energy in a Hybrid Electric Vehicle" (LEW-16876), Vol. 24, No. 4 (April 2000), page 63; "Photovoltaic Power Station With Ultracapacitors for Storage" (LEW- 17177), Vol. 27, No. 8 (August 2003), page 38; "Flasher Powered by Photovoltaic Cells and Ultracapacitors" (LEW-17246), Vol. 27, No. 10 (October 2003), page 37; "Hybrid Power Management" (LEW-17520), Vol. 29, No. 12 (December 2005), page 35; and "Ultracapacitor-Powered Cordless Drill" (LEW-18116-1), Vol. 31, No. 8 (August 2007), page 34. To recapitulate from the cited prior articles: The use of ultracapacitors as energy- storage devices lies at the heart of HPM. An ultracapacitor is an electrochemical energy-storage device, but unlike in a conventional rechargeable electrochemical cell or battery, chemical reactions do not take place during operation. Instead, energy is stored electrostatically at an electrode/electrolyte interface. The capacitance per unit volume of an ultracapacitor is much greater than that of a conventional capacitor because its electrodes have much greater surface area per unit volume and the separation between the electrodes is much smaller.

Eichenberg, Dennis J.↗
DOE OSTI2023

Hierarchical Cellulose Superinsulation Membrane

The environment-friendly components coupled with the ability to mimic the simplicity and originality of nature necessitate advanced sustainable materials with structural capabilities for energy-efficient applications. The use of feedstock deriving from plant-based, renewable organic material to produce nanofibril that embodies enhanced insulating properties and high mechanical strength constitutes an efficient development strategy. In this work, a free-standing, hierarchical superinsulation membrane by leveraging the principle of the bottom-up method is reported. The electrospun cellulose nanofibrils/aerogel-based core layer provides exceptional thermal properties with its thermal conductivity of 10.2 mW m –1 K –1 . The lightweight, flexible, and durable paper-like membrane features a tensile strength of 11.3 MPa and a bending rigidity in the order of 4.6 cN mm –1 . The hydrophobic superinsulation membrane material also exhibits a ΔT of ≈25 °C under continuous sunlight illumination and allows thermal runaway mitigation of rechargeable lithium-ion batteries. All the aforementioned properties position this hybrid superinsulation membrane as a promising material for energy-saving thermal management applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tuning the Kinetics of Zinc-Ion Insertion/Extraction in V 2 O 5 by In Situ Polyaniline Intercalation Enables Improved Aqueous Zinc-Ion Storage Performance

Rechargeable zinc-ion batteries (ZIBs) are emerging as a promising alternative for Li-ion batteries. However, the developed cathodes suffer from sluggish Zn 2+ diffusion kinetics, leading to poor rate capability and inadequate cycle life. Herein, an in situ polyaniline (PANI) intercalation strategy is developed to facilitate the Zn 2+ (de)intercalation kinetics in V 2 O 5 . In this way, a remarkably enlarged interlayer distance (13.90 Å) can be constructed alternatively between the V-O layers, offering expediting channels for facile Zn 2+ diffusion. Importantly, the electrostatic interactions between the Zn 2+ and the host O 2- , which is another key factor in hindering the Zn 2+ diffusion kinetics, can be effectively blocked by the unique π-conjugated structure of PANI. As a result, the PANI-intercalated V 2 O 5 exhibits a stable and highly reversible electrochemical reaction during repetitive Zn 2+ insertion and extraction, as demonstrated by in situ synchrotron X-ray diffraction and Raman studies. Further first-principles calculations clearly reveal a remarkably lowered binding energy between Zn 2+ and host O 2- , which explains the favorable kinetics in PANI-intercalated V 2 O 5 . Benefitting from the above, the overall electrochemical performance of PANI-intercalated V 2 O 5 electrode is remarkable improved, exhibiting excellent high rate capability of 197.1 mAh g -1 at current density of 20 A g -1 with capacity retention of 97.6% over 2000 cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Transferring Liquid Metal to form a Hybrid Solid Electrolyte via a Wettability-Tuning Technology for Lithium-Metal Anodes

Integrating solid-state electrolyte (SSE) into Li-metal anodes has demonstrated great promise to unleash the high energy density of rechargeable Li-metal batteries. However, fabricating a highly cyclable SSE/Li-metal anode remains a major challenge because the densification of the SSE is usually incompatible with the reactive Li metal. Here, a liquid-metal-derived hybrid solid electrolyte (HSE) is proposed, and a facile transfer technology to construct an artificial HSE on the Li metal is reported. By tuning the wettability of the transfer substrates, electron- and ion-conductive liquid metal is sandwiched between electron-insulating and ion-conductive LiF and oxides to form the HSE. The transfer technology renders the HSE continuous, dense, and uniform. The HSE, having high ion transport, electron shut-off, and mechanical strength, makes the composite anode deliver excellent cyclability for over 4000 h at 0.5 mA cm(-2) and 1 mAh cm(-2) in a symmetrical cell. When pairing with LiFePO4 and sulfur cathodes, the HSE-coated Li metal dramatically enhances the performance of full cells. Therefore, this work demonstrates that tuning the interfacial wetting properties provides an alternate approach to build a robust solid electrolyte, which enables highly efficient Li-metal anodes.

li-metal anodes↗
DOE OSTI2022

Suppressing Chemical Corrosions of Lithium Metal Anodes

In this study, the lithium metal anode is essential for next-generation high-energy-density rechargeable Li-metal batteries. Although extensive studies have been performed to prolong the cycle life of Li-metal batteries, the calendar life, which associates with the chemical corrosion of Li metal in liquid electrolytes, has not been quantitatively understood. Here, by combing the titration gas chromatography method and cryogenic focused ion beam, a quantitative relationship between the chemical corrosion rate and electrochemically deposited Li morphology in various liquid electrolyte systems is established. It has been identified that the corrosion rate is dominated by the porosity of the deposited Li. The larger the porosity of deposited Li has, the faster the corrosion rate will be. Strategies to mitigate the chemical corrosion on Li thus to extend the calendar life of Li-metal batteries are further proposed. By strictly controlling the stacking pressure during Li plating, Li deposits with ultra-low porosity can be achieved, suppressing the corrosion rate to 0.08 ± 0.16%/day compared with 1.71 ± 0.19%/day of the high-porosity Li.

25 ENERGY STORAGE↗
DOE OSTI2025

Enhancing Cycle Life in Superoxide‐Based Na–O 2 Batteries by Reducing Interface Reactivity

Abstract Sodium–oxygen (Na–O 2 ) batteries are considered a promising energy storage alternative to current state‐of‐the‐art technologies owing to their high theoretical energy density, along with the natural abundance and low price of Na metal. The chemistry of these batteries depends on sodium superoxide (NaO 2 ) or peroxide (Na 2 O 2 ) being formed/decomposed. Most Na–O 2 batteries form NaO 2 , but reversibility is usually quite limited due to side reactions at interfaces. By using new materials, including a highly active catalyst based on vanadium phosphide (VP) nanoparticles, an ether/ionic liquid‐based electrolyte, and an effective sodium bromide (NaBr) anode protection layer, the sources of interface reactivity can be reduced to achieve a Na–O 2 battery cell that is rechargeable for 1070 cycles with a high energy efficiency of more than 83%. Density functional theory calculations, along with experimental characterization confirm the three factors leading to the long cycle life, including the effectiveness of the NaBr protective layer on the anode, a tetraglyme/EMIM‐BF 4 based electrolyte that prevents oxidation of the VP cathode catalyst surface, and the EMIM‐BF 4 ionic liquid aiding in avoiding electrolyte decomposition on NaO 2 .

Azaribeni, Adel [Department of Chemical and Biolog↗
DOE OSTI2021

Unraveling the formation mechanism of NaCoPO 4 polymorphs

Phosphate compounds are one of the attractive classes of cathodes for their potential application in rechargeable Na-ion batteries (NIBs) due to their high insertion voltages and stabilities. Unlike olivine-LiMPO 4 (M = Fe, Mn, Co, Ni) cathodes, maricite-NaMPO 4 phases are electrochemically inactive due to lack of sodium diffusion channels. Interestingly, cobalt bearing compounds exhibit rich polymorphism (α-, β-, ABW- and Red-NaCoPO 4 ). However, the preparation of NaCoPO 4 single phases is challenging due to their closer enthalpies of formation. In this study, we have attempted to understand the formation mechanism of NaCoPO 4 polymorphs during solid state reaction using two different cobalt precursors (Co(OH) 2 and CoCO 3 ). The temperature dependent in-situ powder X-ray Diffraction (PXRD) studies showed the formation of β-NaCoPO 4 at a much lower temperature compared to the one reported in the literature, whilst we have discovered a new high temperature polymorph gamma-NaCoPO 4 . The choice of precursors was also found to be important for the preparation of ABW-NaCoPO 4 , i.e., the rapid decomposition of Co(OH) 2 has resulted in its formation whilst the high stability of CoCO 3 prohibited the same. The order of the formation of the NaCoPO 4 polymorphs during heating was found to be ABW→β→α →$\gamma$. During normal cooling, gamma-NaCoPO 4 transformed primarily into alpha-polymorph whilst quenching in liquid N-2 resulted in the formation of β-NaCoPO 4 The as-synthesized beta- and ABW-NaCoPO 4 phases exhibited high sodium (de)-intercalation voltages (similar to 4.2 and 4.5 V vs. Na + /Na 0 , respectively), albeit with limited reversible capacities. This study is expected to open new avenues to harvest metastable polymorphs based on other transition metals (Fe, Mn and Ni).

25 ENERGY STORAGE↗
DOE OSTI2021

Correlative analysis of structure and chemistry of Li x FePO 4 platelets using 4D-STEM and X-ray ptychography

Lithium iron phosphate (Li x FePO 4 ), a cathode material used in rechargeable Li-ion batteries, phase separates upon de/lithiation under equilibrium. The interfacial structure and chemistry within these cathode materials affects Li-ion transport, and therefore battery performance. Here, correlative imaging of Li x FePO 4 was performed using four-dimensional scanning transmission electron microscopy (4D-STEM), scanning transmission X-ray microscopy (STXM), and X-ray ptychography in order to analyze the local structure and chemistry of the same particle set. Over 50,000 diffraction patterns from 10 particles provided measurements of both structure and chemistry at a nanoscale spatial resolution (16.6–49.5 nm) over wide (several micron) fields-of-view with statistical robustness. Li x FePO 4 particles at varying stages of delithiation were measured to examine the evolution of structure and chemistry as a function of delithiation. In lithiated and delithiated particles, local variations were observed in the degree of lithiation even while local lattice structures remained comparatively constant, and calculation of linear coefficients of chemical expansion suggest pinning of the lattice structures in these populations. Partially delithiated particles displayed broadly core–shell-like structures, however, with highly variable behavior both locally and per individual particle that exhibited distinctive intermediate regions at the interface between phases, and pockets within the lithiated core that correspond to FePO 4 in structure and chemistry. The results provide insight into the Li x FePO 4 system, subtleties in the scope and applicability of Vegard’s law (linear lattice parameter-composition behavior) under local versus global measurements, and demonstrate a powerful new combination of experimental and analytical modalities for bridging the crucial gap between local and statistical characterization.

25 ENERGY STORAGE↗
DOE OSTI2020

Diffusion and migration in polymer electrolytes

Mixtures of neutral polymers and lithium salts have the potential to serve as electrolytes in next-generation rechargeable Li-ion batteries. The purpose of this review is to expose the delicate interplay between polymer-salt interactions at the segmental level and macroscopic ion transport at the battery level. Since complete characterization of this interplay has only been completed in one system: mixtures of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI), we focus on data obtained from this system. We begin with a discussion of the activity coefficient, followed by a discussion of six different diffusion coefficients: the Rouse motion of polymer segments is quantified by D seg , the self-diffusion of cations and anions is quantified by D self,+ and D self,- , and the build-up of concentration gradients in electrolytes under an applied potential is quantified by Stefan-Maxwell diffusion coefficients, D 0+ , D 0- , and D +- . The Stefan-Maxwell diffusion coefficients can be used to predict the velocities of the ions at very early times after an electric field is applied across the electrolyte. The surprising result is that D 0- is negative in certain concentration windows. A consequence of this finding is that at these concentrations, both cations and anions are predicted to migrate toward the positive electrode at early times. We describe the controversies that surround this result. Knowledge of the Stefan-Maxwell diffusion coefficients enable prediction of the limiting current. We argue that the limiting current is the most important characteristic of an electrolyte. Excellent agreement between theoretical and experimental limiting current is seen in PEO/LiTFSI mixtures. What sequence of monomers that, when polymerized, will lead to the highest limiting current remains an important unanswered question. It is our hope that the approach presented in this review will guide the development of such polymers.

25 ENERGY STORAGE↗
DOE OSTI2021

Modifying Li + and Anion Diffusivities in Polyacetal Electrolytes: A Pulsed-Field-Gradient NMR Study of Ion Self-Diffusion

Polyacetal electrolytes have been demonstrated as promising alternatives to liquid electrolytes and poly(ethylene oxide) (PEO) for rechargeable lithium-ion batteries; however, the relationship between polymer structure and ion motion is difficult to characterize. Here, we study structure-property trends in ion diffusion with respect to polymer composition for a systematic series of five polyacetals with varying ratios of ethylene oxide (EO) to methylene oxide (MO) units, denoted as P(xEO-yMO), and PEO. We first use 7 Li and 19 F pulsed-field-gradient NMR spectroscopy to measure cation and anion self-diffusion, respectively, in polymer/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt mixtures. At 90 °C, we observe modest changes in Li + diffusivity across all polymer compositions, while anion (TFSI - ) self-diffusion coefficients decrease significantly with increasing MO content. At a given reduced temperature (T - T g ), all polyacetal electrolytes exhibit faster Li + self-diffusion than PEO. Intriguingly, P(EO-MO) and P(EO-2MO) also show slower TFSI - anion self-diffusion than PEO at a given reduced temperature. Molecular dynamics simulations reveal that shorter distances between acetal oxygen atoms (O-CH 2 -O) compared to ether oxygens (O-CH 2 -CH 2 -O) promote more diverse, often asymmetric, Li + coordination environments. Finally, Raman spectra reveal that anion-rich ion clusters in P(EO-MO) and P(EO-2MO) lead to decreased anion diffusivity, which along with increased cation diffusivity, support the viability of polyacetals as high-performance polymer electrolytes.

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

Water-In-Glass: A Self-Supporting Inorganic Aqueous Electrolyte

Aqueous rechargeable sodium-ion batteries (ARNIBs) are emerging as cost-effective and safe candidates for large-scale energy storage applications. However, their advancement has been constrained by the narrow electrochemical stability window (ESW) of conventional aqueous electrolytes (1.23 V). Here, in this study, we present a transformative approach using an inexpensive and rapidly dissolvable inorganic glass material, water glass (W-glass), to significantly enhance the ESW and enable the development of solid-state, self-supporting aqueous film (SSA film) electrolytes. These SSA film electrolytes exhibit an extended ESW of up to 3.5 V and a conductivity of ∼10 –4 S/cm at room temperature. Structural analysis using magic-angle spinning nuclear magnetic resonance (NMR) and solution-state NMR reveals that the dissolution of W-glass in water is driven by the interdependent hydrolysis of P–O–P linkages and Na + –H + ion exchange. This work offers a cost-effective and scalable solution for advancing high-performance ARNIB technology, addressing critical barriers to commercial adoption.

Electrochemical stability window↗
DOE OSTI2020

Enhancing the Electrocatalysis of LiNi 0.5 Co 0.2 Mn 0.3 O 2 by Introducing Lithium Deficiency for Oxygen Evolution Reaction

LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), as a cathode material for rechargeable lithium-ion batteries, has attracted considerable attention and been successfully commercialized for decades. NCM is also a promising electrocatalyst for the oxygen evolution reaction (OER), and the catalytic activity is highly correlated to its structure. In this paper, we successfully obtain NCM523 with three different structures: spinel NCM synthesized at low temperature (LT-NCM), disordered NCM (DO-NCM) with lithium deficiency obtained at high temperature, and layered hexagonal NCM at high temperature (HT-NCM). By introducing lithium deficiency to tune the valence state of transition metals in NCM from Ni 2+ to Ni 3+ , DO-NCM exhibits the best catalytic activity with the lowest onset potential (~1.48 V) and Tafel slope (~85.6 mV dec -1 ), whereas HT-NCM exhibits the worst catalytic activity with the highest onset potential (~1.63 V) and Tafel slope (~241.8 mV dec -1 ).

36 MATERIALS SCIENCE↗
DOE OSTI2022

Nanoengineering Porous Silica for Thermal Management

Thermal insulation of solid materials originates from the nanoscale porous architectures to regulate thermal management in energy-critical applications from energy-efficient buildings to heat-sensitive energy devices. Here, we show nanoengineering of porous silica materials to control the architecture transition from mesoporous to nanocage networks. A low thermal conductivity of such a porous silica network is achieved at 0.018 W/(m K) while exhibiting a porosity of 92.05%, specific surface area of 504 m 2 /g, and pore volume of 2.37 cm 3 /g after ambient pressure drying. Meanwhile, the crosslinking of the porous silica and ceramic fiber frameworks show a tensile Young’s modulus of 2.8 MPa while maintaining high thermal insulation, which provides an effective thermal runway mitigation strategy for rechargeable lithium-ion batteries. Furthermore, the nanoengineering strategy reported here would shed light on achieving superthermal insulation of nanostructures for energy-critical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Beyond Volume Variation: Anisotropic and Protrusive Lithiation in Bismuth Nanowire

Materials storing energy via an alloying reaction are promising anode candidates in rechargeable lithium-ion batteries (LIBs) due to their much higher energy density than the current graphite anode. Until now, the volumetric expansion of such electrode particles during lithiation has been considered as solely responsible for cycling-induced structural failure. Here, we report different structural failure mechanisms using single-crystalline bismuth nanowires as the alloying-based anode. The Li–Bi alloying process exhibits a two-step transition, that is, Bi–Li 1 Bi and Li 1 Bi–Li 3 Bi. Interestingly, the Bi–Li 1 Bi phase transition occurs not only in the bulk Bi nanowire but also on the particle surface showing its characteristic behavior. The bulk alloying kinetics favors a Bi-(012)-facilitated anisotropic lithiation, whose mechanism and energetics are further studied using the density functional theory calculations. More importantly, the protrusion of Li 1 Bi nanograins as a result of anisotropic Li–Bi alloying is found to dominate the surface morphology of Bi particles. The growth kinetics of Li 1 Bi protrusions is understood atomically with the identification of two different controlling mechanisms, that is, the dislocation-assisted strain relaxation at the Bi/Li 1 Bi interface and the short-range migration of Bi supporting the off-Bi growth of Li 1 Bi. As loosely rooted to the bulk substrate and easily peeled off and detached into the electrolyte, these nanoscale protrusions developed during battery cycling are believed to be an important factor responsible for the capacity decay of such alloying-based anodes at the electrode level.

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

Atomic Structural Features of Stacking Faults and Domain Connections in the Li- and Mn-Rich Cathode

Li- and Mn-rich layered oxides (LMRs), a class of earth-abundant materials for rechargeable Li-ion battery cathodes, crystallize into layered structures of two different symmetries: C2/m represented by Li 2 MnO 3 and R$\overline{3}$m represented by LiMn 0.5 Ni 0.5 O 2 . Fundamental questions about how the C2/m and R$\overline{3}$m domains spatially correlate within the same oxide grain and how the C2/m stacking faults arrange themselves when this happens still remain. Here, by using integrated differential phase contrast imaging in scanning transmission electron microscopy (STEM-iDPC), we probe the structural and compositional details of a prototypical, cobalt-free LMR material, 0.3Li 2 MnO 3 •0.7LiMn 0.5 Ni 0.5 O 2 (Li 1.13 Mn 0.57 Ni 0.3 O 2 ). The connection between the C2/m and R$\overline{3}$m domains is found to be abrupt, facilitated by the small lattice mismatch between the two structures. Stacking faults in the C2/m domains feature atomic plane shifting that accommodates stacking sequence changes, which explains why the stacking faults form in a random manner. Furthermore, a local disordering mechanism was identified to correlate with the C2/m stacking faults. Chemically, it is found that Ni coexists with Mn at the transition metal sites within the nominal Li 2 MnO 3 domain. As a result, this study demonstrates that STEM-iDPC is a very useful tool for capturing all the elements in a single image, revealing atomic details on domain connections and stacking faults in the LMRs.

Li-rich and Mn-rich oxides↗
DOE OSTI2021

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrO x ) due to its superior performance and excellent stability. However, large scale applications using IrO x are impractical due to its low abundance and high cost. In this work, we report a highly active hafnium-modified iridium oxide (IrHf x O y ) electrocatalyst for OER. The IrHf x O y electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrO x electrocatalyst. The highest intrinsic mass activity of the IrHf x O y catalyst in acid conditions was calculated as 6950 A gIrO x -1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrO x and IrHf x O y catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, and shortening the bond lengths of Ir-O, and lowering free energies for OER intermediates to accelerate the OER process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗