Hafnium Oxynitride Research Articles

Structural, morphological, and chemical composition of ternary ZrHfN thin films deposited by reactive co-magnetron sputtering

This study focuses on a deposited ternary zirconium hafnium nitride (ZrHfN) thin film prepared using the reactive co-magnetron sputtering technique. The effect of the nitrogen (N2) flow rate on the films' structural, morphological, and chemical composition was investigated. The gracing-incidence X-ray diffraction (GIXRD) showed that all ZrHfN thin films exhibited a crystalline face-centered cubic structure with a strong (111) orientation. With increasing the N2 flow rate, the film's crystallography changed based on thermodynamic theory. The films demonstrated uniformity and homogeneity in their ternary nitride composition, as confirmed by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) mapping and electron probe microanalyzer (EPMA). Chemical state analysis using X-ray photoelectron spectroscopy (XPS) indicated the presence of both nitride and oxynitride species in all samples. Notably, the atomic concentration of the main elements (Zr, Hf, and N) remained relatively stable over the controlled N2 flow rate range. However, the N2 flow rate played a crucial role in forming hafnium nitride and oxynitride chemical states, whereas it did not significantly influence the Zr phases, dominated by Zr oxides. Additionally, the hardness of the ternary ZrHfN thin films exhibited enhancement comparable to typical ternary nitrides.

Structural, optical and phonon properties of hafnium oxynitride thin films synthesized using plasma-enhanced atomic layer deposition

Hafnium Oxynitride belongs to the group IVB compounds with high permittivity and large acoustic impedance. In this work, hafnium oxynitride films have been synthesized using plasma-enhanced atomic layer deposition on Si and Quartz substrates. XRD results show the presence of mixed cubic and monoclinic phases with an optimum crystallization occurring at 850 °C. The thin films show strong absorption in the UV–visible spectrum suggesting semiconductor behaviour. The optical properties of the spectrophotometer and spectroscopic ellipsometry agree with the XRD observations. We also report the first observation of experimentally derived photoluminescence (PL) from hafnium oxynitride thin films synthesized using plasma-enhanced atomic layer deposition. The PL spectrum is consistent with the XRD results with two absorption peaks around 576 nm and 705 nm, corresponding to cubic and monoclinic phases, respectively. Also, the PL results match very well with the theoretical value of the band gap of cubic and monoclinic phases of Hf2ON2. The Raman spectrum shows a phonon band gap around 242–263 cm−1, consistent with the theoretically reported value for cubic Hf2ON2.

Laser-synthesized plasmonic HfN-based nanoparticles as a novel multifunctional agent for photothermal therapy.

Hafnium nitride nanoparticles (HfN NPs) can offer appealing plasmonic properties at the nanoscale, but the fabrication of stable water-dispersible solutions of non-toxic HfN NPs exhibiting plasmonic features in the window of relative biological transparency presents a great challenge. Here, we demonstrate a solution to this problem by employing ultrashort (femtosecond) laser ablation from a HfN target in organic solutions, followed by a coating of the formed NPs with polyethylene glycol (PEG) and subsequent dispersion in water. We show that the fabricated NPs exhibit plasmonic absorption bands with maxima around 590 nm, 620 nm, and 650 nm, depending on the synthesis environment (ethanol, acetone, and acetonitrile, respectively), which are largely red-shifted compared to what is expected from pure HfN NPs. The observed shift is explained by including nitrogen-deficient hafnium nitride and hafnium oxynitride phases inside the core and oxynitride coating of NPs, as follows from a series of structural characterization studies. We then show that the NPs can provide a strong photothermal effect under 808 nm excitation with a photothermal conversion coefficient of about 62%, which is comparable to the best values reported for plasmonic NPs. MTT and clonogenic assays evidenced very low cytotoxicity of PEG-coated HfN NPs to cancer cells from different tissues up to 100 μg mL-1 concentrations. We finally report a strong photothermal therapeutic effect of HfN NPs, as shown by 100% cell death under 808 nm light irradiation at NP concentrations lower than 25 μg mL-1. Combined with additional X-ray theranostic functionalities (CT scan and photon capture therapy) profiting from the high atomic number (Z = 72) of Hf, plasmonic HfN NPs promise the development of synergetically enhanced modalities for cancer treatment.

(Invited) Synthesis and Characterization of New Catalyst Materials for Electrocatalysis

In this talk I will discuss some of our recent explorations of new catalyst materials for electrocatalysis for renewable fuels and energy, including the synthesis and characterization of these materials. First, I will briefly describe our report of an efficient acid-stable N2-plasma treated hafnium oxyhydroxide electrocatalyst for hydrogen evolution and oxidation reactions (HER and HOR). Lack of a highly active, stable, earth-abundant electrocatalyst for carrying out HER and HOR in strongly acidic conditions is a major technical challenge for developing economical polymer electrolyte membrane (PEM)-based electrolyzers and fuel cells. We found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material, and we propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt group metal (PGM) catalysts, opening new opportunities to develop technologically and economically viable unitized regenerative fuel cell (URFC) systems and cost-effective PEM electrolyzers. Furthermore, these results have broad implications for using nitrogen incorporation (e.g. by N2 plasma treatment) to activate other non-conductive compounds and films to form new active electrocatalysts.I will discuss our recent observations on the reaction-driven restructuring of defective PtSe2 into an ultrastable electrocatalyst for the oxygen reduction reaction (ORR), which is of interest for accelerating the commercialization of PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). We have found that the restructured defective PtSe2 electrocatalyst had 1.3 times the specific activity and 2.6 times the mass activity of a commercial Pt/C catalyst, and maintained this superior ORR activity even after 126,000 cycles in accelerated durability tests.I will describe our report of a highly active hafnium-modified iridium oxide (IrHfxOy) electrocatalyst for the oxygen evolution reaction (OER), a reaction that limits development of energy efficient and economically viable a wide range of energy conversion devices. Iridium oxide (IrOx) is the current benchmark electrocatalyst for OER due to its superior performance and excellent stability, but large scale applications using IrOx are impractical due to its low abundance and high cost. We have found an IrHfxOy electrocatalyst that demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than an IrOx electrocatalyst. The highest intrinsic mass activity of the IrHfxOy catalyst in acid conditions was calculated as 6950 A gIrOx -1 at an overpotential (η) of 0.3 V. I will also discuss our research on phosphorus-doped Hf-Ru oxide (denoted here as HfxRuyPO) electrocatalysts for OER in which we demonstrated a strategy for efficiently promoting the durability of RuO2. This research was motivated by the need to address the rapid corrosion and precipitation of highly efficient precious metal (Ir, Ru, and Rh) nanoparticle catalysts in the harsh environment of OER in PEM electrolyzers for hydrogen fuel generation. We found that when the HfxRuyPO catalysts possessed a Hf:Ru atomic ratio of 1:2, they exhibited a highly efficient and stable OER activity over 40 hours. By XPS and Raman analyses, the HfPO component was found to play a significant role in improving the durability of RuO2 through the formation of a HfOOH phase, which also possessed a higher OER activity and lower impedance.Most recently we are investigating metal-organic frameworks (MOFs) and MOF-derived (MOF-d) materials as electrocatalysts for renewable fuels and energy applications.

Studies of Electrical Parameters and Thermal Stability of HiPIMS Hafnium Oxynitride (HfOxNy) Thin Films

This work demonstrated the optimization of HiPIMS reactive magnetron sputtering of hafnium oxynitride (HfOxNy) thin films. During the optimization procedure, employing Taguchi orthogonal tables, the parameters of examined dielectric films were explored, utilizing optical methods (spectroscopic ellipsometry and refractometry), electrical characterization (C-V, I-V measurements of MOS structures), and structural investigation (AFM, XRD, XPS). The thermal stability of fabricated HfOxNy layers, up to 800 °C, was also investigated. The presented results demonstrated the correctness of the optimization methodology. The results also demonstrated the significant stability of hafnia-based layers at up to 800 °C. No electrical parameters or surface morphology deteriorations were demonstrated. The structural analysis revealed comparable electrical properties and significantly greater immunity to high-temperature treatment in HfOxNy layers formed using HiPIMS, as compared to those formed using the standard pulsed magnetron sputtering technique. The results presented in this study confirmed that the investigated hafnium oxynitride films, fabricated through the HiPIMS process, could potentially be used as a thermally-stable gate dielectric in self-aligned MOS structures and devices.

Formation of HfOxNy nanorod GLAD films growth by rapid thermal oxidation

In this work, the hafnium oxynitride (HfON) nanorod films on p-type silicon (Si) wafer substrate have been fabricated by reactive magnetron sputtering with glancing angle deposition (GLAD) technique, subsequently with a rapid thermal oxidation process at 500–900 °C under low-vacuum state. The crystallinity and morphology were investigated by glazing-incident X-ray diffraction (GIXRD) and field-emission scanning electron microscopy (FE-SEM), respectively. As-deposited films were amorphous nanorod films. The surface-sensitive X-ray photoelectron spectroscopy (XPS) analytical technique revealed that nitrogen atoms are increasingly replaced with oxygen atoms at the surface. The distribution of nitrogen atoms investigated by X-ray absorption spectroscopy (XAS) revealed the substitution of nitrogen with oxygen at the film surface and molecular nitrogen trapped in the film as the annealing temperature was increased.

Investigating Electronic, Optical, and Phononic Properties of Bulk γ-M2ON2 and β-M7O8N4 (M = Hf and Zr) Insulators Using Density Functional Theory.

Hafnium and zirconium oxynitrides have similar properties, yet a consolidated investigation of their intrinsic properties has not been carried out. In this paper, we perform first-principles density functional theory calculations of γ- and β-phase hafnium and zirconium oxynitrides, which show that the γ-M2ON2 (M = Hf and Zr) is an indirect band-gap (Eg) insulator, while the β-M7O8N4 has a “pseudo-direct” type of Eg. β-phase has higher Eg than γ-phase, with concomitant disappearance of the conduction band tail. Optical properties in γ-M2ON2 show that the anisotropy is negligible, and the optical constant values are in the range of other superhard materials. Phonon calculations present peculiar characteristics such as a small phonon band gap in γ-Hf2ON2 and imaginary phonon frequencies in β-phases relating to lattice instability. The phononic properties are unfavorable for their potential use as an absorber material of the hot carrier solar cell—an emerging photovoltaic concept.

A FORMing-Free HfO2-/HfON-Based Resistive-Gate Metal–Oxide–Semiconductor Field-Effect-Transistor (RG-MOSFET) Nonvolatile Memory With 3-Bit-Per-Cell Storage Capability

We present a nonvolatile resistive-gate metal-oxide-semiconductor-field-effect transistor (RG-MOSFET). An RG-MOSFET comprises a MOSFET and dielectric layers sandwiched between the top and bottom electrode metal-dielectric-metal (MDM). The gate of the MOSFET is electrically connected to the bottom electrode of MDM in series. Dielectric layers of MDM are composed of a resistance-switching layer made by hafnium oxynitride (HfON) and a resistance-nonswitching layer by hafnium dioxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). RG-MOSFET as a memory can be achieved by resistance changes in the HfON. Eight distinguishable resistance values can be programmed randomly, which is performed as a 3 bit per cell through the edge Fowler-Nordheim (FN) mechanism between the top electrode metal and the drain of RG-MOSFET. The external voltage applied to the top electrode is divided by the MDM and the gate dielectric, and the threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) of an RG-MOSFET is modulated by different resistance values of MDM. Therefore, states stored in MDM can be read out from the drain-current level of an RG-MOSFET. FORMing-free characteristic of MDM is required for bipolar operation. A ratio of drain current between the high-resistance state (HRS) and the low-resistance state (LRS) is $10^{{5}}$ . Gradual SET/RESET is executed to tune the drain current by pulse operations; $10^{{5}}$ endurance cycles of RG-MOSFETs are demonstrated, and a retention test can be passed for more than 5 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> (35 days) at 125 °C.

Technology and optimization of hafnium oxynitride (HfOxNy) thin-films formed by pulsed-DC reactive magnetron sputtering for MIS devices

This study is devoted to the technology and optimization of pulsed-DC reactive magnetron sputtering of hafnium oxynitride (HfOxNy) thin-films. The fabrication process of HfOxNy films was optimized employing the Taguchi orthogonal tables approach leading to the material with possible best electrical parameters. During the optimization procedure, the parameters of dielectric films were monitored by means of electrical characterization of MIS structures with hafnium oxynitride as the gate-dielectric. The thermal stability of fabricated HfOxNy layers was also examined. The presented results have shown the improved electrical parameters of fabricated films after thermal treatment. Namely, we have observed beneficial flat-band voltage (Vfb) value, the disappearance of frequency dispersion of CV characteristics, reduced effective charge (Qeff/q), and interface traps (Dit) densities of examined MIS structures. However, the permittivity value is slightly lower as compared to reference samples. The superior stability of HfOxNy layers up to 800 °C was proved. Although the significant increase of crystalline phase in the layer bulk was observed, no deterioration of electrical properties or surface morphology has been noticed. The results presented in this study make the investigated HfOxNy fabricated using pulsed-DC reactive magnetron sputtering the possible candidate as a gate dielectric in MIS structures and devices.

(Invited) Modified Oxide Electrocatalysts for Renewable Fuels and Energy

In this talk I will discuss research to develop and understand modified oxide electrocatalysts for renewable fuels and energy. In the first part, I will focus on catalysts for the oxygen evolution reaction (OER). An important challenge for realizing cheap, stable, and efficient electrocatalysts for producing renewable fuels and energy is to enable transition metal oxides (TMOs) to replace precious metal-based electrocatalysts (e.g. IrOx and RuOx) for OER. Low energy efficiencies of OER resulting from sluggish reaction kinetics and large overpotentials is a major cause of energy losses in photocatalytic systems for solar fuels and in a number of other emergent technologies such as water splitting devices, rechargeable metal-air batteries, and unitized regenerative fuel cells. Insights from two examples of our studies will be discussed, involving a range of spectroscopic techniques for characterization of Ce-modified copper oxide (CuOx) and Ni-modified cobalt (oxy)hydroxides to reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts. In the case of Ce-modified CuOx, a strong promoting effect of Ce4+ for OER was observed for Ce incorporation (7 at%) into CuOx up to a concentration corresponding to CeO2 phase segregation, as probed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. In the case of Ni-modified CoOxHy, operando Raman spectroscopy and electrochemical techniques were used during OER to identify the composition and local structure of electrodeposited CoOxHy and Ni-modified CoOxHy catalyst films. During OER, Ni-modified CoOxHy films with lower initial crystallinity underwent substantial structural evolution that began with an irreversible transformation of a spinel local structure to an amorphous CoO structure at low anodic potentials. Increasing anodic polarization with elevated oxygen evolution rates caused additional structural conversion of the amorphous CoO structure to a complex phase that can be described as an amalgamation of NiOOH and layered CoO2 motifs (NiOOH-h-CoO2). Formation of this structure, independent of the initial cobalt oxide structure, suggests that this could be the universally active structure for NiCoOxHy catalysts. I will also discuss recent work on phosphorus-doped Hf-Ru oxide (denoted here as HfxRuyPO) electrocatalysts for OER in which we demonstrated a strategy for efficiently promoting the durability of RuO2. This research was motivated by the need to address the rapid corrosion and precipitation of highly efficient precious metal (Ir, Ru, and Rh) nanoparticle catalysts in the harsh environment of OER in proton exchange membrane (PEM) electrolyzers for hydrogen fuel generation. We found that when the HfxRuyPO catalysts possessed a Hf:Ru atomic ratio of 1:2, they exhibited a highly efficient and stable OER activity over 40 hours. By XPS and Raman analyses, the HfPO component was found to play a significant role in improving the durability of RuO2 through the formation of a HfOOH phase, which also possessed a higher OER activity and lower impedance.In the second part of the talk, I will briefly describe our report of an efficient acid-stable N2-plasma treated hafnium oxyhydroxide electrocatalyst for hydrogen evolution and oxidation reactions (HER and HOR). We found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material, and we propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt group metal (PGM) catalysts, and these results have broad implications for using nitrogen incorporation to activate other non-conductive compounds and films to form new active electrocatalysts. I will also discuss our recent observations on the reaction-driven restructuring of defective PtSe2 into an ultrastable electrocatalyst for the oxygen reduction reaction (ORR), which is of interest for accelerating the commercialization of PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). We have found that the restructured defective PtSe2 electrocatalyst had 1.3 times the specific activity and 2.6 times the mass activity of a commercial Pt/C catalyst, and maintained this superior ORR activity even after 126,000 cycles in accelerated durability tests.

The electronics transport mechanism of grain and grain boundary in semiconductive hafnium oxynitride thin film

HfOxNy thin film was deposited on oxidized silicon substrate; its physical structure and chemical composition were studied in detail by X-ray diffractometer, scanning electron microscopy, field emission transmission electron microscope and X-ray photoelectron spectrometer. Microtemperature sensors with high sensitivity based on the film were fabricated. To clarify the conduction process of grain, grain boundary (GB) and the whole film, temperature-dependent AC impedances of a sensor were measured and analyzed in 40–300 K. The results show that at all of the measured temperatures, the resistance of grain is much larger than that of GB, and its rising rates with the temperature reduction are also much larger than that of GB, indicating that the resistive property of HfOxNy thin film is determined by grain. In addition, it has been confirmed that the conduction process of both the HfOxNy film and GB is dominated by thermal activation and Mott variable-range hopping (VRH) in relatively high and low temperature range, respectively. The conduction process of the grain obeys Mott VRH in the whole considered temperature range, while the Mott characteristic temperature is changed. These results provide new insights into the performance enhancement of the transition metal oxynitride-based temperature sensors.

(Invited) Hafnium Oxynitride-Derived Electrocatalyst with High Activity and Stability in Strong Acid for Both Hydrogen Evolution and Oxidation Reactions

There is a need to decrease our dependence on fossil fuels for producing energy and chemicals, and there is a great interest in using sunlight to drive fuel-forming reactions to produce solar fuels. Solar fuels are important future energy carriers and provide a way to store solar energy, and this makes for a theme that is continuing to move up the global research agenda. Alternatively, with solar and wind power creating increasingly large amounts of electricity, an important question is how can we take advantage of the expanding increase of renewable electricity for catalysis? By applying a voltage in electrocatalysis, we can harness the flow of electrons to form and break chemical bonds, with the remaining challenge to carry out this catalysis with high selectivity and energy efficiency utilizing sustainable electrocatalyst materials. Renewable H2 technologies could provide alternatives to fossil fuels, and this could be enabled by H2 production from water electrolysis and generation of electricity using H2 fuel cells. However, lack of highly active, stable, earth-abundant electrocatalysts for carrying out the hydrogen evolution reaction (HER) in water electrolyzers and the hydrogen oxidation reaction (HOR) in fuel cells, is a major technical challenge for developing economical polymer electrolyte membrane (PEM)-based electrolyzers and fuel cells. Carrying out these reactions in strongly acidic conditions is desirable because of the advantages of using proton exchange membranes, including compact system design, lack of liquid electrolyte, high current density and energy efficiency, and high gas purity. Recently, we have found a material that demonstrates excellent HER and HOR activities in acids using non-platinum group metal (PGM) catalysts [1]. Processing hafnium oxide with an atmospheric nitrogen plasma forms an acid-insoluble hafnium oxynitride material. We propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt. This opens up new opportunities to develop technologically and economically viable PEM-fuel cells, regenerative fuel cells, and electrolyzers. Furthermore, these results have broad implications for using nitrogen incorporation (e.g. by nitrogen plasma treatment) to activate other non-conductive compounds and films to form new active electrocatalysts. 1. X. Yang, F. Zhao, R.S. Selinsky, Z. Chen, Y.-W. Yeh, N. Yao, C.G. Tully, and B.E. Koel, Nature Communications, 10, 1543−1−8 (2019).

Sensitization of silicon by singlet exciton fission in tetracene.

Silicon dominates contemporary solar cell technologies1. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap2. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton)3-5. Singlet exciton fission in the molecular semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap6-8. When the triplet excitons are transferred to silicon they create additional electron-hole pairs, promising to increase cell efficiencies from the single-junction limit of 29 per cent to as high as 35 per cent9. Here we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using electric-field-effect passivation to enable the efficient energy transfer of the triplet excitons formed in the tetracene. The maximumcombined yield of the fission in tetracene and the energy transfer to silicon is around 133 per cent, establishing the potential of singlet exciton fission to increase theefficiencies of silicon solar cells and reduce the cost of the energy thatthey generate.

New rare earth hafnium oxynitride perovskites with photocatalytic activity in water oxidation and reduction.

RHfO2N perovskites with R = La, Nd and Sm show a GdFeO3-type structure and are semiconductors with band gaps of 3.35, 3.40 and 2.85 eV and relative dielectric constants of 30, 16 and 28 respectively. These compounds have adequate reduction and oxidation potentials to conduct the overall water splitting reaction, and the analogous perovskite LaZrO2N with a band gap of 2.8 eV shows photocatalytic activity under visible light irradiation for O2 evolution.

Structure and properties of Hf-O-N films prepared by high-rate reactive HiPIMS with smoothly controlled composition

Hafnium oxynitride ceramics were prepared in the form of thin films by high-power impulse magnetron sputtering of Hf in various Ar+O2+N2 gas mixtures. Smooth composition control was achieved by maximizing the degree of dissociation in plasma, suppressing the importance of the difference between reactivities of undissociated O2 and N2. The application potential of the films was further enhanced by extremely high deposition rates (e.g. 230nm/min for stoichiometric HfO2; achieved by feedback pulsed reactive gas flow control), low deposition temperatures (<140°C) and not using any substrate bias. We focus on the relationships between elemental composition, phase structure, and optical, electrical, mechanical and hydrophobic properties of the materials. We quantify the evolution of smoothly controlled film properties along the transition from an oxide to a nitride, such as increasing extinction coefficient, decreasing electrical resistivity, increasing hardness or increasing water droplet contact angle.

Oxidation behavior of vacuum plasma-sprayed hafnium–tantalum nitrides

Abstract

Oxygen Reduction on Hafnium/Hafnium Carbide Nanoparticles

Non-precious metal nanoparticles are attractive candidates for oxygen electrocatalytic reduction in electrochemical energy conversion devices, such as fuel cells and batteries. Here we report for the first time electrocatalytic activity of hafnium/hafnium carbide nanoparticles towards the oxygen reduction reaction (ORR). While activity of hafnium oxynitride nanoparticles towards the ORR in acid has previously been reported [1] a little was known about the ability of Hf/HfC nanoparticles to reduce molecular oxygen. Here we demonstrate that molecular oxygen can be reduced on Hf/HfC nanoparticles both in acidic and alkaline environment. However, the reaction mechanism changes from 4 - to 2 - electron pathway, when switching from acidic to alkaline environment. Although the ORR in base occurs through the two-electron parthway, this reaction has a potential for a practical application because Hf/HfC catalyst demonstrates an activity toward the disproportionation of hydrogen peroxide to O2 and H2O. Onset potential for the ORR in 0.1 M KOH for non-optimized catalyst (-0.10 V vs AgAgCl ) is superior to that of the best reported to date MnO2 catalyst (-0.13 V vs Ag/AgCl). In acid, moderate catalytic activity towards the ORR is observed. The results suggests that Hf nanomaterials can be considered as a promising candidate for replacing noble metal catalysts in alkaline air electrodes. [1] CHISAKA, M. & MURAMOTO, H. 2014. Suppression and Evaluation of Hydrogen Peroxide Formation on Carbon-Supported Hafnium Oxynitride. ECS Electrochemistry Letters, 3, F1-F4.

Oxygen Reduction Reaction Activity of Aluminum-Doped Hafnium Oxynitride in Acid Media

Among the non- platinum-group-metal catalysts for polymer electrolyte fuel cells (PEFCs), only oxide and oxynitride catalysts containing either group 4 or 5 metals have been found to be stable in acid media using inductively coupled plasma-atomic emission spectroscopy.1,2 The high stability motivated us to investigate the active sites for oxygen reduction reaction (ORR). To date, ORR-active sites either in the oxide or oxynitride catalysts have been acknowledged as anion defects.3,4 The charge neutrality of the defect-incorporated oxides is compensated by a decrease in the valence of the metal, which results in dissolution as the oxides are stable in acid media when the valence of either group 4 or 5 metals is the highest.5 The substitution of oxygen by an anion with larger valence (below 2–) such as nitrogen and metal by a cation with lower valence (below 4+ and 5+ for group 4 and 5 metals, respectively) are the possible routes to incorporate oxygen defects into oxides without decreasing the valence of either group 4 or 5 metal. The first route has been investigated by several groups, whereas the second one has not been reported so far for either group 4 or 5 metal oxide catalysts. In this study, we report the synthesis of aluminum-doped hafnium oxynitride catalysts supported on carbon black (Hf1-δ Al δ O x N y -C) and its ORR activity. Hf1-δ Al δ O x -C catalysts were synthesized using a previously reported impregnation method for obtaining HfO x -C (denoted as IM-Cl23) with the modification of addition of powdered AlCl3·6H2O precursor into a dispersion.6 The δ was varied over the range of 0.03–0.3. Only Hf0.8Al0.2O x -C catalysts which showed the maximum activity were doped with nitrogen by heating them at 1273 K for 6 h under flowing ammonia gas to obtain Hf0.8Al0.2O x N y -C. The catalyst morphologies, crystal structures, and surface chemical states were evaluated using field-emission transmission electron microscopy (FE-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ORR activities were evaluated by measuring rotating disk electrode (RDE) voltammograms in 0.1 mol dm–3 H2SO4 solution. Figure 1 shows a FE-TEM image of Hf0.8Al0.2O x N y -C. The energy dispersive X-ray (EDX) spectra (not shown) revealed that the black particles contained both Hf and Al atoms whereas grey area contained only C atoms. The Hf0.8Al0.2O x N y particles’ size shown in Figure 1 were nanometer-order; however, it is noted that some particles aggregated into ~15 nm. All the Hf1- δ Al δ O x N y -C catalysts were synthesized using the impregnation method previously reported for the synthesis of Al-free HfO x N y -C catalysts in which the HfO x N y particle size was about a few nanometers,4 lower than that of Hf0.8Al0.2O x N y . The experimental differences between the previous4 and current studies are the ammonia-treatment conditions and presence of Al precursor. Because the former did not affect the particle size,2 the latter could be the source of size enhancement. XRD and XPS analyses confirmed that the trivalent-Al atoms were successfully doped into tetravalent-Hf sites in HfO x -C without lowering the Hf-valence, thereby yielding the cubic-HfO2 phase with oxygen defects. Both the content of the cubic phase and ORR activity showed a volcano type dependence on δ, with an optimum value of 0.2. Nonetheless, the N-doping into O-sites enhanced the activity further. The durability of Hf0.8Al0.2O x N y -C catalyst was investigated using an accelerated degradation protocol recommended by the U.S. Department of Energy.7 It showed high durability: a decay of only 50 mV at 1 mA cm–2 after the 20,000th potential cycling without significant changes in the chemical bonding states on their surfaces confirmed by XPS spectra obtained after the accelerated degradation tests.ReferencesA. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).M. Chisaka, T. Iijima, T. Yaguchi, Y. Sakurai, Electrochim. Acta, 56, 4581 (2011).H. Imai, M. Matsumoto, T. Miyazaki, S. Fujieda, A. Ishihara, M. Tamura and K. Ota, Appl. Phys. Lett., 96, 191905 (2010).M. Chisaka, Y. Suzuki, T. Iijima, and Y. Sakurai, J. Phys. Chem. C, 115, 20610 (2011).M. Pourbaix, Atlas of Electrochemical Equilibra in Aqueous Solutions, Pergamon Press, Oxford, 1966.M. Chisaka and H. Muramoto, ChemElectroChem, in press. doi: 10.1002/celc.201300271U.S. DoE, Testing Oxygen Reduction Reaction Activity with the Rotating Disc Electrode Technique, https://www1.eere.energy.gov/hydrogenandfuelcells/webinars.html AcknowledgmentsThis work was partially supported by a research grant from the Kurata Memorial Hitachi Science and Technology Foundation; Grant-in-Aid for Young Scientists (B), 23760185, from the Japanese Ministry of Education, Culture, Sports, Science and Technology; Adaptable and Seamless Technology Transfer Program through target-driven R&D, AS242Z00224L, from the Japan Science and Technology Agency; a research grant from the Kao Foundation for Arts and Sciences, Japan.

Evaluation of "Intrinsic" Oxygen Reduction Reaction Selectivity on Carbon-Supported Hafnium Oxynitride Catalysts

Non- platinum-group-metal (PGM) catalysts have been extensively developed for use in cathodes of many energy-conversion devices such as fuel cells and metal-air batteries. Some of them showed oxygen reduction reaction (ORR) activities higher than that of platinum in alkaline media;1 however, none of them showed such high activity in acidic environments. Therefore, most researchers developing non-PGM ORR catalysts for use in polymer electrolyte fuel cell (PEFC) cathodes, which operate in highly acidic environments, have focused on improving their activity. Selectivity for ORR, i.e., the number of electrons transferred per unit oxygen molecule, n, is another important factor that should be considered for durability. When ORR proceeded via the so-called 2-electron pathway wherein an oxygen molecule is first reduced to hydrogen peroxide (O2 + 2H+ + 2e– → H2O2), the H2O2 intermediate degraded the proton-conductive perfluorosulfonate-ionomer (PFSI) in catalyst layers and PFSI membranes.2 Thus, non-PGM catalysts on which ORR proceeded via a 4-electron pathway wherein an oxygen molecule was directly reduced to water (O2 + 4H+ + 4e– → H2O) was necessary for the long-term operation of inexpensive PEFCs.In this study, the amount of H2O2 formed on carbon-supported hafnium oxynitride (HfO x N y -C) catalyst3 during ORR was successfully suppressed without a decrease in activity by simply changing the synthesis conditions. A decrease in the NH3 treatment time from 50 to 6 h and NH3-flow rate from 200 to 100 sccm at 1223 K resulted in a decrease in H2O2formation by a factor of 3 at 0.6 V versus a standard hydrogen electrode as shown in Figure 1.The “apparent, not intrinsic” selectivity as well as activity depended on the mass fraction of PFSI, Nafion in catalyst layers, χ N as shown in Figure 2. The n value was almost constant when χ N was increased up to 0.4, and significantly decreased when χ N was increased further to 0.5. The Koutecky-Levich plots of the catalyst layers for two different χ N are shown in the inset of Figure 2 (b). The intercept of the y–axis at ω → ∞ was almost zero for catalyst layers with χ N = 0.3, indicating that the ORR process was not controlled kinetically nor the other factors at the low disk potential, E d of 0.1 V versus SHE, as expected. However, the intercept for catalyst layers with χ N = 0.5 was much larger than zero under identical conditions, suggesting the appearance of Nafion film on the catalyst surface due to the large χ N. Such Nafion film should be a barrier to both O2 and H2O2 transport through catalyst layers, which affect n. These results indicated that both the activity and selectivity of non-PGM catalysts should be evaluated after the optimization of χ Nfor the accuracy. References (1) H. T. Chung et al., Nature Commun., 4, 1922 (2013).(2) S. Hommura et al., J. Electrochem. Soc., 155, A29 (2008).(3) M. Chisaka et al., J. Phys. Chem. C, 115, 20610 (2011). Acknowledgment This work was partially supported by Grant-in-Aid for Young Scientists (B), 23760185, from the Japanese Ministry of Education, Culture, Sports, Science and Technology, the Adaptable and Seamless Technology Transfer Program through target-driven R&D, AS242Z00224L, from the Japan Science and Technology Agency, and a research grant from the Kao Foundation for Arts and Sciences, Japan.

Aluminum‐Doped Hafnium Oxynitride: Effect of Cation Doping on Oxygen‐Reduction‐Reaction Activity in Acid Media

AbstractAl atoms are doped into Hf sites in HfOx nanoparticles supported on carbon black to incorporate O defects without decreasing the Hf valence. The oxygen reduction reaction activity is enhanced by the Al doping, and Al atoms are stable following the N doping into the O sites and accelerated degradation tests involving 20 000 potential cycles.