Structure and electrocatalytic hydrogen evolution performance of Mo2C thin films prepared by magnetron sputtering

Mo₂C, which has a unique electronic structure similar to the electronic structure of Pt, is considered as the material with the greatest potential to replace Pt as a catalyst for the electrocatalytic hydrogen evolution reaction (HER). However, Mo₂C thin films have not attracted enough attention in the field of electrocatalysis. This work proposes a method for preparing Mo₂C thin films as a catalyst for electrocatalytic HER through radiofrequency magnetron sputtering. The HER activity of the Mo₂C thin film in acidic and alkaline media is studied by changing the deposition power of the Mo₂C target and doping Ni for structural modification. Results show that increasing the deposition power of Mo₂C can significantly enhance the HER activity of the films in acidic and alkaline media, and metal Ni doping can further enhance the HER activity of the Mo₂C films. In an alkaline environment at a current density of 10 mA cm⁻², the films demonstrate an overpotential of as low as 163 mV with a Tafel slope of 107 mV·dec⁻¹. In acidic media, the films present the corresponding overpotential of 201 mV and a Tafel slope of as low as 96 mV·dec⁻¹. Moreover, the Ni-doped Mo₂C films have excellent HER stability. The synergy between doped Ni and Mo vacancies optimizes the strength of the Mo–H bond and the adsorption and desorption equilibrium of active H, thus enhancing HER kinetics. This work guides the possible structural design of Mo₂C thin films for electrocatalytic HER.     

Noble fabrication of Ni–Mo cathode for alkaline water electrolysis and alkaline polymer electrolyte water electrolysis

Among the catalysts for hydrogen evolution reaction (HER) in alkaline media, Ni–Mo turns out to be the most active one. Conventional preparations of Ni–Mo electrode involve repeated spraying of dilute solutions of precursors onto the electrode substrate, which is time-consuming and usually results in cracking and brittle electrodes. Here we report a noble fabrication of Ni–Mo electrode for HER. NiMoO₄ powder was synthesized and used as the precursor. After reduction in H₂ at 500 °C, the NiMoO₄ powder layer was converted to a uniform and robust electrode containing metallic Ni and amorphous Mo(IV) oxides. The distribution of Ni and Mo components in this electrode is naturally uniform, which can maximize the interaction between Ni and Mo and benefit the electrocatalysis. The thus-obtained Ni–Mo electrode exhibits a very high catalytic activity toward the HER: the current density reaches 700 mA/cm² at 150 mV overpotential in 5 M KOH solution at 70 °C. This new fabrication method of Ni–Mo electrode is not only suitable for alkaline water electrolysis (AWE), but also applicable to the alkaline polymer electrolyte water electrolysis (APEWE), an emerging technique for efficient production of H₂.     

Porous Ni–Co surface formation and analysis of hydrogen generation by gas sensor

A Ni–Co alloy was used as the test piece. The porous Ni–Co alloy surface was prepared by Al electrodeposition at ?1.4 V and ?1.8 V and Al dissolution at ?0.5 V in a NaCl–KCl-3.5 mol% AlF₃ molten salt. The bath temperature was 750 °C and 900 °C. As a result, a porous Ni–Co alloy could be prepared by Al electrodeposition and Al dissolution on the Ni–Co alloy in the molten salt. It was clarified that a denser surface was formed at the bath temperature of 750 °C than at the bath temperature of 900 °C. Furthermore, it was clarified that the porous layer became thicker when the electrodeposition potential was ?1.8 V than when it was ?1.4 V. The formed porous Ni–Co alloy was evaluated for cathode performance in a 10 mass% KOH solution. Furthermore, the amount of generated hydrogen was measured by a constant voltage and constant current test with a gas sensor using a solid electrolyte. In the cathode polarization curve, the porous Ni–Co alloy showed a higher current density at a lower potential than the untreated Ni–Co alloy. It was shown that the Ni–Co alloy formed under the electrodeposition conditions at the electrodeposition potential of ?1.4 V and bath temperature of 750 °C is a very excellent cathode material. Furthermore, based on the constant voltage test, it was revealed that the porous treated sample generates a higher amount of hydrogen than the untreated sample.     

Metal silicide-mediated microcrystalline silicon thin-film growth for photovoltaics

Microcrystalline thin Si films were grown by the metal-induced growth method. The metal catalyst (Co, Ni, or Co-coated Ni) first reacted to sputtered Si forming a silicide layer. Then a Si film was epitaxially grown above the silicide seed template. The crystallinity of Si films was investigated by X-ray diffraction (XRD) confirming Si film growth with CoSi₂ or NiSi₂ as an intermediate step. The grown Si films were fabricated into Schottky photodiodes. The Co-coated Ni modulated the silicide formation and gave a short-circuit current density (J_sc) of 10.6 mA/cm², which is one order higher than that for the single Co catalyst case.     

One-step synthesis of oxygen incorporated V–MoS2 supported on partially sulfurized nickel foam as a highly active catalyst for hydrogen evolution

Highly active noble-metal-free catalysts for hydrogen evolution reaction (HER) are essential for the sustainable production of hydrogen. MoS₂ based HER catalysts are potentially competitive to noble metals but facing the challenges of low conductivity, density of active sites and intrinsic activity in basal planes. Herein, oxygen incorporated V–MoS₂ supported on partially sulfurized nickel foam (V–Mo(x)S/NF) are synthesized as binder-free HER catalysts via a one-step in-situ hydrothermal method. By controlling the fed atomic ratios of V/Mo, the optimal V–Mo(0.05)S/NF shows the low HER overpotential of ~31 and ~115 at 10 mA cm⁻² and 100 mA cm⁻², respectively in alkaline electrolyte, which is among the most active noble-metal and noble-metal-free HER catalysts reported. In V–Mo(0.05)S/NF, the introduced V and partially sulfurized nickel foam increases the conductivity. The hedgehog-like morphology ensures the exposure of high-density active sites. More importantly, the incorporated surface oxygen can be readily tuned by the fed atomic ratios of V/Mo, which plays the primary role in promoting the intrinsic HER activity for V–Mo(x)S/NF. This work demonstrates the feasibility of boosting HER through the precise control of incorporated surface oxygen in MoS₂ based catalysts.     

Development of Ni–Fe based ternary metal hydroxides as highly efficient oxygen evolution catalysts in AEM water electrolysis for hydrogen production

A number of mixed metal hydroxide oxygen evolution reaction (OER) catalysts i.e. Ni–Fe, Ni–Co, Ni–Cr, Ni–Mo, Ni–Fe–Co, Ni–Fe–Mo and Ni–Fe–Cr were prepared by cathodic electrodeposition and characterised by SEM, TEM, EDS, XPS and micro X-CT. The compositions of selected catalysts were optimised to give lower OER overpotentials in alkaline media. Further optimisation of Ni–Fe based ternary metal hydroxide catalysts such as Ni–Fe–Co and Ni–Fe–Mo was carried out, showing improved performance at high current densities up to 1 A cm⁻² in 1 M NaOH, 333 K. The influence of electrodeposition parameters such as current density, pH, electrodeposition time and temperature on the electrocatalytic performance of ternary Ni–Fe–Co metal hydroxide was further investigated and optimised. The durability of the optimised catalyst was tested at a current density of 0.5 A cm⁻² in an anion exchange membrane (AEM) water electrolyser cell at 4 M NaOH, 333 K, demonstrating stable performance over 3.5 h.     

Multifunctional Co–B–O@CoxB catalysts for efficient hydrogen generation

The development of efficient and non-noble catalyst is of great significance to hydrogen generation techniques. Three surface-oxidized cobalt borides of Co–B–O@Co_xB (x = 0.5, 1 and 2) have been synthesized that can functionalize as active catalysts in both alkaline water electrolysis and the hydrolysis of sodium borohydride (NaBH₄) solution. It is discovered that oxidation layer and low boron content favor the oxygen evolution reaction (OER) activity of Co–B–O@Co_xB in alkaline water electrolysis. And surface-oxidized cobalt boride with low boron content is more active toward hydrolysis of NaBH₄ solution. An alkaline electrolyzer fabricated using the optimized electrodes of Co–B–O@CoB₂/Ni as cathode and Co–B–O@Co₂B/Ni as anode can deliver current density of 10 mA cm⁻² at 1.54 V for overall water splitting with satisfactory stability. Meanwhile, Co–B–O@Co₂B affords the highest hydrogen generation rate of 3.85 L min⁻¹ g⁻¹ for hydrolysis of NaBH₄ at 25 °C.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

A novel Ni–La-Nd-Y flim on Ni foam to enhance hydrogen evolution reaction in alkaline media

The exploration of non-precious metal catalysts has always been a hot topic. Here, a new type of Ni–La-Nd-Y film was electrodeposited on nickel foam (NF). The Ni–La-Nd-Y film shows outstanding HER activity and stability under alkaline conditions. La₂NiO₄ makes the electrode surface present a regular layered structure, which greatly increases the number of electrochemically active sites. Ni–La-Nd-Y only needs 46 mV overpotential driving current density of 10  mA cm^?2 in 1MKOH solution, which is very close to the HER performance of metal Pt. By doping rare earth elements and taking advantage of the shrinkage characteristics of lanthanides, this work took advantage of the shrinkage characteristics of the lanthanide series and found new ideas for improving catalysts through the combination of different rare earth elements.     

CoMo heterohierarchical foam-structured cathode for anion exchange membrane water electrolyzer

It is important to consider the synergy of heterostructures to improve the slow kinetics of water dissociation in the alkaline hydrogen evolution reaction (HER). Herein, we report a simple method to design a heterohierarchical CoMo catalyst. The CoMo catalyst was prepared by simple one-pot electrodeposition on carbon paper (CP). The CoMo/CP catalyst was optimized for the alkaline HER by controlling the electrodeposition bath conditions, potential, and time. The optimized catalyst shows the heterohierarchical structure containing the electrically conductive metallic Co in the bulk and Mo-incorporated Co containing Mo⁴⁺ at the surface. It exhibited a lower HER overpotential of 0.11 V at ?20 mA/cm² compared to those of the others owing to the synergetic effect of the between the Co and Mo incorporated Co. The results highlight the advantages of the simple method developed herein for the design of heterohierarchical catalysts.     

Cu–Ga–Se thin films prepared by a combination of electrodeposition and evaporation techniques

Cu–Ga–Se thin films were prepared using a combination of electrodeposition and evaporation techniques. A Cu–Se/Mo/glass precursor thin film was first prepared by galvanostatic electrodeposition. On top of this film three different thicknesses of Ga were deposited by evaporation. The Cu–Ga–Se thin films were formed by annealing the Ga/Cu–Se/Mo/glass thin film configuration in a tubular chamber with Se powder, at different temperatures. Thin films were characterized by X-ray diffraction (XRD), photocurrent spectroscopy (PS), inductively coupled plasma (ICP) analysis, and scanning electron microscopy (SEM). The detailed analysis from X-ray reveals that after annealing at 550 °C the CuGaSe₂ phase is formed when the thickness of Ga is 0.25 μm, however at 0.5 μm and 1.0 μm Ga the formation of CuGa₃Se₅ and CuGa₅Se₈ phases is observed respectively. Band gap values were obtained using photocurrent spectroscopy.     

Preparation of N-doped nano-hollow capsule carbon nanocage as ORR catalyst in alkaline solution by PVP modified F127

In order to reduce the cost of oxygen reduction reaction (ORR) catalyst, a metal-free N-doped carbon nanocage as ORR catalysts is prepared by using Polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) three-block copolymer (F127), polyvinyl pyrrolidone (PVP) and Zn (OH)₂ as a carbon source, nitrogen source and the morphology retaining agent, respectively, via a self-template method. The structure and microstructure of the N-doped carbon nanocage are characterized via physical characterization methods such as X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller method, and X-ray photoelectron spectroscopy. Electrochemical properties are evaluated in an O₂-saturated alkaline solution. The obtained sample pyrolyzed at 800 °C with 10% content of PVP (C–N-800) presents nano-hollow carbon capsule with a diameter of 100 nm and shell thickness of 8 nm, and has the maximum surface area of 978 m²·g⁻¹. In O₂-saturated 0.1 M KOH solution, C–N-800's onset potential, limit current density, and half-wave potential, which are 0.87 V, 5.73 mA/cm²@0.45 V, and 0.77 V, respectively, are close to those of commercial 20% Pt/C catalysts. Meanwhile, its electrochemical performance only loss 11.75% after 5000 cycles, showing excellent ORR catalytic performance and stability. During self-assembly of F127, the added PVP participated into the process. Pyrolysis at 800 °C would result in the complete decomposition of F127 and PVP's lipophilic. So, the metal-free N-doped carbon nanocage was obtained and behaved excellent ORR catalytic activity and stability.     

High active and easily prepared cobalt encapsulated in carbon nanotubes for hydrogen evolution reaction

We prepared M/CeO₂ (M = Fe, Co or Ni) by coprecipitation method, and then fabricated M@CNT/CeO₂ electrocatalysts through ethanol decomposition on M/CeO₂. Experimental results showed that the activity of Co@CNT/CeO₂ for hydrogen evolution reaction (HER) was much higher than that of Fe@CNT/CeO₂ and Ni@CNT/CeO₂, these experimental results were consistent with the density functional theory (DFT) calculation results. The electrocatalysts from ethanol decomposition on Co/CeO₂ at 800 °C with different time were obtained, and their electrocatalytic activities for HER were discussed. Co@CNT-90 showed higher activity than others, when the reaction time exceeded 90 min, their HER activities declined gradually, because long-term ethanol decomposition caused decreased dispersion and thicker layers of carbon nanotube (CNT). To obtain a current density of 10 mA cm⁻², overpotential of 181 mV was required for Co@CNT-90, and its polarization curve after 8000 cycles retained a similar performance to the initial polarization curve. The high activity and durability of Co@CNT-90 could be explained from the carbon-encapsulated-metal structure, thus metal was protected by carbon layers and prevented metal from contacting with electrolyte directly. XRD patterns, TEM images and experimental results proved that Co was well encapsulated by CNT.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Electrodeposition of Mo-doped NiFex nanospheres on 3D graphene fibers for efficient overall alkaline water splitting

Developing efficient bifunctional catalysts for hydrogen and oxygen evolution reactions (HER/OER) has attracted great interest in hydrogen production from water splitting. In this work, a novel material of Mo-doped NiFe_x nanospheres on 3D graphene fibers (Mo-NiFe_x/3DGFs) has been successfully fabricated through a simple and cheap one-step electrodeposition method. The Mo-NiFe_x/3DGFs possessed ultra-high conductivity and specific surface area, greatly benefiting to electrocatalytic hydrolysis activity. And it was found that Fe element could obviously promote OER process, while Mo doping facilitated both OER and HER reactions. We proved that there existed synergetic roles between Fe and Mo element, which could realize the control of the electronic structure and optimize the adsorption/desorption of intermediates. And electrochemical tests showed that the Mo–Ni/3DGFs exhibited a relatively smaller overpotential of 109.9 mV for HER, while the Mo-NiFe_x/3DGFs presented better OER performance with an overpotential of 240.8 mV at the current density of 100 mA cm^-2 in 1.0 M KOH. Finally, a system for overall water splitting constructed by Mo–Ni/3DGFs||Mo–NiFe_0.68/3DGFs electrodes has a low cell voltage of 1.52 V at 10 mA cm^?2 and long-term stability, exceeding most of literature results. Our findings provide insight into possibilities for the simple synthesis of high-performance and cheap catalysts, and laid the foundation for the practical application of transition metal catalysts.     

A novel co-electrodeposited Co/MoSe2/reduced graphene oxide nanocomposite as electrocatalyst for hydrogen evolution

In this study, a simple and fast electrochemical method was employed to synthesis molybdenum diselenide thin film. The morphology, structure and chemical composition of the nanocomposites were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The progressive effects of transition metal ions including Ni, Cu, and Co were surveyed on the hydrogen evolution activity of MoSe₂ thin films. Co/MoSe₂ nanocomposite thin films has significant electrocatalytic activity as compared to other samples, In order to achieve higher performance, preparing Co/MoSe₂/RGO nanocomposite thin film, two strategies including layer by layer electrodeposition and co-electrodeposition has been employed. The presence of reduced graphene oxide leading to the onset potential shifts to more positive values and increase the current density. Also, results showed that the Co/MoSe₂/RGO nanocomposite prepared by co-electrodeposition exhibits the best electrochemical hydrogen evolution at onset potential of −0.18 with an overpotential of −0.45 V.     

Hydrogen evolution reaction activities of electrodeposited nanocrystalline Ni–Mo thin films in alkaline baths

The hydrogen evolution reaction (HER) electrocatalytic activities normalized to electrochemically active surface area (ECSA) were systematically evaluated and correlated with its composition (i.e., Mo content up to 26 at. %), crystal structure (i.e., face-centered cubic (fcc) to orthorhombic phase and the mixed phases with different phase ratios), and crystallinity. The electrodeposited with mixed phases exhibited highest ECSA (up to 228 cm² per 1 cm² geometric surface area) compared to deposits with single phase, which serves as the dominant factor to enhance exchange current density and overpotential. Low overpotentials per ECSA were observed with Mo content of 5.16 at% (fcc) and 24.1 at% (mixed phases). After normalizing with ESCA, The Tafel slope and exchange current density were indifference with Mo content, crystal phase, and grain size. The metallic Ni–Mo thin films have low mixed potential and overpotential at 10 mA/cm² of 20 mV and 120 mV, respectively.     

Hydrogen production by catalytic decomposition of methane over Fe based bi-metallic catalysts supported on CeO2–ZrO2

Methane decomposition to produce hydrogen was studied over iron based bimetallic catalysts supported on cerium-zirconium oxide in a continuous flow fixed bed reactor at 700 °C. 15 wt% Fe/CeZrO₂ was prepared by wetness impregnation and the promoted Fe catalysts (15 Fe-5 Co/CeZrO₂ and 15 Fe-5 Mo/CeZrO₂) were prepared by co-impregnation technique. Mo promoted Fe catalyst exhibited the maximum surface area of 24.08 m²/g. X-ray diffraction studies revealed that Fe₂O₃, Co₃O₄ and MoO₃ were the phases present in freshly calcined catalysts, while the reduced catalysts consisted of phases including elemental Fe, Mo and Fe–Co alloys. Both X-ray diffraction and temperature programmed reduction studies confirmed the complete reduction of metal oxide species under H₂ at 700 °C. The catalytic activity of Fe/CeZrO₂ was enhanced upon addition of Co and Mo as promoters. The initial hydrogen yield on 15 Fe-5 Mo/CeZrO₂ was ~90% and it decreased with increase in time on stream (TOS), and finally stabilized around ~50% after 125 min of TOS. The Co promoted catalyst exhibited similar activity while the initial hydrogen yield on 15 Fe/CeZrO₂ was ~83% and dropped to ~33% after 125 min of TOS. Graphitic carbon, Fe₃C and Mo₂C phases were observed in the XRD patterns of spent catalysts along with elemental Fe and Fe–Co alloy. It was evident from temperature programmed oxidation results that coke formation which deactivates the catalyst was dominant in 15 Fe/CeZrO₂ when compared to the promoted (Co and Mo) Fe catalysts where carbon nanostructures were dominant. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the formation of carbon nanostructures on the surface of spent catalysts. The Fe based catalysts supported both tip and base-growth mechanisms for the growth of carbon nanostructures.     

Novel,large area scalable polycrystalline Zn1-xCoxO films RF sputtered from a single mixed target for electrochemical water oxidation

The compound semiconductor Zn_1-xCo_xO (x = 0.42; ZnO:Co) has recently gained interest as a highly active electrocatalyst material for water oxidation (WOCs). We report on the significant structural and electronic transformations under anodic oxygen evolution reaction conditions of a novel, large area scalable sputtered ZnO:Co electrocatalyst. ZnO:Co sputtered films showed the transition of the polycrystalline ZnO:Co electrocatalyst material to a partially amorphous nanocomposite (ZnO:Co/Co(OH)₂/CoOOH). A high reactivity of the electrocatalyst material compensates effectively the low specific surface area of sputtered electrocatalyst thin films. The aim of this work is to examine the polycystral morphology of sputtered electrocatalyst thin film material, which can be also produced industrially on large areas and has a high WOC performance.     

Co–Ni/WC-AC catalysts for dry reforming of methane: The role of Ni species

To improve the DRM reaction performance of the catalysts, a series of Co–Ni/WC-AC catalysts are prepared by impregnation using WC-AC as the support. The structural features of the fresh and spent catalysts are characterized by BET, XRD, H₂-TPR, XPS and TG. The results show that the introduction of Ni in the 20Co/WC-AC catalyst promotes the conversion of W species to WC. Further, WC enhances the interaction between the active metal and the support. Thus, the activity and sintering resistance of Co–Ni/WC-AC catalysts are improved. It is also found that the introduction of different ratios of Ni has a significant effect on the chemical environment (oxygen environment) on the catalyst surface.10Co–10Ni/WC-AC catalysts showed high surface O_α and O_β contents of 26% and 53%, respectively. The catalyst shows excellent catalytic performance. The conversion of CH₄ and CO₂ is stable at about 84% and 85% at 800 °C.