Inert Anode Research Articles

Characterisation of oxide-ion diffusivity and phonon vibrations in inert anode materials for molten salt electrolysis

Characterisation of oxide-ion diffusivity and phonon vibrations in inert anode materials for molten salt electrolysis

Pre‐oxidized Recycled MgO–Steel Composite Material for Possible Application in Cryolitic Melts

Recycled MgO–C lining bricks and 316L stainless steel are used to manufacture composite material for inert anode samples for aluminum electrolysis cell. The microstructure of the composite material is characterized after preoxidation thermal treatments at 800, 900, and 1000 °C as well as in its sintered state. Preoxidation (PO) process is designed to enhance the material's corrosion resistance in molten cryolite environments by developing robust Fe–Mg–O, Fe–Cr–O‐ containing phases. Analytical techniques including scanning electron microscopy, electron backscatter diffraction, and energy dispersive X‐ray spectrometry are applied to characterize the phase formation, revealing the potential of these composites for use as inert anodes in aluminum electrolysis cells. PO at 800 °C is not sufficient to form adequate protective oxide layers. Whereas, PO at 900 and 1000 °C leads to the formation of protective oxide layers containing Mg–O Fe–O halite‐like solid solutions and (Cr,Fe)3O4 spinel phase. Sample, preoxidized at 1000 °C is sealed in Mg–Fe–O spinel phase.

Enhanced thermal insulation and corrosion resistance in Ni-Fe cermet through incorporation of high-entropy spinel oxides

The influence of high-entropy spinel oxide (HESO) content on the microstructure, electrical conductivity, and high-temperature corrosion resistance of HESO/NiFe cermets were investigated. The results found that the addition of 10 to 30wt.% HESO enhanced sintering densification and thermal insulation, although it also resulted in reduced electrical conductivity. Isothermal oxidation tests conducted at 900 °C for 10hours revealed that cermets containing 30 to 50wt.% HESO exhibited significantly lower weight gains (0.954 to 1.167mgcm⁻²) and oxidation rate constants (0.1057 to 0.1525mg² cm⁻⁴ h⁻¹). Furthermore, static corrosion tests in AlF₃-KF-Al₂O₃ molten salt demonstrated that HESO effectively mitigated the erosion of the metallic phase by the fluoride molten salt. This study holds significant potential by providing new insights for the design of inert anodes for aluminum electrolysis and exploring the practical applications of high-entropy oxide materials.

Calcium titanate corrosion inhibitor enabling carbon as inert anode for oxygen evolution in molten chlorides

The corrosion inhibition efficacy of titanate (CaTiO3) for carbon anodes in molten salts was investigated through various analytical techniques, including linear sweep voltammetry, X-ray diffraction, scanning electron microscopy, and energy dispersion spectroscopy. The results demonstrate that the addition of CaTiO3 corrosion inhibitor efficiently passivates the carbon anode and leads to the formation of a dense CaTiO3 layer during the electrolysis process in molten CaCl2−CaO. Subsequently, the passivated carbon anode effectively undergoes the oxygen evolution reaction, with an optimal current density for passivation identified at 400 mA/cm2. Comprehensive investigations, including CaTiO3 solubility tests in molten CaCl2−CaO and numerical modeling of the stability of complex ionic structures, provide compelling evidence supporting “complexation−precipitation” passivation mechanism. This mechanism involves the initial formation of a complex containing TiO2·nCaO by CaTiO3 and CaO, which subsequently decomposes to yield CaTiO3, firmly coating the surface of the carbon anode. In practical applications, the integration of CaTiO3 corrosion inhibitor with the carbon anode leads to the successful preparation of the FeCoNiCrMn high-entropy alloy without carbon contamination in the molten CaCl2−CaO.

Iron Production By Molten Sulfide Electrolysis

With urgency and incentives to cut CO2 emissions, alongside increasing demand for steel, there is a need for technologies that use solely electricity for iron ore reduction, eliminating the role of carbon as a reductant. Herein, we propose the electrolytic production of liquid cast iron using a novel sulfide route, molten sulfide electrolysis (MSE). The process operates using sulfide chemistry and an inert anode. Sulfides are well known from non-ferrous metallurgy1 and the exclusion of oxygen supports a virtual elimination of green-house gases(GHG) emissions from the reduction step. The electrolytic decomposition of iron sulfide into iron and elemental sulfur gas operates in a multi-component molten sulfide electrolyte. The underlying thermodynamics suggests the absence of trivalent iron species (Fe3+) in such conditions, supporting the reduction of only divalent iron (Fe2+) and reducing the energy need proportionally, as compared to other oxide-based routes. Iron sulfide deposits or tailings, as well as conventional iron oxide ores after sulfidation2 are some of the suitable feedstocks for MSE.Both thermal only, and galvanostatic electrochemical experiments were carried out on electrolyte droplets (~200mg) in a thermal imaging furnace, to confirm the electrolytic deposition of Fe and the evolution of sulfur in a 2-electrode set-up. Faradaic efficiency estimates based on mass-loss measurements – i.e. with respect to gaseous sulfur anodic evolution – are of the order of 90%. Key electrochemical attributes of MSE to be reported include impedance measurement at various fixed DC potentials, measurements at various current densities up to 2A/cm2, and the role of the total charge passed, to highlight the potential limitations observed in such an experimental set up. Characterization of the electrochemical deposits are also presented.

Ultrafast pressureless sintering of high conductivity Ni-based cermets inert anode for Carbon‐free electrolytic aluminum

Cermet is a promising inert anode material to solve the problem of high carbon emission and bearing C-F harmful gas generation of primary aluminum produced from bauxite using molten electrolysis with carbon anode. However, a major challenge for traditional methods is to ensure that the cermet inert anode simultaneously has good conductivity, corrosion resistance, and strength. Herein, a new ultrafast pressureless sintering (UPS) method is developed to greatly improve the conductivity of metal ceramic inert anode materials while ensuring good corrosion resistance and strength. In a departure from conventional practice that metal phase undergoes local enrichment accompanied by irregular formation of ceramic particles at low heating rates, here UPS achieves rapid diffusion and uniform distribution of metal phase at regular ceramic particle interfaces during liquid phase sintering. The new method is applicable to Ni-based cermet and the production of Ni-based cermet inert anode with excellent conductivity in a few tens of seconds has been demonstrated, which is able to improve the quality of cermet inert anode requiring a low energy input. The conductivity of cermet produced by the applied UPS process is 15-fold higher than that of other pressureless sintering processes with low heating rates. These Ni-cermet inert anodes exhibit high corrosion resistance and density with excellent mechanical properties. This rapid sintering technique facilitates the structural design of inert anodes and develops a pathway of new systems with scalable, efficient, and high-quality sintering of composite materials.

Evaluating Phase Relations in Molten Oxides for Electrochemical Operating Windows for Selective Metal Reduction

AbstractMolten oxide electrolysis is a promising pathway to decarbonize primary metal production. The oxide electrolyte is less hazardous and eco‐toxic than halides (molten salt electrolysis) and, with the use of renewably generated electricity and an inert anode, oxygen is produced as a by‐product instead of CO2. Building fundamental understanding of electrolytic operating windows requires starting with simple chemistries. Here we investigate a simplified binary oxide system as an electrolyte for titanium oxide reduction to metal. The TiO2−Na2O binary system forms a eutectic, reducing the liquidus temperature over a wide composition range. FactSage 8.1 predictions suggested Ti reduction would become favorable over Na reduction for compositions greater than 0.49 mole fraction TiO2. The prediction was validated by the detection of metallic Ti after electrolysis experiments. However, the reduction efficiency was too low (0.24±0.08 % at −0.1 V vs Ti reference electrode) for the TiO2−Na2O system to be a viable industrial electrolyte for Ti production. Scoping for other binary oxide systems was performed for electrolytic production of two critical metals, tantalum and neodymium. Based on TiO2−Na2O system's predicted behavior, candidate binary oxide systems were identified that contained congruently melting line compounds flanked by eutectic reactions from the ACerS‐NIST Phase Equilibria diagrams database.

In-Sn alloy extraction through novel process for recycling of spent ITO targets

The feasibility of utilizing spent indium tin oxide (s-ITO) as an inert anode for the recovery of s-ITO targets through the electro-deoxidation was investigated, which concurrently achieved the avoidance of molten salt pollution caused by graphite anodes and maximized the utilization of resources by using the same material as the anode and cathode. Various techniques including anodic polarization and Tafel polarization were employed to evaluate the corrosion resistance and electrochemical stability of s-ITO. XRD and XPS are used to analyze the phase and valence state of the anodic materials and products, respectively. The practicability of replacing graphite anodes with s-ITO anodes was discussed in detail under identical electrolytic conditions. When using s-ITO as the anode, the carbon content and oxygen content in the electrolytic product are 31 ppm and 57 ppm, respectively, which are lower than those of graphite as the anode. Moreover, based on the results of above investigations, a cathode and anode collaborative electrolytic device was devised to maximize the utilization of resources and recovery of high-value products. The findings indicate that s-ITO has good corrosion resistance when used as an inert anode.

Cu(N2)-Li Battery for Ammonia Synthesis.

The cathodic mechanism of Li-N2 batteries is similar to Li-mediated N2 reduction (LiNR). Herein, the Li-N2, LiNR, and Cu-Li battery were amalgamated to a milliliter-scale Cu(N2)-Li system. The utilization of a lithium anode with lithium oxidation reaction (LiOR), ensures an uninterrupted supply of lithium ions to active N2. LiOR not only enhances electrolyte stability but also reduces voltage by stripping Li ions, in contrast to the inert platinum anode, commonly employed in LiNR. Notably, an unusual observation of ammonia accumulation within the anode chamber elucidates the presence and role of reaction intermediates. The charging process aimed at lithium regeneration faces high polarization, and a cycling procedure involving low-current charging was proposed to improve cycling. This study integrates insights from three distinct research directions to leverage their respective advantages and scientific insights. The Li-N2 battery emerges as a highly advantageous strategy for ammonia synthesis due to the progressiveness of lithium anode.

Inertia of Technology Stocks: A Technology-Explicit Model for the Transition toward a Low-Carbon Global Aluminum Cycle.

Low-carbon technologies are essential for the aluminum industry to meet its climate targets despite increasing demand. However, the penetration of these technologies is often delayed due to the long lifetimes of the industrial assets currently in use. Existing models and scenarios for the aluminum sector omit this inertia and therefore potentially overestimate the realistic mitigation potential. Here, we introduce a technology-explicit dynamic material flow model for the global primary (smelters) and secondary (melting furnaces) aluminum production capacities. In business-as-usual scenarios, we project emissions from smelters and melting furnaces to rise from 710 Mt CO2-eq./a in 2020 to 920-1400 Mt CO2-eq./a in 2050. Rapid implementation of inert anodes in smelters can reduce emissions by 14% by 2050. However, a limitation of emissions compatible with a 2 °C scenario requires combined action: (1) an improvement of collection and recycling systems to absorb all the available postconsumer scrap, (2) a fast and wide deployment of low-carbon technologies, and (3) a rapid transition to low-carbon electricity sources. These measures need to be implemented even faster in scenarios with a stronger increase in aluminum demand. Lock-in effects are likely: building new capacity using conventional technologies will compromise climate mitigation efforts and would require premature retirement of industrial assets.

A Membrane-Free Rechargeable Seawater Battery Unlocked by Lattice Engineering.

Seawater batteries that directly utilize natural seawater as electrolytes are ideal sustainable aqueous devices with high safety, exceedingly low cost, and environmental friendliness. However, the present seawater batteries are either primary batteries or rechargeable half-seawater/half-nonaqueous batteries because of the lack of suitable anode working in seawater. Here, a unique lattice engineering to unlock the electrochemically inert anatase TiO2 anode to be highly active for the reversible uptake of multiple cations (Na+, Mg2+, and Ca2+) in aqueous electrolytes is demonstrated. Density functional theory calculationsfurther reveal the origin of the unprecedented charge storage behaviors, which can be attributed to the significant reduction of the cations diffusion barrier within the lattice, i.e., from 1.5 to 0.4eV. As a result, the capacities of anatase TiO2 with 2.4% lattice expansion are ≈100 times higher than the routine one in natural seawater, and ≈200 times higher in aqueous Na+ electrolyte. The finding will significantly advance aqueous seawater energy storage devices closer to practical applications.

Additive manufacturing multiscale ultrahigh strength and high accuracy nanocrystalline nickel structures and components using ultrafine anode scanning through-mask electrodeposition

Through-mask electrodeposition (TM-ECD) is a widely-used technique for manufacturing mesoscale and microscale precision metal features and components in a mass-production manner. However, in most cases, the traditional TM-ECD processes are very hard to fabricate high-accuracy multiscale (macroscale, mesoscale, microscale) features simultaneously due to the inherent nonuniform distribution of the applied energy fields including electric field, mass transportation field, and temperature field during electrodeposition. Furthermore, nanocrystalline microstructures have not been realized by TM-ECD without additional nonmetallic additives or mechanical stirring. Here, a ultrafine anode scanning through-mask electrodeposition (UASTM-ECD) process operated at DC mode is developed to further trigger the formation of nanocrystalline without additives, in which a ultrafine wire-shaped inert anode embedded in a stirring paddle reciprocates over the top surfaces of the through-masks with a spacing of less than 100 μm. Experimental investigations into this unique TM-ECD were systemically carried out by examining the geometrical profile, surface morphologies, forming accuracy, grain size, and mechanical properties of the fabricated nanocrystalline structures and components. Our findings showed that the UASTM-ECD can facilitate to achieve a significantly improved thickness distribution uniformity and considerably fine nanocrystalline ultrapure microscale and mesoscale complex cross-sectional arrays components over the traditional TM-ECD processes even though the multiscale features and components are simultaneously deposited on the same substrate. These result from the unique deposition manner of the UASTM-ECD in which the metal materials are intermittently filled into the patterned photoresist cavity molds very locally and at the extremely high current densities. This versatile approach eliminates the possible contamination by additives and integration difficulty of mechanical stirring, which can be used to fabricate the multiscale ultrahigh strength and high accuracy ultrapure nanostructured-material features.

Overpotential on Oxygen-Evolving Platinum and Ni-Fe-Cu Anode for Low-Temperature Molten Fluoride Electrolytes

To eliminate climate gas emissions from aluminum electrolysis, modifying a cryolite-based electrolyte partly replacing Na with K reduces liquidus, allowing a process temperature of 800°C. This enables the use of various metallic alloys for oxygen-evolving inert anode technology. This alternative process requires a higher energy efficiency to compensate for an increased reaction voltage, which highlights the importance of evaluating the kinetics and overpotential on oxygen-evolving anodes. This study evaluates anodic overpotentials using steady-state polarization on platinum and three Ni-Fe-Cu-based alloy compositions in a KF-NaF-AlF3-Al2O3(sat.) electrolyte at 800°C. The polarization curve on the platinum anode reveals two linear Tafel regions, while Ni-Fe-Cu anodes exhibit a single Tafel region. Notably, Ni-Fe-Cu anodes treated with high-temperature air oxidation to develop a pre-formed oxide layer exhibit better electrocatalytic activity than untreated anodes of corresponding composition. The kinetic equations, based on a theoretical model for the proposed mechanism of the oxygen evolution reaction, are derived and utilized to simulate overpotential and current, taking into account surface coverage. This model accurately predicts the two experimentally observed Tafel regions on the platinum anode, indicating a two-step charge transfer-controlled mechanism. We illustrate that multiple Tafel slopes can be attributed to the potential-dependent surface coverage of an adsorbate and can be correlated with the particular rate-determining step.

Plastic shaping of NiFe2O4‐based cermets using granular feedstock: microstructure and mechanical properties

AbstractCarbon‐free aluminum electrolysis is crucial for achieving the low‐carbon development and carbon neutrality in the future aluminum industry. NiFe2O4‐based cermet is the potential candidate of inert anode, benefiting from the satisfactory corrosion resistance and high‐temperature conductivity. Herein, complex‐shaped 25(Cu–20Ni)/(NiFe2O4–10NiO) cermets were fabricated via a plastic shaping method using the polyformaldehyde (POM)‐based granular feedstock. The feedstock showed uniform microstructure with evenly dispersed powders wrapped in polymers, which exhibited shear‐thinning behavior during molding. Roles of the feedstock with different powder loadings in the morphology and bending strength of the cermets were investigated. High‐precision gear parts exhibited no deformation, a high relative density over 98%. Investigation of different powder loadings revealed that parts prepared with a 56 vol% loading demonstrate the excellent performance, and possessed an impressive flexural strength of 178.4 MPa. This achievement provided a foundation for the future utilization of complex‐shaped inert anode material components in industrial applications.

Electrolytic Reduction of CO2 Using Perovskite-Type La-Based Oxide Anodes in Chloride Molten Salts

CO2 decomposition in molten salt is an attractive process as a new method for achieving carbon neutrality, for which the development of inert anodes and electrolytic bath suitable for CO2 decomposition is essential. Previous studies mostly investigated the electrode material but did not investigate the relationship between the electrolysis bath and electrode reaction. The effects of the electrolytic bath on anode reaction and the development of inert anode material with high current density were investigated in this study. Electrolytic bath of LiCl–CaCl2 and NaCl–CaCl2 with similar oxide solubilities were compared, finding that LiCl–CaCl2 melts exhibited higher oxidation current density in cyclic voltammetry and more stable potential transition in galvanostatic electrolysis. Among the five anodes with a perovskite-type La-based oxide were investigated, La0.6Sr0.4CoO3−δ showed the highest electrical conductivity and the largest increase in oxidation current density. CO2 decomposition tests were performed in LiCl–CaCl2 with CaO and CaCO3 added as a CO2 source at 873 K using La0.6Sr0.4CoO3−δ anode. Only C and O2 were recovered from CO2 by electrolysis test. The current efficiencies were 78.3% and 54.4%, respectively. The La0.6Sr0.4CoO3−δ anode exhibited excellent corrosion resistance, demonstrating its potential as a promising inert anode for CO2 decomposition.

Tetrabutylammonium Borohydride: A Sacrificial Reductant in Organic Electrosynthesis

In this work, tetrabutylammonium borohydride (TBAB) is investigated as a terminal reductant in reductive organic electrosynthesis as replacement for sacrificial anodes. Sacrificial anodes are typically based on easily oxidized metals, such as Mg, Zn, or Al, and are consumed during the reaction, resulting in stoichiometric metal waste. In contrast, oxidation of TBAB enables the use of inert anodes and results in anodic H2 formation, effectively serving as the inverse of cathodic proton reduction. Our results indicate that TBAB oxidation at carbon-based electrodes can replace the use of sacrificial anodes in several organic electrosynthetic reactions, including Birch reduction, hydrodesulfurization of thioethers, hydrodeoxygenation of alcohols1, and cross-electrophile couplings.References Villo, P.; Lill, M.; Alsaman, Z.; Soto Kronberg, A.; Chu, V.; Ahumada, G.; Agarwala, H.; Ahlquist, M.; Lundberg, H. Electroreductive Deoxygenative C–H and C–C Bond Formation from Non-Derivatized Alcohols Fueled by Borohydride Oxidation. ChemElectroChem. 2023, accepted for publication (a previous version can be found in ChemRxiv, DOI: 10.26434/chemrxiv-2023-tw9l1-v2.) Figure 1

TOWARDS A SUSTAINABLE FUTURE IN ALUMINIUM PRODUCTION: ENVIRONMENTAL AND ECONOMIC BENEFITS OF REVOLUTIONARY INERT ANODE TECHNOLOGY

The aluminium industry currently produces significant greenhouse gas emissions and relies on energy-intensive production technology, substantially burdening the environment. The primary goal of aluminium production is to achieve expertise in carbon-free technologies and alternative energy sources. Global business companies and scientific foundations are crucial in driving progress towards sustainability objectives. Reducing the average 16 tonnes of CO2 emissions and 14,000 kWh of electricity required for every tonne of primary aluminium production presents a significant challenge. Traditional aluminium production can incur environmental costs of up to $1,000 per tonne. Addressing this issue is unfeasible without revolutionary scientific and technological innovations. The development of inert anode technology is considered one of the greatest innovations of the 21st century, potentially transforming the aluminium industry fundamentally. Due to the threat of global climate change, green strategies have become a top priority on the world’s political and economic agendas, with many areas setting net zero CO2 targets for 2050. This scientific study highlights the importance of integrating inert anode technology into the green platform of the aluminium industry as a crucial step towards achieving long-term sustainability.

Molecular Dynamics Simulation of the Effect of Zr, B, C, W, Ti, Hf, Nb, and Ta Elements on the K3AlF6-Na3AlF6-AlF3 Molten Salt System

The criticality of the physicochemical properties of the K3AlF6-Na3AlF6-AlF3 molten salt system to electrolytic aluminum production is a topic of great interest. In this study, molecular dynamics simulations were conducted to investigate the impact of Zr, B, C, W, Ti, Hf, Nb, and Ta on various properties including the radial distribution function, coordination number, viscosity, and conductivity of the 23.4K3AlF6-54.6Na3AlF6-22AlF3 molten salt electrolyte under conditions of 1173 K and standard atmospheric pressure. Significant and noteworthy discoveries were yielded by the simulations. A decrease in the coordination between Al-F and conductivity was brought about by the inclusion of B2-. Likewise, higher Zr4+ content resulted in a decrease in the coordination number between Al-F and viscosity. The impact of carbon was insignificant, while the presence of tungsten reduced the coordination number between Al-F. On the other hand, titanium, hafnium, niobium, and tantalum facilitated the decomposition of Al-F ligands, leading to an increased coordination between Al-F and the ions in the electrolyte. These findings provide valuable insights for future investigations on inert anodes containing Zr, B, Ti, Hf, Nb, and Ta. By understanding the effects of these elements on the physicochemical properties of the molten salt electrolyte, this work lays a solid foundation for further research in the field.

Study of the Process of Electrochemical Oxidation of Active Pharmaceutical Substances on the Example of Nitrofurazone ((2E)-2-[(5-Nitro-2-furyl)methylene]hydrazine Carboxamide)

The effective mineralization of nitrofurazone (10–100 mg L−1) was performed in aqueous solutions in the presence of chloride ions by electrochemical treatment. The destruction of the organic pollutant molecules was due to their interaction with active oxygen- and chlorine-containing species forming at the inert anode (Pt/Ti or BDD) during electrolysis. Measurements of nitrofurazone concentration, chemical oxygen demand (COD) and total organic carbon (TOC) were used to estimate the removal efficiency of the pollutant. Both the pollutant oxidation rate and the degree of its mineralization were higher for the BDD anode due to the higher anode potentials on it in the course of electrolysis, which provides a high rate of active oxidizer species generation. As a result, practically full nitrofurazone molecule destruction (>99%) was achieved in 30 min at an anodic current density of 0.1 A cm−2, a volume current density of 1.33 A L−1 and pH 2 using BDD anodes. On the other hand, the nitrafurazone degradation efficiency was about 95% for Pt/Ti anodes under the same conditions. Additionally, byproducts of nitrofurazone electrooxidation were investigated by means of liquid chromato-mass-spectrometry (LC/MS). It was found that the initial decolorization of nitrofurazone solution, which occurs during the first 5 min of electrolysis, is due to the formation of a dichloro derivative of nitrofurazone, which causes the destruction of the π−conjugated bond system. Further electrolysis resulted in the almost complete destruction of the dichloro derivative within 30 min of electrochemical treatment.

Steel ceramic composite anodes based on recycled MgO–C lining bricks for applications in cryolite/aluminum melts

Novel manufacturing route for composite inert anodes containing 60:40 of 316 L stainless steel and MgO powder obtained from recycled MgO–C brick material has been developed and evaluated. After burnout of residual carbon from the recycled MgO–C powder, MgO and steel were granulated and pre-sintered in order to generate agglomerates of composite material acting as coarse grains within the composite material, and thus lowering the sintering-related shrinkage. The pre-sintered granules were mixed with raw steel and MgO powder in order to achieve a high particle packing and subsequently cold isostatically pressed in the form of electrodes. All manufactured anode samples were subjected to sintering at 1350 °C and pre-oxidation at different temperatures – 800 °C, 900 °C, and 1000 °C. Afterwards, mechanical and electrical properties of the manufactured electrodes were characterized. The results show that upcycling of the MgO–C material enables manufacturing of sophisticated electrode products, which can be applied in the aluminum industry.