Carbon-free Anode Research Articles

Enhancing Anode Reversal Tolerance by Low-Carbon and Carbon-Free Anode Catalyst Layers for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs), as the main hydrogen utilization device, are considered a powerful tool for accelerating a carbon-neutral society. However, durability and cost are the two main obstacles to the commercialization of PEMFCs. Specifically, the anode reversal has been extensively studied as a severe issue that deteriorates the performance and durability of PEMFCs and leads to cell failure. Commercially, iridium oxide (IrOx) or iridium black is intensively utilized as oxygen evolution reaction catalysts to fabricate reversal tolerant anodes (RTAs) despite the slight performance loss. However, previous studies concentrated on investigating the origin or phenomena during the cell-reversal, the key factors and mechanisms affecting the anti-reversal performance, and the interaction among the anode catalyst layer components are still yet to be explored.Hence, we systematically studied the performance using three typical carbon supports with three metal contents and different metal deposition orders by experiments and model calculations. First, we synthesized Ir outside Ir/Pt/C and Pt outside Pt/It/C with uniform nanoparticle size and distribution and characterized them comprehensively. A new parameter, “relative metal coverage”, is put forward to evaluate and predict the anti-reversal ability, showing that higher relative metal coverage results in a longer reversal time and lower polarization performance degradation rate. In addition, Ir outside Ir/Pt/C shows better reversal tolerance than Pt outside Pt/It/C, which has been rarely mentioned in previous studies. Further, the experimental results show that even though the metal coverage is high, the performance degradation caused by carbon corrosion is still inevitable. On this basis, we synthesized a high specific surface area conductive oxide Ti4O7 as the anode catalyst support to eliminate the adverse effects of carbon corrosion. Previously, the initial performance of Ti4O7-supported catalyst was obstructed by the disadvantaged electrical conductivity of Ti4O7. We obtain a comparable polarization performance after optimizing the Ti4O7-supported anode catalyst layer parameters by shortening the electron transfer pathway and increasing metal coverage. Typically, the Ir@IrOx/Pt/Ti4O7-fabricated RTA with low Ir loading displays approximately ten times longer reversal time (6 hours) and two orders of magnitude lower degradation rate than a conventional carbon-supported counterpart. The degradation origin of the Ti4O7-supported anodes is also studied by postmortem characterizations, pointing to the Pt oxidation caused by the formation of TiOx thin layers on the Pt surface. These two works aim to promote the development of highly durable and reversal tolerance anode and provide a bright perspective for practical high-performance, low-degradation, and cost-friendly RTA fabrication.

Bi-Sb Nanocrystals Embedded in Phosphorus as High-Performance Potassium Ion Battery Electrodes.

The development of high-performance potassium ion battery (KIB) electrodes requires a nanoengineering design aimed at optimizing the construction of active material/buffer material nanocomposites. These nanocomposites will alleviate the stress resulting from large volume changes induced by K+ ion insertion/extraction and enhance the electrical and ion conductivity. We report the synthesis of phosphorus-embedded ultrasmall bismuth-antimony nanocrystals (BixSb1-x@P, (0 ≤ x ≤ 1)) for KIB anodes via a facile solution precipitation at room temperature. BixSb1-x@P nanocomposites can enhance potassiation-depotassiation reactions with K+ ions, owing to several attributes. First, by adjusting the feed ratios of the Bi/Sb reactants, the composition of BixSb1-x nanocrystals can be systematically tuned for the best KIB anode performance. Second, extremely small (diameter ≈ 3 nm) BixSb1-x nanocrystals were obtained after cycling and were fixed firmly inside the P matrix. These nanocrystals were effective in buffering the large volume change and preventing the collapse of the electrode. Third, the P matrix served as a good medium for both electron and K+ ion transport to enable rapid charge and discharge processes. Fourth, thin and stable solid electrolyte interface (SEI) layers that formed on the surface of the cycled BixSb1-x@P electrodes resulted in low resistance of the overall battery electrode. Lastly, in situ X-ray diffraction analysis of K+ ion insertion/extraction into/from the BixSb1-x@P electrodes revealed that the potassium storage mechanism involves a simple, direct, and reversible reaction pathway: (Bi, Sb) ↔ K(Bi, Sb) ↔ K3(Bi, Sb). Therefore, electrodes with the optimized composition, i.e., Bi0.5Sb0.5@P, exhibited excellent electrochemical performance (in terms of specific capacity, rate capacities, and cycling stability) as KIB anodes. Bi0.5Sb0.5@P anodes retained specific capacities of 295.4 mA h g-1 at 500 mA g-1 and 339.1 mA h g-1 at 1 A g-1 after 800 and 550 cycles, respectively. Furthermore, a capacity of 258.5 mA h g-1 even at 6.5 A g-1 revealed the outstanding rate capability of the Sb-based KIB anodes. Proof-of-concept KIBs utilizing Bi0.5Sb0.5@P as an anode and PTCDA (perylenetetracarboxylic dianhydride) as a cathode were used to demonstrate the applicability of Bi0.5Sb0.5@P electrodes to full cells. This study shows that BixSb1-x@P nanocomposites are promising carbon-free anode materials for KIB anodes and are readily compatible with the commercial slurry-coating process applied in the battery manufacturing industry.

Behaviour of aluminium oxide in KF-AlF3-Al2O3 melts and suspensions

The article addresses the properties of melts and alumina suspensions based on the molten KF-AlF3-Al2O3 system, the kinetics of alumina dissolution and the rheological properties that determine the sedimentation stability. The study of such suspensions has become topical due to the prospects of their use as electrolytes in the production of aluminium using carbon-free anodes. The effects of the temperature, the particle size and the phase composition of the dispersed material and its volume fraction in the suspension on the dissolution kinetics and the sedimentation velocity are studied. The experiments were carried out over the melts with cryolite ratios 1.3 and 1.5 in the range of 750–850 °C. Three different types of aluminium oxide were used. The Reynolds numbers for sedimentation have indicated the Stokesian regime. Typical alumina dissolution rates were in the range of 0.028–0.167 g kg-1 s-1, which is close to the values reported previously. Sedimentation velocities were in the range of (0.05–3.61)∙10-2 m/s, which is several times higher than those obtained previously for ϕ = 0.24–0.32 at 700 °C. It is shown that mechanical activation of alumina increases the performance of the suspension in terms of the sedimentation stability and the dissolution rate increasing.

Silicon Electrochemistry in Molten Salts.

Silicon electrochemistry has the potential to advance sustainable energy solutions by offering environmentally friendly and secure technologies that can contribute to the low-carbon economy. Electrochemical methods use electrons directly as reducing agents, eliminating the need for harmful chemicals and offering simpler, one-step, process control. Silicon itself is the second most abundant element in the earth's crust, is nontoxic, and is a robust material offering high efficiencies in solar photovoltaics. As such, silicon currently dominates the solar energy market and could continue to do so for the next few decades. This review summarizes recent achievements in the molten salt electrochemistry of silicon, highlighting subjects of technological significance such as the production of silicon by silica electro-deoxidation, the formation of photoactive layers, silicon electrorefining, and the synthesis of semiconductors as well as nanostructures for energy storage applications. The review highlights future opportunities and challenges such as the production of highly pure silicon, the creation of carbon-free anodes for oxygen production, and silicon electrodeposition from gaseous precursors.

Understanding Carbon Corrosion Damage to Fuel Cell Electrodes: Towards Reversal Invincible

Polymer electrolyte membrane fuel cells (PEMFCs) have the advantage of low carbon dioxide emission and high efficiency over internal combustion engines in automobile applications1. However, durability of PEMFCs is one of the major obstacles for a wide range of applications. One such durability issue is the cell reversal caused by hydrogen starvation. It results in severe carbon support corrosion in the anode catalyst layer and dramatically decreases the fuel cell performances2. Material-based solutions to increasing the cell’s tolerance to reversal are preferable since it will not increase the system control complexity. Effective material-based solutions include using more corrosion-resistant carbon supports3 and adding water electrolysis catalysts to promote the oxygen evolution reaction (OER) over carbon corrosion4,5. However, these current solutions are still not resilient enough to guarantee long-term durability of PEMFCs under extreme hydrogen starvation conditions. This present work has examined different anode configurations and their effectiveness in preventing cell performance degradation during hydrogen starvation events. Three different anode structures are studied and compared, including i) a conventional Pt/C gas diffusion electrode (GDE), ii) an anode made of platinum (Pt) black and gas diffusion layer (GDL), and iii) an anode made of Pt black, a thin titanium (Ti) protection layer (10 μm in thickness), and a GDL. Figure 1a shows the change in cell voltage of PEMFCs when hydrogen was cut off while keeping a constant current density of 0.2 A/cm2. During reversals, the Pt/C GDE anode and the anode made of Pt black and a GDL experienced a sudden voltage drop within 5 min which was attributed to severe carbon corrosion damage. In contrast, the cell with Ti protection layer between the Pt black and the GDL was stable for 60 min of reversal without a large voltage drop. As shown in Figire 1b, the cell voltage of the fuel cell with an anode made of Pt black, a Ti protection layer, and a GDL decreased less than 10 % even after the accumulated voltage reversal time of 240 min. Thus the anode with Ti protection layer is much more effective compared with the common reversal tolerant anode approach of adding OER catalyst (i.e., 50 wt% IrO2) to the anode5. Both hydrogen starvation tests and post-reversal electrochemical characterizations demonstrate that using a carbon-free anode with a corrosion-resistant barrier layer is highly effective in preventing cell reversal damage over long periods of time. Figure 1

SnS2 Nanowall Arrays toward High-Performance Sodium Storage.

Cost-effective sodium ion batteries (SIBs) are emerging as a desirable alternative choice to lithium ion batteries in terms of application in large-scale energy storage devices. SnS2 is regarded as a potential anode material for SIBs because of its unique layered structure and high theoretical specific capacity. However, the development of SnS2 was hindered by the sluggish kinetics of the diffusion process and the inevitable volume change during repeated sodiation-desodiation processes. In this work, SnS2 with a unique nanowall array (NWA) structure is fabricated by one-step pulsed spray evaporation chemical vapor deposition (PSE-CVD), which could be used directly as binder-free and carbon-free anodes for SIBs. The SnS2 NWA electrode achieves a high reversible capacity of 576 mAh g-1 at 500 mA g-1 and enhanced cycling stability. Attractively, an excellent rate capability is demonstrated with ∼370 mAh g-1 at 5 A g-1, corresponding to a capacity retention of 64.2% at 500 mA g-1. The superior sodium storage capability of the SnS2 NWA electrode could be attributed to outstanding electrode design and a rational growth process, which favor fast electron and Na-ion transport, as well as provide steady structure for elongated cycling.

Chestnut-Like TiO2@α-Fe2O3 Core–Shell Nanostructures with Abundant Interfaces for Efficient and Ultralong Life Lithium-Ion Storage

Transition metal oxides caused much attention owing to the scientific interests and potential applications in energy storage systems. In this study, a free-standing three-dimensional (3D) chestnut-like TiO2@α-Fe2O3 core-shell nanostructure (TFN) is rationally synthesized and utilized as a carbon-free electrode for lithium-ion batteries (LIBs). Two new interfaces between anatase TiO2 and α-Fe2O3 are observed and supposed to provide synergistic effect. The TiO2 microsphere framework significantly improves the mechanical stability, while the α-Fe2O3 provides large capacity. The abundant boundary structures offer the possibility for interfacial lithium storage and electron transport. The as-prepared TFN delivers a high capacity of 820 mAh g-1 even after 1000 continuous cycles with a Coulombic efficiency of ca. 99% at a current of 500 mA g-1, which is better than the works reported previously. A thin gel-like SEI (solid electrolyte interphase) film and Fe0 phase yielded during charge/discharge cycling have been confirmed which makes it possible to alleviate the volumetric change and enhance the electronic conductivity. This confirmation is helpful for understanding the mechanism of lithium-ion storage in α-Fe2O3-based materials. The as-prepared free-standing TFN with excellent stability and high capacity can be an appropriate candidate for carbon-free anode material in LIBs.

Green Strategy to Single Crystalline Anatase TiO2 Nanosheets with Dominant (001) Facets and Its Lithiation Study toward Sustainable Cobalt-Free Lithium Ion Full Battery

A green hydrothermal strategy starting from the Ti powders was developed to synthesis a new kind of well dispersed anatase TiO2 nanosheets (TNSTs) with dominant (001) facets, successfully avoiding using the HF by choosing the safe substitutes of LiF powder. In contrast to traditional approaches targeting TiO2 with dominant crystal facets, the strategy presented herein is more convenient, environment friendly and available for industrial production. As a unique structured anode applied in lithium ion battery, the TNSTs could exhibit an extremely high capacity around 215 mAh g–1 at the current density of 100 mA g–1 and preserved capacity over 140 mAh g–1 enduring 200 cycles at 400 mA g–1. As a further step toward commercialization, a model of lithiating TiO2 was built for the first time and analyzed by the electrochemical characterizations, and full batteries employing lithiated TNSTs as carbon-free anode versus spinel LiNixMn2–xO4 (x = 0, 0.5) cathode were configured. The full batteries of TNSTs/LiMn2O4 an...

Free-standing molybdenum disulfide/graphene composite paper as a binder- and carbon-free anode for lithium-ion batteries

Two dimensional nanosheets, such as graphene and metal disulfides, have attracted a great deal of attention as anode materials for lithium-ion batteries, owing to their unique capability for lithium-ion storage. In this work, we integrate graphene and MoS2 nanosheets into a free standing film form using a simple vacuum filtration method. As-prepared composite film could be readily employed as a binder- and carbon-free anode for lithium-ion batteries, removing the polymeric binders and conductive carbon additives that are required for the preparation of conventional electrodes. In addition, the interconnected structure of graphene and MoS2 sheets provide a good electrical conductivity to the entire film electrode. When tested electrochemical performance as an anode for lithium-ion batteries, the composite film electrode exhibits superior performance compared to the exfoliated MoS2 electrode, such as 65.8% capacity retention at a high current rate of 1000 mA g−1 and 91.1% capacity retention after 100 cycles.

Film formation in LiBOB-containing electrolytes

Lithium bis(oxalato)borate (LiBOB), a new electrolyte salt for lithium batteries, is actively involved in the formation of the solid electrolyte interphase (SEI) at the anode. Part of this formation is an irreversible reductive reaction taking place at potentials of around 1.75 V versus Li/Li + and contributing to the irreversible capacity of anode materials in the first cycle. Cyclic voltammetry has been performed on several carbon materials as well as on Li 4Ti 5O 12 and pre-treated glassy carbon electrodes in order to achieve a better understanding of the underlying processes. It is found that the intensity of the 1.75 V peak depends on the BET specific surface area and the surface chemistry of the active material and increases with the amount of oxygen-containing surface functionalities. It is not specific to carbonaceous materials but is also observed on carbon-free anodes like Li 4Ti 5O 12. In addition, the effect of several potential impurities and of film-forming additives on the filming behaviour of LiBOB-containing electrolytes has been investigated.

New opportunities for metals extraction and waste treatment by electrochemical processing in molten salts

Molten salt electrolysis is a proven technology for the extraction of metals—all the world's primary aluminum is produced in this manner. The unique properties of molten salts also make them excellent media in which to treat a variety of forms of waste. Of special note in this regard is electrolysis in molten oxides, a concept put forward by the author, initially as a “clean technology” for producing primary metal. However, in the context of waste treatment, elcctrolysis in molten oxides is a process offering the prospect of changing the valence of dissolved heavy metals while making pure oxygen gas as the main by-product. Laboratory tests conducted at a temperature of 1550 °C on chromate sludge dissolved in melt composed of Al2O3, SiO2, CaO, and MgO have confirmed electrochemical production of oxygen on a carbon-free anode.