Use of Hydrogel Electrolyte in Zn-MnO2 Rechargeable Batteries: Characterization of Safety, Performance, and Cu2+ Ion Diffusion

Achieving commercially acceptable Zn-MnO2 rechargeable batteries depends on the reversibility of active zinc and manganese materials, and avoiding side reactions during the second electron reaction of MnO2. Typically, liquid electrolytes such as potassium hydroxide (KOH) are used for Zn-MnO2 rechargeable batteries. However, it is known that using liquid electrolytes causes the formation of electrochemically inactive materials, such as precipitation Mn3O4 or ZnMn2O4 resulting from the uncontrollable reaction of Mn3+ dissolved species with zincate ions. In this paper, hydrogel electrolytes are tested for MnO2 electrodes undergoing two-electron cycling. Improved cell safety is achieved because the hydrogel electrolyte is non-spillable, according to standards from the US Department of Transportation (DOT). The cycling of "half cells" with advanced-formulation MnO2 cathodes paired with commercial NiOOH electrodes is tested with hydrogel and a normal electrolyte, to detect changes to the zincate crossover and reaction from anode to cathode. These half cells achieved ≥700 cycles with 99% coulombic efficiency and 63% energy efficiency at C/3 rates based on the second electron capacity of MnO2. Other cycling tests with "full cells" of Zn anodes with the same MnO2 cathodes achieved ~300 cycles until reaching 50% capacity fade, a comparable performance to cells using liquid electrolyte. Electrodes dissected after cycling showed that the liquid electrolyte allowed Cu ions to migrate more than the hydrogel electrolyte. However, measurements of the Cu diffusion coefficient showed no difference between liquid and gel electrolytes; thus, it was hypothesized that the gel electrolytes reduced the occurrence of Cu short circuits by either (a) reducing electrode physical contact to the separator or (b) reducing electro-convective electrolyte transport that may be as important as diffusive transport.

Room-Temperature Pseudo-Solid-State Iron Fluoride Conversion Battery with High Ionic Conductivity

Li-metal batteries (LMBs) employing conversion cathode materials (e.g., FeF₃) are a promising way to prepare inexpensive, environmentally friendly batteries with high energy density. Pseudo-solid-state ionogel separators harness the energy density and safety advantages of solid-state LMBs, while alleviating key drawbacks (e.g., poor ionic conductivity and high interfacial resistance). In this work, a pseudo-solid-state conversion battery (Li-FeF₃) is presented that achieves stable, high rate (1.0 mA cm^-2) cycling at room temperature. The batteries described herein contain gel-infiltrated FeF₃ cathodes prepared by exchanging the ionic liquid in a polymer ionogel with a localized high-concentration electrolyte (LHCE). The LHCE gel merges the benefits of a flexible separator (e.g., adaptation to conversion-related volume changes) with the excellent chemical stability and high ionic conductivity (∼2 mS cm^-1 at 25 °C) of an LHCE. The latter property is in contrast to previous solid-state iron fluoride batteries, where poor ionic conductivities necessitated elevated temperatures to realize practical power levels. The stable, room-temperature Li-FeF₃ cycling performance obtained with the LHCE gel at high current densities paves the way for exploring a range of architectures including flexible, three-dimensional, and custom shape batteries.

The Role of Electrolyte Composition in Enabling Li Metal-Iron Fluoride Full-Cell Batteries

FeF₃ conversion cathodes, paired with Li metal, are promising for use in next-generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg^-1 compared to 690 Wh kg^-1 for LiNi_0.5 Mn_1.5 O₄ ; however, many successful studies on FeF₃ cathodes are performed in cells with a large (>90-fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full-cell compatibility, the electrolyte must produce both a protective solid-electrolyte interphase and cathode-electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2-dimethoxyethane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full-cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF₃ cell, with both limited Li and high capacity FeF₃ utilization.

Thin Film Electrodes for Anodic Stripping Voltammetry: A Mini-Review

Anodic stripping voltammetry (ASV) is a powerful electrochemical analytical technique that allows for the detection and quantification of a variety of metal ion species at very low concentrations in aqueous media. While early, traditional ASV measurements relied on macroscopic electrodes like Hg drop electrodes to provide surfaces suitable for plating/stripping, more recent work on the technique has replaced these electrodes with thin film metal electrodes generated in situ. Such electrodes are plated alongside the analyte species onto the surface of a primary electrode, producing a composite metal electrode from which the analyte(s) can then be stripped, identified, and quantified. In this minireview, we will explore the development and use of these unique electrodes in a variety of different applications. A number of metals (e.g., Hg, Bi, Sn, etc.) have shown promise as thin film ASV electrodes in both acidic and alkaline media, and frequently multiple metals in addition to the analyte of interest are deposited together to optimize the plating/stripping behavior, improving sensitivity. Due to the relatively simple nature of the measurement and its suitability for a wide range of pH, it has been used broadly: To measure toxic metals in the environment, characterize battery materials, and enable biological assays, among other applications. We will discuss these applications in greater detail, as well as provide perspective on future development and uses of these thin film electrodes in ASV measurements.

A phase transition-induced photocathodic p-CuFeO2 nanocolumnar film by reactive ballistic deposition

Vertical nanocolumnar Cu–Fe–O electrodes synthesized by the reactive ballistic deposition technique followed by heat treatment in an Ar atmosphere undergo a switch for conductivity at elevated temperatures.

Anodized Nickel Foam for Oxygen Evolution Reaction in Fe-Free and Unpurified Alkaline Electrolytes at High Current Densities

To achieve practically high electrocatalytic performance for the oxygen evolution reaction (OER), the active surface area should be maximized without severely compromising electron and mass transport throughout the catalyst electrode. Though the importance of electron and mass transport has been studied using low surface area catalysts under low current densities (∼tens of mA/cm²), the transport properties of large surface area catalysts under high operating current densities (∼500 mA/cm²) for practical OER catalysis have rarely been explored. Herein, three-dimensional (3D) hierarchically porous anodized nickel foams (ANFs) with large and variable surface areas were synthesized via electrochemical anodization of 3D nickel foam and applied as OER electrocatalysts in Fe-free and unpurified KOH electrolytes. Using Fe-free and in situ Fe-doped ANF that were prepared in Fe-free and unpurified electrolytes, respectively, we investigated the interdependent effects of active surface area and transport properties on OER activity under practically high current densities. While activity increased linearly with active surface area for Fe-free ANF, the activity of Fe-doped ANF showed a nonlinear increase with active surface area due to lower electrocatalytic activity enhancement. Detailed investigations on the possible factors (Fe incorporation, mass transport, and electron transport) identified that electron transport limitations played the major role in restricting the activity enhancement with increasing active surface area for Fe-doped ANF, although Fe-doped ANF has electron transport properties better than those of Fe-free ANF. This study exemplifies the growing significance of electron transport properties in large surface area catalysts, especially those with superb intrinsic catalytic activity and high operating current density.

Effect of Selenium Content on Nickel Sulfoselenide-Derived Nickel (Oxy)hydroxide Electrocatalysts for Water Oxidation

An efficient and inexpensive electrocatalyst for the oxygen evolution reaction (OER) must be found in order to improve the viability of hydrogen fuel production via water electrolysis. Recent work has indicated that nickel chalcogenide materials show promise as electrocatalysts for this reaction and that their performance can be further enhanced with the generation of ternary, bimetallic chalcogenides (i.e., Ni_1-aM_aX₂); however, relatively few studies have investigated ternary chalcogenides created through the addition of a second chalcogen (i.e., NiX_2-aY_a). To address this, we studied a series of Se-modified Ni₃S₂ composites for use as OER electrocatalysts in alkaline solution. We found that the addition of Se results in the creation of Ni₃S₂/NiSe composites composed of cross-doped metal chalcogenides and show that the addition of 10% Se reduces the overpotential required to reach a current density of 10 mA/cm² by 40 mV versus a pure nickel sulfide material. Chemical analysis of the composites' surfaces shows a reduction in the amount of nickel oxide species with Se incorporation, which is supported by transmission electron microscopy; this reduction is correlated with a decrease in the OER overpotentials measured for these samples. Together, our results suggest that the incorporation of Se into Ni₃S₂ creates a more conductive material with a less-oxidized surface that is more electrocatalytically active and resistant to further oxidation. Importantly, oxidation does still occur, and the active catalyst is most likely a nickel (oxy)hydroxide surrounding a crystalline, conductive Ni₃S_2-xSe_x core.

Lithium Fluoride Coated Silicon Nanocolumns as Anodes for Lithium Ion Batteries

Silicon (Si) films are promising anode materials in thin-film lithium batteries due to their high capacity of 3578 mAh g^-1, but the huge volume expansion of lithiated Li₁₅Si₄ and the unstable solid electrolyte interphase (SEI) preclude their practical application. Here lithium fluoride (LiF) coated Si nanocolumns are fabricated by glancing angle evaporation to address the obstacle. The LiF coating can elevate the lithium ion diffusion coefficient (LDC) of Si electrodes upon the alloying reaction and reduce the LDC upon the SEI formation. The composition evolution of the outer SEI layer in the LiF/Si electrodes is studied by ex situ X-ray photoelectron spectroscopy. The modified surface and mitigated volume expansion enable the LiF/Si nanocolumns to exhibit superior rate capability and higher cycling stability compared with the pristine Si nanocolumns. This work demonstrates the positive effect of LiF coating for reducing the polarization and forming a robust SEI film on Si anodes.

Spatially Controlled Molecular Analysis of Biological Samples Using Nanodroplet Arrays and Direct Droplet Aspiration

Mass spectrometry (MS) has emerged as a valuable technology for molecular and spatial evaluation of biological samples. Ambient ionization MS techniques, in particular, allow direct analysis of tissue samples with minimal pretreatment. Here, we describe the design and optimization of an alternative ambient liquid extraction MS approach for metabolite and lipid profiling and imaging from biological samples. The system combines a piezoelectric picoliter dispenser to form solvent nanodroplets onto the sample surface with controlled and tunable spatial resolution and a conductive capillary to directly aspirate/ionize the nanodroplets for efficient analyte transmission and detection. Using this approach, we performed spatial profiling of mouse brain tissue sections with different droplet sizes (390, 420, and 500 μm). MS analysis of normal and cancerous human brain and ovarian tissues yielded rich metabolic profiles that were characteristic of disease state and enabled visualization of tissue regions with different histologic composition. This method was also used to analyze the lipid profiles of human ovarian cell lines. Overall, our results demonstrate the capabilities of this system for spatially controlled MS analysis of biological samples.

Probing the Degradation Chemistry and Enhanced Stability of 2D Organolead Halide Perovskites

Recent work on quasi-2D Ruddlesden-Popper phase organolead halide perovskites has shown that they possess many interesting optical and physical properties. Most notably, they are significantly more stable when exposed to moisture when compared to the typical 3D perovskite methylammonium lead iodide (MAPI); direct evidence for the chemical source of this stability remains elusive, however. Here, we present a detailed study of the superior moisture stability of a quasi-2D Ruddlesden-Popper perovskite, n-butylammonium methylammonium lead iodide (nBA-MAPI), compared to that of MAPI, and examine a simple, yet efficient, methodology to improve the stability of MAPI devices through the application of a thin layer of nBA-MAPI to the surface. By employing a variety of analytical techniques (photoluminescence, time-of-flight secondary ion mass spectrometry, cyclic voltammetry, X-ray diffraction) we determine that the improved stability of Ruddlesden-Popper perovskites is a consequence of a unique degradation pathway which produces a passivating surface layer, composed of increasingly stable phases of the 2D perovskite, via disproportionation. Our work establishes that this protective material isolates the bulk of the perovskite from a newly identified hydration layer which is found to accumulate at the C₆₀/perovskite interface of full devices, slowing further hydrolysis reactions that would damage the device. As MAPI devices degrade quickly without any protection, a surface treatment of nBA-MAPI is an efficient way to delay device deterioration by creating an artificial 2D surface layer that similarly inhibits interaction with the hydration layer.

Carbon Nitride Transforms into a High Lithium Storage Capacity Nitrogen-Rich Carbon

We describe here the metal-templated transformation of carbon nitride (C₃N₄) into nitrogen-containing carbons as anodes for Li-ion batteries (LIBs). Changing the template from the carbon- and nitrogen-immiscible Cu powder to the carbon- and nitrogen-miscible Fe powder yields different carbons; while Fe templating produces graphitized carbons of low (<10%) nitrogen content and moderate pore volume, Cu templating yields high defect-density carbons of high (32-24%) nitrogen content and larger pore volume. The Li⁺ storage capacity of the high nitrogen content and larger pore volume Cu-templated carbons exceeds that of the more graphitic Fe-templated carbons due to added contribution from Li⁺ insertion/extraction from pores and defects and to reversible faradaic Li⁺ reaction with nitrogen atoms. The Cu-templated carbon annealed at 750 °C delivers the highest specific capacity of 900 mAh g^-1 at 0.1 A g^-1 and 275 mAh g^-1 at 20 A g^-1, while also achieving a 96% capacity retention after 2000 cycles at 2 A g^-1. The fabrication of higher mass loading electrodes (4.5 mg cm^-2) provided a maximum areal capacity of 2.6 mAh cm^-2 at 0.45 mA cm^-2 (0.1 A g^-1), comparable to the capacities of commercial LIB cells and favorable compared to other reported carbon materials.

Interface Engineering and its Effect on WO3-Based Photoanode and Tandem Cell

During photoelectrochemical (PEC) water splitting, the reactions occur on the surface of the photoelectrode. Therefore, the properties of the interfaces between the various components of the electrode (semiconductor/semiconductor, semiconductor/catalyst, or photoelectrode/electrolyte) affect the PEC performance of the composite material. Notably, surface trap states may hinder charge transfer and transport properties, and also cause Fermi pinning, affecting the quasi-Fermi level and onset potential under illumination, which may in turn influence the PEC performance of the corresponding tandem cells. In this study, plate-like WO₃ array films prepared by an aqueous chemical growth method were employed to highlight the effect of interfacial properties on the performance of a WO₃-based photoanode. The Mott-Schottky and linear sweep voltammetry experiments prove the existence of surface trap states and Fermi pinning for pristine WO₃, which are alleviated after an "etching" treatment and disappeared after surface passivation by a Ga₂O₃ layer. Both etching and passivation increase the oxygen evolution activity and the Faradaic efficiency for the oxygen evolution reaction (OER). After loading a permeable catalyst (FeOOH), the photocurrent is further increased, and there is a synergistic effect between loading of the electrocatalyst with etching or passivation. The onset potentials of the samples follow the trends: etch-WO₃/FeOOH < WO₃/FeOOH ≤ WO₃/Ga₂O₃/FeOOH < etch-WO₃ < WO₃ < WO₃/Ga₂O₃, indicating that the interfacial properties have a significant effect on the PEC performance. Meanwhile, the modified WO₃-based electrode was combined with a dye-sensitized solar cell to fabricate tandem cell, which showed 2.42-fold photocurrent density compared with the pristine WO₃-based tandem cell.

Highly Efficient Photoelectrochemical Water Splitting from Hierarchical WO3/BiVO4 Nanoporous Sphere Arrays

Nanoarchitecture of bismuth vanadate (BiVO₄) photoanodes for effectively increasing light harvesting efficiency and simultaneously achieving high charge separation efficiency is the key to approaching their theoretic performance of solar-driven water splitting. Here, we developed hierarchical BiVO₄ nanoporous sphere arrays, which are composed of small nanoparticles and sufficient voids for offering high capability of charge separation. Significantly, multiple light scattering in the sphere arrays and voids along with the large effective thickness of the BiVO₄ photoanode induce efficient light harvesting. In addition, attributed to ultrathin two-dimensional Bi₂WO₆ nanosheets as the precursor, the synergy of various enhancement strategies including WO₃/BiVO₄ nanojunction formation, W-doping, and oxygen vacancy creation can be directly incorporated into such a unique hierarchical architecture during the one-step synthesis of BiVO₄ without complex pre- or post-treatment. The as-obtained photoanode exhibits a water splitting photocurrent of 5.5 mA cm^-2 at 1.23 V versus RHE under 1-sun illumination, among the best values reported up-to-date in the field.

Activation of a Nickel-Based Oxygen Evolution Reaction Catalyst on a Hematite Photoanode via Incorporation of Cerium for Photoelectrochemical Water Oxidation

There has been debate on whether Ni(OH)₂ is truly catalytically active for the photo/electrocatalytic oxygen evolution reaction. In this report, we synthesized a Ni(OH)₂ cocatalyst on a hematite photoanode and showed that, as has been proposed in other studies, the current density varies as a function of scan rate, which arises due to a photoinduced capacitive charging effect. We discovered that this photoinduced charging of Ni^2+/3+ can be overcome by mixing cerium nitrate into the Ni precursor solution. Under illumination, the NiCeO_x cocatalyst on a hematite photoanode exhibited an approximately 200 mV cathodic shift in onset potential and a ∼53% enhancement in photocurrent at 1.23 V vs RHE. Material characterization by electrochemical impedance spectroscopy revealed that the Ni species create a p-n junction across the charge space region, which facilitates collection of the photogenerated holes by the cocatalyst layer, and core level X-ray photoelectron spectroscopy showed that Ce incorporated into the Ni-based cocatalyst layer may possibly induce the oxidation of the Ni species. In addition, we observed a reduction in binding energies of Ni after photoelectrochemical water splitting reactions, which suggests that the lattice oxygen of the NiCeO_x is consumed in the catalytic cycle, forming oxygen vacancies. The NiCeO_x cocatalyst, however, was incapable of passivating the surface recombination centers of the hematite photoanode, as indicated by the unaltered flat-band potential determined with Mott-Schottky analysis.

NH3-assisted chloride flux-coating method for direct fabrication of visible-light-responsive SrNbO2N crystal layers

SrNbO₂N crystal layers were prepared on niobium substrates by using an NH₃-assisted chloride flux-coating method.