ECS Electrochemistry Letters

A novel electrochemically induced method for ammonia synthesis (eU2A) on demand from urea in alkaline media was demonstrated. A Nickel based electrode was employed as the active catalyst. The effective rate of ammonia generation of the eU2A process at 70°C is ∼28 times higher than the thermal hydrolysis (THU) of urea. The eU2A operates at lower temperature (55% lower) and pressure (6 times lower) than the THU; this could lead to significant energy savings. The process finds applications on selective catalytic reduction (SCR) for the removal of nitride oxide from combustion systems (e.g., diesel vehicles, power plants, etc.).

Lithium-ion batteries are prone to failure at low temperatures and dendrite growth during charging is one suspect. We attempt to understand lithium dendrite growth by observing their number, initiation time and growth rate at ambient and sub-ambient temperatures: −10°C, 5°C, and 20°C using an in-situ optical microscopy cell (Li⁰|Li⁰). We find that while dendrites initiate quickly at −10°C, the cells at 5°C short-circuit most rapidly due in part to a favorable morphology at this temperature. The experimental approach has broad applicability to other electrochemical energy storage technologies where mass transport limitations are present at low temperatures, particularly Li-air, Li-S, and Zn-air batteries.

Improved charge/discharge performance of Iron-ion/Hydrogen redox flow battery (RFB) electrolyte with a mixed FeSO₄ and FeCl₂ is reported. Addition of Cl⁻ ions into a sulfate electrolyte changes the charge/discharge behavior of the sulfate electrolyte leading to a reduction in charging potential for a mixed FeSO₄ and FeCl₂ electrolyte system. This suggests that a sulfate/chloride electrolyte system can lead to improved charge/discharge of the Fe-ion/H₂ RFB. Reverse addition of FeSO₄ to FeCl₂ showed a decrease in the mixed electron transfer efficiency (experimental current relative to theoretical) equivalent to a decrease in electrolyte performance. We deduce that 0.8 M FeCl₂ corrosive electrolyte can be replaced by less corrosive mixture of 46 mol % Cl⁻ in 0.8 M FeSO4 to achieve the same performance that can be obtained using an all chloride system.

Two procedures to introduce a lithium metal reference electrode into commercially manufactured lithium-ion pouch cells (Kokam SLPB 533459H4) are described and compared. By introducing a stable reference potential, the individual behavior of the positive and negative electrodes can be studied in operando under normal cycling. Unmodified cells and half-cells made from harvested electrode material were cycled under identical conditions to the modified cells to compare capacity degradation during cycling and thus validate each modification procedure for degradation testing. A configuration that did not affect the performance of the cell over 20 cycles was successfully developed.

A P2-layered oxide using copper as the active redox metal has been discovered. It has a composition of Na_⅔Cu_⅓Mn_⅔O₂, and can withstand a thousand cycles, maintaining 61% of its original capacity. We demonstrate that copper can enable not only high voltage, but also excellent stability. This work opens up a new avenue of oxide design for high energy, cost effective battery systems.

Electroplating of aluminum (Al) on silicon (Si) substrates has been demonstrated in an above-room-temperature ionic liquid for the metallization of wafer-Si solar cells. The electrolyte was prepared by mixing anhydrous aluminum chloride and 1-ethyl-3-methylimidazolium tetrachloroaluminate. The plating was carried out by means of galvanostatic electrolysis. The structural and compositional properties of the Al deposits were characterized, and the sheet resistance of the deposits revealed the effects of pre-bake conditions, deposition temperature, and post-deposition annealing conditions. It was found that dense, adherent Al deposits with resistivity in the high 10⁻⁶ Ω-cm range can be reproducibly obtained directly on Si substrates.

We synthesized a copper rubeanate metal organic framework (CR-MOF) which has the potential to improve the catalytic activity of electrochemical reduction of CO₂ due to its characteristics of electronic conductivity, proton conductivity, dispersed reaction sites, and nanopores. Synthesized CR-MOF particles were dropped on carbon paper (CP) to form a working electrode. The onset potential for CO₂ reduction of a CR-MOF electrode was about 0.2 V more positive than that observed on a Cu metal electrode in an aqueous electrolyte solution. Our analysis of the reduction products during potentiostatic electrolysis showed formic acid (HCOOH) to be virtually the only CO₂ reduction product on a CR-MOF electrode, whereas a Cu metal electrode generates a range of products. The quantity of products from the CR-MOF electrode was markedly greater (13-fold at −1.2 V vs. SHE) than that of a Cu metal electrode. Its stability was also confirmed.

In situ X-ray diffraction (XRD) technique combined with electrochemical analysis was used for investigating the structural changes of nickel hydroxide catalysts in alkaline media and to provide a better understanding of the reaction mechanism of urea electrooxidation for applications in hydrogen production, fuel cells, and sensors. The evolution of XRD patterns reveals Ni(OH)₂ is electrochemically oxidized to NiOOH at cell voltages from 1.2 to 1.6 V. The generated NiOOH reacts with urea and thus is reduced back to Ni(OH)₂, while urea is concurrently oxidized. The technique can be extended to other electrochemical systems (alkaline rechargeable batteries, supercapacitors, and fuel cells).

This study examined the feasibility of using a sintered Ag/AgCl electrode as a combined reference (RE) and counter electrode (CE) for polarization measurements in thin film solutions. The combined electrode provided uniform current distribution without altering the thin film electrolyte composition. This approach avoids the problems of distorted current distributions inherent in the use of reference and counter electrodes positioned away from the working electrode (WE) under thin film conditions.

Reduced graphene oxide (RGO) is prepared by thermal exfoliation of graphite oxide in air. Symmetric RGO/RGO supercapacitors are constructed in a non-aqueous electrolyte and characterized. The values of energy density are 44 Wh kg⁻¹ and 15 Wh kg⁻¹, respectively at 0.15 and 8.0 kW kg⁻¹. The symmetric supercapacitor exhibits stable charge/discharge cycling tested up to 3000 cycles. The low-temperature thermal exfoliation approach is convenient for mass production of RGO at low cost and it can be used as electrode material for energy storage applications.

We synthesized a copper rubeanate metal organic framework (CR-MOF) which has the potential to improve the catalytic activity of electrochemical reduction of CO₂ due to its characteristics of electronic conductivity, proton conductivity, dispersed reaction sites, and nanopores. Synthesized CR-MOF particles were dropped on carbon paper (CP) to form a working electrode. The onset potential for CO₂ reduction of a CR-MOF electrode was about 0.2 V more positive than that observed on a Cu metal electrode in an aqueous electrolyte solution. Our analysis of the reduction products during potentiostatic electrolysis showed formic acid (HCOOH) to be virtually the only CO₂ reduction product on a CR-MOF electrode, whereas a Cu metal electrode generates a range of products. The quantity of products from the CR-MOF electrode was markedly greater (13-fold at −1.2 V vs. SHE) than that of a Cu metal electrode. Its stability was also confirmed.

Porous flower-like α-Fe₂O₃ nanostructures synthesized by an ethylene glycol mediated self-assembly process are crystalline and porous with BET surface area of 64.6 m² g⁻¹. The discharge capacitance is 127 F g⁻¹ when the electrodes are cycled in 0.5 M Na₂SO₃ at a current density of 1 A g⁻¹. Capacitance retention after 1000 cycles is about 80% of the initial capacitance. The high discharge capacitance and its retention are attributed to high surface area and porosity of the iron oxide. As the iron oxides are inexpensive, the nano α-Fe₂O₃ is expected to be of potential use for supercapacitor application.

In situ X-ray diffraction (XRD) technique combined with electrochemical analysis was used for investigating the structural changes of nickel hydroxide catalysts in alkaline media and to provide a better understanding of the reaction mechanism of urea electrooxidation for applications in hydrogen production, fuel cells, and sensors. The evolution of XRD patterns reveals Ni(OH)₂ is electrochemically oxidized to NiOOH at cell voltages from 1.2 to 1.6 V. The generated NiOOH reacts with urea and thus is reduced back to Ni(OH)₂, while urea is concurrently oxidized. The technique can be extended to other electrochemical systems (alkaline rechargeable batteries, supercapacitors, and fuel cells).

Lithium tetramethyl borate (LTMB, LiB(OCH₃)₄) has been prepared and investigated as a novel cathode film forming additive to improve the performance of LiNi_0.5Mn_1.5O₄ cathodes cycled to high potential (4.25-4.8 V). Addition of LTMB to 1.2 M LiPF₆ in EC/EMC (3/7, v/v) improves the capacity retention of graphite/LiNi_0.5Mn_1.5O₄ cells cycled at 55°C. The added LTMB is sacrificially oxidized on the surface of the cathode during the first charging cycle. Ex-situ surface analysis of the LiNi_0.5Mn_1.5O₄ by X-ray photoelectron spectroscopy (XPS) reveals the presence of a borate based passivating layer which appears to inhibit electrolyte oxidation on the cathode surface.

Lithium-ion batteries are prone to failure at low temperatures and dendrite growth during charging is one suspect. We attempt to understand lithium dendrite growth by observing their number, initiation time and growth rate at ambient and sub-ambient temperatures: −10°C, 5°C, and 20°C using an in-situ optical microscopy cell (Li⁰|Li⁰). We find that while dendrites initiate quickly at −10°C, the cells at 5°C short-circuit most rapidly due in part to a favorable morphology at this temperature. The experimental approach has broad applicability to other electrochemical energy storage technologies where mass transport limitations are present at low temperatures, particularly Li-air, Li-S, and Zn-air batteries.

Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR and solution NMR spectroscopy and the evolved gases have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate and ethylene and lithium propylene dicarbonate and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate and ethane and lithium methyl carbonate and methane, respectively. Lithium carbonate is not observed as a reduction product.

We report on single-electrode electrochemical impedance spectroscopy studies of an all-vanadium redox battery using a dynamic hydrogen reference electrode. The negative electrode, comprising the V²⁺/V³⁺ couple, contributes approximately 80% of the total cell overpotential during discharge. The impedance spectra measured at the negative electrode exhibit high-frequency, semicircular arcs which correspond to the double layer capacitance in parallel with a faradaic resistance. The faradaic resistance decreases in magnitude with increasing polarization. Integration of the current-dependent faradaic resistance quantifies the fraction of the overvoltage that is attributed to the kinetic limitations of the charge transfer reaction.

HF concentration and generation rate in organic LiPF₆-based carbonate electrolytes at 60°C was studied. Various compositions of linear and cyclic carbonates were used to determine the electrolyte degradation mechanism. Diethyl carbonate (DEC) has been identified as a promoter of HF formation, yielding 1 mmol/L of HF in 1 mol/L LiPF₆ DEC after 200 hours at 60°C. Prolonged aging of 1 mol/L LiPF₆, EC:DEC electrolyte at room temperature results in a significantly lower HF formation rate with time. Spiking the aged electrolyte with 500 ppm of water restores the HF formation rate to a level typical for a freshly prepared electrolyte.

Nickel matrix composite coatings were successfully fabricated by the incorporation of h-BN (hexagonal boron nitride) nanosheets using the pulse electrodeposition technique. Electrodeposition was carried out by dispersing 5–20 g/L of h-BN nanosheets into the sulfamate electrolytic bath. Composite coatings have shown smooth surface over pure nickel coating. XRD patterns of the composites have shown mixed orientations of crystallites unlike pure nickel which showed preferred (100) orientation. The results demonstrated that the incorporation of h-BN nanosheets into the nickel matrix significantly improves corrosion resistance in 3.5 wt% NaCl solution.

The expected theoretical energy density of Na₂CoPO₄F as a cathode material for sodium ion batteries surpasses 500 Wh kg⁻¹ even if considering only one-electron utilized per formula unit. It is therefore one of the highest energy density cathode materials for sodium-ion batteries. In this work, pure phase Na₂CoPO₄F/C nanocomposite was successfully synthesized by a spray-drying and high temperature sintering method. The Na₂CoPO₄F/C nanocomposite, when tested vs. sodium metal, delivered a discharge capacity of 107 mAh g⁻¹ with a voltage plateau at 4.3 V.