A fast ceramic mixed OH-/H+ ionic conductor for low temperature fuel cells.

Low temperature ionic conducting materials such as OH- and H+ ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH-/H+ conduction in ceramic materials. SrZr0.8Y0.2O3-δ exhibits a high ionic conductivity of approximately 0.01 S cm-1 at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr0.8Y0.2O3-δ. The OH- ionic conduction of CaZr0.8Y0.2O3-δ in water was demonstrated by electrolysis of both H218O and D2O. The ionic conductivity of CaZr0.8Y0.2O3-δ in 6 M KOH solution is around 0.1 S cm-1 at 90 °C, 100 times higher than that in pure water, indicating increased OH- ionic conductivity with a higher concentration of feed OH- ions. Density functional theory calculations suggest the diffusion of OH- ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices.

Optimization of a Perovskite Oxide-Based Cathode Catalyst Layer on Performance of Direct Ammonia Fuel Cells.

To maximize fuel cell performance, transport pathways for electrons, ions, and reactants should be connected well. This demands a well-constructed microstructure in the catalyst layer (CL). Herein we design and optimize a cathode CL for a direct ammonia fuel cell (DAFC) using a perovskite oxide as the catalyst to reduce reliance on platinum group metals (PGMs). The effects of tailoring carbon, ionomer, and polytetrafluoroethylene (PTFE) content in cathode CLs (CCLs) were explored, and several DAFCs were tested. Using the same catalyst and operating conditions, the lowest maximum current density and peak power density obtained were 85.3 mA cm-2 and 5.92 mW cm-2, respectively, which substantially increased to 317 mA cm-2 and 30.1 mW cm-2 through proper carbon, ionomer, and PTFE optimization, illustrating the importance of an effective three-phase interface. The findings reveal that despite employment of an active catalyst for oxygen reduction at the cathode site, the true performance of the catalyst cannot be reflected unless it is supported by proper design of the CCL. The study also reveals that by optimizing the CCL, similar performances to those of Pt/C-based CCLs in literature can be obtained at a cost reduction.

An Efficient Symmetric Electrolyzer Based On Bifunctional Perovskite Catalyst for Ammonia Electrolysis.

Ammonia is a natural pollutant in wastewater and removal technique such as ammonia electro-oxidation is of paramount importance. The development of highly efficient and low-costing electrocatalysts for the ammonia oxidation reaction (AOR) and hydrogen evolution reaction (HER) associated with ammonia removal is subsequently crucial. In this study, for the first time, the authors demonstrate that a perovskite oxide LaNi0.5 Cu0.5 O3-δ after being annealed in Ar (LNCO55-Ar), is an excellent non-noble bifunctional catalyst towards both AOR and HER, making it suitable as a symmetric ammonia electrolyser (SAE) in alkaline medium. In contrast, the LNCO55 sample fired in air (LNCO55-Air) is inactive towards AOR and shows very poor HER activity. Through combined experimental results and theoretical calculations, it is found that the superior AOR and HER activities are attributed to the increased active sites, the introduction of oxygen vacancies, the synergistic effect of B-site cations and the different active sites in LNCO55-Ar. At 1.23 V, the assembled SAE demonstrates ≈100% removal efficiency in 2210 ppm ammonia solution and >70% in real landfill leachate. This work opens the door for developments towards bifunctional catalysts, and also takes a profound step towards the development of low-costing and simple device configuration for ammonia electrolysers.

N,N-Dimethylacetamide-Diluted Nitrate Electrolyte for Aqueous Zn//LiMn2O4 Hybrid Ion Batteries.

N,N-Dimethylacetamide (DMA) cooperated with LiNO3 salt has previously shown to be a promising electrolyte for a Li//O2 battery, showing good stability against both the O2 electrode reaction and Li stripping/plating. In this work, DMA is hybridized with a concentrated nitrate electrolyte [2.5 m Zn(NO3)2 + 13 m LiNO3 aqueous solution] for better electrochemical stability while using less dissolved salts. The widest electrochemical stability window for this DMA-diluted electrolyte is determined as 3.1 V, the negative critical stability potential of which is -1.6 V versus Ag/AgCl, indicating desirable stability against hydrogen evolution and Zn deposition. The findings can be attributed to the weakened Li+/Zn2+ solvation sheath caused by low permittivity of DMA, as revealed through Raman spectra characterization and molecular dynamics simulation. A Zn//Zn symmetrical cell and Zn//LiMn2O4 hybrid ion batteries are assembled in air directly, attributed to the stability of DMA toward O2. Zn stripping/plating with a dendrite-free morphology is delivered for 110 h and 200 charge/discharge cycles under 1 C rate, achieving 99.0% Coulombic efficiency. The maximum capacity of the battery is 121.0 mA h·g-1 under 0.2 C rate (based on the mass of LiMn2O4), delivering an energy density of 165.8 W h·kg-1 together with 2.0 V working voltage. This work demonstrates the feasibility and validity of utilizing a relatively dilute electrolyte dissolved in oxygen for a highly stable aqueous rechargeable battery.

Investigation of Perovskite Oxide SrCo0.8 Cu0.1 Nb0.1 O3-δ as a Cathode Material for Room Temperature Direct Ammonia Fuel Cells.

Single-phase perovskite oxide SrCo0.8 Cu0.1 Nb0.1 O3-δ was synthesized using a Pechini method. X-ray diffraction (XRD) analysis indicated a cubic structure with a=3.8806(7) Å. The oxide material was combined with active carbon, forming a composite electrode to be used as the cathode in a room temperature ammonia fuel cell based on an anion membrane electrolyte and NiCu/C anode. An open circuit voltage (OCV) of 0.19 V was observed with dilute 0.02 m (340 ppm) ammonia solution as the fuel. The power density and OCV were improved upon the addition of 1 m NaOH to the fuel, suggesting that the addition of NaOH, which could be achieved through the introduction of alkaline waste to the fuel stream, could improve performance when wastewater is used as the fuel. It was found that the SrCo0.8 Cu0.1 Nb0.1 O3-δ cathode was converted from irregular shape into shuttle-shape during the fuel cell measurements. As the key catalysts for electrode materials for this fuel cell are all inexpensive, after further development, this could be a promising technology for removal of ammonia from wastewater.