Structural change of carbon supported Pt nanocatalyst subjected to a step-like potential cycling in PEM-FC

Abstract In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations of the changes in the local geometric and electronic structure of Pt nanoparticles used as a cathode catalyst in proton exchange membrane fuel cell (PEMFC), working under controlled potential cycling conditions. The body of the results obtained suggests that in the first stage of \{PEMFC\} operation, small particle dissolution was a dominant process. Subsequent 100 h of work led to the progressive agglomeration of nanoparticles followed by a pronounced growth of the mean nanoparticle size. At the same time, high-quality \{XAFS\} spectra analysis demonstrated that negligible changes in structural local ordering and a slight increase in Pt 5d-electron density occurred during the whole \{FC\} operation period under consideration

Correlation of Ac-impedance and in situ X-ray spectra of LiCoO2

In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalation of lithium from LiCoO2 using a specially designed electrochemical cell. The AC-dispersions have been correlated with the cell parameters obtained from the X-ray spectra. The correlation confirms previous hypothesis on the interpretation of the AC-dispersions in terms of an equivalent circuit comprising an element that relates the change of the intrinsic electronic conductivity, occurring at the early stages of deintercalation, to the semiconductor to metal transition caused by the change of the cell parameters

Impact of 3-Cyanopropionic Acid Methyl Ester on the Electrochemical Performance of ZnMn₂O₄ as Negative Electrode for Li-Ion Batteries

Due to their high theoretical capacity, transition metal oxide compounds are promising electrode materials for lithium-ion batteries. However, one drawback is associated with relevant capacity fluctuations during cycling, widely observed in the literature. Such strong capacity variation can result in practical problems when positive and negative electrode materials have to be matched in a full cell. Herein, the study of ZnMn2O4 (ZMO) in a nonconventional electrolyte based on 3-cyanopropionic acid methyl ester (CPAME) solvent and LiPF6 salt is reported for the first time. Although ZMO in LiPF6/CPAME electrolyte displays a dramatic capacity decay during the first cycles, it shows promising cycling ability and a suppressed capacity fluctuation when vinylene carbonate (VC) is used as an additive to the CPAME-based electrolyte. To understand the nature of the solid electrolyte interphase (SEI), the electrochemical study is correlated to ex situ X-ray photoelectron spectroscopy (XPS)

Hindered Aluminum Plating and Stripping in Urea/NMA/Al(OTF)3_3 as a Cl-Free Electrolyte for Aluminum Batteries

Conventional electrolytes for aluminum metal batteries are highly corrosive because they must remove the Al$_2$O$_3$ layer to enable plating and stripping. However, such corrosiveness impacts the stability of all cell parts, thus hampering the real application of aluminum-metal batteries. The urea/NMA/Al(OTF)$_3$ electrolyte is a non-corrosive alternative to the conventional [EMImCl]: AlCl$_3$ ionic liquid electrolyte (ILE). Unfortunately, this electrolyte demonstrates poor Al plating/stripping, probably because (being not corrosive) it cannot remove the Al$_2$O$_3$ passivation layer. This work proves that no plating/stripping occurs on the Al electrode despite modifying the Al surface. We highlight how urea/NMA/Al(OTF)$_3$ electrolyte and the state of the Al electrode surface impact the interphase layer formation and, consequently, the likelihood and reversibility of Al plating/stripping. We point up the requirement for carefully drying electrolyte mixture and components, as water results in hydrogen evolution reaction and creation of an insulating interphase layer containing Al(OH)$_3$, AlF$_3$, and re-passivated Al oxide, which finally blocks the path for the possible Al plating/stripping

Immobilization of Polyiodide Redox Species in Porous Carbon for Battery-Like Electrodes in Eco-Friendly Hybrid Electrochemical Capacitors

Hybrid electrochemical capacitors have emerged as attractive energy storage option, which perfectly fill the gap between electric double-layer capacitors (EDLCs) and batteries, combining in one device the high power of the former and the high energy of the latter. We show that the charging characteristics of the positive carbon electrode are transformed to behave like a battery operating at nearly constant potential after it is polarized in aqueous iodide electrolyte (1 mol L−1 NaI). Thermogravimetric analysis of the positive carbon electrode confirms the decomposition of iodides trapped inside the carbon pores in a wide temperature range from 190 ◦C to 425 ◦C, while Raman spectra of the positive electrode show characteristic peaks of I3 − and I5 − at 110 and 160 cm−1 , respectively. After entrapment of polyiodides in the carbon pores by polarization in 1 mol L−1 NaI, the positive electrode retains the battery-like behavior in another cell, where it is coupled with a carbon-based negative electrode in aqueous NaNO3 electrolyte without any redox species. This new cell (the iodide-ion capacitor) demonstrates the charging characteristics of a hybrid capacitor with capacitance values comparable to the one using 1 mol L−1 NaI. The constant capacitance profile of the new hybrid cell in aqueous NaNO3 for 5000 galvanostatic charge/discharge cycles at 0.5 A g−1 shows that iodide species are confined to the positive battery-like electrode exhibiting negligible potential decay during self-discharge tests, and their shuttling to the negative electrode is prevented in this syste

Modification of Al Surface via Acidic Treatment and its Impact on Plating and Stripping

Amorphous Al$_2$O$_3$ film that naturally exists on any Al substrate is a critical bottleneck for the cyclic performance of metallic Al in rechargeable Al batteries. The so-called electron/ion insulator Al oxide slows down the anode\u27s activation and hinders Al plating/stripping. The Al$_2$O$_3$ film induces different surface properties (roughness and microstructure) on the metal. Al foils present two optically different sides (shiny and non-shiny), but their surface properties and influence on plating and stripping have not been studied so far. Compared to the shiny side, the non-shiny one has a higher (~28 %) surface roughness, and its greater concentration of active sites (for Al plating and stripping) yields higher current densities. Immersion pretreatments in Ionic-Liquid/AlCl$_3$-based electrolyte with various durations modify the surface properties of each side, forming an electrode-electrolyte interphase layer rich in Al, Cl, and N. The created interphase layer provides more tunneling paths for better Al diffusion upon plating and stripping. After 500 cycles, dendritic Al deposition, generated active sites, and the continuous removal of the Al metal and oxide cause accelerated local corrosion and electrode pulverization. We highlight the mechanical surface properties of cycled Al foil, considering the role of immersion pretreatment and the differences between the two sides

Metal-oxidized graphite composite electrodes for lithium-ion batteries

The electrochemical behavior of composite electrodes obtained by mixing graphite (Timrex KS-15 by Timcall), partially oxidized by thermal treatment, with nanometric metal particles (Au, Ag, Ni, Cu, Al, Sn) at about 1% (w/o) is presented. The charge–discharge properties of the composite electrodes have been studied in the temperature range 20 to −30 ◦ C in 1 M LiPF6 EC–DEC–DMC (1:1:1). The main effect is a general improvement of the cycling behavior at any temperature. In particular, at −30 ◦ C about 30% of the theoretical intercalation capacity is retained by electrodes containing Cu, Al and Sn. At the same temperature, the composites containing the above metals show evidences of lithium staging. This may indicate that certain metals affect the kinetics of phase transformation that, together with other effects including charge transfer resistance, lithium diffusion coefficient and polarization due to SEI and solvent conductivities, seems to be the main cause of the poor intercalation capacity of graphite anodes at low temperature

An ac impedance spectroscopic study of Mg-doped LiCoO2 at different temperatures: electronic and ionic transport properties

An ac impedance spectroscopic study of LiMg0.05Co0.95O2, prepared by sol–gel process, is presented. The results are interpreted on the basis of the equivalent circuit proposed for LiCoO2 oxide that takes into account the ionic and electronic transport processes. The results demonstrate that the intercalation/deintercalation reaction occurs through two-steps processes involving an adatom mechanism that may be described in terms of partially desolvated lithium ions adsorbed/desorbed from the electrode surface and insertion/loss of an electron from the conduction band of the host followed by diffusion of Li+ to/from the lattice intercalation site. Mg doping of LiCoO2 affects the electrochemical and electronic properties of the pristine oxide by facilitating the insulator-to-conductor transitions that are responsible of the phase transition in the undoped material