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Manganese hexacyanoferrate is a promising cathode material for lithium and sodium ion batteries, however, it suffers of capacity fading during the cycling process. To access the structural and functional characteristics at the nanometer scale, fresh and cycled electrodes are extracted and investigated by transmission soft X-ray microscopy, which allows chemical characterization with spatial resolution from position-dependent x-ray spectra at the Mn L-, Fe L- and N K-edges. Furthermore, soft X-rays prove to show superior sensitivity toward Fe, compare to hard X-rays. Inhomogeneities within the samples are identified, increasing in the aged electrodes, more dramatically in the Li-ion system, which explains the poorer cycle life as Li-ion cathode material. Local spectra, revealing different oxidation states over the sample with strong correlation between the Fe L-edge, Mn L-edge, and N K-edge, imply a coupling between redox centers and an electron delocalization over the host framework.

Rechargeable aqueous Zn-ion batteries (ZIBs) have obtained extensive attention owing to their high safety, low-cost, environmental friendliness and high energy density. Nevertheless, developing suitable cathode materials remains challenging due to requirements for appropriate microstructure. We present a porous N-doped carbon nanosheet/MnO2 (NCS/MnO2) derived from bacterial cellulose (BC) by a simple route. BC chunks were soaked in urea solution and then carbonized under Ar flow at 900 °C. N-doped carbon nanosheets were obtained and MnO2 was added by reaction with NaMnO4. Benefiting from both the conductivity and porosity of the NCS support, the NCS/MnO2 composite delivers a high capacity and long cycling stability (114 mA h g−1 at 2 A g−1 after 1800 cycles). The electrode reaction mechanism was further investigated and the MnO2 dissolution/deposition mechanism was confirmed, with a critical role of zinc sulfate hydroxide (ZSH) to assist the deposition of MnO2

In operando experiments aim to perform in situ measurements of an electrochemical cell on a chip using transmission electron microscopy (TEM) during operation, which is a crucial requirement for understanding the fundamental physical and electrochemical phenomena taking place in a battery. These findings can enhance the understanding of electrochemical cells' behaviour and pave the way for designing more efficient and effective electrochemical energy-storage devices. Hence, the aim of this project is manufacturing a thin-film cell suitable for in operando TEM measurements by testing different chip designs and liquid electrolytes. This kind of cells require electrodes thinner than a few hundred nanometers, but typical commercial active material powders mainly consist of micrometric particles. So, this dissertation is focused on designing a process to prepare suitable cells from commercial materials. To accomplish this, particle size selection of LiMn2O4 was performed using separation methods. Having the ability to both deintercalate and intercalate, LiMn2O4 is used as active material for all electrodes in the thin-film cell. Therefore, nanoparticles are selected by centrifugation suspended in ethanol. Dynamic Light Scattering is employed to characterize their size and concentration to study the effects of the solvent, the centrifugation speed, and the use of filters. Then, adequate particles are drop casted on the chip with Polyvinylidene Fluoride (PVDF) in N-Methyl-2-pyrrolidone (NMP) to adhere them onto the surface. This particle distribution on the chip can be observed in Scanning Electron Microscopy (SEM). It was possible to demonstrate with planar microcells on chips electrochemical features that reproduce those typically shown by macroscopic cells. For example, 5 M LiNO3 had a good CV performance with 70 nA for the oxidation peak and -55 nA for the reduction one when selected depositing LiMn2O4 particles with 500 rpm centrifugation. The cell collectors represen

This thesis is developed at the Nanostructured Interfaces for Electrochemical Energy Storage research line of the Institute of Materials Science of Barcelona (ICMAB, CSIC) and focuses on experimental investigation of activation carbon felts (CF) as vanadium redox flow battery (VRFB) electrodes by electrochemical treatments. Because of the environmental impact of the present days, the Redox Flow Battery has emerged as an energy storage system. In this work, a series of electrochemical treatments were carried out using either sulphuric or phosphoric acids to activate the carbon felts that act as electrodes in these batteries. Following treatments, these were tested using a VRFB, and characterised by SEM and XPS. The felts treated in sulphuric acid slightly improve the thermally activated felt benchmark when placed in the positive electrode. Conversely, treatments in phosphoric acid demonstrated capable of improving results provided by the benchmark at both the negative and positive electrode. Overall, the work shows that electrodes activated by electrochemical treatments with low environmental impact and proper performance can be produced.

Synchrotron radiation based techniques are powerful tools for battery research and allow probing a wide range of length scales, with different depth sensitivities and spatial/temporal resolutions. Operando experiments enable characterization during functioning of the cell and are thus a precious tool to elucidate the reaction mechanisms taking place. In this perspective, the current state of the art for the most relevant techniques (scattering, spectroscopy, and imaging) is discussed together with the bottlenecks to address, either specific for application in the battery field or more generic. The former includes the improvement of cell designs, multi-modal characterization and development of protocols for automated or at least semi-automated data analysis to quickly process the huge amount of data resulting from operando experiments. Given the recent evolution in these areas, accelerated progress is expected in the years to come, which should in turn foster battery performance improvements.

A single-step electrochemical liquid-liquid-solid (ec-LLS) deposition route is developed for growing nanowire-based Ge-Sn nanostructures with tailored morphologies and compositions. Composite films, submicron fibers, nanowires, and also three-dimensional (3D) hierarchical structures are fabricated in the same electrolyte, simply by varying the electrodeposition current density. COMSOL simulations indicate that the current density is able to generate sufficient Joule heating to activate the ec-LLS growth mode. A concomitant reduction of Ge and Sn ions during Sn-doped Ge nanowire growth results in the formation of multicomponent heterostructures comprising non-equilibrium GexSn1-x, pure Sn, and Sn-rich amorphous phases. The proposed deposition technique enables the fabrication of high mass loading electrodes with enhanced Li storage properties. In particular, a 3D hierarchical structure composed of 38 wt % Ge delivers a specific capacity of 938 mA h g-1 after 400 cycles, corresponding to a high areal capacity of 4.95 mA h cm-2. We believe that the present study could be considered as a low-cost procedure for the industrial fabrication of anode materials for high-performance Li-ion battery applications.

Developing low-cost and efficient electrodes for hydrogen evolution reaction (HER) under neutral electrolytes remains an unattained milestone. We report a highly performing binder-free electrode through electroless deposition of Ni–P nanoparticles on bacterial cellulose (BC). Not needing carbonization to provide the electric conductivity, BC can maintain its excellent mechanical properties and thin fiber microstructure. The nanometric cellulose fibers facilitate the formation of small Ni–P nanoparticles, leading to more catalytic active sites. The obtained Ni–P/BC electrode presents remarkable HER activity with an overpotential of only 161 mV at 10 mA cm−2 and a low Tafel slope (141 mV dec−1) in 1 M potassium phosphate-buffered saline (pH = 7) electrolyte. Besides, Ni–P/BC also exhibits good stability for 24 h at 10 mA cm−2. This binder-free, low-cost, and easily fabricated electrode holds excellent promise for HER applications in benign neutral environments.