In nano-sized FeWO 4 electrodematerial, both Fe and W metal cations are suspected to be
involved in the fast and reversible Faradaic surface reactions giving rise to its pseudocapacitive signature. As for any other pseudocapacitive materials, to fully understand the charge storage mechanism, a deeper insight into the involvement of the electroactive cations still has to be provided. The present paper illustrates how operando X-ray absorption spectroscopy (XAS) has been successfully used to collect data of unprecedented quality allowing to elucidate the complex electrochemical behavior of this multicationic pseudocapacitive material. Moreover, these in-depth experiments were obtained in real time upon cycling the electrode, which allowed investigating the reactions occurring in the material within a realistic timescale, which is compatible with electrochemical capacitors practical operation. Both Fe K-edge and W L 3 -edge measurements point out the involvement of the
Keywords: nanostructuration; supercapacitors; cobalt oxyhydroxide; ionic liquids; energy storage;
surface modification; nanomaterial
Nanomaterials are generating an intense research interest in the energy storage area due to their numerous advantages compared to their bulk counterparts [ 1 , 2 ]. Indeed, they offer greatly improved ionic diffusion and electronic conductivity compared to micron-sized particles, which allows faster charge/discharge kinetics [ 1 ]. Additionally, nanoparticles’ size permits them to better accommodate the stress induced by repeated charge/discharge cycles and thus, improves the lifetime of the electrodematerial [ 3 ]. These characteristics make them ideal candidates for electrode materials in hybrid supercapacitor applications, where the high power density and the long cycle life provided by nanostruc- turation are associated with the high capacity generated by redox reactions [ 4 ].
(1) A development of positive and negative homocharges would appear systematically under the effect of a relatively weak electric field (4kV/mm with a clear enhancement above 10kV/mm) and whatever the temperature (cf. Figure 12). The nature of the electrode/insulator interface plays an important role in this first phenomenon. Indeed when gold electrodes are deposited on samples, the injection phenomenon is lowered (cf. Figure 8). In this context, we consider the quality of the electrode/sample contact, and the electrodematerial. Regarding the latter, as far as polyethylene-type insulations were considered, it has been reported in several instances that a semiconducting electrode – i.e. the material used as HV electrode of the PEA system for acoustic impedance adaptation – is much more injecting than a metallic electrode (see e.g. Chen et al. 2001; Fleming et al. 2000). Besides, aluminium electrodes appear to be more efficient for injection than gold electrodes: this has been observed experimentally (Chen et al. 2001) and this is an expected result given the difference in work function for Au (4.70eV) and Al (4.08eV) (Fukunaga et al. 1998). Regarding now the quality of the contact, gold electrodes are in deep contact with the insulation surface since it was deposited by sputtering. For not-metallized samples, films are clamped between the two electrodes so that it is not completely unexpected that microdischarges be generated at the interfaces. In this situation, additional kinetic energy of charges could increase there injection. On the whole, the decrease in charge generation in gold-metallized epoxies is an expected process.
To cite this version : Rozier, Patrick and Sathiya, Mariyappan and Paulraj, Alagar-Raj and Foix, Dominique and Desaunay, Thomas and Taberna, Pierre-Louis and Simon, Patrice and Tarascon, Jean- Marie Anionic redox chemistry in Na-rich Na2Ru1−ySnyO3 positive electrodematerial for Na-ion batteries. (2015) Electrochemistry Communications, vol. 53. pp. 29-32. ISSN 1388-2481
Stainless steel is a promising electrodematerial for anodes of microbial fuel cells
Diana Pocaznoi, Amandine Calmet, Luc Etcheverry, Benjamin Erable and Alain Bergel*
The abilities of carbon cloth, graphite plate and stainless steel to form microbial anodes were compared under identical conditions. Each electrode was polarised at 0.2 V vs. SCE in soil leachate and fed by successive additions of 20 mM acetate. Under these conditions, the maximum current densities provided were on average 33.7 A m 2 for carbon cloth, 20.6 A m 2 for stainless steel, and 9.5 A m 2 for flat graphite. The high current density obtained with carbon cloth was obviously influenced by the three-dimensional electrode structure. Nevertheless, a fair comparison between flat electrodes demonstrated the great interest of stainless steel. The comparison was even more in favour of stainless steel at higher potential values. At +0.1 V vs. SCE stainless steel provided up to 35 A m 2 , while graphite did not exceed 11 A m 2 . This was the first demonstration that stainless steel offers a very promising ability to form microbial anodes. The surface topography of the stainless steel did not significantly affect the current provided. Analysis of the voltammetry curves allowed two groups of electrode materials to be distinguished by their kinetics. The division into two well-defined kinetics groups proved to be appropriate for a wide range of microbial anodes described in the literature.
Capacitance depends on the electrode surface accessible to the electrolyte’s ions. In theory, the highest capacitances may be reached using a material with a high microporous volume due to narrow micropores as these seem to be responsible for charges accumulation. A universal ideal structure for electrodematerial in terms of pore size distribution does not exist, but an ideal relation of pore size to ion size might exist [Ani 2006]. The ideal pore size depends on the electrolyte and thus on the ion size. Chmiola et al. [CYG 2006] propose this might be the desolvated ion size for applications which require longer discharge times. Recent studies show that 0.7 nm and 0.8 nm are adequate pore sizes for aqueous and organic electrolytes respectively [Vix 2005, RaK 2006]. Also, adequate pore sizes for positive and negative electrodes corresponding to the electrolyte’s anion and cation sizes, respectively, have been found to differ [Seg 2010]. However, narrow micropores may restrict or retard the electrolyte diffusion and thereby contribute to high time constants. The time constant [s], determined according to equation V-4, indicates how fast the electrode can be charged/discharged [Fra 2001].
A B S T R A C T
This study relates the sonoelectrochemical production of metallic particles and nanoparticles. The emphasis is on the inﬂuence of electrodematerial and roughness on the morphology of iron electrodeposits and their dispersion from the electrode by ultrasoniﬁcation. Ultrasoniﬁcation is either applied during cyclic voltammetries with solution stirring or after galvanostatic iron electrodeposition; no dispersion was observed when using a gold electrode, whereas dispersion was always observed when using vitreous carbon (VC) substrates. Scanning Electron Microscopy (SEM) imaging of the electro- deposits shows higher iron coverage on gold than on VC electrodes. Iron spreads more on gold than on VC. The values of both the interfacial energy of the iron/electrode interface and the work of adhesion of iron on the electrode are in agreement with the previous observations. Dispersion kinetics on VC were found to be dependent on the electrode surface roughness. Results suggest that dispersion follows a ﬁrst order kinetics, which is coherent with the constant action of cavitation bubbles in the vicinity of the electrode surface. Enhancement of mass-transfer by ultrasound has also been observed.
electrodeposit that has grown on the gold electrode in chloride
medium and under silent conditions ( Fig. 1 , pictures 6) is the only
one to grow as a thin ﬁlm. For all other Fe (II) salts and electrodematerial combinations, electrodeposited iron presents micro-
Keywords—polyimide, passivation, DC stress, space charge, electrodematerial, LIMM
I. I NTRODUCTION
In recent years, high temperature electronics and power electronics applications have emerged needing the use of wide bandgap semiconductor (SiC, GaN…). In these devices, substrates coated with thin polymer films have been widely used. Particularly, polyimide is of great interest owing to its excellent thermal and electrical properties and its easy processing. Some of the most important applications of these materials films are as inter-level dielectric insulators and as electronic device surface passivation .
TCO stands for Transparent Conducting Oxides layer, EC1 and EC2 for Electrochromic layers and Elec. for electrolyte. In our group, aiming at reducing the cost and ease the process of ECDs, the combination of layer properties (i.e conductivity and electrochemical/electrochromic activity) was chosen. Such ap- proach leads to a reduction of the number of layers, from five to three when using conductive electrochromic active or/and counter electrodematerial. In this paper, the fabrication of a printable electrochromic device using PEDOT on ITO/PET or PEDOT:PSS on PET and silver metal film with a PMMA poly- mer ionic liquid at room temperature is discussed. Further im- provement in the electrolyte composition and process allows the realization of ECDs on paper substrate of which activation is demonstrated thanks to a modern life everyday tool, namely a smartphone. In addition, very preliminary understanding of the mechanism of the silver based device is investigated using x- ray photoelectron spectroscopy.
The next step appears even much more challenging than that overpass during the last twenty years, to develop successfully the promising Li-rich layered oxides to propose the next generation of higher energy density positive electrode materials for Lithium-ion batteries. Up to now, we were playing with the phase modifying its composition, structure and morphology and we were limited to deliver successfully less than 70% of the theoretical capacity in most cases (i.e. less than 180 mAh/g). Struggling to reach higher reversible capacity means that the chemical and thermal instabilities of layered oxides will be exacerbated, for the layered oxides involving only the transition metal ions into the redox processes, and even more those involving the oxygen anion.
“A” can be either monovalent (Li + or Na + ) or divalent (Ni 2+ ,
Mg 2+ , Ca 2+ , Mn 2+ , etc.) ions and M is a transitional metal (Ti,
Fe, V, Zr, Sc, etc.). 10 Among these materials, A x Ti 2 (PO 4 ) 3 is
usually considered as a negative electrode because of its low working voltage versus Li + /Li ( ∼2.5 V). 11 , 12 However, similar to other phosphate-based materials such as olivine cathodes, 13 the use of the NASICON-type phosphates is limited because of the low electronic conductivity resulting from the long M − M distance (existence of −M−O−P−O−M− linking) in their structural framework. To overcome this inconvenience, carbon coating and chemical substitution (or doping) are considered as e ﬃcient methods in order to improve the electrochemical performance of these materials, mainly their rate capabil- ity. 11 , 13 , 14
d OCP, Innovation, BP. 118, El Jadida, Morocco
Table S1: Refinement resulting atomic position of the Li 0.5 Ni 0.5 Ti 1.5 Fe 0.5 (PO 4 ) 3 material
Li 0.5 Ni 0.5 Fe 0.5 Ti 1.5 (PO 4 ) 3 Atoms x y z B Occ. Mult
Li 0.0000 0.0000 0.0000 2.4(3) 0.5 6
1.7mm thickness) and the potential (-0.9 to -1.6 V/ MSE ) was applied by use of a VersaSTAT 3
potentiostat from Ametek/Princeton Applied (Elancourt, France). The solution (50 mg L -1 of
alachlor in 0.05 M NaOH) percolated the porous electrode thanks to a Gilson minipuls 3
peristaltic pump (Middleton, WI, USA). The typical electrolysis duration was 100min. At predetermined time intervals, 200 μL samples of solution were withdrawn from the cathode compartment for HPLC analysis. The conversion of alachlor was calculated as the molar ratio
In this work, operando 57 Fe Mössbauer spectra were collected during the electrochemical cycling of NaFeO 2 vs. Na metal using a specifically designed in situ cell, and analysed using an alternative and innovating data analysis approach based on chemometric tools such as Principal Component Analysis (PCA) and multivariate curve resolution (MCR).[5,6] This approach, which allows the unbiased extraction of all possible information from the operando data, enabled the stepwise reconstruction of the “real” spectral components occurring during the cycling of NaFeO 2 . In this way, a clear description of the electrochemically active iron species could be obtained, allowing a clearer comprehension of the cycling mechanisms of this material vs. sodium.
displayed in Figure 3 and experimental setup peaks are in- dexed in blue (copper grid and beryllium window). Most of the remaining peaks are indexed as the Li 3 N-type hexago-
nal structure (space group P6/mmm, indexation in hkl*) with the following cell parameters: a = 3.729 (1) Å and c = 3.556 (1) Å. These values are slightly different from the pris- tine powder (a = 3.760 (1) Å and c = 3.538 (1) Å) and suggest that a chemical oxidation took place during the electrode preparation. Indeed, even with drastic precautions used during handling and manipulations (acetylene black dried under argon atmosphere at 800°C, PTFE dried overnight at 200°C under primary vacuum, all manipulations per- formed under high purity argon atmosphere), such a reac- tion cannot be avoided. This kind of oxidation has already been evidenced in similar lamellar phases Li 3-y Fe y N
nm) was deposited by physical vapor deposition on both sides of the electrolyte. The electrode ink was deposited on both sides of the electrolyte followed by sintering in air at 1100 °C for 3h. The current collectors consist of gold grid discs (A = 0.95 cm 2 ) connected to the electrodes and linked to the external current and voltage circuits. The discs were placed into the open- flange setup TM provided by the Swiss company Fiaxell as described previously 46 . It contains an oven and an Inconel 600 & 601 support in order to maintain the cell in the furnace. EIS was performed in potentiostatic mode using a VersaSTAT device and associated VersaStudio software in the 0.1-10,000 Hz frequency range at open circuit voltage (OCV) conditions with an AC signal amplitude of 20 mV or 10 mV; the amplitude value was optimized for each measurement in order to get the best signal to noise ratio without loss of the transfer function linearity.
a 11 mm-diameter disc was cut and then pressed on nickel foam (current collector) at 100 MPa; the electrode was finally dried at 60 C for 12 h. A typical electrode is around 200 mm thick, has poros- ity close to 10%, and density close to 4.9 g.cm ÿ3 . Electrochemical impedance spectroscopy and cyclic voltammetry were completed using a VMP3 potentiostat (Biologic). Impedance tests have been carried out at 0 V between 10 mHz and 200 kHz. Cyclic voltamme- try was performed at 5 mV/s between ÿ 0.5 and 0.65 V.
The present work is focused on the study of Pr 4 Ni 3 O 10+δ as a new cathode material for Solid
Oxide Fuel Cells (SOFCs). The structural study leads to an indexation in orthorhombic
structure with Fmmm space group, this structure being thermally stable throughout the
temperature range up to 1000 °C under air and oxygen. The variation of oxygen content (10+δ)
Before testing the SiNWs as active electrodematerial against Li alloying, we have evaluated the possible electrochemical con- tribution of the Au droplets still present at the top of the silicon wires after synthesis. Indeed even when only a little is known on the electrochemical properties of Li–Au alloys at room tempera- ture, the Au electrode has been recently proved to accommodate Li in the 0.02–0.5 V potential range with a poor reversibility [40,41]. The discharge–charge curves obtained for a 50 nm thick gold film are shown in Fig. 4. Two voltage plateaus corresponding to the Li alloying process into two different phases are observed at 0.2 and 0.1 V while Li removal from the alloy takes place in two steps at 0.18 and 0.4 V in good agreement with Refs. [40,41]. The poor effi- ciency of the charge process is clearly evidenced since only 50% of the faradaic yield involved in the alloying reaction is recovered at