Electrolytes nickel-based alloys

Air(02) + C02,Cathode(+)/y - LiA102 + M tC03/anode(-), fuel(H2) (Nickel oxide) (Matrix/electrolyte) (Ni-based alloy)... 

The corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

HEE is the degradation observed in mechanical properties during the plastic deformation of alloys exposed to, usually, gaseous hydrogen. Similar behavior sometimes occurs when the hydrogen is provided by a gas such as H2S, by electrolytic charging, or by corrosion. HEE has been exhibited by ferritic steels, nickel-base alloys, and metastable austenitic steels. 

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

In the early days of the MCFC, the electrode materials used were, in many cases, precious metals, but the technology soon evolved during the 1960s and the 1970s saw the use of nickel-based alloys at the anode and oxides at the cathode. Since the mid-1970s, the materials for the state-of-the-art electrodes and electrolyte structure (molten... 

M. Bojinov, A. Galtayries, P. Kinnunen, A. Machet, P. Marcus, Estimation of the parameters of oxide film growth on nickel-based alloys in high temperature water electrolytes, Electrochim. Acta 52 (26) (2007) 7475-7483. 

Figure 7.5 Thermal expansion curves of CrFeS Y O) 1, the ferritic steel XlOCrA 118 and for comparison a nickel-based alloy (Ni 20Cr, VA Chromium) as well as the two mostly used supporting components in planar SOFCs, the electrolyte (8YSZ)and the anode substrate (NiO/ YSZ).

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

Different alloys display reactivities that, due to ion adsorption, vary with the nature of the electrolyte. In HCIO4, the catalytic activity order for Pt-based alloys with cobalt and nickel was PtsCo... 

The capacity of the 18,650 cell appears to have reached its practical limit of 2.9 Ah based on the present graphite and planar nickel-based cathode in 2007. Further improvement in capacity is expected to be realized from the development of a silicon alloy type anode with a capacity of 700 mAh/g or more and the planar lithium-nickel-cobalt-aluminum and nickel-manganese-cobalt cathode materials with capacities approaching 250 mAh/g. New electrolytes and/or additives also are under development. 

The cell producers accomplished the performance improvements through engineering improvements in cell design, new electrode materials, and automated high-speed production to reduce the cost. The capacity of the 18650 cell had reached 2.9 Ah in 2007 based on treated graphite anode and planar-nickel-based cathode and with several kinds of electrolyte additives [12]. With further continuous improvements in all the cell components that includes silicon alloy-type anode materials, lithium-nickel-cobalt-aluminum and nickel-manganese-cobalt cathode materials, novel electrolyte and/or additives, some cell manufacturers are currently able to achieve a maximum capacity of up to 3.4 Ah for the same 18650 cell design. 

A planar-design SO electrolyzer is fairly similar, in stractural terms, to a PEM electrolyzer. Figure 2.12 shows the diagram of a typical layout of such a device. We see a sandwich-type structure, in which the solid electrolyte (a solid oxide which is a ceramic) is bordered on both sides by two porous electrodes (a ceramic is used for the anode and a ceramic/metal alloy (cermet) for the cathode). The interconnectors which serialize the cells are based on metallic materials. Depending on the design of the stack, channels are scored into the intercormectors or are replaced by flow-fields (pre-electrodes, so to speak), which are usually nickel-based. 

This paper presents a brief review of the literature of nickel-based cermet electrodes for application in solid oxide cells at temperature from 500 to 1000 °C. The applications may be fuel cells or electrolyser cells. Variables that are used for controlling the properties of Ni-cermet-electrodes are (1) Ni/electrolyte volume ratio, (2) additives, e.g. alloying of the Ni or infiltration of the composite with nanoparticles of other elements or compounds, (3) the chemical composition of the electrolyte component and (4) porosity and particle size distribution, which is mainly affected by raw materials morphology, application methods and production parameters such as milling and sintering possibly followed by infiltration of nanosized electrocatalytic active particles. The various electrode properties are deeply related to these parameters, but also much related to the atomic scale structure of the Ni-electrolyte interface, which in turn is affected by segregation of electrolyte components and impurities as well as poisons in the gas phase. 

Numerous proprietary electrolytes have been developed for the production of harder and brighter deposits. These include acid, neutral and alkaline solutions and cyanide-free formulations and the coatings produced may be essentially pure, where maximum electrical conductivity is required, or alloyed with various amounts of other precious or base metals, e.g. silver, copper, nickel, cobalt, indium, to develop special physical characteristics. 

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