Molten carbonate anodic hydrogen oxidation

A factor that has been ignored in Figure 7.1 is that in the molten carbonate and solid oxide fuel cells, as we shall see, the product steam ends up at the anode - essentially the hydrogen is replaced by steam. So, if the partial pressure of the hydrogen decreases, the steam partial pressure will increase. Studying equation 7.4, we see that a rise in the partial... 

FLINAK is purified by treatment with the HF released by ammonium bifluoride (NH4HF2) the HF converts oxide impurities in the melt to H20 [7]. In this purification procedure, the fluoride salt mixture is combined with 15 wt% NH4HF2 and heated to about 500°C in a graphite crucible. The molten mixture is poured into a platinum container and heated to 750°C. Hydrogen is passed through the molten mixture for approximately 2 days. Further purification can be achieved by con-trolled-potential electrolysis at an applied potential of about 3 V between a tungsten cathode and glassy carbon anode. 

Carbon monoxide, trace metals, and sulfur compounds, such as HjS, COS, mercaptans, and thiophenes, exist in hydrogen produced from coal gasification and used in molten carbonate Hj/Oj fuel cells. In addition, nitrogen compounds from coal, such as HCN and HCNS can be present or they might oxidize to corrosive NO. While carbon monoxide is reactive in these cells, the rest impurities can either poison the Ni anode or they can attack chemically cell and electrodes 249), for example, HjS sulfidizes nickel and stainless steel. HjS could also undergo oxidation to deposit sulfur 250) ... 

Molten Carbonate Fuel Cells—The anode fuel is hydrogen, with the following oxidation reaction ... 

A molten carbonate fuel cell is an energy conversion device that converts chemical energy in fossil fuels into electricity. The operating temperature of an MCFC is 600 to 700°C.The principal anode reaction is the oxidation of hydrogen or carbon monoxide ... 

The MCFC anode operates under reducing atmosphere, at a potential typically 700-1000 mV more negative than that of the cathode. Many metals are stable in molten carbonates under these conditions, and several transition metals have electrocatalytic activity for hydrogen oxidation. Nickel, cobalt, copper and alloys in the form of powder or composites with oxides are usually used as anode materials. Ceramic materials are included into the anode composition to stabilize the anode structure (pore growth, shrinkage, loss of surface area) at the time of sintering. An alloy powder of Ni + 2-10 wt% Cr can be used. The initial formation of CrjOs, followed by surface formation of LiCr02, can stabilize the anode structure. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

Two parts are treated one is the physical and chemical features of materials of molten carbonate fuel cells (MCFCs), and the other is performance analysis with a 100 cm class single cell. The characteristics of the fuel cell are determined by the electrolyte. The chemical and physical properties of the electrolyte with respect to gas solubility, ionic conductivity, dissolution of cathode material, corrosion, and electrolyte loss in the real cell are introduced. The reactirm characteristics of hydrogen oxidation in molten carbonates and materials for the anode of the MCFC are reviewed. The kinetics of the oxygen reduction reaction in the molten carbonates and state of the art of cathode materials are also described. Based on the reaction kinetics of electrodes, a performance analysis of MCFCs is introduced. The performance analysis has importance with respect to the increase in performance through material development and the extension of cell life by cell development. Conventional as well as relatively new analysis methods are introduced. 

One of the major problems with the MCFC is that the nickel oxide state-of-the-art cathode material has a small, but significant, solubility in molten carbonates. Through dissolution, some nickel ions are formed in the electrolyte. These then tend to diffuse into the electrolyte towards the anode. As the nickel ions move towards the chemically reducing conditions at the anode (hydrogen is present from the fuel gas), metallic nickel can precipitate out in the electrolyte. This precipitation of nickel can cause internal short-circuits of the fuel cell with subsequent loss of power. Furthermore, the precipitated nickel can act as a sink for nickel ions, which promotes the further dissolution of nickel from the cathode. The phenomenon of nickel dissolution becomes worse at high CO2 partial pressures because of the reaction... 

Basically, an MCFC consists of fwo porous elecfrodes, separafed by a molten electrolyte held in place by a matrix as seen in Figure 9.13. In the three-phase region on the anode side, hydrogen oxidation reaction occurs and hydrogen combines with carbonate ions, producing water and carbon dioxide while... 

Magnanini studied the absorption spectrum and A. Speransky found that the electrical conductivity of aq. soln. shows that only a small proportion of the salt is ionized. The soln. of the violet modification conducts electricity three times better than that of the green. G. Gore electrolyzed a cone. soln. of chromic fluoride acidified with hydrofluoric and hydrochloric acids, and found that the liquid became hot no gas was liberated at the cathode, but chlorine and ozone were liberated at the platinum anode which was not corroded. C. Poulenc showed that the salt is reduced by hydrogen at dull redness. The heat of formation is 230-95 Cals, per mol—vide infra, the dichloride. Steam transforms chromic fluoride into chromic oxide. Chromic fluoride is insoluble in water, and alcohol hydrogen chloride transforms it into chromic chloride hot hydrochloric, sulphuric, and nitric acids attack chromic fluoride only a little hydrogen sulphide converts it into black sulphide and molten alkali nitrate or carbonate converts it into chromate. A. Costachescu prepared complex pyridine salts. 

The electrolyte used by the fuel cell is a solid gas—impermeable zirconia known as zirconium oxide (ZrOj). This ZrOj is doped with calcium oxide (CaO) to supply enough oxide ions to carry the cell current. The oxidant air or oxygen is bubbled through the molten silver cathode, which is held inside the zirconia cup. At the fuel electrode or the carbon-based anode electrode, the oxide ions are combined with carbon monoxide (CO) and give up their electrons to an external circuit. The cell by-products CO and hydrogen, which are formed in the initial fuel decomposition, are burned outside the cell to keep the fuel cell at operating temperature. The hydrogen is not involved in the electrochemical cell reaction. 

In alkaline solution formaldehyde can be electrolytically oxidized to formic acid and carbon dioxide. When a specially prepared copper or silver anode is employed, Muller reports that pure hydrogen is liberated in equal amounts from both cathode and anode. Under these circumstances sodium formate is produced in theoretical quantity and two equiva lents of hydrogen are liberated for each faraday of electricity. The anode is best prepared by treating copper or silver with molten cuprous or silver chbride respectively and then subjecting to cathodic reduction in a solution of sodiu m hydroxide. Approximately equivalent results were obtained vith 0.6 to 2N alkali containing about 17 per cent dissolved formaldehyde. The reactions taking place are shown below as postulated by Muller. 

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