Nickel anodes coking

Electrorefining Nickel Metal Anodes In the refining of nickel metal anodes, the principal reaction at the anode is the dissolution of nickel metal as nickel ions. The principal cathodic reaction is the reduction of nickel ions from solution. Nickel anodes are made by reducing nickel oxide with coke at temperatures up to 1550 °C and casting in molds. The practice is designed to obtain anodes with the desired strength and crystal size. 

The activity of the nickel anode decreases due to sintering and coke formation when carbon-containing fuels are used. The ceramic parts can easily break if vibrational forces are present. This is one reason why SOFCs are best suited for stationary applications rather than mobile applications. 

INCO produced electrolytic nickel at its refinery in Port Colborne, Ontario, Canada. The production started in 1926. The anodes were made by reducing nickel oxide with coke, and the anodes contained about 93.5% Ni, 4% Cu, and 1% Co. The sulfur content was low, about 0.6%. The approximate composition of the purified electrolyte was 60 g L-1 Ni2+, 95 g L-1 S042-, 35 g L-1 Na+, 55 g L-1 Cl , and 16 g L 1 boric acid, and the temperature was 60 °C. The current density of the process was 16 A/sq.ft (approximately 170 A m-2) and the cell voltage was about 1.6 V. At the normal cell operating voltage, the principal impurities - iron, cobalt, lead, arsenic, and copper - dissolved into the solution with nickel. Silver, gold, the PGMs, sulfur, selenium, and tellurium fell to the bottom of the cell as an insoluble slime. The produced cathodes... 

Nickel is used as the anode because it is economical and exhibits high performance, although due to reasons of adherence and different expansion coefficients, it flakes off easily from the electrolyte unless it is mixed with zirconia, creating a cermet. Ni-YSZ anodes allow a rapid and clean connection to the fuel and are good electronic conductors although Ni is susceptible to become coated with a carbon layer when it reacts with carbon-based fuels. Coke formation usually impedes further reaction from... 

For the anode, a nickel-oxide-YSZ cermet is applied, though the limitations of the current state of that type with respect to redox stability, coking, and sulfur tolerance are known. More recently, the integration of oxide anodes into the concept has been analyzed and tested [78]. 

The main problem with the nickel-based anodes is their propensity to coke, that is to become coated with a carbon layer on reacting with hydrocarbon fuel. This carbon layer has two deleterious effects it can disrupt the anode by pushing the nickel particles apart and it can form a barrier at the nickel surface, preventing gas reactions. Typically, if a hydrocarbon such as methane is fed directly into an SOFC anode, then it may not remain functional after as little as 30 minutes as the coking proceeds. Additives to the Ni+YSZ cermet such as 5% ceria or 1% molybdena can inhibit this process [19]. Alternatively, metals other than nickel can be employed [20]. 

It was demonstrated in the 1960s that hydrocarbons could be injected directly into SOFCs if steam was supplied [37]. The steam can beneficially be obtained from the spent fuel stream. The main problem with direct use of hydrocarbons is that coke can form to block up and contaminate the anode. There are two damaging reactions which can occur on the nickel ... 

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