Alkaline electrolyte, oxygen reduction

In an alkaline medium, oxygen reduction and hydrogen oxidation involve OH- ions. The OH-species formed by the cathodic reduction of oxygen move through the electrolyte to the anode, where recombination with hydrogen, oxidized at the anode, produces water. 

Alkaline electrolytes offer further advantages over acidic electrolytes. Oxygen reduction is considerably faster in alkaline electrolytes, which implies that the working potential of the oxygen electrode is more positive, and a larger cell voltage can be realized. Also, apart from the advantages in catalyst selection, less severe corrosion conditions allow nickel and alloys of iron to be used as structural materials in AFCs. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

The Huron-Dow Process. The Huron-Dow (H-D) process is a refinement of the cathodic reduction of oxygen in an alkaline electrolyte yielding low strength hydrogen peroxide directiy. Earlier attempts reHed on neutralizing the excess caustic or forming insoluble metal peroxides (92). The two reactions involved are... 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

Carbon shows reasonable electrocatalytic activity for oxygen reduction in alkaline electrolytes, but it is a relatively poor oxygen electrocatalyst in acid electrolytes. A detailed discussion on the mechanism of... 

Markovic NM, Tidswell IM, Ross PN. 1994. Oxygen and hydrogen peroxide reduction on the gold(lOO) surface in alkaline electrolyte the roles of surface structure and hydroxide adsorption. Langmuir 10 1-4. 

Being thermally decomposed onto the surface of carbon, this complex is expected to form very small catalytically active NiCo204 spinel centers. Thus, we have studied the catalytic activity of the products of pyrolysis at different temperatures toward two electrochemical reactions -reduction of oxygen in alkaline electrolyte and intercalation of lithium into carbons in aprotic electrolyte of Li-ion battery. To our knowledge, the catalytic effect of the metal complexes in the second reaction was not yet considered in the literature. 

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

The first chelate found to be electrocatalytic was cobalt phthalocyanine x>, which functions as an oxygen catalyst in alkaline electrolytes. Soon afterwards we were able to show 3,4,10,11) -that several phthalocyanines are also active in commercially important, sulfuric acid containing media. A comparison of various central atoms showed that activity increased in the order Cu Ni iron phthalocyanine, the nature of the carbon substrate plays a very important part FePc is more active on a carbon substrate with basic surface groups than on one with acid surface groups3). This property is however specific to phthalocyanines (Pc). 

Cobalt pkthalocyanine as catalyst in alkaline electrolyte. Jasinski 1 2> was the first to show that CoPc in alkaline solution showed a pronounced activity for the cathodic reduction of oxygen. The compound was mixed with powdered nickel in a ratio of 1 10 w/w and pressed into a stainless steel tube (geometrical surface of the electrode 0.5 cm2). The tube served for both current conduction and gas supply. 

G. Silver Cathodes for Oxygen Reduction in Alkaline Electrolyte... 

Fig. 22, Comparison of current voltage curves of cathodic oxygen reduction in alkaline electrolyte (a) PTFE-bonded nonactived soot cathode, (b) PTFE-bonded Pt-activated soot cathode, (c) Silflon cathodes consisting essentially of submicrometer PTFE particles covered by a 0.1-fim Ag layer.

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

Carbon and graphite are often used as supports for electrocatalysts, but they also have an electrocatalytic function in electrode reactions such as oxygen reduction in alkaline electrolytes, chlorine alkali industry, and SOCI2 reduction in lithium-thionyl chloride batteries. 

This low-temperature fuel cell uses H2 and O2 reactants and a highly alkaline aqueous KOH electrolyte. The advantages of this fuel cell are the faster oxygen reduction reaction in the alkaline electrolyte and the possibility of using low-cost, nonprecious metal electrode catalysts, such as Ag-loaded carbon powder. The greatest problem with alkaline fuel cells is that the electrolyte reacts with traces of CO2 to produce insoluble carbonates. 

Which course oxygen reduction takes depends upon the electrolyte media, its composition, and the electrode used. Kinetic and mechanistic studies on the electroreduction of oxygen have been performed in both acidic [25-31], and alkaline media [31-38] using different metals as well as various types of carbon electrodes. Studies on the production of hydrogen peroxide in significant amounts by the electroreduction of oxygen have been done using carbon cathodes in alkaline electrolytes (i.e. reaction (7)). 

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