Electrochemistry Downs cell

Aqueous, alkaline fuel cells, as used by NASA for supplemental power in spacecraft, are intolerant to C02 in the oxidant. The strongly alkaline electrolyte acts as an efficient scrubber for any C02, even down to the ppm level, but the resultant carbonate alters the performance unacceptably. This behavior was recognized as early as the mid 1960 s as a way to control space cabin C02 levels and recover and recycle the chemically bound oxygen. While these devices had been built and operated at bench scale before 1970, the first comprehensive analysis of their electrochemistry was put forth in a series of papers in 1974 [27]. The system comprises a bipolar array of fuel cells through whose cathode chamber COz-containing air is passed. The electrolyte, aqueous Cs2C03, is immobilized in a thin (0.25 0.75 mm) membrane. The electrodes are nickel-based fuel cell electrodes, designed to be hydrophobic with PTFE. 

Several applications of electrochemistry to medical and biological matters were presented. For example, the possibility that Oj (a superoxide ), which is an intermediate in some mechanisms of the reduction of oxygen, could cause degenerative diseases by adsorbing on cell surfaces and decomposing DNA was examined. The idea was turned down, but a related one emphasized Oj can combine with protons and other radicals, and these (e.g., peroxy radicals) are very reactive and may indeed cause trouble. 

There is another way in which electrons can be rearranged in a chemical reaction, and that is through a wire. Electrochemistry is redox chemistry wherein the site for oxidation is separated from the site for reduction. Electrochemical setups basically come in two flavors electrolytic and voltaic (also known as galvanic) cells. Voltaic cells are cells that produce electricity, so a battery would be classed as a voltaic cell. The principles that drive voltaic cells are the same that drive all other chemical reactions, except the electrons are exchanged though a wire rather than direct contact. The reactions are redox reactions (which is why they produce an electron current) the reactions obey the laws of thermodynamics and move toward equilibrium (which is why batteries run down) and the reactions have defined rates (which is why some batteries have to be warmed to room temperature before they produce optimum output). 

The three-dimensional electrochemical cell is a hypothetical device that illustrates how some of the advances in microscale and nanoscale electrochemistry over the past two decades may be applied to its construction (Figure 6.1). The three-dimensional electrochemical cell is a conventional battery in the sense that it has a cathode and anode, but they are configured in an interpenetrating array with electrodes anywhere from micron dimensions if they are prepared using lithographic techniques down to the nanometer scale. 

It can be seen that electrochemical reaction engineering has much in common with chemical reaction engineering. Scaled-down versions of full-sized cells are readily constructed and the system behavior can be characterized in detail using electrochemical techniques. One can anticipate therefore that there will be a valuable interplay between electrochemical and conventional chemical reaction engineering. It is relevant in this context that whereas some reactions are carried out on very large scales, many desired products are made on the scale of a few tons to a few thousand tons per annum. The investigation of the chemical engineering of small systems is a prerequisite for the successful implementation of such process, a field which can be readily explored in the area of electrochemistry. 

Surface area It is advantageous if the surface area can be high. However, it must be high in the correct way. For example, the usable area of deep pores may be limited firstly because they may allow contact of the electrolyte with the underlying substrate, and secondly because current lines in an electrode do not penetrate completely down the pores (8) and the length of the pores which remain active is limited, a subject well known in fuel cell electrochemistry. SEM s of polymer surfaces are shown in Fig. 6. 

---