Technology
Electrochemical cell shunt currents eliminated Shunt currents—which cut
series. This current bypasses some of the cells in the center of the series. At the very least, shunt currents decrease the efficiency of the cell battery, since the center cells receive less current than the cells at either end of the series. However, shunt currents also can cause other prob lems. When the cells are being used for chemical synthesis, these currents allow some of the anode product to mix with some from the cathode, in troducing impurities and in some cases even causing explosions. They also can greatly increase corrosion and deposition problems at the end electrodes and in the electrolyte channels. This last problem can be particularly serious, since free circu lation of electrolyte is essential to the proper functioning of batteries ar ranged in this manner. The scientists solve the problem rather simply: They find that passing a separate current through the elec trolyte in the same direction as elec trolyte flow essentially eliminates shunt current problems. Basically, this current eliminates the potential difference between the cells and the electrolyte channel, so a current doesn't flow. How much current is needed can be calculated mathemat ically from an analysis of the electrical circuits involved, or it can be deter mined by trial and error.
battery cell efficiency and cause other problems—can be eliminated by passing separate current through electrolyte A simple but effective way to elimi nate shunt currents and the problems they cause in electrochemical devices has been developed by scientists at Exxon Research & Engineering Co., Linden, N.J., and at Massachusetts Institute of Technology. Patrick Grimes, a chemist and chemical en gineer in Exxon's advanced energy system's laboratory, and MIT elec trical engineering professor Markas Zahn described the technique, which they call protective current, in com panion talks at the Electrochemical Society meeting last week in Holly wood, Fla. Shunt currents occur whenever a number of electrochemical cells are arranged in series and share a com mon, flowing electrolyte, Grimes ex plains. In such systems, part of the current may flow out of early cells through the electrolyte channels eventually reaching the appropriate electrode in a cell further down the
Shunt currents bypass central electrodes by entering electrolyte channels Shunt current Electrolyte
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C&EN Oct. 13, 1980
Shunt current
Electrolyte
The Exxon group, which includes Zahn as a consultant, discovered this protective current effect by devel oping a careful mathematical model of a system of cells connected in series with a common electrolyte. Though the model itself is fairly straightfor ward, the calculations involved to determine the value of the shunt current from it are laborious. Other investigators often have used either simplifying assumptions or a com puter to handle this problem. The Exxon group decided, instead, to use what Grimes calls a "brute force" approach—they solved the equations completely and by hand. They found that the magnitude of the shunt current could be expressed as the difference between two terms, one related to the overall cell voltage and to the resistance of the electrolyte and the other proportional to a hy pothetical external current passing through the electrolyte channel. "The key here is the minus sign," Grimes explains. That sign told the researchers that there was some ex ternal current that could cancel out the first term and eliminate the shunt current. "We tried it, and it works," Grimes says. Earlier calculations may have failed to notice this negative term ei ther because of simplifying assump tions that hid or distorted it or be cause the actual calculations were done by a computer, which would not appreciate the significance of the sign, Grimes suggests. The first application to which Exxon has put the protective current is its own developmental program for a zinc-bromine battery. The battery is one of several types of near-term, higher-energy-density storage bat teries now being developed in the U.S. for use in electric vehicles (C&EN, Sept. 10,1979, page 27). Shunt currents in this system were causing a serious problem. Metallic zinc dendrites grew out into the bat tery's electrolyte channels, quickly clogging them and eventually shortcircuiting the whole battery, explains Exxon's senior project head for the battery project, Richard Bellows. With the appropriate protective current running through the electro lyte channels, this is eliminated. "Without protection, we'd be
Exxon's Patrick Grimes tests shunt current protection in zinc-bromine battery
dead," Grimes says about the effect of protective currents for the zinc-bro mine battery. "With it we are doing very nicely. We now have no problems with shunt current. Each cell now acts as if it were independent of its neighbors. We could, in principle, stack as many plates as we wanted into a single series." Exxon's zinc-bromine battery uses a system of bipolar electrodes con nected in series with a common elec trolyte flowing past them. This con-
figuration was chosen for a number of reasons. One important one is that it gives a low current and high volt age—a combination that allows the electrodes to be made from light weight and low-cost carbon plastic rather than highly conducting metals such as silver or titanium, Bellows explains. However, the configuration leads to shunt currents that increase in importance as the number of cells in the series increases. Relatively long series of cells will be needed to make batteries with enough storage capac ity to be practical in cars. Similar cost considerations make series batteries and flowing electro lytes attractive for many other elec trochemical processes, Grimes says. These include chlor-alkali produc tion, production of metals such as aluminum or magnesium, many or ganic and inorganic chemical syn theses, fuel cells, and electrodialysis. In all these systems, shunt currents cause problems, he says, that protec tive currents may help to solve. At least Exxon hopes so. The com pany recently has patented the pro tective current approach to elimi nating shunt current effects [U.S. Patent 4,197,169 (Zahn)] and hopes to license it broadly. Rebecca Rawls, Washington
Soviet underground coal gasification on the rocks The latest look at Soviet underground coal gasification indicates that the Soviet program has all but ceased. Appraisers at Lawrence Livermore Laboratory note that in the early 1950's the Soviets had developed highly successful operational tech niques for both flat-seam and steeply dipping-seam gasification. The orig inal plans had called for a major effort to supply up to 41 Χ 109 eu m per year of fuel gas with a typical heating value of 1000 kcal per eu m by 1958. This was considered a full-scale commer cial development plan that would consume up to 300,000 tons per year of coal, mostly in fields located near Moscow, Tashkent, the Donets Basin, and Siberia. In fact, gas production peaked out in 1966 at about 2 billion eu m per year and has dwindled ever since. One reason for declining gas pro duction from underground coal gas ification in the Soviet Union is be lieved to be poor performance from · several test burns at Angren near Tashkent. Most of the problems ap pear to center on lack of proper spacing of the holes drilled into the coal seams, according to the Lawrence Livermore experts. Bores spaced on 30- to 40-m grid spacings are consid ered commercially attractive, but the
evidence suggests that the Soviets used much closer spacing, meaning greater drilling costs. The expense may have foredoomed the project. The heating value from the Angren tests and elsewhere does not appear to have achieved predicted values by any more than 20%. This adds to the economic penalties. One of the most crucial problems cited by the Lawrence Livermore appraisers may have been caused by unfavorable geology in the test fields. Gas losses through a porous over burden have been as much as 26% when calculated by non-Soviet methods. If true, this would add an other 33% to the cost of the product fuel gas. Most data on Soviet experience with underground coal gasification come from a few Soviet publications and from reports by western techni cians who have visited the Soviet test sites on a reciprocal basis. The Soviets told Canadians in 1975 that under ground gasification in the Soviet Union was having trouble competing economically with natural gas and with open-pit lignite mining. Likewise the low heating value of the fuel gas did not permit its being transported very far away from the burn site. Since most of the burn sites were in
remote areas, this could make serious commercialization impossible. The Lawrence Livermore analysts calculate that in the U.S., an under ground burn conducted under favor able circumstances could provide a fuel gas with a heating value of 125 Btu per scf at a cost about 35% less than that from a conventional Lurgi gasifier, which is more or less a ruleof-thumb standard for such calcula tions. Though admittedly optimistic, such values generally have been re alized in the U.S. trials conducted so far. Applying the same calculations to the available Soviet data indicates that the Soviet gas products cost as much as 132% of the standard Lurgi value. The Soviets are saying nothing about all of this, and the Lawrence Livermore analysts admit to consid erable speculation. But the fact re mains that anticipated Soviet activity has failed to materialize. The demise of the Soviet effort is doubly sur prising considering the many major technical contributions from Soviet workers over the past 20 or 30 years. Other reasons offered for this de mise are a lack of good underground diagnostics before and during a burn, and lack of a good laboratory support program. Lack of good diagnostics is usually manifested in erratic results from the field, where gas quality varies greatly. This has been the case in some of the Soviet trials. Likewise, laboratory support in instrumenta tion and control permits optimization of the field layout as well as control of combustion below ground. D
Catalysts allow CO, C0 2 comethanation Catalysis chemists at Japan's Kyoto University have developed a series of composite catalysts that permit si multaneous methanation of carbon monoxide and carbon dioxide in mixtures with hydrogen. Aside from the considerable simplifications that this development may provide for Fischer-Tropsch syntheses, it also may indicate a long-sought route toward direct use of carbon dioxide as a recyclable material in industrial processes. The methanation of carbon mon oxide with hydrogen classically takes place over stable nickel catalysts at high temperatures, and the reaction is highly exothermic. This reaction is basic to many reaction sequences using synthesis gas. It also has func tioned in applications such as purifi cation of reactant gases for ammonia synthesis, in which carbon monoxide has been shown to inhibit some amOct. 13, 1980 C&EN
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