Electrochemical Energy Conversion
Fuel Cells and Electrochemical Energy Storage Anthony F. Sammells Eltron Research, Inc., 710 E. Ogden Avenue, Naperville, IL 60540
The inhrrmt feature 111de\.ices for electrurhemical enerhy conversion lien in t h ~ilict . thl~tc.hct~~ical energy i~ runverred directlv to electricitv with the absence of anv movine Darts or Carnot efficiency limitations, as is the case with conventional electrical energy generators. As a consequence, fuel cells and rechargeable batteries provide a high efficiency route for the respective generation and convenient storage of electrical energy. Present fossil fueled generating plants typically operate with overall efficiencies of 2035%. This compares to the burning via electrochemical oxidation in a fuel cell, of hydrocarbon derived fuels for which overall efficiencies between 50% and close to theoretical can, in principle, be envisioned. When we consider the increasing demands placed upon the earth's limited resources as a consequence of an expanding population and dwindling fossil reserves, there is a clear necessity to identify and develop new technologies for the more efficient eeneration and usaee of enerev. -" I t can be e x ~ e c t e d that a clearer understanding of electrochemical processes will lead to the more ranid and inevitahlv wider accevtance of such devices to realize this goal. Apart from the rather obvious near term social and economic incentives for makina greater use
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even more imperative in the future. This is the slow buillup of COz in the atmosphere, (1,2)which appears to he occurring presently as a result of the increasing consumption of fossil fuels in relatively inefficient power plants and internal combustion engines. Such a build up of COz in the atmosphere could result in a n increase in the earth's average temperature because of the "greenhouse effect" where the longer wavelength radiation reflected from the earth hecomes absorbed by COz. A partial melting of existing ice caps could result. This could have disasterous effects on present coastal communities because of the resultant rise in sea level which could be anticipated, together with a potentially dramatic change in the earth's climate patterns. To gain an appreciation for the efficiency gains that can be realized from electrochemical energy convertors let us first 2Hz0, where consider the chemlcal reaction, 2Hz Oz hydrogen is burned in the presence of oxygen to form water. If the energy resulting from this reaction were used to power an internal combustion engine operating an electrical generator, overall efficiencies for the conversion of this chemical energy to electricity of about 30% might be expected. If we were to perform this same reaction electrochemically in a fuel cell at two metallic electrodes separated by an electrolyte, then the overall reaction could be separated into two partialor half reactions. Hydrogen would become oxidized at the anode via the half reaction
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whereas oxygen would become reduced at a cathode via the reaction O2
+ 4Hf + 4e
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2Hz0
Here the overall chemical reaction has become separated into two components via an external wire (for electron transfer) 320
Journal of Chemical Education
which connects the anode (where oxidation occurs) and the cathode (where reduction occurs). The free energy change occurring from this overall reaction can he efficiently extracted via the electric current which flows in the wire, which can then. of course. be used directlv useful work. "to verform . The overall efficiency for such a process would be 70-80%. In fuel cells, therefore, chemical enerev con... is s~ontaneously . verred ru elecrricity by I he doi~urionof elertro~isat an unodr h i m ,in uxidi.ul~lrmaterial and the arreptance oielectruns via a reducible species a t a cathode povided the overall reaction has a negative AG. Although the principle of the fuel cell was first experimentally demonstrated as early as 1839 (31, it has only been in the last several vears that thev have shown the notentid for reduction to practical devices. The most dramatic demonstration of this was the HdOq - - fuel cell used on the Anollo moon flights, which operated continuously for 440 hours, eenerated 292 Kw-hr of electrical enerw and. incidentallv 100 i of water. This fuel cell used an aqueous KOH electrolyte, specially prepared porous nickel electrodes, and operated at around 200°C. Major emphasis today is, however, directed toward fuel cells for terrestrial avvlications. Such fnel cells are classified according to the nature of their electrolyte and include mainly the phosphoric acid, molten carbonate and solid oxide systems (4). Although i t is well recognized that the electrochemical kinetics for the reduction of oxygen at a fnel cell cathode are, in general, more rapid in alkaline than with acid electrolytes (thereby leading to lower "overpotential" energy losses) a t a given temperature, the presence of COXin atmospheric oxygen will in the former electrolvte lead to a build uv of carbonate both within the electrolytditself and the electrke pores. This will result in limiting the useful life of the cell. Since oxygen derived from the air is the preferred (cheapest) reducible svecies in such fuel cells, this has dictated an emohasis on fuel cells with acid electrolytes. Types of Fuel Cells The phosphoric acid fuel cell is the most highly developed fuel cell today with a 4.8 megawatt unit installed in New York City (5).This fuel cell operates at 180°C and uses highly disptricd pliitirnun on or;iphitizt.d c.whon us the rlectrodei. I n diviih~alwlls opvmte nt currtwr drnsirirs 1r;il m h cni >ill ;I cell \ d t a r c 1,irr.7> V. Rc:irarch is t~rl..rnrlv hrinr " directed toward minimizing the amount of platinum required to adeauatelv catalvze the s~ontaneouselectrochemical reaction. .'dso, n triil~itmnnrthancsull'~mirarid ;ire t)eing rescarrhcd hemuae rd'thcir higher ikmicconductivitv and thepossibility of improved electrode kinetics, both df which would lead to lower internal cell enerav losses and. therefore, higher energy conversion efficiencies. This phosphoric acid fuel cell can be o ~ e r a t e donlv in the absence of carbon monoxide a t the fuel electrode (anode) since its presence will result in a poisoning of the platinum and a consequent rapid deteriorkion in cell performance. The molten carbonate fuel cell shown in Figure 1is often termed "the second generation fuel cell" because of its presumed commercial demonstration after the phosphoric acid system. I t operates at 650°C and consists of a porous nickel
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"I"
1
rpi -A"
COMPRESSOR
Figure 2 Molten carbonate fuel cell ~ntegratedwith coal gasifier
,ON
mNn"t,m
&m
ANODE
ElTlTkO* I " I " L I I 0 I
CATHODE
Figure 1. Schematic diagram of molten carbonate fuel cell.
anode together with a porous nickel oxide cathode, hoth of which are pressed against what is called the "electrolyte tile." This tile contains hoth an inert matrix (LiAlOd into which can he sunnorted a varietv of alkali carbonate mixtures, which become molten at the operating temperature of the fuel cell. In this fuel cell the use of precious metal catalysts in the phosphoric acid fuel cell have been substituted for a system where rapid electrode kinetics are achieved by use of a high temperature of operation. From the half reactions shown in Figure 1.C07 wroduced at the anode is transferred via a cat& t i c burnerto the cathode where it is reduced with oxygen to form hack the carbonate anion. Carbon monoxide present in the fuel cell anode at 650°C is subject to the water-gas shift equilibrium CO Hz0 e COz Hz and, hence, will supply hydrogen for the electrochemical oxidation reaction at the anode ( 6 , 7). Small cells have operated for 40,000 hours although scale up, particularly of the electrolyte tiles, remains a problem. The inherent feature of this fuel cell is its potential to integrate with a coal gasifier to achieve greater efficiencies for converting coal gasification products to electricity when compared to- conventional power generating stations. A schematic diagram of how such a system might be envisioned is shown in ~ k u r 2. e The solid ox;de fuel cell is based upon the solid electrolytes yttria or calcia-stabilized zirconia which can effectively act as highly ionic conducting materials for oxygen ions at temoeratures of 900 to 1000°C. The fuel is oxidized at a nickeleoated anode within the center of a tube made of the solid electrolvte. and the oxidant (atmospheric oxygen) is reduced .. H I d t i n rIup
