Kraft lignins can be pyrolyzed to durable, electrically conducting solids. These pyrolysates are able to function as cathodes in metal-lignin batteries. Voltage-time characteristics and fuel cell capabilities are discussed: results show sufficient promise to warrant further study

Steelink, Cornelius

Wilkinson, Alan

English

Wood and Fiber Science (E-Journal)

ELECTRICAL ENERGY FROM INDUSTRIAL LIGNIN: A PRELIh4INARY EVALUATION1 Alan IVilkinsotz and Cn~nelius Steelink I)c,pnrtn~rnt of Che~ l l i s t r~ ,  Univc>rsity of  Arizona, Tncson, AZ 85721 Al3STRACT Kraft lignins (,an I)c. pyrolyzetl to dl~ral)l(>, electrically conducting solids. These l?yrolysates arc. ablc. to fnnction as cathoc1c.s in metal-lignin I~attcrics. Vo1tagc:-time cl~aracteristics and fnc.1 cc.11 cal)abilitics are d isc~~sscd:  rcsnlts sliow s1ifficie:lt pronrise to warrant fi~rtlier s t l~dy.  Arlrlitior~c~l ke!lzcorrl.c.: Pyrolyzed lignin, zinc-lignin hattvrics, redos properties, voltaic cells, kl-aft lignin utilization. Ilespitc several decades of intense ap- plied rcscxarch into the properties of lignin, uses have, been found for only a small portion of the many millions of tons of lignin that accumulate a1111ually as a by- prodnct of the wood pulp industry. A few specialty chemicals, such as vanillin and dimc~tl~yl sulfouide, are derived from lignin, but thesc coi~stitutc a very sinall fraction of the total anrlual production. A large frac- tion of the comlnercial liguin is burned as an industrial fuel. Harclly any of tlie com- mcrcial bvurod~icts take advantarre of the , A <, highly organized structure of the lignin inacromoleculc. Kr'lft and alkali lignins contain cyuinonc- hyctroquinonc structures in the macromole- cule ( Stec,link 1966; Tollin and Steelink 1966). As \uch, they repre5:wt potential source3 of elvctrical cnergy, in particular, as regc~nerntivc~ c,lectrodes tor primary bat- teries. If thesc lignins could be made con- ductive. onc might obtain niaterials that - could 11c used in a number of stationary po\r7csr supplies, such as signal dcvices, eniclrgency power for buildings, railroad \\\/itching deviccs, and many other exam- ples. Thc market for such power supplies \vo~lld not absorb the bulk of the l i ~ n i n  " production, but it coulcl create another out- let for the specialty uses of lignin. 111 addition to its redox properties, alkali lignin possvsses a numl~cr of properties that favor usage as a potential cathode in a battery. These are: 1. I t  is thermoplastic and can be molded. 2. After it is pyrolyzed, i t  becomes con- ductive. 3. After pyrolysis, it is a cohesive solid that nlaintains its shape and is fairly resistant to mechanical shock. 4. I t  is functionalized and therefore can be chemically modified prior to pyrolysis. 5. I t  is abundant and ch?ap. A11 of the abovc prompted us to cxaminc some industrial lignins as potential cathodes in voltaic cells. The use of organic materials for voltaic cells is not new, of course. Quinone-liydroquinone monomers, when rnixed with coilductiilg charcoal and resins, perforin quitc well in metal-orqanic voltaic cells ( Glicksman and Morehouse 1959). Another type of cell has attracted consider- able attention: the metal-air battery. This cell utilizes a special charcoal-noble metal cathode that catalyzes the rcdliction of oxygen (Collins 1971; Mantel 1970; Schu- rnacher 1971). Other experimental cells use quinone materials that can he regenerated by air (Sandstede 1972). All of tlie organic conlponellts in the abovc batteries are fairly exotic and expensive. 111 theory, lignin should be able to per- form the cathodic functions of the batteries described above. The following exploratory ' 'I'his work was s l ~ ~ ~ ~ o r t e d  by a University of Arizona Institutional Grant, for which we are expcrimeilts were carried out to test this gratc:ful. assumption. They represent a material eval- WOOD hN1) FIBER 12 Sl'HING 1075, V. 7 ( 1 )  ELECTRICITY ~ ~ 0 x 1  LIGNIN 13 uation, in \vhicli cliffcrcnt industrial lignins werc sul~jected to heat and chemical mod- ification. Thesc cxpcrimeilts do not iilclude engineering studies, which would be rc- (1uirc.d to convert a suitable electrode ma- terial to an  economic battery. l.:X1'EI<IXfENTAL ANII RESULTS Rerlox properties Hardwood and softlvood kraft lignins' were dissolvetl in aqueous alkali and placed ill separate coulpartnicnts. These were con- nected by a salt bridge, fitted with platinurn charge collectors and chargcd overnight \vith a DC voltage of 2 volts. This wet asscinbly was able to hold a charge and discharge through a load for one hour. Al- thongh this is a c~~nlbersorne d vice, it did confirn~ our earlier prediction that ligni~i possvsses rcdox functionality, which is capa- 111~. of dclivering electrical energy. A nl~rnl~cr  of kraft lignins wcre heatcd in an atmosphere of nitrogen to teinpera- tmes up to 1000 C. The pyrolysates were low tlensity solids. If the pyrolysis tempcra- ture was raised above 600 C, the materials 1)c.came electrically conducting (i.e., resis- tance 1c.s~ tlian 5 o h ~ n / c n ~ ) .  The pyrolysates ]lad good ~ncchanical strengtll and could I)c machined, drillecl, or sanded. A material of superior mechanical strength could be o1)tained Ily mixing po\vdered pyrolysatc with untreated lignin and repyrolyzing the 111isturc.. A ~ l u ~ n l ~ e r  of simple chc~mical modifica- tions werc) carried out with softwood kraft lig~iin in an attempt to improve the ouida- tion potrntial and conductivity. Also, we were interested in determining the cxtcnt of incorporation of certain element\ into the Iigniil macromolec~~le. Subsecluent to thc. cl~c,mical treatment, all of tl-le products \irc.rc. heated to 800 C for one hour 11nder nitrogen. The followi~~g e~pc,riinents were carried out. ' Kindly tlonatctl by \f'cyst\';~c.o, North Charles- toll. Soi~th C;lrolin;l. (1. Softwood lignin3 (Product A )  was stirred with dilute nitric acid f :~r  two days. The nlixture was filtered, the precipitate dissolved in aqueous ammonia and the ni- trated ammonium lignin salt recovered by evaporation to dryness at room temperature ( Product R ) . I) .  Soft~vood lignin ( A )  was stirred with dilute nitric acid for two days. The mixture was filtered, thc precipitate dissolved in 25% aqueous NH4S,, warmed to 80°C and t he~ i  evaporated to dryness (Product C ) .  c. Product B was dissolved in an aqueous solution of Cr ( NOR ) R. The resulting pre- cipitate was filtered, washed and dried ( Product D ) . Pyrolysate Products A-D were analyzed for the elenlents C, H, N, S, and Cr. Con- ductivity n~easurements were made on thc po~vdcrcd saniples. These data arc shown in Table 1. The results show that even at 800 C, the pyrolysate retains a large residue of func- tionality. Under mild conditions of chem- ical treatment, N, S and transition metal arc incorporated and retained at high tmpe ra -  turc. Co~lductivity can be drastically altered by chemical moclificntion. The resistance values shown in Table 1 are much higher than those for solid blocks of material. For example, Lignin R has a resistance of 2 ohms when in the solid form, as compared to 155 ohms under powdered conditions. No doubt, contact is poor between powder particles; however, comparison under stan- dard conditions is easier with powders than with solid blocks. Catlrorle behavior To test the pyrolysates as potential cathodes, we constructed a simple voltaic cell. The anode was a 1 cm' square of zinc sheet (1.25 grams). The cathode was a disc of pyrolysate, 2.0 cm. in diameter weiglling 2.0 grams. The two electrodes were separated by an asbestos sheet soaked in an electrolyte (n5ually saturated NH,CI). A wick connected the asbestos sheet to a 'Kindly tlonatetl hy Donlt.lr Hcsrarcl~ Center, Rloi~trc,ll, C;~ntlda. 14 ALAN WILKINSON AND CORNELIUS STEELINK TABLE 1. Properties of modified lignin pyroly.~utes Lignin Sample Elemental Analysis H N - Resistance a Cr Ohms a ~ e s i s t a n c e  measured across a cylinder of powder, 1 cm X 0.2cm2, packed under 1.0 l b  pressure.  Powder t o  pass 80 mesh screen or  l e s s .  well of electrolyte solution. Voltages across the zinc-pyrolysate electrodes were Inca- surcd at various time intervals and recorded as shown ill Tahle 2. Thc voltage Vr, was mcnsurcd across a load of 100 ohms, which corresponds to the load of a small lamp. Indeed, thesc batteries developed enough powc:r to light a srnall bulb for one half hour, or until the voltage dropped below 1.0 ~ o l t .  The recovcry cyclc of these cells sug- gc~5ted that the ligniil cathode inight be catalyzing the reduction of i-nolecular oxy- gen ill the air. To test this assumption, we measured the voltage of the lignin A cell \vhile the latter was encased in a nitrogen bag. Under these conditions, the V,,, was 0.71 volt and the VT, was 0.42-0.29 volts (compare to 1.22, 1.00-0.53 in air).  The original voltages were restored when the cell was placed in air. Thus the pyrolysate had surface properties somewhat compa- rable to commercial renewable cathodes, made from conducting charcoal imprcg- nntcd with noblr nletal and resin. Compurison t o  commercial dr!/ cell For a perforil~a~lce omparison to a single Evcready Dry Cell (1.5 volt), we con- strlicted a 65 gram ligl~iil pyrolysatc block. Whcn this WAS wrapped with imprcgnated asbestos and zinc sheet, it approxiinatcd the size of a dry cell. Thc voltage properties of this assemblage wcre: V,, = 1.24; VI, = 1.20-1.04 ( 2  hours discharge). Under the same conditions, the Eveready Cell (85 gr'uns) performed as follows: V,,, = 1.50 volt\; Vr, = 1.48-1.44. CONCLUSION Lignin pyrolysates are capable of func- tioning as cathodes in voltaic cells. They TABLE 2. Lignin-zinc cell vtrltagcs Lignin Sample vEC Recovery Period "k A 1.22 1 . 00-0.53 12 Hours Tom1 in i t e3  1.24 0.50-0.17 B 1.39 1.06-0.73 Hardwood 1.24 1.04-0.53   raft^ aOpen c i r c u i t  voltage. b ~ o l t a g e  measured across 100 ohm load. F i r s t  number i s  i n i t i a l  voltage; f i na l  number i s  voltage a f t e r  two hours discharge under load. b 'voltage across 100 ohm load a f t e r  recovery period, measured as  fo r  V L  . ELECTRICITY FHO1\1 LIGNIN 15 are susceptible to simple cl~cnlical modifi- cation prior to pyrolysis: these modifica- tions drainatically alter their electrical prop- erties. Because of the thcrmoplastic nature of lirniil and its chemical modifications. <, solid cohc.sivc electrodes can be cast in a n u ~ n l ~ e r  of desirable configurations. I11 ad- dition, the pyrolysates appear to have cata- lytic surface properties that promote the rvductioii of oxygen at room tc>mperaturc. These could 1,e optimized by appropriate ~nodification ex~eriments. If such a surface A co~lld 11e acliicved, one would obtain a truly reiic,wable (or permanent) electrode: i.e., :I tuel cell using only air for the cathode. At prcsent, these- crude zinc-lignin bat- teries represent oilly a feasibility study. Certainly, they do not colilpnre to commer- cial dry cells. Bl~lt they do perform well vnough to justify further study. I11 this c,~ploratory work, a number of parameters IlLn~e been ideritifiied. These will constitute th(3 basis for a s!fsteniatic investigation of the electroche~nical properties of alkali lignins. REFERENCES COLLINS, D. 11. (ed . )  1971. Primary power sourccs #3. Oriel Press, London. G L I C K ~ ~ ~ A N ,  R., AND C. K. R~OREHOUSE. 1959. Investigation of the elcctro-chemical charac- teristics of organic componnds IV. Quinonc componnds. Electrochein. Soc. 106 ( 9 )  :741- 745. ~ I A N T E L ,  C. L. 1070. Batteries and energy sys- tems. hfcGraw-Hill Book Co., New York. pp. 59-63. SANIISTEDE, G. (ecl.) 1972. From electrocatnlysis to fnel cells. Univ. Wash. Press, Seattle. S~:HU~IACHEH, E. A. 1971. I n  G. W. Heise and N. C. Cahoon, eds. The primary battery, v. I. J. M7iley ancl Sons, New York. STEELINK, C. 1966. Stable free radicals in lignin and lignin oxidation products. Adv. Chcnl. 59:51-64. TOLLIS, G., AND C. STEELINK. 1966. Biological polymers related to catechol: EPR and IR studies of melanin, tannin, lignin, humic acid and I~ycl~ox~qninones. Biochirn. Biophys. Actn 112:977-379. 

Electrical Energy from Industrial Lignin: A Preliminary Evaluation

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Electrical Energy from Industrial Lignin: A Preliminary Evaluation

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