CA1097481A - Bi.sub.2¬m in2-x xxbi.sub.x|o in7-y xx compounds wherein m is ru, ir or mixtures thereof, and electrochemical devices containing same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Novel bismuth-rich pyrochlore-type compounds are described having the formula:

Description

~748'1 BACXGROUND OF THE INVENTION AND STATEMENT OF PRIOR ART
The present invention is directed to novel bismuth-rich pyrochlore-type compounds having the formula:

Bi2[M2 xBix107 y wherein M is selected from RU, Ir and mixtures thereof, 0 <x < 1.0 and 0 < y < 1Ø The present invention is al-so directed to electrochemical devices containing these com-pounds as electrocatalyst materials for oxygen electrodes.
A number of various types of electrochemical devi-ces have been developed over the past few years for the pro-duction of electrical energy by electrochemical reaction and obversely for the consumption of electrical energy to ef-fectuate electrochemical reactions. Many of these devices rely upon a reaction involving oxygen (or air~ as part of the mechanism to accomplish the desired result. For example, such devices may contain oxygen elect:rodes which are oxygen reducing cathodes in which oxygen is catalytically elec-tro-reduced. Alternatively, such devices may contain oxygen electrodes which catalyze the evolution of oxygen from water.
In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, metal-air bat-teries, fuel cells, electrolyzers, metal electrowinning de-vices, etc., are among the well-known electrochemical de-vices which may contain oxygen electrodes. Typically, such devices contain electrocatalyst materials at one or more of _ 2 ~

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their electrodes and preci.ous metals, such.as platinum (on carbon support) and silver (on carbon and other supportsl, are frequently employed as electrocatalysts.
In addition, various electrocatalytic alloys, com-pounds and compound mixtures have been developed for these electrochemical devices to achieve more desirable .

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systems. For eY~ampleg U.S~. Paten~ No. 3,536,533 ~Ri~:am~ra)

2 describes the use o ~.n alloy of gold~ silve.r9 pallad:ium and

3 at least oIIe of platinumg rhodium and rutheni~m as a fuel

4 cell electrode electrocatalyst~ and U.S. Paterlt No. 3~305,402 (Jones et al) describes the use of a combinatiorl of platin~
6 and ru~hen7um oxl.des as an elec~roca~alys~. However, bo~h 7 references describe these catalysts as fuel call anoc~e (or 8 fuel o~idat:ion) cat~lystss O'Grady et al9 Technical Report 9 No~ 37j, I'Rul:h~nlum Oxid* Catalysts for the Oxygell Electrodet';, Contrac~ No., N0014-67~ 0404~0006 (AD-779-~99) 0~ e of 11 Naval Research, May 1974 (National Technical Informati.on 12 5ervic~) describes the ~se of ruthenium oxide as an ele~c~o~
13 chemical ca~alys~ for both ~he genera~ion of oxygell and the 14 reducti.on of o2~y~en, U.S. Patent No. 3,40S,,010 (Kordesch e~ al~ t:eaches that splnel type~el.ect.xccle ca~alys~:s have 16 been found to produce better activati.oII of th~ elect:r~cle 17 and improved electrolyte repeLlenc~ of th~ electro~e by ~8 t~ inclusion of rutheniump 19 The foregoing prior ært sle`scribes ~rarious ~ypes of electrodes includin~3 ~hose which ucil~ze iridium and/or 21 rutheni~m-containing catalysts, Howe~er, n~ne of these 22 references teaches or renders ol~vio~s the novel compou2lds 23 R t~e presen~ invention or the`electrochemical devices of 24 the pre~en~ inve~tion havi.ng ~hese speciied pyrochlore 2s type co~npounds as the eleetroca~alysits.
26 Heretofore9 many pyrochlore compolmds such as the 27 pyrochlore compounds P~2Ru2O7 y tla~tice parame~er o~ ¦ :
28 10.253A), Pb~r2Q7 y (lattice parameter o lO,X7LA)~ ~¦
29 ~i2Ir~07~yg Bi~h~O7~y7 Pb2Rhzo7-y9 Pb2P~2Q7-y ~rld Cd2Re2070y, con~m~nly referred to as lead ru~hena~:e, lead 31 iridate9 blsn~u~h lrid~te, bismuth r~oda~e9 lead rhod~J
32 lead pl~tinat:e and cadmi~n rhenat~, re~pe.c~-.L~ely, hav~ ~een :~

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1 knOWn. F~r eXaTnP1e9 LnngO~ ~aCCah and GOOdenOUgh9 Mat, ReS, 2 BU11., VO1. 49 PP. 191~2029 (~.~69) l1a~e deSCrib~d the COm-3 POUndS Pb~RU207_~ and Pb~Ir207_Y ~nd thei~ PrePa~atin at 4 e1eVated temPeratUreS WhiCh are in eXCeSS O 700C. S1eight3 Mat~ ReS,_BU11 " ~O1. 6, P. 775 (1971) haS a1SO deSCribed 6 the COr~OUndS Pb2RU207_Y ~nd Pb2lr2O7_Y (in~LUdi~ the 7 PYrOCh1Ore COmPOUnd Pb2RU26 t 5 ha~ing a ~a~tiCe Paramet~r 8 Of 10.27 ~) and ~he-Lr PreParatiOn at 700C and 300Q atmOS-9 pheres of PreSSUre, U,S. Patent NQ. 39682~40 (Van ~oan) deSCribeS thP PreParatiOn of lead rUthenate at temPeratUreS
11 Of 800C and hi~h2rr These re~er~rlces do nOt teaCh t~1at 12 bis~nuth-rich compouIlds of the present inven~ion exis~ or that 13 they may have superi.or physical proE~e~t-le~. Fur~her, t~.ey 14 do not. teach tha~ theSe novel pyrochlol-e compounds may be useul a6 e1eCtrOCata1YStS in e1e~CtrOChemiC~1 deV1CeS SUCh 16 as in tha Pr~Sen~ inVentiOn.
17 United States P~ents No~; i 3, 7699 3~2 (IC~Io e~ al) 18 and 3y 951, 672 (~angley et al? both clisclose the prep~r tior 19 o lead ruthenate and lead irida~e using ~arious cechnique~
at ~emPeratUreS o a~ least about 6n~oc, ~nd preferahly a~
21 higher tempera~ures~ ~owever~ ~h~e reforences fail ~o 22 recognlz2 that the bismuth-rich psrrochlores of t~e present 23 in~ention are obtained at generaliy lower temperatures or 24 that sueh pyrochlores have improved physic$1 properties.
Fur~her~ these re~erences alsp fail to ~each or render 26 obvious ~he use of thes~ novel pyrochlore compounds as 27 electr~ca~alysts in elec~rochemical devi~es s~eh as in the 28 present inventi.on.
29 Bouchard and Gillson, Mat. Res, Bull.. ~ Vol . 6;
pp, 669-680 (1971) describe Bi2Ru~ 7 ~nd Bi2Xr~07 prepar~ :
31 tion and properties, including the f~c~: that these compouncls 32 have high conductiv:i.ty a;nd sma1 1 Seebeck coe~i.cients .

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However, there is no teaching that these compounds are useful electrocatalysts in electrochemical devices or that the bismuth-rich compounds of the present invention even exist.
Derwent's sasic Abstract Journal, Section E, Chemdoc, Week NoO
Y25, Abstract No. 320 (August 17, 1977), Derwent Accession No.
44866Y/25 describes electrodes for electrolysis of alkaline and carbonate solutions which comprise nickel plated steel strips coated with high conductivity layers containing Cd2Re207 Pb2Re207 or Ni2Re207. These compounds are prepared by im-pregnating perrhenic acid and a metal nitrate such as Cd nitrate onto a nickel strip and baking at 350C. However, these compounds are all rhenates and are not ruthenates or iridates of the type claimed herein as novel compounds and in electrochemical devices. National Bureau of Standards, Wash., D.C. Inst. for Mat. Research, Abstract of Rept. No. NBSIR-75 742 (1975) describes the use of mixed oxides as oxygen reducing electrocatalysts in acid fuel cells, including the use of barium ruthenate. However, of all materials suggested for such electrocatalysts, none are of the pyrochlore type structure, much less of the bismuth-rich pyrochlore structure as in the present invention.
Trehoux, Abraham and Thomas, Journal of Solid State Chemistry, Vol. 21, pp. 203-209 (1~77) and C.R. Acad. Sc.
Paris, t. 281 pp. 379-380 (1975) describe the solution pre-paration of a pyrochlore compound of the formula ` ,,` ~; ~ , .:

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1.14 Q.27 Lio. 27Bil,73~ ro4 gHl ~ HQ 8- The synthesis is carried out by adding a bismuth nitrate solution to a sol~
ution of 17% potassium hydroxide containing an excess of pot-assium hypochlorite. The reaction is carried out in this medium for 2 hours in a reflux type of apparatus at a temper-ature slightly highter than 100 C. The method of synthesis and the product prepared are different in many respects from the synthesis method and described products herein. The com-pound prepared in the cited reference is not an oxide but rather an oxy-hydroxide which has a significant amount of pro-tons incorporated into the bulk structure. Proton nuclear magnetic resonance experiments on the materials of the present invention show that they are oxides which do not have signifi-cant amounts of protons incorporated into the structure. The pyrochlore synthesized by Trehoux et al is not a ruthenium or idirium-containing compound and, in ~act, is believed not to be an electrically conductive pyrochlore. The po-tassium hy-droxide solution used in the Trehoux reference serves not only as a reaction medium but also as a constituent in the reaction since potassium is incorporated into the A site of the pyrochlore. The compounds of the present invention are made by a method in which the alkali solution employed is solely a reaction medium with no measurable amount of alkali metal cations incorporated in the pyrochlore compound which results from the- synthesis.

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Morgenstern-Badarau and Michel, Ann. Chim., Vol.
6, pp. 109 et seq. (especially at 109-113) (1971), and C.R.
Acad. Sc. Paris., Vol. 271, Seire C. pp. 1313-1316 (1970) report the solution preparation of pyrochlore compounds hav-ing the formula Pb2Sn206-xH20 where a< x ~ 1. The conditions of preparation are strictly defined as follows: equimolar quantities of lead and tin are reacted from solution in the presence of the complexing agent nitrilo-triacetic acid (NITA) such that the concentration of [NITA~/ ~Pb ~ = 2.
The pH of the reaction medium is fixed at 11 and the reaction is carried out for several hours at 80C. The compound pre-pared by Morgenstern-Bardarau et al is - 6a -:X
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a hydrated b~-lde ~ wllereas p~rochlore bismuth compounds o:E th~
2 present ~nvention 2re ~xi.~les- The pyrochl~re prepared in 3 ~his reference ~ while it do~Qs corltain lead,, is not a lead-4 rich pyrochlore and is no~ siDiilal^ to the materials of ~ne

5 presellt im7er2tion. In fact~ the pyrochlore prepared by

6 Mor~erlstern~Badaraw and Mi.ch~l is not a ru~.henilml or iridiLm-

7 conl:aining oyrochlore and is believed not to be e~e-trical~

8 conduct:ive, Mor~enstern-Bad~rau et al also speciically

9 state that their preparation ml2thod is one which gives a

10 solid produc~ con~.aining Pb~. The compoundis o~ ~he E~resen1 ~n~ention~, orl ~he o1 e~ harld,~ ~re such that the~ corlta r 12 ~ Ul:h C~ ;XlS~, Wllile t1:~e presence of ~ comple~c~ g agen~
13 is required in ~he syrlthesis described in the ci~ed refer-14 ence, no such com~)lex:Lng a~;ent is required in the m2thod o:~
15 prepara~ion o tl:se compounds o:E~the present invention, 16 Fur~:hermore~ th~ speci:Ei~d xange o pH vf l:he syn~hesls t medium in the me~hod o pre~ara~:ioII of l:h2 cvmpounds of ~:he 1~ pres~nt invention clearly di:Efers ~rorn the range of p~
19 within which ~he ~nethod of ~he cited reference will ~pera~2 20 In fact" the Morgenstern~Badar~u and Michel9 An~ Chim.9 21 Vol~ 6~ pp. lO~l2~ il971~i re~arence clearly st~tes that no 22 solid product compound can be c~btained if c :?nditions w~ich 23 are coinciden~ with th~se specified for the present invena 24 tion (pll ~ 13.5, tem~erat~re - 80~C~ zero concentration ~f 25 complexir~g agen~) are çmployed~
26 In s~nary~ there exis~s a fo3:midable body 27 prior ar~ d~sc~ibirtg the existence of various pyrochlores"
28 ~hei~ po~.en~ial us~s inclulling uses as dielectric ma~rials" I ;
2~ and descri~In~ ~rcsrio~s ~het:a:l.s and rmetal o~ides as elSectro~
30 cat~lys~ ma ~erl~ls . No~w~ starl~ng such prior art, tl~eL e 31 i.s no su~gesl:i.o~ ~r teachin~ that ~a~ the bismuth-rich 32 py~ochloxe COllipOIlT1~3S vf th e ~reserlt invencion e~en exist~
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1 or that (b) ~h~se specific pyrochlores may be useful electro-2 catalyst ma~erials in electrochemical devices as claimed in 3 the present invention~
4 DESC~IPTION OF_THE INVF.NTXQN
The present in~ention is directed to no~el bismuth-6 rich pyroc~lllore~ty~e compounds having the formula:
7 ~i2[M2~XBix~o7~y (l) 8 ~herein M is selected from the group consisting of Ru~ Ix 9 and mixtures thereof~ and wherein x is great~r tllan zero and less than or eqlal to about 1. O and y is gre~ter than

11 or equa.l to zero and less than or equal to about ~.O~

12 The presellt invention is also directed to electro~

13 chemical devices co~ntaining these compounds as electrocat~

14 lytic ma~erials for o~ygen electrodes.
In one preferred emho~iment o~ the pre~ent inver ~6 tion, t.he variabl.e M i~ rutl~enium ~nd the compounds are 17 ~epresented by the formula:
18 Bi2~U~Bix]o7=y (2 19 wherein x and y are ~s described.
In another eml~odiment of the ~resent inverltiorl9 21 the variable M is iridium and the compounds are repl-esented 22 by the fore~lao 23 Bi2 ~ Ir~ ~xB ix] ~ -y (3 ) 24 wherein x and y are as described, In desired embodim~nts of the present in~entiorl, 26 the variable ~{ is within the range vf about Q . l to ab~ut 27 0.8, preferably about 0.25 to abou~ 0.6. Also9 the variable 2~ y is, as ment~oned9 wlthin the range o~ about 0 ~:o abou~ lO0 29 This variable represents ~n average value and takes int:o accolmt ~he Eacc that a fractlon o~ anion sites may he 31 vacant in the crystal structure.
32 '~ corl.pounds ~f ~n~ l~re~;e~ irl~enti~ll; a~

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represented by formulas (1), (2) and t3) above, display the pyrochlore crystal structure. Pyrochlore structure oxides are represented by the general formula A2B2060' wherein A
and B are metal cations. A detailed description of their crystallographic structure may be found in Structural In-organic Chemistry, Fourth Edition by A.F. Wells, Clarendon Press, Oxford, 1975. Briefly, oxides of this type display a face-centered cubic structure having a unit cell dimension of about 10g. The B cations are octahedrally coordinated by oxygen anions (0). The structural framework is formed by a three-dimensional array of these corner shared octahedra, each sharing corners with six others. This framework has the composition B20~. As Wells describes, this Eramework of octahedra is "based on the diamond net r having large holes which contain the 0' and two A atoms, which themselves form a cuprite-like net A20' interpenetrating the octahedral framework". The octahedra are actually arranged in tetra-hedral clusters. These clusters of octahedra are then tetra-hedrally arranged so as to form the large holes in the structure described by Wells. Each of these large holes may also be defined by four tetrahedrally arranged puckered, hex-agonal rings which are formed by the corner shared octahedra.
The A cations reside in the center of these puckered hexa-gonal rings and are coordinated by the six 0 anions which de-fine the rings plug two more 0' cations at a slightly dif-_ g _ X

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~L~97~81 ferent distance. These 0' anions reside at the center of the large holes in the octahedral framework. It is the 0' anions which may be partially or totally absent, leading to -the general pyrochlore oxide formula A2B207_y where O~y<l. Thus, the compounds of the present invention are referred to as pyrochlore compounds, albeit they are not stoichiometric pyrochlores, but rather are bismuth-rich compoundsof the formulas as above.
It has been discovered that the pyrochlore com-pounds of the present invention exhibit an expanded lat-tice, believed to be a result of the increased amount of bismuth in the crystal s-tructure. Although merely a hy-pothesis at the present time, it is believed that there may exist a direct correlation between the ex-tent of expansion of the lattice and the amount of excess bismuth relative to a bismuth to metal ratio of 1`.0:1.0 in the crystalline structure. Thus, it is believed that all things otherwise being equal, the greater the lattice parameter, the greater the amount of bismuth and therefore the larger the variable x in the formula representing the compounds of the present invention. This hypothesis is supported by the fact that all compounds of the present invention, which have been prepared and lattice parameters determined, do indeed exhibit expanded lattices.
In certain pyrochlore structures, a slightly ex-panded lattice is obtainable by partial occupancy of the X

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anion vacancies that may he present in the pyrochlore struc-ture. For example, Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) prepared a slightly expanded Pb2Ru207 y lead ruthenate by carrying out the synthesis at 700C and 3000 atmospheres of pressure. This high pressure synthesis resulted in de-creasing the magnitude of the variable y in the compound formula Pb2Ru207 y or, alternatively expressed, increased the fraction of 0' anion sites which were occupied by oxygen ions.
Thus, in the case of Pb2Ru207 , a slight lattice parameter lQ increase was achieved by increasing the anion content of the lattice (by filling up vacant anion sites) while still main-taining a 1.0:1.0 lead to ruthenium ratio. In the case of Bi2Ru207 y, the A cations are trivalent and preservation of electroneutrality therefore requires that all anion .

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l sites be ~ccup,i.ed; thus, y in the form~la Bi2Ru~07_y must 2 equal zero fQr the kn~wn stoi.chiometric compound ~Bouchard 3 and Gillsorl Mat. Res~ ~ull., Vol. 6, pp. 669-6809 1971~.
4 The only conceivable ~xplanation ~or the expanded latti~e exhibited by the compounds of tIle presen~ inven~ion is a 6 bismuth-rich pyrochlore or~ expressed differen~ly~ a substi 7 tute of ruthenium by trivaleTIt or a combination o~ ~ri.- and 8 pent~valent bismuth ions, The substitution o~ these larger q B ions (ionic radii of 1.0~ and 0.72R for Bi3~ and Bi5+
respectively as compared to 0.~2A for ~u~? will result in a s ig~i~icaTltly expatlded lattice as is observed in the 12 r~terials clescrlbed ln the present invention.
13 In general terms) the n~vel compounds o~ the 14 present inveIItion are prepared by a m~thod which invol~Ps reacti.ng bismuth and M c~tions to yield a pyrochlore oxide lh by precipit~ion o~ bismuth al~d M c~lt.ion~ f`l-om an aqueous l7 solution sou~ce o th--~se cations in a liquid alkaliIle medium l~ in the presence of an oxygen source a~ a temperature helo.
l9 about 200C for a ~ufficleTlt time for reacti~n to oceur.
~- The aqueous solu~ion so~rce of re~c~ant (bismu~h 21 and ~) cations i~ meant by deinitioT1~ to includ2 ~ny aqueous 22 sol~tion whic~ will d:issol~re i~nic bism~ and M cations.
23 This metal cation containing solution m~y be prepared using 24 ~ismuth source m~t:erials which inclu~e bismuth nltrate;
25 bismu~h chloride~ bismuth oxide, bismuth oxalate and bismuth 26 oxychloride. Among the mentioned bismuth source m~terials~
27 preferred is bismuth nitra~e. The M source ma~erials used 28 in preparing the aqueous solution source o ~ismuth ~nd M
29 cations i.nclude rut~enium.chlori.de~ rut}lenium nitra~e, ruthenium nitrosyl ni~rate, i.ri.clium chlor:i.de, irldium 3l hydroxide arld iri.dium oxalic acicl~ as well as mixtures 32 thereo~. l)esirably~ the M source ~naterial is either a ~LQ~748~

ruthenium source or an iridium source, although mixtures thereof may be employed. The preferred M source materials include ruthenium nitrate and iridium chloride.
The aqueous solution source of bismuth and M
cations is prepared by dissolving appropriate amounts of bis-muth source material and M source material in aqueous solvent.
Usually the bismuth and M source materials are dissolved in ;~
aqueous acid solutions, the acid solutions being just strong enough to cause the bismuth and M source materials to dis-solve. Acids such as nitric or hydrochloric may be used but nitric acid is preferred.
The bismu-th source material and M source material are dissolved in relative amounts so as~-to achieve, in ge~er-al, an initial reactant bismuth to M ion ratio o at least about 1.0:1Ø Desirably, this rati.o is within -the range of about 1.05:1.0 to about 10.0:1Ø ~n the preferred embodi-ments, the bismuth to M ion ratio is in the range of about 1.2:1.0 to about 5Ø1Ø As a practical matter, the reactants may be used in a bismuth to M ion ratio appreciably higher than the ratio of bismuth to M in the final pyrochlore product.
Preparation of the aqueous solution source of bis-muth and M cations in the manner just described assures atomic scale mixing of the cations and thereby provides favorable kinetics for the low temperature, solution medium synthesis .
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that follows.
The liquid alkaline medium is meant by definition to include any liquid alkaline medium which will promote reaction between the bismuth ions and M ions from the men-tioned aqueous solution source of bismuth and M cations, and will effect the precipitation of -the desired pyrochlore phase. The liquid alkaline medium may be any which satisfies , .

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thi.s deill:it-i.on artc~ includes a].kali me~al hydro~idf~s 9 and 2 especially tt!eir aqueous basic solutions. Thus, the li.quid 3 alkaline me-lium may desirab~y be an aqueous basic solution ~:
4 containing a base se:Lected from the group consisi:ing o~
sodiutn hydroxide9 rubidium hydroxide9 cesiurn hydroxide~
6 pot:assium hydroxide ar~d mixtures thereo~. Su:Eficiea~ ~ase 7 is included so as to render a liquid alkaline medium h~vin~
8 a p~ o at least clbout 13 . S . Preferablyt suficient base i.
9 e~ployed so ag to produce a liquid all;alins~ medium having a pH of between about 14 and l5 . 5 . Exa.ct amounts o:E base 11 material need not be speciied since pH detex1l~ination i.s 12 withi.n t:he purvlew of the artisan.
13 It is also :~ound 1.o 1~e help.~ althou~r,h not ncces~
14 saryl to sa~:ula'ce the alkali.lle reacl~i.on medi~lm w~ h respec~
to one or more of the reactant ~a1:ions (and especially with 16 resp~ct~. i;o the most all;~li svluble. ca~:ion reactan~) prior ~o 17 conlbination o the c~.quev~1s solu~.it:sn source ol A and B cati~ris 18 with the ~lkaline rPaetion medium~ This mc~y be done s~ as 19 to avoi~ large discrepancies l~etwe~n cation ra~ic~s in tn~
reacted product and in ~he initial reactant mix~ure due to 21 possible solu~ y in tlle ~lk~line reaetion medium n~ one 22 or more of the reac~ant cations.
23 The alkaline mediu~ acts solely as a reaotion 24 m~dium and not as a constit~ent in the reaction. Ih-ls i~
supported by the fæc~ that the pyrochlores made by the 26 me~h~d of ~.his inven~ion sho~ less ~han 0.02% ~by ~eigh~
27 alkali metal cation as measured by ~r.omic a~sorption.
28 The oxygen s~urce is rneant to include ~y deiniti~n 29 any sour~e whi.eh will pro~ride the nxygen needed ~o ~c~t~i the pyrochlore compound. The oxygen source n)~y ~e an~ o~ ~:he 31 bismuth source materidl, the M SVl.lX'Ce material, the alkt.~line 32 Lic~uid medit~m or colrit inations thereoE . Alternat:i~ely or ~ 13 ~

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1 additionallyf t:he o~ygen source m~y be or include indepe.ndent 2 o~ygell~con~ributing material, e.~. bub~led o~gen or oxygen~
3 containing sal~s or o~her addi~ives~ In any event, an essen~
4 tlal aspect o~ the present lnves~tion co~lpound preparation is the inclusiol1 of adequate o~ygen to permit the foru~tion 6 of ~he py~-ochlore structure~
7 No critic~lity ex~sts as to whether the ac~ueous 8 solution source o bis~nuth and M cations is added to ~he 9 alka1.i.ne medium ox whether the alkaline medium is added to 10 the aqtseous source of reactant cations. However9 the ~ormer 11 is usually practiced to insure that all o the cations see 12 an excess of alk~line medium. In general3 at le.ast about 13 l.O li.~er of llquid alkaline mPdium is used per sum tot~l :
14 mole of s~letal ca~ion reactant. As snentioned~ ~h~ re.action .
m~y be carx:i.ed ou~ at tempera~uræs below about 200C.
16 Desirably9 the reaction temperature is within the xange o~
17 a~out lO ~o about 100C~ Pref~rahly~ ~he reactlan ls ca~rl~
18 out at temperatures within the range o about 50 to about 19 8~C~
~urin~ tne rea~tiQn p~.ri~d9 the alkal~ne medium 21 ~ay be replaced wi~h ~resh alk~line medium al~lough this is 22 no~ ne~essa~y Eor ~uccessful prac~ice of the invention, 23 The reaction is carried out for a time sufficient for re æ ~
24 ti.on ~o occur. With many reactant co~nbinationsy at leas~ a partial re~c~ion occurs almost instantly. In any event9 the 26 ~eng~h of t~le over which the reaction should be allowed ~o 27 proceed ls a matter o~ choice. Within limi.~s~ however, the 2~ longer the reac~ion time9 the greatar the extent of reactionO
29 As a ~ractical matter~ a significant am~unt of reac~lon prG~
3~ duct is obtsined by reaotlng ~or about l day9 and generally 31 a reac~ion ~ime o~ ~bout 3 to abqut 7 days is advan~ageou~, 32 Af~er ~he reactlon ls completed9 the reaction 74~

product may be separated by known separation means. These separation techniques include filtration and centrifugation.
Various post treatments may be employed as desired. These might include heat treatments to improve the crystallinity of the product and/or washing in various media in order to leach out any unreacted metal species. The reaction product in-cludes one or more of the pyrochiore compounds of formula (1) above. when preferred amounts of reactants are employed, com-pounds of formula (1) may be obtained wherein o Cx ~ 1Ø
~s mentioned, the above pyrochlores of the present invention have an expanded lattice and exhibit a high surface area. Further, these compounds display high electronic con-ductivity, thus making them particularly useful for electrode applications, e.g. as oxygen electrodes. Thus, the present invention is also directed to an electrochemical device, as mentioned.
The electrochemical device of the present inven~ion contains one or more of the mentioned novel bismuth-rich pyro-~ chlore compounds as an electrocatalyst material. More specif-ically, the device is one which contains an oxygen electrode containing one or more of the mentioned compounds as the electrocatalyst material. The device, therefore, may be any electrochemical device having this oxygen electrode, including metal oxygen batteries, metal-air batteries, other types of batteries containing one or more oxygen electrodes, fuel cells,

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electrowinn~ng devices and electrolyzers. These include both the oxygen-reducing electrode devlces as well as the oxygen-producing electrode devices. For example, these devices in-clude those which contain oxygen-reducing cathodes which con-sume or electrocatalytically reduce oxygen in an oxygen-containing gas.

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: :~ , . : ~ .

8~

1 Also included are t:ne oxygen~producing anode devices wherein 2 oxygen i5 lib~rated or prod~ced by electrocatalytic oxi~ation 8 of oxygen-b~aring compounds. The devices of the present 4 invention contain the mentioned oxy~en electrode or electrod2s and othe~ise may be the same as conventional devices which 6 are well known to the artisc~n, Thus, t'ne p~esent invention 7 contemplates all known electrochemical de~ices having oxy~en 8 electrodes except that one or nDre of`th~ above compounds is 9 used as the electrocatalyst material of the oxygen electrodes.
10 BRIEF DESCRIPTXON OF Tl-E D~AWIN~S
11 Figure 1 illustrates oxygen electro~ duction pera 12 forma~ce curves ~or the novel compound o~ the pr2sen~ .T1 13 tion having the :Eonnula Bi2[Ru~xBi~O7~y a~ employed 1n an 14 elec~rochemical device oE the present invention3 and or 10%
pla~:lnum on carbon as tested in a similal~ device and~

16 Fi~ure 2 is ~n oxygen evolutlon curv~ for

17 Bi2[Ru2-xBix~o7

18 DET~ ,D ~C~IPTTON OF T~E D~AWIN~ AND ~ EXAMP~E~

19 The present i.nven~ion will be ms~e fully appr~ci~t:e.d ln view of the followin~ examples~ However, these exam~les 21 are presenLed for illustrative purposes, arld ~h~ presen~ in~
22 ventiqn sllould not be collstrued to he limited theretsso 23 EJ~y~
____ 24 A bism~th rich pyrochlore, e.gO Bi~LRu~ ~Bi~;~t)7~y~ . `
of th,o present invention is prep~red as ollows:
26 ~i(N03)3-5H20 and ~u(Ns)33~ are combirled in aqueous 27 solu~ion in an approxis~ ely l.0:l.0 molar ratio oE bismu~:h 2~ to rutheniumO That is, about 2~425 g~ams o:E Bi(N~3)~5H2~
29 and about l~435 grams o R~(NO3)3 (in aql~eous solul:ion) a~ e added to 100 ml of water and 5û ml of coneentrated nitric 31 aeid to obtain an equ;.molar bismuth ruthel1ium aq~leous solu^
3~ tion, This solucion i5 th~n addc~d, wi~ h stirri.ng~ ~o 60~1 ml - 16 -~

48~

of ~N l~O~ which is he~ted l:o appro~imately 75"C~ Precipita tion of a solid occurs ir~nediate1y~ The reaction is carried 3 out, with StiL'rin,g~ or approxitnately 16 hours in this medium.
4 The soli.d is th~n separated tby vacuum filtra~ion ~nd reacted for another 16 hc,urs in 600 ml vf approximately 6N K~O'~ at 6 75G. The solicl is then again separated by ~raCUL~ il~ration 7 and rea~tecl for an addi~ional 65 hou~s in ~lO ml G:~ 3N KOH
8 at 85C. Ihe reacted product after separation by filt.rat-lvn 9 is washed with distil1ed wat:er and dried. X~ray diffraction shows t:hat the reac!:ed procluct :is ~ crystal1ine materia1 `11 e~ibitin~ thf~ pyrochl~re eryst:al str~lcture. The ind1ces ~nd 12 1.nt:erp1anar spacill~s :Eor l~he X~ray di:Eracti.on r~ 1ections 13 for this compo~md are given in Tab1e I below 9 ComE)ar1son l'~ with ~e X-ray clif:Er~ction pat:tern o ~he previous1y kn~rl m~teria1 J3i2Ru207~y (given In Tab1e II) shvws th~t the ccm p~und of the presen~ xamp1e has ~ cryst~1 lat~ice. t:hat is 7 exp~rlded re1ative to Bi~Ru~2~7~y. and9 thus3 is a ~igrlif1canL1y di~erent. ~nd new materia1, The sur~ce area" me~suLre(l by 19 the BET N2 ahsorption m~thod9 is 178 m~/g.
A~er a ~ ho~r i~iring a~: 40û'C o:E ~he reacted 21 produc~ X-ra~ dirac~ n shows ~ch~t ~ second ~e of Ru3~
22 has cryst~ lzed a7~cl iS admi~d with the pyrochl~r~ ph~s~. !
23 The pyrcschlc3re phase re~ ins unchanged by thi.s hea~ treat 24 ment. The l~ismuth to rutheni~rn ratio3 as determined expe~i-mentally by X~ray f~uorescence/ i~ 1,07+.05:1~0~ Si.nce l:he 26 pyroch1Ore plt~se with the ~panded latl~iee (relltive to 27 Bi2RU2o7~) is admixed with an impurit~ [~ase of Rul~ the 28 bismll~h ~o ru~henium ratio in the e~p~nded pyrc)chlore ~ cs~
29 be s`ignif:Lcan~1y greater ~han l.. 0, l, 0 alid is ~herefore 30 proper~.y reerrf~d to as a bismut:h~ri ch pyr~clt ' ore, e ~ .
3l Bi2 ~ Ru2 -xBix] Q7 ~Y

~ ~7 741~

~BLF. l- .
2X P~AY DIFFRACTION POWDER FOR Bi2~Ru~{Bix~07_~
, 3 h k 1 d~
4 (~22) 3 . (~7~
~400) 2.59 (~40). 1 . 83 (331) 2.38 . ~ (622) 1~56 ~00) 1 . 30 o (662) 1 . 1 . .
....

~ 748~1 ~BLl~
2 X-~LY DIFFR.. ~'.CTION POI~OER PATTERN FOR Bi2Ru207.

3 h 1{ 1 d (A) (111) S . 9~6 (311) 3 . ~1?5 6 (222) 2 . 973 7 (400) 2.575 8 (331) 2 . 363 9 (333)~ 82 11 (511) J
12 (44~) 1. 821 "`
13 (531) 1 . 7~
14 (6~2) 1 . 553 (~ 4) 1.~87 16 (553)~ l. 3~1 18 (73 1) 19 (~00) 1 . ~87 (733) 1.258 21 (662) 1 . 1~1 22 (8~0) 1. 152 - 19 - ~ .

7g~8~L `

E~MPLE ~.
2 To illustrate the util.ity oE the compo^.md which is 3 obtained by the method of E~ample 1~ electrocatalytic per-4 formance curves are obtain~d in 3N ICOH at 75 C . In these S tests ~ the rnaterial is fabricated ino test electrocles con~
6 sisting o.~ the catal-yst9 a binder~ a wetproofing agen~ and 7 a support. Te10r~serves as bc>th a binder and w~2tpl00fing 8 agen~ or a:l 1 the ~lect:rodes tested. Gold expallded metal 9 screen is used as the s,upport.
Electrodes are fabr~cated by mixin~3 a weighed 11 amotmt o~ cal:alysl: wi~h a few drop~ o water ad~ing a mea~
12 sured ~rolume of T~flon~2 suspensîon9 and mixing v:igorously 13 to preclpitate t he Te~lon~ Thc gummy product is t:hen spread l~l on weighed gold E2~met screen and i.s pressed dry be~ween lS i~ er paper, l~e elec:t:rode is~then cold pressed for 0.5 16 Inin. al~ 200 psL9 ls al.lowed to al7 dr~ ~or 30 min. and is 17 then b.c~t pressed in an lnert a~mos~lere ~t 3~5C~ S00 psi l8 ~or 0.5 min~ After cooling9 the electrode is we7ghed to 19 de~e~mine i~s loading and ~en placed in th~ elec~rochemical cell for testir.g. -- ~
21 The electrochemi.cal half~ll used for testing i~
22 of the interface maintaining type and consists o~ a jacketed 23 liquid phase~ cell compaxtment. The liquid side contains the 24 platlnum wire coun~er ~lectrod~ a saturate.d calomel refer-ence electrode (in conkact by Lugin çapillary)9 and magnetic 26 s~irrer~ The gas side contains (oxygen~ inlet a~d outlet and 27 a s~opcock ~o draitl o~ any condensate. The working elec~
2~ trode is held ln place ~be~Yeen the twn compartmen~s) between 29 two Te~lon discs with a gold ourrent collector pressing against it.
3l The c.ell is connected to a Pr~nceton Applied 32 Resea~ch ~lo~ 3 p~ten~los~at: wi~h a p~og7:a~ner ~r~d ~ 20 ~
~r~

74~1 :Logarlthrll.i.c currer~t: converter. Constant rate potenti~l 2 sweep mf~asureEI~F~ts are corlducted~ Outputs of pot:ential and 3 log o:~ current are ~ecorded ~n an x-y plotter, ~nd t:he 4 resultirlg potent:ial vs~ log curren~ d2n5i1-y plo~ referred t:o as a p~r:ormance curve~ is used to evaluate the electrode 6 activi~y, 7 Fi~ure 1 shows per~c,rmance cu~Jes or the ~lectro^
catalytic reduction o oxygen ln 3N K~H a~ 75~C U5illg the 9 bisllluth r ich pyrochlore of Example 1 and using platirlum sup~
ported ~10% by weight~ on act-ivP carbon. The plat~ u 1 carbon eleccxode is typical o:~ conventionally used supported 12 noble me~al elect:rocat~lysts~ The results shown in the 13 Volt~;e -C~lrrent ~ens ity plot: o~ Fi.gure 1 est~lîsh t:~at the 14 bismuth~rich pyrochlore compo~mcl of Exclmpl.e 1 h~s o~y~en electro~reduction c~pa.bility su~rior to ~ conventiollal 16 su.ppo~t~d plat:înum e3.ectrocatalyst: in th~ ~est~.d erlvLronment~
17 l~lso i1.lc:1udecl in Figur~3 1 is the p~L~o~m~ e CUl^~re for pure.
18 RuO~ w}tich has,here~ofore been recogni2ed as an o~y~en 19 electrocat~lyst. A ~parison o~ the c~lrves in Figure 1 ~o will ~h~w ~h~ ~he ~a~orable eleetrochemical acti~i~y w~ich 21 is ob~ain~d with the electrode ~abrica~ed from t~-~ m.a~e~ial 22 of Example 1 is not due to the presence o an impurity phase 23 o Ru02, 24 Figure 2 shows a per~ormQnc~. curve for the electro~
ca~uly~ic e~olu~ion o~ ~xygen from 3N K~H at 75C using the 26 electrode a~ricated rom the material of Example 1. Also 27 included for comparlson are oxygen evolution performarlce 28 cur~es for Ru02 ~nd pla~inum black. It is eviden~ that .he 29 o~cygen o~er~potential (anodic polari2at.lon~ at any cur~ent ~ensity L~ mt~.ch greater for plaLlnum or Rul~2 than it is or 31 ~he bismuth-rich pyrQchlorc, demonstrating ~h~t the used ~:
32 bismuth rich pyrochlore cornpoulld o the present in~Jen~i.On 33 remains ~ stl.perior electroca~alys~, ~Q1974~

A bismuth-rich pyrochlore, e.g. Bi2 ~U2 XBi ~07 y, of the present invention is prepared as follows:
Bi(N03).5H2Q and Ru(N03)3 are combined in aqueous solution in an approximately 1.5:1.0 molar ratio of bismuth to ruthenium. That is, about 3.60 grams of Bi(N03)3-5H20 and about 2.12 grams of Ru(N03)3 (in aqueous solution) are added to 75 ml of distilled water and 15 ml of concentrated nitric acid to obtain a bismuth-ruthenium aqueous solution. This solution is then added, with stirring, to 150 ml of 12N KOH
at room temperature. Precipition of a solid occurs immediate-ly. The solution is heated to 90C and the reaction is car-ried out, with stirring and with oxygen being bubbled through the solution, for approximately 20 hours. The solid is then separated by vacuum filtration, washed in distilled water, and dried at 100C. The measured latticeparameter of this material is approximately 10.465~ and demonstrates that the lattice is clearly expanded relative to Bi2Ru207 y which has a lattice parameter of 10.299A. The surface area, measured by the BET N2 absorption method, is 136 m /g.
After a two-hour firing at 400C of the reacted product, X-ray diffraction shows that no second phases have crystallized. Thus, the pyrochlore is not admixed with any impurity phase. The pyrochlore phase remains unchanged by this heat treatment. The bismuth to ruthenium ratio, as ~ .
- ~ ~

~0~7~8~L

determined experimentally by X~ray fluorescence, is 1.78:
1Ø The formula for this pyrochlore may therefore be ex-P Pb2 ~U1.443io.s~ 7_y- The fact that the bi5m~lth to ruthenium ratio in the reacted product pyrochlore is higher than in the initial reactant mixture can be explained by the solubility of ruthenium in the strongly alkaline reaction medium. This is confirmed by the presence of ruth-enium in solution in the filtrate that is obtained when the pyrochlore product is separated from the reaction medium.
The subject matter of this application is related to that of commonly assigned copending Canadian Applications S.N. 315,786, 316,046 and 324,813.

_ .

,, ~

, ~ . . . .

Claims (34)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A compound having-the formula:
Bi2 [M2- xBix] O7-y wherein M is selected from the group consisting of Ru, Ir and mixtures thereof, wherein x is greater than zero and less than or equal to about 1.0 and wherein y is greater than or equal to zero and less than or equal to about 1Ø

2. The compound of claim 1 wherein M is Ru.

3. The compound of claim 1 wherein M is Ir.

4. The compound of claim 1 wherein x is within the range of about 0.1 to about 0.8.

5. The compound of claim 4 wherein M is Ru.

6. The compound of claim 4 wherein M is Ir.

7. The compound of claim l wherein x is within the range of about 0.25 to about 0.6.

8. The compound of claim 7 wherein M is Ru.

9. The compound of claim 7 wherein M isIr.

10. In an electrochemical device having an oxygen electrode containing an electrocatalyst material, the improvement com-prising using as said electrocatalyst material one or more compounds selected from the group consisting of those com-pounds having the following formula:

Bi2[M2-xBix]O7-y wherein M is selected from the group consisting of Ru, Ir and mixtures thereof, wherein x is greater than zero and less than or equal to about 1.0, and wherein y is greater than or equal to zero and less than or equal to about 1Ø

ll. The electrochemical device of claim 10 wherein said device is a fuel cell.

12. The electrochemical device of claim 10 wherein said device is a metal-air battery.

13. The electrochemical device of claim 10 wherein said device is an electrolyzer.

14. The electrochemical device of claim 10 wherein said device is a metal electrowinning device.

15. The electrochemical device of claim 10 wherein M is ruthenium.

16. The electrochemical device of claim 15 wherein said device is a fuel cell.

17. The electrochemical device of claim 15 wherein said device is a metal-air battery.

18. The electrochemical device of claim 15 wherein said device is an electrolyzer.

19. The electrochemical device of claim 15, wherein said device is a metal electrowinning device.

20. The electrochemical device of claim 10 wherein M is iridium.

21. The electrochemical device of claim 20 wherein said device is a fuel cell.

22. The electrochemical device of claim 20 wherein said device is a metal-air battery.

23. The electrochemical device of claim 20 wherein said device is an electrolyzer.

24. The electrochemical device of claim 20 wherein said device is a metal electrowinning device.

25. The electrochemical device of claim 10 wherein x is within the range of about 0.1 to about 0.8.

26. The electrochemical device of claim 25 wherein said device is a fuel cell.

27. The electrochemical device of claim 25 wherein said device is a metal air battery.

28. The electrochemical device of claim 25 wherein said device is an electrolyzer.

29. The electrochemical device of claim 25 wherein said device is a metal electrowinning device.

30. The electrochemical device of claim 10 wherein x is within the range of about 0.25 to about 006.

31. The electrochemical device of claim 30 wherein said device is a fuel cell.

32. The electrochemical device of claim 30 wherein said device is a metal air battery.

33. The electrochemical device of claim 30 wherein said device is an electrolyzer.

34. The electrochemical device of claim 3.0 wherein said device is a metal electrowinning device.