CA1048601A - Fuel cell electrodes with finely divided platinum catalyst - Google Patents

Abstract

Abstract of the Disclosure This disclosure deals with novel very fine, particulated colloidal platinum of the 15-25 Angstrom size range of unusual catalytic activity and particu-larly adapted for adsorption or other deposition upon carbon for use as fuel cell catalytic electrodes and the like and produced from platinum colloids and sols including complex platinum sulfite compounds and sols derived therefrom.

Description

1~486(~i The present invention relates to new platinum compounds, sols and particulated platinum deposits derived therefrom and to methods of preparing the same, being specifically, though not exclusively, concerned with use in fuel cell electrode prepa~ation and the like.
This application is related to Canadian Patent No.
982,782, issued February 3, 1976.
The art is, of course, replete with numerous compounds and processes employed to provide platinum deposits for use as catalysts in a myriad of applications including oxidation, hydro-genation, dehydrogenation, reforming, cracking, chemical reaction-aiding, contaminant burning, electrochemical cell ele.ctrode operation and the like, all herein after generically connoted by reference to "catalytic" usage. Particulated platinum has been employed to provide increased effective surface area, as by adherence to rough substrata, such as carbon, alumina and other substances, such as platinum tetrachloride, chloroplatinic acid and the like. As described, for example, in Actes_Du Deuxieme Congres International De Catalyse, Paris, 1960, pp. 2236, 2237, the average particle size 2n of such particulated platinum lies in the range of from about 45 to 250 Angstroms, and it has not proven possible commercially to pro-vide much smaller particles and thus obtain vastly increased catalytic efficiency.
In accordance with discoveries underlying the present invention, however, it has, in summary, now been found possible consistently to produce excellently adhering particulated platinum deposits in the much finer 15-25 Angstrom range; and it is to new methods, compounds and sols for producing the same that the present invention is accordingly primarily directed.

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In one particular aspect the present application, which is a division of co-pending Canadian Application No. 223,881 filed Ap~il 4, 1975, is concerned with the provision in the method of preparing electrodes for fuel cells and the like comprising platinum-on-carbon electrocatalyst, the steps of producing an aqueous colloidal platinum-containing sol having an average platinum particle size substantially of the order of 15-25 Angstroms, depositing said platinum contained in said sol on an electrically-conducting carbon substrate~ and controlling the depositing to cause the carbon to nucleate the deposit and limit the formation of platinum particles on said carbon to said size.
In another particular aspect the present application is concerned with the provision of a catalytic fuel cell electrode comprising an electrically conductive high surface area carbon substrate on which has been deposited platinum particles of the '' order of substantially lS to 25 Angstroms in particle size and in which said particles load the electrode surface in the range of .
from substantially 0.04 milligrams/cm2 to 0.5 milligrams/cmZ.

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1~4~6Ul Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims.
A first discovery underlying a part of the invention resides in the rather unexpected fact that a novel complex platinum sulphite acid void of chlorine may be prepared from chloroplatinic acid and particularly adapted for the formation of a colloidal sol from which extremely finely particulated platinum may be deposited.
While prior experience had led those skilled in the art to consider either that adding SO2 to chloroplatinic acid would invariably result in reduclng the platinum to the "2" state, without replacing chloride in the complex with S03=, yielding chlorophatinous acid ~see, for example, H. Remy, Treatise on Inorganic Chemistry, Vol. 2, p. 348~, or that the reaction of SO2 with a platinum compound resulted in its reduction to the metallic or zero valence state ("Applied Colloidal Chemistry", W.N. Bankcroft, McGraw Hill, 1926, p. 54), it has been discovered that through appropriate pH and other controls, a complex platinum acid containing sulphite (and to the complete exclusion of chloride) is decidedly achievable. And from such complex acid, unusual colloidal sols depositing particulate platinum in the 15-25 Angstrom range can readily be obtained, and thus vastly superior catalytic performance attained.
Specifically, one of the preferred methods for the preparation of this novel complex platinum acid jl/ _3_ --- 10~t~601 (represented substantially by a formula containing two mole of S03 per mole of platinum) involves the neutralizing of chloroplatinic acid with Qodium carbonate, forming orange-red Na~ Pt (Cl)~.
Sodium bisulfite is then added, dropping the pH to about 4, and with the solution changing to pale yeLlow and then to a substantially colorless shade. Adding more sodium carbonate brings the pH back to neutral (7), and a white precipitate forms in which the platinum has been found to be contained in excess of 99% of the platinum contained in the chloroplatinic acid starting sample. It was believed (now confirmed) that thls precipitate contains six atoms of sodium and four moles of S03 per atom of platinum. It is slurrled with water, and then enough strong acid resin is added (~uch as sulfonated styrene divinyl ben ene in the hydrogen from --DOWEX~-50, for example), to replace three-of the Na atoms. The solution is filtered to remove resin and then passed through an ion-exchange column with sufficient of~the said acid resin to re-place the other three Na atoms. Inherently, duri~g th~ two-step cation exchange, copious quantities of SO2 are liberated, amounting to a loss of substantially two moles of SO~/mole Pt. Boiling to concentrate the solution, results in the novel complex sulfite platinum acid compound above discussed containing groups of (OH~
and H,Pt(SO3)2, free of excess unbound SOl.
Proof of the above-stated complex character of this novel platinum acid has been obtained by reacting 0.0740 g-mole of ~ -chloroplatinic acid in the form of the commercial material containing 40~ by weight of Pt to form the "white precipitate" precisely in accordance with the method described abo~e. The "white precipitate"
weighted 48.33 g, after filtering, washing and drying at 150C
(to constant weight). The filtrate contained 40 ppm platinum, as ,' -, ' . . ' ;~ .

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t-. 10'18601 determined by atomi~ adsorption, showing that more than 99% of the orlginal platinum contained in the sample of chloroplatinic acld was presene in the precipitate. Thus, the precipitate has an empirical formula weight of about 653 based on one atom of Pt {o807340}~ 653. Chemical analysis showed that the salt contained 21.1% Na :by atomic adsorption), 2g.9~ Pr (by atomic adsorption) and 43.7% S03 (by oxidative fusion and BaSO~ precipitation and by KMnO~ titration), thereby confiriming the presence of subseantially 6 Na and 4 S03 per Pt atom.
The precipitate was then converted to the comple~ acid solution in accordance with the precise procedure described above.
It waq boiled to a concentration approximately 2 molar in Pt t2 g atoms Pt/liter of solution).

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When the acid was concentrated to this strength, SO2 was no longer evolved.
(1) A sample of substantially water-free complex platinum acid, prepared by distillation under high vacuum, was found to contain 52% Pt by weight determined by thermogravimetric analysis~

(2) A sample of complex platinum acid (in solution) was found to have a sulfur content of 42.6% by weight, as S03 ~ determined by oxidative fusion and BaSO4 precipitation and by oxidometric titration with KMnO4, i.e. 2 moles of sulfite/mole Pt.

(3) Titration of a sample of the complex platinum acid with standard base showed a characteristic titration curve with three titratable hydrogen lons per atom of Pt, amounting to 0.8% by weight, two of which were strongly acid (i.e. completely dissociated) and the third quite weakly acid ; (Ka~ 10 8 for the third H ion).
; (4) A sample of complex platinum acid was found to contain one OH group per atom Pt, or 4.54% by weight OH, determined by neutralizing the three acid hydrogens with NaOH to pH 9.5, then reacting with excess sodium sulphite solution of natural pH = 9.5, thereby gradually reforming white precipitate having the above des~ribed composition, and raising the pH of the reaction mixture above 12, and back-titrating with H2SO4 to pH 9.5.
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(5) A sample decomposed at about 400C in nitrogen yielded only oxides of sulfur (S02 and S03 ) and water in the gas phase, and Pt metal residue.
(6) Addition of silver nitrate to the acid yielded a yellow product insoluble in dilute sulfuric acid.
From these experiments, the following is concluded:
(1) The acid contains only H, 0, Pt and S. (The replacement of Na by H in the ion exchange step cannot introduce any other element); Cl is absent.
(2) The acid contains Pt and S in the ratio of 1:2.
(3) The sulfur is present as sulfite as shown by the analysis and by the high temperature decomposition of the acid in nitrogen.

(4) The sulfite has to be complexed because (a) the complex acid (no S02 odor) is completely dis-sociated whereas the ionization constants of HzS03 (which is odorous) are 1.54 X 10 2 and 1.02 ~ 10 ', respectively; (b) the complex acid is more soluble in water than HzS03 at the boiling point (max.
solubility of S0z is 5.8g/1 or 0.07 molar in H2S03 at 100C vs. the 2 molar acid produced by the method of this invention); and (c) silver sulfite is soluble in dilute sulfuric acid, whereas the silver salt of the new complex platinum acid is insoluble in dilute sulfuric acid.

(5) The acid is trivalent, having two strongly acidic and a third weakly acidic hydrogen as evidenced by a characteristic titration curve. An unusual kinetic effect occurring during titration of jl/ -7-. . : . , :, -- ~
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the third hydrogen suggests the possibility thatit could be part of the sulfite ligand. Turning back, now, to the said "white precipitatell~attention ;
is lnvited to "The Chemistry of the Co-ordination Compounds", edited by John C. Bailar Jr., ACS Monograph, Reinhold Pulbishing Co., 1956, ~r p. 57-58, where a compound of composition Na6pt(so3) is disclosed (with no reference to any utility), but as baving to be prepared by the complicated process of making the appropriate isomer of a platinum amine chloride, Pt (NH3)2Cl2, and then converting it to Na6Pt(SO3)~. -~.
Thls further points up the highly novel and greatly simplified high-yield technique of the present invention, starting with chloroplatinic acid and preparing the sodium platinum sulfite complex "white precip-ltate" (for which the present invention has found and taught important utility in the development of the novel complex platinum acid of the lnventlon), substantially quantitatively.
Fron this novel complex platinum acid, a new colloidal sol may be prepared by decomposing the acid by heating it to dryness in air (oxidizing) and holding the temperature at about 135C for about an hour, producing a black, glassy material which, when dispersed in water, yields a novel colloidal platinum-containing sol having an average finely divided platinum particle size of from about 15-25 Angstroms, with substantially all the platinum particles consistently lying within this range. Some platinum metal and sulfuric acid may be present and may be respectively removed by filtering (and re-cycling use of the metallic platinum) and by treating with hydroxide resin ffuch as DOWEX~ 2 or the like. A ~et black colloidal sol with these flne size particles is thus obtained.
From this novel product, a host of vastly improved catalytic surfaces have been obtained.

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As a~first example, the sol has been deposited or adsorbed on a carbon black substrata tsuch as electrically conductive Norit~ A) to form a catalytic electrode structure (by means well known in the art and comprising a conventional current collector). One of the uses of such an electrode structure for example, is as a cathode electrode in fuel cells and the like. This has been effected by reducing the adsorbed metal of the sol with hydrazine; forming on thc -carbon, platinum metal crystals of measured approximately 20-Angstrom size. For use as an oxygen cathode electrode in an air-hydrogen 135C
fuel cell with phosphoric acid electrolyte and a platinum anode, with both electrode sizes about l inch by l inch, about 2-10% by weight of adsorbed platinum was so reduced with about 10% solution of hydrazine to form and adhere the fine partlculate platinum on the electrically conductive carbon substrate, the electrode structure exclusive of con-ventlonal components being about 70% by weight of Norit~ A carbon and 30% by weight of Teflon ~ (i.e. a typical fluorinated hydrocarbon polymer) emulsion, such as TFE 30. Most remarkable cathode performance was obtained in this fuel cell, with cathode loading of only 0.25 milligrams/cm~ of platinum, as follows:
, Current Volta&e 100 amperes/ft2 660 millivolts Thls improved performance is evident from the fact that ln an ldentlcally operating cell with the cathode formed by adhering to the carbon substrate platinum particles from platinum black of nominal surface area of 25 metérs~/gram, such cell performance could only be obtained with ten times the platinum loading (i.e. 2 milli-`
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104~6(~i grams/cm2). Similar performance cou]d also be obtained in the samecell with the platinum deposited on the carbon from platinum tetra-chloride and chloroplatinic acid (approximately 40-80 Angstrom particles). but only with three to four times the platinum loading.
Prior phosphoric acid fuel cell operation with other platinum catalysts is described, for example, by W.T. Grubb et al, J. Electrochemical Society III, 1015, 1964, "A High Performance Propane Fuel Cell Operating in the Temperature Ran8e of 150-200C". Prior methods of fabricating fuel cell electrodes are described, for example, in U.S_ Letters Patent No.3,388,004.
As another example, similar electrochemical cell electrodes were operated as air cathodes in the same cell as the first example with as little as 0.04 milligrams/cm2 platinum loading, and with as much as 0.5 milligrams/cm2. The respective cell performance characteristics were 100 amperes/ft2 at 530 millivolts, and 100 amperes/ft 2 at 690 millivolts.
The above-described catalytic electrode structures have other advantages, for example when used as hydrogen anode electrodes in fuel cells and the like. As an illustration, the electrode struc-ture described above as a first example, was used as novel hydrogenanode electrode in the above mentioned air-hydrogen fuel cell in lieu of the (conventional) platinum anode also above mentioned. Remarkable anode performance was obtained in this fuel cell with low loadings between 0.05 and 0.25 milligrams of platinum per cm of anode area, particularly with respect to improved tolerance of carbon monoxide.
One known commerical method of producing low-cost hydrogen is by steam reforming of hydrocarbons followed by the shift reaction, which process yields an impure hydrogen containing typically of the order of 80% hydrogen, the remainder being CO2, excess steam and of the :. . : .

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- order of lX-2X carbon monoxide. It i8 well ~nown ln the fuel cell art that carbon monoxide ls a polson for anodic platlnum and that such poisoning is temperature dependent, the loss of anode performance being the more trastlc, the lower the temperature. Uslng such low cost hydrogen, lt is thus generally advantageous to operate the above phosphoric acid fuel cell at hlgher temperatures, for example in the rsnge of 170C to 190C. Remarkable anode performance in the presence of C0 lmpurity, was obtalned ln this fuel cell, especlally at hlgh current densltles, wlth an anote loadlng of 0.05 mllllgrams/cm2 of platinum whem compared to the performance of an anode having a conven-tlonal platinum catalyst tprepared by reaction of chloroplatlnic acid nnd deposlted ln ~ubstantlally the same manner) and havlng the same load~ng of 0.05 mllllgrams/cm~, as shown ln the followlng table.

Cell Current Denslty Loss of Voltage (millivol~s) Temperature (Amps/sq ft) by Polarization Due to 1.6%
C0 in ~ydro~en Novel AnodeConventional Anode 190c 400 10 28 190c 300 9 14 lq5C 500 66 118 In connectlon with the examples above, moreover, not only has greatly improved catalytlc efficiency been obtained as a r~sult of the extremely hlgh surface area provlded by such flne colloldal partlcles, but thls enhanced ac~lvlty was found to be malntalnable over several thousand hours of operatlon wlth no detectable decay ln cell performance.

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; :1()486~1 As a further example, such catalytic structures for electrode use have also been prepared without the step of converting the complex platinum sulfite acid to the sol. Specifically, the acid was adsorbed on the carbon substrate, decomposed with air, and re-duced with hydrogen. During such retention, it was Gbserued that H2S evolved, indicating the retention of sulfide materials, but the H2 reduction at 400C was found to remove substantially all sulfides.
Again particles in the 20-Angstrom range were produced with similar electrode performance to that above-presented.
A still additional example is concerned wlth deposition or adhering to a refractory non-conductive substrate of alumina.
Sufficient complex platinum sulfite acid to contain 200 milligrams of platinum was applied to 50 cc of insulative eta~alumina pellets, about 1/8 inch by 1/8 inch. The mixture was dried at 200C and, to effect decomposition and adsorption, was held at 600~C in air for about fifteen minutes. This resulted in a very uniform distribution of fine platinum particles (approximately 20 Angstroms) throughout the alumina surface structure, but not within the same~ This was reduced by ll2 at 500C for about half an hou,r, providing a significantly improved oxid-ation catalyst having the following properties, considerably improved from Houdry Platinum-on-Alumina Catalyst Series A, Grade 200 SR, a typical present-day commercial product, under exactly comparable con-ditions:
Ignition Temperature For Invention Houdry 1. Methane 355C 445C
2. Ethanol 85C 125C
3. Hexane 145C 185C
Another example, again bearing upon this oxidation catalyst application, involves the same preparation as in the immediately jl/ -12-:

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1~486~31 previous example, but with two and a half times the amount of partic-ulated platinum (i.e. 500 milligrams). The following results were obtained:
Ignition Temperature For Invention 1. Methane340C
2. Ethanol30C (room temperature) 3. Hexane130C
Still another example, identical to the previous one, but with 2 grams of platinum adhered to the 50 cc alumina, was found to produce the following results:
Ignition Temperature For Invention 1. Methane250C
2. Ethanol30C (room temperature) 3. Hexane~0C
Still another example, 200 milligrams of the preformed sol was adsorbed on alumina, and reduced with H2 and found to produce the following results:
Ignition Temperature For Invention 1. Methane310C
2. Ethanol` 45C
3. Hexane110C
Por the usage of the last four examples, a range of platinum of from about 0.01% to 5% may be most useful, depending upon the economics and application.
As still a further example , the deposition or adsorption described in the Iast four examples, above, may also be effected on other refractory oxides in similar fashion, including silica and zirconia.
Lastly, other refractories, such as zeolites, calcium ~1 -13-~: ~
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10486~1 phosphate and barlum sulfaee, may be similarly coated by the processes of the last four examples.
Whlle the novel complex platinum compounds, acid and/or sol may be prepared by the preferred method previously described, it has been found that the acid may also be prepared from hydroxyplatinic acid (H~Pt(OH)s) by dissolving the same cold in about 6X aqueous H2SO3, and evaporating to boil off excess SO2. This appears to yield the complex platinum sulfite acid material, also (identified by its characteristic titration curve). While this process involves a lower pH, it should be noted that chloride is excluded by the starting materlal.
The above-descrlbed methods for the preparation of . several plstinum compounds of unexpected utility as sources of superior catalysts for fuel cells, oxidation catalysts, etc. have proven quite satlsfactory, speclfically, for producing (I) the water-insoluble salt characterlzed to have the composition of Na6Pt(SO3)4; (Il) the complex sulflte-platinum compound, soluble in water, and having an empirical formula and composition represented substantially by H,Pt(SO3)20H; and (III) the colloldal dlspersion or sol of a platinum compound of unknown composltion, but formed by the oxldative, thermal decomposition of (II).
Among the important before-described uses for these compounds is the preparing of fuel cell catalysts, consisting of platlnum supported on carbon, havlng superior electrocatalytic pro-perties .
Subsequent work has revealed new, unexpected and slmpllfled means and steps of preparlng such superlor forms of fuel ~ -cell catalysts. The basls for all of the syntheses of a carbon-supported platinum fuel cell catalyst ls the formation of a platlnum collold, capable of belng deposited on carbon to yield platinum ~l/ -14-:: : `
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". :, ' ' ' ' lOg8bOl ' supported on c-arbon of average particles size range of substantially of the order of 15-25 Angstroms, either as a colloid, as before described, which can be subsequently contacted with finely divided carbon, or as hereinafeer described, as colloid generated in the presence of such carbon, thereby causing the colloidal platinum particles to be formed and deposited on the carbon in a single step.
We will now describe in detail one especially advantageous technique whlch involves, typically, the step of oxidizing the sulfite ligand of the preferred complex platinum compounds (I) and (II) to sulfate, in aqueous solution, by means of a non-complexing oxidant, it being understood that other platlnum complexes containing ligands capable of being oxidized to substantially non-complexing products are also suitable, as later dlscussed.
Techniques for preparing a fuel cell catalyst, equi-Yalent to that found from the complexes (I) and (II), have been dis-covered, wherein chloroplatinic acid (CPA) and sulfite are reacted, to yield (II), but wherein, unlike the before-described methods, the complex acid (II) i8 never separately isolated, but is converted to a catalyst directly, and without isolation from by-products, such as HCl and NsCl. .
An lllustration of the synthesis of a carbon-supported platinum fuel cell catalyst i8 the observation of the oxidizing reaction of the complex platinum sulfite acid (II) with N202. When H202 is added to a dllute solution of the complex acld (lI), the sulfite present ln the sulflte-platlnum complex, is oxidized. The solution's color 810wly changes from a faint yellow, to orange. Following the appearance of the orange color, a faint Tyndale effect is noted. With time, thls becomes more pronounced, the solution becomes cloudy, and finally, precipitation occurs. While the material precipitated is of ~ 15-, .
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unknown exact composition, it i8 believed to be a hydrated oxide of platinum, since it is soluble in base much as is hydrated platinum hydroxide or platinic acid, H2Pt(OH)6. In any case, treatment of the complex platinum sulfite acid (II) with H202 yields a meta-stable colloid of a platinum compound. The sequence of reactions described - , above are hastened with heat, and proceed more slowly uith increasin~
scidity, as from the addition of sulfuric acid.
Whereas in the earlier-described methods, the platinum colloidal sol is first formed and then applied to the carbon particle substrate, if the reaction described immediately above is performed in the presence of the high surface area carbon, the~carbon particles act both as nuclei and as a support for the extremely small particles of the platinum compound, as they are formed, and they are deposited on the carbon rather than coalesclng to yield a lower surface area pre-cipltate. It has been found that this carbon nucleation of the platinum partlcles permlts the restrictlon of the platinum deposits to partic- :
ulate catalytic particles of the said preferred 15-25 Angstrom size range.
It has slso been found that the same reaction occurs if the complex sodium platinum sulflte precipitate (I) is acidulated by dissolving in dilute sulfuric acid, and is then ox~dized by treatment with H~02, or if CPA i8 reacted with NaHS03 or H2S0,, to yield a ;
sulfite-platinum complex, and then oxidizingly treated with H202.
Several examples of the use of the reactions observed above are given below. Basically, however, they all depend upon the oxldation of the sulfite present in a platinum-sulfite complex, with H~0~ being the preferred oxidant, although other non-complexing oxidants, such as potassium permanganate, persulfuric acid and the like have been used. The term "non-complexing oxidant", as used in this specification ~ 16-, ' . :
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1~4~6~1 and in appended claims, means an oxidant which does not introduce groups capable of forming strong complexing ligands with platinum.
Also while any high surface area carbon ls suitable, the carbon black Vulcan~ XC-72 ~Cabot Corp.), has been found to yield an excellent catalyst, but the fact that thls carbon ls used in the examples to be cited does not imply that other carbons cannot be used. Nor, since the carbon is merely a support onto which to deposit the colloidal partlcles of platinum as they are formed, should it be thought that carbon ls the only support upon whlch the deposit can be made. Other materials such as AL203, BaSO4, SiOz, etc. can be used as supports for a high surface area platinum, as previously described, but are, of course, useful for other catalytiC properties rather than for fuel cells, electrodes and the like, because af their high electrical re-sistance. We shall now proceed to a further series of examples.

EXAMPLEI
To a liter of water, sufficient of complex platinum - sulfite acid (II) ls added to give a platinum concentration of 2.5 g/l.
To this solutlon is added 22.5 grams of Vulcan~ XC-72. The solution has an initial pH of about 1.8 whlch ls unaltered by the addltlon of carbon. The solution is stlrred vigorously, so as to keep the carbon well dispersed. Add 50 ml of 30% H2O2, while continuing the vigorous stirring. Maintaln the stirrlng for about one hour. The pH wlll drop slowly, indicating that hydrogen ions are being generated. Next, heat the solution to boiling, while maintaining the stirring. Filter the carbon, wash it well with water, and dry the carbon in an oven set to 100-150C. This air-dried material is now ready for use without further treatment. Platinum uptake is about 98% with the remainder being discharged to the filtrate. The resulting carbon, containing 9.9 -9.8% platinum shows platinum crystallites of S-20 Angstroms ln diameter ~1/ -17-.: ~. ~ . . - . : : . :

6()1 Y electron microscopy. Fuel cell performahce _- was measured using Teflon~ bonded anodes and cathodes having platinum loadings of 0.25mg/cm2 of electrode area. Performance with H2 and air, at 190C
in a phosphoric acid fuel cell, was measured and found to give 200 Amperes per square foot (ASF) at .670 - .680 V. The resistance loss was about 0.02 volts at this current density, so the IR-free per-formance was about .700 Volts as 200 ASF.

The reaction was conducted as in Example 1, but rather than heating the solution after one hour, stlrring was continued for 24 hours at ambient temperature. Platinum uptake was 97-98%, and - physical and electrochemical properties substantially identical to the produce described in Example 1 were obtained.

.
The reaction of the complex platinum sulfite acid (II) with H202 was conducted much as in Example 1, except the pH of the solution was adjusted to 3 with NaOH, prior to the addition of H2O2.
After the one hour reaction period, the pH was again brought to 3 with NaOH, and the solution boiled. The carbon was filtered, washed, and dried, as previously described. Platinum uptake was substantially quantitative, and the physical and electrochemical properties of the product substantially identical to those described in Examples 1 and 2.

In 100 ml of Hz0, sufficient of the complex sodium platinum sulfite salt (I) was dissolved to yield a platinum concent-ration of 25 g/l. The salt was put in solution by the addition of sufficient H2SO4 to drop the pH to 2. This solution was diluted with H20 to volume of one liter, and reacted as described in Example 3.
Platinum uptake was quantitative and the physical and electrochemical jl/ -18-r - . .

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properties of the product substantially identical to those already described in the previous examples.
Before proceeding to Example 5, which describes a process that does not require the isolation of either of the complexes (I) or (II) but rather uses CPA heated with sulfite, it may be useful to hypothesize upon the mechanism of the reactions taking place in Examples 1-4, since they have a bearing on the reaction of Example 5, and will help to explain some of the difficulties of control noted in Example 5, though the invention is not dependent upon the accuracy of such hypothesis, it being sufficient to describe the steps that do indeed work and produce the results of the invention.
It is believed, however, that when H202 is added to either the sodium platinum sulfite complex (I) or the like, dissolved in dilute H2SO4, or to a solution of the platinum sol (III), the sulfite or like ligand is destroyed. Since it is the complexing power of ~ sulfite which is the stabilizing force in maintaining an ionic platinum species, its oxidation to sulfate destroys this stabilizing force.
Sulfate is, at best, a feeble complexing agent for platinum, whether it is Pt I or PtIV. With the removal of the sulfite, there does not exist a favorable environment for maintaining a soluble species of platinum, and the platinum species just formed upon the destruction of the stabilizing sulfite must slowly hydrolize and in the process has a transient existence as extremely small colloidal particles. It is these particles which are deposited on the carbon yielding the active catalytic structure. It is believed that the reactions of Examples 1-3 can be adequately described as being substantially:
(1) and (2) H3Pt(So3j2oH + 3H202 > 2H2SO4 + PtO2 + 3H20 (3) NazHPt(SO3)20H + 3H202 > Na2SO4 + PtO2 + 3H20 + H2SO4 Example 4 is somewhat different, in that the starting 3~

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..104t~ 11 material ls different. However, it would appear that when the complex salt of composition Na~Pt(SO3)4 is dissolved in H2SO~, the complex acld of composition H~Pt(SO,)20H i8 formed, since there is a vigorous evolution of SO2, and when the SO2 is evolved, the characteristic tltration curve of H9Pt(SO~)20H is observed. Hence, the reaction of Example 4 is apparently similar to that of Example 3.
In Example 5 presented below, however, CPA is reacted , ~ -with NaHSO3 to yield a complex believed to be the complex acid of ~ -composltion H3Pt(SO3)20H, and HCl and NaCl are formed. One possible reactlon is substantially as foliows:
H~PtCl, + 3NaHSO~ ~ 2H20 ~ H3Pt(SO3)20H + Na2SO4 + NaCl + 5HCl bowever, when this mixture is treated with H202, the ;
presence of chloride, along with the hlgh acidity, leads to the form-ation ln part, of H2PtCl~, rather than the desired colloidal species.
lo minimize this effect, the platinum concentration must be kept low (in order to keep the chloride concentration low) and the pH closely controlled.
EXA~PLE 5 Dlssolve 1 gram of CPA (0.4 gm Pt) in lOOml water.
20 Adt 2 grams of NaHSO3 ant heat until the solution turns colorless. ;
Dllute to 1 liter with water and ad~ust the pH to 5 with NaOH. Add 3.6 grams of Vulcan~ XC-72, and while stirring add 50 ml of 30% H202.
Contlnue to stlr and as the pH changes, add NaOH to maintain the pH
between 4 and 5. When the pH hasstabilized, heat the solution to boil, nd filter and wash the carbon. Platinum pickup is variable, but in general is about 90Z. Increasing the platinum concentration decreases the percentage of platlnum deposlted upon the carbon since the con-~erslon of H2PtCl~ is favored. The catalyst formed in this way, has been found to be substantially identical in performance to that made in ~1/ -20-.... ' ~ ' - .

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-~` 104~6Ql Examples 1-4.
As compared with the earlier described methods of said prior application, also embodied herein, the additional methods, supra, avoid the conversion of the compound having the composition of Na6Pt(SO3)4 to that of composition H3Pt(SO3)20H, and then to the colloidal sol material. This latter colloid, in turn, must then be applied to carbon, filtered, dried, and reduced in H2, in accordance with the earlier methods. As described in Example 4, however, the compound of composition Na6Pt(SO3)4 is dissolved in acid, reacted with H202 in the presence of carbon, the product filtered, washed and dried and with no H2 reduction necessary, since the sintering temper-ature required to prepare the electrodes is ample to decompose the adsorbed species to the catalytically-active platinum particles.
EXAMPLE 6_ -5 g of the precipitate having the composition corres-ponding to Na6Pt(SO3) 4 iS suspended in about lOOcc of water and reacted with a large excess of the ammonium form of Dowex~ 50 (a sulfonated copolymer of styrene and divinylbenzene) cation exchange resin iD
bead form until the precipitate is dissolved. The pH of the resulting solution is about 4. After filtration, the solution is passed through a column of Dowex~ 50 in the ammonium form until all of the sodium is removed. The resulting platinum sulfite complex in solution is then oxidized with hydrogen peroxide in the presence of finely divided carbon, using the procedure of Example 1, yielding a nearly equivalent electro-catalyst.
Similar results are obtainable by first neutralizing to pH 9 a solution of the complex compound corresponding to H3Pt(SO3)20H
with aqueous ammonia which neutralization requires almost five moles of NH3 (instead of only 3 moles in the case of neutralization of NaOH), jl/ -21-. , - . ~ . :
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1~4~6~)1 then acidifying the solution to pH 3 with sulfuric acid, and oxidizing with H202 in the presence of carbon, again using the procedure of Example 1.
In both the earlier methods of the said applications and the additional methods supplementarily discussed herein, however, common over-all steps are involved of forming the complex sodium plat-inum sulfite precipitate from CPA, acidifying the same and developing the complex platinum sulfite acid and oxidizing such into a platinum colloidal sol, which is applied to the carbon particle substrate and reduced to form the conduction catalytic fuel cell or related electrode.
While the above examples relate to a complex platinum sulfite as the starting material for an appropriate platinum colloid, other platinum complexes comprising oxidizable ligands can be similarly used, as before stated, to produce suitable platinum colloids by means of a non-complexing oxidant, as illustrated in the next Example 7.

Four grams of platinic acid, H2Pt(OH)6, were dissolved in 25 milliliters of 1 molar NaOH. Six grams of sodium nitrite were dissolved in this solution and then the mixture was diluted to a volume of 800 milliliters with water. The pH was then reduced from about 11 to pH of 2 with H2SO4. During this process, a precipitate formed and then re-dissolved as the pH approached 2, thereby forming a platinum nitrite complex. To this solution, 18 grams of finely divided carbon (Vulcan~ XC-72) were added, and while vigorously stirring, 200 millileters of 3% H202 were added. The pH dropped to 1.4 substantially instantaneously The resulting platinum-catalyzed carbon was filtered, washed and dried. Fuel cell performance for 0.25 milligram per square centimeter electrodes of this material in a phosphoric acid fuel cell at 190C, was 640 millivolts at 200 amperes 3~, jl/ -22 -: .~
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~04~
per square foot, with hydrogen and air.
In thls case, the lower performance of this platinum nltrlte complex, as compared with the platinum sulfite complex, appears attrlbutable to the fact that the colloidal state ls rapidly produced and perslsts only for a very short time, followed by precipitation, whereas ln the case of the platinum sulfite complex, the oxidation proceeds slowly and the collo~d is stable over long periods of time.
As before explained, in general, suitable electro-catalysts are prepared by depositing platinum of the 15-25 Angstrom particle size on finely divided conducting carbon. It has also been found possible to prepare colloidal solutlons, though not quiee so efficaclous, by the use of solutions of non-complex platinum salts from which colloidal solutions can be made, for example, by the use -of an appropriate hydrolysis technique, as illustrated by Examples 8 and 9.

Four grams of platinlc acid, H2Pt(OH)4, were dissolved ln lO m.llileters concentrated H NO,. This solutlon was slowly added to one liter of water containing 18 grams of finely divided carbon ~;~
(Vulcan~ XC 72) while vigorous stirring was maintained for one hour, and then the pH was ad~usted to 3 with NaOH, while continuing stirring.
The dlsperslon was then bolled, whlle stirring. This colloid was thus produced by hydroli~ing a non-complex platinum salt solution at the above appropriate pH. The resulting platinized carbon was filtered, washed and dried. Puel cell electrodes were fabricated therefrom havlng a platlnum loading of 0.25 milligrams per square centimeter and phosphoric acid fuel cell constructed. Performance with hydrogen and alr at 190C was 660 millivolts at 200 amperes per square foot.
EXA~PLE 9 ~ 23-- - ~ . '-':' '.-', ' .:': ':
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486r1 The experiment of Example 8 was repeated except 6 molar H2SO4 was substituted for nitric acid, this time producing the colloid by hydrolyzing the non-complex platinum salt resulting from the H2S04 reaction at the same pH of about 3. Fuel cell performance under similar conditions as in Example 8 was 667 millivolts at 200 amperes per square foot.
The platiniæed carbon electrodes produced with the non-complex platinum sols of Examples 8 and 9, while most useful for the purposes described, have given somewhat lower fuel cell voltages at the same current densities than electrodes made from the preferred platinum sulfite complex, before discussed, apparently because of the difficulties involved in controlling the hydrolysis conditions required for the non-complex platinum salt processes.
As before stated, while only illustrative electrode and other catalytic uses have been described, the lnvention is clearly applicable to a wide variety of electrodes, oxidation, hydrogenation, dehydrogenation, reforming, cracking, chemical reaction-aiding, con-taminant burning and other uses, as well. Further modifications will also occur to those skilled in this art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.
This application is a division of co-pending Canadian Patent Application No. 223,881 filed April 4, 1975.

jl/ -24-

Claims (6)

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

1. In the method of preparing electrodes for fuel cells and the like comprising platinum-on-carbon electrocatalyst, the steps of producing an aqueous collodial platinum-containing sol having an average platinum particle size substantially of the order of 15-25 Angstroms, depositing said platinum contained in said sol on an electrically-conducting carbon substrate, and controlling the depositing to cause the carbon to nucleate the deposit and limit the formation of platinum particles on said carbon to said size.

2. A catalytic fuel cell electrode comprising an electric-ally conductive high surface area carbon substrate on which has been deposited platinum particles of the order of substantially 15 to 25 Angstroms in particles size and in which said particles load the electrode surface in the range of from substantially 0.04 milligrams/cm2 to 0.5 milligrams/cm2,

3. A catalytic fuel cell electrode as claimed in Claim 2 and in which said electrode is a cathode disposed in a phosphoric acid electrolyte of an air-hydrogen fuel cell.

4. A catalytic fuel cell electrode as claimed in Claim 3 and in which the platinized carbon is admixed with a fluorinated hydrocarbon polymer.

5. A catalytic fuel cell electrode as claimed in Claim 2 and in which said electrode is an anode disposed in a phosphoric acid electrolyte of an air-hydrogen fuel cell, said hydrogen com-prising carbon monoxide impurity.

6. A catalytic fuel cell electrode as claimed in Claim 5 and in which the platinized carbon is admixed with a fulorinated hydrocarbon polymer.