CA1257646A

CA1257646A - Fuel cell

Info

Publication number: CA1257646A
Application number: CA000488914A
Authority: CA
Inventors: Wolfgang Habermann; Ernst-Heinrich Pommer; Peter Hammes; Hubert Engelhardt; Wolfgang Geiger; Werner Simmler; Guenther Huber
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1984-08-18
Filing date: 1985-08-16
Publication date: 1989-07-18
Also published as: US4670360A; EP0172505A2; DE3430485A1; ATE50888T1; EP0172505A3; DE3576436D1; EP0172505B1

Abstract

Abstract of the Disclosure: In a fuel cell which has an activated carbon-containing anode and an activated carbon-containing cathode and is preferably used for the oxida-tive treatment of waste waters containing oxygen or oxygen-containing compounds, the cathode has been subjec-ted to partial anodic oxidation in a mineral acid at a potential .epsilon.h of from +1.3 to +10V and then doped with molybdenum(VI) and/or tungsten(VI) and/or vanadium(V) compounds, and the anode has been subjected to partial anodic oxidation in an aqueous mineral acid, cathodically reduced in an aqueous mineral acid and then impregnated with cobalt hydroxide and/or nickel hydroxide and/or cop-per hydroxide and/or zinc hydroxide, the anode and the cathode being separated by a porous polyelectrolyte through which the waste water flows.

Description

- 1 - O.Z. 0050/37282 Fuel cell The present invention relates to a fuel cell which has an activated carbon-containing anode and an activated carbon-containing cathode and is preferably used for the oxidative treatment of waste waters containincJ oxygen or oxygen-containing compounds.
In industry, the oxidative treatment of waste water is carried out using not only biological methods but also chemical or electrochemical oxidation processes.
In biological activated sludge plants, substantial degra~
dation of harmful substances can be effected only with inadequate space-time yields. For example, humic acids, fulvic acids, aromatics and chlorohydrocarbons are scarcely degraded by this technique. Better space-time yields are obtained with processes in which air or oxygen is used in combination with activated carbons. The degree of degradation corresponds to that of the biological acti-vated sludge plants. Virtually complete degradation is achieved by means of chemical processes in which ozone or hydrogen peroxide is used as the oxidizing agent. The high costs and the production of small amounts of chloro-hydrocarbons as by-products of the oxidation are disadvan-tages of this technique. Electrochemical oxidation by electrolysis has the same advantages and disadvantages.
It is an object of the present invention to pro-vide a process ~hich makes it possible to submit harmful substances in waste water to oxidative degradation in a manner ~hich causes little pollution and is economical.
We have found that this ob~ect is achieved, in accordance with the invention, by a fuel cell of the type stated at the outset ;n which the cathode has been sub-jected to part;al anodic oxidation in an aqueous mineral acid at a potential ~h of from ~1.3 to +10 V and then doped with molybdenum(VI) and/or tungstentVI) and/or vanadium(V) compounds, and the anode has been subjected to part;al anod;c oxidation in an aqueous mineral acid, cathodically reduced in an aqueous mineral ac;d and then ~L2~

- 2 - 0.~. 0050/37282 impregnated with cobalt hydroxide and/or nickel hydroxide and/or copper hydroxide and/or zinc hydroxide, the anode and cathode being separated by a porous polyelectrolyte through which the waste water flo~s.
The subclaims relate to further features of the invention.
Examples of suitable carbon-containing materials for the electrodes are graphite and active carbon.
The anodic oxidation of the carbon carrier is lD carried out, for the cathode and the anode, in an aqueous mineral acid, eg. nitric acid, phosphoric acid, sulfuric acid or perchloric acid, at a potential ~h of from +1.3 to +10 V, preferably from +1.8 to +2.5 V.
2-80, preferably 30-65, % strength by weight aqueous nitric acid is particularly suitable. The anodic oxidation is effected at from -2C to +100C, preferably from 110 +50C, and at a current density of from 0.1 to 1û kA/m2. In the anodic oxidation, the most advantageous current density is from 0.5 to ~ kA/m2 of outer surface area of the carbon.
The oxidation time can be from 2 seconds to 2 hours, preferably from 5 to 30 minutes. The amount of oxygen bound to the carbon surface should be about 30 atom per cent after the anodic oxidation.
Doping and impregnation of the partially oxidized cathode material are carried out using dissolved or finely dispersed compounds of molybdenum, tungsten or vanadium.
Examples of su;table molybdenum compounds are ammonium dimolybdate, ammonium heptamolybdate, ammonium decamolybdate, sodium molybdate, potassium molybdate, molybdenum tetrachloride and molybdenum oxytetrachloride, examples of suitable tungsten compounds are sodium tungst-ate, potassium tungstate, tungsten hexachloride and tung~
sten oxytetrachloride, and examples of suitable vanadium compounds are sodium vanadate, potassium vanadate, alkali metal divanadates and tetravanadates, sodium ammonium ~25'71Ei46 ~ 3 - O.Z. 0050/37282 vanadate and vanadium oxytrichloride.
Preferably, alkali metal molybdates are used for doping the partially oxidized cathode material. When vanadium compounds are used, care must be taken to ensure good fixing on the partially oxidized carbon carrier sur-face so that no vanadium passes into the waste water.
Hence, vanadium compounds are preferably fixed in com-bination with tungsten or molybdenum(VI) compounds and/or by conversion to iron titanium or zirconium vanadates.
To carry out doping, the molybdenum, tungsten or vanadium compounds and other additives are dissolved or dispersed in water, an alcohol, eg. methanol or ethanol, an ether, eg. methyl ethyl ether, or a chlorohydrocarbon, eg. chloroform or carbon tetrachloride.
Water is preferably used as the solvent. The concentration of the tungsten, molybdenum or vanadium com-pounds in the solvent can be from 0.01% by weight to satu-ration limit, preferably from 0.3 to 5% by weight.
After impregnation with the dopants, fixing can be promoted by drying the carbon carrier, halides being dehydrolyzed beforehand. The actual fixing is effected with a dilute aqueous mineral acid or an acidic alkali metal salt. This is preferably done using from 0.1 to 4X
strength by weight aqueous nitric acid or sulfuric acid.
Good fixing is achieved if fixing is carried out at from +15 to +30C during a residence time of from 0.1 to 3 hours.
Fixing can be followed by additional doping and partial reduction of the molybdenum(VI), tungsten(VI) and vanadium(V) compounds with sulfide or hydrogen sulf~
ide. Aqueous ammonium sulfide or alkali metal sulfide solutions, eg. sodium sulfide or potassium sulfide, are preferably used for this purpose. The concentration of these solutions can be from 0.1 to 10, preferably from 1 to 6, % by weight of alkali metal suLfide. To carry out doping, the catalyst material is immersed for a few min-utes, preferably from 1 to 8 minutes, in the alkali metal - 4 - O.Z. 0050/37282 su~fide solution, separated off and then freed from excess sulfide ~ith a dilute aqueous mineral acid.
Instead of usinc7 sulfides, partial reduction may also be effected using other reducing agents, eg. hydra~
zine hydrate, hydroxylamine, hydroquinone or hydrogen, or by means of cathodic reduction.
Particularly suitable activation and fixing fol-lowing the anodic oxidation of the carbon carrier has proved to be doping by means of molybdenum(VI) or vana-dium~V) compounds and with titanium~IlI) or titanium(IV)compounds and/or with iodine(VII) and~or with iodine(V) and/or with tellurium(VI) compounds. In this activation, the atomic ratio of molybdenum or vanadium to titanium should be 2:1, that of molybdenum or vanadium to iodine should be 1:1 and that of molybdenum or vanadium to tellu-rium should be 6:1. Where mixtures are used, ~he amounts should be adapted to the ratios.
- Preferably used titan;um compounds are titanium trichloride and titanyl sulfate. Iodine is preferably used in the form of the alkali metal iodates, or tellu-rium is preferably employed in the form of alkali metal tellurites.
Instead of tellurium compounds, it is also pos-sible to use selenium compounds. For toxicological reasons, however, the use of these compounds is avoided in most cases.
Oxygen or air is preferably used as the oxidizing agent for the cathode. In special cases, however, it is also poss;ble to use other oxidizing agents, eg. hydrogen 3û peroxide, peroxydisulfates, perborates, chlorates, chlor-ites, chlorine dioxide, ozone, nitric acid, nitrous gases.
nitrogen dioxide, iron(III) salts, iron(III) salt/hydrogen peroxide mixtures, nitric acid/hydroyen peroxide mixtures or nitric acid/hydrogen peroxide/iron~III) salt mixtures.
When these oxidizing agents are used, the cathode must be polarized to a potential ~h of C r1.34 V in order to avoid the formation of chlorohhydrocarbons.

~2~7~6 - 5 - O.Z. 0050/37232 In producing the anodes, the carbon carriers anodically oxidized in the nitric acid are first subjected to cathodic reduction in an aqueous mineral acid in order to remove residual oxidizing agent from the pores.
Examples of suitable mineral acids are aqueous sulfuric acid, phosphoric acid and hydrochloric acid, 5-20%
strength by weight aqueous sulfuric acid preferably being used. The cathodic reduction is preferably carried out at a current density of from 0.2 to 2 kA/m2 and at from +10 to +50C. The reduction time can be from 10 minutes to 2 hours, preferabLy from 15 to 30 minutes.
Instead of cathodic reduction, it is also pos-sible to carry out chemical reduction with a reducing agent, eg. nitrogen, hydrazine hydrate, hydroxylanine or hydroquinone.
The carbon carriers pretreated in this manner are doped using dissolved or finely dispersed compounds of cobalt, nickel, copper or zinc, ~hich are subsequently converted to the hydroxides with alkaline precipitating agents.
Examples of suitable cobalt compounds are cobalt(II) sulfate, cobalt(II) nitrate ancl cobalt~II) chloride, examples of suitable nickel compounds are nickel(II) chloride, nickel(II) sulfate, nickel(II) nit-rate and nickel ammonium sulfate, examples of suitablecopper compounds are copper(II) halides, copper(II) ammo-nium chloride, copper(lI) nitrate and copper(II) sulfate, and examples of suitable zinc compounds are zinc(II) halides, zinc(II) ammonium chloride, zinc(II) nitrate, zinc ammonium sulfate and zinc sulfate.
Cobalt hydroxide, nickel hydroxide or a nickel hydroxide/zinc hydroxide mixture is preferably used.
To carry out doping, the cobalt, nickel, copper or zinc compounds are dissolved or dispersed in water or an alcohol, eg. methanol or ethanol. Water is preferably used as the solvent~ The concentration of the cobalt, nickel, copper and zinc compounds can be from 0.02æ by ~7 6~

- 6 - O.Z. 0050/372~2 weight to saturation lim;t, preferably from 0.5 to 5% by weight.
After impregnation with the dopants~ the carbon carriers for the anode material can be dried in order to promote fixing. The actual fixing is effected us;ng a d;lute aqueous alkal; metal hydroxide, ammon;um hydrox-ide, ammonium sulfide, an ammonium hydroxide/ammonium sulfide mixture, an alkali metal sulfide or hexamethylene-tetramine. Where nitrates are used, precipitation may furthermore be effected by cathodically reducing the n;trate or convert;ng it to ammonia with a reducing agent.
Examples of suitable reducing agents for this purpose are hydroquinone and lithium aluminum hydride. Fixing of the dopants with an aqueous alkali metal sulfide has proven the most advantageous method, an aqueous solut;on contain-ing from 0.2 to 5X by weight of sodium sulfide or potas-sium sulfide being preferred for this purpose. ~ighly active anodes may furthermore be obtained if the carbon carriers doped with the salts or hydroxides are used in

3% strength by weight sodium sulfate solution which con-tains 0.1æ by ~eight of dextrose and 0.1% by weight of urea and is inoculated with adapted sulfate-reducing micro-organisms of the Desulfovibrio desulfuricans type. The residence time in this medium should be a few days, pre-ferably from 3 to 10 days.
A porous polyelectrolyte was used to separate theanode from the cathode. Suitable polyelectrolytes are ;norganic anion exchangers and cation exchangers, eg.
titanium oxide hydroxide, zirconium oxide hydroxide, kaolinite, montmorillonite, apatite, synthetic hydroxyl-apatite, magnesium oxide hydroxide, aluminum oxide hydrox-ide, and alum;num z;rconium ox;de hydrox;de, and inorga-nic anion exchangers and cation exchangers, eg. polymers or copolymers of styrene, styrene and divinylbenzene, styrene and maleic anhydride~ acrylates and div;nylbenz-ene, methacrylates and d;v;nylbenzene~ olefins, perfluo rinated olefins, and vinyl chloride and aldehydes, 5~6~6 resorcinol and aldehydes, and anisole and aldehydes which contain, as charge-carrying groups, sulfo and/or carboxyl and/or quaternary ammonium and/or primary, secondary or tertiary amino groups.
Synthetic hydroxylapa-tite, zirconium aluminum oxide hydroxide, zirconium oxide hydroxide, titanium oxide hydroxide and macroporous exchangers consisting of styrene and divinylbenzene or copolymers based on vinyl chloride which contain primary, secondary or tertiary amino groups or sulfo groups as charge-carrying groups are preferred.
The polyelectrolytes are used in the fuel cell, preferably in the form of granules, as a bed for separating the anode and the cathode. To avoid blockages, the particle size should preferably be from 2 to 6 mm. In the case of waste waters which.do not con-tain any suspended solid particles, the particle size may be smaller. In this case, porous open-cell polyelectrolytes in sponge form can also be used. Moreover, some of the polyelectrolytes may be replaced with sand, dolomite, limestone, peat or clay-containing or sand-containing earth.
However, the bed be-tween the anode and the cathode should contain no less than 20 vol. % of polyelectrolyte.
For industrial operation, graphites having an open porosity of >12% and ~28% are used as cathode materials.
The fuel cell can be provided with tubular or plate-like electrode materials. In special cases, it is also possible to employ beds of granules. The oxidizing agent, eg.
oxygen, is forced through the porous graphite cathodes toward the waste water side.
In the accompanying drawings:
Figure l shows a fuel cell according to the inven-tion, for the oxidation of waste water; and Figure 2 shows ano-ther fuel cell according to -the invention.

,;

- 7a - ~2 ~7~

The fuel cell shown in figure 1 is used for the oxidation of waste water and comprises a porous tubular oxygen reduction electrode 1 made of graphite, and an impermeable tubular graphite anode 2. The two electrodes 1 and 2 are separated by a bed of macroporous polyelec-trolyte granules 3. During operation of the cell, oxygen is forced from the inside of the cathode through the latter to the waste /

~5~

- 8 - O.Z. 0050/3728Z
water side, while the waste water 4 fLows through the porous polyelectrolyte. The oxygen feed is such that the amount of oxygen forced through the cathode is about 5-10%
more than the amount o~ oxygen consumed by the oxidation of the harmful substances.
Figure 2 shows a fuel cell in which the tubular oxygen reduction cathode 1 and the oxidation anode Z, each of which may be of any form, dip into a trough 5 made of porous polyelectrolyte material 3 which is per-meable to water. The oxygen feed for the fuel cell inFigure 2 is similar to that in Figure 1, while the waste water 4 flows through the trough 5 containing the porous polyelectrolyte 3.
The advantages of such arrangements are that the conductivity of the waste water is not critical owing to the presence of the polyelectrolyte, the fuels are stored by adsorption of the harmful substances (fuels) on the polyelectrolyte, and contamination is avoided as a result of the cathode being flushed with oxygenO

A fuel cell arrangement having the structure shown in Figure 1 consists of an oxygen reduction elec-trode of porous electrographite, an oxidation anode of compacted electrographite. The open porosity of the cathode mater;al is ~ 16%, while that of the anode mate-rial is ~ 10X. The cathode has an internal diameter of 40 mm, an external diamter of 60 mm and a length of 300 mm, and the anode has an internal diameter of 80 mm, an exter-nal diameter of 100 mm and a length of 300 mm. The cath-ode is activated by the following steps:Anodic oxidation of the outside of the tubular graphite cathode in 5ûX strength by weight aqueous nitric acid at room temperature for about 10 minutes.
Subsequent impregnation with 5% strength by weight aque-ous sodium molybdate solution- Followed by dry;ng at ~80C for about 8 hours ~ Treatment with 5% strength by weight aqueous sodium ~2~7~

- 9 - O.Z. 0050/37282 sulfate solution at pH 1 for about 5 mi.nutes - Washing with water and treatment with 0.5% strength aqueous sodium sulfide solution for about 2 minutes - Final removaL of the residual sulfide by treatment with 0.5% strength by weight aqueous hydrochloric acid.
To activate the anode, the following steps are carried out:
- Anodic oxidation in 30% strength by weight aqueous nit-ric acid at 2 kA/m2 for about 10 m;nutes - Subsequent cathodic reduction in 10% strength by weight aqeuous sulfuric acid at 1kA/m2 for about 30 minutes - Followed by impregnation with saturated aqueous nickel sulfate solution - Precipitation of the nickel, as nickel hydroxide, onto the graphite surface by treatment with 5~ strength by weight aqueous sodium hydroxide solution - Aging of the nickel hydroxide at +80C for about 2 hours - Treatment of the graphite surface with 2~ strength by weight aqueous sodium sulfide solution for about 2 minutes.
The polyelectrolyte used is a mixture of granules which has a particle size of from 2 to 4 mm and consists of 55 per cent by volume of a synthetic hydroxylapatite and 45 per cent by volume of a zeolite.
During operation of the fuel cell, oxygen is forced from the inside of the tubular graphite cathode through the porous electrode material, and waste water is passed through the polyelectrolyte granules.
The waste water used is a water which contains about 500 mg/l of TOC in the form of fulvic acids~
120 mg/l of alkali metal sulfide, ~û mg/l of alkali metal sulfite and 0.8 mg/l of chlorohydrocarbons, predominantly chloroform.
With this waste water, the resulting equilibrium potential is ~ û.85 V. In batchwise operation, a cur-rent of 2 A/dm2 flows initially, but current ceases to 7 ~ ~

10 - 0.Z. 0050/37282 flow when the harmful subst3nces have been degraded.
When there is no longer any flow of current~ the waste water is found to contain 12 mg/l of TOC, ~1mg/l of sul-fide, ~ 1 mg/l of sulfite and ~ 0.1 mg/l of chlorohydro-carbons.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fuel cell which has an activated carbon-containing anode and an activated carbon-containing cathode, wherein the cathode has been subjected to a partial anodic oxidation in an aqueous mineral acid at a potential .epsilon.h of from +1.3 to +10 V and then doped with molybdenum (VI) and/or tungsten (VI) and/or vanadium (V) compounds, and the anode has been subjected to partial anodic oxidation in an aqueous mineral acid, cathodically reduced in an aqueous mineral acid and then impregnated with cobalt hydroxide and/or nickel hydroxide and/or copper hydroxide and/or zinc hydroxide, the anode and the cathode being separated by a porous polyelectrolyte through which the waste water flows.

2. A fuel cell as claimed in claim 1, wherein a bed of macroporous ion exchanger granules is used -to separate the anode from the cathode.

3. A fuel cell as claimed in claim 1, wherein the porous polyelectrolyte consists of a mixture of an anion exchanger and a cation exchanger.

4. A fuel cell as claimed in claim 1, wherein oxygen and/or air or hydrogen peroxide is or are fed to the cathode as an oxidizing agent.

5. A fuel cell as claimed in claim 1, wherein the anode and the cathode are tubular, and the porous cathode which is gassed or charged from the inside with the oxidizing agent dips into the impermeable tubular anode, and the porous polyelectrolyte in the annular gap serves as a separator.

6. A fuel cell as claimed in claim 1, wherein tubular oxygen reduction cathodes and oxidation anodes, each of which may be of any form, dip into a trough of porous polyelectrolyte particles.