1026 DENKI KAGAKU Chlor－Alkali Electrolysis with a Zero－Ga，P Type Oxygen Catho（1E

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1026 DENKI KAGAKU Chlor－Alkali Electrolysis with a Zero－Ga，p Type Oxygen Catho（1e

Takahiro ASHIDA，Shuhei WAKITA，Masashi TANAKA， Yoshinori NISHIKI＊and Takayuki SHIMAMUNE

Received June2，1997，Accepted August1，1997

Anewtypeofoxygengasdiffusioncathode（DCPTM），whichisplacedontoanion exchange membrane（the so－caHed，zero－gap type），was propose（1and tested in a chlor－ alkalicelL The cellpe㎡omancewas measu旭dundervarious elecαolydcco繭tions， such as temperat皿e and concen江ation of the electrolyte as well as current（1ensity．By optimizing the components an（1the stmctu爬of the electrode，the cell voltage attI麺ned 2．05V at30A酒dm2，which comespon（1s to a1℃duction of about l V vs。a conventional hy（廿ogen evolution cathode．The cen has been operated stably over700（1ays．

1．INTRODUCTION A schematic cross sec沈ional view of a It has been sロ圏essed that the cell voltage hl chlor－alkali cell with a conventional oxygen chlor－alkali membrane or diaphragm pr㏄esses cathode is shown in Figure1，where two can be greatly reduce（1using an oxygen gas catho（1e chambers composed of both the diffusion catho（le，instead of a conventional catholyte and the gas compartments are hydrogen evolution catho（ie．In order to reali2e require（1．B㏄ause oxygen gas must be supplied this new el㏄trolysis system，development of Im from the back side of the cathode，passing excellent gas diffusion elec1■ode is essendal． through the hy（h巴o－phobic porous layer in the However，in spite of a great deal of effort，such cathode and reaching the 旭action layer，this an elect■ode has not yet been obtained on a cathode must possess a perfect gas－liqui（1 commercial scale1・2）．On the other hand，a impemeable sαucture。However，itisgenerally lIwge－scalecellhavingahydrogengasdiffusion difficult to avoid leakage of the NaOH solution anode of about l m2 area，known as into the gas phase（1uring long－time operation Hy面na田，hadbeensuccessfully・peratedina because of prcssure imbalance along the height commercial plant foHecovehng acid an（1a皿副i direction，causing inhomogeneous oxygen gas 丘om efflue血t sodium sulfate3）．A new type of supPly to由e cathode． oxygen gas（1iffusion catho（1e，named the On the other han（1，0ur cathodemust have DCPTM（Depola直zed Cathode Package），whele good pe㎜eability for both gas an（11iqui（1．A thecathodeisplacedontotheionexchange schematic cross－sectional view of a zero－gap membrane，has been propose（14・5），based on type of oxygen gas depolarized cathode thisconcept。 proposed in this paper is shown in Figure2， whe爬the catho（1e is m㏄hanically attache（10nto Pemlelec Electro〔1e Lt（1。（2023－15Endo， the membrane an（10nly one chamber of both the Fujisawa－city，Kanagawa Pref。，252，Japan） catholyte and由e gas phase is present at its back κ8yWo眺：Oxygengasdiffusioncathode， side・Whentheel㏄江olysisisst頒ed，oxygenis Chlor－alkali membrane cell，Durability test supPlie（1to血e porous catho（1e，and at the sanle 65，No．12（1997／） 1027

十望12 02 NaOH CI2 02＋H20 ↓ ↑ ↑［NaCI ↓ ↑一～aα

02 NaOH Na＋ H20 Na＋％ O2 OH一 NaOH H20 NaOH

↓ ↓

02＋Drain 難cI 02＋NaOH ：喩DCPcM

GDE CM Anode DCP CM GDE ．Gas DiIfusion Ebctrode DCP胴＝Depolarized Cathode Paゆkage CM l Cation Ex（；hange Membrane CM ＝Ca廿on Ex（；hange Membrane Fig．1 Schematic drawing of conventional Fig。2Schematic drawing of zero gap cell th1℃e compartment GDE cell． with DCPTM．

Table l Cell specification and elec甘olysis condi重ions

49mm in（1iameter（18．8cm2） Elech℃des size 5cm in width｛md25cm in height（125cm2） Anode 臨淵襟濫，） Memb翼me Cadlode current coUector Silver perforated plate Ca廿10de subsロ・ate Nickelfonn（SumitomoDenkoCo甲．，Cemet⑱） Catalyst layer 1γrFE－bonded’silver powder Cun’ent density below30A／dm2 Anolyte 2009バNaCl Catholyte 31～33wt％NaOH

time caustic soda produced in the c飢hode is and the subsnOate in 亡he proposed cel1．It is remove（1through the cathode itself against the expected from thermodynamics that silver is gas flow d血ection． extremely stable undcr the electrolysis This new cell structu爬is simpler than that conditions． of a conventional one，which has two chambers In this paper，the results obtained from a for liquid and gas in the catho（le side・Henc弓it small－sized chlor－alkali㏄11with a zero－gap type is considere（1that the fonner has merits for ・f・xygencath・de肛erep・鵬d4・5）．hi劇 scale－up because pressure control between the perfo㎜ance for a cell of25cm in height is also two chambers could be omitted． presented。 The optimization ofthe cathode materials is 2．EXPERIMENTAL also a key factor in at惚ining long durability．In Electmlytic conditions a】℃liste（1」血丁旨ble1． conventional type cells，a carbon－based caωyst The anode an（1the catho（1e chambers of the was nomally used in order to form a五ne small－sized cell，having a19cm2electrolysis porous s血cture，but it is often observed to be 肛ea of ci1℃ular shape，were mspectively made conrodcd in highly concen江ated causUc so（1a in of Pyrex glass and acryl resin．Those of125 the presence of oxygen。Nickel and silver cm2electrolysis area of lectImgular shape were materials are used for both the cathode catalyst respectively made of titanium an（1nickel metal．

r 一

1028 DENKI K：AGAKU

The anode was a typical DSE with RuO2and flow by gravitation。 BD2catalyst on a pelfbrate（1titanium plate．Fine Electrolysis of a hy（1rogen evolution silver pa∫ticles of100～1000A diameter we爬 system，using an activated cathode composed of used as the cathode catalyst，which was pa血a皿y RuO2powder，wasc面記outundern・㎜al loaded on a silver plated nickel foam with PTFE conditions in order to con行m the value of resin（Mitsui DuPont Fluon㏄hemical Coψ．， reducdon of ce皿voltage as a1℃fe1℃nce． 30J）。The foam was heat－treated at a high The cathode potential was meas皿ed temperat皿eof330～380℃for10～30 ag田nst SCE using a cun障nt inteπupter（Hokuto minutes in air in order to enhance the contact Denko Manufacturing，Ltd．，HC－113）and a s㏄ng山oflhe紀sintothecatalystandtoob面n （1igital oscilloscope（lwatsu Corp。，DS91）．The good hy（1rophobicity．Thr㏄diff¢rent samples， luggin capillary tip was immerse（1in the anode No．1，No．2 an（l No．3，were prepare（1by chambeL Hence，the measured cathode pαendal varying the manufacturing conditions and the after current interruption contained 山e kinds of catalyst．The size of the silver pa血cle memb貰me potential．The hydrophobicity of the became smaller in o面er ofthe sample numbcL cathode su㎡ace was measure（1by a contact Homogeneity of the mixtu爬of the catalyst an（1 angle scope（Elmer Corp．，G－1－1000）．Gas the resin was also㎞prove（1in this orde凱 pemeability was evaluatOdby a densometer（ Oxygen gas was f¢d ffom a gas cylinder at Mitsutoyo Seiki Corp。，Garley type）．Liquid twice the rate theoletically requested。The CO2 p㎝rneability was investigated by observing the content in the oxygen gas fe（1to the ca山ode liquid pemleation through an electmde that was chamber must be kept un（1er a level of l ppm to placed at the bottom of a glass tube（5cm hl pl℃vent the deposidon ofsodium carbonate。The diameter and100cm in height），with water fee（1gas was humidified by passage through a fillhlg in廿1e tube． water vessel outside the cell．The water was Because hydrogen genemtion㏄cu㎡ng at heated at temperatures of70～96℃to examine high cuπent densities or（1ue to dle lack of the effect on the concen廿・ation of the causdc oxygen fee（1was very dangerous，the hydrogen so（1a produced．In order to avoi（l steam content of the catho（1ic gas outflow was liquefying in the cell chamber，it is measured with a sensor（Stec Corp．，GS－30） Iecommende（1to heauhe cathode chamber f士om and was conf㎞e（1to be below1000ppm． the back side．For this purpose，some additional 3．RESULTS AND DISCUSSlON lheaUng apParatus was prepared． The physical parameters of the DCPTM It is desired that the cathode should were compare（1with those of other homogencously contact the membrane to commercially available gas electrode，as listed in remove the produce（1catholyte from the Table2．The hydrophobicity of the DCPTM was membrane－cathode gap，opposing the down almost the same as the latter but the permeability Table2Physical p肛ameter ofgas（1iffusion electrodes。 Item DCPTM Commercially available GDE C・甑tangleαtalystside 115～130 130～150 （deg爬e） Gasside 110～140 115～150 Gas penneability （cm／sec） 10～30 0．003～0．03 possible impossible Liquid pe㎜ination at10cmH20 at80cmH20 65，No，12（1997） 1029 3．0 0

2．8 ぴ『 onventional ce” 2．6 円＿0．2 ＞ハ q ツ 2．4 ω Φ 切 o き σ 2．2 ＞一〇．4 望o 》ご 2．0 邸 oΦ ほs 1．8 5－0．6 α 1．6

1．4 一〇．8 0 10 20 30 40 1 10 100 Current density（A！dm2） Current densi》（Aldm2） Fig．3Cell voltages fbr DCPTM ofNo．1（△）， Fig．4Pola逝zation plots fbr DCF「M ofNo．1 No．2（○）and No．3（●）in the small－（△），No．2（○）Imd No．3（●）in small－ sized cell． sized cel1．

2．3 100

● ● 2．2 ぷ ●D● ● ハ ● ＞ 0》95 Φ ζ ロ o 9 2、1 ．9 5 と＞ 2 Φ §go o 2．0 oも

1．9 85 20 25 30 35 40 45 20 25 30 35 40 45 Concentration of NaOH（％） Concentration of NaOH（％） Fig．5Relation between NaOH concentration Fig．6Relation between NaOH concentration andcellvoltage。 and current ef行ciency． for both gas and liquid was clearly diffe1℃nt， the former section．The decrease in the cen which in（1icates that it is suitable for zeK＞gap vohage is supPose（1to be rela威I to面ucdon of type electfolysis。 the ohmic resistance of the membrane and the The cell voltage amd the potential for th1℃e solution by the change in cu∬ent dis廿ibution， samples a貰》plotted against廿1e cur旭nt densi汐in which was also cause（1by th¢optimization of Figures3and4．Compahng the No．1sample the structure。It was confimled that廿1e with No．3at30A！dm2，the cell voltage was re（1uc廿on in the cell voltage by dle use of廿1e shown to be reduced from2．4to2．05Y in DCPTM was about l Y compa爬dto the voltage spite of the fact that the ovewoltage was only ofaconvenUon証hyd1Dgenevoludoncathode， re（luced by O．2 V The reduction of the as shown in Figure3． overvoltage is supPosed to be due to both Figure5 shows the effect of NaOH enlargement of the catalyst su㎡ace area and concentrationon the cellvoltage．The enhmcement of the hydrohobicity by changing relationship reflects the depen（1ency of the mi㏄O S血1CtUre Of山e eleCt1て》de，aS me面On鉛in ohmic resistance of the membrane and the 」

1030 DENKI KAGAKU

2．5 100 ●．露・華織踊、邑go 9 む苛 ◎qg 留 5 モ…2・0 ’§80 ……i o ≧ 芒る o o ヒ70 oコ

1．5 60 0 200 400 600 800 0 200 400 600 800 Days in operation（day） Days in operation（day）

Fig．7 Result of ceU vokage in the durability Fig。8 Result of cuπent efnciency孟n the test for DCPTM of No．2（○）md No。3 durability test for DcPTM ofNo．2（O）（●）inthesmall－sizedce11s． an（1No．3（●）in the small－sized cells． overvoltage of the cathode．In Figure6，it was both cells recovered to their initial values．A found that the current efficiency was almost similar recovery of the cell voltage was constant at about 96 ％ regardless of d｝e observe（1after the shutdown，which could be concentration，which means that the membrξme related to the disapPearance of flooding in the remained in good con（1ition．The heat generation electrode．There seems to be a爬lationship due to the mcmbrane ohmic resistance an（1the between the cell voltage and the cuπent high overvoltage of血e ca血o（1e a爬so large廿1at efficiency，that is，when the cuπent efficiency the cell voltage was fbun（1to be independent of decreased，the cell voltage incrcased．This the temperature of the feed gas，though it tendency was explained as the detehomtion of depended on the anolyte temperature． the membrane，because blisters were found hl The Cl－ion in the catholyte was bclow50 the membrane after a long operation，which ppm，which is within the basic cha皿ctehstics of might be caused by the condensation of the this membπme． catholyte between the mcmbrane and the The爬sultsofadurabilitytest跡eplottedin catho（1e。It was suggested that the selecdon an（1 Figures7and8．The cell voltages gra（1ually the（1evelopment of a membrane suitable for the increase（1with operating time．Though the DCPTM，as well as severe control of the membranes were changed after200an（1300 operational con（1itions，are of impoltance in （1ays operadon f6r No．2and No．3resp㏄dvely， order to obtain long durability． in addidon to several cun，ent interruptions，both Thoughitwasthemodynamicallyexpcctod elect1Ddes were able to be operate（1for700days that nickel metal would be corroded under those without critical damage。Their cell voltages in electrolysis con（1itions，the nickel component this investigation increased by O．1～0。2V f廿om was not seriously det㏄ted insi（1e the membrane the initial values．The cument efficiencies we㎎ a負er elec跡olysis．Instead of nickel，silver could 理adually decrcased from95to85％．A rather beusedforboththecathodecatalystandthe steep decrease observed in the data for No．2 protection of the substrate with a cument during the iniUal200days occurred due to the collcctor composed of nickel or carbon contamination with some minerals due to the mate貞als，because silver is stable under t耳e insufficient pretreatment of the brine．A負er electrolysis conditions as expected from changing the membrane，the perfomance of themodynamics．For example，a silver－platα1 65， No，12 （1997） 1031

2．5 2．5

2．4 一2，2 ＞＞ママ 8 Φ 2．3 要2．0 o ＃o 望＞ 2．2 Φ Φ 0 1．8 Q 2．1

1．5 2．0 0 10 20 30 40 0 10 20 30 Currentdensity（Aldm2） Days in operation（day） Fig．9Cell voltage for DCPTM of No・2in Fig．10Result of the d皿ability test fbr DCPTM 出e medium－siz甜cel1。 ofNo。2in the medium－sized cell． nickel subsロ・ate is considered to be a suitable DCPTM cathode shoul（1be easy to scale up．A 飢temadve，which is also being evaluated in the pilot－scale cell of 100cm in height is to be test㏄ll and showing good perfo㎜ance． evaluated． The cell voltage of a me（1ium－sized cell， References using the same cathode as No。2， is plotted 1）H．Aikawa，50吻侃4ChZoη。πε，45，85 ag＆inst cun℃nt density in Figure 9． The （1994）． perfb㎜ance was almost the same as that of the 2）H．Aikawa，So4ααn4（7hZon’π8，47，192 Smal1－SizedCellS．（1996） Figure10shows the vanation in the㏄11 3）G．Faita，Pr㏄eedings of7th Intema丘onal voltages obtained ffom a25cm high cell，which Forum on Electrolysis in the Chemical showed goo（1stability over25 days。The In（1ustry，Electrosynthesis（1993） current efficicncy was about95 ％．This 4）T．Shimamme，Y Nak句ima，Y Nishiki， suggests that the elec荘ode could be operate（1in an（1T．Ashi（1a，Pr㏄eedings of18th Japam a cell of25cm in height without any ch｛mge in Soda琵chnicalMee亘ng（1994）．由e s血CUlre． 5）T．Ash三da，S．Wakita，M．肱naka，Y Nishiki， 4．CONCLUSIONS and T．Shimamune，Pr㏄㏄dings of19th A zero gap－type oxygen cathode，DCPTM， Japan Soda琵chnicalM㏄廿ng（1995）． was foun（1to be applicable to a chlor－alkali membrane electrolyzer with considerable reduction in㏄ll voltage in comparison with the hydrogen evolution cathode process。Smal1－ sizedcellsc・uldbe・perated・ve∫700days though the membrane had to be change（1during thetestduetothegenerationofblisters。 Suitable selection of the membrane an（1the operational con（五tions is of interest to at面n long durability of the DCPTM in the fut皿e．A me（1ium－sized cell of25cm in height was easily operate（1，which suggests that the cell with a