JP2009158118A - Separator material for solid polymer fuel battery and manufacturing method for the separator material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator material for a solid polymer fuel battery which has superior properties required as a separator material, such as gas impermeability, strength property, electrical physical property, and releasability in molding, and to provide a manufacturing method for the separator material. <P>SOLUTION: The separator material is made of a graphite/resin cured compact made by binding graphite powder using as binders a phenol resin curing agent containing an epoxy resin as a main agent and a high-paranovolac phenol resin at a content ratio of 50% or more and a mixture resin containing an imidazole curing accelerator as an essential element. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte fuel cell separator and a method for producing the same.

  A fuel cell directly converts chemical energy contained in fuel into electrical energy, and has a high conversion efficiency into electrical energy. In particular, solid polymer fuel cells have a lower temperature than fuel cells such as phosphoric acid fuel cells. In addition, because it is capable of high-power generation, it is expected to be used as a small mobile power source including automobile power sources.

  The polymer electrolyte fuel cell is usually an electrolyte membrane made of a polymer ion exchange membrane such as a fluororesin ion exchange membrane having a sulfonic acid group, and a catalyst electrode carrying a catalyst such as platinum on both sides thereof, A stack in which a single cell composed of a separator provided with a groove serving as a gas flow path for supplying a fuel gas such as hydrogen or an oxidant gas such as oxygen or air is stacked on each electrode, and 2 provided outside the stack. It consists of two current collectors.

  The structure of the single cell is, for example, a pair of electrodes (cathode, anode) disposed with a solid polymer electrolyte membrane made of an ion exchange membrane of a fluororesin, a separator sandwiching this from both sides, a separator It is comprised from the sealing material installed in the edge part.

The separator is formed with a plurality of linear or grid-like grooves, and a space formed between the grooves and the cathode serves as an oxidant gas flow path, and a space formed between the grooves and the anode serves as a fuel gas flow. It becomes a road.
Therefore, the separator needs to be supplied to the electrode in a state where the fuel gas and the oxidant gas are completely separated from each other, so that a high degree of gas impermeability is required, and a high electric conductivity is required to increase the power generation efficiency. In order to perform stable power generation for a long time, it must have high corrosion resistance, low impurity elution, etc., and carbonaceous materials have been used as separator materials that require such material properties. In general, a separator made of a carbon / resin cured molded body obtained by binding and molding a carbon powder such as graphite with a thermosetting resin as a binder is used (see Patent Document 1).

  As a binding material, a material to which a phenol resin is applied (see Patent Document 2) has been proposed. However, when a phenol resin is applied, a separator material having very excellent characteristics from the viewpoint of heat resistance, corrosion resistance, and the like. However, when a molding material using phenolic resin is heat-molded and cured, condensed water is generated during the curing reaction of the phenolic resin, and a part of the condensed water remains in the molded body and is liable to cause structural failure ( Generation of voids), and the material strength also decreases, so in order to secure high gas impermeability and to obtain a highly reliable strength material, set the temperature low and take time not to leave voids, There are drawbacks such as molding by repeatedly degassing.

  On the other hand, epoxy resin is a thermosetting resin that does not generate condensed water or ammonia during the curing reaction, and therefore has the advantage that it does not easily cause voids and other structural defects when hot-pressed in a short time. A separator material using an epoxy resin as a thermosetting resin to be a binding material has been proposed (see Patent Document 3).

  When an epoxy resin is used as the binder, an amine curing agent, an acid anhydride curing agent, or a phenol curing agent is used as the curing agent. In a separator using an amine curing agent, a fuel is used. During battery power generation, ammonium ions elute into the battery cell, and the eluted ammonium ions have a problem of deteriorating battery performance. When an acid anhydride curing agent is used, the acid anhydride reactivity Has a problem that since the curing reaction proceeds at a very high temperature, the curing rate is slow, the molding time of the separator is increased, and not only the monomer remains in the molded body but also the amount of dissolved organic substances increases.

From such points, a phenolic curing agent is useful as the curing agent (see Patent Document 4). However, the phenol novolac resin that is usually used is a phenol resin in which ortho positions are bonded, and when this resin is used, there is a problem that a molded cured product is brittle, and fracture strain and toughness are low.
JP 2000-021421 A JP 2004-127646 A JP 2001-261935 A International Publication WO2006 / 049319A1

  The inventor is a phenolic as a curing agent in the process of studying improvement of material properties of a separator made of a carbon / resin cured molded body obtained by binding and molding carbon powder such as graphite with an epoxy resin as a binder. As a result of tests and examinations focusing on the novolak type phenol resin among the curing agents, it was found that the properties of the novolak type phenol resin curing agent greatly affect the mechanical properties of the separator material.

  The present invention has been made on the basis of the above findings, and its purpose is excellent in properties such as gas impermeability, strength properties, electrical properties, and mold release properties required for a separator material. The object is to provide a separator for a polymer electrolyte fuel cell and a method for producing the same.

  A separator for a polymer electrolyte fuel cell according to claim 1 for achieving the above object is a phenol resin curing agent and an imidazole curing accelerator containing an epoxy resin as a main component and 50% or more of a high paranovolak type phenol resin. It is characterized by comprising a graphite / resin-cured molded body in which graphite powder is bound with a mixed resin having an essential component as a binder.

  The method for producing a separator for a polymer electrolyte fuel cell according to claim 2 comprises a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin, an imidazole curing accelerator and a dispersant as an organic solvent. A slurry is prepared by dispersing graphite powder in a mixed resin solution dissolved in a slurry, and a green sheet is prepared from the slurry by a doctor blade method, and the green sheets are laminated and hot-pressed.

  A method for producing a polymer electrolyte fuel cell separator according to claim 3 is prepared by dissolving a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin and an imidazole curing accelerator in an organic solvent. After kneading the mixed resin solution and graphite powder, the organic solvent is volatilized and removed, and then the molding powder obtained by pulverizing the kneaded material is filled into a pre-molding die, and the preform is formed by pressing and pre-molding. It is produced, and the preform is inserted into a mold and hot pressed.

  According to the present invention, there are provided a separator material for a polymer electrolyte fuel cell excellent in characteristics such as gas impermeability, strength characteristics, electrical properties, and mold release properties required for a separator material, and a method for producing the same. Provided. In other words, by using a high paranovolak type phenolic resin as a curing agent, improved separator toughness, improved heat resistance, improved mold release during separator molding, improved productivity by shortening molding hardening time, and reduced impurities eluted from the separator Long-term stability of fuel cell power generation can be obtained.

  The separator material for a polymer electrolyte fuel cell according to the present invention combines a mixed resin containing an epoxy resin as a main component and a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin and an imidazole curing accelerator as essential components. It consists of a graphite / resin-cured molded body with graphite powder bound as a material, and is particularly characterized by using a phenol resin curing agent containing 50% or more of a high paranovolak type phenol resin.

  A normal novolac type phenol resin is a high ortho novolac type phenol resin as shown below, which is represented by the following chemical formula 1, and has a methylene bond at the ortho position as viewed from the phenol hydroxyl group.

  Because the hydroxyl group of the phenolic resin is close to the methylene group to be cross-linked to the commonly used high-ortho novolak type phenolic resin, it is sterically crowded and there is a steric repulsion against the epoxy group of the epoxy resin. The curing reaction rate with the epoxy resin is not so fast, and there are many unreacted hydroxyl groups in the components constituting the resulting molded article, and the crosslinking density does not sufficiently increase. In particular, when the partner epoxy resin is bifunctional and has a large molecular weight and a large epoxy equivalent, the reactivity of the epoxy resin is low, so that the crosslink density does not increase and the heat resistance is lowered. Moreover, since the hydroxyl group remained, the problem of sticking to a metal mold | die at the time of hot press molding was also seen.

  On the other hand, a high paranovolak type phenol resin can be represented by the following chemical formula 2.

  The high paranovolak type phenol resin is one in which methylene bonds are all connected in the para position as viewed from the hydroxyl group of phenol. In the case of this resin, the hydroxyl group, which is a reaction point that reacts with the epoxy group, comes out from the whole molecule, and the steric repulsion to the epoxy group is also reduced, so the curing speed is fast, the crosslink density of the cured product is increased, and the toughness is high. (High strength / high breaking strain) and high temperature strength can be improved. Furthermore, since the curing speed is higher than that of the orthonovolak type phenolic resin, the molding time can be shortened and the amount of remaining hydroxyl groups is small, so that the mold release property is excellent and the productivity is improved. There is also an advantage that the amount of elution of the total organic carbon component from the obtained molded cured product can be suppressed.

  Normally, a novolak type phenolic resin is produced in a state where ortho-linked novolak and para-bonded novolak are mixed, and when the ratio of the para-bonded novolak (high paranovolak type phenolic resin) is 50% or more, A high product can be obtained, but if the proportion of the high paranovolak type phenol resin is less than 50%, a sufficient toughness improving effect cannot be obtained.

The high paranovolak type phenolic resin can be produced by changing the ratio of phenol and formaldehyde which are the raw materials of the resin, or adjusting the catalyst type, reaction temperature and time. Further, the content of high para novolak type phenolic resin, 810 cm ^-1 due to the absorbance peak and para bond wavenumber 760 cm ^-1 due to the ortho-position binding in the Fourier transform infrared spectrometer of the resin (FT-IR) The ratio can be calculated by the intensity ratio of the absorbance peaks.

  As the high paranovolak type phenol resin, a cresol novolac type para-bonded phenol resin having an aromatic ring with a methyl group can be used in addition to the structural formula (2). When the cresol type is used as a separator, it becomes difficult to absorb water.

  The main epoxy resin is biphenyl type, bisphenol type, novolac type, naphthalene type, anthracene type, phenanthrene type, pyrene type, triphenylmethane type, benzylic ether type, allylbenzene type, from the viewpoint of strength and heat resistance. Aromatics such as vinylbenzene, bromobenzene, chlorobenzene, and phthalate ester, or cyclic hydrocarbons such as cyclohexane, dicyclopentadiene, isobornyl, norbornene, and adamantane, imide, and triphenyl A polyfunctional epoxy resin having a cyclic amine-based molecular skeleton such as an amine type, an isocyanurate type, an azophenyl type, and an azine type can be used.

  In order to obtain a flexible material and a separator material having a large breaking strain, a mixed epoxy resin of an alcohol ether type bifunctional epoxy resin and a polyfunctional epoxy resin containing an aromatic ring can be used. The cured product containing this alcohol ether type epoxy resin has a low strength at high temperatures. However, when this high paranovolak type phenol resin is used, the strength at high temperatures is improved and the molding is performed at the time of molding. The releasability from the mold is also improved.

  Imidazoles are used as the curing accelerator. When an amine curing accelerator is used, ammonium ion elution occurs from the separator material, which affects power generation performance. When a phosphorus curing accelerator is used, water absorption of the separator material increases, and the separator easily swells during use, and elution occurs to affect the power generation performance, and the adhesion between the separator and the sealing material is poor. There are difficulties.

  A first method for producing a separator for a polymer electrolyte fuel cell according to the present invention comprises a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin, an imidazole curing accelerator and a dispersing agent as a main component. A slurry is prepared by dispersing graphite powder in a mixed resin solution dissolved in an organic solvent, a green sheet is prepared from the slurry by a doctor blade method, the green sheets are laminated, and hot pressing is performed.

Hereinafter, the first manufacturing method will be described.
Preparation of resin solution:
An epoxy resin as a main agent, a high paranovolak type phenol resin as a curing agent, an equivalent ratio of a phenolic hydroxyl group to an epoxy group in a weight ratio of 0.5 to 1.5, and further a predetermined amount of a curing accelerator and graphite powder The minimum amount of dispersant that can be dispersed is put into an organic solvent such as MEK and dissolved by stirring well to obtain a resin solution. In addition, the total resin solid content of the main agent, the curing agent, and the curing accelerator is 10 to 35 parts by weight with respect to 100 parts by weight of the graphite powder.

Slurry preparation:
100 parts by weight of graphite powder is added to the resin solution, and the graphite powder is dispersed in the resin solution using a stirring / dispersing machine such as a universal mixer to prepare a slurry. As graphite powder, artificial graphite, natural graphite, etc. can be used, but considering the mechanical properties (strength, fracture strain) of the separator material, artificial graphite powder alone or a mixed powder of artificial graphite and natural graphite is preferably used. Is done. Moreover, even if the amount of the solution is reduced, the graphite powder has a good fluidity and becomes a stable slurry, and does not have cracks due to drying shrinkage when forming the green sheet in the next process. Is preferably used. By blending the large particles and the small particles, there is a filling effect that the small particles enter the gaps between the large particles, and a dense green sheet can be obtained. The addition of small particles (presence) affects the mechanical properties in addition to preventing dry cracking at the time of forming the green sheet and obtaining a dense green sheet, and can be expected to improve strength and break strain.

  The slurry prepared in this manner is adjusted by adding an appropriate amount of an organic solvent so that the viscosity becomes 100 to 2000 mPa · s. This slurry is appropriately subjected to centrifugal degassing and vacuum degassing to degas air entrained by stirring and mixing.

Green sheet production:
The slurry prepared as described above is applied onto the film by a doctor blade method. In the application of the slurry, after adjusting the gap between the doctor blade and the film, the slurry is poured into a slurry hopper of the doctor blade, and the slurry is applied on the film coated with the release agent to a uniform thickness. Next, it is cut into a suitable length, blown dry and pre-dried. When the surface is in a dry state, it is cut into a predetermined shape with a punching die or the like. Further, the green sheet is peeled off from the film by drying or cooling.

Separator molding:
The green sheets produced as described above are appropriately stacked according to the thickness of the separator material to be manufactured, set in a mold, and hot-press molded at a temperature of 150 to 250 ° C. and a pressure of 10 to 100 MPa. Thus, a fuel cell separator is manufactured.

  A second method for producing a polymer electrolyte fuel cell separator according to the present invention is to dissolve a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin and an imidazole curing accelerator in an organic solvent. The prepared mixed resin solution and graphite powder are kneaded, the organic solvent is volatilized and removed, and then the molding powder obtained by pulverizing the kneaded material is filled into a pre-molding die, pressed and pre-molded. A reform is produced, and the preform is inserted into a mold and hot-press molded.

Hereinafter, the second manufacturing method will be described.
Preparation of resin solution:
The second manufacturing method is the same as the first manufacturing method except that the dispersant is not included.
Preparation of molding powder:
The produced resin solution and graphite powder are mixed and kneaded uniformly. Mixing and kneading are sufficiently kneaded using an appropriate kneader such as a kneader, screw-type kneader, or universal mixer to prepare a uniform kneaded product. After kneading, the organic solvent is volatilized and removed from the kneaded product by vacuum drying or blow drying. The graphite powder to be used is preferably used by adjusting the particle size to an average particle size of 50 μm or less and a maximum particle size of 100 μm or less, and artificial graphite, natural graphite, or a mixed powder thereof can be used. Since the surface of the kneaded material is covered with a resin film, the organic solvent is volatilized and dried so that the graphite part is exposed to suppress the decrease in conductivity of the molded product, and the preform can be uniformly filled. The kneaded product is pulverized to form a molding powder. For the pulverization, a machine capable of pulverizing to about 0.1 to 1 mm, such as a free pulverizer or a cutter pulverizer, is used.

Pre-form:
The obtained molding powder is uniformly filled into the cavity of the preforming mold, and the upper mold heated to a temperature equal to or higher than the melting point of the resin, for example, (resin melting point + 10 ° C.) is placed and preformed at a pressure of 1 to 10 MPa. Then, a plate-shaped preform is produced.

Separator molding:
A plate-like preform is inserted by applying a mold release agent into a mold having a concavo-convex portion that forms a groove serving as a gas flow path of the separator, and is hot-pressure molded at a pressure of 10 to 100 MPa and a temperature of 150 to 250 ° C. By doing so, a fuel cell separator is manufactured. If necessary, after-curing is performed at a temperature of 150 to 250 ° C.

  Hereinafter, the implementation of the present invention will be described in comparison with comparative examples, and the effects will be demonstrated. In addition, these Examples show one embodiment of this invention, and this invention is not limited to this.

Examples 1 to 3, Comparative Examples 1 and 2 (first manufacturing method)
Novolac type epoxy resin (manufactured by Nippon Kayaku Co., Ltd .: EOCN-103S) and biphenyl type epoxy resin (manufactured by Nippon Kayaku Co., Ltd .: NC-3000H) as the main agent, and high para novolak type phenolic resin (Maywa Kasei Co., Ltd.) as the curing agent Company: HF-1M, HF-3M) and orthonovolak-type phenol resin (Maywa Kasei Co., Ltd .: H-1, H-4), equivalent ratio of epoxy group of epoxy resin to phenolic hydroxyl group of phenol resin So as to be 1.0. The blending amount is shown in Table 1.

The content (%) of the hyparanovolak ^- type phenol resin was determined by measuring the absorbance (A) in the infrared region of 700 cm ^{−1 to} 4000 cm ⁻¹ of the resin sample with a Fourier transform infrared spectrometer (FT-IR) of the resin. From the obtained data, the absorbance peak at 760 cm ⁻¹ due to the methylene bond crosslinked at the ortho position (A ₇₆₀ ) and the absorbance peak at ₈₁₀ cm ⁻¹ due to the methylene bond crosslinked at the para position (A ₈₁₀ ) The ratio can be calculated by the intensity ratio. Therefore, the high paranovolak type phenol resin content ratio is (A ₈₁₀ / A ₇₆₀ ) × 100, and the high ortho novolac phenol resin content ratio is (A ₇₆₀ / A ₈₁₀ ) × 100.

  The above mixed resin is 22 parts by weight of graphite powder, 2-ethyl-4-methylimidazole (curing accelerator) is 1 part by weight of resin, and an anionic surfactant (polycarboxylic acid type polymer) is used as a dispersant. 0.5 parts by weight of graphite powder was mixed, and methyl ethyl ketone (MEK) was further added as an organic solvent. To this resin solution was added 100 parts by weight of artificial graphite powder whose particle size was adjusted to a ratio of 50 parts by weight of an average particle size of 50 μm, 10 parts by weight of 10 μm, and 40 parts by weight of 3 μm. Prepared.

  After adjusting the gap between the doctor blade and the film, the slurry was poured into a hopper of a doctor blade molding machine, and the slurry was applied onto the PET film coated with a release agent. Next, it was blown and dried to volatilize MEK as a solvent and cut to a predetermined size, and then released from the film to produce a green sheet having a thickness of about 0.3 mm.

  The obtained green sheet is punched into a predetermined shape, and a site is placed in a molding die having an outer shape of 270 × 270 mm in which a groove-shaped portion having a width of 1 mm and a depth of 0.6 mm is engraved within a range of 200 × 200 mm. In accordance with the above, a predetermined number of green sheets are laminated and hot-press molded at a pressure of 40 MPa and a temperature of 180 ° C. to obtain a separator material having a thickness of 200 × 200 mm, a thickness of 0.8 mm, and a thinnest wall thickness of 0.30 mm. Manufactured.

  A test piece (test material) was prepared from the manufactured separator material, and bending strength at room temperature and 80 ° C., fracture strain at room temperature and 80 ° C., specific resistance, contact resistance, 80 by the following methods. The amount of increase in water absorption after 336 hours by immersion in hot water at 0 ° C. and the releasability during molding were evaluated. The evaluation results are shown in Table 2.

Measurement of bending strength (MPa): Measured according to JIS R1601 (room temperature and 80 ° C.).
Measurement of fracture strain (%): Measured according to JIS R1601 (room temperature and 80 ° C.).

Measurement of specific resistance (mΩ · cm): Measured according to JIS C2525.
Measurement of contact resistance (mΩ · cm ² ): Measurement was performed at an energization amount of 1 A while contacting the test pieces with a pressure of 1 MPa.

  Measurement of increase in water absorption (%): A test piece having a thickness of 50 mm × 30 mm × 0.8 mm was immersed in distilled water, and heated with a heating device set at 80 ° C. for 336 hours. At this time, the weight of the test piece before and after immersion is measured with an electronic balance that can read up to the fourth decimal place, and the water absorption increase rate is calculated by {(weight after immersion−weight before immersion) / weight before immersion} × 100. did.

  Evaluation of releasability at the time of molding: As described above, an outer shape 270 in which a green sheet is punched into a predetermined shape, and a groove-shaped portion having a width of 1 mm and a depth of 0.6 mm is engraved within a range of 200 × 200 mm. In the case where a predetermined number of green sheets are laminated in a molding die of 270 mm in a mold and hot-pressure molded at a pressure of 40 MPa and a temperature of 180 ° C., when released immediately from the molding die The release property was evaluated as good.

  As shown in Table 2, all of Examples 1 to 3 (when using a high paranovolak type phenolic resin) according to the present invention have high bending strength and breaking strain, low specific resistance and contact resistance, and are immersed in hot water. The amount of increase in water absorption was small, the mold releasability at the time of molding was excellent, the thickness accuracy was also good, and the fuel cell separator material had satisfactory characteristics.

  On the other hand, in Comparative Examples 1 and 2, since a high ortho novolak type phenol resin was used as the curing agent, the room temperature fracture strain was low. Moreover, the increase amount by the water absorption after warm water immersion was also large, and the performance as a separator for fuel cells was inferior. Furthermore, in Comparative Example 2, a part of the separator surface adhered to the mold during hot press molding, and the separator was depressed, leaving a problem during mass production.

Examples 4 to 5 and Comparative Examples 3 to 4 (second manufacturing method)
The same resin solution as in the first production method and artificial graphite powder having an average particle size of 50 μm were mixed so that the weight ratio of the resin solid content to the graphite powder was 20:80, and sufficiently kneaded with a kneader. The blending ratio is shown in Table 3.

  The obtained kneaded product was aerated and vacuum dried to volatilize and remove the organic solvent, and then the kneaded product was pulverized and the particle size was adjusted to obtain a molding powder of 0.1 to 0.5 mm. Next, the molding powder is put into a preforming die, preformed at a temperature of 70 ° C. and a pressure of 3 MPa for 10 seconds to produce a plate-like preform, and using the same molding die as the first manufacturing method, Manufactured.

  A test piece (test material) was prepared from the manufactured separator material, and the bending strength at room temperature and 80 ° C., the breaking strain at room temperature and 80 ° C., and the specific resistance were performed in the same manner as in Examples 1-3. The contact resistance, the amount of increase in water absorption after 336 hours by hot water immersion at 80 ° C., and the releasability during molding were evaluated. The evaluation results are shown in Table 4.

  As shown in Table 2, each of Examples 4 to 5 (in the case of using a high paranovolak type phenolic resin) according to the present invention has high bending strength and breaking strain, low specific resistance and contact resistance. The amount of increase in water absorption was small, the mold releasability at the time of molding was excellent, the thickness accuracy was also good, and the fuel cell separator material had satisfactory characteristics.

  On the other hand, in Comparative Examples 3-4, since the high ortho novolak type phenol resin was used as a hardening | curing agent, the room temperature fracture | rupture distortion was low and the performance as a separator for fuel cells was inferior. Further, in Comparative Example 3, a part of the outer periphery of the separator was stuck at the time of hot-pressure molding, and the material on the separator surface adhered to the mold, causing the separator to peel off, leaving a problem during mass production.

Claims (3)

From a graphite / resin-cured molded body in which graphite powder is bound with a mixed resin containing epoxy resin as a main component and a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin and an imidazole curing accelerator as essential components A separator material for a polymer electrolyte fuel cell, characterized by comprising: A slurry is prepared by dispersing graphite powder in a mixed resin solution in which an epoxy resin is a main component and a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin, an imidazole curing accelerator and a dispersing agent are dissolved in an organic solvent. A method for producing a polymer electrolyte fuel cell separator, comprising producing a green sheet from the slurry by a doctor blade method, laminating the green sheets, and hot pressing. An epoxy resin is the main ingredient, a phenol resin curing agent containing 50% or more of a high paranovolak type phenolic resin and an imidazole curing accelerator are dissolved in an organic solvent, and the prepared mixed resin solution and graphite powder are kneaded, and then the organic solvent is added. Volatilizing and removing, then crushing the kneaded product, filling the molding powder into a preforming mold, pressurizing and preforming to produce a preform, and then inserting the preform into the mold and hot pressing A method for producing a polymer electrolyte fuel cell separator.