Electrochemical gaseous diffusion half cell

This invention relates to an electrochemical half cell having a gas diffusion electrode as an anode or cathode, in which the gas space behind the gas diffusion electrode is optionally partitioned, for pressure equalization, into gas compartments which are joined to the gas diffusion electrode and which can be removed from the half cell or changed as a unit.

The use of gas diffusion electrodes in pressure-equalised electrochemical
 cells of large overall height necessitates the partitioning of the gas
 space into segments situated one above another, which are termed gas
 compartments, and which in the respective edge region have to be in
 contact with the electrode and sealed so that they are gas-tight. A half
 cell which has a basic construction such as this is described in DE 44 44
 114 Al. A disadvantage of the mode of construction disclosed in DE 44 44
 114 is the comparatively elaborate manner of making contact with and
 sealing the gas diffusion electrode. It is desirable for the electrode to
 be placed in electrical contact with the gas compartment and sealed in the
 smallest possible space, in order to keep the area of the electrode which
 is inactive as regards the electrode reaction as small as possible.
 The object of the present invention is to provide an electrochemical gas
 diffusion half cell which makes it possible to utilise the active
 electrode area as extensively as possible, wherein the electrode together
 with the gas compartment is optionally fashioned as a module so that it is
 removable, thus permitting prior electrode installation in the gas
 compartment so that the edge contact is gas-tight, and so that making
 electrical contact for the operation of the gas diffusion electrode in the
 half cell is simplified.
 A further object of the invention is to fashion the half cell and the
 electrode- and gas compartment module in particular so that modules of the
 half cell can also be replaced in a simple manner by a conventional
 electrode, for example a nickel electrode which produces hydrogen.
 This object is achieved according to the invention by providing
 compartments fitted with gas diffusion electrodes such that said
 compartments can be removed from the half cell, optionally even
 individually, and can be detached from their gas supply or pressure
 equalisation means and from the electrical connections. In this respect,
 the gas compartments are manufactured as compact shallow elements, the
 front face of which is blanked off by the gas diffusion electrode so that
 when the electrode is installed in the half cell no electrolyte emerges
 into the gas space and no gas emerges in the opposite direction from the
 gas compartment.
 The present invention relates to an electrochemical half cell based on a
 gas diffusion electrode as cathode or anode, having a gas space, which is
 formed from one or more gas compartments, for the gas diffusion electrode,
 having an ion exchange membrane, a holding structure for the installation
 of the electrode, electrical connections for the electrode, a gas inlet to
 the gas compartment and a pressure-equalised gas outlet to the gas
 compartment, a electrolyte feed line and an electrolyte discharge line,
 and having a housing for receiving the cell constituents, which is
 characterised in that the gas diffusion electrode is joined to the gas
 compartment to form a module which is detachably fastened to the holding
 structure, wherein the gas inlet and the gas outlet form a detachable
 connection to the gas compartment.
 In one preferred embodiment, the gas space is partitioned into a plurality
 of gas compartments which are optionally supplied with electrode gas
 independently of each other, and which are optionally pressure-equalised
 in relation to the electrolyte space located in front of the gas diffusion
 electrode.
 A half cell is particularly preferred in which, when there is a plurality
 of individual gas compartment modules, the individual modules are
 detachably fastened to the holding structure independently of each other.
 Embodiments are also particularly preferred in which the gas diffusion
 electrodes are fastened to the gas compartment in an easily detachable
 manner so as to permit their replacement by modified electrodes.
 The gas inlet and gas outlet are preferably designed as a flexible hose
 connection to which the gas compartment is attached, and which makes it
 easier to remove the gas compartments after the ion exchange membrane has
 been removed.
 Alternatively, it is also possible in particular to employ gas inlets
 manufactured as a coupling seal and designed as bubble channels, and
 correspondingly pressure-equalising immersed elements.
 In another preferred form of the half cell according to the invention the
 gas compartment modules are positioned in the half cell with the aid of
 structural elements, are electrically connected to the external power
 supply and are optionally sealed to such an extent that electrolyte from
 the electrolyte gap through which flow occurs cannot overflow in an
 uncontrolled manner into the rear, pressure-equalised space of the gas
 compartment, wherein the structural elements position the gas compartment
 modules so that the gas diffusion electrodes can serve simultaneously as
 an electrolyte gap for the passage of the electrolyte into the electrolyte
 space between the gas diffusion electrode and the ion exchange membrane.
 The electrical contact between the gas compartment modules and an external
 source of electric current, e.g a source connected to the half cell
 housing, can be improved in that, in addition to the press contacts in the
 edge region, the modules are brought into electrical contact with the aid
 of an auxiliary structure, which is in pressed contact with the half cell
 housing for example, from the rear face thereof, namely the face remote
 from the gas diffusion electrode, via flexible, electrically conducting
 contact elements (e.g. spring contacts).
 A low resistance supply of electric current to the electrode is made
 possible due to the electrical contact in the edge region and due to the
 contact of the electrode, which is distributed over the area of the
 electrode.
 In another preferred variant of the half cell according to the invention,
 the gas diffusion electrode is brought into electrical contact on its side
 facing the gas space of the gas compartment with the aid of a support grid
 on its face. The support grid is thereby in electrical contact with the
 gas compartment rear wall.
 A possible additional channel which is formed at the lower edge between the
 gas diffusion electrode and the gas compartment can serve to receive
 condensate which may possibly arise. The condensate can be discharged into
 the rear electrolyte space, together with the excess gas, via the gas
 outlet situated at the lowest point.
 In one preferred variant, any heterogeneity in the supply of fresh
 electrolyte to the electrolyte space situated in front of the gas
 diffusion electrode can be prevented by providing the half cell behind the
 electrolyte feed line with an additional electrolyte distributor which
 homogenises the flow of electrolyte over the width of the half cell. In
 this embodiment, there is optionally no flow through the
 pressure-equalising rear space. The rear space only communicates with the
 flow of electrolyte at the top end of the half cell.
 One particular advantage of the mode of construction of the half cell
 according to the invention is the possibility of easily operating the half
 cell also, when the gas compartment module is replaced, using conventional
 electrodes, particularly electrodes which produce hydrogen, e.g. nickel
 electrodes. In one preferred variant, the gas compartment modules are
 therefore fashioned so that they can be replaced by conventional
 electrodes, wherein the gas feed line to the gas compartment can
 optionally be blanked off or removed.
 The present invention also relates to the use of the electrochemical half
 cell according to the invention in an electrochemical cell particularly an
 electrolysis cell, for operation according to choice with gas compartment
 modules or with conventional electrodes, particularly with electrodes
 which produce hydrogen, especially activated nickel electrodes.

EXAMPLES
 Example 1
 An electrochemical half cell is constructed as follows:
 A mounting structure 11 and an auxiliary structure 12, which is connected
 to an external power lead (not shown), are fixedly anchored in a
 shell-like housing 13 which is impervious to electrolyte (see FIG. 3). A
 gas line 21 (see FIG. 1), an electrolyte feed line 20 and a combined
 electrolyte and gas outlet line 22 are led into the housing. The gas
 compartment unit 1, which comprises the gas diffusion electrode 5, is
 fixed on the holding structure 11 in the housing 13 by means of vertical
 clamping strips 8 and horizontal clamping strips 9. The half cell is
 closed by the membrane 40, which is seated on the surrounding edge of the
 housing 13 via a seal 41 (see FIG. 2), and is pressed, so that it is
 impervious to electrolyte, on the surrounding edge of the housing 13, e.g.
 by the flange-mounting of a further half cell, which is not shown. The
 flexible lines 6a, 6b, 6c, 6d (see FIG. 1) which lead off from the gas
 distributor are connected to the gas compartment unit via flanged
 connections to supports 6e in each case.
 The embodiment shown in FIG. 1 is partitioned into four gas compartments
 2a, 2b, 2c, 2d, which are each provided with pressure equalisation via gas
 overflow lines 7. The gas compartment 1 consists of the metal housing 2,
 which is connected to the current-carrying auxiliary structure 12 at
 pre-arched structural parts 16 via current contacts, e.g. spring contacts
 14 or brush contacts 15, or by direct contact (see the details shown in
 FIG. 4). The further supply of current is effected via the metal process
 contacts between the clamping strips 8, 9 and the holding structure 11,
 via the edge region of the housing 2. The contact resistance can be
 reduced by surface treatment, e.g. by gold plating. The gas compartment
 unit 1 also comprises feed lines and outlet lines 6e and 7 for the
 electrode gas, a supporting structure 3 for supporting the gas diffusion
 electrode 5, and an electrode support grid 4 on which the entire surface
 of the gas diffusion electrode 5 rests. Each of the four gas compartments
 1a, 1b, 1c, 1d comprises a gas diffusion electrode 5, which is attached in
 a gas-tight manner in the edge region of the gas compartment module and
 which at the same time is electrically connected here. When gas diffusion
 electrodes are employed which have a conductive back face, there is
 complete electrical contact via the electrode support grid.
 In operation, as shown in FIG. 5a, the electrolyte enters the electrolyte
 space 27 in front of the gas diffusion electrode 5 from the electrolyte
 feed line 20 through the electrolyte distributor 23, via holes 24 and
 further holes 10 in the horizontal clamping strips. The electrolyte flows
 through the electrolyte space 27 along the four gas compartment modules 1a
 to 1d and flows through holes in the uppermost horizontal clamping strip 9
 into the electrolyte collecting duct 26. From there, the electrolyte flows
 off via holes 25 into the rear space behind the gas diffusion electrode
 and is discharged from the half cell together with excess electrode gas
 via a downpipe 22 (see FIG. 5b). In operation as an electrolysis cell, the
 half cell is closed on its front face by an ion exchange membrane 40,
 which ensures the passage of ions corresponding to the electrolysis
 reaction concerned from the half cell according to the invention to a
 further connected half cell or optionally in the reverse direction.
 The electrode gas enters the half cell at feed line 21 and is distributed
 via a gas distributor 19 on to the feed lines 6a, 6b, 6c and 6d, which
 lead, via constrictions into the individual gas compartment modules for
 better gas distribution, to the four gas compartment modules 1a, 1b, 1c,
 1d (see FIG. 1). The electrode gas flows through the gas compartments in
 the longitudinal direction, and the unconsumed excess, together with any
 condensate arising, emerges from the gas compartments at the opposite end,
 through gas overflow connection pieces 7, towards the electrolyte in the
 electrolyte gap. Pressure equalisation is thereby effected. From the gas
 overflow lines 7 the gas bubbles upwards through the space situated behind
 the gas compartment modules and collects in the space above the end of the
 connection 22. From there the gas is discharged from the half cell through
 the downpipe 22.
 EXAMPLE 2
 For operation with a conventional electrode, the gas compartment modules 1a
 to 1d, together with the clamping strips 8 and 9, can be removed as
 indicated in FIG. 3 and replaced by a nickel cathode 30 comprising an
 auxiliary structure 31 which serves as a power lead and as a support. The
 gas inlets 6a to 6d are most usefully closed. The conventional electrode,
 e.g. an activated nickel electrode, is fixed to the mounting structure 11
 of the half cell by screwed connections 32 (see FIGS. 6 and 7). The
 hydrogen formed in the electrode reactions can be discharged, together
 with electrolyte draining off, via the downpipe 22.