Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This inven~ion relates to a hydronium (H30+) polycrys-
talline superionic conductors and a method of making the same,
and to a proton conducting cell.
In one aspect, the invention employs an ion exchange
method of producing a hydronium polycrystalline superionic
conductor from suitable precursor materials; namely, NYS or
NGS where NYS = Na5YSi4012 and NGS = Na5GdSi4012.
These precursor solid ceramics are produced in accor-
dance with the inventors' co-pending Canadian Patent Applica-
tion Serial No. as yet unknown, filed 6 February, 1984 entit-
led "SILICATE SUPERIONIC CONDUCTORS AN~ METHOD OF MAKING
SAME".
Previously known ionic conductors such as ~/~" alumina
proton conductors have molecules in which there are a pair of
coplanar conducting planes; thus, when they are sintered as a
solid polycrystalline ceramic, they exhibit intermolecular
non-conductive boundaries, when the coplanar conducting planes
of one grain do not allign with an adjacent grain.
The precursor solids NYS and NGS of this invention have
molecules with 3-dimensional conductivity; thus, when
molecules are in juxtaposition forming a solid polycrystalline
ceramic, they do not exhibit any non-conductive boundaries
between adjacent grains. This and other features of the
invention make thç precursors, when converted, a highly suit-
able proton conductor~
The invention contemplates therefore converting NYS and
NGS by various means, into proton conductors with the general
formula
(~30 ,Na+)s(Re)si4ol2
where Re = Y or Gd.
sub.nom. HNYS or HNGS
According to the invention, the precursor solids, NYS
and NGS are converted into an appropriate intermediary precur-
sor of the following formula
(x~Na+)5(Re)si4ol2
where X+ is preferably either K+, Cs+ or mixtures of K+ and
Cs+ .
-- 2 --
Generally, X+ is an ion in the lA group of the Periodic
Table with an atomic weight greater than that of sodium, ie.
greater than 23.
These intermediary precursors, which themselves are
solid polycrystalline ceramics, are further converted to the
target proton conductor or hydronium ion conductor having the
following general formula; namely,
(H30+~Na+)5(Re)si40l2
where Re = Y or Gd
The need to establish the intermediary precursors of the
general formula
(X+,Na+)5(Re)si40l2
aforesaid stems from the need to establish a crystal lattice
structure for each of the molecules of the polycrystal such
that the physical size thereof and particularly of (X+,Na+) is
approximately that of hydronium (H30~). The X+ ion is at a
later stage replaced by H30+. Expanding the molecular lattice
this way to form the intermediary precursor, the X+ ion can
then be replaced by the hydronium without over-stressing the
lattice structure during creation of the target hydronium
conducting ceramic.
Those skilled in the art should know that the size of
various ions is as follows:
Ion Size
H30+ 1.5-~
Na+ 0.9 A
K+ 1.4 R
CS+ 2 . 67 A
Note, that K+ and Cs+ molecularly are sized equal to or
greater than that of H30+. Thus X+ is preferably K+ or Cs+.
To alter the interstitial space ofvthe molecular lattice
of the feed ceramic, the the invention contemplates a method
of ion exchange of the feed precursors, NYS or NGS, in melts
possessing certain specific mole fractions of sodium replacing
ions (members of the lA group of the Periodic Table) and par-
ticularly potassium and cesium. The melts are chlorides or
nitrates. As a further example, mixtures o~ potassium
~1,.3~
chloride and cesium chloride may be used as well as those mix-
tures in combination with sodium chloride, or corresponding
nitrates.
In all cases there are two steps of ionic exchange. The
feed ceramic is converted first into the intermediate ceramic
by immersing the same in an appropriate chloride melt afore-
said.
The immersion of the feed ceramic in the chloride or
nitrate melts is iterated through successive steps whereby the
dimensional spacing of the molecules of the feed ceramic are
altered and it is converted thereby into the intermediate
ceramic. At each iteration, the mole cOnCentratiGns of the
cations of the melt are changed.
The intermediate ceramic is then subjected to a field
assisted ionic exchange to achieve the target ceramic.
The target ceramic may then be employed directly as an
element in a proton conducting cell but preferably its sur-
faces are polished in order to eliminate boundary scaling
prior to its employment in such environment.
The invention therefore contemplates an intermediary
precursor being a solid polycrystalline ceramic of the general
formula
(x+lNa+)s (Re)Si412 i,
where Re = Y or Gd and X is an element ip Group lA of the
Periodic Table wi~h atomic rate greater tha~n 23. Preferably
X+ is K+ or Cs+ or mixtures of K+ and Cs+.
The invention also contemplates a proton conducting cell
comprising;
(a) the proton conductor (H30+,Na+)5(Re)Si4012 immersed
in a medium selected from steam and an aqueous solution;
and,
(b) electrodes attached to opposite sides of the cera-
mic, wherein Re = Gd or Y.
The invention also contemplates a process ~or producing
an hydronium conducting solid polycrystalline ceramic compri-
sing the steps of;
(a) selecting as a feed ceramic one of a group of pre-
cursor polycrystalline ceramics comprising Nasysi4ol2
3'7~
_ - 4 -
and NasGdSi4ol2;
(b) immersing the said feed ceramic of said step (a)
into a chloride or nitrate melt for a period of between
10 to 20 hours whereby to exchange, some of the sodium
ions thereof by ca~ions of the melt whereby to create an
intermediate ceramic of the general formula
(X+Na+)s(Re)si4Ol2
where X+ is K+, Cs+, or K+/Cs+
(c) immersing the said intermediate ceramic of step (b)
into an ion exchange solution of acid, while,
(d) subjecting the intermediate ceramic to a potential
difference across its surfaces while so immersed; and,
(e) maintaining the concentration of free hydrogen on
one side of the ceramic for a period of at least 3 hours
whereby to create a target ceramic of the general
formula
(H3o+lNa+)5(Re)si4ol2
where Re = Y or Gd
The acid aforesaid is preferably sulphuric or acetic~
The invention will now be described by way of example
and reference to the accompanying drawings in which;
Figure 1 is a flow chart of the method according to the
invention;
Figure 2 is the field assisted ionic cell that is emp-
loyed to execute the field assisted ionic exchange converting
the intermediate ceramic to the target ceramic.
Figure 3 is a distribution map of Na+,K+ and Gd3~ ions
in NKGS (an intermediate ceramic) as provided by electron
probe micro analysis (EPMA).
Figure 4 are x-ray diffraction patterns of the interme-
diate ceramics, figure 4A of NKGS and figure 4B of NCYS;
figure 4B(l) with unpolished surfaces; figure 4B(2) with
polished surfaces.
Figure 5 is a plot of weight change of NYS during itera-
tive immersion in CsCl melts of various cation concentrations,
according to the invention in an electrolysis cell;
Figure 6 plots temperature dependence for proton migra-
~L3'7~
-- 5 --
tion rate of a specimen target HNYS.
Figure 7, located with figure 2, is a proton cell emp-
loying the target proton conducting ceramics of the invention.
Referring to figures 1 and 2, a suitable feed ceramic is
composed of either NGS or NYS and is preferably created accor-
ding to our co-pending patent application aforesaid. This
feed ceramic which is selected at step llO in figure 1, is
placed, by step 1?0 into a chloride melt, where the anions
thereof are K+, Cs+ or melts thereof. In this respect, and
referring to figure 5, depending upon the chemical composition
and molar concentration of the melt 18, sodium ions in the
feed ceramic 10 are displaced by cations of the melt, in this
case mixtures of cesium and potassium. We prefer, however, to
"slow down" the reaction when the feed ceramics are placed in
the chloride melt. This can be simply achieved by mixing the
desirable chloride melts with sodium chloride. This makes for
a much less vicious exchange of the cations, since within the
melt there are already some sodium ionsO
Thus, preferably, we select chloride melts being either
potassium chloride and/or cesium chloride, and sodium chloride
to make the exchange less vicious. In the former case potas-
sium displaces the sodium ions in the feed ceramic and in the
latter cesium displaces the sodium ions. The chloride melt
can also be a mixture of sodium and cesium chloride of various
concentrations. The intermediate ceramic has thus a mixture
of Cs+ and K~ anions with Na+. Depending upon the molar con-
centrations of the chloride melt on the anions an intermediate
ceramic will result with various ratios of cesium ions or of
potassium ions or of both having displaced most or the sodium
ions in the feed ceramic. Figure 5 depicts, by the dash line,
such events when the feed ceramic, N(Re)S of step 110 is sel-
ected as Na5YSi4O12 and for various molar concentrations of
CsCl eg. (Cso.sNaO.s)Cl. The immersion time for step 120
ranges from 30 to 43 hours and is dependent upon the molar
fraction of the cation. (See Table I)
An identical linear plot is achieved when the feed cera-
mic step 110 is selected as NasGdSi4O12.
~2~,3'7~
-- 6 --
Where, in step 120, a melt of potassium chloride is
used, the linear relationship 55 is achieved and the resultant
intermediate ceramic has various displacements of the sodium
ion by the potassium ion from
(KgONa3g6)5YSi4012 to (Kg-ONa9o)sysi4ol2 that is
similar to figure 5. Similarly when the feed ceramic of step
is Na5GdSi4O12 identical plots are achieved. Thus the
intermediary precursor ceramics have the general formula
(X+Na+)5(Re)si40l2
where X+ preferably is K+, Cs+, or K+/Cs+ and Re is Y or Gd.
Accordingly, in the aforesaid procedure there are three diffe-
rent types of intermediary precursors fathered from NYS and
three additional types of intermediary precursors fathered
from NGS each resulting from which type of chloride melt is
used; cations of cesium; potassium; cesium and potassium; and
upon the actual melt used at step 120 and their respective
molar concentrations.
We prefer that the melt be of sodium and potassium at
20 step 120 and maintained at a temperature of 800C so that the
molar fraction is [K+]/([Na+] + IK-~]) = 0.45.
Referring to figure 3, the same represents the distri-
bution mass of an intermediate ceramic sample of KNGS, resul-
ting from step 130, when probed by EPMA for ions of Na+,K+ and
Gd3+ respectively as indicated on the chart. The regions
indicated "S.L." are the surface layers on either side of the
material sample and each shows, in the case of Gd3~ a marked
increase thereat. (The plot for a sample of NKYS is identi-
cal.) By polishing, step 140, these surfaces with jewellers'
rouge, the surface layers are removed and disappear. Confir-
ming the same and, referring to figure 4A with the interme-
diate as KNGS, when subjected to x-ray diffraction, the same
indicates a surface layer peak as approximately 33. This
peak disappears after the polishing, step 140.
For intermediate ceramics of CNYS, the x-ray diffraction
patterns of figure 4Bl and 4B2 more clearly show, in the
former, the surface layer, and in the latter, the removal of
~37~
_ 7 _
the surface layer by the disappearance of the secondary peak
at approximately 33 1/2 after polishing with jewellers'
rsuge .
From the foregoing, it is apparent, therefore, that
polishing of the surfaces of the intermediate ceramic, in
order to establish a heterogeneous quality thereof should be
conducted prior to the field assisted ion exchange step 150 of
figure 1 as a separate polishing step. As will become appa-
rent, when the potassium ion, or the cesium ion, or mixtures
of those are replaced with hydronium, hydronium ion concentra-
tion at the surface layer is avoided by having the surface
layer first removed by polishing prior to the ion exchange
step 150. If polishing does not take place, as it can be
omitted, the field assisted ion exchange step following, is
extraordinarily long in duration. Hence the polishing step
140 is to be preferred. After the field assisted ion exchange
occurs, step 150, the target ceramic (H3O+,Na+)s(Re)Si4O12
results as step 160.
Whether the feed ceramic be NYS or NGS and the melts
used in step 120 be cations of potassium or of cesium or mix-
tures thereof with sodium, in each instance, the intermediate
ceramic is either KNGS or KNYS; (K,Na)s(Re)Si4O12, where Re=Y
or Gd and exhibits a plot almost identical to that of figure 3
(KNGS) save and e~cept the peak for Gd3+, K+ and Na~ for KNYS
are shifted 50 to 100 microns (~m) as illustrated by arrow 30
in the figure, while that of Na+ is shifted to the right,
arrow 40, by the equivalent magnitude.
Now referring again to figure 1, after the intermediate
precursor is created at step 130 and the surfaces polished at
step 140, the intermediate precursor sample 10 is placed in a
field assisted ion exchange cell 15, figure 2, as step 150 of
figure 1. The ion exchange cell 15 has two compartments 12
and 14 and the intermediate precursor ceramic 10, is placed
therebetween in the centre as shown. Each side of the cell is
filled with an acid, preferably acetic, nitric, sulphuric or
mixtures thereof. Into both compartments extend platinum
electrodes 13 and 16 placed on either side of the sample 10
3~7~
- 8 --
with electrode 16 having hydrogen gas bubbled over its
surface. This ensures that the pH of the solution is main-
tained low. In due course, the target ceramic H3O+Na+(Y/G)S,
at step 160 is achieved. In this environment the voltage
across the electrodes 13 and 16 is between 1 and 240 volts and
the hydrogen gas is bubbled over the electrode 16 throughout
the duration of the exchange, preferably 20 hours at 40 volts.
At the end of 20 hours, the specimen 10 is converted
into the target ceramic HNYS, step 160. The identical proce-
dure is followed if the intermediate precursor ceramic speci-
men 10 is any of the NGS derivatives whereupon the target cer-
amic achieved is HNGS. Chloride melts of ion mixtures of pot-
assium, cesium and of sodium may also be used effectively.
Referring specifically now to figures 4B(l) and 4B(2)
one of the four intermediate cesium ceramics, CNYS, was selec-
ted for x-ray diffraction analysis and the pattern of figure
4B(l) emerged showing a secondary peak of 2~ at approximately
34 1/2. This represented a surface layer crust of impurity
product on the sample (Cs,Na)3 (Re)Si3Og. The sample surfaces
were polished using jeweller's rouge and, the x-ray diffrac-
tion results of the polished sample are shown in figure
4B(2). Note the former surface layer peak at 34 1/2 now dis-
appears.
The target ceramics of HNYS or HNGS are thus achieved at
step 160 from the appropriate intermediate ceramic 130 via
step 140 whatever the 6 intermediate ceramics be.
Returning to step 120, where cesium is used as a cation
in the melt, it is held at approximately 680C with a molar
concentration of [Cs+]/([Na+~ + [Cs+]) = 0.5 mole.
Where the mixture of cesium, potassium and sodium
cations in the chloride are used, as the melt 18 of step 120,
(Cs,K,Na)Cl is held to approximately 600C with a ratio of
[Cs+]:[K+]:LNa+] = 40:12:48 mole ~6. Alternatively, the
ratios can be [Cs+]:[K+]:[Na+]: = 30:35:35 and an appropria-
tely resulting target ceramic is achieved at 150, is either
HNGS type or HYGS type.
The aforesaid procedures are summarized in Table I.
t7~-~
_ g _
Referring to figure 7 the target ceramic achieved at
step 160 may be used in a hydrolysis cell 50 as the active
element for hydrolysing water, steam, into its constituent
components. The glass cell chamber 51 contains, therefore,
water over a burner or flame so as to generate in chamber 51,
steam or superheated steam, and a water condensation column 53
communicating therewithO The column has a water cooling
jacket through which as vla arrows 53 and 53' cooling water
flows. A sample of target material 10' resulting at ste2 160
of figure 1 is located at the lower orifice of an intermediate
capture chamber 54 whose lower end extends into the chamber
51 but above the water therein. The upper end of the capture
chamber 54 communicates through a water trap 56 into a collec-
tion column 59 where hydrogen gas is recovered.
Electrodes of platinum are attached to opposite surfaces
of the sample 10' with one of the conductors passing through a
sealing grommet 57 in the vessel wall 51 to the positive ter-
minal of a voltage source V. The negative terminal of the
source V is connected b~ a second conductor through a second
sealing grommet 58 disposed in the wall of the capture chamber
54 and thence passes to the opposite surface of the sample
10'. A voltage of 1 to 240 volts, preferably 50 volts or so,
is applied across the sample and hydronium ions migrate across
the sample from the steam 65 in chamber 51 into the collection
column 54 to become neutralized by the electric charge of the
voltage source V; releasing hydrogen gas into the collection
column 54.
The following two equations define the reaction, equa-
tion 1 in the glass chamber 51 and equation 2 in the capture
chamber 54.
3H20 - 2e~ ~ 2H30+ + 1/2 2~ (1)
2H30+ + 2e~ -~ 2H20(steam~) + H2~ (2)
From the foregoing it can be seen that electrons are
taken off of the water (steam 65) molecules in chamber 51
causing the molecules to turn into hydronium ions which then
migrate across the hydronium conductor 10'. Electrons are
~ ;.
- 10 --
given back to the hydronium ions (reassociated at the electode
in collection chamber 54) generating steam and hydrogen in the
collection chamber 54. In order to capture H2~, the steam
must be cooled down hence the water bath 56 through which the
steam is bubbled, condenses the same and the hydrogen gas H2~
is collected at the top of the collection column at pipette
59. The upper portion of the vessel 51 communicates through a
cooling tower or condenser with cooling water jacket flowing
via 53, 53'. Oxygen and steam are collected at the upper end.
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