Environmental Engineering Reference
In-Depth Information
At any electrolysis temperature, T
STEP
, and at unit activity, the reaction has electro-
chemical potential, E
◦
T
. This may be calculated from consistent, compiled unit activity
thermochemical data sets, such as the NIST condensed phase and fluid properties data
sets, (Chase 1998) as:
E
T
=−
G
◦
(T
E
ambient
≡
E
T
(T
ambient
);
=
T
STEP
)
/
nF;
25
◦
C,
here T
ambient
=
298
.
15K
=
and:
y
G
◦
(T
c
i
(H
◦
(C
i
,T)
TS
◦
(C
i
,T))
=
T
STEP
)
=
−
i
=
1
x
r
i
(H
◦
(R
i
,T)
TS
◦
(R
i
, T))
−
−
(8.2.3)
i
=
1
Compiled thermochemical data are often based on different reference states, while a
consistent reference state is needed to understand electrolysis limiting processes, includ-
ing water (Light et al., 2005; Licht, 1987). This challenge is overcome by modification
of the unit activity (a
1) consistent calculated electrolysis potential to determine the
potential at other reagent and product relative activities via the Nernst equation (Licht,
1985; Licht et al., 1991). Electrolysis provides control of the relative amounts of
reactant and generated product in a system. A substantial activity differential can also
drive
STEP
improvement at elevated temperature, and will be derived.
=
The potential variation with activity, a, of the reaction:
i
=
1
r
i
R
i
→
y
i
1
c
i
C
i
,is
=
given by:
RT
nF
ln
i
=
1
a(R
i
)
ri
E
T
−
E
T,a
=
·
y
i
(8.2.4)
1
a(C
i
)
ci
=
Electrolysis systems with a negative isothermal temperature coefficient tend to cool
as the electrolysis products are generated. Specifically in endothermic electrolytic pro-
cesses, the Equation 8.2.4 free-energy electrolysis potential, E
T
, is less than the enthalpy
based potential. This latter value is the potential at which the system temperature would
remain constant during electrolysis. This thermoneutral potential, E
tn
, is given by:
H(T)
nF
E
tn
(T
STEP
)
=−
;
(8.2.5)
b
a
H
(
T
STEP
)
=
c
i
H(C
i
,T
STEP
)
−
r
i
H(R
i
,T
STEP
)
i
=
1
i
=
1
Two general STEP implementations are being explored. Both can provide the ther-
moneutral energy to sustain a variety of electrolyses. The thermoneutral potential,
determined from the enthalpy of a reaction, describes the energy required to sustain
an electrochemical process without cooling. For example, the thermoneutral potential
we have calculated and reported for CO
2
splitting to CO and O
2
at unit activities,
from Equation 8.2.5, is 1.46(
0.01) V over the temperature range of 25-1400
◦
C. As
represented in Scheme 8.2.3 on the left, the standard electrolysis potential at room
±
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