4.1.2.1.2.1 Operating Principles of AEMs in VRFBs
Generally, AEMs are notorious for their low conductivity compared to PEMs due to
the high mobility of protons compared to other ions in solution. Indeed, the proton
conductivity of N212 at room temperature was found to be 69 mS cm-1
while the SO42-
conductivity of the AEM at room temperature was only 5 mS cm-1
. This difference in
ionic conductivity poses a critical challenge to deploy AEMs as alternatives to PEMs
for electrochemical applications. Fortunately, the high concentration of SO42-
in the
electrolytes of VRFBs mitigates the low conductivity of AEMs due to uptake of free
SO4 -
ions into the membrane. Furthermore, the existence of protons in the anolyte and
catholyte may also help lower the resistance of AEMs. To support these ideas, we
measured the conductivity of N212 and the experimental AEM after equilibration in 1
M VOSO4 + 2.5 M H2SO4 solution for 24 h. The apparent conductivity of N212 was
44 mS cm-1
, lower than its proton conductivity in pure water. This difference can be
rationalized by considering that some of the proton sites were occupied by vanadium
ions which had much lower mobility. Also, increased acid concentration has been
observed to decrease the conductivity of solutions and membranes [85-86].
Interestingly, under these conditions the measured conductivity of the AEM had
increased to 20 mS cm-1
, almost half of the conductivity of N212, making it promising
for practical applications.
The transport properties of vanadium ions through the AEM and N212 were
investigated by measuring their VO2+
permeability. For N212, the permeation of VO2+
could be visually observed by the change of the vanadium deficient solution from
clear to blue within 1 h. The VO2+
3.2 × 10-12 m2
s-1
, similar to literature values [16]. For the AEM, there was no solution
color change after one month measurement time and also no detectable VO2+
ion by
UV-vis analysis. This absence of VO2+
crossover indicated that the AEM has
excellent capability to repulse the transport of vanadium ions. Therefore, the AEM in
a VRFB can transport both proton and sulfate ions but retains vanadium ions, as
depicted in Fig. 4.6.
Figure 4.6 Functions of an AEM in a VRFB.
4.1.2.1.2.2 VRFB Performance under Different Current Densities
The charge-discharge curves of VRFBs assembled with AEM or N212 membranes at
the current density of 20 mA cm-2
are shown in Fig. 4.7. The VRFB assembled with
N212 had lower charge voltage than the VRFB assembled with the quaternary
ammonium functionalized poly(fluorenyl ether) AEM. This difference in charge
voltage was attributed to the lower resistance of N212 as discussed above since both
membranes had similar thickness. The average discharge voltage for both the VRFBs
were similar even though the discharge curve of the VRFB assembled with N212 was
slightly lower than the discharge curve of the VRFB assembled with the AEM. Since
N212 had lower resistance, its corresponding VRFB was expected to afford higher
average discharge voltage than the AEM counterpart. Nevertheless, N212 suffered !
from large vanadium permeation resulting in certain amount of short-circuit reactions,
which negatively affected the discharge voltage and thus offset its lower resistance.
Figure 4.7 Charge-discharge curves of VRFBs at 20 mA cm-2. The membranes used in the
VRFBs are indicated in the figure.
The coulombic efficiency (CE) of the VRFB assembled with N212 at this current
density was 81.2%, similar to literature values [87]. Surprisingly, the CE of the VRFB
assembled with the AEM was 100%, or as near to 100 % as we could measure,
suggesting there was no vanadium crossover or side reactions during cell operation.
Generally, the CE of a VRFB is influenced by vanadium permeation, electrode
corrosion, and side reactions of vanadium ions with oxygen or other solution
contaminants. For this experiment, the upper limit of charging voltage was kept to 1.7
V, which has been shown to avoid corrosion of the electrodes [88]. Furthermore,
electrolyte tank was sealed and purged with inert N2 gas carefully to eliminate side
the AEM, it is reasonable to achieve 100% CE, which paves the way for very high
efficiency VRFBs under a wide range of conditions, even during long periods of test.
Figure 4.8 Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of VRFBs at various current densities. The membranes used in the VRFBs are indicated in the figure.
The influence of current density on coulombic efficiency (CE), voltage efficiency
(VE) and energy efficiency (EE) of the VRFBs are shown in Fig. 4.8. It can be seen
that the VRFB assembled with AEM achieved 100% CE for all the current densities
tested. The CE of the VRFB assembled with N212 increased with increasing current
density, which was due to the decreased time allowed for vanadium permeation. The !
VE of both the VRFBs decreased with the increasing current density because of the
higher ohmic polarization at higher current densities. The VRFB with the N212
separator had a more pronounced advantage in VE with increasing current density as
compared to the VRFB assembled with AEM, suggesting a larger influence of
membrane resistance at higher current densities, as expected. The EE of the VRFB
assembled with the AEM decreased monotonically from 20 mA cm-2
to 80 mA cm-2
due to the decreasing VE, while the EE of the VRFB assembled with N212 exhibited
a peak value at 40 mA cm-2
which was the combination effect of its CE and VE. It is
worthwhile to mention that the VRFB assembled with the AEM had higher energy
efficiency than the VRFB assembled with N212 at current densities lower than 60 mA
cm-2
, making it highly desirable for medium/low current density VRFB developments.
4.1.2.1.2.3 VRFB Cycling Performance
Capacity change and CE change versus cycle number were investigated in VRFB
cycling tests, Fig. 4.9. Capacity fade of the VRFB assembled with N212 was observed,
while the VRFB assembled with the AEM showed no measureable capacity fade. This
observation correlated well with no vanadium crossover or no self-discharge in the
VRFB assembled with the AEM and was in good agreement with the 100% CE of the
battery. The absence of capacity fade guaranteed maintenance-free operation of the
VRFB, which has not been met so far in other separator work. The CEs of the VRFBs
were very stable during the cycling, Fig. 4.9. This phenomenon is typical in flow
Figure 4.9 Normalized capacity changes and coulombic efficiencies of VRFBs in the cyling test. The membranes used in the VRFBs are indicated in the figure.