Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 72 Volume 2, Issue 1
energy efficiencies have been plotted against time at
different applied voltages in Figure 3(a) and 3(b),
respectively. Time required to attain steady state is higher
at lower voltages and the steady values of current
efficiency are close to 100%. The steady values of current
efficiencies are almost same at all applied voltages and
also independent of it. That value may be considered as a
“saturation” value of current efficiency using simulated
wastewater containing ammonia in 0.50% (w/v) KOH.
Figure 3(b) shows that the steady state energy efficiency
reaches the highest value, i.e. 48.3% at an applied voltage
of 3 V. The figure also shows that the energy efficiency
decreases with increase in applied voltage obeying the
relation − energy efficiency inversely proportional to the
applied voltage for constant current efficiency.
3.2. Aqueous Urea solution with KOH
The variations of current efficiency for production of
hydrogen in alkaline medium of 0.50% (w/v) KOH
with/without urea at an applied voltage of 3 V have been
depicted in Figure 4(a). At different concentrations of
urea solution, the current efficiencies for production of
hydrogen have been found to vary from 59.9 to 76.3% at
1 hr, and to reach steady state values of 81.4–98.1% in 2
hr.
Figure 4: Effect of concentration of urea on (a) current
efficiency, and (b) energy efficiency for production of H
2
in 0.50% (w/v) KOH solution at 3 V.
Time required to attain steady state is independent of
concentration of urea, but the steady state values of
efficiencies decrease with increase in urea concentration.
The steady state current efficiency for production of
hydrogen with 0.50% (w/v) KOH have been found to vary
from 92.0 to 98.1% using 0.75–2.00% (w/v) urea
solutions, while the same varies from 81.4 to 93.7% using
2.50% (w/v) urea solution. The current efficiencies have
always been found to be well below 100% using higher
concentrations of urea. AAS analysis report of
electrolytes for pre- and post electrolysis shows that the
amount of Fe to increase from almost zero to about
0.15±0.01 mg/l (Table 1). The reason for not attaining
100% current efficiency at steady state in these cases may
therefore be the dissolution of Fe from electrode(s) to
electrolyte.
Table 1: Concentration of Fe quantified by AAS analysis
in the electrolytes – simulated alkaline wastewater (SAW)
containing ammonia/urea – before and after electrolysis.
Current efficiency obtained during electrolysis of urea in
simulated wastewater is almost constant. It has been also
found that lower applied voltage is favorable for getting
higher energy efficiency for constant current efficiency.
Investigations have therefore been carried out using
simulated wastewater containing urea at applied voltage
of 3 V only.
Figure 4(b) shows that the steady state energy efficiencies
vary from 45.5 to 48.5% using 0.75–2.00% (w/v) alkaline
urea solutions; but at 2.50% (w/v) urea in KOH, it
decreases slightly to 39.9–46.2%. The energy efficiency
achieved in this study is comparable with that of
electrolysis of dilute aqueous KOH solution alone as well
as using alkaline simulated ammonia wastewater.
FESEM (Field Emission Scanning Electrode
Microscopy) analyses of the surfaces of fresh material as
well as materials used as anode and cathode during
electrolysis of alkaline simulated wastewater containing
urea are presented in Figure 5. Three rows of the figure
denoted by (a), (b), and (c) are for fresh material, material
used as anode and cathode, respectively. Five columns are
for different magnification levels, i.e. 5, 25, 80, 100 and
120 kx, respectively. Surface of the fresh material is free
of grains and flakes; however, small grains and flakes
were found to form on the surfaces of the anode and
cathode. Growth of grains on the surface of anode is
higher than that on cathode, while number of flakes on the
surface of cathode is higher compared to that on anode.
The grains are assumed to be oxides of Fe, i.e. Fe
2
O
3
and
flakes are considered to be as hydroxide of Fe, i.e.
Fe(OH)
3
. Iron, in form of Fe(OH)
3
, might be leached out