Appunti di laboratorio di Chemical technologies for the energy transition with lab

Comments about the experiences

Experience 1​

circuit overview and measurements​

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The first experiment was structured in two main sections, aimed at familiarizing ourselves

with both the Autolab system and the manual instrumentation.

Digital

The Autolab system was programmed to perform potentiostatic measurements, following a

specific set of instructions.​

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These included a procedure based on Linear Sweep Voltammetry (LSV).​

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Two different resistive loads (225 Ω and 950 Ω) were tested. The resulting current–voltage

curves clearly demonstrated a perfectly linear relationship between potential and current

across both configurations.

As an additional validation, a Cyclic Voltammetry (CV) test was performed by replacing the

LSV step with a CV cycle. The voltage sweep was configured as follows: 0 V → 1 V → –1 V

→ 0 V. The resulting cyclic voltammogram was almost completely symmetrical, indicating no

significant difference between the forward and reverse scans. This symmetry suggests the

absence of direction-dependent electrochemical behavior under the tested conditions.

Analytical​

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A similar set of measurements was conducted using manual instrumentation. In each case,

a known resistance was inserted into the circuit, and either the voltage across the system or

the current through it was controlled, depending on whether potentiostatic or galvanostatic

mode was employed. In both operational modes, the voltage–current characteristics

remained linear, confirming Ohm’s law behavior across the resistor.​

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Additionally, measurements of the voltage drop across the power source were compared

with those across the resistor. This allowed for an estimation of the residual resistance

contributed by the remaining circuit elements.​

This parasitic resistance was found to be very low, typically in the range of a few ohms, and

tended to decrease at higher current levels.

Experience 2​

Water Electrolysis

This experiment was carried out over two separate sessions. The first day was dedicated to

assembling and testing a basic electrolyser made of two water chambers separated by a

proton exchange membrane (PEM), into which electrodes were submerged. The second

session involved the use of a pre-assembled, optimized electrolyser, designed for higher

performance.

Unoptimized Electrolyser

The system was filled with approximately 150 mL of sulfuric acid solution. Likely due to the

initial heat generated by the dissolution of the acid, the temperature of the system gradually

dropped during the early phase of the current ramp. At this stage, the electrolysis process

had not yet generated sufficient heat through overpotential to offset the initial cooling.

However, as the current increased, the system began to heat up due to resistive and

electrochemical heating, and the temperature during the descending current scan was

consistently higher than during the ascending one.​

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This temperature rise had a noticeable impact on performance. From the current–potential

curve, it is evident that higher temperatures slightly improve the electrolyser’s efficiency: at

equal voltages, more current passes through. This can be attributed to enhanced electron

transfer kinetics and increased conductivity of the electrolyte at elevated temperatures.

Additionally, in the final backward scan points, the current becomes negative as the potential

drops below the threshold necessary for water electrolysis. At that point, the system begins

to behave like a fuel cell rather than continuing electrolysis.

Optimized Electrolyser

When the optimized electrolyser was activated, a gradual increase in current over time was

observed while voltage and temperature also slowly rose. This behavior mirrored what had

been seen in the unoptimized setup, suggesting a similar dynamic of thermal and

electrochemical development.​

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In both experimental runs (up to 3 A and 4 A), the electrolyser displayed consistent

performance, with slightly better efficiency during the descending scan phase. This trend is

likely due to reduced internal resistance as the cell temperature increased.​

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For the 4 A run, the current–potential curve revealed the characteristic shape associated

with the three main types of overpotential: an exponential region at low currents due to

charge-transfer limitations, a linear region at moderate currents, and finally, a tapering off at

higher currents indicating transport limitations. However, in this case, the transport-limited

plateau was not very pronounced.​

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In both tests, once the target current was reached and held steady, the power vs.

temperature curve displayed a downward trend. This observation supports the notion of

improved performance at elevated temperatures. Specifically, the internal resistance of the

cell decreases due to multiple factors, such as enhanced proton conductivity in the

membrane or accelerated electrochemical reaction kinetics. Altogether, these effects lead to

a drop in internal resistance, enabling the system to sustain the same current with a slightly

lower voltage.​

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Comparison​

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The comparison of the two electrolysis setups reveals substantial differences in terms of

energy efficiency and hydrogen production capability. The unoptimized system required a

significantly higher voltage, around 15 V, to reach a current of 1 A. This resulted in elevated

values for both specific energy and specific power, indicating a larger energy demand per

mole of hydrogen produced. Such a high operating voltage implies major ohmic losses, likely

due to poor ionic conductivity and an inefficient design. Specifically, in the unoptimized

configuration, protons generated during electrolysis had to traverse the bulk of the solution,

introducing substantial resistance.

In contrast, the optimized electrolyser, featuring a smaller PEM, tailored electrode materials

(Pt for the hydrogen evolution reaction and IrO₂ for the oxygen evolution reaction), and

improved architecture, was able to reach 4 A with only ~2 V applied. This setup significantly

reduced energy consumption per mole of hydrogen and led to much higher productivity,

thanks to increased current density and better electrochemical kinetics. The lower voltage

required is a direct indicator of improved charge transfer and ion transport processes.​

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In conclusion, while the unoptimized system consumes more energy per mole of hydrogen,

the optimized electrolyser achieves greater hydrogen production rates at a much lower

energy cost, demonstrating the advantages of refined design and material selection.

Experience 3​

Fuel cell​

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The fuel cell employed in this study was interfaced with a non optimized electrolyser, similar

to the one utilized in the prior experiment. This electrolyser provided a continuous supply of

hydrogen and oxygen, with flow rates regulated via a galvanometer. Upon system activation,

an initial delay was observed before reaching steady-state conditions at the open circuit

potential. This lag corresponded to the time required for the reactants to displace residual air

within the fuel cell chambers. Once equilibrium was achieved, the open circuit voltage

stabilized at approximately 0.84 V lower than the theoretical standard potential of 1.229 V for

water formation, as anticipated.​

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Subsequent tests were conducted by varying the external load resistance across the fuel cell

while maintaining a constant current supplied to the electrolyser. The resulting

current-voltage profiles exhibited two distinct regimes: an initial exponential region at low

current and high voltage, attributed to activation overpotentials associated with electron

transfer kinetics (analogous to the behavior observed in electrolyser), followed by a linear

region at higher current levels where ohmic losses dominated.​

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Overall, variations in the hydrogen and oxygen feed rates appeared to have minimal

influence on the power output and polarization curves across different trials. However, a

noticeable performance enhancement was observed during the second run. This

improvement is likely linked to progressive thermal stabilization and increasing membrane

hydration due to water generation within the cell. The relatively minor role of reagent flow

rates on cell performance may be explained by the absence of mass transport limitations;

even at a low external resistance of 1Ω, the operating conditions did not approach

diffusion-limited overpotentials. In fact, during the trial in which the electrolyser operated at

0.5 A, only 19% of the generated hydrogen was actually consumed.​

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For the analysis of specific energy and power metrics, three main criteria were considered.

First, energy values were normalized with respect to the mass of the fuel cell itself, reflecting

the fact that fuel (hydrogen) can be stored separately. Second, a theoretical energy density

was calculated based solely on hydrogen consumption (oxygen being assumed available

from ambient air) yielding a high specific energy of 32.7 kWh/kg, but a low volumetric energy

density of 2.9 Wh/L. Lastly, the energy output was normalized to the active surface area of

the electrodes, acknowledging that scaling up the system would primarily involve increasing

this area.

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These two apparatuses perform the same reaction in two opposite ways, and for both parts

of the energy that is either required or produced. In the following graph are represented the

curves for potential-current and power-current for the fuel cell and the unoptimized

electrolyser.

As can be seen from the graph above, the potential moves away from the theoretical E0

(1.229 V) for the electrochemical reaction, in positive for the electrolyser, as the resistances

have to be exceeded to perform the water splitting, and in negative for the fuel cell since the

potential is reduced by the internal resistances.​

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The electron transfer overpotential seems more impacting for the electrolyser than the fuel

cell, this is probably due to the electrode geometry, that in the case of the fuel cell was made

by a porous carbon support with dispersed active sites of platinum, while in the electrolyser it

was simply a platinum plate.

Experience 4​

Photovoltaic solar panels​

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This experience focused on the evaluation of the performance of commercial solar panels.​

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Halogen Lamps

Under both ambient light and halogen lamp illumination, we observed high values of both

series resistance (R ) and shunt resistance (R ). This is consistent with the graphical data:

s sh

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