I-V Characteristic and Performance of Three Electrode Alkaline Electrolyzer

Hydrogen production by electrolysis of water is seen as a promising technique as it is environment friendly and it can use renewable energy source for the production of hydrogen gas. However, this technology has less than 4% contribution to the production of commercial hydrogen in the market. This is due to the high electricity consumption of the water splitting reaction. The main challenge to make this technology ecient and economically viable is to develop cost effective and highly ecient electrolyzer. Here we have developed a three electrode electrolyzer in which an extra electrode is inserted between conventional electrodes: cathode and anode. This novel electrolyzer utilizes an extra voltage source which reduces the overpotential and increases the anode current of the cell, which is responsible for the hydrogen production. Furthermore, we observed that, the operating resistance of the cell decreases under the application of the new voltage source. Our results demonstrate that the introduction of third electrode improves the performance of electrolysis by consuming less power as compared to the traditional or conventional two electrode electrolyzer system.


Introduction
The non-renewable energy sources are limited, but the global demand of the energy is increasing. So, the challenge to meet the demand is more prominent than ever. The optimum utilization of renewable power sources is unquestionably single way to address the exponentially growing demand [1]. Hydrogen fuel is considered as one of the potential answers to the ever-increasing demand of the sustainable energy.
Hydrogen fuel has highest energy density value. It is highly e cient in energy conversion. When hydrogen is used in a fuel cell. It exhibits several advantages. Technologies related to hydrogen is looked upon by many. The hydrogen fuel employs various sustainable energy sources like solar and wind energy for the hydrogen production. This leads to uninterrupted supply of energy without the emission of carbon pollutants [2,3]. But, the 90% of total hydrogen fuel is extracted from the fossil fuels. This extraction process involves the release of harmful carbon components as byproduct. So, in order to improve the popularity of hydrogen fuel, the production process should be sustainable and e cient [3][4][5][6][7][8] Water electrolysis generally utilizes renewable energy sources to produce hydrogen fuel. So, it is taken as one of the ideal method for production of hydrogen [9,10]. The required input for the electrolysis are: water and electricity.
Hydrogen production by splitting of water is one of the important technologies. This process enables to convert electrical energy to pure hydrogen fuel. Furthermore, no harmful carbon derivative pollutants are released. So, researchers all around the world are motivated to develop the advanced water splitting cells for sustainable production of hydrogen.
Hydrogen production by electrolysis is technologically very simple. Furthermore, the gas collected during this process is pure [11,12]. So, the cost and effort needed for further puri cation is reduced. This process is also liked and recommended in the current situation because it is environment friendly and cost effective. To make the system even more e cient, the energy loss should be reduced and the cost of the equipment should be cut down. So, the prospect of hydrogen production regarding the economy and improved technology is a matter of interest [13][14][15][16][17][18][19][20]. However, the contribution of electrolysis technology to the overall hydrogen in global market is not more than 4%. The main reason for this is: the cost associated with the electricity required to overcome the energy losses that occurs to achieve the splitting of water [21]. Hence, it is important to come up with a novel idea of electrolyzer which increases the e ciency of hydrogen production.
In the present work, we rst time to our knowledge introduced three electrode system electrolyzer. We have modi ed the conventional two electrode electrolysis system to three electrodes by inserting a third electrode (grid) between cathode and anode. The introduction of this extra electrode was done so that the e ciency of the electrolyzer can be increased, thereby increasing the hydrogen production. Two voltage sources were given as input during the electrolysis process. This helped to overcome the ohmic loss of the reaction and thus decreased the overpotential, thereby increasing the hydrogen production.

Results
Three electrode electrolyzer design: The schematic diagram of the used three electrode electrolyzer is shown in Figure 1.
The components used in assembling the electrolyzer is shown in Fig. 2.
The circuit diagram for our experiment is shown in Fig. 3. Here, V1 and V2 are two power supplies. V2 supplies non pulsating dc voltage whereas, V1 supplies Half Wave Recti ed Pulsating DC (HWRPD) voltage. Since, negative terminal of V1 is connected to electrode 1, it acts as cathode. The potential difference between electrode 1 and grid due to V1 termed as Vcg. The positive terminal of V2 is connected to electrode2 and it acts as anode, and the potential difference across it is termed as Vag.
Similarly, the current through cathode, anode and grid are called Ic, Ia and Ig respectively. Since, grid electrode is common to both power supplies V1 and V2, we call this connection as common grid con guration.
The whole system can be viewed as two electrolyzers: one at LHS powered by V1 and other at RHS powered by source V2. Therefore, when V1=0 (made by removing V2), it acts as two electrode system ( RHS electrolyzer). Now when voltage V1 is applied, it acts as three electrode system and we study effect of V1 on I-V characteristics of electrolysis at RHS. Similarly, when V1= 0 it acts as two electrode electrolyzer at LHS, and we study the effect of voltage V2 on I-V characteristics of electrolysis at LHS. Here, Vcg = 0 have been made by plugging out the power supply Vcg. In the gure, it is seen that when Vcg = 0V, Ia starts to appear from Vag =2.35V, which is the reaction starting voltage (Vs) of water electrolysis. Above 2.35V, Ia increases linearly. As Vcg increases, Ia starts to appear at lower Vag values. Therefore, as Vcg increases Vs decreases and above Vs, Ia increases linearly as in Vcg = 0V case. Theoretically water starts to split at 1.23V, therefore, for Vcg = 0V case, the over potential is 1.12V. In similar ways, values for the reaction starting voltage and over potential are calculated for all other Vcg values and plotted in Figure 5.

I-V Charactersistics for Different HWRPD Voltage Vcg
In the gure it is seen that Vs decreases from 2.35V at Vcg=0V to 0.84V at Vcg=5V. similarly, the over potential is also observed to decrease with increase in Vcg. This shows that this external voltage source Vcg forces to reduce over potential of water electrolysis. Fig. 6 shows the plot for operating resistance of the cell versus applied HWRPD voltage Vcg. The operating resistance of the cell is calculated from the slope of linear t of Ia-Vag plot (Fig. 3). In the gure, it is seen that the operating resistance of the cell decreases with increase in Vcg. This shows that the application of external HWRPD voltage source Vcg results in reduction of operating resistance of the electrolyzer and hence improving its performance.
In Fig. 4, at xed voltage Vag along the vertical line (say Vag = 4V), it can be seen that Ia increases with the increase in Vcg. This shows that Ia and hence performance of the cell can be improved by the application of external HWRPD voltage Vcg.
In order to see effect of external voltage source Vag on cathode current density (Ic), we applied xed HWRPD voltage Vcg in left compartment of the cell (see Fig. 3) and measured Ic by varying Vag. Fig. 7 shows the variation of Ic with Vag at different HWRPD voltage Vcg. In the gure, it is seen that for Vcg < 0.5V, the cathode current density (Ic) is almost zero for all Vag voltage. When Vcg = 1V, 1.3V, 1.5V and 1.8V, cathode current density (Ic) starts to appear at Vag = 6.4V, 4.6V, 4V and 2.2V respectively. This infers that current density (Ic) appears even at Vcg < 2V (below Vs) when Vag is high. This shows that, the application of external voltage Vag forces the reaction to starts at lower Vcg value. shows that the application of high external voltage Vag forces to start splitting of water to its components at lower Vcg voltage reducing the over potential of reaction.
From the slope of best t line of Ic Vs Vcg for different xed Vag voltage curve, operating resistance of the cell is determined and plotted against Vcg and shown in Fig. 9. From the gure, it is seen that the operating resistance of cell decreases as Vag increases. Fig. 10 shows the variation of grid current density Ig with Vag at different HWRPD voltage Vcg. In the gure it is observed that for the xed voltages Vcg = 0V to 1.8V, on increase of Vag, the value of Ig decreases from the initial value zero (Ig = 0A) to negative value. At Vcg = 2V to 5V, Ig decreases from positive value to zero and then to negative values. This implies that depending on Vcg values the grid current (Ig) can be positive, zero and negative. The change in sign of grid current density (Ig) can be explained with the help of Fig. 3 above. In the gure, it is seen that due to voltage source Vcg, the current Ig ows in the left compartment in clockwise direction as shown. Similarly, due to voltage source Vag, current Ig ows in right compartment of cell in clock wise direction. Thus, direction of current ow in grid electrode due to Vcg is opposite to Vag. When Vcg > Vag, Ic > Ia, this results in positive grid current. Similarly, when Vcg < Vag, Ig becomes negative. When Vcg = Vag, Ig becomes zero. Therefore, depending on value of Vcg and Vag, Ig can be positive, negative or zero.
In order to see the behavior of each current: Ic, Ig and Ia towards Vcg, all three currents are plotted with variation of Vcg for two different Vag values(Vag=1V & 5V). Figure 11shows the variation of Ic, Ig and Ia with Vcg at xed Vag=1V and 5V. From the gure and also, analysing the data pro le, it is seen that these current Ic, Ia and Ig follow reaction Ic=Ia+Ig where Ic and Ia always take positive value and Ig can be positive or negative as discussed above.
In Fig. 11 (left), it is seen that when Vcg is low(Vcg=1V), Ig is always positive and current Ic= Ig +Ia. Therefore, when Vag is low ow of current in each compartment of cell is shown in the Fig. 12 (left). Similarly, when Vcg is high (Fig. 11(right)), current Ia is given as Ia = Ic + Ig. Current ow in each compartment in this case is shown in Fig. 12 (right).

Variation of total current density ( Ia+Ic ) with Power Input (Pin)
In Fig. 12, it can be seen that current Ia ows in RHS compartment and current Ic ows in LHS compartment of the cell. Therefore, current Ic produces oxygen and hydrogen in LHS compartment of cell and current Ia produces oxygen and hydrogen in RHS compartment of the cell because LHS side compartment of cell grid electrode acts as anode and in RHS side compartment of cell it acts as cathode. Therefore, due to voltage source Vcg, current Ic ows in the LHS compartment of cell producing H 2 at cathode and O 2 at the LHS of grid electrode. Similarly, current Ia ows in the RHS compartment of cell producing O 2 at anode and H 2 at the RHS of grid electrode (see Fig. 2). Thus Ic+Ia represents the total H 2 or O 2 producing current. In order to see the performance of cell with the application of HWRPD voltage Vcg, total H 2 or O 2 producing current (Ia+Ic) is plotted against total electrical power input Pin supplied to the cell for different xed Vcg values and shown in gure 12, Pin is calculated using relation Pin = Vcg×Ic + Vag×Ia. Here, it can be seen that for all Vcg, when Pin is increased, the total oxygen or hydrogen gas producing current density (Ic+Ia) is also increased (non-linearly). In gure it can be seen that for the xed Pin, say Pin = 5W, total current density (Ic+Ia) increases from 0.311A/cm 2 to 0.502A/cm 2 with the increase of Vcg. Increase in current density means increase in hydrogen and oxygen gas production. This shows that application of external supply Vcg results in high improvement of H 2 and O 2 production indicating high improvement of the e ciency of electrolyzer. In the Figure 12, the lowermost curve, is Ia + Ic vs Pin curve for Vcg=0 case. When Vcg=0, Ic=0. So, this is basically plot for two electrode system (RHS compartment of the cell). In the gure, (Ic+Ia) is lowest for all Pin, when Vcg=0V ( i.e. for two electrode system). As Vcg increases, curve shifts to higher (Ic+Ia) values. This implies that the total current density for three electrode system is always high as compared to two electrode system. Therefore, for xed Pin, Ic +Ia and hence, e ciency of the three electrode system electrolyzer increases with increase in external voltage source. However, this does not hold good for all Pin and Vcg values. For example, at Vcg=5V and Pin < 4.5W, the (Ic +Ia) curve (green lled ) deviates downward below (Ic +Ia) case for Vcg=4V case.
In order to see the effect of type of voltage source used for Vcg, all experiments mentioned above case were repeated by replacing HWRPD source Vcg wwith using Full wave recti ed dc as well as non pulsating dc source. The plot of all curve shows similar trend (not shown in the gure) in all cases and the best performance were seen with HWRPD source were used for Vcg.
Below we show only comparative study of (Ia + Ic) Vs Pin for HWRPD voltage replaced with DC voltage for Vcg is shown below. Fig. 13 shows the comparative plot of total current (Ic+Ia) with Pin for Vcg = 0V, 2V, 4V and 5V for DC and HWRPD voltage source. In the gure, it is seen that for xed Pin (say 5W), the total current density (Ic+Ia) is high when HWRPD voltage source is used for Vcg. This means that hydrogen or oxygen gas production e ciency is slightly higher at high Pin when HWRPD voltage source (Vcg) is used in place of non-pulsating DC source. This shows that electrolyzer e ciency for half wave recti ed voltage source is better than for non-pulsating DC voltage source.
We have also repeated all with grid (plate) electrode replaced b grid (steel mesh) electrode (to be published). We have also replicated whole experient in comman cathode con guration (to be published), and In all cases, the performance of the cell was improved.

Discussion
In the Figure 4 and 5, it is observed that the reaction starting voltage and hence the overpotential of the reaction decreases with the increase in external voltage Vcg. It is due to the fact that the overpotential in water electrolysis is cumulative effect of concentration potentials due to transportation of reactant and product, the activation overpotential of both cathode and anode and ohmic overpotential caused by the resistance of the electrolyte. Since, the operating resistance of the cell decreases with increase in Vcg, this results in decrease in reaction starting voltage and overpotential of the reaction. Also as discussed earlier, when Vcg=0, the system acts as two electrode system cell & as usual the reaction starting voltage is 2.3 V. When Vcg is increased from 0 to 2.3V, (Vcg < Vs), water electrolysis reaction does not occur in left compartment of the cell. Therefore, no marked change in the current (Ic) can be observed. Since, Vcg is in series with Vag, the total voltage Vcg + Vag appears across anode and cathode electrode. Therefore, even for the values of Vcg< Vs, the total voltage Vcg + Vag across cathode and anode forces the water splitting reaction to start even at lower Vcg values.
In conclusion, this work shows that the introduction of the third electrode in the conventional electrolyzer boosts the electrolysis process. It does so by reducing the reaction starting voltage and decreasing the overpotential of the electrolyzer. Moreover, the introduction of third electrode with certain power supply connection induces the increment in both Ic and Ia. These currents are responsible for the production of hydrogen in the cell. Thus, the addition of an extra electrode increases the hydrogen production of the system e ciently. Our work a show the practical approach to enhance the performance of the electrolytic cell for the production of carbon neutral hydrogen fuel.

Methods
Three electrode electrolyzer set up Our electrolyzer was constructed using two insulating polymer blocks and three steel plates. Three steel plates were used as electrodes. The insulating gasket were used in order to make the electrolyzer water tight (see Figure 2). The effective area of these electrodes is 1.5cm × 1.5cm. The polymer blocks were drilled at the top of the cell and gas pipes were tted on this so that the hydrogen and oxygen gases produced during the electrolysis can be collected at the burette. A thin plastic lm was adjusted at the upper part of the cell (not shown in the gure) between two gas pipes, to prevent the mixing of the gases.

I-V characteristics measurement:
We have used power supplier (BAKU BK-1502DD) for dc power supply and an AC variate with half wave recti er circuit for HWRPD source. The I-V characteristics for Ia, Ig and Ic versus Vag were taken by xing the Vcg at 0V, 0.3V, 1V, 1.3V, 1.5V, 1.8V, 2V, 2.5V, 3V, 4V and 5V.In order to measure the current and voltage, two FLUKE 15B+, one FLUKE 17B+ and one DT 830 D multimeters were used. The DT 830 D was calibrated according to FLUKE 15B+.

Chemicals used
1 Molar NaOH solution was used as the electrolyte to carry out the experiment. The electrodes were cleaned with Ethyl Alcohol (C 2 H 5 OH) before carrying out the electrolysis.

Declaration:
All these datas are authentic and can be shown upon request. Picture of homemade three electrode electrolyzer using locally available materials Power supply connection for three electrode electrolysis in common grid con guration.

Figure 4
Page 12/20 Variation of anode current density (Ia) with Vag at different xed HWRP DC voltage Vcg for three electrode electrolyzer system. Variation of operating resistance (R) of the electrolyzer ( The resistance R is derived from Ic-Vag characteristic curve)

Figure 10
Variation of grid current (Ig) with variable Vag at different xed HWRPD voltage Vcg.

Figure 11
Page 18/20 The variation of Ic, Ig and Ia with Vcg at xed Vag=1V ( left) and 5V(right) Figure 12 Direction of ow of currents when grid current is positive (right) and negative (left) Page 19/20

Figure 13
Variation of total current density (Ia +Ic) Vs Pin for HWRPD Vcg.

Figure 14
Comparison of total current density (Ic+Ia) Vs Pin for DC (un lled) symbol and pulsating DC ( lled) symbol