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Lignin augmented hydrogen absorption by palladium cathode
~ ) Pergamon
Int. J. Hydrogen Energy, Vol. 19, No. 3, pp. 219 222, 1994
InternationalAssociationfor Hydrogen Energy Elsevier ScienceLtd. Printed in Great Britain 0360,3199/94 $6.00 + 0.00
LIGNIN A U G M E N T E D H Y D R O G E N ABSORPTION BY PALLADIUM CATHODE S. MURALIDHARAN,M. ANBU KULANDAINATHANand V. KAPAL! Radio Electrochemistry Section, Central Electrochemical Research Institute, Karaikudi--623 006, India (Received for publication 1 April 1993)
Abstract In alkaline water electrolysis, the incorporation of lignin is found to yield a faradaic efficiency of over 95% at constant potential at a Pt electrode. Cyclic voltammetry and hydrogen permeation studies carried out at the Pd/ 0.IM NaOH interface bring out the fact that lignin enhances the adsorpion of hydrogen on Pd during the cathodic polarization. The kinetics of the cathodic reaction is under mass transfer control in the case of the Pt/NaOH interface, whereas the same is controlled by the adsorption of hydrogen on the electrode surface in the case of the Pd/NaOH interface. Hence, Pt is preferable to Pd in alkaline water electrolysis, but Pd is to be preferred for efficient hydrogen storage.
INTRODUCTION Lignin is an aromatic noncarbohydrate polymer, and is secondary to cellulose as a principal constituent of wood, averaging about 26%. This polymer consists of hydroxyl and methoxyl groups [ 1]. Its electrochemistry is not fully known. Chum and Baizer thoroughly reviewed the literature published until then on the electrochemical oxidation and reduction of lignin, including delignification and other electrochemical treatments on lignin and related biomass materials [2,3]. The voltammetric study of various lignins have revealed that their half-wave potentials ranged from 0.23 to 0.33 V vs NHE, thereby demonstrating the ease of electrochemical oxidation of lignin [4, 5]. Recently, Lalvani and Rajagopal have experimented on the possible use of lignin in the augmentation of alkaline water electrolysis with the specific aim of increasing the faradaic efficiency of hydrogen production [6]. In fact, they have observed that hydrogen production from an aqueous solution of lignin dissolved in 1M N a O H is achieved at a high faradic efficiency (of the order of 95-100%) during constant potential electrolysis at Pt mesh electrodes. The rates of reaction are found to increase with applied electrode potential. The anodic current is found to increase with the increase of dissolved lignin. Cyclic voltammetry of the Pt electrode in the three ranges (i) 0- + 1.0 V, (ii) + 1.0- -- 1.0 V and (iii) -- 1.0-0 V vs _Ag/AgC1 electrode in 1M N a O H solutions containing 40 g of dissolved lignin in 1 litre has revealed three irreversible peaks, one anodic and two cathodic. Further, the three peak currents, when plotted against the square root of the scan rate give rise to a straight line with correlation factor of 0.99, indicating that the observed limiting current is mass transfer controlled and not due to 219
adsorption. In a conventional water electrolysis cell, hydrogen is produced at the cathode at low overpotentials, but it is the evolution of oxygen at the anode with high overpotentials that causes the cell to require a relatively high voltage. If the oxygen evolution overvoltage is suppressed and replaced by another thermodynamically and kinetically more favourable anodic reaction then it may be possible to produce hydrogen at overall lower cell voltages. In addition to this, good depolarization behaviour by lignin on the Pt anode was observed during electrolysis which increases with temperature, concentration of lignin and applied potential, and thus lignin augmented water electrolysis is achieved. The present investigation using Pd in the place of the Pt cathode in alkaline media has yielded opposing results in the following sense. It has been noticed that lignin augments hydrogen absorption during cathodic polarization of Pd in 0.1M NaOH. Though Pd and Pt belong to the same family, their individual kinetics and mechanisms of hydrogen evolution, adsorption and absorption are different from one another. This fact has been confirmed by cyclic voltammetric and hydrogen permeation studies. From these studies, it is inferred that Pd is a better electrode for the absorption (i.e. storage) of hydrogen than Pt, which is more suitable for the hydrogen evolution because of its difference in kinetics and mechanism.
EXPERIMENTAL A conventional three-electrode glass cell was used. A polycrystalline Pd disc of area 0.014 cm 2 and a Pt foil served as the working and counter electrode, respectively. All potentials are measured with respect to Hg/HgO/
220
S. MURALIDHARAN et al.
0.1M N a O H electrode. The cyclic voltammograms are recorded using a computerised electrochemical analyser BAS 100A. The working electrode, namely a Pd disc of purity 99.5% was polished mechanically using successively finer grades of emery sheets. All solutions were deaerated for 1 h by purging with pure nitrogen before their actual use. A.R grade reagents and battery grade lignin (99~o) were used for the study. Triple-distilled water was used for the preparation of all the solutions. Hydrogen permeation measurements were carried out using Devanathan and Stachurski's two compartment glass cells as described elsewhere [7, 8]. Hydrogen permeation currents were recorded using an X - Y - t Rikadenki recorder in the presence and absence of lignin. The Pd membrane of purity 99.5~o and of thickness 0.025 mm was used as such without any surface preparation for this experiment. All the experiments were carried out at a constant temperature 28 _ I°C.
7
- 2.86
rE v .. ~ ~~ ~"~ g ~-
1
~ i
I
*0.600
0 Potential /V
vs
-1.0 Hgl
HgO 1 0 . 1 M
NaOH
Fig. 1. Cyclic voltammogram of the Pd/0.1M NaOH interface for different concentrations of lignin (gpl) at sweep rate of 25 mV s -~. (1) Blank; (2) 0.5; (3) 1.0; (4) 5.0; (51 10.0.
RESULTS A N D D I S C U S S I O N Cyclic voltammetric studies
Cyclic voltammetric studies of the system Pd/0.1M N a O H was carried out between + 600 and - 1000 mV vs Hg/HgO/0.1M N a O H electrode at a fixed scan rate of 25 mV s 1, both in the presence and absence of lignin, whose concentration was varied from 0.5 to 10 grammes per litre (gpl). Figure 1 gives cyclic voltammograms of Pd/0.1M N a O H with lignin concentration varying from 0 to 10 gpl. Anodic and cathodic peaks indicate hydrogen adsorption and ionization, respectively. The organic species (lignin) does not undergo any oxidation or reduction within this potential range [4]. The adsorption ionization peaks (redox peaks) are reversible in nature. The cyclic voltammogram obtained in this study is quite akin to that obtained by Czerwinski and Marassi [9], especially in the case of the Pd/0.1M N a O H system. In Table 1, the values of anodic and cathodic peak potentials obtained during the above cyclic voltammetric study in the presence of different concentrations of lignin dissolved in 0.1M N a O H are given. It is quite interesting to note that both anodic and cathodic peak potentials become more cathodic with increasing concentration of dissolved lignin. Cathodic potential shifts are greater, of
the order of 80 mV in the case of anodic peaks and only of the order of 16 mV for cathodic peaks. In this connection, Srinivasan et al. [8] have established that the ionization potential for hydrogen at Pd/0.1M N a O H is - 300 mV vs Hg/HgO/0.1M N a O H electrode. Hence, the above cathodic peak is the desorption or ionization peak ( - 334- - 350 mV) and the anodic peak is the adsorption peak ( - 5 0 0 - - 5 8 0 m V ) . The organic species, namely lignin, is not found to affect the cathodic peak as it has affected the anodic peak. Perhaps this is another proof for the fact that lignin's adsorption also varies with concentration. In fact, up to a concentration of 10 gpl of lignin, the adsorption peak potential shifts up to - 6 0 0 from - 5 0 0 m V , and thereafter it slightly reduces to - 5 8 0 m V , remaining there up to a concentration of 10 gpl of lignin. In the case of Pd/0.1M NaOH, the peak currents at all concentrations of lignin are found to be increasing with increase of scan rate. But the graph of ip vs the square root of scan rate is not a straight line passing through the origin at all concentrations of lignin. Thus, the Pd/0.1M N a O H interface is completely different from the Pt/1M N a O H interface. Table 2 gives the cathodic and anodic
Table 1. Shift of peak potentials with various concentrations of lignin at scan rate of 25 mV s- 1
Concentration of lignin (gpl) 0 0.5 1.0 5.0 10.0
Peak potential w.r.t.
(Hg/HgO/0.1M NaOH)
Anodic (mV)
Cathodic (mV)
- 500 --556 --600 -- 580 --580
--334 -346 -350 - 348 -- 350
LIGNIN AUGMENTED HYDROGEN ABSORPTION
221
Table 2. Peak currents at peak potentials for different concentrations of lignin (gpl) from cyclic voltammetry Sweep rate (mV s - l )
Blank (mA cm 2)
0.5 (mA cm 2)
1.0 (mA cm 2)
5.0 (mA cm 2)
10.0 (mA cm 2)
Cathodic peak 10 20 50 100 250
2.4 2.8 4.2 9.5 15.8
2.4 4.1 4.3 9.8 18.6
2.4 4.3 5.9 10.6 17.9
1.9 4.1 6.9 9.1 17.6
1.9 4.1 7.3 9.2 17.3
Anodic peak 10 25 50 100 250
3.6 4.8 4.5 10.8 12.3
3.8 4.6 13.5 16.2 19.7
3.6 4.5 17.0 18.1 21.8
4.6 5.2 15.8 18.3 21.8
4.3 5.1 15.3 17.8 21.4
peak currents, respectively, for the Pd/0.1M N a O H interface in the absence and presence of dissolved lignin (from 0.5 to 10 gpl) at different scan rates (10-250 mV s-1 t. With the increase of scan rate at the same concentration of lignin, the cathodic and anodic peak currents are found to increase. At any concentration of lignin and at any scan rate, the cathodic peak current values are far less than the corresponding anodic peak current values.
Table 3. Permeation current (Ip) for different concentrations of lignin
Hydrogen permeation current measurement In order to have a better insight into the phenomenon that is taking place from - 3 0 0 to - 3 5 0 mV at the Pd/ 0.1M N a O H interface in the presence and absence of lignin dissolved in the electrolyte, Devanathan and Stachurski's technique was adopted to measure steady-state permeation currents (Ip), both in the presence and absence of lignin. The cathodic charging compartment and the anodic ionization compartment were filled with 0.1M N a O H solution. In the cathodic compartment, the concentration of lignin in 0.1M N a O H was varied from 0 to 40 gpl and this solution was employed for the cathodic charging of Pd with hydrogen at a fixed current density of 100/tA cm -2 for all the experiments. Steady-state permeation currents were recorded for each trial and the average was calculated. The average steady-state permeation currents for the Pd/0.1M N a O H interface in the absence and presence (different concentrations) of lignin are given in Table 3. The reproducibility of the percentage permeation current is _+ 2~o. Table 3 clearly indicates that as the concentration of lignin increases, the steady-state permeation current value also increases. But, one interesting trend is observed that at zero concentration of lignin, the permeation current is about 48/~A cm -2 and at the concentration of lignin of 0.5 gpl, the corresponding permeation current has reduced to a value of 3.6/~A cm -2. However, beyond a concentration of lignin of 0.5 gpl up to 40 gpl, the permeation current is increasing linearly. Both the decline in lp between 0 and 0.5 gpl concentration of lignin and the sudden linear
5.0 10.0 30.0 40.0
Concentration of lignin (gpl) Blank 0.5 1.0
Permeation current (,uA cm- 2) 48.0 3.6 7.2 42.0 96.0 198.0 268.0
increase of lp in the range of concentration of lignin of 0.5-40 gpl are brought out clearly in Fig. 2. Therefore, it can be inferred from the above observations that lignin at low concentrations (below 0.5 gpl) acts as a retarder of hydrogen uptake by Pd cathode. Beyond this concentration (namely 0.5 gpl) the same lignin acts as a promoter of hydrogen uptake by the Pd cathode in alkaline media. When I is reduced in the concentration range of lignin of 0-0.5 gpl, the mechanism of the H E R (hydrogen evolution reaction) at Pd/0.1M N a O H is one with the hydrogen adsorption step acting as the rate-determining step, while at the same interface in the presence of a higher concentration of lignin ( > 0.5 gpl), the mechanism is different in the sense that now the recombination step (chemical or electrochemical) is the rate-determining step. In the concentration range of 0.5 0 g p l of lignin, its effects on the anodic and cathodic peak potentials and on the respective peak current values in the cyclic voltammogram of Pd/0.1M N a O H are not so pronounced as in the case of hydrogen permeation experiments. This point needs more detailed investigation. However, a definite and indirect indication is evident in the increasing trend
222
S. MURALIDHARAN et al.
2601 ~
and scan rates in cyclic voltammetry, and in the increase in steady-state permeation currents. However, the initial fall in permeation current values up to and below a critical concentration of 0.5 gpl of lignin in solution in the case of the Pd/0.1M N a O H interface can be attributed to the HER mechanism [6], where the rate-determining step is hydrogen adsorption. Based on the permeation and voltammetric studies, it can be said that beyond the above initial concentration of lignin (0.5 gpl) the mechanism changes suddenly into a new one with a recombination (electrochemical or chemical or mixed) step as the rate-determining step. Hence, it is concluded that the lignin augments hydrogen absorption in Pd cathodes in alkaline media whereas it augments hydrogen evolution at the P t / N a O H interface.
6
180
}l0r / / ~
iO0
60
20 0
2
4
6 8 TIME(MIN)
10
12
14
Fig. 2. Hydrogen permeation current vs time graph for the Pd/ 0.1M NaOH interface for different concentrations of lignin (gpl). (1) Blank; (2) 0.5; (3) 5.0; (4) 10.0; (5) 30.0; (6) 40.0.
authors thank the Director, CECRI and Dr S. V. K. Iyer for their interest and for permission to publish this work. The authors (SM, MAN) thank CSIR for the award of Fellowship. Acknowledgements--The
REFERENCES of anodic and cathodic peak current values with the increase of concentration of lignin at any fixed scan rate, such that the concentration of lignin affects the mechanism and the kinetics of the HER at the P d / N a O H interface by altering the rate-determining step from adsorption to recombination. It is seen from the cyclic voltammograms that the anodic peak currents are far higher than the corresponding cathodic peak currents at all concentrations of lignin. Further, the anodic and cathodic peak currents increase with increasing scanning rates at any fixed concentration of lignin, indicating a situation of diffusion-cum-charge transfer controlled kinetics which is more pronounced with the anodic peak currents rather than the cathodic peak currents. CONCLUSION O n the basis of cyclic voltammetric and hydrogen permeation experiments, it can be concluded that the increase of concentration of lignin augments adsorption of hydrogen in the Pd/0.1M N a O H system (totally in contrast to that of the P t / N a O H interface) which is manifested in the increase of anodic and cathodic peak current values with increase of concentration of lignin
1. G. R. Chatwal, Organic Chemistry of Natural Products, Vol. II, p. 578. Himalaya, New Delhi (1986). 2. H. L Chum and M. M. Baizer, The electrochemistry of biomass and derived materials. ACS Monooraph 183, 246 (1985). 3. H. L. Chum and R. A. Osteryoung, SERI Report No. SERI/ TR-322-417, August (1981); and December (1982). 4. Yu. V. Vodzinskii, V. F. Koryttseva, N. P. Skvortsov, in J. Stradins (Ed.) Nov. Polarogr., Tezisy Dokl. Vses. Soveshch. Polarogr., 6th, p. 131. Zintane, Riga (1975); Chem. Abstr. 86, 47,028 (1977); Yu. V. Vodzinskii, V. F. Koryttseva, N. P. Skvortsov and V. V. Budylina, Tr. Khim., Khim. Tekhnol 3, 81 (1974); Chem. Abstr. 83, 149,361 (1975). 5. V. F. Koryttseva, Yu. V. Vodzinskii, N. P. Skvortsov, Khim. Drev. 6, 79 (1978); Chem. Abstr. 90, 77,525 (1979); V. F. Koryttseva, Yu. V. Vodzinskii and N. P. Skvortsov, Khim. Drev. l, 87 (1979); Chem. Abstr. 90, 188,745 (1979); V. F. Koryttseva, Yu. V. Vodzinskii, and N. P. Skvortsov, Khim. Drev. 2, 45 (1979); Chem. Abstr. 91, 78,555 (1979). 6. S. B. Lalvani and P. Rajagopal, J. Electrochem. Soc. 139, L1 (1992). 7. M. A. V. Devanathan and Z. Stachurski, Proc. R. Soc., Lond. A270, 90 (1962). 8. K. N. Srinivasan, R. Subramanian, V. Kapali and S. V. K. Iyer, B. Electrochem. 2, 547 (1986). 9. A. Czerwinski and R. Marassi, J. Electroanal. Chem. 322, 373 (1992).
Int. J. Hydrogen Energy, Vol. 19, No. 3, pp. 219 222, 1994
InternationalAssociationfor Hydrogen Energy Elsevier ScienceLtd. Printed in Great Britain 0360,3199/94 $6.00 + 0.00
LIGNIN A U G M E N T E D H Y D R O G E N ABSORPTION BY PALLADIUM CATHODE S. MURALIDHARAN,M. ANBU KULANDAINATHANand V. KAPAL! Radio Electrochemistry Section, Central Electrochemical Research Institute, Karaikudi--623 006, India (Received for publication 1 April 1993)
Abstract In alkaline water electrolysis, the incorporation of lignin is found to yield a faradaic efficiency of over 95% at constant potential at a Pt electrode. Cyclic voltammetry and hydrogen permeation studies carried out at the Pd/ 0.IM NaOH interface bring out the fact that lignin enhances the adsorpion of hydrogen on Pd during the cathodic polarization. The kinetics of the cathodic reaction is under mass transfer control in the case of the Pt/NaOH interface, whereas the same is controlled by the adsorption of hydrogen on the electrode surface in the case of the Pd/NaOH interface. Hence, Pt is preferable to Pd in alkaline water electrolysis, but Pd is to be preferred for efficient hydrogen storage.
INTRODUCTION Lignin is an aromatic noncarbohydrate polymer, and is secondary to cellulose as a principal constituent of wood, averaging about 26%. This polymer consists of hydroxyl and methoxyl groups [ 1]. Its electrochemistry is not fully known. Chum and Baizer thoroughly reviewed the literature published until then on the electrochemical oxidation and reduction of lignin, including delignification and other electrochemical treatments on lignin and related biomass materials [2,3]. The voltammetric study of various lignins have revealed that their half-wave potentials ranged from 0.23 to 0.33 V vs NHE, thereby demonstrating the ease of electrochemical oxidation of lignin [4, 5]. Recently, Lalvani and Rajagopal have experimented on the possible use of lignin in the augmentation of alkaline water electrolysis with the specific aim of increasing the faradaic efficiency of hydrogen production [6]. In fact, they have observed that hydrogen production from an aqueous solution of lignin dissolved in 1M N a O H is achieved at a high faradic efficiency (of the order of 95-100%) during constant potential electrolysis at Pt mesh electrodes. The rates of reaction are found to increase with applied electrode potential. The anodic current is found to increase with the increase of dissolved lignin. Cyclic voltammetry of the Pt electrode in the three ranges (i) 0- + 1.0 V, (ii) + 1.0- -- 1.0 V and (iii) -- 1.0-0 V vs _Ag/AgC1 electrode in 1M N a O H solutions containing 40 g of dissolved lignin in 1 litre has revealed three irreversible peaks, one anodic and two cathodic. Further, the three peak currents, when plotted against the square root of the scan rate give rise to a straight line with correlation factor of 0.99, indicating that the observed limiting current is mass transfer controlled and not due to 219
adsorption. In a conventional water electrolysis cell, hydrogen is produced at the cathode at low overpotentials, but it is the evolution of oxygen at the anode with high overpotentials that causes the cell to require a relatively high voltage. If the oxygen evolution overvoltage is suppressed and replaced by another thermodynamically and kinetically more favourable anodic reaction then it may be possible to produce hydrogen at overall lower cell voltages. In addition to this, good depolarization behaviour by lignin on the Pt anode was observed during electrolysis which increases with temperature, concentration of lignin and applied potential, and thus lignin augmented water electrolysis is achieved. The present investigation using Pd in the place of the Pt cathode in alkaline media has yielded opposing results in the following sense. It has been noticed that lignin augments hydrogen absorption during cathodic polarization of Pd in 0.1M NaOH. Though Pd and Pt belong to the same family, their individual kinetics and mechanisms of hydrogen evolution, adsorption and absorption are different from one another. This fact has been confirmed by cyclic voltammetric and hydrogen permeation studies. From these studies, it is inferred that Pd is a better electrode for the absorption (i.e. storage) of hydrogen than Pt, which is more suitable for the hydrogen evolution because of its difference in kinetics and mechanism.
EXPERIMENTAL A conventional three-electrode glass cell was used. A polycrystalline Pd disc of area 0.014 cm 2 and a Pt foil served as the working and counter electrode, respectively. All potentials are measured with respect to Hg/HgO/
220
S. MURALIDHARAN et al.
0.1M N a O H electrode. The cyclic voltammograms are recorded using a computerised electrochemical analyser BAS 100A. The working electrode, namely a Pd disc of purity 99.5% was polished mechanically using successively finer grades of emery sheets. All solutions were deaerated for 1 h by purging with pure nitrogen before their actual use. A.R grade reagents and battery grade lignin (99~o) were used for the study. Triple-distilled water was used for the preparation of all the solutions. Hydrogen permeation measurements were carried out using Devanathan and Stachurski's two compartment glass cells as described elsewhere [7, 8]. Hydrogen permeation currents were recorded using an X - Y - t Rikadenki recorder in the presence and absence of lignin. The Pd membrane of purity 99.5~o and of thickness 0.025 mm was used as such without any surface preparation for this experiment. All the experiments were carried out at a constant temperature 28 _ I°C.
7
- 2.86
rE v .. ~ ~~ ~"~ g ~-
1
~ i
I
*0.600
0 Potential /V
vs
-1.0 Hgl
HgO 1 0 . 1 M
NaOH
Fig. 1. Cyclic voltammogram of the Pd/0.1M NaOH interface for different concentrations of lignin (gpl) at sweep rate of 25 mV s -~. (1) Blank; (2) 0.5; (3) 1.0; (4) 5.0; (51 10.0.
RESULTS A N D D I S C U S S I O N Cyclic voltammetric studies
Cyclic voltammetric studies of the system Pd/0.1M N a O H was carried out between + 600 and - 1000 mV vs Hg/HgO/0.1M N a O H electrode at a fixed scan rate of 25 mV s 1, both in the presence and absence of lignin, whose concentration was varied from 0.5 to 10 grammes per litre (gpl). Figure 1 gives cyclic voltammograms of Pd/0.1M N a O H with lignin concentration varying from 0 to 10 gpl. Anodic and cathodic peaks indicate hydrogen adsorption and ionization, respectively. The organic species (lignin) does not undergo any oxidation or reduction within this potential range [4]. The adsorption ionization peaks (redox peaks) are reversible in nature. The cyclic voltammogram obtained in this study is quite akin to that obtained by Czerwinski and Marassi [9], especially in the case of the Pd/0.1M N a O H system. In Table 1, the values of anodic and cathodic peak potentials obtained during the above cyclic voltammetric study in the presence of different concentrations of lignin dissolved in 0.1M N a O H are given. It is quite interesting to note that both anodic and cathodic peak potentials become more cathodic with increasing concentration of dissolved lignin. Cathodic potential shifts are greater, of
the order of 80 mV in the case of anodic peaks and only of the order of 16 mV for cathodic peaks. In this connection, Srinivasan et al. [8] have established that the ionization potential for hydrogen at Pd/0.1M N a O H is - 300 mV vs Hg/HgO/0.1M N a O H electrode. Hence, the above cathodic peak is the desorption or ionization peak ( - 334- - 350 mV) and the anodic peak is the adsorption peak ( - 5 0 0 - - 5 8 0 m V ) . The organic species, namely lignin, is not found to affect the cathodic peak as it has affected the anodic peak. Perhaps this is another proof for the fact that lignin's adsorption also varies with concentration. In fact, up to a concentration of 10 gpl of lignin, the adsorption peak potential shifts up to - 6 0 0 from - 5 0 0 m V , and thereafter it slightly reduces to - 5 8 0 m V , remaining there up to a concentration of 10 gpl of lignin. In the case of Pd/0.1M NaOH, the peak currents at all concentrations of lignin are found to be increasing with increase of scan rate. But the graph of ip vs the square root of scan rate is not a straight line passing through the origin at all concentrations of lignin. Thus, the Pd/0.1M N a O H interface is completely different from the Pt/1M N a O H interface. Table 2 gives the cathodic and anodic
Table 1. Shift of peak potentials with various concentrations of lignin at scan rate of 25 mV s- 1
Concentration of lignin (gpl) 0 0.5 1.0 5.0 10.0
Peak potential w.r.t.
(Hg/HgO/0.1M NaOH)
Anodic (mV)
Cathodic (mV)
- 500 --556 --600 -- 580 --580
--334 -346 -350 - 348 -- 350
LIGNIN AUGMENTED HYDROGEN ABSORPTION
221
Table 2. Peak currents at peak potentials for different concentrations of lignin (gpl) from cyclic voltammetry Sweep rate (mV s - l )
Blank (mA cm 2)
0.5 (mA cm 2)
1.0 (mA cm 2)
5.0 (mA cm 2)
10.0 (mA cm 2)
Cathodic peak 10 20 50 100 250
2.4 2.8 4.2 9.5 15.8
2.4 4.1 4.3 9.8 18.6
2.4 4.3 5.9 10.6 17.9
1.9 4.1 6.9 9.1 17.6
1.9 4.1 7.3 9.2 17.3
Anodic peak 10 25 50 100 250
3.6 4.8 4.5 10.8 12.3
3.8 4.6 13.5 16.2 19.7
3.6 4.5 17.0 18.1 21.8
4.6 5.2 15.8 18.3 21.8
4.3 5.1 15.3 17.8 21.4
peak currents, respectively, for the Pd/0.1M N a O H interface in the absence and presence of dissolved lignin (from 0.5 to 10 gpl) at different scan rates (10-250 mV s-1 t. With the increase of scan rate at the same concentration of lignin, the cathodic and anodic peak currents are found to increase. At any concentration of lignin and at any scan rate, the cathodic peak current values are far less than the corresponding anodic peak current values.
Table 3. Permeation current (Ip) for different concentrations of lignin
Hydrogen permeation current measurement In order to have a better insight into the phenomenon that is taking place from - 3 0 0 to - 3 5 0 mV at the Pd/ 0.1M N a O H interface in the presence and absence of lignin dissolved in the electrolyte, Devanathan and Stachurski's technique was adopted to measure steady-state permeation currents (Ip), both in the presence and absence of lignin. The cathodic charging compartment and the anodic ionization compartment were filled with 0.1M N a O H solution. In the cathodic compartment, the concentration of lignin in 0.1M N a O H was varied from 0 to 40 gpl and this solution was employed for the cathodic charging of Pd with hydrogen at a fixed current density of 100/tA cm -2 for all the experiments. Steady-state permeation currents were recorded for each trial and the average was calculated. The average steady-state permeation currents for the Pd/0.1M N a O H interface in the absence and presence (different concentrations) of lignin are given in Table 3. The reproducibility of the percentage permeation current is _+ 2~o. Table 3 clearly indicates that as the concentration of lignin increases, the steady-state permeation current value also increases. But, one interesting trend is observed that at zero concentration of lignin, the permeation current is about 48/~A cm -2 and at the concentration of lignin of 0.5 gpl, the corresponding permeation current has reduced to a value of 3.6/~A cm -2. However, beyond a concentration of lignin of 0.5 gpl up to 40 gpl, the permeation current is increasing linearly. Both the decline in lp between 0 and 0.5 gpl concentration of lignin and the sudden linear
5.0 10.0 30.0 40.0
Concentration of lignin (gpl) Blank 0.5 1.0
Permeation current (,uA cm- 2) 48.0 3.6 7.2 42.0 96.0 198.0 268.0
increase of lp in the range of concentration of lignin of 0.5-40 gpl are brought out clearly in Fig. 2. Therefore, it can be inferred from the above observations that lignin at low concentrations (below 0.5 gpl) acts as a retarder of hydrogen uptake by Pd cathode. Beyond this concentration (namely 0.5 gpl) the same lignin acts as a promoter of hydrogen uptake by the Pd cathode in alkaline media. When I is reduced in the concentration range of lignin of 0-0.5 gpl, the mechanism of the H E R (hydrogen evolution reaction) at Pd/0.1M N a O H is one with the hydrogen adsorption step acting as the rate-determining step, while at the same interface in the presence of a higher concentration of lignin ( > 0.5 gpl), the mechanism is different in the sense that now the recombination step (chemical or electrochemical) is the rate-determining step. In the concentration range of 0.5 0 g p l of lignin, its effects on the anodic and cathodic peak potentials and on the respective peak current values in the cyclic voltammogram of Pd/0.1M N a O H are not so pronounced as in the case of hydrogen permeation experiments. This point needs more detailed investigation. However, a definite and indirect indication is evident in the increasing trend
222
S. MURALIDHARAN et al.
2601 ~
and scan rates in cyclic voltammetry, and in the increase in steady-state permeation currents. However, the initial fall in permeation current values up to and below a critical concentration of 0.5 gpl of lignin in solution in the case of the Pd/0.1M N a O H interface can be attributed to the HER mechanism [6], where the rate-determining step is hydrogen adsorption. Based on the permeation and voltammetric studies, it can be said that beyond the above initial concentration of lignin (0.5 gpl) the mechanism changes suddenly into a new one with a recombination (electrochemical or chemical or mixed) step as the rate-determining step. Hence, it is concluded that the lignin augments hydrogen absorption in Pd cathodes in alkaline media whereas it augments hydrogen evolution at the P t / N a O H interface.
6
180
}l0r / / ~
iO0
60
20 0
2
4
6 8 TIME(MIN)
10
12
14
Fig. 2. Hydrogen permeation current vs time graph for the Pd/ 0.1M NaOH interface for different concentrations of lignin (gpl). (1) Blank; (2) 0.5; (3) 5.0; (4) 10.0; (5) 30.0; (6) 40.0.
authors thank the Director, CECRI and Dr S. V. K. Iyer for their interest and for permission to publish this work. The authors (SM, MAN) thank CSIR for the award of Fellowship. Acknowledgements--The
REFERENCES of anodic and cathodic peak current values with the increase of concentration of lignin at any fixed scan rate, such that the concentration of lignin affects the mechanism and the kinetics of the HER at the P d / N a O H interface by altering the rate-determining step from adsorption to recombination. It is seen from the cyclic voltammograms that the anodic peak currents are far higher than the corresponding cathodic peak currents at all concentrations of lignin. Further, the anodic and cathodic peak currents increase with increasing scanning rates at any fixed concentration of lignin, indicating a situation of diffusion-cum-charge transfer controlled kinetics which is more pronounced with the anodic peak currents rather than the cathodic peak currents. CONCLUSION O n the basis of cyclic voltammetric and hydrogen permeation experiments, it can be concluded that the increase of concentration of lignin augments adsorption of hydrogen in the Pd/0.1M N a O H system (totally in contrast to that of the P t / N a O H interface) which is manifested in the increase of anodic and cathodic peak current values with increase of concentration of lignin
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