3.2.2 Glucose electro-oxidation in alkaline medium
The glucose electro-oxidation performance on bismuth modified electrode surface was studied in 1 M NaOH + 1 mM glucose solution. Figure 6a represents the CV curves for glucose oxidation on Bi-2 and Bi-4 modified electrode surface at 100 mVs− 1 scan rate. Large and wide oxidation and reduction peaks were observed in this study. The enhanced current values indicate the electro-oxidation of glucose. The oxidation peak currents were measured as 34.61 mAcm− 2 at -0.34 V and 33.73 mAcm− 2 at -0.11 V for Bi-4 and Bi-2 electrocatalyst suggesting the slightly better kinetics of glucose electro-oxidation on bismuth nanosphere electrocatalyst of smaller diameter. Inset shows the broader view of oxidation current on Bi-4 tailored GCE surface. We clearly noticed two anodic peaks A1 and A2 at -0.39 V and − 0.34 V respectively. The A1 peak is mainly allocated to the dehydrogenated adsorption of glucose on the active surface of Bi [6, 28, 29]. The next anodic peak A2 is attributed to the further electro-oxidation of gluconate to 2-keto-gluconate via four-electron transfer pathway [6]. The proposed electro-oxidation mechanism of glucose on bismuth modified electrode surface has been reported by the following equations [6, 30].
Bi + OH− \(\to\) Bi-(OH)ads(1−n) + ne− (2)
C6H12O6 + Bi-(OH)ads \(\to\) Bi-(C6H11O6)ads + H2O (3)
Bi-(C6H11O6)ads \(\to\) C6H10O6 + Bi + e− + H+ (4)
From the above equations, it is clear that Bi-(OH)ads active sites plays an important role on glucose electro-oxidation. Bi-(OH)ads sites on the Bi surfaces act as the active species for glucose oxidation. Once the electrolyte solution is in contact with the Bi catalyst surface, the OH− ions in test solution easily adsorb on the electrode surface to form Bi-(OH)ads. The glucose molecules adsorb into the Bi-(OH)ads sites in which they can be oxidized [28]. With further positive sweep of potential, a dramatic decrease in oxidation peak currents were noticed for both eelectrocatalysts because of the formation of Bi oxides on the electrode surfaces which reduce the Bi-(OH)ads sites and may limit the oxidation of glucose and intermediates [28]. In backward scan the cathodic peaks observed at -1.01 V and − 1.21 V for Bi-4 and Bi-2 respectively, this indicates the reduction of bismuth oxide to metallic bismuth [6]. The schematic diagram of glucose electro-oxidation in alkaline medium on Bi/GCE surface is demonstrated in Fig. 7.
We performed CV at various scan rates to investigate the effect of scan rate on glucose electro-oxidation kinetics. Figure 8a displays the CV curves for Bi-4 catalyst in 1 M NaOH + 1 mM glucose solution at different scan rates. It is clear that electrochemical kinetics enhances with scan rates. Both anodic and cathodic peaks shift to the more anodic and cathodic direction respectively. Forward and backward peak currents plotted against the square root of scan rates which is presented in Fig. 8b. The plot of peak potentials versus scan rates is also exhibited in Fig. 8c. The proper linear relationship in Fig. 8b and Fig. 8c was observed. The linear relationship between peak current and square root of scan rate satisfies the following Randles–Sevcik equation.
ip = 0.4463nFAC (nFνD)1/2/(RT)1/2 (5)
Where ip = current maximum in amps, n = number of electrons transferred in the redox event, A = electrode area in cm2, F = Faraday Constant in C mol− 1, D = diffusion coefficient in cm2/s, C = concentration in mol/cm3, ν = scan rate in V/s, R = Gas constant in J K− 1 mol− 1, T = temperature in K. The current satisfying Eq. 5 i.e proportional with square root of scan rate indicates that the electron transfer at the working electrode surface is fast and the current is limited by the diffusion of electrolyte species to the electrode surface [31–33].
The concentration of glucose is a significant factor for glucose electro-oxidation. The effect of glucose concentration on electrocatalytic performance of Bi-4 for glucose oxidation was examined. Figure 9 is the CV representation of Bi-4 for 1 mM to 10 mM glucose concentrations in 1 M NaOH solution at 100 mVs− 1 scan rate. We observed significant enhancement of current with the increase of glucose concentration. The oxidation peak current was noticeably raised to 79.78 mAcm− 2 when the concentration of glucose was increased to 6 mM. This explains that as the glucose molecules become more concentrated near the catalyst surface, it boosts up glucose oxidation performance [2]. When the glucose concentration was 10 mM, a slight decrease of current was noticed. This is blamed on too much glucose molecules and glucose oxidation reaction intermediates which can occupy the oxidation active sites. Beside, the highly concentrated glucose solution can enhance the internal resistance and interrupt the mass transport of hydroxyl species and reacting glucose molecules to the Bi-4 electrocatalyst’s surface [2]. The values of oxidation peak currents and peak potentials are summarized in Table 2. Based on the CV performances in different glucose concentrations, we can conclude that the electrocatalytic behavior of Bi-4 electrocatalyst for glucose oxidation is better than some reported electrocatalysts as detailed in Table 3. Shi’s group reported nanoporous bismuth (NPB) electrocatalyst for glucose oxidation [6]. Among their prepared catalysts, NPB3 showed highest activity towards glucose electro-oxidation. They reported 7.50 mAcm− 2 peak current in 0.5 M NaOH + 0.5 M glucose solution at 50 mVs− 1 scan rate. In our study Bi-4 showed 15.03 mAcm− 2 i.e double oxidation peak current even at 20 mVs− 1 scan rate in lower (1 mM) glucose concentration. Pd-Bi/C (1 : 0.14) showed 16.4 mAcm− 2 peak current at 50 mVs− 1 in current in 0.5 M NaOH + 0.5 M glucose solution [2]. In much lower glucose concentration (1 mM), Bi-4 showed 22.99 mAcm− 2 oxidation peak current even at comparatively smaller scan rate 40 mVs− 1. Basu’s group reported PtPdAu/C as a carbon supported trimetallic electrocatalyst for glucose oxidation which showed 3.4 mAcm− 2 oxidation peak current in 0.5 M KOH + 0.05 M Glucose at 20 mVs− 1 [34]. Our prepared catalyst is also a far better material as compared to that trinary noble metal electrocatalyst for glucose oxidation. This higher kinetics is principally attributed to the greater active surface area and higher number of active sites due to the synthesis process used here.
Table 2
The oxidation peak currents and peak potentials for Bi-4 modified GCE in different glucose concentrations in 1 M NaOH solution.
Glucose concentration | Oxidation peak current (mAcm− 2) | Oxidation peak potential (V) |
1 Mm | 34.61 | -0.34 |
3 Mm | 41.47 | -0.12 |
4 mM | 42.20 | -0.07 |
5 Mm | 42.90 | -0.11 |
6 mM | 79.78 | -0.12 |
10 mM | 79.22 | -0.10 |
Table 3
The glucose electro-oxidation activity of few reported catalysts.
Electrocatalyst | Test protocol | Oxidation peak current | Reference |
NPB1 | 0.5 M NaOH + 0.5 M Glucose at 50 mVs− 1 | 5.95 | 6 |
NPB2 | 0.5 M NaOH + 0.5 M Glucose at 50 mVs− 1 | 6.65 | 6 |
NPB3 | 0.5 M NaOH + 0.5 M Glucose at 50 mVs− 1 | 7.50 | 6 |
PtPdAu/C | 0.5 M KOH + 0.05 M Glucose at 20 mVs− 1 | 3.4 | 34 |
Pd-Bi/C (1 : 0.14) | 0.5 M NaOH + 0.5 M Glucose at 50 mVs− 1 | 16.4 | 2 |
The temperature of the test solution plays a vital role in electro-oxidation kinetics. The improvement of electrochemical kinetics for glucose oxidation because of the rise of mass transfer rate of glucose molecules and conductivity of hydroxyl species was shown in previous reported investigations [2, 35]. The effect of temperature for the electrocatalytic performance of Bi-4 toward glucose oxidation in 1 M NaOH + 1 mM glucose solution was tested. Figure 10 displays the forwardic spike of CV of Bi-4 at various temperatures in the mentioned solution at 100 mVs− 1 scan rate. In our study, we noticed a surprising decline in electrocatalytic performance of Bi-4. In our test solution the concentration of NaOH (1 M) was tremendously higher than glucose. With the increase of temperature, the generation of OH− becomes high. The too high OH− species reduces the active sites for glucose adsorption on catalyst surface [2].
Chronoamperometry (CA) is another important tool to explore the electrocatalytic performance of a catalyst. Figure 11a represents the CA profile of Bi-2 and Bi-4 electrocatalysts in 1 M NaOH + 1 mM glucose solution at -0.4 V. Due to the absorbance of species on the electrode diffusion layers, a rapid current decline was seen in the early stages of the CA profile [32]. The specific surface currents from CA profile for Bi-2 and Bi-4 were evaluated as 25 µAcm− 2 and 56 µAcm− 2 respectively at 600 s. The higher specific surface current value of Bi-2 electrocatalyst suggests its better electrocatalytic performance toward glucose oxidation in alkaline medium. The stability of the electrocatalyst is a key factor to use as anode material. To examine the stability of Bi-4 electrocatalyst we measured CV up to 100 cycles at 100 mVs− 1 at room temperature in 1 M NaOH + 1 mM glucose solution. In this experiment, we used higher amount of Nafion™ NR 50 in catalyst ink. 8 mg Bi-4 catalyst was dispersed uniformly in 2 ml binder solution (50 mg Nafion™ NR 50 in 40 ml ethanol). The reason of using quite higher amount of Nafion™ NR 50 was to maintain the long time (up to 100 cycles) stability of catalyst powder on glassy carbon electrode surface. Figure 11b exhibits the anodic hump of CV of 1st, 5th, 10th, 15th, 20th and 25th cycles. Figure 11c displays the activity (η) up to 100 cycles. In Fig. 11b we observe the rise of current up to 20th cycle and from 25th cycle a decline of current is noticed. The increase in peak current is due to intensity decrease of electrode’s corrosion damage with the potentiodynamic cycle number increase in alkaline solution [1, 36]. From 25th cycle we noticed a nonstop current decay. This is because of the continuous fuel consumption and poisonous effect due to absorption of species on diffusion layer [33]. The preserved activity (η) was evaluated using the ratio of peak current density at nth cycle (Ifn) to that at initial cycle (If0). The η value of Bi-4 electrocatalyst was 0.795 at 100th cycle that means the catalyst can retain 79.5% of its electrocatalytic activity after 100 cycles.
Along with stability, the activation energy (Eapp) is another important parameter of a electrocatalyst. Activation energy of a electrocatalyst is that minimum energy which is required to activate a electrocatalyst for electrochemical reaction. An efficient electrocatalyst should have lower value of Eapp. To find out Eapp value of Bi-4 catalyst, we performed CA measurement at different temperature in 0.1 M NaOH + 1 mM glucose solution at -0.4 V and used the following Arrhenius equation.
$$\frac{{\partial \log j}}{{\partial T}}= - \frac{{{E_{app}}}}{{R{T^2}}}$$
5
The symbols R, T and j in Arrhenius equation define the molar gas constant (8.314 J mol− 1 K− 1), temperature (in Kelvin) and current density (mA.cm− 2) respectively. The chronoamperometry at different temperature and corresponding Arrhenius plot ( log j vs 1/T) were demonstrated in Fig. 12a and Fig. 12b respectively. Using the slope of the Arrhenius plot we evaluated the activation energy value of Bi-4 for glucose oxidation as 35.9 kJmol− 1.