A new cobalt(II) coordination polymer: electrocatalytic hydrogen evolution reaction and electrochemical sensing of ascorbic acid in glassy carbon electrodes
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Xia Tang
Abstract
A cobalt(II) coordination polymer, formulated as [Co(bpbe)2(terephthalate)(H2O)2] n (where bpbe = bis(1-(pyridin-3-ylmethyl)-benzimidazol-2-ylmethyl)ether), has been synthesized and characterized by elemental analysis, IR and UV/Vis spectroscopy and single-crystal X-ray diffraction. The structural analysis shows that the compound has a chain structure. The electrocatalytic activity towards hydrogen evolution reaction (HER) and ascorbic acid sensing were investigated by fabricating glassy carbon electrodes (Co/GCE) containg the new polymer. The results of HER measurements showed that the overpotential (η10298K) and Tafel slope (b298K) were −700 mV and 151.5 mV·dec−1 for Co/GCE, but −941 mV and 277.9 mV dec−1 for bare/GCE. The η10298K and b298K of Co/GCE were significantly positive shifted and significantly reduced compared with bare/GCE, indicating that Co/GCE has a good electrocatalytic HER activity. In addition, the recognition performance of Co/GCE for ascorbic acid was studied by chronoamperometry. The results show that Co/GCE has specific recognition performance for ascorbic acid. The detection limit is 0.3 μm in the response range of 0.5 μm–4 mm. The new electrode has long-term stability and good selectivity. This work provides a meaningful reference for the application of non-noble transition metal coordination polymers in electrocatalysis.
1 Introduction
In recent years, with the high consumption of fossil energy, it is urgent to seek for alternative sources of energy. 1 , 2 , 3 Hydrogen has a high combustion calorific value and high energy density and it is therefore considered as one of the best energy sources to replace traditional petrochemical energy sources. 4 At present, large-scale hydrogen production methods include steam methane reforming, coal gasification, natural gas cracking and others, but these techniques lead to environmental pollution and greenhouse gas emissions. 5 , 6 , 7 Among various hydrogen production strategies, the hydrogen evolution reaction (HER) by electrolysis of water is one of the simplest and cleanest methods to obtain high-purity hydrogen. 8 , 9 Catalysts are needed in the process of hydrogen production by electrolysis of water, among which noble metal-based catalysts have been most widely used because of their high activity and good selectivity. However, due to the high price and low abundance in mineral resources, the large-scale application of precious metal catalysts is limited. 10 , 11 Therefore, it is of great significance to design and develop new efficient and cheap catalysts for HER. 12 , 13 , 14 , 15
Coordination polymers (CPs) are a class of inorganic-organic hybrid materials formed by linking metal ions to ligands. 16 , 17 CPs not only may have a variety of structures, but also great potential for applications in many fields, such as luminescence, 18 , 19 catalysis, 20 , 21 , 22 , 23 sensing, 24 , 25 biology 26 and adsorption. 27 Among them, coordination polymers as electrode materials have attracted much attention in the field of electrocatalysis in recent years. Wang et al. 28 reported nickel CPs to achieve high selectivity and excellent detection sensitivity for glucose (sensitivity increased from 140 to 2,125 mA cm−2 mm−1). Song et al. 29 constructed a novel cobalt-based coordination polymer that showed good electrocatalytic hydrogen evolution performance in acidic media (overpotential 115 mV, Tafel slope 89 mV·dec−1). Wang et al. 30 synthesized a series of cobalt-based coordination polymers showing excellent electrochemical sensing capabilities for the detection of Cr(VI) and Fe(III) (the low limit of detection (LOD) of Cr(VI) and Fe(III) being 6.5 μm and 5.8 μm, respectively). According to the volcanic map 31 of HER activity, the non-precious metal cobalt occupies a high position, showing that cobalt has good electrocatalytic HER activity. Consequently, the design and synthesis of new cobalt(II) CPs with potential HER activity is an important research topic.
Benzimidazole-N-pyridine derivatives have abundant coordination sites and multiple coordination modes, making them one of the best ligands for constructing CPs. 32 In this paper, one new Co(II) CP was constructed by the solvothermal method in a water-DMF solvent using the bis(1-(pyridine-3-ylmethyl) -benzimidazole-2-ylmethyl) ether (bpbe) ligand, terephthalic acid and a cobalt(II) salt as reagents. Its structure was characterized by single crystal X-ray diffraction, elemental analysis, infrared (IR) and ultraviolet-visible (UV/Vis) spectroscopy. A cobalt(II) coordination polymer modified electrode (Co/GCE) was prepared. The electrocatalytic HER and ascorbic acid detection performance was studied by cyclic voltammetry (CV), variable temperature linear sweep voltammetry (VT-LSV) and electrochemical impedance spectroscopy (EIS). This study provides a feasible strategy for exploring non-noble metal molecular electrocatalysts.
2 Experimental
2.1 Materials and methods
All chemicals and solvents used in the tests were of reagent grade and put to use without being further purified. The ligand bpbe was bought from Jinan Henghua Sci. & Tec. Co. Ltd. The Carlo Erba 1,106 elemental analyzer was used to perform the C, H, and N elemental studies. The IR spectra were obtained using KBr pellets in a Nicolet FT-VERTEX 70 spectrometer in the 4,000–400 cm−1 range. A Lab-Tech UV Bluestar spectrophotometer was used to record electronic spectra.
2.2 Synthesis of cobalt(II)coordination polymers
A mixture containing Co(NO3)2·6H2O (14.55 mg, 0.05 mmol), bpbe (11.51 mg, 0.025 mmol), terephthalic acid (8.31 mg, 0.05 mmol), DMF (1 mL) and H2O (4 mL) was sealed in a Teflon-lined stainless-steel vessel (15 mL), which was heated at T = 90 °C for 72 h and then cooled to room temperature at a rate of 5 K h−1. Pink needle crystals were obtained. Yield: 61 % (based on Co). – Elemental analysis for C64H58CoN12O8: calculated C 65.02, H 4.95, N 14.22; found C 64.93, H 4.93, N 14.19 %. – Selected IR data (KBr, cm−1): ν = 3,431(m), 1,567(s), 1,465(m), 1,423(m), 1,380(m), 1,291(w), 1,096(w), 1,006(w), 760(m). – UV/Vis (DMF): λ = 279, 287 nm.
2.3 Preparation of modified electrode
Modified glassy carbon electrodes (GCE) were prepared according to the method described in the literature. 33 , 34 Before using the GCE, the surface of the GCE was polished with polishing powder to a mirror-like surface, and then cleaned and dried with anhydrous ethanol and distilled water. The procedure for the preparation of the modified electrode was as follows: 0.5 mg of acetylene black and 2 mg of Co(II) CP were thoroughly ground in an agate mortar for half an hour and the mixture transferred to a sample tube, then adding 0.5 mL of 0.25 % Nafion solution (Nafion™ perfluorinated resin solution) and ultrasonic shaking for 30 min. 5 μL of the liquid was uniformly applied to the surface of the GCE. The modified electrode Co/GCE was ready after drying. For comparison, a glassy carbon electrode (bare/GCE) without cobalt(II) coordination polymer was created using a similar approach.
2.4 Electrochemical measurement
Cyclic voltammetry (CV), chronoamperometry (CA) and linear sweep voltammetry (LSV) were used to collect data on a LK2005A electrochemical workstation. GCEs were employed as the working electrodes (3 mm diameter), Ag/AgCl (saturated KCl solutions) as the reference electrode, and graphite electrodes (3 mm diameter) as the counter electrode in a three-electrode system. According to E (vs. RHE) = E (vs. Ag/AgCl) + 0.059 pH + 0.198 V (pH = 0.16), the obtained potentials connected to the Ag/AgCl electrode were converted to the RHE scale. 35 Under the same conditions, the electrochemical impedance spectroscopy (EIS) method was used to conduct experiments. The measurement frequency range was 100.0 kHz to 0.1 Hz, and the modulation amplitude at different voltages was 5 mV. The EIS spectrum and related component parameters were obtained by the Z-SIMPWIN software (Dr. Brian A. Sayers, University of Manchester: Manchester, U. K.) fitting. Before evaluating the working electrode’s electrocatalytic performance, CV was repeatedly run at a scan rate of 0.05 V s−1 until no change in the redox peaks was noticed. Utilizing CV, LSV, and EIS in a 0.5 m H2SO4 (pH = 0.16) solution, the electrocatalytic activity of Co/GCE for HER were evaluated. All electrochemical measurements were repeated 10 times with a maximum error of 5 % in order to ensure the repeatability of the experimental data.
2.5 X-ray structure determination
A suitable single crystal was mounted on a glass fiber. At T = 153 K, graphite-monochromatized MoKα radiation (λ = 0.71073 Å) was used to measure intensity data on a Bruker APEX-II area detector. Data reduction and cell refinement were performed using the programs Saint. 36 The absorption corrections are carried out with Sadabs. 37 The structure was solved by Direct Methods and refined by least-squares methods against F2 using the program Shelxl 38 as implemented in the Olex-2 program system. 39 Because the crystal easily weathered, the disorder of solvent water molecules in the crystal is too difficult to be determined. As a consequence, the solvent water molecules were masked with the routine provided in Olex-2. Some diffraction points are missing because some low-angle diffraction points have large fitting errors, which may be due to the instrument. Table 1 summarizes the crystal data and experimental parameters pertinent to the structure determination. The relevant bond length and bond angle data are listed in Table 2.
Crystal data and structure refinement of the cobalt(II) coordination polymer.
| Molecular formula | C64H58CoN12O8 |
| Molecular weight Mr | 1,182.15 |
| Crystal system | Triclinic |
| Space group |
|
| a / Å | 11.3886(10) |
| b / Å | 11.8799(10) |
| c / Å | 13.5302(12) |
| α / deg | 113.2120(10) |
| β / deg | 99.119(2) |
| γ / deg | 97.4360(10) |
| V / Å3 | 1,623.6(2) |
| Z | 1 |
| ρcald / g cm−3 | 1.21 |
| Absorption coefficient / mm−1 | 0.3 |
| F(000) / e | 617 |
| Crystal size / mm3 | 0.5 × 0.5 × 0.5 |
| 2θ range for data collection / deg | 3.366–50.000 |
| Reflections collected | 8,341 |
| Independent reflections | 5,686 |
| Rint; Rsigma | 0.0195; 0.0399 |
| Index ranges hkl | −13,+13; −9,+14; −16,+15 |
| Refinement method | Full-matrix least-squares |
| Data; restraints; parameters | 5,686; 36; 386 |
| Goodness-of-fit on F2 | 1.087 |
| Final R1; wR2 [I > 2 σ(I)] | 0.0432; 0.1269 |
| Final R1; wR2 (all data) | 0.0535; 0.1334 |
| Largest diff. peak; hole / e Å−3 | 0.27; −0.33 |
Selected bond lengths (Å) and angles (deg) of cobalt(II) coordination polymera.
| Bond lengths | |||
| Co1–O21 | 2.0556(14) | Co1–O31 | 2.1214(16) |
| Co1–O2 | 2.0557(14) | Co1–N1 | 2.1791(17) |
| Co1–O3 | 2.1214(16) | Co1–N11 | 2.1792(17) |
| Bond Angles | |||
| O21–Co1–O2 | 180 | O21–Co1–N11 | 89.85(6) |
| O2–Co1–O31 | 89.95(6) | O3–Co1–O31 | 180 |
| O21–Co1–O31 | 90.05(6) | O31–Co1–N11 | 92.32(6) |
| O2–Co1–O3 | 90.05(6) | O3–Co1–N1 | 92.32(6) |
| O21–Co1–O3 | 89.95(6) | O31–Co1–N1 | 87.68(6) |
| O2–Co1–N1 | 89.85(6) | O3–Co1–N11 | 87.68(6) |
| O21–Co1–N1 | 90.15(6) | N1–Co1–N11 | 180 |
| O2–Co1–N11 | 90.15(6) | ||
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aSymmetry operation: 1: 1–x, 1–y, 2–z.
CCDC 2413795 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3 Results and discussion
A new cobalt(II) coordination polymer was synthesized by the solvothermal method and characterized. The results of elemental analysis are consistent with the theoretical composition. The polymer is insoluble in methanol, ethanol, acetonitrile, and water, but slightly soluble in dimethylformamide (DMF), dimethylacetamide (DMA) and dimethylsulfoxide (DMSO).
The IR spectra of the ligand bpbe and of the cobalt(II) coordination polymer are shown in Figure S1 (Supporting Information available online). The IR absorption peaks of bpbe at 1,292 and 1,079 cm−1 can be attributed to the tensile vibrations v(C–N) and v(C–O–C). 40 , 41 Compared with the ligand bpbe, the bands of v(C–N) and v(C–O–C) of the cobalt(II) coordination polymer are shifted by only about 1 and 13 cm−1, respectively, indicating the coordination between bpbe and the metal center. 42 , 43 , 44 , 45 Figure S2 shows the UV/Vis spectra of bpbe and the cobalt(II) coordination polymer in DMF solution at 298 K. Due to the n-π* and π-π * transitions of benzimidazole, the ligand bpbe produces two maximum at 287 and 279 nm, 46 , 47 , 48 , 49 which are slightly red-shifted for the complex. 50 , 51
3.1 Description of the crystal and molecular structure
The compound crystallizes in the monoclinic system with space group P
The crystal structure of cobalt(II) coordination polymer. (a) Chain structure of the cobalt(II) coordination polymer (all H atoms have been omitted for clarity).(b) Molecular structure and atom numberings. Displacement ellipsoids have been drawn at the 30 % probability level (H atoms omitted); (c) Coordination of Co(II) ion.(d) The coordination mode of the bpbe ligand.
3.2 Cyclic voltammetry investigations
As described in the literature, 52 , 53 CV curves in the non-Faraday range were measured under acidic/neutral conditions. The CV curves of the working electrode were obtained of samples in 0.5 m H2SO4 at different scanning rates (10–100 mV s−1) (Figure S3). The electrochemical double-layer capacitance (Cdl) was obtained by fitting a linear relationship as shown in Figure 2. The electrochemically active surface area (ECSA) value of the working electrode was calculated from Eq. (1). 54 The voltammetric charge (Q) of the working electrode was determined in a PBS (phosphate buffer) solution of pH = 7 at a scan rate of 50 mV s−1 over a potential interval of −0.2–0.6 V (Figure 3). 55 , 56 The number of active sites (n) on the working electrode can be calculated from Eq. (2): 57
where Cs is taken as 0.040 mF cm−2 in 0.5 m H2SO4.
Electrochemical double layer capacitances determined from CV measurements.
Voltammetric charge measurements of bare/GCE (a) and Co/GCE (b).
And where Q indicates the voltammetric charge, C; F = 96485 C mol−1.
The ECSA, Q and n data are important parameters to evaluate the electrocatalytic performance of the electrode materials. According to the literature, 54 , 55 , 56 , 57 higher values of these three parameters manifest the higher the catalytic activity of the electrode material. As shown in Table 3, the ESCA, Q, and n of Co/GCE values are greater than those of the bare/GCE, which indicates that Co/GCE possesses better electrocatalytic activity.
Parameters relevant for cyclic voltammetric studies of working electrodes.
| Electrodes | Cdl / mF | ECSA / cm2 | Q / C | n / mol |
|---|---|---|---|---|
| Bare/GCE | 0.16 | 4.05 | 7.75 × 10−4 | 4.02 × 10−9 |
| Co/GCE | 0.66 | 16.48 | 8.29 × 10−3 | 4.30 × 10−8 |
3.3 Electrochemical hydrogen evolution characteristics
3.3.1 Variable temperature linear sweep voltammetry (VT-LSV)
The VT-LSV curves of bare/GCE and Co/GCE were determined in 0.5 m H2SO4 at different temperatures (293, 298, 303 and 308 K). 58
3.3.1.1 Polarization curves and Tafel curves
The cathodic polarization curves were obtained by transforming the LSV curves. The horizontal coordinate is the overpotential (η) obtained by transforming the voltage value of the LSV curve, and the vertical coordinate is the current density (j) obtained by dividing the current by the geometric surface area of the electrode (Figure 4a and b). The Tafel curve is obtained by transforming the cathodic polarization curve. The vertical coordinate is the negative value of the overpotential (–η), and the horizontal coordinate is the logarithm of the absolute value of the current density (log|J|). Its Tafel curve at different temperatures was obtained by intercepting the linear segments and fitting them (Figure 4c and d). The kinetic parameters are derived from Eq. (3): 58
where a refers to the Tafel intercept; b stands for the Tafel slope; α indicates the cathodic charge transfer coefficient; J0 denotes the exchange current density; N represents the number of electrons transferred for generating a molecule of product (for H2, N = 2); R, T and F have their usual meanings (R = 8.314 J mol−1 K−1; F = 96,485 C mol−1).
The cathodic polarization curves of the working electrode bare/GCE (a), Co/GCE (b) at different temperatures; corresponding Tafel curves for bare/GCE (c), Co(/GCE (d).
In general, under standard operating conditions (current density = 10 mA cm−2, T = 298 K), the electrocatalytic HER performance is evaluated by comparing overpotential (η10298K) and Tafel slopes (b298K). 59 , 60 Positive overpotentials (η10298k) and smaller Tafel slopes (b298K) indicate that the electrode material has better electrocatalytic HER activity. 59 As shown in Table 4, the η10298K and b298K values of bare/GCE are −941 mV and 277.9 mV·dec−1, and for Co/GCE are −700 mV and 151.5 mV dec−1, respectively. The electrode modified by cobalt(II) coordination polymer is thgus found to have a good electrocatalytic HER activity. The above conclusions can also be drawn from experiments carried out under other temperature conditions.
Electrocatalytic HER performance of working electrodes at different temperaturesa.
| Electrodes | T / K | η10 / mV | Δη10 / mV | b / mV·dec−1 |
|---|---|---|---|---|
| Bare/GCE | 293 | −1,049 | – | 299.1 |
| 298 | −941 | – | 277.9 | |
| 303 | −877 | – | 276.2 | |
| 308 | −830 | – | 223.0 | |
| Co/GCE | 293 | −738 | 311 | 201.0 |
| 298 | −700 | 241 | 151.5 | |
| 303 | −664 | 213 | 124.4 | |
| 308 | −616 | 214 | 112.9 |
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aAnnotation: Δη10, Co/GC,T – bare/GCE,T.
The electrocatalytic performance of different electrocatalysts for HER. The electrolyte is 0.5 m H2SO4 in all casesa.
| Electrocatalysts | η10 / mV | b / mV·dec−1 | Ref. |
|---|---|---|---|
| Commercial Pt/C | −21 | 28.5 | 61 |
| CoMoS4 | −515 | 309 | 62 |
| P/CoS2-1 | −209 | 68 | 63 |
| P/CoS2-3 | −151 | 61 | 63 |
| CoS/PCE | −190 | 66 | 64 |
| CoCP/CPE-1 | −695 | 210 | 65 |
| CoCP/CPE-2 | −780 | 273 | 65 |
| CoCP/CPE-3 | −801 | 236 | 65 |
| Co /GCE | −700 | 151.5 | This work |
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aCP, coordination polymer; GCE, glassy carbon electrode; PCE, plastic chip electrode; CPE, carbon paste electrode. Table 5 lists two important parameters (η10 and b) related to the HER electrocatalytic performance of the catalysts prepared in this paper and other catalysts that have been reported in recent years. The comparison shows that although the HER electrocatalytic activity of the former is at a moderate level.
3.3.1.2 Arrhenius curve and Turnover frequency curve
In order to verify the electrocatalytic hydrogen evolution performance of the fabricated electrode materials, the Arrhenius and turnover frequency (TOF) curves were obtained by cathodic polarization transformation. The reciprocal of the temperature was taken as the horizontal coordinate of the Arrhenius curve (T−1), and the logarithm (log|JIE|) of the current density at different temperatures (293, 298, 303 and 308 K) at the same overpotential was taken as the vertical coordinate with the overpotential at a particular temperature as the standard. The Arrhenius curves were finally obtained by linear fitting (Figure 5a). The final values of activation energy (Ea) calculated according to Eq. (4) are listed in Table 6. 66 The corresponding TOF curves were obtained by fitting the cathodic polarization curves with the 293 K overpotential η as the horizontal coordinate and the TOF values as the vertical coordinate (Figure 5b). The final TOF values calculated according to Eq. (5) are listed in the Table 6: 67
where JIE is the current density under a certain overpotential (η) at different temperatures (T); R = 8.314 J mol−1 K−1.
where J is the current density in mA cm−2; A indicates the geometric surface area of the working electrode, in cm2; N is the number of electrons transferred (HER: 2); n represents the number of active sites in mol; F = 96,485 C mol−1.
Arrhenius curve (a) and TOF curves (b) for bare/GCE (a) and Co/GCE (b).
The TOF values and activation energies of bare/GCE and Co /GCE.
| Electrodes | Ea / kJ mol−1 | TOF / S−1 |
|---|---|---|
| Bare/GCE | 69.2 | 0.014 |
| Co /GCE | 58.1 | 0.054 |
According to the transition state theory of chemical kinetics, 66 , 67 , 68 the smaller Ea and the larger TOF show that the new electrode materials have higher electrocatalytic HER activity. Experimental data is listed in Table 6. Compared with bare/GCE, the Ea of Co/GCE obviously decreases and the TOF value increases significantly. The results further prove that the fabricated electrocatalysts Co/GCE have a good electrocatalytic HER activity.
3.3.2 Electrochemical impedance spectroscopy
An electrochemical impedance spectroscopy (EIS) investigation was also carried out to further study the electrocatalytic activity of Co/GCE for HER. The diameter of a semicircle at high frequencies in the Nyquist plot, which becomes typically inversely proportional to the activity of the electrocatalyst, was used to determine the charge-transfer resistance (Rct) at the surface of the working electrode. 69 Figure 6 shows the Nyquist plots of bare/GCE and Co/GCE obtained at a bias potential of −0.6 V (vs. RHE). Relevant parameters are listed in Table 7.
EIS spectra for bare/GCE and Co/GCE.
EIS related parameters of bare/GCE and Co /GCE.
| Electrodes | Rct / Ω cm−2 |
|---|---|
| Bare/GCE | 81.2 |
| Co/GCE | 48.7 |
Generally it is true, that a lower Rct value indicates a stronger conductivity of the modified working electrode and an increased rate of charge transfer between the electrode surface and the electrolyte interface, hence a better electrocatalytic activity. 69 , 70 The Rct value of Co/GCE is 48.7 Ω cm−2, significantly lower than that of bare/GCE (81.2 Ω cm−2), revealing that the working electrode Co/GCE has ultrafast charge transfer and surface reaction kinetics at the interface between the working electrode Co/GCE and the electrolyte. This conclusion is consistent with the results of the CV and LSV experiments.
3.3.3 Stability
An electrocatalyst to be used in actual applications must be stable. To test the stability of the prepared molecular electrocatalysts, 500 cycles of cyclic voltammetry were performed at a sweep rate of 0.05 V s−1 71 Figure 7 shows that Co/GCE exhibits good long-term stability since the LSV polarization curves before and after 500 cyclic voltammetric scans nearly overlapped.
Stability studies for Co/GCE over 1 and 500 cyclic voltametric scans.
3.4 Electrochemical sensing of ascorbic acid
3.4.1 Selection of working voltage
In order to study the effect of the electric potential on the Co/GCE response current of the electrode, the actual voltage of ascorbic acid was screened by the chronocurrent method and the experimental parameters were optimized. Figure 8 shows the current response curve of Co/GCE to 1 mm ascorbic acid at an applied potential of 0.3–0.6 V (vs. Ag/AgCl). Considering the fast response time, low noise background and obvious current signal, 0.4 V was selected as the potential to be applied for the identification of ascorbic acid.
Current response curve of the working electrode to 1 mm ascorbic acid at different voltages.
3.4.2 Titration experiment and confidence level
At the selected sensing potential, the current response of the working electrode to different concentrations of ascorbic acid was tested by chronoamperometry. As shown in Figure 9, bare/GCE and bpbe/GCE produce only a weak current response to high concentrations of ascorbic acid, while Co/GCE shows a current response even to lower concentrations of ascorbic acid. The cobalt(II) coordination polymer obviously plays a major catalytic role in the identification of ascorbic acid.
Current-time response of bare/GCE, bpbe/GCE and Co/GCE to different concentrations of ascorbic acid.
Electrochemical sensors with excellent performance have faster response times, higher sensitivities and lower detection limits (LOD). 72 , 73 Figure 10a shows the current response of Co/GCE to ascorbic acid with different concentrations (0.5 μm–4 mm) at 0.4 V. The response current gradually increases with the increase of ascorbic acid concentration, and the steady-state current can be reached within 4 s. As shown in Figure 10b, there is a linear relationship with the high correlation coefficient (0.994 ≤ R2 ≤ 0.999) in the concentration range of 0.5 μm ∼ 4 mm and 0.5 μm ∼ 200 μm for Co/GCE, and the slope of the fitting curve represents the sensitivity (7.818 μA mm−1) of the new ascorbic acid sensor. The detection limit (LOD) is the theoretical minimum of an electrochemical sensor. The LOD calculated according to Eq. (6) is 0.31 μm (Co/GCE). 25 The performance parameters of Co/GCE and some other ascorbic acid electrochemical sensors reported in the literature are listed in Table 8. The results show that Co/GCE exhibits good electrochemical recognition performance for ascorbic acid.
where 3 represents the signal-to-noise, σ corresponds to the standard deviation of the blank, and k is the sensitivity.
Chronoamperometric titration. (a) At 0.4 V, the current response of Co/GCE to different concentrations of ascorbic acid (0.5 μm ∼ 4 mm); the inserted graph illustrates the current response in the low concentration range. (b) The fitting curve between the current response of Co/GCE and the concentration of ascorbic acid (0.5 μm ∼ 4 mm); the insert shows the linear relationship within the concentration range of 0.5 μm ∼ 200 μm.
Different modified electrodes for ascorbic acid detection.
| Electrode materials | Linear range | LOD / μm | Ref. |
|---|---|---|---|
| Co3O4/GCE | 0.05 μm–3 mm | 1 | 74 |
| Co2P hybrid | 0.1 mm–4.5 mm | 0.34 | 75 |
| CoTMPyP/TaMoO6/GCE | 0.22 mm–2.11 mm | 28 | 76 |
| ZnCo2O4 NA/CC | 10 μm–500 μm | 7.79 | 77 |
| Co3O4–CuNi/RGO/GCE | 2.5 μm–100 μm | 0.34 | 78 |
| Co(II)CP/GCE | 0.5 μm–4 mm | 0.31 | This work |
Under the selected conditions, the current responses of Co/GCE to ascorbic acid of 30 μm, 0.4 mm and 3 mm concentrations were tested by chronoamperometry. The actually detected ascorbic acid concentration was calculated from the linear relationship shown in Figure 11b. The error between the actually added concentration and the actually detected concentration is small (≤6.7 %) (Table 9), which reflects the high confidence of electrochemical sensor sensitivity.
Stability and anti-interference. (a) The stability test for Co/GCE; (b) the current responses of Co/GCE when 1 mm interfering substances were added sequentially.
Relevant data of the reliability test for electrochemical identification of ascorbic acid.
| Electrodes | Actual addition concentration | Actual detected concentration | Error |
|---|---|---|---|
| Co/GCE | 0.030 mm | 0.031 mm | 3.3 % |
| 0.400 mm | 0.393 mm | 1.8 % | |
| 3.000 mm | 2.982 mm | 0.6 % |
3.4.3 Stability and anti-interference experiment
For non-enzyme electrochemical sensors, anti-interference and stability are the key indexes to evaluate whether it has practical application value. For stability, Co/GCE can maintain more than 96 % of the initial current response after 1,500 s (Figure 11a). For selectivity, 1 mm glucose, d-fructose, uric acid, adenine, K2SO4, NaCl, sucrose, and tyrosine were added in sequence. 79 Figure 11b clearly describes that Co/GCE can produce a significant current response to 1 mm ascorbic acid, while producing almost no current response to interfering substances, indicating that the prepared sensor has a specific ability to recognize ascorbic acid. The experimental results show that the prepared electrochemical sensor has high specific recognition performance and long-term stability for ascorbic acid.
4 Conclusions
In summary, a new cobalt(II) coordination polymer was synthesized using the solvothermal method. The electrocatalytic activity for HER and the electrochemical sensing performance for ascorbic acid were investigated by preparing modified glassy carbon electrodes (Co/GCE). The overpotential (η10298K) and Tafel slope (b298K) of Co/GCE were significantly positive shifted and decreased compared with the bare/GCE, indicating that Co/GCE has a good electrocatalytic activity for HER. Moreover, Co/GCE has a specific recognition performance for ascorbic acid with low detection limit, quick response time, good stability and anti-interference ability. Therefore, Co/GCE can be used as an effective bifunctional electrocatalyst, which provides a meaningful theoretical reference for the application of non-noble metal coordination polymers in electrocatalysis.
5 Supporting information
The IR and UV/Vis absorption spectra of bpbe and the cobalt(II) coordination polymer as well as electrochemical double-layer capacitance measurements of bare/GCE and Co/GCE are given as supplementary material available online (https://doi.org/10.1515/znb-2025-0017).
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Research ethics: Not applicable.
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Informed consent: Not applicable.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Use of Large Language Models, AI and Machine Learning Tools: None declared.
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Conflict of interest: The authors declare no conflicts of interest regarding this article.
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Research funding: Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong University (Grant No. 152022).
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Data availability: The raw data can be obtained on request from the corresponding author.
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Supplementary Material
This article contains supplementary material (https://doi.org/10.1515/znb-2025-0017).
© 2025 Walter de Gruyter GmbH, Berlin/Boston
Artikel in diesem Heft
- Frontmatter
- In this issue
- Research Articles
- Molecular and electronic structures of molecules and ions with a linear chain of four carbon atoms: polyyne or cumulene?
- A new cobalt(II) coordination polymer: electrocatalytic hydrogen evolution reaction and electrochemical sensing of ascorbic acid in glassy carbon electrodes
- A switch from homogeneous mixed-valent to trivalent cerium in the solid solutions CeNi1–x Pd x Zn
- The metal-rich phosphide β-ZrCr6P4 with β-UCr6P4-type structure
- Palladium-coordinated Al4 butterfly clusters in the palladium-rich aluminides RE 4Pd11Al8 (RE = Y, Sm, Gd–Lu)
- Attempts to crystallize salts of thiocyameluric acid C6N7S3H3 and the crystal structure of the first hydrogen thiocyamelurate [Sr(H2O)6][HC6N7S3]
- Deriving the Goldschmidt-like tolerance factors for solids: a simplified polyhedral approach
Artikel in diesem Heft
- Frontmatter
- In this issue
- Research Articles
- Molecular and electronic structures of molecules and ions with a linear chain of four carbon atoms: polyyne or cumulene?
- A new cobalt(II) coordination polymer: electrocatalytic hydrogen evolution reaction and electrochemical sensing of ascorbic acid in glassy carbon electrodes
- A switch from homogeneous mixed-valent to trivalent cerium in the solid solutions CeNi1–x Pd x Zn
- The metal-rich phosphide β-ZrCr6P4 with β-UCr6P4-type structure
- Palladium-coordinated Al4 butterfly clusters in the palladium-rich aluminides RE 4Pd11Al8 (RE = Y, Sm, Gd–Lu)
- Attempts to crystallize salts of thiocyameluric acid C6N7S3H3 and the crystal structure of the first hydrogen thiocyamelurate [Sr(H2O)6][HC6N7S3]
- Deriving the Goldschmidt-like tolerance factors for solids: a simplified polyhedral approach
