The formic acid oxidation reaction (FAOR) is a key anodic process in direct formic acid fuel cells and requires efficient electrocatalysts with high activity and resistance to catalyst poisoning. In this study, a graphitic carbon nitride (g-C3N4)-modified glassy carbon electrode (g-C3N4/GC) was fabricated and evaluated for FAOR in acidic media. The electrocatalytic performance of the modified electrode was investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and atomic force microscopy (AFM). Compared with the bare glassy carbon electrode, the g-C3N4/GC electrode exhibited significantly enhanced electrocatalytic activity toward FAOR, characterized by substantially higher peak current densities, lower charge-transfer resistance, and improved long-term stability. EIS analysis revealed faster interfacial electron-transfer kinetics following surface modification, while CV studies indicated an increase in electrochemically active surface area and improved accessibility of catalytic sites. Chronoamperometric measurements further demonstrated enhanced durability and reduced activity loss during prolonged operation. The improved electrochemical performance is attributed to the nitrogen-rich structure, increased conductivity, and abundant active sites provided by g-C3N4. These findings demonstrate that g-C3N4-modified glassy carbon electrodes represent a promising metal-free electrocatalyst platform for formic acid electrooxidation and may contribute to the development of cost-effective direct formic acid fuel cell technologies.
The electrochemical oxidation of small organic molecules has attracted considerable attention because it is important for developing low-temperature fuel cells and sustainable energy conversion technologies [1]. Among these molecules, formic acid (HCOOH) is regarded as one of the most promising liquid fuels for direct formic acid fuel cells (DFAFCs) owing to its high theoretical energy density, low toxicity, ease of storage and transportation, and relatively simple electrochemical oxidation pathway [2]. In addition to being an attractive energy carrier, formic acid can be produced from biomass-derived feedstocks and carbon dioxide, making it a renewable and environmentally benign fuel candidate [3].
The anodic oxidation of formic acid proceeds through two principal reaction pathways: a direct dehydrogenation pathway, which produces carbon dioxide and releases electrons efficiently, and an indirect dehydration pathway, which generates adsorbed carbon monoxide (CO) as an intermediate. The latter pathway is undesirable because strongly adsorbed CO blocks catalytic active sites and significantly reduces catalyst activity and durability [4,5,6]. Consequently, identifying electrocatalysts that favour the direct oxidation pathway while suppressing CO formation remains a major challenge in DFAFC research.
Platinum- and palladium-based materials remain the benchmark electrocatalysts for formic acid oxidation because of their excellent intrinsic catalytic activity. However, their widespread application is limited by high cost, scarcity, catalyst poisoning by CO intermediates, and gradual performance degradation during prolonged operation [7]. Various strategies have therefore been explored to overcome these limitations, including alloying platinum with secondary metals, engineering catalyst morphology, and tailoring the electronic structure of catalyst surfaces [8,9,10,11]. Although these approaches have improved catalytic performance, they continue to rely heavily on expensive noble metals, highlighting the need for alternative and more sustainable electrocatalytic materials.
In recent years, carbon-based materials have emerged as attractive alternatives or catalyst supports because of their high electrical conductivity, large specific surface area, excellent chemical stability, and tunable surface chemistry. Materials such as graphene, carbon nanotubes (CNTs), and other nanostructured carbon architectures have been widely employed to enhance electron transport and increase the electrochemically active surface area of electrocatalysts for formic acid oxidation [12,13,14,15]. Despite these advances, most reported systems still depend on noble-metal nanoparticles as the active catalytic component. Consequently, increasing attention has shifted toward metal-free carbon materials capable of providing intrinsic catalytic activity while maintaining the advantages of low cost, environmental friendliness, and long-term stability.
Graphitic carbon nitride (g-C3N4) has recently emerged as one of the most promising metal-free carbonaceous materials for electrochemical applications. Owing to its two-dimensional conjugated structure, high nitrogen content, abundant surface-active sites, and excellent chemical stability, g-C3N4 exhibits favourable electronic properties that promote efficient charge transfer and strong interactions with adsorbed molecules [13,14,15,16,17]. These characteristics have led to its successful application in photocatalysis, sensing, energy conversion, and electrocatalysis. Nevertheless, comparatively few studies have investigated the ability of g-C3N4 to enhance the electrocatalytic oxidation of formic acid on glassy carbon electrodes, and the fundamental mechanisms responsible for its catalytic enhancement remain insufficiently understood.
Computational methods, particularly Density Functional Theory (DFT), have become valuable tools for elucidating adsorption behaviour, electronic structure, charge-transfer characteristics, and reaction mechanisms of electrocatalytic materials at the atomic scale. When combined with electrochemical experiments, DFT calculations provide molecular-level insights that complement experimental observations and facilitate the rational design of improved electrocatalysts.
In this work, a graphitic carbon nitride-modified glassy carbon electrode (g-C3N4/GCE) was fabricated and evaluated as a metal-free electrocatalyst for the formic acid oxidation reaction. The electrocatalytic performance of the modified electrode was systematically investigated using cyclic voltammetry, electrochemical impedance spectroscopy, chronoamperometry, atomic force microscopy, and Raman spectroscopy. Furthermore, molecular modelling was employed to investigate the electronic properties, frontier molecular orbitals, and adsorption behaviour of formic acid on the modified surface, thereby providing mechanistic insight into the experimentally observed enhancement in electrocatalytic performance. The combined experimental and computational approach establishes a comprehensive understanding of the role of g-C3N4 in promoting formic acid electrooxidation. It demonstrates its potential as a low-cost, metal-free electrocatalyst for future direct formic acid fuel cell applications.
Materials and MethodReagents
All chemicals used in this research were of analytical grade. Sulfuric acid (H2SO4, 97-98 wt%) and formic acid (HCOOH, 88.0 wt%, molecular weight 46.03) were purchased from Guangzhou Jinhuada Chemical Reagent Sci-Tech Co. Ltd, China. We prepared using 0.1 M, 0.5 M, and 1.0 M formic acid mixed with 10 mM H2SO4. Britton-Robinson buffer solutions with pH values of 4.0, 7.0, and 9.0 were employed as supporting electrolytes.
Electrode Pretreatment and Modification
Graphitic carbon nitride (g-C3N4) nanoparticles were purchased from [Flamingo, Shanghai, China] and used as received without further purification.
Prior to catalyst deposition, a glassy carbon (GC) electrode (diameter: 3.0 mm, Bioanalytical Systems, USA) was mechanically polished using 0.05 μm alumina slurry and subsequently cleaned by sonication in 0.1 M HNO3, 0.1 M H2SO4, and ultrapure water for 10 min each.
For electrode modification, 13.5 mg of g-C3N4 nanoparticles were dispersed in 5 mL of ethanol. Subsequently, 40 μL of a 20 wt% Nafion solution was added as a binder to improve the adhesion and mechanical stability of the catalyst film on the electrode surface. The suspension was sonicated for 20 min to obtain a homogeneous catalyst ink. Thereafter, 10 μL of the catalyst ink was drop-cast onto the polished GC electrode using a Hamilton microliter syringe and allowed to dry at room temperature. The surface morphology of the prepared electrodes was examined using atomic force microscopy (AFM) (Stramini-Go DIY Scanning Probe Microscope Controller).
Electrochemical Measurements
All electrochemical measurements were carried out using a three-electrode system with an Autolab SP-200 Potentiostat controlled by EC-Lab software (Bio-Logic, Netherlands). The working electrodes were the prepared GC and g-C3N4/GC electrodes. A saturated calomel electrode (SCE) as reference electrode and a platinum coil counter electrode completed the setup. All potentials reported are referenced to the SCE reference electrode.
Formic acid oxidation was investigated in acidic media (10 mM H2SO4 solution) containing varying concentrations of formic acid (0.5 M, 1.0 M, and 5.0 M). Cyclic voltammetry (CV) scans were recorded between -0.3 V and 1.2 V (vs. SCE) at scan rates of 10, 20, 30, 40, and 50 mV/s. The influence of temperature and solution pH on the peak current density (Ip) and peak potential (Ep) was also explored.
We complemented CV measurements with chronoamperometry and electrochemical impedance spectroscopy (EIS). Chronoamperometric experiments were conducted at three different HCOOH concentrations (0.1 M, 0.5 M, and 1.0 M). Potential values ranged from 0.3 V to 0.9 V, and the resulting current was monitored over 10 minutes. The current measured at the end of this period reflects the steady-state current at the applied potential. EIS measurements were performed using the same Bio-Logic SAS potentiostat (model SP-200). The impedance spectra were recorded from 100 kHz to 10 mHz with an AC signal amplitude (rms) of 10 mV.
Molecular Mechanics Calculations
Molecular mechanics calculations were performed using the Forcite module in BIOVIA Materials Studio. The glassy carbon electrode (GCE) surface was constructed from a graphite crystal structure (C.cif) and subsequently modified with graphitic carbon nitride (g-C3N4) to obtain the nanocomposite electrode model. The geometry of the isolated formic acid molecule and the electrode models was first optimized using the COMPASS III force field. Geometry optimization employed the Smart algorithm with convergence criteria of 2.0 × 10-5 kcal mol-1 for energy, 0.001 kcal mol-1 Å-1 for maximum force, and 1.0 × 10-5 Å for maximum displacement. The optimized formic acid molecule was positioned above the electrode surface, and adsorption geometries were obtained through further energy minimization. Periodic boundary conditions were applied in the surface plane, while a vacuum layer of approximately 20 Å was introduced normal to the surface to eliminate interactions between periodic images.
The interaction between formic acid and the electrode surface was evaluated using the adsorption (binding) energy calculated as (Equations 1 and 2):
Eads=EHCOOH/GCE-(EGCE+EHCOOH)(1)Eads=EHCOOH/(g-C3N4/GCE)-(Eg-C3N4/GCE+EHCOOH)(2)Result and DiscussionCyclic Voltammetry (CV) at a Bare GCE
Cyclic voltammetry (CV) is a valuable technique for determining whether an electrochemical reaction is controlled by diffusion or kinetics. Figure 1 illustrates the cyclic voltammogram of a glassy carbon (GC) electrode in a solution of 1 M formic acid (FA) and 10 mM sulfuric acid (H2SO4) across potential scan rates from 10 to 50 mV/s.
(a) CV on bare GCE in 1 M HCOOH + 10 mM H2SO4 at different scan rates; (b) Peak current dependence on the square root of the scan rate.
From Figure 1a, it is evident that as the scan rate increases, the reaction occurring at the electrode becomes less reversible, suggesting a transition towards a quasi-reversible process. This behavior is likely due to diffusion control, attributed to the electrode’s porosity, which hinders natural convection and the movement of reactants from the reaction zone to the bulk solution.
The diffusion-controlled nature of the process is confirmed by the linear relationship between the peak current and the square root of the scan rate (ν1/2), with a high correlation coefficient (R2 = 0.9997). This linearity indicates that diffusion governs the electrochemical process. Additionally, plotting the logarithm of the peak current against the logarithm of the scan rate yields a straight line with a slope of 0.42. This value is close to the theoretical slope of 0.5, which is characteristic of an ideal diffusion-controlled electrode process [18,19].
The Effect of Varying the Concentration of Formic Acid
Figure 2 presents cyclic voltammograms for formic acid oxidation at different concentrations (ranging from 0.1 M to 1.0 M) recorded at a scan rate of 50 mV/s. The corresponding peak current densities and potentials are summarized in Table 1.
CVs on bare GCE in solutions of different FA concentrations (0.1 M, 0.5 M and 1 M plus 10 mM H2SO4) at 50 mVs-1.
Electrochemical parameters obtained from three different concentrations of FA on the bare Glassy Carbon Electrode (GCE).
Concentration of HCOOH
ECSA (cm2)
Coefficient of Diffusion
(cm2 s-1)
Peak current density
(mA cm-2)
0.1 M
3.09 × 10-6
12.89 × 10-12
6.83 × 104
0.5 M
2.83 × 10-6
12.85 × 10-12
1.52 × 105
1 M
1.41 × 10-6
13.00 × 10-12
3.06 × 105
As evident from the figure, the anodic peak current density for formic acid oxidation progressively increases with increasing formic acid concentration.
This trend indicates a direct correlation between the number of formic acid molecules available and the rate of the electrocatalytic reaction. At higher concentrations, more formic acid molecules are present near the electrode surface, increasing the probability of collisions with the electrode and participation in the oxidation process. This translates to a higher observed current density [20,21,22].
Electrochemical Impedance Spectroscopy (EIS) Properties of Bare GC Electrode
EIS was used to evaluate charge transfer resistance (Rct), which reflects a catalyst’s activity for formic acid electrooxidation [23,24]. Figure 3 presents EIS data measured at open-circuit potential in a 10 mM H2SO4 solution with 0.1, 0.5, and 1.0 M formic acid (FA) for unmodified and modified electrodes. The data are presented in both (a) Nyquist and (b) Bode formats. As observed in Figure 3a, the diameter of the semicircular arcs in the Nyquist plots decreases with increasing formic acid concentration for both unmodified and modified electrodes. A smaller arc diameter signifies lower Rct, indicating faster charge transfer kinetics at the electrode-electrolyte interface [25,26,27,28]. This trend is further supported by the Bode format plots (Figure 3b), where a decrease in the phase angle at lower frequencies is observed with increasing FA concentration.
EIS data sets presented in (a) Nyquist and (b) Bode plots, obtained for the bare GC electrode in different concentrations of FA in 10 mM aqueous solution.
These observations are consistent with the cyclic voltammetry (CV) results (Section 3.1.2), where higher FA concentration led to increased peak current densities. This suggests that a higher concentration of formic acid molecules facilitates faster reactant interaction with the electrode surface, leading to a lower Rct and improved catalytic activity. The combined results from EIS and CV analyses demonstrate that the modification enhances the electrode’s conductivity and promotes faster charge transfer kinetics, leading to the anticipated improvement in catalytic activity for formic acid oxidation.
Effect of Electrode ModificationCyclic Voltammetry Analysis
This section analyzes cyclic voltammetry (CV) data obtained for bare and g-C3N4 modified glassy carbon electrodes (GCEs). Figure 4 illustrates the CV characterization of the modified GCE. In contrast, Figure 5 presents the CV data used for half-wave peak analysis in a 0.5 M formic acid (FA) solution containing 10 mM H2SO4 at a scan rate of 50 mV/s. Table 2 summarizes and compares the key CV parameters for both electrodes.
(a) CV at the modified GCE in 0.5 M HCOOH + 10 mM H2SO4; (b) Inset linear fitting of peak current density vs. square root of scan rate.
CV at half wave potential for both bare and modified GCE in 0.5 M FA in 10 mM H2SO4 at 50 mVs-1 scan rates.
Electrochemical parameters obtained for bare and g-C<sub>3</sub>N<sub>4</sub>-modified glassy carbon electrodes during formic acid oxidation.
Concentration (M)
Electrochemical Active Surface Area
(ECSA) (cm2)
Peak current density
(mA cm-2)
Bare GCE
0.1 M
3.09 × 10-6
6.83 × 104
0.5 M
2.83 × 10-6
1.52 × 105
1 M
1.41 × 10-6
3.06 × 105
Modified GCE
0.1 M
7.26 × 10-6
6.62 × 106
0.5 M
2.41 × 10-6
9.23 × 106
1.0 M
3.10 × 10-6
13.58 ×106
In electrochemistry, peak current density (jp) signifies the highest level of current measured during a species’ oxidation or reduction at the electrode’s surface. It’s commonly expressed in milliamps per square centimeter (mA cm-2) and considers the electrode’s size. This allows for a more precise comparison of electrocatalytic activity between different electrodes. Generally, higher peak current densities correspond to improved electrocatalytic performance, suggesting a more efficient catalyst [29,30]. As shown in Table 2, the peak current density values exhibit a minimum tenfold increase across all concentrations due to the modification.
The electrochemical surface area (ECSA) represents the portion of the electrode surface actively participating in the electrochemical reaction. A larger ECSA typically means more active sites available for the catalytic reaction, potentially leading to improved performance [31,32]. The higher ECSA for the modified electrode would suggest the presence of more active sites for formic acid oxidation compared to the bare GCE.
The diffusion coefficient of formic acid was estimated from the Warburg impedance coefficient obtained from electrochemical impedance spectroscopy (EIS) measurements. The diffusion coefficient was calculated according to Equation (3), where R is the universal gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons transferred, F is the Faraday constant, C is the concentration of formic acid, and σ is the Warburg coefficient obtained from equivalent circuit fitting, which simplifies to (Equation 3):
D=0.5(RTAn2F2Cσ)2(3)
The results presented in Table 2 demonstrate that modification of the glassy carbon electrode with g-C3N4 significantly enhanced its electrochemical performance toward formic acid oxidation. The modified electrode exhibited substantially higher peak current densities and lower charge-transfer resistance (Rct) compared with the bare electrode, indicating improved electrocatalytic activity and faster interfacial electron-transfer kinetics. In addition, the larger electrochemically active surface area (ECSA) observed for the modified electrode suggests more accessible catalytic sites for formic acid oxidation [33,34,35].
Electrochemical Impedance Spectroscopy Analysis
A detailed analysis of the Nyquist plots in Figure 6 reveals a significant reduction in the diameter of the semicircle for the modified electrode compared to the bare GCE. This 77% decrease suggests enhanced electron-transfer efficiency at the electrode-electrolyte interface. This improvement can be attributed to the increased conductivity of the modified surface due to the incorporation of g-C3N4. The corresponding decrease in charge-transfer resistance (Rct) for the modified electrode further supports this conclusion. The lower Rct signifies faster charge transfer kinetics during the formic acid electrooxidation reaction (FAEOR) on the modified electrode. These findings align with previous studies by Jung et al. [36] and Gao & Goodman [37], which highlight the importance of electronic modifications in improving electrocatalytic performance. One possible explanation for the improved electron transfer efficiency could be the creation of additional pathways for electron transfer between the electrode and the reactants due to the incorporation of g-C3N4.
Impedance spectra of the bare and modified GCE electrodes.
Figure 7 illustrates the equivalent electrical circuit employed to fit the EIS data obtained for both the bare GC and the modified GC electrode. This circuit comprises the electrolyte solution resistance (Rs), the double-layer capacitance (Cdl), the charge-transfer resistance (Rct), and the Warburg impedance (W). The Warburg element accounts for the diffusion of ions from the bulk electrolyte towards the electrode interface.
A simple Randles equivalent electrical circuit (EEC) used in fitting the EIS data.
Chronoamperometry
We used chronoamperometry to assess further the electrocatalytic performance and stability of the bare GCE and g-C3N4-modified GCE (GC/g-C3N4) for formic acid oxidation (FAO) (Figure 8). This technique involves applying a constant potential and monitoring the resulting current over time. As observed in Figure 8, the current initially decreases for both electrodes after applying the potential. This initial decay is common in FA oxidation. It is often attributed to the accumulation of adsorbed intermediates, such as CO, which can block active sites on the electrode surface and reduce catalytic activity [38,39].
CA plots showing the comparable stability of the two electrode systems.
After this initial decrease, both electrodes show a more stable, sustained current, indicating their ability to maintain some activity for FA oxidation. Importantly, the chronoamperometry data reveals that the GC/g-C3N4 electrode exhibits a higher steady-state current density compared to the bare GCE over the extended time period. This observation suggests that the g-C3N4 modification improved the catalyst’s resistance to poisoning by CO intermediates, enhancing long-term stability for FA oxidation.
Surface Characterization of the Glassy Carbon Electrodes
Figure 9 presents atomic force microscopy (AFM) images in topography mode for (a) a freshly polished GCE, (b) g-C3N4-modified GCE, and (c) the bare GCE after a few runs in the electrolyte. All images are shown at the same scale for easier comparison, as in Figures 9b and 9c. Both the modified and bare GCEs after use exhibit heterogeneous surface features, characterized by a roughness that is not apparent on the freshly polished GCE Figure 9a. This surface roughness likely arises from electrochemical processes during formic acid oxidation. Figure 9c also shows some areas with a lower surface profile compared to the rest. These areas could potentially be uncovered regions of the glassy carbon substrate or microcracks that formed during the experiment. One possible explanation is a non-uniform g-C3N4 layer during modification, resulting in incomplete coverage of the underlying glassy carbon.
AFM topographic images of (a) freshly polished GCE, (b) g-C3N4-modified GCE, and (c) bare GCE after electrochemical measurements.
Raman Spectral Analysis
The Raman spectra in Figure 10 show distinct differences between the bare GCE and the g-C3N4/GCE electrode, confirming that surface modification significantly altered its vibrational characteristics. Compared with the bare GCE, the g-C3N4/GCE electrode exhibits enhanced Raman intensity together with broader spectral features, indicating the successful incorporation of graphitic carbon nitride onto the electrode surface. The observed spectral changes are consistent with the formation of a nanostructured, conjugated carbon nitride network and suggest increased structural disorder arising from the interaction between g-C3N4 and the glassy carbon substrate. Such structural modification is expected to increase the electrochemically active surface area, expose additional catalytic active sites, and facilitate electron transfer at the electrode/electrolyte interface. Furthermore, the conjugated framework of g-C3N4 is anticipated to promote stronger adsorption of formic acid molecules, thereby enhancing the kinetics of the formic acid oxidation reaction. These structural characteristics align well with the superior electrocatalytic performance observed for the g-C3N4/GCE electrode in the electrochemical measurements.
Raman Spectroscopy images of (A) Bare GCE (B) g-C3N4/GCE.
Frontier Molecular Orbital Analysis
The optimized graphitic carbon slab employed as the computational model of the glassy carbon electrode is shown in Figure 11. Geometry optimization preserved the hexagonal graphitic framework without significant structural distortion, confirming the structural stability of the model and its suitability for subsequent electronic structure and adsorption calculations.
Optimized graphitic carbon (GCE) model.
The optimized structures together with the frontier molecular orbitals (HOMO and LUMO) of formic acid and nano-g-C3N4 are presented in Figure 12. Geometry optimization maintained the structural integrity of both species. The nano-g-C3N4 retained its characteristic conjugated carbon-nitrogen network, while formic acid preserved its expected molecular geometry. These optimized structures formed the basis for evaluating their electronic properties.
Optimized Structures and frontier molecular orbitals (HOMO and LUMO) of formic acid and g-C3N4 (Triazine). (legend: Carbon = gray; Oxygen = red; Nitrogen = blue; Hydrogen = white) (BIOVIA Materials Studio, version 2021).
The calculated HOMO and LUMO energies of nano-g-C3N4 were -5.051 and -1.932 eV, respectively, corresponding to a HOMO-LUMO energy gap (Eg) of 3.119 eV. In comparison, formic acid exhibited HOMO and LUMO energies of -6.912 and -1.418 eV, respectively, giving a much larger energy gap of 5.494 eV.
As shown in Figure 12, the frontier molecular orbitals of nano-g-C3N4 are extensively delocalized over the conjugated carbon-nitrogen framework, whereas those of formic acid are mainly localized around the carboxyl functional group. In contrast, those of formic acid are mainly localized around the carboxyl functional group.
The considerably smaller energy gap of nano-g-C3N4 indicates that electrons require less energy to be excited from the HOMO to the LUMO, reflecting improved electronic conductivity and enhanced charge-transfer capability. Moreover, extensive orbital delocalization across the nano-g-C3N4 framework provides continuous pathways for electron migration, which is advantageous for electrocatalytic oxidation reactions. These electronic characteristics suggest that nano-g-C3N4 can facilitate rapid electron transfer between the electrode surface and adsorbed formic acid molecules, thereby improving the kinetics of formic acid oxidation.
The optimized adsorption configurations of formic acid on the bare graphitic carbon and nano-g-C3N4-modified graphitic carbon surfaces are presented in Figure 13. The side-view structures show the layered arrangement of the graphitic substrate, the nano-g-C3N4 modifier, and the adsorbed formic acid molecule. In contrast, the top-view structures reveal the preferred adsorption sites on the respective surfaces. The calculated adsorption energy of formic acid on the nano-g-C3N4-modified surface was -83.672 kcal mol-1, substantially more negative than on the bare graphitic carbon surface (-65.365 kcal mol-1). This result demonstrates that nano-g-C3N4 significantly strengthens the interaction between formic acid and the electrode surface, thereby increasing the stability of the adsorbed species.
Optimized adsorption configuration of formic acid on the nano-g-C3N4/GCE composite: top and side view perspectives.
Overall, the DFT calculations provide molecular-level support for the electrochemical results. The lower HOMO-LUMO energy gap and highly delocalized frontier orbitals of nano-g-C3N4 facilitate efficient electron transport. At the same time, the stronger adsorption of formic acid on the modified surface increases the availability of electroactive species at the electrode/electrolyte interface. These synergistic effects account for the enhanced oxidation current observed in cyclic voltammetry, the lower charge-transfer resistance in electrochemical impedance spectroscopy, and the superior electrocatalytic performance of the nano-g-C3N4-modified glassy carbon electrode toward formic acid oxidation.
Proposed Mechanism for Formic Acid Oxidation on Nano-g-C<sub>3</sub>N<sub>4</sub>/GCE
Figure 14 presents the proposed mechanism for electrocatalytic formic acid oxidation on the nano-g-C3N4-modified glassy carbon electrode. Formic acid molecules are initially adsorbed onto the g-C3N4-modified surface through interactions with the nitrogen-rich active sites, consistent with the more negative adsorption energy obtained from the molecular modelling calculations. The conjugated π-electron network of g-C3N4 facilitates rapid electron transport from the adsorbed formic acid molecules to the underlying glassy carbon electrode, thereby accelerating the oxidation reaction.
Proposed mechanism for formic acid oxidation on nano-g-C3N4/GCE.
The enhanced adsorption of formic acid on the modified surface increases the concentration of electroactive species at the electrode/electrolyte interface. At the same time, the smaller HOMO-LUMO energy gap and extensive orbital delocalization of g-C3N4 promote efficient charge transfer. These electronic characteristics agree with the electrochemical results, which showed significantly higher oxidation current densities and lower charge-transfer resistance for the modified electrode compared with the bare GCE. The oxidation of formic acid ultimately produces carbon dioxide with the release of protons and electrons according to: The proposed mechanism therefore suggests that the principal role of nano-g-C3N4 is not to serve as a reactant but to provide abundant adsorption sites and efficient electron-transfer pathways that lower the kinetic barrier for formic acid electrooxidation, leading to the observed enhancement in electrocatalytic performance.
Conclusion
This study demonstrated enhanced formic acid oxidation reaction (FAOR) by modifying a glassy carbon (GC) electrode with g-C3N4 nanoparticles (NPs). Key electrochemical techniques, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry (CA), were employed to characterize the modified electrodes. The analysis revealed superior electrocatalytic performance for the g-C3N4/GC electrode compared to the bare GCE. This improvement was evident in higher peak current densities in CV and lower Rct values in EIS. The modified electrode also displayed a larger electrochemically active surface area and facilitated faster charge-transfer kinetics at the electrode-electrolyte interface. These characteristics can be attributed to the increased conductivity and presence of more active sites due to g-C3N4. Overall, the findings suggest that g-C3N4/GC electrodes hold promise for advancing formic acid fuel cell technologies.
Although the present study focused on acidic media relevant to direct formic acid fuel cells, future investigations should evaluate the electrocatalytic performance of the g-C3N4-modified electrode under alkaline conditions to assess its broader applicability and reaction behavior across different electrolyte environments.
Acknowledgments
This work was supported by African Centre of Excellence in Future Energies and Electrochemical Systems Federal University of Technology Owerri (ACE-FUTO | FUTO) Nigeria, under the sponsorship of World Bank.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data Availability Statement
Data will be made available on request.
AI-Assisted Technologies Statement
AI disclosure: This manuscript was conceived, designed, interpreted, and written by the authors. All scientific analyses, experimental procedures, data interpretation, and conclusions are entirely the work of the authors. Artificial intelligence tools (OpenAI ChatGPT) were used solely for grammar checking, language editing, and improvement of readability, and ChatGPT assisted with the reorganization of graphical elements and visual refinement of Figure 14. No AI tool was used to generate scientific data, perform analyses, interpret results, or formulate conclusions. The authors carefully reviewed and edited all AI-assisted text and took full responsibility for the content of this manuscript.
ReferencesKoperMT,
LaiSC,
HerreroE.
Mechanisms of the oxidation of carbon monoxide and small organic molecules at metal electrodes. In:
Fuel cell catalysis: A surface science approach.
Hoboken, NJ:
John Wiley & Sons;
2009. pp.
159-207.
Ferre-VilaplanaA,
Perales-RondónJV,
FeliuJM,
HerreroE.
Understanding the effect of the adatoms in the formic acid oxidation mechanism on Pt (111) electrodes.
ACS Catal.
2015;
5:
645-654.
WangZL,
YanJM,
WangHL,
PingY,
JiangQ.
Pd/C synthesized with citric acid: An efficient catalyst for hydrogen generation from formic acid/sodium formate.
Sci Rep.
2012;
2:
598.
EppingerJ,
HuangKW.
Formic acid as a hydrogen energy carrier.
ACS Energy Lett.
2017;
2:
188-195.
TingSW,
ChengS,
TsangKY,
van der LaakN,
ChanKY.
Low activation energy dehydrogenation of aqueous formic acid on platinum-ruthenium-bismuth oxide at near ambient temperature and pressure.
Chem Commun.
2009;
45:
7333-7335.
FangZ,
ChenW.
Recent advances in formic acid electro-oxidation: From the fundamental mechanism to electrocatalysts.
Nanoscale Adv.
2021;
3:
94-105.
Vidal-IglesiasFJ,
Solla-GullonJ,
HerreroE,
AldazA,
FeliuJM.
Formic acid oxidation on Pd-modified Pt (100) and Pt (111) electrodes: A DEMS study.
J Appl Electrochem.
2006;
36:
1207-1214.
CantilloNM,
Solla-GullónJ,
HerreroE,
SánchezC.
Ethanol electrooxidation on PtSnNi/C nanoparticles prepared in water-in-oil microemulsion.
ECS Trans.
2011;
41:
1307-1316.
MatsumotoF,
RoychowdhuryC,
DiSalvoFJ,
AbruñaHD.
Electrocatalytic activity of ordered intermetallic PtPb nanoparticles prepared by borohydride reduction toward formic acid oxidation.
J Electrochem Soc.
2008;
155:
B148-B154.
RoychowdhuryC,
MatsumotoF,
ZeldovichVB,
WarrenSC,
MutoloPF,
BallesterosM,
.
Synthesis, characterization, and electrocatalytic activity of PtBi and PtPb nanoparticles prepared by borohydride reduction in methanol.
Chem Mater.
2006;
18:
3365-3372.
Busó-RogeroC,
Perales-RondónJV,
FariasMJ,
Vidal-IglesiasFJ,
Solla-GullonJ,
HerreroE,
.
Formic acid electrooxidation on thallium-decorated shape-controlled platinum nanoparticles: An improvement in electrocatalytic activity.
Phys Chem Chem Phys.
2014;
16:
13616-13624.
YiY,
WeinbergG,
PrenzelM,
GreinerM,
HeumannS,
BeckerS,
.
Electrochemical corrosion of a glassy carbon electrode.
Catal Today.
2017;
295:
32-40.
DongG,
ZhangY,
PanQ,
QiuJ.
A fantastic graphitic carbon nitride (g-C3N4) material: Electronic structure, photocatalytic and photoelectronic properties.
J Photochem Photobiol C Photochem Rev.
2014;
20:
33-50.
IdrisAO,
OsegheEO,
MsagatiTA,
KuvaregaAT,
FeleniU,
MambaB.
Graphitic carbon nitride: A highly electroactive nanomaterial for environmental and clinical sensing.
Sensors.
2020;
20:
5743.
AziziN,
EdrisiM.
Preparation of choline sulfate ionic liquid supported on porous graphitic carbon nitride nanosheets by simple surface modification for enhanced catalytic properties.
J Mol Liq.
2020;
300:
112263.
LiuW,
IwasaN,
FujitaS,
KoizumiH,
YamaguchiM,
ShimadaT.
Porous graphitic carbon nitride nanoplates obtained by a combined exfoliation strategy for enhanced visible light photocatalytic activity.
Appl Surf Sci.
2020;
499:
143901.
ÇögenliMS,
YurtcanAB.
Graphene aerogel supported platinum nanoparticles for formic acid electro-oxidation.
Mater Res Express.
2018;
5:
075513.
HabibiB,
MohammadyariS.
Facile synthesis of Pd nanoparticles on nano carbon supports and their application as an electrocatalyst for oxidation of ethanol in alkaline media: The effect of support.
Int J Hydrogen Energy.
2015;
40:
10833-10846.
YilmazS,
SadikogluM,
SaglikogluG,
YagmurS,
AskinG.
Determination of ascorbic acid in tablet dosage forms and some fruit juices by DPV.
Int J Electrochem Sci.
2008;
3:
1534-1542.
XuJ,
YuanD,
YangF,
MeiD,
ZhangZ,
ChenYX.
On the mechanism of the direct pathway for formic acid oxidation at a Pt (111) electrode.
Phys Chem Chem Phys.
2013;
15:
4367-4376.
SyaifarizA,
KurniawanC.
Electroanalysis of formaldehyde using electrodeposited nickel on glassy carbon electrode.
Sainteknol.
2021;
19:
16-27.
ErasmusE,
ClaassenJO,
Van Der WesthuizenWA.
Catalytic wet peroxide oxidation of formic acid in wastewater with naturally-occurring iron ore.
Water SA.
2016;
42:
442-448.
KimJH,
HaHY,
OhIH,
HongSA,
KimHN,
LeeHI.
Electrochemical studies of DMFC anodes with different ionomer content.
Electrochim Acta.
2004;
50:
801-806.
QuWL,
WangZB,
JiangZZ,
GuDM,
YinGP.
Investigation on performance of Pd/Al2O3-C catalyst synthesized by microwave assisted polyol process for electrooxidation of formic acid.
RSC Adv.
2011;
2:
344-350.
TanwarS,
SharmaA,
MathurD.
A graphene quantum dots-glassy carbon electrode-based electrochemical sensor for monitoring malathion.
Beilstein J Nanotechnol.
2023;
14:
701-710.
LiuX,
JacobT,
GaoW.
Progress of fundamental mechanism of formic acid decomposition and electrooxidation.
J Energy Chem.
2022;
70:
292-309.
KuralayF,
ErdemA,
AbacıS,
ÖzyörükH.
Electrochemical characterization of redox polymer modified electrode developed for monitoring of adenine.
Colloids Surf B Biointerfaces.
2013;
105:
1-6.
LinH,
YangJ,
LiuJ,
HuangY,
XiaoJ,
ZhangX.
Properties of Pd nanoparticles-embedded polyaniline multilayer film and its electrocatalytic activity for hydrazine oxidation.
Electrochim Acta.
2013;
90:
382-392.
BulutA,
YurderiM,
AlalO,
KivrakH,
KayaM,
ZahmakiranM.
Synthesis, characterization, and enhanced formic acid electrooxidation activity of carbon supported MnOx promoted Pd nanoparticles.
Adv Powder Technol.
2018;
29:
1409-1416.
FolkmanSJ,
González-CobosJ,
GiancolaS,
Sánchez-MolinaI,
Galán-MascarósJR.
Benchmarking catalysts for formic acid/formate electrooxidation.
Molecules.
2021;
26:
4756.
LiY,
YaoMS,
HeY,
DuS.
Recent advances of electrocatalysts and electrodes for direct formic acid fuel cells: From nano to meter scale challenges.
Nano-Micro Lett.
2025;
17:
148.
DmitrievDS,
TenevichMI.
Evaluation of the electrochemical active surface area for carbon felt and nanostructured Ni coatings as electrocatalysts for hydrogen evolution reaction.
Nanosyst Phys Chem Math.
2023;
14:
590-600.
RadhiMM,
IbrahimAI,
Al-HaidarieYK,
Al-AsadiSA,
Al-MullaEA.
Rifampicin: Electrochemical effect on blood component by cyclic voltammetry using nano-sensor.
Nano Biomed Eng.
2019;
11:
150-156.
RadhiMM,
MossaAA,
Al-MullaEA,
LaftaAN.
Electrochemical study of modified glassy carbon electrode with polyaniline nanoparticles using cyclic voltammetry.
Bull Chem Soc Ethiop.
2022;
36:
687-696.
ZanelloP,
NerviC,
Fabrizi de BianiF.
Fundamentals of electrode reactions. In:
Inorganic electrochemistry: Theory, practice and application.
Cambridge, UK:
Royal Society of Chemistry;
2003. pp.
3-41.
JungWS,
HanJ.
Enhanced stability of PdPtAu alloy catalyst for formic acid oxidation.
Korean J Chem Eng.
2021;
38:
2229-2234.
GaoF,
GoodmanDW.
Pd-Au bimetallic catalysts: Understanding alloy effects from planar models and (supported) nanoparticles.
Chem Soc Rev.
2012;
41:
8009-8020.
ZhangLY,
ZhaoZL,
LiCM.
Formic acid-reduced ultrasmall Pd nanocrystals on graphene to provide superior electocatalytic activity and stability toward formic acid oxidation.
Nano Energy.
2015;
11:
71-77.
ZhangLY,
WangF,
WangS,
HuangH,
MengX,
OuyangY,
.
Layered and heterostructured Pd/PdWCr sheet‐assembled nanoflowers as highly active and stable electrocatalysts for formic acid oxidation.
Adv Funct Mater.
2020;
30:
2003933.