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Article

Sustainable Fe3C/Fe-Nx-C Cathode Catalyst from Biomass for an Oxygen Reduction Reaction in Alkaline Electrolytes and Zinc–Air Battery Application

by
Shaik Gouse Peera
1,*,
Seung-Won Kim
1,
Shaik Ashmath
2 and
Tae-Gwan Lee
2,*
1
Natural Science Research Institute, College of Natural Sciences, Keimyung University, 1095, Dalseo-gu, Daegu 42601, Republic of Korea
2
Department of Environmental Engineering, College of Engineering, Keimyung University, 1095, Dalseo-gu, Daegu 42601, Republic of Korea
*
Authors to whom correspondence should be addressed.
Inorganics 2025, 13(5), 143; https://doi.org/10.3390/inorganics13050143
Submission received: 11 April 2025 / Revised: 23 April 2025 / Accepted: 28 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Carbon Nanomaterials for Advanced Technology, 2nd Edition)
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Abstract

Realistic applications of zinc–air batteries are hindered by the high cost of Pt/C cathode catalysts, necessitating the search for alternative, sustainable electrocatalysts. In this work, we developed a sustainable Fe3C/Fe-Nx-C cathode catalyst from waste coffee biomass for an oxygen reduction reaction (ORR) in alkaline electrolytes and zinc–air battery applications. The Fe3C/Fe-Nx-C cathode catalyst was synthesized via a mechanochemical synthesis strategy by using melamine and an EDTA–Fe chelate complex, followed by pyrolysis at 900 °C. The obtained Fe3C/Fe-Nx-C catalyst was evaluated for detailed ORR activity and stability. The ORR results show that Fe3C/Fe-Nx-C displayed excellent ORR activity with an E1/2 of 0.93 V vs. RHE, a Tafel slope of 68 mV dec−1, 3.95 e− transfer for the O2 molecule, and high ECSA values. In addition, the Fe3C/Fe-Nx-C catalyst exhibited excellent stability with a loss of 75 mV for 10,000 potential cycles, and a loss of ~14% of relative currents in the chronoamperometric test. When applied as a cathode catalyst in zinc–air battery, the Fe3C/Fe-Nx-C catalyst delivered a power density of 81 mW cm−2 and admirable electrochemical stability under galvanostatic discharge conditions. Furthermore, the practical application of the Fe3C/Fe-Nx-C catalyst was demonstrated by a panel of LEDs illuminated with a dual-cell zinc–air battery connected in a series, clearly validating the practically developed catalysts for use in various energy storage and electronic devices.

1. Introduction

The utilization of a high amount of electricity has become a major factor in the rapid industrialization and advancement of society, especially for electric automotives, flexible electronics and smart grids [1,2]. At present, the high demand for electricity is met by the use of non-renewable energy sources such as fossil fuels and coal, which has led to increased levels of CO2 in the atmosphere, a major environmental problem. Therefore, researchers have undertaken a quest for alternative energy technologies such as fuel cells, supercapacitors, solar cells, lithium-ion batteries, dual-ion batteries, and microbial fuel cells [3,4,5,6,7]. In this regard, Zn–air batteries (ZABs) have become increasingly promising technologies due to their high specific energy density (1086 Wh kg−1, including oxygen) and their potential applications in wearable electronics and hybrid electric vehicles [8]. The successful implementation of ZABs is very much possible due to the abundance of Zn in the Earth’s crust, being almost 300 times higher in abundance than Li, and Zn is the fourth most abundant element [9]. However, the commercialization of ZABs is hindered by slow and sluggish reactions, especially at the air–cathode interface, which is composed of gas diffusion layers and electrocatalysts that catalyze the reduction of O2 into OH¯ ions [10]. The sluggish nature of an oxygen reduction reaction (ORR) at the cathode leads to high overpotentials and low output power density, necessitating highly active electrocatalysts such as Pt/C, which remain as the standard and benchmark catalysts for the ORR due to their excellent ability to break the strong O=O bond (498 kJ mol−1). However, due to its high cost and scarcity in the Earth’s crust, the use of Pt/C-based catalysts is challenging in commercial applications [11]. In addition, the low stability of Pt/C in an alkaline environment results in the performance degradation of Zn–air batteries, mainly due to the electrochemical corrosion of the carbon support. Therefore, to improve the performance and stability of ZABs, it is paramount to develop cathodic electrocatalysts composed of cost-effective metals with high electrocatalytic activity comparable with commercial Pt/C standards [12].
Among several alternative catalysts, transition metals (Fe, Co, Mn, Cu, Ni, etc.) in combination with heteroatoms such as N-, S-, B-, P- and F-based catalysts have been most widely assessed in recent years owing to their abundance and cost-effective nature. Transition metal catalysts have been employed for use in various electrochemical reactions such as HER, OER, NRR, and the electrooxidation of methane [13,14,15,16], due to their similar ORR kinetics to those of Pt/C catalysts. Fe-N4-C and Fe3C are thought to be two highly active catalysts from the transition metal catalyst category [17]. Several experimental and theoretical studies suggest that Fe-N4-C catalysts boost ORR performance owing to the presence of highly dispersed atomic FeNx moieties, which effectively optimize the adsorption energy of oxygen-containing species on active centers and have great potential use as catalyst alternatives to Pt/C [18]. The ORR activity of Fe-N4 catalysts is attributed to several complex factors such as the density of Fe-Nx moieties and the electronic nature of the carbon support, with dopants providing either an electron-withdrawing/donating property affecting the d-band center of Fe or changes to the spin states of Fe [19]. On the other hand, researchers also showed that the co-presence of the metallic Fe and/or Fe3C phase together with Fe-Nx further boosts ORR performance [20,21]. The ORR activity of Fe3C and N-doped carbon layers involves electron transfer from N-doped carbon to the Fe metallic active site, enhancing the electronic density around Fe and thus reducing the work function, therefore favoring efficient ORR kinetics [22,23]. In addition, with the high electronic conductivity arising from N-dopants, which modifies the electronic structure of Fe, the bending strength of Fe3C is further enhanced [24]. Based on these assumptions, several studies suggest that the co-existence of Fe3C and Fe-N4-C active sites synergistically enhances the ORR activity and stability of fuel cell electrocatalysts [25,26,27,28,29]. Therefore, it is reasonable to assume that Fe3C/Fe-N4-C catalysts are worth investigating for ORR, aiming to achieve ORR kinetics similar to those in Pt/C catalysts. In addition, with regard to both economic and environmental issues, the best electrocatalyst material for cathodes should be derived from naturally occurring sources.
Recently, electrocatalysts derived from renewable biomass have been attracting increasing attention due to their availability from various sources and the ease with which they can be transformed into carbon-based materials via high-temperature carbonization [30]. Furthermore, the incorporation of heteroatoms such as N, P, S and F, etc., and/or hybridization with other transition metals, is possible due to their simultaneous presence during high-temperature carbonization, which results in highly active electrocatalysts [31]. The resulting electrocatalysts were found to have large surface areas, high porosity, enhanced electronic conductivity, and high thermal stability. The utilization of electrocatalysts derived from biomass as the ZAB’s air electrodes has the potential to significantly reduce the manufacturing cost, which in turn provides enormous economic benefits and offers multiple applications with regard to air electrodes, including as the backing layer, the gas diffusion layer, and the catalyst support. According to this perspective, a variety of biomasses have been employed as sources of carbon and electrocatalysts, such as agricultural crops and residues, forest crops, industrial and poultry waste, marine wastes and domestic wastes, etc. [32]. Among various types of biomasses, waste coffee-based biomass has attracted tremendous attention recently due to its large consumption rate worldwide, resulting in an estimated amount of ~10 million tons of coffee waste being generated annually. When these waste coffee biomasses are disposed of in a landfill, they undergo bacterial decomposition, emitting greenhouse gases such as methane and CO2, which negatively impact the environment [33]. Therefore, employing coffee waste and transforming it into porous carbons and electrocatalysts not only reduces the costs of the raw materials but also protects the environment. Numerous initiatives have been undertaken to utilize the coffee waste-derived carbons and electrocatalysts for various electrocatalytic reactions, such as ORR, OER, HER, supercapacitor, photocatalyst reactions, and for hydrogen storage [34,35,36]. Consequently, it is evident that used coffee grounds could be one of the best choices to use as raw materials for synthesizing porous carbon nanomaterials with metallic active sites, which can themselves be used as sustainable ORR catalysts.
We have recently developed sustainable, atomically dispersed Fe3C/Fe-Nx-C electrocatalysts mediated by Fe-complexation, utilizing waste coffee grounds [37]. Ethylenediaminetetraacetic acid (EDTA), a well-known hexadentate ligand, has been used to create Fe-Nx coordination sites together with melamine as the N-source. In this process, a mixture of processed waste coffee grounds, the EDTA–Fe co-ordination complex and melamine is subjected to pyrolysis at 900 °C to obtain the Fe3C/Fe-Nx-C catalyst. The synthesized Fe3C/Fe-Nx-C catalysts’ ORR performance is assessed in 0.1 M HClO4, and it was found that the optimized Fe3C/Fe-Nx-C catalyst demonstrated a half-wave potential of 0.85 V vs. RHE with excellent stability through a loss of 50 mV in half-wave potential over 10,000 potential cycles. When utilized as cathode catalysts in microbial fuel cells, a volumetric power density of 95 mW m−3 was attained, alongside an organic matter degradation of 68% over a period of 30 days of continuous operation.
Inspired by the obtained ORR activity of the biomass-derived Fe3C/Fe-Nx-C catalyst in 0.1 M HClO4 electrolytes in our earlier study [37], this study evaluated the detailed ORR activity of the previously synthesized Fe3C/Fe-Nx-C catalyst in the 0.1 M KOH electrolyte targeting its application in Zn–air batteries. We have systematically assessed the ORR activity of previously synthesized Fe3C/Fe-Nx-C catalysts by evaluating the ORR activity in terms of half-wave potentials, electrochemical surface area measurements (ECSA), stability, Zn–air battery performance and electrochemical stability, and demonstrated the practical applications of the Zn–air battery by utilizing sustainable Fe3C/Fe-Nx-C catalysts derived from waste coffee grounds.

2. Results

The Fe3C/Fe-Nx-C catalyst is synthesized as per our previous report [37]. As comprehensive physicochemical characterizations of the Fe3C/Fe-Nx-C catalyst have been conducted in our prior studies, we ask the readers to refer to our earlier report for the XRD, BET, SEM, TEM and XPS characterization of the catalysts [37]. This study exclusively examines the ORR of Fe3C/Fe-Nx-C catalysts in 0.1 M KOH electrolyte and Zn–air battery applications (Figure 1).
This section will succinctly address the synthesis process and physical characterizations of the catalysts outlined in our earlier report [37]. The purified coffee waste grounds were subjected to pyrolysis in the presence of melamine to obtain N-doped carbon catalyst (C–N–C). The EDTA and Fe nitrate are ground with the help of a mortar and pestle to obtain the EDTA–Fe complex, which results in an octahedral co-ordination complex wherein the amine and carboxylic groups of EDTA form co-ordination bonds with Fe. The resulting EDTA–Fe complex is then mixed with waste coffee powder and melamine in a mortar and pestle to obtain the final catalyst precursor. The precursor is then pyrolyzed in a tubular furnace at 900 °C in N2 atmosphere for 1 h to obtain the Fe3C/Fe-Nx-C catalyst. The XRD results for the Fe3C/Fe-Nx-C catalyst indicate low-intensity peaks of Fe3C phases, while the HR-TEM images show no visible nanoparticles of Fe3C/metallic Fe particles, indicating that the Fe3C phase likely exists in the form of nanoclusters, with the majority of Fe atoms atomically dispersed on the N-doped carbon derived from the waste coffee powder. The Fe3C/Fe-Nx-C catalyst exhibits a BET surface area of 370 m2/g, characterized by a significant presence of mesopores in the range of 3.82 nm. The XPS analysis of the Fe3C/Fe-Nx-C catalyst reveals the presence of pyridinic-N, pyrrolic-N, and graphitic-N carbons, as well as the existence of pyridinic-N-Fe. Multiple studies suggest that the pyridinic-N’s unpaired electrons can potentially bond with transition metal atoms, such as Fe (Figure S1). This may generate highly active Fe–N4–C active sites within the catalysts [38,39]. The resultant Fe3C/Fe-Nx-C catalyst is then used as the ORR catalysts in the 0.1 M HClO4 electrolyte, demonstrating commendable ORR activity with a half-wave potential of 0.85 V vs. RHE in 0.1 M HClO4 electrolyte, and when applied as a cathode catalyst in the microbial fuel cell, it attained a volumetric power density of 95 mW cm−3. This study employed identical catalysts to examine the ORR in alkaline electrolytes, with the objective of utilizing the Fe3C/Fe-Nx-C catalyst as a cathode in Zn-air batteries. Figure 2a shows the cyclic voltammetric profiles of C–N–C and several Fe3C/FeNx-C catalysts with differing Fe wt. % in the catalyst synthesis. The electrochemical studies were carried out in an O2-saturated 0.1 M KOH solution. All the catalysts exhibited a characteristic CV curve featuring a peak at the reduction current that can be ascribed to a reduction in gaseous O2 [40]. It is observed that the O2 reduction peak potentials shifted to higher potential for all the Fe3C/FeNx-C catalysts compared to the C–N–C catalyst, indicating that the presence of Fe in conjunction with N atoms in the carbon matrix enhances electrocatalytic activity. Figure 2b illustrates the magnified sections of the CV curves depicting the O2 reduction peak potentials, which clearly show the peak potentials were consistently shifted to higher values with increasing amounts of Fe wt. %. This clearly suggests that the density of Fe in the catalyst is paramount for enhancing the number of active sites in the catalyst. Quantitatively, the O2 reduction peak potential for C–N–C is 0.67 V vs. RHE, while the highest O2 reduction peak potential for Fe3C/Fe-Nx-C-10 is 0.83 V vs. RHE, a gain of 160 mV with the introduction of Fe into the C–N–C matrix. Figure 2c shows the linear sweep voltametric analysis of the C–N–C and Fe3C/Fe-Nx-C. All the catalysts exhibited typical LSV curves with kinetic, mixed kinetic–diffusion and limiting currents, thereby indicating that all the catalysts are electrocatalytically active in relation to ORR.
The superiority of the synthesized electrocatalysts has been assessed through analyzing their half-wave potentials (E1/2) [41]. The measured E1/2 potentials are 0.72, 0.77, 0.79, 0.87, 0.93 and 0.90 V vs. RHE for C–N–C, Fe3C/Fe-Nx-C-2, 5, 8, 10 and 15 catalysts, respectively. Consistent with the CV observations (Figure 2b), the half-wave potentials steadily raised while increasing the Fe content in the catalyst layer, reaching a maximum of 0.93 V vs. RHE for the Fe3C/Fe-Nx-C-10 catalysts (Figure 2d). The high density of Fe in conjunction with N-doped carbon promotes the formation of Fe-Nx coordination sites and their improved distribution, leading to an increased density of ORR active sites and consequently elevated half-wave potentials. However, the ORR activity of catalysts with a higher wt. % of Fe, i.e., 15 wt. % (Fe3C/Fe-Nx-C-15 catalyst), was inferior to that of the Fe3C/Fe-Nx-C-10 catalyst, despite having a high quantity of Fe as active sites. We presume that a higher wt. % of Fe might may result in poor dispersion and Fe3C cluster aggregation, which could lead to a slightly smaller accessible surface area, and hence a reduction in ORR activity.
With the highest ORR reduction potentials from CV and highest half-wave potentials from the LSV curves, it can be concluded that the Fe3C/Fe-Nx-C-10 catalyst is the optimum catalyst synthesized with 10 wt. % of the Fe precursor. The superiority of the Fe3C/Fe-Nx-C-10 catalyst is further contrasted with that of the commercial Pt/graphitized carbon (40 wt. %) catalysts, as shown in the Figure 2e. Notably, the Fe3C/Fe-Nx-C-10 catalyst exhibits an ORR activity comparable to that of Pt/graphitized carbon (40 wt. %), with half-wave potentials of 0.93 V vs. RHE in a 0.1 M KOH electrolyte. This suggests that the Fe3C/Fe-Nx-C-10 catalyst may serve as a viable alternative to conventional Pt-based catalysts for ORR in zinc–air batteries. To further understand the ORR kinetics, Tafel plots were derived, as shown in Figure 2f. The measured Tafel slopes are 70 and 68 mV dec−1 for the Pt/C and Fe3C/Fe-Nx-C-10 catalyst, respectively, suggesting that both the catalysts have similar ORR kinetics, with the formation of *OOH as the rate determining step [42].
The C–N–C, Fe3C/Fe-Nx-C and Pt/C catalysts were further examined for their kinetic and diffusion-controlled ORR activity by recording the LSV curves at different rpms, as shown in Figure 3a–g. The onset potentials of the ORR were not affected by any of the catalysts, but the limiting current regions increased with the increased rotations per minute (rpm), which indicates that the ORR occurred in the diffusion-controlled reaction. This indicates that the intrinsic catalytic activity relies on the mass transport of oxygen from the bulk electrolyte to the electrode surface [43]. Furthermore, the number of electrons transferred in the O2 reduction pathway is further derived from the different rpm data and plotted against the inverse of the current density vs. the inverse of the root square of rpm, yielding what is known as K-L plots, as shown in Figure 3h and Figure S2a–g. The K-L plots exhibit a linear relationship at various potentials, indicating that all catalysts adhere to analogous ORR kinetics across different potentials.
Comparative K-L plots of the catalysts at 1600 rpm suggest that all the Fe3C/Fe-Nx-C show nearly similar trends to the commercial Pt/C catalysts, suggesting that Fe3C/Fe-Nx-C and Pt/C catalysts have similar ORR kinetics (Figure 3h). The numbers of electrons derived from K-L plots are found to be 3.65, 3.75, 3.82, 3.91, 3.95, 3.95, 3.95 and 3.98 for C-NC, Fe3C/Fe-Nx-C-2, 5, 8, 10 and Pt/C catalysts, respectively. From the “n” values, it is clear that the presence of Fe in the C–N–C catalysts shifts the ORR process more efficiently due to the presence of Fe-Nx-C-type active sites in the catalysts, evident from the higher “n” for Fe3C/Fe-Nx-C catalysts than the C–N–C catalysts. Further, it is observed that the “n” value steadily raises with an increase in the Fe content, which suggests that a higher Fe content enhances the ORR activity, peaking at 3.95 e− for the Fe3C/Fe-Nx-C-8, 10 and 15 catalysts. The “n” of nearly 4 for the Fe3C/Fe-Nx-C-8, 10 and 15 catalysts suggests a direct reduction of O2 to OH−, which is desirable for the ORR catalysts in Zn–air batteries. To assess the enhanced ORR activity of the Fe3C/Fe-Nx-C catalysts, the electrochemical surface area of the catalysts was measured by developing CV curves in N2 saturated 0.1 M KOH electrolyte at different scan rates in a non-faradaic region. All catalysts exhibited an elevated current with increased scan rates, signifying that a higher scanning rate enhances the electron transfer reaction rate on the electrode material’s surface [44]. The double-layer capacitance (Cdl) is determined from the slope of the capacitive current, which is derived by averaging the anodic and cathodic currents at the selected potential (Figures S3 and S4). Subsequently, the electrochemical active surface area (ECSA) is calculated by dividing Cdl by the specific capacitance (F/cm2) of the N-CC carbon [45]. The obtained Cdl and ECSA values are given in Table 1. The table indicates that the elevated Cdl and ECSA values for Fe3C/Fe-Nx-C-10 catalysts signify a high density of available active sites, implying that Fe3C/Fe-Nx-C-10 exhibited superior ORR activity, as we see from the highest half-wave potential of 0.93 V vs. RHE.
In addition to the high ORR activity, the stability of the Fe3C/Fe-Nx-C-10 and Pt/C catalysts was evaluated by cyclic voltmeter and chronoamperometric studies. The GC electrode with the catalyst deposited on it was subjected to potential cycling between 0 and 1.23 V vs. RHE, and the degradation of the catalyst was assessed through the half-wave potential analysis by recording LSV curves after 10,000 potential cycles. Figure 4a,b shows the CV and LSV curves of the Pt/C catalyst, showing that the area under the CV curves represents the ECSA of the Pt nanoparticles, which were diminished with respect to the potential cycles. This suggests that the Pt/C catalyst undergoes a slight degradation, potentially attributed to several phenomena, such as carbon corrosion and Pt nanoparticle dissolution/agglomeration, the latter of which decrease the effective ECSA of Pt, thereby reducing the available active sites and hence leading to the lower ORR activity [46]. The LSV curves show a slight negative shift in the half-wave potential of 60 mV compared to the original half-wave potential of 0.93 V vs. RHE. In contrast, the Fe3C/Fe-Nx-C-10 catalyst under similar testing conditions experienced a decline of 75 mV in the half-wave potential following 10,000 potential cycles. (Figure 4c,d). In addition to the potential cycling, chronoamperometric stability of the Fe3C/Fe-Nx-C-10 and Pt/C catalyst has been evaluated in the O2 saturated 0.1 M KOH electrolyte at the potential of 0.60 V vs. RHE, and the results obtained are shown in Figure 4e. After a continuous 10 h chronoamperometric test, it was found that both Fe3C/Fe-Nx-C-10 and Pt/C catalysts deliver comparable performances, with a relative current loss of approximately 14%, which suggests that the developed Fe3C/Fe-Nx-C-10 catalyst performs similarly to the commercial Pt/C catalysts, thereby positioning it as a viable alternative to conventional Pt-based catalysts for the oxygen reduction reaction in zinc–air battery applications.
Following a comprehensive analysis of ORR kinetics and a stability analysis of the Fe3C/Fe-Nx-C-10 and Pt/C catalysts, their practical application has been evaluated in a custom-built Zn–air battery cell, as shown in Figure 5. The practical application of the Fe3C/Fe-Nx-C-10 catalyst has been evaluated by i–v and galvanostatic discharge curve analysis. Figure 6a shows the OCV values of the Fe3C/Fe-Nx-C-10 and Pt/C catalysts, which stand out with the maximum potentials of 1.45 and 1.41 V, respectively. Figure 5b shows the i-v curves of the single-cell zinc–air battery, which shows a typical i-v curve of a Zn–air battery. They obtained maximum power density outputs of 81 and 98 mW for Fe3C/Fe-Nx-C-10 and Pt/C catalysts, respectively. This indicates that the Fe3C/Fe-Nx-C-10 catalyst may serve as an excellent alternative to conventional Pt/C catalysts, potentially providing comparable performance to commercial Pt/C catalysts. Alongside the analysis of power density output, the Fe3C/Fe-Nx-C-10 and Pt/C catalysts are assessed for voltage stability, rate performance and electrode stability by a galvanostatic discharge test, which serves as the fundamental mode of evaluation for the developed catalysts in the zinc–air battery set up. To evaluate this, the galvanostatic discharge curves were obtained by discharging a definite amount of current at different intervals of time, as shown in Figure 5c,d. From the galvanostatic discharge curves, it can be observed that at each given discharge current, the obtained potential remained stable over time, indicating that both Fe3C/Fe-Nx-C-10 and Pt/C catalysts exhibited excellent electrochemical stability under different load currents. The stable voltage at different current loads further suggests efficient ORR kinetics, as well as structural and electrochemical stability suitable for practical zinc–air battery applications. To further demonstrate the practical efficacy of the Fe3C/Fe-Nx-C-10 catalyst, we assembled two zinc–air batteries connected in series, and their abilities were evaluated via the i–v and galvanostatic discharge curves, as shown in Figure 6. The dual-cell zinc–air battery delivered a maximum voltage OCV of 2.87 V and a maximum power density of 120 mW cm−2 with Fe3C/Fe-Nx-C-10 as the cathode catalyst. Furthermore, the dual-cell zinc–air battery demonstrated stable galvanostatic discharge curves recorded at different load currents, indicating that the dual-cell zinc–air battery exhibited excellent electrochemical stability under different load currents. Figure 6d shows a practical demonstration of using the Fe3C/Fe-Nx-C-10 as a cathode catalyst by illuminating the light emitting diode (LED), which clearly demonstrates the practical application of the developed catalysts in various energy storage and electronic devices.
The enhanced ORR activity of the Fe3C/Fe-Nx-C-10 is ascribed to the synergistic interaction between Fe3C nanoclusters and the Fe-Nx-C active sites within the catalysts. In our previous study [37], we clearly established the presence of Fe3C nanoclusters from the XRD analysis of various Fe3C/Fe-Nx-C catalysts. Furthermore, the N1s XPS analysis of the Fe3C/Fe-Nx-C-10 catalyst revealed the presence of pyridinic-N-Fe, which clearly hints at the presence of Fe-Nx active sites in the catalyst. Based on these observations, we can infer the factors contributing to the enhanced ORR activity of the Fe/Fe3C-Fe-N-C catalyst shown in the existing open-access literature, as follows. Several studies suggest that the presence of the Fe/Fe3C phase implies the ORR activity of the adjacent Fe-Nx active sites via an interfacial electron transfer from Fe3C to Fe-Nx active sites [25,26,27,28,29]. Liu et al. [47] demonstrated that the electron-rich Fe3C phase donates 0.06 e− to the electron-withdrawing Fe–N4–C active sites, thereby influencing the adsorption and desorption of O2 intermediates on the neighboring Fe–N4–C active sites. The enhanced electron transfer from Fe3C to Fe–N4–C increased electronic interactions between Fe–N4–C and O2* and OH*, thereby facilitating the formation of the OOH* intermediate, which promotes the direct reduction of O2 to H2O.
The Gibbs free energy (ΔG) of the oxygen ORR and its intermediates reveals that the energy barrier for the Fe3C/Fe–N4–C active site is significantly lower than that of the Fe–N4–C and Fe3C active sites for the rate-determining steps of ORR, specifically O* and OH*. The enhanced electron density and O2 adsorption energy on the Fe3C/Fe–N4–C active site was found to be higher than on the Fe-N4-C active sites, clearly suggesting that the Fe3C/Fe–Nx–C active sites could serve as better ORR active sites than the Fe-Nx-C active sites alone (Figure 7a). In another study, Ren et al. [48] demonstrated that the efficient electron transfer from Fe3C to Fe-N-C resulted in spontaneous thermodynamic exothermic processes for ORR. The (ΔG) at 1.23 V, the energy barrier for the rate-determining step (RDS) of the reaction OH* + e¯ → OH¯, is 0.82 eV for Fe-N-C, whereas for the Fe/Fe3C-Fe-N-C catalyst, it is 0.71 eV (Figure 7b,c). Electron transfer from Fe3C to Fe-N4-C was observed, which is believed to enhance ORR performance by modulating the adsorption and desorption of oxygen-related intermediates on the Fe-N4-C. This outcome substantiates the hypothesis, and collectively suggests that Fe3C can significantly improve the ORR kinetics at the Fe–N–C active site sites (Figure 7d). Further, the orbital overlapping study suggests that the overlap between O 2p orbitals and Fe 3d orbitals for the O* intermediate is smaller, whereas the overlap between the orbitals of OH* active intermediates is greater for the Fe/Fe3C-Fe-N-C catalyst than the Fe-N-C catalyst. This suggests that OH* active intermediates have a lower desorption energy barrier, and that Fe/Fe3C@N-doped C has a better adsorption effect on O2. Therefore, Fe3C can be added to the Fe-N-C structure to lower the ORR active energy barrier, improve the adsorption–desorption process of O* and OH* intermediates, and make the Fe/Fe3C-Fe-N-C catalyst exceptionally active in the ORR. In summary, the presence of the Fe3C phase adjacent to the Fe–N4–C active sites is advantageous for enhancing the kinetics of the ORR. The ORR activity and Zn–air battery performance of the Fe/Fe3C-Fe-N-C catalyst are compared with analogous catalysts derived from biomass, and the comparative data are presented in Table 2. The Fe/Fe3C-Fe-N-C catalyst derived from waste coffee biomass is recognized as a promising alternative to the Pt/C catalyst.

3. Materials and Methods

The Fe3C/Fe-Nx-C catalyst was synthesized based on our previous research [37]. Briefly, the overall synthesis process comprises three steps. First, the waste coffee grounds are collected from the local coffee shop on our campus, Keimyung University, Daegu. The collected coffee waste is rinsed with an ample amount of distilled water to eliminate any impurities and dissolved solids. The collected coffee waste is subsequently dried in a hot air oven and then powdered in a mortar and pestle. In the second step, the dried coffee powder is mixed with melamine (obtained from Sigma-Aldrich, South Korea) (C3H6N6 as N-source) in a 1:1 molar ratio in a mortar and pestle. The mixed solids are then transferred into a graphite boat, which is then placed in the center of the tubular furnace. The pyrolysis of the melamine+coffee waste mixture is conducted at the temperature of 900 °C in a continuous flow of N2 at a temperature ramping speed of 5 °C min−1 for 1 h. The resulting powder was designated as N-doped carbon (C–N–C). In the third step, 2, 5, 7, 10 and 15 wt. % of Fe nitrate nonahydrate (Fe(NO3)3·(H2O)9) melamine (obtained from Sigma-Aldrich, South Korea) and 100 mg of ethylene diamine tetra acetic acid (EDTA) melamine (obtained from Sigma-Aldrich, South Korea), melamine and washed coffee waste (100 mg) is added, and then ground in a mortar and pestle for about 10 min. The resulting mixture is then transferred into a graphite boat, which is then placed in the center of the tubular furnace. The pyrolysis of the Fe-EDTA+melamine+coffee waste mixture is performed at the temperature of 900 °C in a continuous flow of N2 with the temperature ramping speed of 5 °C min−1 for 1 h. The obtained black powder was here named Fe3C/Fe-Nx-C catalyst-2, 5, 8, 10 and 15, representing the wt. % of Fe used in the synthesis, and this was then used for the ORR studies.
The C–N–C and Fe3C/Fe-Nx-C catalysts were evaluated for ORR in an alkaline electrolyte (0.1 M KOH). A traditional three-electrode system was used for the RDE studies, with a glassy carbon electrode (GCE) with a geometrical area of 0.1257 cm2 as the working electrode, a saturated calomel electrode as the reference electrode, and a graphite rod as the counter electrode. The catalyst ink was prepared by dispersing 4 mg of the catalyst in 1 mL of ethanol:water mixture (1:3), followed by ultrasonication for 30 min, to which a Nafion solution (5 wt. %) of 15 µL was added, after which it was further ultrasonicated for 30 min. A 7 µL sample of the resultant catalyst ink was then deposited onto the glassy carbon electrode and allowed to dry at room temperature (catalyst loading: 280 µg cm−2). For comparison, the platinum on graphitized carbon Pt/C catalyst (40 wt. %) with a catalyst loading of 80 µg cm−2 was also deposited on the GCE. Several electrochemical techniques were used for the evaluation of ORR kinetics, such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV), chronoamperometry (CA), and i–v studies for the Zn–air battery set up. The CV values were recorded in both N2- and O2-saturated 0.1 M KOH electrolyte to assess the O2 redox potentials and for the stability analysis of the catalysts from 0 to 1.23 V vs. RHE. Linear sweep voltametric analysis was used to assess the ORR activity of the catalysts. The ECSA values of the catalysts have been determined from the CV curves. The power density of the in-built Zn–air battery was used to assess the i–v curves and stability of the catalysts in the Zn–air battery set up. The detailed procedures and methods are described in the Supporting Information of this article.

4. Conclusions

The practical application of zinc–air batteries is impeded by the high cost of Pt/C cathode catalysts, prompting research into the exploration of alternative, sustainable electrocatalysts. One of the most sought-after alternatives is represented by metal-based catalysts. This study presents the development of a sustainable Fe3C/Fe-Nx-C cathode catalyst derived from waste coffee biomass for use in oxygen reduction in alkaline electrolytes and zinc–air battery applications. The Fe3C/Fe-Nx-C cathode catalyst was synthesized through a mechanochemical strategy utilizing melamine and an EDTA–Fe chelate complex, followed by pyrolysis at 900 °C. The synthesized Fe3C/Fe-Nx-C catalyst was assessed for comprehensive ORR activity and stability. The ORR results indicate that Fe3C/Fe-Nx-C exhibited superior ORR activity, characterized by a half-wave potential of 0.93 V vs. RHE, a Tafel slope of 68 mV dec−1, 3.95 electrons transferred per O2 molecule, and elevated ECSA values. The Fe3C/Fe-Nx-C catalyst demonstrated remarkable stability, exhibiting a potential loss of 75 mV over 10,000 cycles and a mere 14% reduction in relative currents during chronoamperometric testing. The Fe3C/Fe-Nx-C catalyst, utilized as a cathode catalyst in a zinc–air battery, produced a power density output of 81 mW and exhibited stable electrochemical performance during galvanostatic discharge analysis. Its practical applicability is further illustrated by the panel of LEDs illuminated by the dual-cell zinc–air battery connected in series, effectively showcasing the utility of the developed catalyst for use in diverse energy storage and electronic devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13050143/s1. Electrochemical characterizations of the C–N–C and Fe3C/Fe-Nx-C catalysts, as well as Zn–air battery performance analysis, Figure S1. (a,b) XRD (c,d) XPS (e-m) HR-TEM images of the Fe3C/Fe-Nx-C catalysts. Figure S2. K-L plots of the C–N–C, Fe3C/Fe-Nx-C and Pt/C catalysts, Figure S3. Cyclic voltammograms of the C–N–C, Fe3C/Fe-Nx-C and Pt/C catalysts, Figure S4. Capacitance curves derived from the C–N–C and Fe3C/Fe-Nx-C catalysts.

Author Contributions

Conceptualization, S.G.P. and S.A.; methodology, S.G.P. and S.A.; validation, S.G.P.; formal analysis, S.G.P.; writing—original draft preparation, S.G.P. and S.A.; writing—review and editing, S.-W.K. and T.-G.L.; funding acquisition, S.G.P. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the National Research Foundation of Korea (NRF) funded by the Korean government, Ministry of Science, and ICT (MSIT) (No. 2021R1F1A1046648), Republic of Korea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00143 g001
Figure 1. Schematic representation of the Fe3C/Fe-Nx-C catalysts synthesis and its applications in microbial fuel cells and Zn–air batteries.
Figure 1. Schematic representation of the Fe3C/Fe-Nx-C catalysts synthesis and its applications in microbial fuel cells and Zn–air batteries.
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Figure 2. Electrochemical studies of various C–N–C and Fe3C/Fe-Nx-C catalysts in 0.1 M KOH electrolyte—(a,b) cyclic voltammograms, (c) linear sweep voltammograms (LSV), (d) half-wave potentials, (e) LSV curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts, (f) Tafel plots of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. The blue-colored values are the slope values in V.
Figure 2. Electrochemical studies of various C–N–C and Fe3C/Fe-Nx-C catalysts in 0.1 M KOH electrolyte—(a,b) cyclic voltammograms, (c) linear sweep voltammograms (LSV), (d) half-wave potentials, (e) LSV curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts, (f) Tafel plots of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. The blue-colored values are the slope values in V.
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Figure 3. LSVs recorded at different rotation per minute values; (a) C–N–C, (b) Fe3C/Fe-Nx-C-2, (c) Fe3C/Fe-Nx-C-5, (d) Fe3C/Fe-Nx-C-8, (e) Fe3C/Fe-Nx-C-10, (f) Fe3C/Fe-Nx-C-15 catalysts, (g) Pt/C and (h) K-l plots of all the catalysts at 0.6 V and 1600 rpm.
Figure 3. LSVs recorded at different rotation per minute values; (a) C–N–C, (b) Fe3C/Fe-Nx-C-2, (c) Fe3C/Fe-Nx-C-5, (d) Fe3C/Fe-Nx-C-8, (e) Fe3C/Fe-Nx-C-10, (f) Fe3C/Fe-Nx-C-15 catalysts, (g) Pt/C and (h) K-l plots of all the catalysts at 0.6 V and 1600 rpm.
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Figure 4. Electrochemical stability of the catalysts assessed by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry techniques; (a,b) CV and LSV curves of Pt/C, (c,d) CV and LSV curves of Fe3C/Fe-Nx-C-10, (e) chronoamperometric curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. Chronoamperometric measurements were done at the potential of 0.6 V vs. RHE, in O2-saturated 0.1 M KOH electrolyte.
Figure 4. Electrochemical stability of the catalysts assessed by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry techniques; (a,b) CV and LSV curves of Pt/C, (c,d) CV and LSV curves of Fe3C/Fe-Nx-C-10, (e) chronoamperometric curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. Chronoamperometric measurements were done at the potential of 0.6 V vs. RHE, in O2-saturated 0.1 M KOH electrolyte.
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Figure 5. Zn–air battery performance analysis of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. (a) OCV of the single-cell Zn–air battery with Pt/C and Fe3C/Fe-Nx-C-10 catalysts as cathodes, (b) polarization curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts, (c,d) galvanostatic curves of the Pt/C and Fe3C/Fe-Nx-C-10 catalysts in Zn–air battery configuration (discharge current in mA cm−2).
Figure 5. Zn–air battery performance analysis of Pt/C and Fe3C/Fe-Nx-C-10 catalysts. (a) OCV of the single-cell Zn–air battery with Pt/C and Fe3C/Fe-Nx-C-10 catalysts as cathodes, (b) polarization curves of Pt/C and Fe3C/Fe-Nx-C-10 catalysts, (c,d) galvanostatic curves of the Pt/C and Fe3C/Fe-Nx-C-10 catalysts in Zn–air battery configuration (discharge current in mA cm−2).
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Figure 6. Dual-cell Zn–air battery performance analysis with Pt/C and Fe3C/Fe-Nx-C-10 catalysts. (a) OCV of the dual-cell Zn–air battery with Fe3C/Fe-Nx-C-10 catalyst as cathode; (b) polarization curves of Fe3C/Fe-Nx-C-10 catalyst; (c,d) galvanostatic curves of the Fe3C/Fe-Nx-C-10 catalysts in Zn–air battery configuration (discharge current in mA cm−2); (d) demonstration of the performance of the dual-cell zinc–air battery by illuminating the LED panel.
Figure 6. Dual-cell Zn–air battery performance analysis with Pt/C and Fe3C/Fe-Nx-C-10 catalysts. (a) OCV of the dual-cell Zn–air battery with Fe3C/Fe-Nx-C-10 catalyst as cathode; (b) polarization curves of Fe3C/Fe-Nx-C-10 catalyst; (c,d) galvanostatic curves of the Fe3C/Fe-Nx-C-10 catalysts in Zn–air battery configuration (discharge current in mA cm−2); (d) demonstration of the performance of the dual-cell zinc–air battery by illuminating the LED panel.
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Figure 7. (a) Electron density plots for Fe–N4–C/Fe3C model with adsorbed O2 (O2*) and OH− (OH*). Cyan and yellow contours signify charge depletion and accumulation in real space, respectively (Ref. [47], open access). (b) ORR Gibbs free energy diagrams of Fe/Fe3C@N-doped C and Fe-N-C at U = 0 V and 1.23 V. (c) Charge density distributions for Fe-N-C/Fe3C. (d) Electron difference density plots of O2* and OH* intermediate adsorption. (Ref. [48], open access).
Figure 7. (a) Electron density plots for Fe–N4–C/Fe3C model with adsorbed O2 (O2*) and OH− (OH*). Cyan and yellow contours signify charge depletion and accumulation in real space, respectively (Ref. [47], open access). (b) ORR Gibbs free energy diagrams of Fe/Fe3C@N-doped C and Fe-N-C at U = 0 V and 1.23 V. (c) Charge density distributions for Fe-N-C/Fe3C. (d) Electron difference density plots of O2* and OH* intermediate adsorption. (Ref. [48], open access).
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Table 1. ORR electrochemical data and Zn–air battery performance data of the C–N–C, Fe3C/Fe-Nx-C and Pt/C catalysts.
Table 1. ORR electrochemical data and Zn–air battery performance data of the C–N–C, Fe3C/Fe-Nx-C and Pt/C catalysts.
CatalystEredox Potentials (V vs. RHE)E1/2
(V vs. RHE)		‘n’ESCA
(cm−2)		Loss in E1/2 Potential (10,000 Cycles mV vs. RHE)Loss of Relative Current (%)Zn–Air Battery Performance
OCV
(V)		Power Density
(mW cm−2)
C–N–C0.680.723.650.00777---
Fe3C/Fe-Nx-C-20.700.773.750.01016---
Fe3C/Fe-Nx-C-50.750.793.820.01797---
Fe3C/Fe-Nx-C-80.780.873.910.03406---
Fe3C/Fe-Nx-C-100.830.933.950.0786175131.4181
Fe3C/Fe-Nx-C-150.820.903.950.07614
Pt/C-0.934.0-60141.4698
Table 2. ORR kinetic data and Zn–air battery performance of electrocatalysts derived from various biomass types.
Table 2. ORR kinetic data and Zn–air battery performance of electrocatalysts derived from various biomass types.
Biomass SourceCatalystsE1/2
(V vs. RHE)		Tafel Slope
(mV dec−1)		Number of Electrons (n)Zn–Air Battery Performance
(Aqueous)		Ref.
OCV
(V)		Power Density
(mW cm−2)
Garlic biomassFe@G-800/1000.91693.891.4820[49]
Loofah SpongeFeCo@NC-9000.81713.44–3.641.49103[50]
Eucalyptus pulpFe-N-C-10000.84NR~41.49125[51]
Sodium alginateFeNC-900-80.88633.871.67125[52]
SoybeansFe-NC-8000.91743.991.53220[53]
Natural woodSAC-FeN-WPC0.8583~41.53152[54]
Corn silkFe SA/NCZ 0.80703.91.44101[55]
WoodCo/CoO@NWC0.85963.90–3.951.3828[56]
ChitosanFe3C/NCA-10000.83793.6NR253[57]
Rotten woodNRW-10000.87703.81.53118[58]
Water hyacinthsWHNC-A0.84922.891.4380[59]
fruits of glossy privetGPNCS0.92NR3.8–3.91.4368[60]
Eichhornia crassipesFe2N@NCNTs0.86673.87–3.91.53135[61]
WoodFeP-NWC0.86743.77–3.901.50144[62]
WoodFe3C@NPW0.87983.78–3.891.48125[63]
Waste coffee groundsFe3C/Fe-Nx-C-100.93 3.951.4181This work
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Peera, S.G.; Kim, S.-W.; Ashmath, S.; Lee, T.-G. Sustainable Fe3C/Fe-Nx-C Cathode Catalyst from Biomass for an Oxygen Reduction Reaction in Alkaline Electrolytes and Zinc–Air Battery Application. Inorganics 2025, 13, 143. https://doi.org/10.3390/inorganics13050143

AMA Style

Peera SG, Kim S-W, Ashmath S, Lee T-G. Sustainable Fe3C/Fe-Nx-C Cathode Catalyst from Biomass for an Oxygen Reduction Reaction in Alkaline Electrolytes and Zinc–Air Battery Application. Inorganics. 2025; 13(5):143. https://doi.org/10.3390/inorganics13050143

Chicago/Turabian Style

Peera, Shaik Gouse, Seung-Won Kim, Shaik Ashmath, and Tae-Gwan Lee. 2025. "Sustainable Fe3C/Fe-Nx-C Cathode Catalyst from Biomass for an Oxygen Reduction Reaction in Alkaline Electrolytes and Zinc–Air Battery Application" Inorganics 13, no. 5: 143. https://doi.org/10.3390/inorganics13050143

APA Style

Peera, S. G., Kim, S.-W., Ashmath, S., & Lee, T.-G. (2025). Sustainable Fe3C/Fe-Nx-C Cathode Catalyst from Biomass for an Oxygen Reduction Reaction in Alkaline Electrolytes and Zinc–Air Battery Application. Inorganics, 13(5), 143. https://doi.org/10.3390/inorganics13050143

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