Nowadays, human beings are facing serious environmental problem like excessive CO₂ emission and energy crisis. In order to solve these problems, researchers have made great effort to develop efficient and effective techniques for energy conversion and storage such as fuel cells, supercapacitors, lithium ion batteries and so on. In particular, fuel cells can utilize clean energy hydrogen (Hao et al., 2022; Zhang et al., 2022) and oxygen as gas fuels to convert chemical energy to electricity and heat without generating pollutants or CO₂ emission. Typically, the hydrogen is oxidized at the anode and the oxygen is reduced at the cathode during the working of fuel cells. The kinetics of oxygen reduction reaction (ORR) is quite slow (Guo et al., 2022; Zaman and Chen, 2023) and Pt/C has been used as the commercial electrocatalyst for ORR (Zaman et al., 2021). However, Pt/C catalysts still suffers from some disadvantages including their high cost, and susceptible to the methanol/CO poisoning effect (Guo et al., 2022; Zhang et al., 2023). Therefore, it is necessary and important to develop novel cost-effective, high-performance ORR catalysts to replace the commercial Pt/C catalysts for practical applications in fuel cells.

Carbon nanomaterials (Yan et al., 2023) have attracted worldwide attention since they have demonstrated great potential in energy storage (Fu et al., 2023; Gadipelli et al., 2023; Gao et al., 2023; Mo et al., 2023; Senokos et al., 2023) and conversion (Zhang et al., 2022; Kim et al., 2023; Lv et al., 2023; Peng et al., 2023) to relieve the environmental problem and energy crisis (Gao et al., 2022; Marangon et al., 2023; Yan et al., 2023). Carbon nanomaterials also play a significant role in the field of electrocatalysis of ORR due to their low cost, high performance and excellent stability (Zaman et al., 2022a). For instance, N-doped carbon nanotubes (Ma et al., 2022; Choi et al., 2023) and N-doped graphene (Zaman et al., 2022b; Li et al., 2023) have been demonstrated to exhibit good ORR performance in alkaline medium. According to previous work (Shui et al., 2015; Guo et al., 2022; Ma et al., 2022; Yang et al., 2022; Li et al., 2023; Peng et al., 2023), it has been demonstrated that heteroatom doping of nitrogen is an effective way to improve the ORR electrocatalytic activity of carbon nanomaterials (Li et al., 2022). Apart from the heteroatom doping, the structure of the materials (Yu and Wuttig, 2023) also greatly affect the electronic and chemical properties, catalysis abilities and so on. Carbon materials with hierarchical porous structures are also extremely promising for energy storage and conversion. The porosity is a critical factor that affects the properties of the nanocarbons. According to the pore size, microporous carbons are defined as porous carbon materials (PCMs) with pore size smaller than 2 nm in diameter, mesoporous carbons are PCMs with pore size in the range of 2–50 nm, and macroporous carbons are PCMs with pore size larger than 50 nm (Wu et al., 2012). Hierarchical porous carbons (HPCs), a new family of PCMs, can be considered as carbon materials with multi-level pores. Comparing to the conventional PCMs, HPCs have a unique hierarchical porosity which allows to achieve the minimized diffusive resistance to mass transport from macropores and the high surface area for active site dispersion from micropores or mesopores. From the aspect of material structure, HPCs effectively construct a network of micropores, mesopores and/or macropores, utilizing the advantages of each scale pores with a synergistic effect to efficiently enhance their performances in catalysis (Jiao et al., 2023), energy storage (Wang T. et al., 2022a), adsorption and so on (Wang Y. et al., 2022b).

In recent years, biomass has become important sources for the synthesis of porous carbon nanomaterials due to their low cost, easy accessibility, large quantity, rapid regeneration and simple processing (Song et al., 2022; Han F. et al., 2023a; Han T. et al., 2023b; Chen et al., 2023; He et al., 2023; Jiao et al., 2023). Although researchers have made great effort to convert different biomass into porous carbon materials for ORR, few of these materials achieved good electrocatalytical performance in acidic medium (Wang et al., 2013; Chen et al., 2014; Guo et al., 2014; Pan et al., 2014; Guo et al., 2015; Liang et al., 2015). Thus, it still remains great challenges to develop novel high-performance oxygen reduction electrocatalysts from natural biomass. In this work, fresh daikon was selected as the precursor to synthesize three dimensional (3D) nitrogen-doped carbons with hierarchical porous architecture by simple annealing treatment and NH₃ activation. The daikon-derived material Daikon-NH₃-900 exhibits excellent electrocatalytical performance towards oxygen reduction reaction in both alkaline and acidic medium. Daikon-NH₃-900 was further applied as the cathode catalyst for proton exchange membrane (PEM) fuel cell and shows promising performance with a peak power density up to 245 W/g.

The morphology of the porous carbons was first investigated by field emission scanning electron microscopy. The white dried daikon slices turns into black foam-like Daikon-NH₃-900 after Ar annealing and NH₃ activation as shown in Supplementary Figures S1A, B. The SEM images of as-synthesized sample without HCl treatment are presented in Figure 1A, Supplementary Figures S1C, D. As can be seen, Daikon-NH₃-900 exhibits an interesting 3D porous network structure with uniform macro pores of ∼20 μm. These voids originate from the porous structure of the biomass materials daikon. The wall of these voids shown in Figure 1B is demonstrated to be hierarchical porous carbon sheets by further TEM investigation. Figures 1C, D; Supplementary Figures S2A, B presents the TEM images of Daikon-NH₃-900 washed by HCl. As can be seen in the TEM images in Figure 1C, the carbon sheets of Daikon-NH₃-900 process a highly porous structure. Abundant mesopores are uniformly distributed in the few carbon layers. Figure 1D with a higher magnification shows the average size of these mesopores ranges from 10 to 20 nm. These mesopores are mainly introduced by the ammonia activation. TEM images of Daikon-Ar-900 without ammonia activation are shown in Supplementary Figures S2C, D. The Daikon-Ar-900 is consisting of carbon sheets without porous structure, while Daikon-NH₃-900 exhibits a hierarchical porosity as can be seen in Supplementary Figures S2A, B. The BET analysis further demonstrates the contribution of ammonia activation. According to Figure 2C, Daikon-Ar-900 has a very low specific surface area of ∼87.5 m²/g; surprisingly, the specific surface area of Daikon-NH₃-900 is 1,107.4 m²/g which is about 12 times of Daikon-Ar-900. With the short time NH₃ activation, the surface area of daikon-derived carbons is greatly increased by introducing large amounts of mesopores and micropores. Figure 1E shows the pore size distribution of Daikon-NH₃-900. Apart from the macropores and mesopores, a large amount of micropores of ∼1.8 nm can be observed, resulting from the corrosive gas activation. Thus, the Daikon-NH₃-900 is demonstrated to process a hierarchical porous structure consisting of macropores (∼20 μm), mesopores (3–20 nm) and micropores (∼1.8 nm) according to all the morphology investigations and BET analysis. The hierarchical porosity of Daikon-NH₃-900 plays a great role in the energy storage and conversion applications. The abundant multi-level pores can effectively shorten the diffusion pathways and offer minimized diffusive resistance to mass transport on a large electrode/electrolyte interface. Besides, the hierarchical porosity can also offer rapid ion transport with improved rate capability. On the other hand, high porosity introduce defects and heteroatoms to further increase available active sites and effectively modulate their electronic and chemical characteristics.

The structure of Daikon-NH₃-900 and Daikon-Ar-900 were then studied by Raman, XRD and TGA analysis as can be seen in Figures 2A, B, D, respectively. According to the XRD patterns in Figure 2B, both Daikon-NH₃-900 and Daikon-Ar-900 show a broad C (002) peak at 26.3° and a weaker C (001) peak at 43.2°, indicating that the daikon have been successfully converted to graphitic carbons with amorphous domains during the carbonization process. The NH₃ activation can further, etch the amorphous domains, resulting in a better graphitic crystal structure evidenced by Raman results. As shown in Figure 2A, Daikon-Ar-900 exhibits very broad D band at ∼1,356cm⁻¹ and G band at 1,588cm⁻¹. After NH₃ activation, Daikon-NH₃-900 shows well defined D band at ∼1,356cm⁻¹ and G band at ∼1603cm⁻¹, respectively. The observed redshift of G band in the Daikon-NH₃-900 could be attributed to the nitrogen-doping effect via NH₃ treatment (Das et al., 2008; Jorio, 2012). Compared to Daikon-Ar-900 (I_D/I_G ≈ 0.93), the increasing ratio of D-band to G-band in the Daikon-NH₃-900 (I_D/I_G ≈ 0.94) is also related to the nitrogen-doping effect. Thus, simple NH₃ activation step not only increases the surface area and porosity as discussed previously, but also introduces the nitrogen heteroatoms into the carbon planes of Daikon-NH₃-900. Thermogravimetric Analysis (TGA) was employed to investigate the thermal stability of Daikon-NH₃-900 and Daikon-Ar-900 in air condition. As can be seen in Figure 2D, both Daikon-NH₃-900 and Daikon-Ar-900 show about 11% weight loss before 400°C, corresponding to the removal of oxygen-containing groups and moisture. Then a dramatic weight loss of the daikon-derived carbons at around 435°C can be observed. The Daikon-Ar-900 exhibits a decomposition temperature at around 581°C whereas the Daikon-NH₃-900 decomposed at a higher temperature 653°C. The better thermal stability of Daikon-NH₃-900 is probably associated with the higher graphitization and nitrogen doping effect. Both Daikon-NH₃-900 and Daikon-Ar-900 exhibit 0.0 wt% after decomposition without any residues, demonstrating that they are metal-free materials.

To further study the elemental composition of Daikon-NH₃-900, X-ray photoelectron spectroscopy (XPS) analysis was carried out. Figure 3A shows the survey spectra (0–950 eV) of Daikon-NH₃-900 and Daikon-Ar-900. As can be seen, the XPS spectra include C 1s at ∼ 280 eV and O 1s at ∼ 533 eV with/without N 1s at ∼ 400 eV and no other impurities were detected. It is clear that NH₃ activation successfully introduces nitrogen heteroatoms into Daikon-NH₃-900 and the atomic percentage of N is as high as 9.9%. The high doping level of N in the Daikon-NH₃-900 is closely related to the excellent electrochemical performance in the electrocatalysis and energy storage. To further investigate the state of C and N, high resolution XPS spectra of Daikon-NH₃-900 were carried out. The high resolution XPS C 1s spectrum can be fitted into C-C (284.5 eV), C-N (285.7 eV) and C-O (288.9 eV) as shown in Figure 3B.

Consistently, the O 1s spectrum can be convoluted to two peaks of C-OH (533.4 eV) and O=C-OH 531.5 eV as can be seen in Supplementary Figure S3. For the high resolution N XPS spectrum in Figure 3C, the N 1s can be fitted into two main component peaks located at 398.3, and 400.3 eV, which could be assigned to pyridinic nitrogen and pyrrolic nitrogen, respectively. According to the XPS analysis, the proposed chemical structure of Daikon-NH3-900 is presented in Figure 3D. The electrons accumulate on the pyridinic and pyrrolic nitrogen atoms, because the N (χ = 3.04) has a higher electronegativity than that of C (χ = 2.55). The charge redistribution further changes the adsorption mode of O₂ and facilitate the reduction of O₂. The XPS results also demonstrates that both Daikon-NH₃-900 and Daikon-Ar-900 are carbon materials without metal impurities, which is in good agreement with the previous TGA analysis.

With the understanding of the structure and composition, Daikon-NH₃-900 with hierarchical porosity and rich nitrogen-doping can be considered as an excellent candidate for energy conversion and storage systems. Previously nitrogen-doped carbon nanomaterials have been demonstrated to process good ORR catalytic performance in alkaline medium such as 0.1 M KOH electrolyte. However, their ORR activities in acidic electrolyte were rarely reported which still remained challenges. Besides, to the best of our knowledge, none of the biomass-derived carbons show ORR activity in acid. In this work, for the first time, we investigated the ORR catalytic performance of daikon-derived 3D hierarchical porous carbons in the acidic medium.

The ORR electrocatalytic activity of Daikon-NH₃-900 was first tested by conventional three-electrode cyclic voltammetry (CV) in O₂ and N₂ saturated 0.5 M H₂SO₄ aqueous solutions, respectively. Figure 4A shows a characteristic ORR peak at about 0.40 V (vs. Ag/AgCl) for oxygen reduction on the Daikon-NH₃-900 electrode in the O₂ saturated electrolyte, but not in the N₂ saturated electrolyte. To investigate the kinetics of ORR, we used rotation ring-disk electrode (RRDE) to evaluate ORR performance of the Daikon-NH₃-900. As shown in Figure 4B, the ORR onset potential (E_onset) and half-wave potential (E_1/2) of the Daikon-NH₃-900 is about 0.69V and 0.49 V (vs. Ag/AgCl), respectively. According to the current densities collected on the ring and disk, the electron transfer number was calculated to be about 4.0 as shown in the insert of Figure 4B (Luo et al., 2023; Wang et al., 2023). A series of linear sweep voltammograms (LSVs) for Daikon-NH₃-900 were measured on a rotating disk electrode (RDE) from 400 to 1600 rpm, showing an increased current density with increasing rotations speeds in Supplementary Figure S4A, indicating an excellent electrocatalytical performance with desired four-electron ORR process in consistent with RRDE analysis. To better understand the excellent electrocatalytical activity of Daikon-NH₃-900, the LSVs of Daikon-Ar-900 without NH₃ activation was measured for comparison. As shown in Figure 4C, the Daikon-Ar-900 exhibits negligible ORR performance in 0.5 M H₂SO₄ with a poor E_1/2 and extremely low current density. Thus, the electrocatalytical active sites of Daikon-NH₃-900 should be considered as the nitrogen heteroatoms. The excellent ORR performance of Daikon-NH₃-900 in the acidic medium is highly related to the high N doping level of 9.9 at%. After confirming the important influence of nitrogen doping from NH₃ activation, different thermal annealing and activation temperatures including 800°C, 900°C, and 1000°C were selected to further optimize the ORR performance. Figure 4C; Supplementary Figures S4B, C present the electrochemical reduction behaviors of daikon derived carbons under different temperatures. According to Figure 4C, the LSVs in show an increased ORR onset potential and limited current density in the order of Daikon-NH₃-800< Daikon-NH₃-1000< Daikon-NH₃-900, indicating that 900°C is the best reaction temperature for the carbonization of daikon. The corresponding ring current density and electron transfer number curves of daikon derived at different temperatures are shown in Supplementary Figures S4B, C. The different ORR catalytic performance of these daikon-derived carbons (Daikon-NH₃-800, Daikon-NH₃-900 and Daikon-NH₃-1000) are caused by the structures as shown in Supplementary Figure S4D. According to the Raman spectra of Daikon-NH₃-800, annealing treatment at 800°C of the daikon only results in a low graphitization, and this is responsible for the worst electrocatalytic performance. When increasing the thermal annealing temperature to 900°C and 1,000°C, the well-defied D-band and G-band indicate the good graphitic crystal structures of Daikon-NH₃-900 and Daikon-NH₃-1000. However, the low I_D/I_G ratio (≈0.89) of Daikon-NH₃-1000 indicates that the Daikon-NH₃-1000 has a relatively low doping level probably due to the overheating. Thus, in the carbonization of the daikon, a modest annealing temperature of 900°C is appropriate for both graphitization and nitrogen doping, making Daikon-NH₃-900 the best ORR electrocatalyst. It should also be noted that the ORR performance of the Daikon-NH₃-900 is even comparable to commercial Pt/C in acidic electrolyte according to the LSVs in Figure 4C.

Figure 4D shows the stability of the Daikon-NH₃-900 and Pt/C electrodes under a constant voltage of 0.3 V (vs. Ag/AgCl). It is obvious that the Daikon-NH₃-900 exhibits excellent ORR stability with a very slow attenuation and a high current retention of 93.4% after 30,000 s, which is much better than that of the commercial Pt/C catalyst. The Daikon-NH₃-900 was further tested by the possible methanol crossover and carbon monoxide poisoning effect. As shown in Figure 4E; Supplementary Figure S6A, the original cathodic ORR current of Daikon-NH₃-900 under 0.3 V (vs. Ag/AgCl) does not show any significant changes after the addition of 3.0 M methanol or CO, suggesting that Daikon-NH₃-900 is free from the methanol crossover effect and CO poisoning. In comparison, the commercial Pt/C suffers severe methanol crossover and CO poisoning problem in 0.5 M H₂SO₄.

Then, we further investigated the catalytic performance of the Daikon-NH₃-900 in the proton exchange membrane fuel cell (PEMFC). The Daikon-NH₃-900 and the Daikon-Ar-900 were applied as cathode catalyst and fabricated into the single cell, respectively. Figure 4F shows the device performance of the membrane electrode assemblies fabricated with the Daikon-NH₃-900 (MEA-Daikon-NH₃-900). MEA-Daikon-NH₃-900 has a high open circuit voltage of ∼0.98 V. The peak power density is as high as ∼245 W/g and the limiting current density reaches ∼1303 mA/cm². Supplementary Figures S7A–C present the performance comparison of MEA-Daikon-NH₃-900 and MEA-Daikon-Ar-900. MEA-Daikon-NH₃-900 shows a higher OCV and larger power density than that of MEA-Daikon-Ar-900 due to the nitrogen doping effect. The excellent device performance such as large limiting current and power density are attributed to the hierarchical porosity and rich heteroatom doping of Daikon-NH₃-900.

The ORR electrocatalytical performance of the Daikon-NH₃-900 in alkaline medium was then investigated using the typical electrolyte 0.1 M O₂-saturated KOH. As shown in Figure 5A, a very strong ORR peak is observed at about −0.20 V (vs. SCE) on the Daikon-NH₃-900 electrode in the O₂ saturated KOH. The LSV and ORR kinetics of Daikon-NH₃-900 was investigated on RRDE as shown in Figure 5B. The E_onset and E_1/2 of Daikon-NH₃-900 is 0.03 V vs. SCE and −0.14 V vs. SCE, respectively, while the electron transfer number is 4 as shown in the insert of Figure 5B. Figure 5C displays the LSVs of Daikon-NH₃-900, Daikon-NH₃-900 and commercial Pt/C electrodes at 1,600 rpm. As expected, the ORR performance of Daikon-Ar-900 is really inferior, and this is in good agreement with the results obtained in acidic electrolyte. The Daikon-NH₃-900 with nitrogen doping exhibits even better ORR activity than Pt/C in alkaline electrolyte, showing more positive E_onset and E_1/2. More electrochemical results of ORR are presented in Supplementary Figure S5. As can be seen in Supplementary Figure S5A, Daikon-NH₃-900 achieves the diffusion limiting current plateau at a positive potential of 0.2 V vs. SCE, indicating a high density of electroactive sites for the reduction of oxygen. Supplementary Figures S5B–D presents LSV curves, the corresponding ring currents and electron transfer numbers of Daikon-NH₃-800, Daikon-NH₃-900, Daikon-NH₃-1000, respectively. As expected, the Daikon-NH₃-900 exhibits the best ORR performance, which is in consistent with the results in acidic medium. Daikon-NH₃-800 with a poor graphitic structure shows the worst electrocatalytic activity towards ORR in basic electrolyte. It should be noted that the ORR performance of Daikon-NH₃-1000 including the E_onset, E_1/2 and reduction current density is similar to that of Daikon-NH₃-900, as can be seen in Supplementary Figure S5B. As discussed, the Daikon-NH₃-1000 processes a similar graphitization to Daikon-NH₃-900 with a lower nitrogen doping level. Compared to the ORR in acid, the catalytic performance of Daikon-NH₃-1000 in alkaline is better. This is because that the nitrogen heteroatoms in the Daikon-NH₃-1000 are sufficient to catalyze ORR in the KOH, while more electroactive sites are need for the acidic ORR process. However, as Supplementary Figure S5D showed, the electron transfer number of Daikon-NH₃-900 is still higher than that of Daikon-NH₃-1000 due to a higher density of nitrogen active sites, indicating a more efficient oxygen reduction process on the Daikon-NH₃-900 electrode. As Figure 5D; Supplementary Figure S6B show, the Daikon-NH₃-900 exhibits very good CO and methanol tolerance in alkaline medium, showing great advantages than the commercial Pt/C catalyst. The durability test was also conducted in 0.1 M O₂ saturated KOH. According to Supplementary Figure S6C, the Daikon-NH₃-900 shows excellent stability with the retention of 97.6% after 30,000 s, while Pt/C only had a retention of 70.7%. Daikon-NH₃-900 exhibits outstanding long-term stability in both acidic and basic electrolytes. Daikon-NH₃-900 has been demonstrated to be high-performance ORR electrocatalyst in acidic and alkaline medium on the three-electrode system.

3D nitrogen-doped carbon materials with hierarchical porous architecture were derived from biomass by simple annealing treatment and NH₃ activation. The daikon-derived material Daikon-NH₃-900 exhibits excellent electrocatalytical performance towards oxygen reduction reaction in both alkaline and acidic medium. It shows very positive oxygen reduction onset and half-wave potential, excellent CO and methanol tolerance in different electrolytes. Daikon-NH₃-900 was further applied as the cathode catalyst for proton exchange membrane (PEM) fuel cell and shows promising performance with a peak power density up to 245 W/g, demonstrating excellent performance among ORR electrocatalyst made from biomass.