Figure 1 illustrates the two pollen sources utilized in this study and their correponding carbon architecture. The cattail pollens were obtained from locally grown cattail plants, while the bee pollens were initally collected from flowers by foraging bees. The colored scanning electron microscopy (SEM) images of ACP and ABP reveal the distinct morphological difference of the two pollen derived carbons. ACP carbon resembles the shape of collapsed spheres with uniform diameter of ~20 μm (Fig. 2b). The uniformity is due to the utilization of a single pollen source. On the other hand, ABP exhibits unique morphologies of different pollen grains such as spiky spheres (~15 μm in diameter), brain-like spheres (~50 μm) and small smooth spheres (~10 μm) (Fig. 2a). The rich morphologies of ABP carbon can be attributed to the diverse pollen sources visited by the foraging bees. It is important to note that change of seasons and sourcing locations are expected to significantly impact the composition of bee pollens due to the change in available pollen species24. Such variation in bee pollen derived carbon microstrutures could pose considerable challenges to battery anode quality control; however, controllable carbon mictrostructures with monodispersity could be attained via careful selection of pollen sources. For scale-up production of pollen carbon, it is possible to obtain structural uniformity through utlization of bee pollens harvested from commerical pollination beehives with singular target crop.

Figure 1 Preparation of pollen derived carbon microstructure via solid state pyrolysis of two distinct pollen sources. Full size image

Figure 2 Characterization of pollen derived carbons. SEM images of (a) ABP and (b) ACP; (c) TEM images of ACP; (d) XRD and (e) Raman patterns of ABP and ACP; (f) XPS spectra of ground ABP and ACP samples. Full size image

Transmission electron microscopy (TEM) images were taken to reveal high degree of disorderness in ACP carbon as shown in Fig. 2c. Due to the diverse carbon microstrutures presented in ABP sample, TEM images of small sampling points would not be representative of the entire ABP sample hence its TEM were not collected.

Instead, X-ray diffraction and Raman spectroscopy were utlized to charaterize the bulk samples (Fig. 2d,e). XRD spectra reveal that ABP and ACP have very similar crystal structures with (002) peaks at 24.6° and 25.1° respectively, corresponding to interlayer spacing of 0.358 nm and 0.360 nm (vs. 0.335 nm for graphite20). The larger interlayer spacing is a common feature of hard carbon and is expected for these pollen samples since they were both synthesized at low temperatures. Moreover, Raman spectra of ABP and ACP both shows I G /I D ratios of ~1.00, indicating the equivalent presence of both disordered and graphitic carbon in the samples. The similarities of XRD and Raman spectra between these two samples suggest analogous carbon composition and carbon crystal structure in both samples.

X-ray photoelectron spectroscopy (XPS) was carried out to further examine their compositional difference and the results are presented in Fig. 2f and Table 1. Since XPS is surface sensitive technique, prior to XPS measurements; both ABP and ACP samples were mechanically ground to expose their core composition20. Both ABP and ACP samples contain comparable amount of C contents (~83.5%) and slight deviations in O (~11.5%) and N (~3.0%) contents as listed in Table 1. The high level of oxygen presence in both samples likely resulted from air activation. It is known that air activiation in hard carbon improves lithiation capacity via formation of additional surface pores for additional lithiation25. Similar improvements in capacity have also been observed in current work as described in the following section. Additionally, trace amount of phosphorus (~1%), calcium (~0.5%) and potassium (~0.5%) are detected in both samples, as pollens naturally contain these components.

Table 1 XPS elemental composition comparison of pollen carbons. Full size table

Long cycling of electrode comprising ACP vs. Li half cells were studied at C/10 rate within voltage window of 0 to 3 V at both 50 °C and 25 °C. Voltage profiles of both cells were constructed to compare their cycling behavior (Fig. 3a). Both cells show sloping voltage profiles that are characteristic of hard carbon5; low-voltage plateaus that is typically seen in graphite anode is not observed here, suggesting the risk of lithium plating could be reduced. Furthermore, cattail pollen-derived anode cycled at elevated temperature (ACP_50 °C) exhibit slightly higher potential onset at 1st and 40th discharges than a similar anode cycled at room temperature (ACP_25 °C), suggesting a greater tendency for ACP to lithiate at elevated temperature.

Figure 3 Electrochemical analysis of ACP vs Li half-cells. (a) Voltage profiles from 0 to 3 V at C/10 rate for both 50 °C and 25 °C cells; (b) corresponding dQ/dV plots, inset is the enlarged charge plots; (c) capacity as a function of cycle number at C/10 rate; (d) rate studies of ACP vs Li at room and elevated temperatures. Full size image

Differential capacity (dQ/dV) plots of the 1st and 40th cycles for each cell further reveal the difference in their cycling behavior. As shown in Fig. 3b, the 1st discharge curve of ACP_25 °C cell appears to be erratic from 1.0V to 0.2V; this can be correlated to the 1st cycle solid electrolyte interphase (SEI) formation26. On the contrary, ACP_50 °C cell exhibits rather smooth discharge curve, suggesting very different reaction pathways to form the initial SEI layer, likely promoted by the temperature elevation. The continuous capacity fading observed in the first 10 cycles of ACP_50 °C cell suggests that this 1st cycle SEI layer is not stable (Fig. 3c). The 40th discharge curves for both cells represent stable discharge profiles at both 50 °C and 25 °C; and their comparable profiles suggest similar lithium intercalation mechanisms into the ACP carbon.

On the other hand, 1st and 40th charge profiles of both cells are shown on the inset of Fig. 3b to reveal distinctly different deintercalation features at the two temperatures. The peaks centered at 1–1.3 V region are narrower and more intense for the 50 °C cell than the 25 °C cell, indicating a more moderate sloping profile at higher temperature27 as observed in Fig. 3a. Small peaks appears at 2–3 V in the 1st charge curve of the 50 °C cell may be attributed to the extraction of Li from oxygen functional groups that reside on the surface of carbonaceous materials28,29,30. These peaks disappear upon cycling likely due to the deactivation of surface functional groups.

Figure 3c shows the long cycling performance of both ACP_50 °C and ACP_25 °C cells at C/10 rate. ACP_50 °C cell exhibits high capacity of 590 mAh/g with minimal capacity fading after the first 10 cycles, while ACP_25 °C cell reaches stable reversible capacity of 382 mAh/g after the 1st cycle. The higher capacity of ACP_50 °C cell can be attributed to faster charge transfer/Li ion diffusion at elevated temperature enabling better utilization of the active material. The longer time it took to reach stable capacity is a result of more side reactions taking place on the carbon surface at elevated temperature until a thicker and more stable SEI layer is formed31. This phenomena is also reflected on the lower coulombic efficiency of ACP_50 °C cell (~97%).

Further studies were conducted to evaluate ACP cells’ rate capability at 50 °C and 25 °C. As shown in Fig. 3d, ACP_50 °C cell displays significant improvements in capacities (up to 200 mAh/g more capacity) at slow cycling rates, likely due to the fast charge transfer/Li ion diffusion at elevated temperature. However, the advantage of high temperature gradually diminishes at faster cycling rates. In terms of cycling stability, 25 °C cell outperforms 50 °C cell at slow cycling rates as a result of having fewer side reactions at lower temperature26,31.

ABP half-cells were also cycled to evaluate their performance against ACP cells. As shown in Fig. 4a, the rate study of ABP cells at both 50 °C and 25 °C reveal encouraging results considering the fact that these active material morphologies were not optimized or controlled. When compared to the ACP cells data (Fig. 4b), ABP cells, as expected, exhibit lower capacities (~150 mAh/g lower at C/10 rate for both high and low temperatures) at all rates and at each temperature. The difference in electrochemical performance could be a result of morphological difference between ABP and ACP carbon. As revealed in SEM images, ACP contains particles with smooth surface while ABP carbon contains spheres with various morphologies including ones with spikes which could have led to high local current density that exacerbates side reactions and excessive SEI buildup32. Moreover, this morphological difference likely gives rise to the larger BET surface area of ABP carbon (303.9 m2/g) in comparison to ACP carbon (237.6 m2/g) as listed in Table 2 below (also see Supplementary Fig. S2). The larger surface area could translate to larger electrolyte-electrode reaction interface, which had been linked to more SEI formation26. Consequently, ABP cells show higher 1st irreversible capacity losses at both 50 °C and 25 °C.

Table 2 Surface characterization of pollen carbons vs. 1st cycle irreversible capacity loss. Full size table

Figure 4 (a) Rate studies of ABP half-cells from 0 to 3 V at both 50 °C and 25 °C; (b) comparison of all rate studies of ACP and ABP cells. Full size image

Electrochemical impedance spectrpscopy (EIS) was employeed to probe the evolution of SEI and charge transfer kinetics of both pollen carbons at room temperature with respect to time. The inset of Fig. 5 illustrates the equivalent circuit model used to fit the EIS data. R SOL is the resistance of the electrolyte solution and R SEI is the SEI film resistance. The constant phase element (CPE) was used instead of double-layer capacitor to account for surface nonideality (roughness) of the particle33. R CT is the charge transfer resistance at the particle surface. And Z W is the Warburg impedance that describes solid state diffusion of lithium ions into carbon. All the EIS and fitting data are summerized in Fig. S3. R SOL calculated from the model remain nearly constant (~2.6 Ω) for all the measurements. The results for R SEI and R CT are summerized in Fig. 5. Both ABP and ACP cells experienced high film resistance (R SEI = 150 Ω and 135 Ω, respectively) during the 1st lithiaion as a result of initial SEI formation. The slightly greater value of ABP cell may come from its larger surface area as discussed previously. The film resistance for both cells dropped significantly after the initial discharge and quickly stablized to ~5 Ω after 3 cycles, suggesting the formation of stable SEI layers on both pollen derived carbon. The charge transfer resistance R CT exhibited very similar behaviors for both cells during the first two cycles and stablized at ~39 Ω for ABP and ~16 Ω for ACP afterward. In other words, ABP pacticles experiences more than twice the charge transfer resistance of ACP. Since the film resistances are very similar in both cells, difference in SEI is unlikely the root cause for difference in their charge transfer resistance34. One possible explanation is that ABP derived carbon is low in N contents, which have been known to improve surface reactivity and electron conductivity in carbon35,36. Consequently, ABP should have a lower electron conductivity leading to poorer charge transfer than ACP. However, the opposite observation may also be true if high P content (also benefits charge transfer37) in ABP is taken into account. Nevertheless, the difference in R CT is in good agreement with the galvanostatic cycling results (Fig. 4).

Figure 5 SEI and charge transfer resistances at 0.5 V discharges as a function of cycles. The inset shows the equivalent circuit used to model the EIS data. Full size image

In summary, carbon microstrutures with unique morphologies have been successfully sythesized from bee pollens and cattail pollens through a facile, one-step solid-state pyrolysis route. XRD and Raman spectroscopy were employeed to confirm their hard carbon nature; and XPS elemental analysis revealed that both samples contain high levels of O content (~11.5%), which contributes to their excellent capacities and rate capabilities. ACP carbon were found to deliver high capacities of 590 mAh/g at 50 °C and 382 mAh/g at 25 °C when cycled at C/10 rate, they also exhibit excellent rate capabilities with 1C rate. ABP cells were cycled under the same conditions but were found to deliver less capacities than ACP cells. The capacity difference between ABP and ACP are up to 150 mAh/g for both temperatures. EIS model fitting shows that ACP carbon benefits from having lower charge transfer resistance which is likely influenced by the presence of conductivity-enhancing elements. This study explored the potential of utilizing pollens as renewable carbon sources for energy storage application and found that high reversible capacities are achievable with careful selection of pollen morphology.