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Article

Biomass-Based Oxygen Reduction Reaction Catalysts from the Perspective of Ecological Aesthetics—Duckweed Has More Advantages than Soybean

by
Meiping Zhang
1,*,
Yanqi Zhang
2,
Jiajia Cui
2,
Zongyao Zhang
2 and
Zaoxue Yan
2,*
1
College of Liberal Arts, Yangzhou University, Yangzhou 225012, China
2
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(15), 9087; https://doi.org/10.3390/su14159087
Submission received: 4 June 2022 / Revised: 7 July 2022 / Accepted: 19 July 2022 / Published: 25 July 2022
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Abstract

:
Ecological aesthetics encourages the harmonization of humans and nature. In this paper, we integrate ecological aesthetics into the development of oxygen reduction reaction (ORR) catalysts of H2/O2 fuel cells. Moldy soybean and duckweed as raw materials are adopted to prepare biomass-based ORR catalysts, both of which have advantages in activity, stability, environmental protection and resource richness over the conventional expensive and scarce noble metal-based catalysts. Therefore, duckweed is more environmentally friendly, entails a simpler preparation process and has a better catalytic performance, ultimately being more in line with ecological aesthetics.

1. Introduction

The development of science and technology has brought about a series of improvements to human life, as well as environmental pollution, resource depletion and ecological crises. These problems require humans to treat the development of science and technology with the idea of sustainability [1]. Ecological aesthetics requires humans to coordinate with nature, i.e., people’s production and life do not destroy nature [2]. For examples, the components of lunar soil and its internal porous structure can be directly used for catalytic electrolysis of water and catalytic hydrogenation of carbon dioxide [3]. Atom economy requires that each atom in the raw material be included in the final product as much as possible, which would not cause pollution and waste. Ecological aesthetics coincides well with the concept of sustainability [4].
At present, an important reason for some unsustainable problems is that the efficiency of the widely used energy conversion technology is too low. To conform to the idea of ecological aesthetics, it is necessary to develop efficient and clean energy conversion devices. Fuel cells are devices that support ecological aesthetics. Solar energy, wind energy and excess electric energy are transformed into H2 for storage, and then electricity is generated through fuel cells with H2 as fuel. The whole process is environmentally friendly and meets the idea of ecological aesthetics. It is obvious that fuel cell technology is intended to solve the problems of industrial pollution and resource depletion from the perspective of ecological aesthetics. However, there is still environmental pollution or resource limitation in the preparation of fuel cell assembly such as bipolar plates, diaphragms and electrodes. In particular, the cathodic oxygen reduction reaction (ORR) kinetics of low-temperature fuel cells is very slow, then high loadings of noble metals are needed as active components [5,6]. Considering the very low reserves in the Earth’s crust and high price of noble metals, the use of noble metals as cathode materials is unsustainable.
In order to solve the unsustainable problems of noble metal catalysts, biomass materials such as rice husk, soybean root, black fungus, acorn shell, waste wood, lotus and egg white have been studied widely as ORR catalysts [7,8,9,10,11,12,13]. Biomass contains a lot of water, which will form pores after drying, and will further decompose to form hierarchical pores with a highly specific surface area during carbonization, providing rich active sites and favoring mass transportation [14]. To further produce pores, KOH or NaOH as a pore-forming agent was used to corrode carbon [15,16]. In addition, biomass materials contain heteroatoms such as N, P, O and C. It is reported that heteroatom-doped carbons display similar ORR activity to Pt/C [17,18]. Heteroatoms can change the surface polarity of carbon matrix by electron donation, thus leading to high ORR activity [19,20].
As a kind of biomass, soybean is rich in elements such as C, N, O and P, which can be directly carbonized to be heteroatom-doped carbon. China imports up to 100 million tons of soybeans every year, while quite a lot of soybeans become moldy due to improper storage. We consider transforming moldy soybeans to heteroatom-doped carbon as an ORR catalyst, which would not compete for human food. To obtain a more eco-aesthetic ORR catalyst, duckweed is also studied to prepare an ORR catalyst and is compared with the moldy soybean. Duckweed is a kind of water plant, which has a high water content and leaf protein content (1.90 wt.%), indicating abundant pores and high N content after carbonization. Herein, duckweed is directly carbonized to hierarchical porous N-doped carbon. Both of the resultant catalysts from moldy soybean and duckweed show much higher ORR activity and stability than commercial Pt/C in an alkaline membrane fuel cell. Thus, duckweed exhibits more advantages in terms of respecting ecological aesthetics due to its simpler preparation process and more environmentally friendly nature.

2. Materials and Methods

2.1. Synthesis of Moldy Soybean Carbons (MSCs)

Moldy soybeans were dried at 80 °C for 24 h. The dried soybeans (5 g) were immersed in 50 mL saturated NaCl solution at 25 °C, then the soybeans were separated, dried at 80 °C for 1 h and heated in a tubular furnace under N2 atmosphere from room temperature to 700 °C at a heating rate of 5 °C min−1. The temperature of 700 °C was kept constant for 2 h. After cooling to room temperature, the product was ground into a powder, stirred in a 1 mol L−1 HCl solution and washed using a suction filtration device with deionized water until the washings showed pH 7. Finally, the solid was dried at 80 °C to obtain the final sample. The samples (MSCs) with the immersion time in NaCl solution of 0 h, 18 h, 36 h and 72 h were defined as MSC-0 h, MSC-18 h, MSC-36 h and MSC-72 h, respectively.

2.2. Synthesis of Duckweed Carbons (DWCs)

Duckweeds (5 g) were washed, dried at 80 °C, placed in a tubular furnace and heated at a heating rate of 5 °C min−1 under N2 atmosphere from room temperature to 600 °C, 750 °C or 900 °C and kept at that temperature for 2 h. After cooling to room temperature, the product was ground into a powder and washed using a suction filtration device with deionized water. Finally, the solid was dried at 80 °C to obtain the final sample. The samples which carbonized at 600 °C, 750 °C and 900 °C were defined as DWC-600, DWC-750 and DWC-900, respectively.

2.3. Physical Characterization

The morphology and elements of the samples were determined using a scanning electron microscope (SEM, S-4800, Hitachi, Japan) with energy dispersive X-ray spectroscopy (EDX, Oxford Instruments, Oxford, UK) and a transmission electron microscopy (TEM, JEM-2010, JEOL, Tokyo, Japan). The crystal structures were determined using an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å). The degree of graphitization and content of carbon defects were determined using a Laser Raman spectroscopy (DXR, Thermo Fisher, Waltham, MA, USA) with a 514.5 nm laser source. The surface and pore structure were measured by nitrogen adsorption/desorption isotherms on a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA) at −196 °C. The element chemical states and contents were analyzed using an XPS spectrometer (ESCALAB 250XI, Thermo, USA).

2.4. Electrochemical Measurements

In total, 1 mg of the catalyst sample (MSC, DWC or 40 wt.% Pt/C) was added into a solution (2.0 mL 0.125 wt.% Nafion/ethanol) with sonication (30 min) to form uniformly dispersed catalyst ink. In total, 25 μL of catalyst ink was dropped onto the surface of a glassy carbon rotating disk electrode (0.25 cm2) and dried at room temperature. The typical loading of the catalyst is 50 μg cm−2. To obtain higher catalyst loadings, the catalyst ink can be added and dried repeatedly. Electrochemical measurements including linear sweeping voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a three-electrode working station (Autolab CHI660D, Chenhua Instruments, Shanghai, China) at 25 °C. The counter electrode was a graphite rod and the reference electrode was an Hg/HgO. All the presented potentials are based on reversible hydrogen electrodes (RHE). The conversion of potential based on different reference electrodes is listed in the Supplementary Materials.

2.5. H2/O2 Fuel Cell Characterization

A fuel cell working station (Fuel Cell Technologies Inc., Albuquerque, NM, USA) was adopted to test the ORR activity and stability of the synthesized samples. The synthesized MSC or DWC, ionomer solution (AS-4, 5 wt.%, Tokuyama Corporation, Tokyo, Japan), deionized water and 1-propanol were ultrasonically mixed to be catalyst ink. Then, the ink was sprayed on one side of a piece of Tokuyama-A201 alkaline membrane to form a cathode, where the content of ionomers was 20 wt.%. PtRu/C catalyst (60 wt.%, Johnson Matthey, London, UK) was sprayed on the other side of the membrane to form anode, where the metal loading was 0.4 mg cm−2. The membrane coated with catalyst was clamped between two carbon papers (AvCard GDS3250) to form a membrane electrode assembly (MEA). H2 as fuel and O2 as oxidant both flowed at 200 mL min−1.The active area of a single cell was 4 cm2. The fuel cell was activated at 60 °C in potentiostatic mode. Under the same operating conditions, the performance of fuel cells with Pt/C (40 wt.%) as the cathode catalyst was compared.

3. Results and Discussion

In this section, the structure and ORR performances of the MSCs and DWCs as catalysts are discussed, respectively, and then the fuel cell performances and the sustainability relative to ecological aesthetics of the two kinds of catalysts are compared.

3.1. Structure and ORR Performances of the MSCs

Figure 1 shows SEM and TEM images of the MSCs. With the increase in immersion time in the NaCl solution, the MSCs become more and more porous, with the macropores (100–300 nm, SEM images of Figure 1(c1,d1,c2,d2)) and mesopores (5–20 nm, TEM images of Figure 1(c3,d3,c4,d4)) becoming more and more concentrated. The micropores can also be observed as tiny white dots in Figure 1(a4–d4), where the number of micropores seems to increase with the extension of immersion time in NaCl solution. The above results prove the pore-forming effect of the NaCl template [21]. On the other hand, when prolonging the immersion time from 36 h to 72 h, the morphology of MSCs hardly changes, indicating that the immersion of 36 h almost leads to the highest NaCl concentration in the soybean. In addition, it is found that the moldy soybeans favor the formation of more macropores and micropores, seen through the comparison of fresh soybeans (Figure S1) and moldy soybeans (Figure 1(a1–a4)). Due to the existence of hierarchical pores, more active sites can be exposed and mass transfer is believed to be improved, both favoring catalysis [22].
Figure 2a shows XRD patterns of the MSCs. The increased intensity in the XRD patterns at small angles (e.g., 2θ = 10°) highlights the improved concentration of micropores [23]. It can be seen that the MSC-36 h and MSC-72 h show the highest intensities and similar patterns at 2θ approaching 10°, proving that a longer immersion time in NaCl solution leads to more micropores.
Raman spectra (Figure 2b) were performed to characterize structural defects. There are five fitted Gaussian peaks in the spectrum between 800 cm−1 and 2000 cm−1, which are two major peaks of G (~1600 cm−1) and D (~1350 cm−1), and three minor peaks of D′ (~1620 cm−1), D″ (~1490 cm−1) and I (~1220 cm−1) [24]. The G peak represents in-plane stretching vibration of the sp2 hybridization of C atoms; the D peak represents disordered C atoms [25]; the I peak represents the concentration of impurities or heteroatoms (e.g., S, P and N); and the D′ and D″ represent carbon defects [26,27]. The ID/IG (intensity ratio of D peak to G peak) generally indicates the degree of carbon defects [28]. Figure 2c displays that with the immersion time of soybean in NaCl solution, the peak areal percentages of the D, D″ and D′ peaks obviously increase, while the percentage of the G peak decreases. Consequently, the ID/IG value increases with the immersion time. The above results mean that more immersion time in NaCl solution leads to more disorder and more defects in the carbon matrix; however, the above parameters show almost no changes when the immersion time exceeds 36 h, which are consistent with the SEM, TEM and XRD results. In addition, the areal percentage of I peak of all the MSCs shows almost no changes, indicating that the immersion time does not obviously affect the contents of heteroatoms.
Figure 2e,f show N2 adsorption-desorption and pore size distribution curves. The specific surface area, pore volume and diameter are summarized in Table 1. The results show that with the increase in immersion time in NaCl solution, the micropore surface area, mesopore surface area, micropore volume, mesopore volume and average pore diameter increase. Typically, the average pore diameters of the MSC-36 h and MSC-72 h (7.2 nm for both) are very consistent with the TEM results. The MSC-36 h and MSC-72 h also show similar surface areas and pore structures to each other, proving that 36 h of immersion in NaCl solution is adequate for forming the maximum concentration of pores.
XPS spectra are obtained to analyze chemical states and contents of various atoms in the materials. Figure 2g shows that C, N, P and O elements are included in all the MSCs, which is consistent with the EDX mapping of a typical MSC-36 h (Figure S2). Figure 2h and Table S1 show that the immersion time in NaCl solution does not affect the contents of N and P in the MSCs. However, the content of O increases with the increase in immersion time (i.e., with the increase in the surface area of the samples), which is due to the easy absorption of the O species on the large surface area of the MSCs. The O species would benefit ORR activity due to hydrophilicity [29]. Figure 2i and Figure S3 and Table S2 show that with the increase in immersion time, pyridinic N and pyrrolic N (on edge of carbon layer) increase, while the graphitic N decreases, indicating that immersion in NaCl solution leads to more edges (defects) in the carbon matrix, which is consistent with the TEM, XRD and Raman results. Usually, heteroatoms on the edge of a carbon matrix favor the ORR process [30].
Figure 3a shows LSV curves for ORR on the electrodes of the MSCs with Pt/C (40.wt% Pt, TKK, Tokyo, Japan) being compared. Half-wave potential (E1/2) is usually used to compare ORR activities [5]. The E1/2 of the electrode has the following order: MSC-72 h (0.858 V) > MSC-36 h (0.856 V) > Pt/C (0.851 V) > MSC-18 h (0.818 V) > MSC-0 h (0.704 V). The activity order of the MSCs is exactly the same as the length of immersion time. Similar to their surface and pore structures, the activities of the MSC-36 h and MSC-72 h are also very close to each other. The MSC-36 h and MSC-72 h have, respectively, 7 mV and 5 mV more positive E1/2 than the Pt/C. When the loadings of the MSCs all increase to 250 μg cm−2 (Figure 3b), all the E1/2s are increased and they are in the following order: MSC-72 h (0.868 V) > MSC-36 h (0.864 V) > MSC-18 h (0.828 V) > MSC-0 h (0.750 V). Both the MSC-72 h and MSC-36 h at higher loadings (250 μg cm−2) show more advantages in E1/2 than the Pt/C (50 μg cm−2), indicating that more loadings of the MSCs could be adopted at low price to obviously improve the output voltage and power density of fuel cells.
The area of CV circles obtained in N2-saturated electrolyte can be used to analyze electrochemical active surface area of the electrode materials that have similar components. Figure S4 and Table S3 show that the MSC-36 h and MSC-72 h have the largest and similar areas, accounting for their highest ORR activities. The MSC-18 h has a smaller area and the MSC-0 h has the smallest area, accounting for their poor ORR activities. The peak potential and peak current density in the CV curve (at ~0.9 V) also reflect ORR activity. As expected, the orders of the peak potentials and peak current densities of the MSCs (Figure S4) are exactly the same as their E1/2 order (Figure 3a).
EIS was adopted to characterize diffusion and transport kinetics. Figure 3c displays Nyquist plots on these MSC electrodes; inset is the corresponding equivalent circuit, where Rs represents intrinsic electric resistance of the electrode, Rct represents charge transfer resistance, Rw represents Warburg diffusion resistances and the CEP is the constant phase angle element. The initial Z’-intercept in the high frequency region of Figure 3c corresponds to the Rs [31]. MSC-36 h and MSC-72 h have smaller Rs (~5.7 Ω) than the MSC-0 h and MSC-18 h (7.9 Ω and 9.0 Ω), accounting for the lower onset potentials of the former (Figure 3a). The highly porous structures of MSC-36 h and MSC-72 h have denser contact points than MSC-0 h and MSC-18 h which have relatively smooth surfaces; therefore, the former have lower electronic conductivities. In the middle frequency region, the EIS curves of the MSC-36 h and MSC-72 h possess the smallest semicircle diameters, corresponding to their lowest Rcts (3.6 Ω for MSC-36 h, 3.5 Ω for MSC-72 h, 5.0 Ω for MSC-18 h and 6.2 Ω for MSC-0 h) [32]. The lower Rct means lower interfacial resistance between the electrolyte and electrode, inferring fast ion transportation [33,34]. In the low frequency region, the MSC-36 h and MSC-72 h have lower Rws (0.6 Ω for both) than the MSC-18 h (0.7 Ω) and MSC-0 h (0.8 Ω), indicating that the former have short diffusion paths and low diffusion resistances, which is due to their hierarchical porous structures. On the whole, the MSC-36 h and MSC-72 h have relatively low equivalent series resistance (ESR, which is the sum of Rs, Rct and Rw), indicating both their excellent electronic conductivity and excellent ion transportation properties [35], which greatly favor ORR activity.
Catalyst stability is critical for fuel cell application. The MSC-36 h, MSC-72 h and Pt/C were cycled at a scan rate of 200 mV s−1 in O2-saturated 0.1 mol L−1 KOH solution from 0.1 to 1.1 V for 10,000 circles. Figure 3d–f and the insets display LSV and CV curves before and after the above 10,000 CV cycles. The MSC-36 h and MSC-72 h show hardly changes in both the LSV and CV curves with minute ΔE1/2s of 5 mV and 6 mV, respectively, indicating excellent electrochemical stabilities. However, obvious changes occurred on the Pt/C for both the LSV and CV curves with ΔE1/2 of 35 mV. The Pt/C suffers Pt dissolution, migration, agglomeration and abscission during catalytic processes [36], resulting in poor catalytic stability. The MSCs avoid these problems, resulting in excellent stabilities.

3.2. Structure and ORR Performances of the DWCs

The typical DWC-750 is characterized by SEM and TEM (Figure 4). The SEM image (Figure 4a) shows that the DWC-750 has a block morphology with a coarse surface. The TEM images (Figure 4b,c) show that the DWC-750 is composed of macropores and mesopores with thin carbon layers. Figure 4d shows that the carbon layers have the thickness of ~1.0 nm with abundant micropores; the mesopores are also clearly presented here. This hierarchical porous structure with thin carbon layers can greatly promote mass transfer and expose dense active sites [37].
Figure 5a shows XRD patterns of the DWCs. All the DWCs show increased intensities at 2θ approaching 10°, meaning abundant micropores. The DWC-750 shows the highest intensities at 2θ near 10°, indicating its rich micropores. Raman spectra (Figure 5b) were carried out to characterize defects and heteroatoms of the DWCs. Figure 5c concludes that with the increase in heating temperature, the peak areal percentages of D, D″ and I peaks decrease. As expected, the percentage of G peak and the ID/IG value increases with the increase in heating temperature. The above results mean that a higher heating temperature leads to less disorder, less impurities, less heteroatoms and less defects in the carbon matrix.
Figure 5e,f display N2 adsorption-desorption and pore size distribution curves. The surface and pore data are summarized in Table 2. The results display that the DWC-750 has the highest micropore surface area, mesopore surface area, micropore volume, mesopore volume and average pore diameter, which is consistent with the XRD and Raman results. The lower specific surface area of the DWC-600 is due to the incomplete pyrolysis of the duckweed at low temperature; conversely, the relatively low specific surface area of the DWC-900 is due to the sintering of the carbon matrix at a high temperature.
XPS spectra in Figure 5g show that all the DWCs contain the elements of C, N and O, being consistent with the EDX mapping of the typical DWC-750 (Figure S5). Figure 5h shows that the heating temperature greatly affects the contents of N and O species in the DWCs. The N and O species are easily decomposed and flow away at high temperature, leading to their decreased contents. The high contents of the N and O species are beneficial to ORR due to more active sites and hydrophilicity [29]; however, too high contents of N and O affect electronic conductivity. Therefore, moderate N and O species are expected to show higher ORR activity.
Figure 5i and Figure S6 show that with the increase in heating temperature, the N species on the edge of the carbon layer (pyridinic N and pyrrolic N) decreases, while the graphitic N increases, indicating that a high heating temperature leads to fewer edges (defects) and a high graphitization degree in the carbon matrix, which is consistent with the XRD and Raman results. Usually, heteroatoms on the edge of the carbon matrix favor catalyzing ORR [30].
The DWCs are characterized as electrocatalysts for ORR. Figure 6a shows LSV curves on the electrodes of the DWCs and Pt/C (47.8 wt.% Pt, TKK, Japan). The electrodes have the following E1/2 order: DWC-750 (0.859 V) > Pt/C (0.851 V) > DWC-900 (0.812 V) > DWC-600 (0.742 V). The activity order of the DWCs is completely consistent with the order of specific surface area, indicating that the porous structure plays a critical role in ORR activity since the DWCs have the same ingredients. Notably, the DWC-750 has 8 mV more positive E1/2 than the Pt/C. When the loadings of the DWCs all increase to 250 μg cm−2 (Figure 6b), all the E1/2s are increased and they are in the following order: DWC-750 (0.869 V) > DWC-900 (0.823 V) > DWC-600 (0.765 V). The DWC-750 (250 μg cm−2) shows 18 mV more positive E1/2 than the Pt/C (50 μg cm−2), indicating that more loadings of the DWCs could be adopted at a low price to improve the output voltage and power density of the fuel cells. In addition, the sonication time may affect the ORR activity due to different dispersion degrees of the active sites. Figure S7 indicates that the 30 min of sonication is adequate to display the highest ORR activity for the DWC-750.
Figure 6c–e and Table S3 show that the DWC-750 has the largest CV curve area, highlighting its high ORR activities. The DWC-900 and DWC-600 have smaller areas, highlighting their inferior ORR activities. The ORR peak potentials and peak current densities (at ~0.9 V) of the CV curves in an O2-saturated electrolyte are also used to compare ORR activities. As expected, both the orders of peak potentials and peak current densities of the DWCs (Figure 4) are completely consistent with the E1/2 order (Figure 6a). In addition, all the DWCs have higher areas than that of the Pt/C (inset of Figure 3f), proving the higher specific surface areas of the DWCs than that of the carbon powder (Pt/C). In addition, the Nyquist plots in Figure 3c show that the DWC-750 has higher electronic conductivity (Rs = 5.4 Ω), better electron transfer (Rct = 3.4 Ω) and ion transportation properties (Rw = 0.6 Ω) than the MSC-36 h, which favor ORR activity better.
The catalytic stabilities are characterized. The DWC-750 and Pt/C were cycled at a scan rate of 200 mV s−1 from 0.1 to 1.1 V for 10,000 times in 0.1 mol L−1 KOH solution with O2 saturation. The DWC-750 shows almost no changes in the LSV and CV curves with a slight ΔE1/2 of 5 mV (Figure 6f), indicating excellent cyclic stability, which is also much better than those of the Pt/C (Figure 3f).

3.3. H2/O2 Fuel Cell Performances with MSC-36 h and DWC-750 as Cathodes

The ORR activity and stability were tested on a fuel cell working station using an alkaline MEA at relative humidity (RH) of 100%, temperature of 60 °C and back pressure of 0.1 MPa. Figure 7a shows that the fuel cells with MSC-36 h and DWC-750 as cathodes have the peak power densities (PPDs) of 1.12 W cm−2 and 1.14 W cm−2, respectively, which is 1.03 times and 1.05 times that of the Pt/C as a cathode (1.09 W cm−2). The cell durability with MSC-36 h, DWC-750 and Pt/C as cathodes was studied. Figure 7b displays that the MSC-36 h retains 99.1% of the initial PPD (1.11 W cm−2) after 10,000 cycles and retains 92.9% (1.12 W cm−2) after 30,000 cycles. Figure 7c shows that the DWC-750 retains 98.2% of the initial PPD (1.12 W cm−2) after 10,000 cycles and retains 93.8% (1.07 W cm−2) after 30,000 cycles. However, Figure 7d shows that the Pt/C keeps 84.4% of the initial PPD within 6000 cycles (from 1.09 W cm−2 to 0.92 W cm−2). The MSC-36 h and DWC-750 show excellent durability, while the DWC-750 shows superior advantages.

3.4. Comparison of Soybean and Duckweed as Catalyst Sources from the Perspective of Ecological Aesthetics

Firstly, transforming moldy soybeans can prevent food waste. Secondly, the ORR catalyst prepared from moldy soybean could replace noble metals, resulting in potential economic value. Moreover, compared to noble metals, the MSCs obtained from moldy soybeans are not only environmentally friendly, but are also rich in raw material resources. During the process of preparing the MSC, NaCl is used as pore-forming agent, which avoids the use of KOH as a polluting pore-forming agent. It is deduced that the use of moldy soybean to prepare ORR catalysts is more sustainable than noble metals and better conforms to the concept of ecological aesthetics.
However, the use of NaCl as a pore-forming agent increases process complexity and energy consumption. Furthermore, after the optimization of soybean storage technology, there may be no adequate moldy soybeans for producing the corresponding ORR catalyst; moreover, the moldy degree of soybean affects catalyst performance, which will affect the operation stability of fuel cells. In addition, the MSC needs HCl treatment to remove impurities before obtaining the high ORR activity (Figure 8a and Figure S8), which would lead to pollution and process complexity. Duckweed, as a non-human food, has a high water content, high porosity and high N content and would thus conquer the above problems. Through one-step carbonization, N-doped porous carbon can be obtained; therefore, the HCl treatment and even the water washing process could be avoided (see Figure 8a and Figure S8). There will also be no need for pore-forming agents such as NaCl or KOH, which greatly reduce energy consumption and pollution. In addition, duckweeds can absorb N-containing nutrients and other substances in the water and purify the water body. Therefore, the application of duckweed can combine the purification of the water body with the production of a fuel cell catalyst, which is more in line with ecological aesthetics and the concept of sustainability.

4. Conclusions

Guided by the idea of ecological aesthetics, with moldy soybean and duckweed as raw materials, high activity and high stability biomass-based ORR catalysts (MSC and DWC) with environmentally friendly and excellent performances are successfully prepared, providing better activity than most reported biomass-derived carbon catalysts (Table S4). The MSC and DWC are used as fuel cell cathodes, showing a PPD of 1.14 W cm−2 and 1.12 W cm−2, respectively, which is 1.05 and 1.03 times that of commercial Pt/C. Their operation stabilities are both much higher than that of Pt/C. Therefore, the DWC has more advantages, such as a simple preparation process, low energy consumption, no pollution (no use of other chemical reagents), no occupation of human food and purification of water during raw material growth. Compared to MSC, the DWC is more in line with ecological aesthetics and sustainability. The heteroatom dopants, high specific surface area and hierarchical pores of the carbon matrix as well as some possible residual metal elements would all contribute to the high ORR activities. The uncertainty of the composition of biomass materials affects the structure, composition and performance of the obtained materials; therefore, the control of the components in biomass materials may become an important research direction in the future. Social industrial civilization is gradually transforming to ecological civilization. This study provides a new case for ecological aesthetics.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su14159087/s1, K-L (Koutecky-Levich) equation; Potential conversion equation between RHE and HgO/Hg electrodes; Figure S1: SEM image (a) and TEM image (b) of the carbonized fresh soybean; Figure S2: EDX mapping of the MSC-36 h for (a) C, (b) N, (c) O and (d) P; Figure S3: High-resolution XPS spectra of N1s for the materials of (a) MSC-0 h, (b) MSC-18 h, (c) MSC-36 h and (d) MSC-72 h; Figure S4: CV curves on various electrodes in 0.1 mol L−1 KOH solution with N2 (solid line) and O2 (dashed line) saturation; Figure S5: EDX mapping of the DWC-750 for (a) C, (b) N and (c) O; Figure S6: High-resolution XPS spectra of N1s for the materials of (a) DWC-600, (b) DWC-750 and (c) DWC-900; Figure S7. (a) LSV curves on the DWC-750 with different time of sonication in O2-saturated 0.1 mol L−1 KOH solution with the scan rates of 10 mV s−1 and electrode rotation speeds of 1600 rpm. (b) CV curves on the DWC-750 with different time of sonication in O2-saturated 0.1 mol L−1 KOH solution with the scan rates of 10 mV s−1. Figure S8. EDX patterns of the MSC-36h and DWC-750 with different treatments (with or without HCl or H2O treatment). Table S1: Element contents of the materials from XPS spectra; Table S2: Contribution of various N species from high-resolution XPS spectra of N1s; Table S3: Summarized closed loop areas of CV curves of the materials from Figures S4 and S6; Table S4: Comparison of ORR activities of the MSC-36 h and DWC-750 with other biomass-derived carbons in the reported literature. References [7,8,9,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64] are cited in Supplementary Materials.

Author Contributions

Conceptualization, M.Z. and Z.Y.; methodology, M.Z., Z.Z. and Z.Y.; validation, M.Z., J.C. and Z.Y.; investigation, M.Z., Y.Z. and Z.Y.; resources, Z.Y.; writing—original draft preparation, M.Z. and Y.Z.; writing—review and editing, Z.Y.; supervision, Z.Y.; project administration, Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Mingqian Yan for providing the duckweed.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. SEM (top two rows) and TEM (bottom two rows) images with different magnifications of the MSCs. (a1–a4) MSC-0 h, (b1–b4) MSC-18 h, (c1–c4) MSC-36 h and (d1–d4) MSC-72 h.
Figure 1. SEM (top two rows) and TEM (bottom two rows) images with different magnifications of the MSCs. (a1–a4) MSC-0 h, (b1–b4) MSC-18 h, (c1–c4) MSC-36 h and (d1–d4) MSC-72 h.
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Figure 2. (a) XRD patterns and (b) Raman spectra of the MSC-0 h, MSC-18 h, MSC-36 h and MSC-72 h. Percentage of various Raman peaks (c) and ID/IG value (d) versus immersion time (in NaCl solution) curves. (e) N2 adsorption-desorption isotherms and (f) the corresponding pore size distributions. (g) XPS spectra of the MSCs, (h) element percentage versus immersion time curves and (i) N species percentage versus immersion time curves from XPS spectra.
Figure 2. (a) XRD patterns and (b) Raman spectra of the MSC-0 h, MSC-18 h, MSC-36 h and MSC-72 h. Percentage of various Raman peaks (c) and ID/IG value (d) versus immersion time (in NaCl solution) curves. (e) N2 adsorption-desorption isotherms and (f) the corresponding pore size distributions. (g) XPS spectra of the MSCs, (h) element percentage versus immersion time curves and (i) N species percentage versus immersion time curves from XPS spectra.
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Figure 3. LSV curves on MSC-0 h, MSC-18 h, MSC-36 h, MSC-72 h and Pt/C electrodes in 0.1 mol L−1 KOH solution with O2 saturation; the electrode rotation speed is 1600 rpm: (a) the loadings are 50 μgcat. cm−2 for all; (b) the loading is 50 μgcat. cm−2 for Pt/C and 250 μgcat. cm−2 for the others. (c) Nyquist plots on the MSC-0 h, MSC-18 h, MSC-36 h, MSC-72 h and DWC-750, inset is corresponding equivalent circuit. LSV curves before and after 10,000 CV cycles on (d) MSC-36 h, (e) MSC-72 h and (f) Pt/C in 0.1 mol L−1 KOH solution with O2 saturation, the electrode rotation speed is 1600 rpm; insets are the CV curves before and after the 10,000 CV cycles without electrode rotation. All the scan rates are 10 mV s−1.
Figure 3. LSV curves on MSC-0 h, MSC-18 h, MSC-36 h, MSC-72 h and Pt/C electrodes in 0.1 mol L−1 KOH solution with O2 saturation; the electrode rotation speed is 1600 rpm: (a) the loadings are 50 μgcat. cm−2 for all; (b) the loading is 50 μgcat. cm−2 for Pt/C and 250 μgcat. cm−2 for the others. (c) Nyquist plots on the MSC-0 h, MSC-18 h, MSC-36 h, MSC-72 h and DWC-750, inset is corresponding equivalent circuit. LSV curves before and after 10,000 CV cycles on (d) MSC-36 h, (e) MSC-72 h and (f) Pt/C in 0.1 mol L−1 KOH solution with O2 saturation, the electrode rotation speed is 1600 rpm; insets are the CV curves before and after the 10,000 CV cycles without electrode rotation. All the scan rates are 10 mV s−1.
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Figure 4. (a) SEM, (b,c) TEM and (d) HRTEM images of the DWC-750.
Figure 4. (a) SEM, (b,c) TEM and (d) HRTEM images of the DWC-750.
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Figure 5. (a) XRD patterns and (b) Raman spectra of the DWCs. (c,d) Percentage of peak area and ID/IG value versus heating temperature, summarized using Raman spectra. (e) N2 adsorption-desorption isotherms and (f) the corresponding pore size distributions of the DWCs. (g) XPS spectra of the DWCs. (h) N and O element percentages versus heating temperature and (i) N species percentage versus heating temperature, summarized using XPS spectra.
Figure 5. (a) XRD patterns and (b) Raman spectra of the DWCs. (c,d) Percentage of peak area and ID/IG value versus heating temperature, summarized using Raman spectra. (e) N2 adsorption-desorption isotherms and (f) the corresponding pore size distributions of the DWCs. (g) XPS spectra of the DWCs. (h) N and O element percentages versus heating temperature and (i) N species percentage versus heating temperature, summarized using XPS spectra.
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Figure 6. LSV curves on the DWCs and Pt/C electrodes in 0.1 mol L−1 KOH solution with O2 saturation, the electrode rotation speed is 1600 rpm; (a) the loadings of catalyst are 50 μg cm−2 for all; (b) the loading is 50 μg cm−2 for Pt/C and 250 μg cm−2 for the others. CV curves on the electrodes of (c) DW-600, (d) DWC-900 and (e) DWC-750 in 0.1 mol L−1 KOH solution with N2 (solid line) and O2 (dashed line) saturation. (f) LSV curves before and after 10,000 CV cycles on DWC-750 in 0.1 mol L−1 KOH with O2 saturation, the electrode rotation speed is 1600 rpm. Insets are the CV curves before and after the 10,000 CV cycles without electrode rotation. All the scan rates are 10 mV s−1.
Figure 6. LSV curves on the DWCs and Pt/C electrodes in 0.1 mol L−1 KOH solution with O2 saturation, the electrode rotation speed is 1600 rpm; (a) the loadings of catalyst are 50 μg cm−2 for all; (b) the loading is 50 μg cm−2 for Pt/C and 250 μg cm−2 for the others. CV curves on the electrodes of (c) DW-600, (d) DWC-900 and (e) DWC-750 in 0.1 mol L−1 KOH solution with N2 (solid line) and O2 (dashed line) saturation. (f) LSV curves before and after 10,000 CV cycles on DWC-750 in 0.1 mol L−1 KOH with O2 saturation, the electrode rotation speed is 1600 rpm. Insets are the CV curves before and after the 10,000 CV cycles without electrode rotation. All the scan rates are 10 mV s−1.
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Figure 7. (a) H2/O2 fuel cell polarization and power density curves using MSC-36 h, DWC-750 and Pt/C as cathodes, all with a catalyst loading of 1 mgcat. cm−2, a back pressure of 0.1 MPa and an RH of 100%. Polarization and power density curves for MSC-36 h (b), DWC-750 (c) and Pt/C (d) at different cycles.
Figure 7. (a) H2/O2 fuel cell polarization and power density curves using MSC-36 h, DWC-750 and Pt/C as cathodes, all with a catalyst loading of 1 mgcat. cm−2, a back pressure of 0.1 MPa and an RH of 100%. Polarization and power density curves for MSC-36 h (b), DWC-750 (c) and Pt/C (d) at different cycles.
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Figure 8. LSV curves on the electrodes of (a) the MSC-36 h and (b) DWC-750 in O2-saturated 0.1 mol L−1 KOH solution with the scan rates of 10 mV s−1 and electrode rotation speeds of 1600 rpm. The MSC-36 h and DWC-750 samples were treated (washed) with or without HCl solution or water to remove impurities.
Figure 8. LSV curves on the electrodes of (a) the MSC-36 h and (b) DWC-750 in O2-saturated 0.1 mol L−1 KOH solution with the scan rates of 10 mV s−1 and electrode rotation speeds of 1600 rpm. The MSC-36 h and DWC-750 samples were treated (washed) with or without HCl solution or water to remove impurities.
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Table
Table 1. Surface and pore data of the materials.
Table 1. Surface and pore data of the materials.
SampleStotal (a)
(m2 g−1)		Smicro (b)
(m2 g−1)		Vtotal (c)
(cm3 g−1)		Vmicro (d)
(cm3 g−1)		Dpore (e)
(nm)
MSC-72 h210210533.220.537.2
MSC-36 h205910223.210.527.2
MSC-18 h14037761.810.375.8
MSC-0 h6135440.290.231.3
(a) Total specificsurface area; (b) micropore surface area; (c) total pore volume; (d) micropore volume; (e) average pore diameter.
Table
Table 2. Surface and pore results of the DWCs.
Table 2. Surface and pore results of the DWCs.
SampleStotal (a)
(m2 g−1)		Smicro (b)
(m2 g−1)		Vtotal (c)
(cm3 g−1)		Vmicro (d)
(cm3 g−1)		Dpore (e)
(nm)
DWC-750210811212.220.638.2
DWC-90010374661.10.325.4
DWC-60014967681.520.437.8
(a) Total specific surface area; (b) micropore surface area; (c) total pore volume; (d) micropore volume; (e) average pore diameter.
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Zhang, M.; Zhang, Y.; Cui, J.; Zhang, Z.; Yan, Z. Biomass-Based Oxygen Reduction Reaction Catalysts from the Perspective of Ecological Aesthetics—Duckweed Has More Advantages than Soybean. Sustainability 2022, 14, 9087. https://doi.org/10.3390/su14159087

AMA Style

Zhang M, Zhang Y, Cui J, Zhang Z, Yan Z. Biomass-Based Oxygen Reduction Reaction Catalysts from the Perspective of Ecological Aesthetics—Duckweed Has More Advantages than Soybean. Sustainability. 2022; 14(15):9087. https://doi.org/10.3390/su14159087

Chicago/Turabian Style

Zhang, Meiping, Yanqi Zhang, Jiajia Cui, Zongyao Zhang, and Zaoxue Yan. 2022. "Biomass-Based Oxygen Reduction Reaction Catalysts from the Perspective of Ecological Aesthetics—Duckweed Has More Advantages than Soybean" Sustainability 14, no. 15: 9087. https://doi.org/10.3390/su14159087

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