Carbon Energy. 2020;2:561–581. wileyonlinelibrary.com/journal/cey2
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561 R E V I E WBiomass
‐derived nonprecious metal catalysts for oxygen
reduction reaction: The demand
‐oriented engineering of
active sites and structures
Lei Du
1,2|
Gaixia Zhang
1|
Xianhu Liu
3|
Amir Hassanpour
1|
Marc Dubois
4|
Ana C. Tavares
1|
Shuhui Sun
11Center of Energy Materials and
Telecommunications, Institut National de la Recherche Scientifique, Varennes, Quebec, Canada
2School of Chemistry and Chemical
Engineering, Harbin Institute of Technology, Harbin, China
3Key Laboratory of Materials Processing
and Mold, Ministry of Education, Zhengzhou University, Zhengzhou, China
4CNRS, SIGMA Clermont, Institut de
Chimie de Clermont‐Ferrand, Université Clermont Auvergne, Clermont‐Ferrand, France
Correspondence
Gaixia Zhang and Shuhui Sun, Center of Energy, Materials and
Telecommunications, Institut National de la Recherche Scientifique, Québec J3X 1S2, Canada.
Email:gaixia.zhang@emt.inrs.ca(G. Z.) andshuhui@emt.inrs.ca(S. S.)
Funding information
Fonds de Recherche du Québec‐ Nature et Technologies, Grant/Award Number: 274384; Centre Québécois sur les Materiaux Fonctionnels; Natural Sciences and Engineering Research Council of Canada; Office of China Postdoctoral Council, Grant/Award Number: 20180072; National Natural Science Foundation of China, Grant/Award Number: 21805064; ECS‐Toyota Young Investigator Fellowship; Institut national de la recherche scientifique
Abstract
Oxygen reduction reaction (ORR) is an important electrochemical process for renewable energy conversion and storage applications such as fuel cells and metal‐air batteries. ORR is sluggish in kinetics and requires a large amount of platinum group metal (PGM)‐based catalysts to facilitate its slow reaction rate. Application of precious metals raises the cost and decreases the competitivity of these devices in the market. To address this challenge, PGM‐free ORR catalysts have been intensively investigated as an alternative to replace the PGM‐based catalysts and to promote the deployment of ORR‐related applica-tions. In particular, the biomass holds promising potential to be used as the precursor material for PGM‐free ORR catalysts. This pathway has gained more and more attention in recent years. In this review, recent advances regarding biomass‐derived ORR catalysts are summarized with a focus on the rational design of both active sites and porous structures which are the two key factors in determining ORR performance of catalysts. At the end, the perspectives of development of biomass‐derived catalysts is discussed.
K E Y W O R D S
active sites, biomass, oxygen reduction reaction, porous structures
-This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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I N T R O D U C T I O N
Electrochemical energy conversion and storage devices including fuel cells, unitize regenerative fuel cells (URFCs) and the rechargeable metal‐air (oxygen) bat-teries are key nodes in the renewable energy develop-ment and utilization.1‐8 These devices use the oxygen reduction reaction (ORR), at the cathode of fuel cells and during the discharge process of metal‐air batteries.9‐15
The performance of fuel cells and metal‐air batteries is significantly limited by the sluggish ORR kinetics.16‐18 Therefore, tremendous efforts in both academia and in-dustry have been made on developing advanced electro-catalysts and/or electrode materials for the cathode of these devices.19‐22The state‐of‐the‐art ORR catalysts are platinum group metal (PGM)‐based materials.23‐27 The
cutting‐edge Pt ORR catalyst can achieve higher than 10 A mgPt−1 at 0.9 V in mass activity by rotating disk electrode evaluation28; even in membrane electrode as-semblies, the PtCo catalyst can reach over 1.5 A mgPt−1at 0.9 V.29 However, the biggest challenge for PGM‐based ORR catalyst is the scarce resource of PGM and its high cost, so that intensive efforts have been made on devel-oping advanced PGM‐free ORR catalysts in the last dec-ades.30‐38
Two of most representative types of PGM‐free ORR catalysts, the heteroatom (eg, N) doped carbon39‐42 and the transition metal (eg, Fe) coordinated with N in car-bon matrix11,43‐45 have been well accepted as the pro-mising candidates to replace PGM‐based materials. In both cases, carbon is the major component46; even for the PGM‐based ORR catalysts, carbon support is an im-portant investigation topic.47 Therefore, study of carbon support is crucial for the research and development of PGM‐free ORR catalysts. Different kinds of carbon pre-cursors have been employed in synthesizing of PGM‐free ORR catalysts, for example, metal‐organic framework‐ derived carbon,30carbon black,31 and graphene.48 Parti-cularly, the biological materials (ie, biomass) have been recently emerging as promising carbon precursors. This is based on the renewable and sustainable property of biomass, leading to abundant sources of raw materials, that is, low cost.49,50Besides, biomass is characterized by many interesting properties in terms of composition and structure due to the various natural environments to which the biology has to adapt.51‐53 Therefore, biomass materials possess a large number of required elements and some unique coordination environments for building ORR active site. It should be mentioned that in addition to the active site, mass transfer is another key parameter determining the ORR performance. The mass transfer problem can be easily remitted by employing porous electrocatalysts and electrodes.45 In this regard, the
biomass materials that have well‐organized macro‐, meso‐, and micro‐structures are promising to provide tailorable template for material synthesis.54
Recently, review papers have summarized some critical achievements on biomass‐derived materials for electrochemical energy storage and conversion devices.52,53,55,56‐60 Most of these papers focused on the activation and conversion methods of biomass materials but rarely discussed the reasonable strategies to engineer active sites and porous structure. In this report, we re-view recent progress of biomass‐derived materials for electrocatalytic ORR, aiming at the demand‐oriented engineering of both active sites and structure.
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C O N V E R S I O N O F B I O M A S S T O
C A T A L Y S T S
The raw biomass materials are usually required to be pretreated and converted to catalytic composites which have well‐defined active sites, high chemical and electrochemical stability, and superior electronic conductivity—the fundamental requirements for elec-trochemical ORR. As mentioned above, the conversion of biomass into catalysts has been well reviewed.52,53,55,56‐60 Briefly, the conversion of biomass into catalysts con-ventionally includes washing or leaching to remove impurities, activation to modify the surface chemistry and/or porosity, and (hydro)thermal treatment to carbo-nize the materials to obtain high active site density and high electronic conductivity. The detailed conversion strategies are not the scope of this paper. We would emphasize that these protocols should be demand‐ oriented selected, saying that the recipe should depend on the nature of biomass itself and the property that engineers target in the final electrocatalytic materials. For example, the bamboo as a precursor is composed of or-iented cellulose fibers and ligneous matrix. If the oror-iented carbon nanofibers are expected, the bamboo should be hydrothermally treated with 3M KOH to remove lignin (leaving the targeted cellulose fibers) and then be carbonized to obtain the carbon nanofibers.61 Another example is the peanut shells. If high surface area is pre-ferred in the final catalysts, the KOH treatment before pyrolysis of peanut shells is an efficient way to generate porous structure with more micropores.62
To further explain the demand‐oriented conversion concept, we would like to highlight an example in Reference [63] because it shows how to reasonably convert the biomass into desirable electrode materials although it is not directly related to the ORR electro-catalysis. The purpose of this study is to establish the ion capacitors involving Na+ and ClO4− as carriers by using
peanut‐shell derived materials. The storage and release mechanisms for Na+ and ClO4− are different in carbo-naceous materials: the intercalation and deintercalation reactions store and release Na+ ions; while for ClO4−, adsorption and desorption are the primary mechanisms. Therefore, the anode requires inter‐dilated graphene layers to host the sites for intercalation and deintercala-tion of Na+instead of high surface area; while the cath-ode requires large surface area for ClO4−adsorption and desorption, as well as the electrolyte access. To meet such requirements, the biomass peanut shells should be de-signed, accordingly. The inner shells of peanut consist of three‐dimensional, highly cross‐linked polyphenolic polymer (lignin), which is the homogenous portion in peanut shells. In contrast, the outer shells of peanut are highly heterogeneous, primarily composing of inter-connected cellulosic fibril networks. Therefore, the inner and outer shells of peanut are reasonably selected as the precursors for anode and cathode, respectively (Figure1). To obtain inter‐dilated anode graphene layer, the inner shells were carbonized at 1200°C in Ar before a mild low‐ temperature activation at 300°C in air (Figure 1A). The post‐low‐temperature treatment is to introduce sufficient porosity without destroying the layered architecture (Figure 1C). In contrast, the outer shells were hydro-thermally treated followed by KOH activation at 800°C to 850°C in Ar (Figure 1A) to obtain well‐defined cathode nanosheet materials (Figure1B). According to the above example, per different requirements, such as surface area,
porous structure, and surface chemistry for anode and cathode, the biomass and treatment protocols should be reasonably selected and modified.
In the following sections, we are going to discuss how to engineer the active sites and porous structures for ORR catalyst which are derived from biomass through appro-priately selecting biomass precursor as well as the treat-ment protocols. Table1 summaries the porous structure parameters and ORR activity of some representative biomass‐derived catalysts.
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E N G I N E E R I N G O F A C T I V E
S I T E S
As mentioned in Section 1, the heteroatom doped carbon (representative: N‐C) and transition metal coordinated by N in carbon matrix (representative: single atom Fe‐N‐C) have been accepted as the most promising PGM‐free catalysts for ORR. However, their overall activity toward ORR is still lower than Pt‐based catalysts. Therefore, more efforts should be devoted to engineering the active sites of PGM‐free catalysts to obtain a competitive per-formance to the market ready catalysts. In this section, the strategies to engineer active sites by using the bio-mass as precursor will be discussed as the representative of heteroatom doped carbon and single transition metal atom, which is coordinated with N in the carbon matrix M‐N‐C catalysts.
F I G U R E 1 A, A synthetic illustration for the cathode and anode materials derived from different parts of the peanut shells. B, A
typical transmission electron microscopy image of the peanut shell‐derived cathode material with the morphology of the carbon
nanosheets. C, A typical scanning electron microscopy image of the peanut shell‐derived anode material with the layered structure.63
TABLE 1 The representative ORR catalysts derived from biomass: The porous structure and ORR activity Biomass precursor Derived catalyst Porous structure E1/2 (catalyst) − E1/2 (Pt/C) Electrolyte References Soybean shells N ‐doped graphene Surface area: 1152 m 2 /g − 14 mV 0.1M KOH [ 64 ] Pore volume: 0.67 cm 3/g Ginkgo leaves N ‐doped carbon nanosheets Surface area: 1436.02 m 2 /g − 19 mV 0.1M KOH [ 65 ] Euonymus japonicus leaves N ‐doped carbon nanosheets Surface area: 842.33 m 2 /g 30 mV (in onset potential) 0.1M KOH [ 66 ] Micropore surface area: 424.45 m 2 /g Mesopore surface area: 417.88 m 2/g Pore volume: 0.633 cm 3 /g Glucose, cellulose and lignin N ‐doped carbon Surface area: 1273 ‐1834 m 2/g N/A N/A [ 67 ] Pore volume: 1.41 ‐3.08 cm 3/g Mesopore volume proportion: 69 ‐97.5% Roots, stems, leaves, flowers, and fruits of various plants Heteroatom ‐doped carbon Surface area: 1394 m 2 /g Not compared with Pt/C 0.1M KOH [ 68 ] Pore volume: 0.96 cm 3 /g Typha orientalis N ‐doped carbon nanosheet Surface area: 646 m 2 /g 0 m V 0.1M KOH [ 69 ] Pore volume: 0.36 cm 3 /g Micropore volume: 0.24 cm 3 /g Soybean N ‐doped carbon Surface area: 1749 m 2 /g Not compared with Pt/C 0.1M KOH [ 70 ] Prawn shells N ‐doped carbon fiber aerogel Surface area: 526 m 2 /g 54 mV (in onset potential) 0.1M KOH [ 71 ] Beech wood N ‐doped carbon Surface area: 1589 m 2 /g Not compared with Pt/C 0.1M KOH [ 72 ] Pore volume: 0.826 cm 3 /g Micropore volume: 0.643 cm 3/g Mesopore volume: 0.183 cm 3 /g Cattle bones N,P co ‐doped carbon Surface area: 1516 m 2/g 12 mV 0.1M KOH [ 73 ] Pore volume: 0.983 cm 3 /g Reed Si ‐N ‐C Surface area: 1264 m 2 /g 40 mV 0.1M KOH [ 74 ] β‐ cyclodextrin Highly ordered mesoporous carbons Surface area: ∼ 781 m 2/g N/A N/A [ 75 ] Pore volume: 0.41 cm 3/g Micropore volume: 0.27 cm 3/g
3.1
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Metal‐free heteroatom doped
carbon catalysts
N‐doped carbon (N‐C) materials are the most renowned metal‐free ORR catalyst. For N‐C catalysts, the nitrogen dopant plays an important role in promoting the ORR kinetics. As shown in Figure2, there are different kinds of configurations for N dopant in the N‐C catalysts, including pyrrolic N, pyridinic N, graphitic N (ie, qua-ternary N), and so forth.81The efforts have been made to reveal how the N dopant configurations influence the intrinsic ORR activity although the contributions of different N configurations to the ORR activity are still in dispute. In a recent study, it was concluded that the activities of the catalysts with different type nitrogen dopants follow the sequence of pyridinic N > pyrrolic N > graphitic N > oxidized N > C (carbon) in terms of ORR activity.82 In another report, the relationships be-tween N dopant configuration and ORR activity for a series of model catalysts reveal that the limiting current density of ORR process is highly dependent on the graphitic N, while the onset potential is determined by the pyridinic N.83 Besides, it has been experimentally demonstrated that the electron‐donating quaternary N structure determines the ORR.41Note that in the above discussions, the N configuration is related to the ORR activity, which, however, does not mean that the N do-pant is the active sites. Actually, the improved ORR ac-tivity should be attributed to the heteroatom dopant that creates the net charge on the adjacent carbon atoms.84 For example, the active sites were believed as the ad-jacent carbon atoms next to pyridinic N85and quaternary N.41 In spite of these challenges in understanding the role of N dopant, these results indicate the importance of rational synthesis of desired N dopant in N‐C catalysts. Another important point should be emphasized is the stability of different possible highly active N configura-tions. Even a type of N dopant, for example, pyridinic or quaternary N, primarily contributes to the initial ORR activity, it will not be regarded as the main contributor if
TABLE 1 (Continued) Biomass precursor Derived catalyst Porous structure E1/2 (catalyst) − E1/2 (Pt/C) Electrolyte References Porphyra and hemin Single ‐atomic Fe on N ‐doped carbon Surface area: 1449.1 m 2 /g 30 mV 0.1M KOH [ 76 ] Micropore surface area: 1301.5 m 2/ g Mesopore surface area: 197.6 m 2/g Reed Si ‐Fe/N/C Surface area: 534.9 m 2 /g 90 mV (in onset potential) 0.1M KOH [ 77 ] Peanut shells FeNi alloy and N ‐doped carbon Surface area: 1864 m 2 /g 0 m V 0.1M KOH [ 78 ] Pore volume: 1.19 cm 3/g Seaweed (Ni,Co)/CNT nanoaerogels Surface area: 193 m 2 /g − 40 mV 0.1M KOH [ 79 ] Pore volume: 1.055 cm 3 /g Peach gum Hollow Co oxide embedded in N ‐doped carbon nanosheets Surface area: 307 m 2 /g − 40 mV 0.1M KOH [ 80 ] Abbreviation: ORR, oxygen reduction reaction.
F I G U R E 2 The illustrations of possible nitrogen dopant
configurations in the pyrolyzed N‐C catalysts.81
Reproduced
it decays fast. Therefore, the stability of different active N configurations should also be paid attention within the ORR operation.
Some biomass materials are rich of nitrogen element, for example, soybean,86,87 chitosan,88 so that they can be directly used as carbon and nitrogen precursors to synthe-size N‐C catalysts for ORR. Such a synthetic method is simple to yield PGM‐free catalysts. For example, the soy-bean was employed as the single precursor to prepare N‐doped porous carbon as the ORR catalyst. The conver-sion of soybean was carried out by the carbonization, in the presence of silica template (deriving N‐doped porous car-bon, NPC) which was then removed by KOH activation (deriving activated NPC, ANPC).70By using different KOH/ NPC mass ratios (ie, 1, 2, and 3), ANPC‐1, ANPC‐2, and ANPC‐3 can be obtained. On one hand, higher KOH con-tent during activation process leads to higher surface area, especially the micropore surface area in the final catalysts; on the other hand, higher KOH content results in more total N loss in the ANPC catalysts. In addition, the prawn shells as an animal biomass can act as the precursor to synthesize N‐doped carbon catalyst (denoted as Chitin‐900) by carbonization, as shown in Figure3A.71The Chitin‐900 catalyst demonstrated a total N doping level of up to 5.9% (Figure 3B), including 39.6% of pyridinic‐N, 38.8% of pyrrolic‐N, 21.5% of graphitic‐N (Figure 3C). Due to the high pyridinic‐N content, the Chitin‐900 catalyst presented a more positive ORR onset potential than Pt/C catalyst by about 54 mV (Figure3D).
In some cases, additional nitrogen precursors are necessary to optimize the N dopant configurations and improve the ORR activity even though the biomass pre-cursors contain nitrogen elements. Besides, some widely used biomass materials have lack of nitrogen, for ex-ample, lignin, thus they also requiring additional nitro-gen precursors. In the following parts, we will focus on the effect of additional nitrogen precursors on the ORR activity of the N‐doped carbon catalysts.
High nitrogen content can be achieved by employing a second nitrogen‐containing precursor followed by a series of treatments, for example, pyrolysis.67To further enhance the affinity between nitrogen and carbon, and improve the homogeneity of the final catalysts, the engineering strate-gies at the molecular level are expected to be explored. Reference [72] provides an interesting method to introduce nitrogen functional groups to the lignin molecules. Lignin is one of the most abundant natural aromatic polymers which can be extracted from plant cell walls with an up to 30% in mass ratio. Since lignin is largely accessible with low cost, its application in electrochemical devices has attracted tre-mendous attention. However, the lignin itself has lack of nitrogen so that additional nitrogen precursors are required. The covalent functionalization of lignin by different
nitrogen‐involving functional groups was proposed.72
As shown in Figure4A, a two‐step method was employed for synthesis of N doped carbon catalyst by using lignin which was derived from beech wood. The most important step is the nitrogen introduction, that is, an aromatic nitration process, because lignin (denoted as L in this study) does not contain nitrogen as mentioned above. As shown in Fig-ure 4B, the L was first nitrated and called nitrated lignin (NL) which was then deacetylated as nitrated deacetylated lignin (NDL) and eventually aminated as aminated lignin (AmL). Till then, the lignin can be functionalized with different N‐containing functional groups. Compared with the NDL‐ and AmL‐derived catalysts, the N content in NL‐ derived catalyst (NL‐C) can increase up to 6.7 wt%, that is, about 50% of aromatic rings in pristine lignin are nitro‐ functionalized. The content of active pyridinic and graphitic N is much higher in NL‐C than the NDL‐C and AmL‐C (Figure 4C). Accordingly, NL‐C catalyst demonstrates better ORR activity (Figure4D,E) and higher four‐electron selectivity (Figure 4F) than L‐C, AmL‐C, and NDL‐C catalysts.
The gaseous reactants can easily penetrate into the porous structures of the solid biomass precursors, so the ammonia is a promising candidate as a source of nitrogen precursor for this application. It has been reported that the ammonia pyrolysis can efficiently introduce the nitrogen in catalysts.65,69Recently, our group developed the ammonia pyrolysis to introduce nitrogen in the bio-mass derived materials as a PGM‐free ORR catalyst.74
As shown in Figure5A, a biomass, the reed waste, was used as the starting material and derived porous Si‐C sample; after pyrolysis in ammonia gas for different period of time (ie, 4, 6, and 8 minutes), the porous Si‐N‐C catalysts were obtained.74As shown in Figure5B,C, the optimal Si‐N‐C 6 minutes catalyst has a layered and porous structure with graphitic layers at the edges. Importantly, as shown in Figure 5D, the ammonia method successfully in-troduces nitrogen dopant in the final biomass‐derived catalysts. It is interesting that a longer treatment time (eg, 8 minutes) leads to the nitrogen loss. Reasonably, the optimal Si‐N‐C 6 minutes catalyst presents good ORR activity, even superior to Pt/C (Figure 5E), as well as excellent stability (Figure 5F). In particular, a practical Zn‐air battery using this catalyst at cathode demonstrated a good discharging power density (Figure5G), as well as discharging stability (Figure5H).
3.2
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Single transition metal atom
catalysts
As an important type of PGM‐free ORR catalysts, the single transition metal atom, which is coordinated with N
in the carbon matrix, that is, M‐N‐C catalysts, have been attracting more and more attention in the past decades. In the M‐N‐C ORR catalysts, the nitrogen coordinated transition metal moieties, for example, FeNx, have been well accepted as the most promising PGM‐free active site for ORR.89‐92The surrounding carbon matrix works as a host for FeNx active site, not only stabilizing FeNx and conducting electrons but also likely participating into the reduction reaction of oxygen. For example, three possible configurations including FeN4C10, FeN2+2C4+4, and FeN4C12 moieties were modeled and fitted. The fitting results were compared with the experimental X‐ray ab-sorption near edge structure (XANES) of a real single atomic Fe‐N‐C catalyst. Due to the best match between the modeled sites and the experimental results, the FeN4C12moiety was believed as the most possible activity sites for ORR (Figure6A‐C); besides, the FeN4C12 struc-ture benefits from the four‐electron selectivity toward ORR (Figure6D‐F).93
As discussed above, the biomass is a good carbon and nitrogen resource to synthesize the N‐C materials; they are thus believed to be good substrate candidates for Fe‐ N‐C catalysts. Some efforts have been made using the biomass as precursors to generate high‐performance PGM‐free ORR catalysts. As shown in Figure 7A, a
biomass waste, porphyra, was first activated and carbo-nized into the hierarchically N‐doped porous carbon (NHPC). It was then combined with another biomass, Hemin as a natural Fe recourse, and pyrolyzed to form the Fe‐N‐C catalyst.76 The obtained NHPC which is de-rived from porphyra biomass demonstrates a good ther-mostability as shown in Figure7B, that is, only 6.7% in weight is lost below 800°C, in contrast to the pure hemin, which suffers up to 44.7% weight loss. Interestingly, as shown in Figure7B, the composite including both hemin and NHPC can remarkably decrease the weight loss per-centage, likely through the strengthened interaction be-tween hemin and NHPC. By applying different pyrolysis temperatures, the single atomic Fe‐N‐C (SA‐Fe/NHPC) catalyst, Fe‐N‐C catalyst with Fe nanoparticles (NP‐Fe/ NHPC) can be obtained. As shown in Figure 7C, the SA‐Fe/NHPC catalyst presented well‐defined and uniformly dispersed single Fe atoms. Reasonably, the biomass‐derived SA‐Fe/NHPC catalyst demonstrated superior ORR activity as compared with other catalysts (Figure7D,E). This sug-gests that the biomass‐derived catalyst is a competitive candidate compared with the Fe‐N‐C catalysts by conven-tional methods.
Recently, the peach gum containing only C, O, and H was employed as a biomass precursor to synthesize a
F I G U R E 3 A, Procedures for the synthesis of N‐doped aerogel fibers using prawn shells. B, Full X‐ray photoelectron
spectroscopy (XPS) spectrum of Chitin‐900. C, Narrow N1s XPS spectrum of Chitin‐900. D, Oxygen reduction reaction activity curves
of Chitin‐900 and Pt/C at 1600 rpm and 10 mV s−1.71
hybrid Co/N‐Pg catalyst in the presence of cobaltous ni-trate as Co precursor and melamine as N precursor.80As shown in Figure8A, the Co/N‐Pg catalyst showed hollow cobalt oxide nanoparticles. Figure 8B clearly demon-strated the confined cobalt oxide particle with carbon layers. The ORR activity tests in Figure8Cindicated that the introduction of Co and N significantly enhanced the activity, approaching to that of the state‐of‐the‐art Pt/C catalyst. This Co/N‐Pg catalyst was then fabricated in a Zn‐air battery, showing the maximum power density of 119 mW cm−2, which is 94.6% of the case using Pt/C catalyst (Figure 8D). Importantly, the Co/N‐Pg catalyst demonstrated much better stability than that of Pt/C in terms of the rechargeable Zn‐air battery (Figure8E).
In addition to the nitrogen and carbon elements, which are indispensable to build ORR active sites, other elements that can be provided by the biomass waste also likely benefit the active sites. As discussed in Reference74, the reed waste was converted into porous Si‐N‐C, and the presence of Si significantly improved the ORR activity. Such a Si‐N‐C material can also be used as a new carbon precursor to synthesize Si‐Fe‐N‐C catalyst.77
Due to the highly exposed activity site density, the catalyst shows not only excellent ORR activity but also the four‐electron selectivity feature.
Importantly, the presence of Si promotes the formation of graphitic structure in the final Si‐Fe‐N‐C catalyst, that is, high graphitization degree, which leads to high stability under harsh ORR environment.
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E N G I N E E R I N G O F P O R O U S
S T R U C T U R E S
The engineering of active site is usually revolved around the application of the PGM‐free ORR catalysts. Herein we would like to emphasize on the engineering of porous structures, which are crucial for the ORR activity.17The ORR process follows Equations (1) and (2) in acidic and alkaline media, respectively94:
→
In acidic media:O + 4e + 4H2 − + 2H O.2 (1)
→
In alkaline media:O + 2H O + 4e2 2 − 4OH .− (2)
Based on these equations, the soluble oxygen mole-cules are necessary as the reactant in both acidic and alkaline media. Besides, the water is produced in acidic media and consumed in alkaline media. At the same
F I G U R E 4 A, A synthetic illustration of the nitrogen‐doped mesoporous carbon materials derived from beech wood lignin. B,
The converting process to introduce nitrogen‐containing functional groups in lignin, including the conversion of lignin (L) into nitrated lignin (NL) and the conversion of NL into aminated lignin (AmL). C, Absolute amount of nitrogen sites for the N‐doped
carbon catalysts including AmL‐C, NDL‐C, and NL‐C. D, Oxygen reduction reaction activity curves of the catalyst samples in O2‐
saturated 0.1M KOH with a sweep rate of 5 mV s−1, 1800 rpm. E, The half‐wave potentials and (F) the transfer electron number of
time, a large number of free ions (H+ or OH−) are con-sumed (in acidic media) or produced (in alkaline media). Therefore, the access of reactants and removal of pro-ducts require efficient channels for mass transfer.
The porous structure of the catalysts significantly determines the ORR activity. For the PGM‐free ORR catalysts, it has been well accepted that the micropores host the majority of the active sites while the mesopores
F I G U R E 5 A, An synthetic illustration of the porous Si‐N‐C catalysts derived from reed waste. B and C, The typical transmission
electron microscopy images of Si‐N‐C 6 minutes catalyst. D, The specific contents of N dopant configurations in different Si‐N‐C catalysts.
E, Oxygen reduction reaction (ORR) activity curves of different catalysts in O2‐saturated 0.1M KOH at a scan rate of 10 mV s−1and a
rotation rate of 1600 rpm. F, The stability results including the ORR polarization curves of the catalysts before and after 5000 potential
cycles in O2‐saturated 0.1M KOH. G, Discharging polarization and power density curves in a Zn‐air battery. H, Long‐term galvanostatic
discharge curves of Zn‐air batteries at 20 mA cm−2.74
primarily play the role of mass transfer channel. In this regard, the ORR activity is governed by both micropores and mesopores. Taking the N‐doped porous carbon cat-alyst as an example, the calculated relationship between ORR activity and porosity is shown as Figure 9A. Ob-viously, as the microporosity increases, the ORR activity increases and then decreases, indicating the importance of the balanced micropores and mesopores. Besides, higher total porosity significantly improves the ORR ac-tivity.95As for the M‐N‐C catalyst, there should also be a balanced ratio of micropore to mesopore, even macro-pore.96 As shown in Figure 9B, in addition to Fe%, the representative Fe‐N‐C catalyst presents the dependent ORR activity on the ratio of micropore to mesopore.97 It is noteworthy that the microporosity in Figure 9A is defined based on the pore volume and Figure 9B uses the micro‐to‐mesopore ratio based on surface area.
Even though, these two examples demonstrate the im-portance of the balanced porous structures for ORR activity. In addition to the surface area, pore volume, and pore size distribution, the pore connection and mor-phology, though rarely studied, are very import to the mass transfer for ORR. The relationships between the ORR activity and the porous structure for different catalysts are complicated and require more effort.
The catalyst porous structure could be more important for practical devices, that is, fuel cells and metal‐air bat-teries.79,98This is primarily due to the much thicker catalyst layers in fuel cells and metal‐air batteries than those in the three‐electrode system (rotating disk electrode); particu-larly, the PGM‐free cathode catalyst layers are much thicker than PGM‐based catalyst layer.36,99‐101 The large thickness
of the PGM‐free catalyst layer significantly hinders the mass transfer. The relationship between the porous structure of
F I G U R E 6 Comparison between the
experimental X‐ray absorption near edge structure spectrum at K‐edge of a model single atomic Fe‐N‐C catalyst (black dashed lines) and the fitting spectra (solid red lines) by using different theoretical configuration
structures including (A) FeN4C10,
(B) FeN2+2C4+4, (C) FeN4C12, (D) FeN4C12
adsorbing one end‐on oxygen molecule,
(E) FeN4C12adsorbing two end‐on oxygen
molecules, and (F) FeN4C12adsorbing one
side‐on oxygen molecule. The brown, blue, gray, and red spheres represent the iron,
nitrogen, carbon, and oxygen atoms.93A to F,
Reproduced with permission: Copyright 2015,
catalyst layer and device performance might be more complicated as the pores in the catalyst layer include the porous structure of catalysts as well as the secondary pores composited with catalyst and ionomer. In this review, we will not discuss the porous structure con-sisting of catalysts and ionomer. The engineering of the porous structure is vital for the rational design of PGM‐free ORR catalysts.97,102‐104 Biomass itself has various porous structures that can be used as a self‐ template. Porous structures can also be created through artificial templates.
4.1
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Natural porous structure derived
from biomass precursors
Biological organisms represent various structures from below nanometer to centimeter orders of magnitude (Figure10), which provides excellent templates for ma-terial science.54In particular, the materials derived from biomass can well inherit the macro‐, meso‐, and micro‐ porosity of the original biological precursors after ap-propriate conversion processes.64,66,105This paves a way for biomass precursors to engineer the porous structures in PGM‐free ORR catalysts.
Reference [106] provides a good example of using the pomelo peels as the starting material, which was dried and impregnated in phosphoric acid followed by
high‐temperature carbonization. As shown in Figure11A, the original pomelo peels have a microstructure with smooth and large sheets. By pyrolysis at high tempera-tures, the pomelo peels with and without phosphoric acid treatment were converted into corresponding carbon ma-terials. As shown in Figure11B, without the phosphoric acid treatment, the original large sheet structure was broken into smaller sheets with smooth surfaces; inter-estingly, as shown in Figure 11C, if the phosphoric acid was applied, the derived carbon material demonstrated large sheet morphology with much rougher surfaces, containing abundant interconnected porous structure. Combining with the structural characterizations such as X‐ray diffraction (Figure 11D) and Raman spectroscopy (Figure11E), it can be concluded that the phosphoric acid treatment partially destroys the graphitic structure of the biomass‐derived carbon materials.
This study illustrated in Figure 11indicates the feasi-bility of utilizing the natural porous structures in biomass to derive well‐defined porosity in the carbon materials. To the best of our knowledge, there has been little work done on directly using the biomass‐derived porous materials as PGM‐free catalysts for the ORR activity without any addi-tional procedure. It might be difficult for biomass‐based precursors to simultaneously meet all criteria of desired composition and structure to build high‐performance active sites, as well as suitable porous structure to fabricate effi-cient channels for mass transfer. Therefore, an artificial
F I G U R E 7 A, A synthetic illustration of the SA‐Fe/NHPC catalyst. B, The thermogravimetric analyzer curves of N‐doped porous
carbon (NHPC), hemin, and Hemin/NHPC samples. C, A typical high‐angle annular dark‐field‐ scanning transmission electron
microscopy image of SA‐Fe/NHPC catalyst. D, Oxygen reduction reaction polarization curves of different catalysts in O2‐saturated
0.1M KOH at a sweep rate of 5 mV s−1with 1600 rpm. E, The half‐wave potential, kinetic current density, and diffusion‐limited
porous structure is required to be engineered in the biomass‐derived ORR catalysts.
4.2
|
Artificial porous structure derived
from biomass precursors
The artificial template method is a promising approach to control the hierarchical porous structure of materials which are derived from biomass. ZnCl2 has been con-firmed to play important roles in porous structure en-gineering.107Recently, ZnCl2and Mg5(OH)2(CO3)4were employed as the dual template to control the macropores and mesopores in the catalysts.68 In this experiment, glucose and urea were used as carbon and nitrogen
precursors, respectively. By optimizing the mass ratios of urea to glucose as 0.54, ZnCl2 to glucose as 3, and Mg5(OH)2(CO3)4to glucose as 1, the derived catalyst can be obtained by pyrolyzing at 900°C, which is named as N0.54‐Z3/M1‐900 in this study.
68
During the pyrolysis process, as shown in Figure 12A, the urea provides ni-trogen element; the dual templates including ZnCl2and Mg5(OH)2(CO3)4 are converted into ZnO and MgO, re-spectively. In particular, the glucose starts to convert into carbon at 600°C, which can reduce the ZnO into Zn; Zn will be evaporated at higher temperatures, for example, >907°C, thus leaving abundant macro‐, meso‐ and micro‐porous structures in the final catalysts. In con-trast, the MgO will be removed by acid leaching (Figure12A), deriving abundant porous structures as well.
F I G U R E 8 A, Transmission electron microscopy (TEM) image (inset 1: the particle size distribution; inset 2: the enlarged TEM
image of Co/N‐Pg). B, High‐resolution transmission electron microscopy image of the Co/N‐Pg hybrid (inset: the SADE pattern of
Co/N‐Pg). (C) Oxygen reduction reaction activity curves of Co/N‐Pg, N‐Pg, Co/Pg, and Pt/C in O2‐saturated 0.1M KOH solution. D,
Discharging polarization and power density curves of Zn‐air batteries with 20 wt% Pt/C and Co/N‐Pg as air electrode, respectively. E,
Charging/discharging cycling curves of Zn‐air batteries at a current density of 10 mA cm−2.80
A to E, Reproduced with permission:
F I G U R E 9 A, Theoretical relationships among the relative oxygen reduction reaction activity, total porosity, and microporosity.95
Reproduced with permission: Copyright 2019, Wiley.95B, Relationships among the kinetics current density at 0.9 V, Fe content, and the
ratio of micropore/mesopore.97Reproduced with permission: Copyright 2017, American Chemical Society97
F I G U R E 1 0 An overview of biological
materials, the biomass, demonstrating a wide range in length scales and structures according to their critical dimensions. On the left side, the original biological structures are given while on the right side, the
corresponding synthesized materials are
provided.54Reproduced with permission:
Reasonably, the resultant N0.54‐Z3/M1‐900 catalyst de-monstrates the 3D interconnected carbon walls, forming hierarchically porous structures (Figure12B,C). As shown in Figure 12D‐F, the detailed observation unravels the exsitence of micropores and mesopores on the carbon walls. As shown in Figure 12G, such a derived catalyst demonstrated significantly improved ORR kinetics and diffusion limiting current density. This engineering strategy is designed to control the porous structures in biomass‐derived PGM‐free ORR catalysts.68
Importantly, this method is universal for many types of biomass materials including roots, stems, leaves, flowers, and fruits (Figure13).
Essentially, the above discussion is based on the hard template, that is, ZnO and MgO during the pyrolysis process. Similarly, the soft template methods efficiently control the porous structure in the final catalysts. For example, theβ‐cyclodextrin derived from biomass can be used as the carbon source. As shown in Figure 14A, in the presence of poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) (PEO‐PPO‐PEO) copolymer as the soft template, theβ‐cyclodextrin can form a highly ordered structure. Accordingly, theβ‐cyclodextrin can be converted into carbon materials with highly ordered
mesoporous structures through successive hydrothermal and carbonization procedures (Figure14B,C).75Although the biomass‐derived carbon material was not evaluated as ORR catalyst in this study, the strategy of using soft template is expected to be helpful in preparation of other biomass‐derived PGM‐free ORR catalysts.
It should be noted that the conversion methods are also crucial for the porous structure formation. For ex-ample, the treatment approaches using alkali (eg, KOH as mentioned in Reference [108]), acid (eg, phosphoric acid as mentioned in Reference [109]) and corrosive gas (eg, ammonia as mentioned in Reference [110]) can generate more porous structures.
5
|
S U M M A R Y A N D O U T L O O K
5.1
|
Summary
ORR has been playing a crucial role in electrochemical energy conversion and storage applications including fuel cells, URFCs, and metal‐air batteries which are important technologies for building a sustainable society. One of the greatest short comes of ORR is its sluggish kinetics,
F I G U R E 1 1 The scanning electron microscopy images of (A) natural pomelo peels after vacuum freeze‐drying; (B) carbon
material derived from pomelo peels without phosphoric acid treatment; and (C) carbon material derived from pomelo peels with phosphoric acid treatment. D, The X‐ray diffraction patterns of the carbon materials derived from pomelo peels with and without phosphoric acid treatment. E, The Raman spectra of the carbon materials derived from pomelo peels with and without phosphoric
F I G U R E 1 2 A, A schematic illustration of the synthesis mechanism of hierarchically porous nitrogen/oxygen‐doped carbon
materials. B to D, The scanning electron microscopy images of N0.54‐Z3/M1‐900 catalyst. E and F, The transmission electron
microscopy images of N0.54‐Z3/M1‐900 catalyst. G, Oxygen reduction reaction activity curves of N0.54‐Z3/M1‐900 catalyst compared
requiring a large amount of catalysts to achieve desired performance. The state of the art PGM‐based catalysts have superior intrinsic ORR activity but suffer from the scarce resource and high cost. To address this challenge, the PGM‐free catalysts have been regarded as a promising alternative to replace the PGM‐based materials and have been intensively investigated for decades.
Biomass materials can be seen everywhere; in parti-cular, some biomass materials are even regarded as waste. Due to the various composition and structure, the biomass is promising to be used as the precursors for PGM‐free catalysts to further decrease the cost of the cathodes. Be-sides, the intrinsic ORR activity of PGM‐free catalysts is still lower than the PGM‐based catalysts. Therefore, much more PGM‐free catalysts are needed to be used in the cathodes at device level to compensate the low activity per unit mass or unit area, which usually leads to a thicker catalyst layer and higher mass transport resistance. Well‐defined porous structure is thus needed to allow fast mass transportation. Therefore, the rational engineering of active sites and por-ous structure is focused in this review paper.
The N‐doped carbon and FeNx moieties are the most promising active sites for ORR. Most of biomass materials are rich of carbon element which is the ideal precursor for the carbon supports for PGM‐free catalysts. For the biomass enriched with N element, it can be directly converted into the final N‐doped carbon catalysts. For the biomass without
or with only little N and/or Fe, additional N and/or Fe precursors are needed. In particular, the unique elements in biomass materials, for example, Si as discussed before, sometimes are helpful in building the ORR active sites with improved activity. The advantage of using biomass as the starting materials to tune the porous structure is the various original structures from nanoscale to macroscale in biolo-gical materials. By rationally selecting the biomass materials as precursors, the demand‐oriented structure can be ex-tracted from the biomass precursors. The artificial methods to create porous structures on the basis of biomass have been proposed, which will shed a light on research and development of biomass‐derived PGM‐free ORR catalysts in the future. The biomass and even biomass waste‐derived catalysts have shown good ORR stability, even for the practical devices such as Zn‐air battery in some cases, pre-senting great promise of this biomass‐to‐catalyst pathway.
5.2
|
Outlook
The conversion procedures of biomass materials into cata-lysts are not usually straight forward. For most biomass materials, clean, decomposition, and/or ferment, and so forth. are needed before using them as the precursors. Much efforts should be focused to simplify the conversion processes since more procedures lead to higher fabrication cost.
F I G U R E 1 3 A to E, The photos of various biomass materials. F to O, The transmission electron microscopy images of the
To enhance the competitiveness of biomass materials, enhancing the intrinsic ORR activity of the biomass‐ derived catalysts is urgently required. The selection and utilization of biomass as precursors highly depend on the understanding of the active sites as well as the relation-ship between porous structure and mass transfer for ORR. Knowing these mechanisms will help to know what types of biomass materials should be selected and what conversion procedures should be performed. To deeply understand the active site, porous structure, and mass transfer, especially in practical devices, the models for oxygen molecule transportation, adsorption, reduc-tion, as well as the desorption and removal of products, should be revisited. To achieve insights into such scien-tific questions, advanced physical characterizations and in‐situ technology, more accurate theoretical works are needed. Advanced materials science and technology are needed to develop to enable the synthesis of the desired active sites and porous structure. To fill the gaps between fundamental research and the R&D of renewable energy devices, the stability of the biomass‐derived catalysts should be focused in the future investigations; while the translation of good catalyst activity from half‐cell tests into the corresponding devices such as fuel cells, URFCs, and metal‐air batteries should be intensively carried out. In this review, the biomass is discussed with the focus of their application in synthesizing ORR catalysts. We should mention that the biomass materials can also be used in other fields, for example, oxygen evolution reaction,78 hy-drogen evolution reaction,111 nitrogen reduction reac-tion,[112] batteries,[113,114] and supercapacitors,75 and so
forth. Some knowledge and experiences regarding the en-gineering of active site and porous structures can be shared in the mentioned fields. The community should further dig into the potential applications of biomass in different fields rather than ORR catalysts. In this way, the biomass waste can be really turned into treasure and promote the estab-lishment of a sustainable society.
A C K N O W L E D G E M E N T S
This study is financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche du Québec—Nature et Technologies (FRQNT), Centre Québécois sur les Mate-riaux Fonctionnels (CQMF), Institut National de la Recherche Scientifique (INRS), and National Natural Science Foundation of China (Grant No. 21805064). SS acknowledges the ECS‐Toyota Young Investigator Fel-lowship. LD acknowledges the scholarship under the International Postdoctoral Exchange Fellowship Program by the Office of China Postdoctoral Council (Grant No. 20180072) and FRQNT for the Postdoctoral scholarship (V2, file number: 274384) in Quebec Canada.
O R C I D
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A U T H O R B I O G R A P H I E S
Lei Du is currently a postdoctoral fellow in Prof. Shuhui Sun's group at the Institut National de la Recherche Scien-tifique (Canada). He works as a post-doctoral fellow/lecturer at Harbin Institute of Technology (China). He received his PhD degree in 2017 from Harbin Institute of Technology. He also studied at the Pacific Northwest National Laboratory and Washington State University as a visiting student during his PhD period. His main research interests include chemical conversion in electrocatalysis, mechanism, and devices.
Gaixia Zhang is a research fellow at the Institut National de la Recherche Scien-tifique (INRS), Center for Energy, Mate-rials, and Telecommunications, Canada. She received her PhD degree from École Polytechnique, Université de Montréal, followed by postdoctoral training at the University of Western Ontario. Her current research interests are focused on nanomaterials (eg, graphene, MOF‐derived, and biomass‐based materials) for PEM fuel cells, metal‐ ion and metal‐air batteries, as well as wastewater treatment.
Xianhu Liu is currently an associate professor at the National Engineering Research Center for Advanced Polymer Processing Technology in Zhengzhou University, China. He received his Ph.D. degrees respectively from Zhengzhou Univer-sity, China, and Friedrich‐Alexander‐University Erlangen‐Nuremberg, Germany. His research inter-ests focus on the development of new polymer processing technology, polymer rheology and
processing, advanced polymer composites as well as polymer porous materials.
Amir Hassanpour is a post‐doctoral fellow at the Institut National de la Recherche Scientifique (INRS), Canada. His specialty is engineering of nanoma-terials for use in micro‐photonic devices and composite membranes.
Marc Dubois is a Full Professor at the Université Clermont Auvergne and Insti-tute of Chemistry of Clermont‐Ferrand. His research focuses on inorganic fluorine chemistry applied to (nano)carbons, inor-ganic materials and polymers. These materials fluori-nated either in the bulk or at their surface only are used for energy storage, lubrication, and other applications where fluorine atoms add a functionality (gas barrier, (super) hydrophobicity, low retention,…).
Ana C. Tavares is a Full Professor at the Institut National de la Recherche Scientifique (INRS), Center for Energy, Materials, and Telecommunications, Canada. Her research interests cover the development of electrocatalysts for fuel cells and electrolyzers, as well as functional materials for electrochemical sensors.
Shuhui Sun is a Full Professor at the Institut National de la Recherche Scien-tifique (INRS), Center for Energy, Mate-rials, and Telecommunications, Canada. His research focuses on functional na-nomaterials for energy conversion/storage and envir-onmental applications, such as PEM fuel cells (low‐Pt and Pt‐free catalysts), Li‐/Na‐/Zn‐ion batteries, metal‐ air batteries, solar H2production.
How to cite this article: Du L, Zhang G, Liu X, et al. Biomass‐derived nonprecious metal catalysts for oxygen reduction reaction: The demand‐ oriented engineering of active sites and structures. Carbon Energy. 2020;2:561–581.
