Design of advanced aerogel structures for oxygen reduction reaction electrocatalysis

Oxygen reduction reaction (ORR) is considered the bottleneck reaction in fuel cells. Its sluggish kinetics requires the use of scarce and expensive platinum group metal (PGM) catalysts. Significant efforts have been invested in trying to find a PGM‐free catalyst to replace Pt for this reaction or reduce its loadings. One interesting family of materials that has shown great promise in doing so is aerogels, which are based on covalent frameworks. The aerogels’ high surface area and porosity enable good mass transport and high catalyst utilization that is expected to lower PGM loadings or replacing them completely. This review summarizes recent research in this field, introducing methods of using aerogels as cathodes for ORR, from carbon to metal aerogels. The catalytic sites vary from nanoparticles to atomically dispersed metal ions embedded in carbon aerogels that form all‐in‐one platform which can serve as both the support and the catalyst.


INTRODUCTION
Fuel cells' technologies rely on oxygen reduction reaction (ORR) at the cathode for their operation. The most common fuel cells are proton exchange membrane fuel cells (PEMFC), in which the cathode usually consists of platinum group metal (PGM) nanoparticles (NPs) as catalysts, dispersed on a carbon support. [1][2][3] Unfortunately, the low abundance and the derived high price of Pt is a major cause for the slow market penetration of these technologies. 4 Hence, alternative solutions to reduce or replace Pt are under extensive research. [5][6][7][8][9] The ORR sluggish kinetics is considered one of the major challenges in this field. 10,11 Some of the proposed solutions focus on lowering the PGM content by using Pt alloys or eliminating dimensional networks are typically synthesized via sol-gel chemistry. Owing to these unique properties, aerogels can perform as advanced hierarchical catalyst support platforms, enabling improved mass transport, catalyst loading, and dispersity in fuel cells' electrodes. Carbon aerogels (CAs) have been often used owing to their high electrical conductivity. 29 These aerogels are formed by polymerization and crosslinking to form a wet gel that is later dried under supercritical conditions and heat-treated to result in a hierarchical, porous, high surface area carbon support. 30 In many cases, CAs are made via the resorcinol-formaldehyde (RF) process, which involves the polycondensation of these molecules. 31 Other types of electronically conductive aerogels are metal aerogels that can also perform as self-supported catalysts. 32 This review offers an overview on recent developments and trends in aerogels-supported ORR catalysts, including different aerogel structures, and self-standing aerogels that serve as the support and catalyst in one platform.

Surface area and porosity effect on catalytic activity
The surface area, especially the pore size distribution of catalysts' supports, can have a significant impact on their catalytic activity. [33][34][35] Micropores restrict the access of protons and oxygen, which will impact the catalyst utilization and overall fuel cell performance. Mesopores contribute to electrolyte wetting of the surface area and therefore make it reachable for reactants and active sites, and the macropores decrease mass transport resistance. 36 Therefore, hierarchical porous structure, which includes the optimal pore size distribution, plays an important role in the general performance of the electrode and can significantly influence the cell performance. Therefore, aerogels, which have a porous structure that can be relatively easily tuned, are a good candidate for catalysts' support, more than other traditional carbon supports that do not offer this flexibility. Several methods to control the pore size and surface area were presented in the literature.
The simplest approach to tune the aerogel's pores size is to adjust the reactants and solvent ratio during its synthesis. [37][38][39][40] For example, Wang et al. studied atomically dispersed Fe-based catalysts in RF-modified aerogels. The effect of pore size was correlated to the Fe percent and Fe precursor, and the synthesis condition. The Fe content has affected mostly mesopores pore volume, whereas micropores pore volume remains unchanged. This work suggested that high pore volume, especially mesoporous, is promoted by an increase in Fe content, resulting in improved catalytic activity, due to enhanced accessibility to the catalytic sites. 41 Other approaches have been taken to tune the pores size and create well-defined aerogel structures. For example, porosity induced by templating materials, which is also referred to as hierarchical pore method. 42 Although conducted artificially, this process enables one to create controllable and hierarchical pores. In order to design the pore structure, silica NPs and zinc salts are often used as fillers during the synthesis. They are then etched and evaporated to obtain the desired porosity. 43 Using this approach, polymers such as starch, gelatin, polyethylene, and chitosan are transformed into hierarchical pore aerogels. [44][45][46][47] The aerogel's porosity also affects the distribution of the catalyst. For example, mesoporous structure constricts NPs within the pores of the support, inhibiting their aggregation, which leads to better catalyst utilization and distribution in the substrate. For example, Rafailovich et al. tuned the pores' size in RF aerogels during the synthesis by changing the resorcinol/catalyst ratio and later deposited Pt NPs on the aerogel. They showed that at its optimal pores size, the aerogel reaches an increase in the power density in PEMFC over carbon black support. Among others, this was attributed to the uniform size distribution of small catalyst NPs, which led to high electrochemical surface area, as shown in Figure 1. 48 Another method to increase the total pore volume and surface area was demonstrated using H 2 O 2 treatment of graphene aerogels (GAs). This effect was examined by preforming linear sweep voltammetry and electrochemical impedance spectroscopy tests in alkaline solution. Inplane pore formation allowed reducing the charge transfer resistance (R ct ) and lowering the diffusion resistance (R D ) ( Figure 2). 49 In conclusion, the ability to control the surface area and the pore size distribution can help fine-tune the performance of various aerogel-catalysts systems, and increase the catalyst utilization via improved mass transport, and thus gives the aerogel an advantage over other supports in this regard.

2.2
Structure and catalytic site

Carbon aerogels as catalyst support
The catalyst-support interactions are also known to have significant impact on the performance of fuel cells. 30 The synthesis of aerogels allows the formation of finecontrolled structures, an advantage over other carbons used as catalyst supports. 50 This increased the interest in aerogels as supports for various catalysts, with emphasis on Pt and PtM (M = firstrow transition metal) NPs. 51 For example, Yi et al. studied different Ni-Pt alloys deposited on CAs. They showed that Pt 3 Ni/CA has higher catalytic activity than commercial Pt/C. 52 The Lee group synthesized some interesting well-defined NP structures in aerogels, replacing Pt NPs with other PGM NPs. Their synthesis included adding metal precursors (M n+ ) during the sol-gel process, and then calcinating the aerogel, after a freeze-drying process, under reducing atmosphere. They were able to create well-defined intermetallic Pd 3 Pb NPs in reduced graphene oxide (rGO)-carbon nanotube (CNT) aerogel. Their catalyst showed a small improvement in the activity and better stability when compared to commercial Pt/C and Pd/C due to the well-defined NPs and the porous rGO-CNT structure. 53 This method was also applied to make Ni-MnO PGM-free NPs. 54 The research on combination of transition metals with rare-earth metal oxides, such as cerium, has also been very interesting in the context of aerogels. Co-CeO 2 and CeO 2 -Fe 2 O 3 bimetallic NPs demonstrated enhanced catalytic activity when compared to the single-metal-based NPs. This improvement was attributed to the rich oxygen vacancies in the rare metal oxides (CeO 2 ) that can promote the chemisorption and activation of O 2 . 55,56 Different studies have been focusing on well-dispersed PGM-Free bimetallic NPs, 54,57 for example, CoFe NPs on CA. The metal precursors, cyanometalate, were used both as metal source and as a cross-linker. The particles that were obtained in this synthesis were tightly wrapped by a graphene carbon shell, and that contact can help mitigate particles aggregation and even accelerate electron transfer in the reaction. This type of catalyst exhibits long cycle life and good stability in Zn-air batteries. 57,58

2.2.2
Carbon aerogels containing atomically dispersed PGM-free metal ions CAs have also been suggested as support materials for atomically dispersed metal active sites. To enable the introduction of metal ions to the aerogel framework, coordinating atoms are usually added to the surface of the aerogels. There has been some work on the doping of CAs' surface with nitrogen, oxygen, and sulfur for this purpose. In most cases, this was done using chemical vapor deposition, by exposing the aerogel to a flow of a gas containing the desired atoms, during or after the synthesis or heat-treatment of the aerogel. 16 After this step, the metal, usually a first-row transition metal, was added from a solution containing the metal salt. The salt can vary depending on the metal species (Cu, Fe, Co, etc.), its oxidation state and the counter ions. For example, when introducing iron ions, a commonly used metal in PGM-free catalysts, the use of salts such FeCl 2 , FeCl 3 , Fe(acetate) 2 , and Fe(acetylacetonate) 3 were demonstrated. 41,45,[59][60][61] The structure of RF-aerogels has been modified in different ways to serve both as catalyst and catalysts' support. For example, the metal coordination in a melaminemodified RF-aerogel, iron salt was added to the reaction mixture. 41,59 After critical point draying (CPD) process followed by heat treatment, and additional heat treatment under ammonia, to embed nitrogen atoms in the aerogel surface, which served as a coordinating agents for the metal ions and allowed to the formation of Fe-N x catalytic centers. 41,59 A less conventional way to insert metal species into RF-aerogels is done during the CPD process. Interestingly when introducing cobalt(III) acetylacetonate during the CPD, both atomically dispersed Co and metallic Co NP encapsulated by carbon shells were formed. To remove the metallic species, carbon etching by NaOH was necessary before acidic treatment for NP removal, followed by a reduction process using H 2 . 62 To improve the control over the coordinating heteroatoms' dispersity, heteroatoms have been introduced during the synthesis of the aerogel itself, prior to its drying. For this purpose, nitrogen-and oxygen-rich backbone aerogels, such as gelatin and chitosan, were fabricated. 46,47,63 Other nitrogen-rich organic molecules, such as melamine (Me) and phenanthroline (phen), have been added to the reaction mixture as additives that serve as metal ion coordination sites ( Figure 3). [45][46][47] Another interesting example is the iron-cobalt bimetallic catalytic system that was synthesized by Chen et al. 47 The aerogel structure was based on gelatin as organic support, additional nitrogen source, metal-phenanthroline (M-phen) complexes that serve as catalysts, and SiO 2 NPs as pore templates. Their hypothesis was that there is a trade-off between durability and activity, as iron single atom catalysts offer high ORR activity whereas atomically dispersed cobalt catalysts enables improved durability. When comparing between the mono-metal-to the bimetal-based CAs, they were able to enhance the catalyst performance, which was attributed to synergistic interactions of FeN 3 and CoN 3 centers. This was supported by X-ray absorption spectroscopic measurements.
When focusing on graphene or CNT aerogels, it is possible to avoid heat treatment due to their already high electrical conductivity as is. A mixture of the desired metal salt, the carbon (GO, CNT) and a heteroatom dopant source, has been used to form gels. An example is cobalt and nitrogen co-doped graphene-carbon nanotube aerogel (Co-N-GCA) that was synthesized using cobalt(II) acetate as metal source and urea as nitrogen dopant. 64 A different method of metal ion insertion is used by utilizing the "egg box" structure of sodium alginate and performing ion exchange with metal ions such as Co and Fe. Melamine has been used to better anchor the metal ion and produce MN x sites. This method was demonstrated in a combination with zinc evaporation or by itself. 65,66 In general, CAs have proven to be very useful for binding atomically dispersed metal ions, which can later serve as active sites for ORR. Although several methods have been proposed, it is not clear if one is better than the others.

2.2.3
Carbon aerogels with embedded metal ligands as PGM-free catalytic systems Similar to doping aerogels with coordinating atoms is the addition of certain molecules that can serve as ligands on their surface. This could be done either chemically during the gelation or physically, by adsorption after the aerogel synthesis. To chemically insert ligands in the aerogel's structure, certain molecular ligands can serve as monomers during the synthesis of the aerogel and thus are an integral part of the aerogel. This approach usually allows very good and uniform dispersion of the ligand and thus of the catalytic centers after the addition of metal ions.
Ligands such as bipyridine, porphyrin, and phthalocyanine have been used in different studies conducted by the Elbaz group ( Figure 4A,B). 60,67,68 This strategy is based on adjusting the ligands with the suitable functional groups to enable their polymerization. For example, terminal alkyne bonds were added to bipyridine and phthalocyanine to enable their polymerization by Glaser coupling, 67,68 and terminal amines were used to synthesize porphyrin aerogels by polycondensation with dialdehydes. 30 In these procedures, the metal was inserted at different stages, depending on the polymerization process. For porphyrins (P) and phthalocyanines (Pc), the polymerization was performed after complexation, whereas for bipyridine, the complexation was performed during polymerization. These aerogels were heat-treated to increase their electronic conductivity, but the overall active sites (M-N x ), and their distribution was retained. In these papers, the introduction of ligands as monomers enables high metal loading, reaching very high active site densities of up to 9.7 × 10 20 sites g −1 . 60   A different approach is by anchoring metal ligands to the aerogel platform using axial ligands. In this method, sites such as FeN 5 that mimic heme-based enzyme active sites are created, containing the central iron ion coordinated to a ligand with four nitrogen atoms on the same plane, and an additional axial ligand. Functionalizing the carbon structure with groups that can anchor the metal center in molecular catalysts was demonstrated on CNTs, 69-71 graphene sheets, 72 and aerogels. [73][74][75][76] For example, anchoring Fe-phthalocyanine (FePc) on graphite aerogels was performed by using a surface bound amine-based ligand of pyridine as an axial ligand before adding FePc. The advantage of using functionalized GAs is that there is no need to perform heat treatment; thus the catalytic sites remain unchanged and are very well defined. This is owing to their high electrical conductivity. In these cases, according to the chosen synthetic pathway, the core ligand structure is not covalently attached to the aerogel platform but attached by proxy through the coordination of the metal to a surface-bound ligand ( Figure 4C). 76 From the various approaches presented, atomically dispersed metal sites enable higher metal utilization than NPs. As adding the metal ions to the reaction mixture is simpler, the bottom-up method, which is taken when modifying the aerogels' structure, allows the creation of well-defined catalytic sites that are embedded within the support and relatively easily controlled.

Electrocatalytic ORR activity
The latest studies on aerogels as PGM-free ORR catalysts mainly focus on the electrocatalysis of ORR under alkaline conditions. This is most probably due to the fact that the PGM-free MN 4 ORR catalysts are more active and durable at high pH. 77,78 Most studies show promising results at these conditions. Thus, only few studies were performed with aerogel-based PGM-free ORR catalysts in acidic conditions, some of the results from the most interesting examples are summarized in Table 1.
One example is of an iron phthalocyanine aerogel that was studied in 0.5 M H 2 SO 4 and showed promising activ-ity, with onset potential of 0.9 V versus RHE. 68 Another example is from the work of the Berthon-Fabry group performed in moderate acidic conditions (0.05 M H 2 SO 4 ). They studied the effect of different parameters on the catalytic activity of modified RF-aerogels with incorporated iron and highlighted the role of the nitrogen moieties in the aerogel structure. 41 The importance of nitrogen source was also shown for alkaline conditions. 62,79 As in other ORR catalysts, 80 heat treatment can be used to increase the electrical conductivity and ORR activity of aerogel-based catalysts. 67 The heat-treatment temperature can be optimized to increase activity. 81 But it was also shown that very high temperatures can induce pore collapse and chemical decomposition, reducing the surface area and overall catalytic activity. 41,49 In order to enable the use of the aerogels' advantages, as explained before, and to express them in ORR electrocatalysis, further improvements are required to increase their performance in acidic conditions.

Durability and stability
Durability is one of the most severe issues in ORR electrocatalysis. Carbonaceous catalysts' supports suffer from corrosion during fuel cell operation, which causes significant performance losses. In order to increase the durability, some treatments have been done with aerogels, for example, heat treatments, chemical treatments (HNO 3 , H 2 SO 4 , etc.), or use of carbon-free aerogels. 82 One possible treatment is to protect the carbon with a thin metal oxide coating. A thin layer of coating can, on one hand, keep the main morphology and the electrical properties of carbon underneath, and on the other hand, provide better resistance for corrosion. 83 Doping is also a possible treatment to increase durability. 58,[84][85][86][87] It can change the electronic configuration of the carbon substrate, prevent the aggregation of NPs, and promote synergistic effects that improve the overall performance. Lu et al. synthesized iron NPs on CA doped with both N and P. Their catalyst showed good activity when compared to Pt/C and the same material without any dopants. It exhibited very good stability in Zn-air battery. 88 Accelerated stability and degradation tests (AST or ADT) have been instrumental in deducing aerogel's stability and durability. Some aerogel structures have shown impressive stability under alkaline conditions. One example is aminopyridine GA anchoring iron-phthalocyanines (APAG-FePc). 76 This aerogel showed a decrease in E 1/2 by 16 mV after 6000 continuous cycles, as well as retention of 90% of the initial current density in chronoamperometry, after 4 h, which showed better stability when compared to a commercial Pt/C electrode. The N-doped carbon aerogel mentioned before also showed good stability after 4000 CV cycles, especially for the Co-phenanthroline-based aerogel ( Figure 5). 47 Modified RF-aerogels examined in low acidic environment showed improved stability after performing acid wash treatment. This stage was added to eliminate the inactive and unstable species such as metallic NPs and iron carbides and did not affect the initial catalytic performance ( Figure 6). 59 Even though the use of PGM-free catalysts in acidic fuel cells is highly sought for, their durability needs to be improved in order to work in PEMFCs. 89 One of the limiting factors has been the lack of ability to measure PGM-free catalysts' degradation in fuel cell. A possible solution was recently offered by the use of Fourier transform alternating current technology that allows to monitor the concentration of the active sites in PGMfree ORR catalysts during the operation of fuel cells. 90 Better understanding of the aerogel-based catalytic systems would hopefully allow further improvement in their durability.

METAL AEROGELS AS ORR CATALYSTS
Along the extensive research of improving the durability of the carbon supports, mainly due to corrosion-related issues, there have been many studies that focus on carbon-free supports. 8,[91][92][93][94][95][96][97] Synthesizing metal-based aerogels enables self-supported catalysts and eliminates the use of carbon in the structure while retaining high electrical conductivity. 98 These aerogels also include PGMs, and their high surface area allows better materials' utilization, and thus reducing the PGM loadings. 99 Latest studies on metal-based aerogels focus on bimetallic alloys 100 that reduce catalyst cost even more 101 and improve ORR activity by optimizing the surface binding energies of Pt with adsorbed O-species. 102 An interesting example is Pt-Cu alloy aerogels that were synthesized by a co-reduction of Pt and Cu salts in aqueous solution using NaBH 4 . To control Pt:Cu ratio, different Pt salts at different concentrations were used, H 2 PtCl 6 or K 2 PtCl 6 , to obtain Pt 3 Cu and PtCu, respectively. The massspecific ORR activities measured by RRDE in 0.1 M HClO 4 electrolyte were 1.5-2-folds higher than the 30% Pt/C catalyst reference. Acid wash performed in order to mimic the conditions in PEMFC operation caused Cu leaching and decreased the mass-specific activity by 10% and 25% for Pt 3 Cu and PtCu, respectively (Figure 7). 103 In a different work, nitrogen modification of PtCu aerogel surface using N-methyl pyrrolidone was done in order to improve the ORR activity and stability, by improving oxygenated species adsorption on the aerogel surface, showing onset and half-wave potentials of 1.01 and 0.93 V versus RHE, respectively. Accelerated stress tests (AST) showed a decrease of only 11 mV after 30 000 CV cycles. 104 Another example is of Pt 3 Ni aerogels that were synthesized by Henning et al., in a similar manner to Pt-Cu aerogels. 101 In this work, the characterization was performed in PEMFCs. The mass transport was improved by increasing porosity of the catalytic layer, using K 2 CO 3 as filler in the aerogel ink. Additional washing step enabled K 2 CO 3 to dissolve in acidic conditions and produce CO 2 that further increased the porosity and overall performance. 105 The aerogel's durability improved in startstop degradation study due to the absence of carbon, whereas load-cycle degradation experiments showed inferior stability due to Ni leaching. 99 Figure 8 shows TEM images of Pt 3 Ni aerogel and Pt/C before and after ASTs. For start-stop degradation, small change in nanochain structure and porosity decrease due to carbon corrosion were observed ( Figure 8B,E). In load-cycles degradation, chain and particle increase were observed in Pt 3 Ni aerogel and Pt/C, respectively ( Figure 8C,F). 99 Another interesting example is AuCu aerogels. Different ratios of Au:Cu were produced by changing the concentration of the precursors during synthesis. Au 52 Cu 48 aerogel showed the best performance, with onset and half wave potentials of 1.00 and 0.87 V versus RHE, respectively, in 0.1 M KOH. 106 Salting-out gelation method of metal NP solutions allowed to extend the variation of bi-and tri-metallic noble metal aerogels. 107 Au-Ag, Au-Rh, Au-Pd, Au-Pt, Au-Pd-Pt aerogels were synthesized by the Eychmüller group and studied for ORR catalysis, in both acidic and basic aqueous solutions. In both media, Pt-Au showed superior performance with low peroxide yield (1%-4%), onset and half-wave potentials (respectively) of 1.04 and 0.91 V versus RHE in KOH; and 1.07 and 0.86 V versus RHE in HClO 4 (Figure 9). These samples were also compared to the corresponding noble metal NPs that showed inferior performance and emphasize the advantage of the aerogel porous structure enabling active site accessibility and improved mass and electron transfer. 108 Although impressive ORR activity is reached using metal aerogels, there is still room for improvement in durability and metal utilization. Increasing the surface area may offer a solution.

SUMMARY AND CONCLUSIONS
Aerogels serve as a platform for ORR cathodes that allow price reduction of fuel cells systems due to their unique high surface area, porous structure. These properties improve metal utilization by high active site distribution and accessibility, in addition to preventing aggregation. They also promote mass transport and reduce charge transfer and diffusion resistance. Introduction of the catalytic sites in different manner was discussed, some in the form of traditional NPs, atomically dispersed metal ions, covalently bound complexes, and metal aerogels. Major differences were shown between carbon and metal aerogels, emphasizing that improvements are required in both fields. CAs can eliminate the use of PGMs completely and show good performance and durability in alkaline conditions, whereas metal aerogels have significantly lower surface area but enable high durability and activity in acidic conditions due to high catalyst utilization and durability.

A C K N O W L E D G M E N T S
This work was conducted as part of the Israeli Fuel Cells Consortium and supported by the Israeli Ministry of Energy (grant number: 221-11-058 and 219-11-132), the Israeli Science Foundation (grant number: 238/21), and The Israeli Ministry of Science and Technology (grant number: 3-16020).

C O N F L I C T O F I N T E R E S T
The authors declare no conflicts of interest.