Recent progress of carbon dots and carbon nanotubes applied in oxygen reduction reaction of fuel cell for transportation

Applied Energy 257 (2020) 114027

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Recent progress of carbon dots and carbon nanotubes applied in oxygen reduction reaction of fuel cell for transportation ⁎

Mohamedazeem M. Mohideena, Yong Liua, , Seeram Ramakrishnab, a b

T

⁎

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore 1157, Singapore

H I GH L IG H T S

overview of the state-of-the-art in oxygen reduction reaction is presented. • An discussion of carbon dots and their practical applications in fuel cell. • In-depth • An analysis of recent progress in carbon nanotubes applied on a fuel cell.

A R T I C LE I N FO

A B S T R A C T

Keywords: Fuel cell Oxygen reduction reaction Electrocatalyst Carbon dots Carbon nanotubes Transportation

Proton exchange membrane fuel cell (PEMFC) is receiving strong attention in sustainable energy conversion and storage systems. Its high efficiency, low emission, and environmental friendliness enable hydrogen fuel cell electric vehicle (FCEV) for day to day transportation. High costs, limited durability and inefficient oxygen reduction reaction (ORR) have been identified as key challenges for the widespread commercial success of fuel cell transportation vehicles. ORR is constrained by slow kinetics of cathode catalyst and must be improved by suitable nanoengineering of low-cost electrocatalysts. Replacing platinum group metals with carbon-based nonprecious metal is a promising solution. Recently, advances in zero-dimensional carbon dots are showing promise for challenging energy-oriented issues. In addition, the one-dimensional carbon nanotubes are also found suitable for efficient ORR. However, carbon-based electrocatalyst showing improved performance in electrochemical measurements are not up to the expectation in the practical application of FCEV. This review highlights the advancement and future opportunities for improving performance of ORR using carbon dots and carbon nanotubes nanostructures. We discussed the application of carbon dots and carbon nanotubes in the fuel cell. Their corresponding electrochemical performance was presented. Followed by, practical application of carbon nanotubes in the real operating condition of PEMFC was discussed. Finally, the current status and future target of FCEV were comprehensively addressed.

1. Introduction 1.1. General outline to PEMFC and its application to energy conversion In the modernized world, the economy, transportation and day to day life, all originate within the circumstance of energy. Since the population increasing, the expenditure of available energy confronts problems globally, and also, emission of greenhouse gases polluted the environment [1,2]. So to bridge a gap between the increase in population and demand for energy, a renewable energy source such as biomass, solar cells, fuel cells are a neater solution [3]. From the past decade, the proton exchange membrane fuel cell (PEMFC) has attracted

⁎

widespread interest in both academia and industries. The fuel cell electric vehicle (FCEV) has tremendous attention in automobile industries due to its high efficiency of about 60% in energy conversion and more than 90% pollution-free [4]. PEMFC consists of anode, cathode, and proton exchange membrane as main components, where hydrogen is fed as fuel at anode and oxygen is oxidized at the cathode layer and produces water as a by-product [5]. Among various classes of fuel cells, PEMFC has attracted attention in a sustainable energy system due to its high efficiency of 60% in energy conversion, more than 80% of generating electrical and thermal energies and almost zero pollution [6]. Also, low-temperature operation, easy and fast start-up and high power density are emerged PEMFC as

Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (S. Ramakrishna).

https://doi.org/10.1016/j.apenergy.2019.114027 Received 13 May 2019; Received in revised form 8 October 2019; Accepted 14 October 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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and passing its electron to the cathode via an external circuit to produce electricity and supplying oxygen from the air at the cathode and combining with the hydrogen ions passing from PEM to produce H2O as a by-product. The generated electricity power the motor for smooth acceleration, making vehicles to run [12].

potential clean energy conversion device and also offers as promising commercial viability [7]. The application of PEMFC is focused on portable, stationary and transportation devices. Notably, in a portable device, PEMFC is applied in space applications mainly in missions such as Gemini, Apollo, space shuttle and lunar vehicles. Recently, the paramount benefits of PEMFC attracted NASA to replace already existing alkaline fuel cells with PEMFC [8]. Whereas in stationary applications, as already mentioned above, its cogeneration of electrical and thermal energies has high energy efficiency for small scale residential to large scale industrial purposes [9]. So in the areas discussed above, PEMFC has a promising application, beyond that it majorly focused on transportation for domestic purposes using hydrogen as energy carriers. PEMFC vehicles have many advantages than compared to other electric vehicles which made FCEV more flourishing and viability as follow [10]:

1.2. PEMFC- barriers and solution As an introduction to PEMFC given in the above section, there is no doubt from that, PEMFC is a promising energy conversion power source, but still, it suffers from some barriers which hindrance its commercial availability in a wide area for day to day purpose as like battery vehicles [13]. In PEMFC, the membrane electrode assembly (MEA) is an important part that includes anode and cathode catalyst layer at both ends. In these catalyst layers, a hydrogen oxidation reaction occurs at the anode layer and oxygen reduction reaction accelerates in the cathode catalyst layer. According to the US Department of Energy (DOE), nearly 56% of the total cost of the fuel cell was due to the precious platinum metal utilization in both catalyst layers [14]. The utilization of platinum in cathode and anode layers were in 1:10 ratios, but still, sluggish kinetics of ORR occurs, which was almost six times slower than the kinetics of hydrogen oxidation reaction. The ORR is the most prominent reaction which occurs during the discharging process in PEMFC. In the cathode layer, oxygen molecules adsorb or diffuse at the surface of electrocatalyst with strong O]O bond energy of 498 kJ mol−1 [15]. Since, the electron transport from anode to cathode reduces oxygen molecule by splitting O]O bonds and different reactions pathway with various oxygen-containing intermediates such as OOH*, O*, and OOH* in aqueous solutions. Generally, the ORR reaction has two different pathways, as shown in Eqs. (1) and (2): (i) fourelectron pathway and (ii) two-electron pathway. The kinetics of ORR in four-electron pathway represent direct reaction to forming water (H2O) as by-product at a constant electrode potential of E0 = 1.23 V. The twoelectron pathway represent indirect reaction to forming hydrogen peroxide (H2O2) as by-product at an electrode potential of E0 = 0.682 V. The indirect reaction can be further reduced, when H2O2 is adsorbed to form H2O, it depends upon the catalyst material as shown

• Reduction or zero-emission of greenhouse gases, • High well-to-wheel efficiency, • Fast refueling in 3–5 min (whereas in electric vehicles it took 4–6 h to recharge), • Can travel 350 miles per refuel, • Environment-friendly by the less noisy and quick start-up. From all these potential capabilities of FCEV, it was widespread in the public sector such as a car, buses, scooters, high-speed trains, and aircraft. Initially, the manufacturing of FCEV started in 2004, but their commercial availability was very extremely questionable in past decades due to some barriers which include economic status and performance. Presently, there are various leading automobile companies involved in mainstream FCEV for domestic transportation by overcoming the hindrance through R&D [11]. The schematic illustration of hydrogen FCEV is shown in Fig. 1. The heart of FCEV is the fuel cell stack, which holds a series of single PEMFC and each consists of multiple components. Hydrogen is an alternative to gasoline which is environmental friendly element create from a wide range of sources. In FCEV, hydrogen, and oxygen combine to produce electricity without burning hydrogen. In fuel cell stack, by supplying hydrogen as fuel at the anode

Fig. 1. Schematic illustration of PEMFC vehicle. 2

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2. Carbon dots-0 dimensional

in Eq. (3). Among this reaction, four-electron pathway is the most favorable reaction, but its reaction mechanism is slow due to a strong O] O bond [16,17]. Direct 4e- pathway O2 + 4H+ + 4e− → 2H2O E0 = 1.23 V

From a nanomaterials point of view, 0-dimensional materials are commonly called quantum dots [24]. In 2004, the new class of quantum dots was accidentally discovered during the purification of CNT and became a member of the carbon material group namely, carbon quantum dots (CDs) [25]. CDs are quasi-spherical nanoparticles composed of sp3 hybridized carbon and oxygen/nitrogen groups in it. Recently, the zero-dimensional carbon dots with size ranging between 1 and 10 nm have received tremendous attention due to its promising characteristics and feasible synthesis methods which popular its application into the various field such as bioimaging [26], drug delivery [27], sensors [28], photocatalysis [29], environment sensing [30] and so on. This broad application of CDs mainly relies on their inherent characteristics such as excellent optical properties, low toxicity, secure synthesis methods [31]. Now at present, CDs widespread application in sustainable energy conversion and storage devices and advancing swiftly as a new promising method to overcome the hindrance with energy at low cost and in an environment-friendly manner [32]. From the literature analysis, the increase in the number of publications related to CDs mainly due to its unique properties and its feasible synthesis approach. To accomplish well-controlled size, geometry, and the dimensionality of CDs, synthesizing methods play a vital part in it [33]. Top-down and bottom-up are the two traditional strategies to fabricate 0-dimensional nanomaterials. From Fig. 2 SEM and TEM morphology of different 0-dimensional nanomaterials are shown [34]. Recently, CDs have been used in energy-oriented applications especially as electrocatalyst for ORR in fuel cells. From the literature, several research articles being published mainly in the utilization of CDs in photocatalysts [26,35]. In summary, in electrical energy conversion devices especially in a fuel cell, there is a limited number of research works carried on CDs for electrocatalytic ORR and herein we described a small portion of it in this review article. Feasible synthesizing methods are one of the significant advantages of the rapid development of CDs in a short time. Our review exhibits current approaches in the synthesis of CDs based composite and their mechanism in electrochemical studies of ORR and its application in energy conversion fuel cells by examining their potential possibilities. Generally, the synthesis or fabrication of CDs was approached by top-down or bottom-up methods. The top-down methods are breaking of bulk carbonaceous materials into 0-dimensional material by chemical, physical and electrochemical methods [36]. Whereas, the bottom-up method was in contrast to the top-down. Heteroatoms doped CDs exhibits superior electrocatalyst activity or in other words, heteroatoms doped CDs, especially in N-CDs, have a large number of defects on their surface and edges which leads to the availability of more catalytic sites. From literature analysis the short and crispy mechanism of heteroatoms doped CDs were already discussed by other research groups [37,38] so that some general synthesizing methods of doped CDs shown in the flowchart below for a quick overview, as shown in Scheme 1.

(1)

-

Indirect 2e pathway O2 + 2H+ + 2e− → H2O2 E0 = 0.682 V

(2)

-

The reduction reaction of 2e pathway H2O2 + 2H+ + 2e− → H2O E0 = 1.77 V

(3)

From the above introduction to ORR, we can conclude that the whole barrier is in lies with the electrocatalysts in the cathode layer. Although the reduction in platinum loading can decrease the cost per kW of PEMFC, but their electrochemical performance in MEA was questionable. Therefore, some alternative methods needed to reduce platinum at the same time without affecting the stability, durability and ORR performance of PEMFC [18]. Presently, appropriate electrocatalysts can be prepared in the following ways:

• Alloying Pt with other transition metal to form a hybrid nanostructure • Replacing precious metal by introducing non-precious metal • Metal-free carbon-based electrocatalyst The non-precious metal catalyst consists of transition metal (Fe, Co or Mn) and heteroatoms (mostly nitrogen) co-doped carbon (M-N-C) which presents outstanding results, yet their activity and stability need to be improved substantially to make them viable. M-N-C for energy storage and conversion devices efficiently increase their performance, especially in FCEV, at 0.0625 g m−1 loading of PGM free catalyst achieved 1 W cm−2 power density [19,20]. Carbon materials were extensively studied as electrode material for their prominent physical and chemical properties. Among the carbonaceous materials, carbon nanotubes (CNT) was already existing and still utilizing carbon support due to its remarkable defect sites present at the edges, end and side walls of CNTs, results in high electrocatalytic activity [21]. Recently, a new class of carbon material, carbon dots (CDs) have promising features due to its superior electron transfer ability which favors enhanced catalytic activity for ORR and even for oxygen evaluation reaction (OER). Moreover, the surface of CDs contains eOH, eCOOH and eNH2 groups which stimulate more active sites on heteroatoms doping [22,23]. The main aim and objective of this review to address CDs and CNT as supporting electrocatalyst material for fuel cells. The structure of this review is presented as follow:

• In Section 2, we described the advantages of CDs and their pro•

• •

mising characteristics of sustainable energy conversion and storage devices. In Section 2.1, the application of CDs in fuel cells and their performance towards ORR in the rotating disk electrode (RDE) were reviewed. In Section 3, a brief review of CNT and their physical and chemical properties are discussed and Section 3.1 presents the application of CNT on ORR of PEMFC from electrochemical measurements. Followed by Sections 3.1.1, 3.1.2, and 3.1.3 discussed the influence of CNT support with platinum, platinum alloy, non-precious metal, metal oxides, conductive polymers, and N-doped CNT and their RDE performance towards ORR. In Section 4, we introduce the performance of electrocatalyst in both RDE and the real operating condition of PEMFC In Section 5, the current and future status of FEVC from industrial point view was addressed in a crispy manner.

2.1. Application of CDs on ORR of fuel cell In fuel cells or any other energy-related devices, ORR plays a significant role. From recent research, it is noteworthy that, metal-free electrocatalyst showed enhanced electrocatalytic activities than Pt/C electrocatalyst. With the support of various dimensional carbonaceous materials, especially with graphene, the performance of ORR has been significantly improved by showing enhanced power density. Besides the good result possessed by graphene, there are some disadvantages mainly from their synthesis routines such as aggregation, non-uniform dispersions and low content of doped atoms, which decreases the energy efficiency of the electrocatalyst. These drawbacks arise as a peak point for researchers to develop a green and environmentally friendly catalyst with the 0-dimensional carbon dots. By their fast electron 3

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Fig. 2. Typical scanning electron microscope (SEM) and transmission electron microscope (TEM) images of different types of 0 D NSMs, synthesized by several research groups. (A) Quantum dots, (B) nanoparticles arrays, (C) core–shell nanoparticles, (D) hollow cubes, and (E) nanospheres [34].

Scheme 1. General synthesizing methods of heteroatoms doped carbon dots. 4

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Fig. 3. Schematic illustration the one-pot synthesis of nitrogen-doped carbon dots decorated graphene oxide hybrid [40].

promotes a more active site to improve the electrochemical system [42]. Concerning the line, Zhou et al. handled new ideas to fabricate high efficiency and low-cost electrocatalyst for ORR via a two-step synthesis method. CD-M/rGO prepared by a microbial reduction process followed by a simple hydrothermal method. In briefly, M-rGO mixture was prepared by microbial reduction method. The novel approach to this study was the formation of CDs from Shewanella oneidensis bacterial cell, which was composed of N, S and P sources. During hydrothermal treatment, the bacterial cells undergo degradation and changed its rod liked structure into small particles like pellets along with partially polymerization and carbonization to form CDs with a size range between 2 and 6 nm. The as-prepared CDs were self-formed and self-decorated over M-rGO, and their corresponding TEM image is shown in Fig. 5(a). The schematic illustration of the synthesis method was shown in Fig. 5(b and c). The electrochemical method demonstrated that CD-M/rGO has excellent electrocatalyst activity towards ORR in an alkaline environment. The LSV measurement on RDE revealed that Eonset and E1/2 of CD-M/rGO were considerably positive than C-rGO and M-rGO and comparable to Pt/C. At 800 to 2400 rpm the electron transfer was between 3.7 and 4.2 and led to a four-electron pathway. Moreover, during the accelerated degradation test over 10,000 cycles, CD-M/rGO showed 3% loss in current density whereas 15% loss in Pt/C. The more favorable result of CD-M/rGO strongly suggested to higher stability [43]. In continuous to the previous report, co-doping of nitrogen and sulfur in carbon dots have graphitic N and eCeSeCe on the surface of the particles which was highly active and their synergistic effects with carbon dots also improve the ORR performance exponentially. So with respect to this context, more recently, Zhang et al., prepared heteroatoms doped CDs anchored on reduced graphene oxide (rGO) (N-CDs/ rGO, S-CDs/rGO, and N, S-CDs/rGO) as remarkable electrocatalyst for ORR via simple hydrothermal method as shown in Fig. 6 (a). Their corresponding SEM and TEM images were shown in Fig. 6(b). Herein, Ethylenediamine and α-lipoic acid used as nitrogen and sulfur sources. The CDs:rGO molar ratio plays an essential part in improving catalytic sites. The molar ratio (1:1) was maintained throughout the experiment to avoid electroconductivity of the composites and to increase catalytic sites. The ORR performance of all samples investigated through LSV on RDE revealed that N, S-CD/rGO exhibited superior ORR activity by having higher kinetic current density and positive E1/2 than directly doped N, S-rGO. This improvement observed due to the synergistic effect of both N and S doped with CDs, which presents more active sites on edges of catalyst rather than on the plane, which bridges to access more oxygen molecules [44].

transfer and high surface area, CDs possessed excellent behavior in their electrochemical applications. Moreover, having eCOOH, eOH, eNH2 functional groups on their surface promotes enhanced multicomponent electrocatalyst [39]. With this inspiration, Niu et al. prepared nitrogendoped CDs decorated graphene oxide hybrid catalyst for the first time via a simple one-step synthesis method including in situ formation of polymer-like intermediate and carbonized to decorate N-CDs over GO as efficient low-cost metal-free electrocatalyst towards ORR. Fig. 3 showed a schematic illustration of the preparation of N-CDs/GO. The resulting N-CDs/GO hybrid catalyst exhibited improved electrical conductivity and a current density of about 1.4 times, respectively than those of commercial Pt/C [40]. From the outstanding electrochemical performance of N-CDs/GO, we can able to visualize that there may be an excellent interaction between N-CDs and GO which encourage the intermolecular electron transfer to favor high electrocatalytic activity. The route to synthesis heteroatoms doped porous material through carbonization or in other words deriving 3D porous material from less dimensional materials via high temperature carbonization is very mature. Recently, Chen et al. designed and constructed 3D nitrogen-doped porous carbon framework (NCF) from CDs and melamine by a simple one-step self assemble method, as shown in Fig. 4(a). Herein, CDs and melamine carbonized at various high temperatures (700, 800, 900 °C) and their resultant catalyst obtained at NCF-800 display high power density. On pyrolysis at high temperature, two significant reactions were occurred to bridge NCF: (i) During carbonization, CDs were selfassembled over a surface of randomly interconnected carbon sheets (ii) melamine starts to decompose, as a result, a vast amount of gases was release under high pressure, which leads the pathway to construct NCF. Fig. 4(b and c) shows SEM image and (d, e) shows TEM and HRTEM images of 3D-NCF and their design and formation of thin carbon sheets. The ORR measurement of prepared catalyst examined through electrochemical and linear sweep voltammetry (LSV) tests on RDE, the onset potential Eonset, half-wave potential E1/2, and current density of NCF-800 (0.87 V, 0.74 V, and 16.1 mA cm2) was almost closer to commercial 20% Pt/C (0.93 V, 0.81 V, and 21.73 16.1 mA cm2), meanwhile the average electron transfer of NCF-800 was 3.92 and favored a fourelectron pathway on reduction of oxygen. Furthermore, the cyclic voltammetry (CV) test of NCF-800 revealed better stability, durability and methanol tolerance than those of Pt/C [41]. Generally, the mono-doping of nitrogen in carbonaceous materials breakthrough the electroneutrality of the material, thereby adsorption of O2 molecules and conversion of reactive species occurs. Apart from that, co-doping of heteroatoms such as nitrogen, sulfur and phosphorous applied in carbon dots has also recognized to enhance the performance of energy-oriented devices. Moreover, it can alter the physical and chemical properties of the synthesized electrocatalyst and 5

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Fig. 4. (A) Schematic illustration of the synthesis of NCF; (B, C) SEM image of NCF-800; (D) TEM image and (E) HRTEM image of NCF-800 [41].

3. Carbon nanotubes – 1 dimensional

unique characteristics which overwhelm conventional carbon black such as (i) high surface area (1315 m2 g−1), (ii) less impurity, (iii) high electrical conductivity, (iv) free from deep crack, and (v) high catalytic activity due to excellent interaction between catalytic metals and CNT supports [54-56]. In earlier decades, exploring a proper synthesizing method for energy conversion and storage devices is highly challengeable. Generally, CNTs fabricated via two methods: (i) physical method (arc discharge and laser ablation) and (ii) Chemical methods (Chemical vapor deposition and aerosol synthesis) [57]. At present, CVD was the most utilized method for the production of CNT due to its low cost, low temperature and large scale production [58]. In the chemical vapor deposition method, decomposing hydrocarbon gas at 600–1200 °C can act as carbon precursor source which passes over a surface of the metal nanoparticles (Fe, Co or Ni) and these metal particles initiate nucleation site for CNT growth [59]. Generally, methane [60], ethane [61], ethylene [62], acetylene [63], and benzene [64] are commonly used carbon source for the production of CNT via CVD method. They are specific parameters have to maintain, which plays critical aspects in the growth of CNT are atmosphere, pressure, carbon source composition, growth temperature, catalyst concentration and feed rate [65]. Since more review articles discussed the growth mechanism of CNT via CVD methods, for further reference to readers, refer articles [66–68]. Besides, CNT is known to be one of the most durable carbon material due to its tubular structure and high aspect ratio. Table 1 shows some excellent properties of CNT, which leads to widespread applications in the biological field, ceramic field, energy storage and as a catalyst supporting material.

In the development of the global economy, green and environmentally friendly materials are essential to enhance the energy efficiency of sustainable systems. From the past decade to present much progress have made in energy conversion and storage devices to improve the energy power density, stability and durability mainly in a fuel cell, choosing appropriate materials for electrocatalyst is an important aspect. In the carbonaceous group, CNT is a promising electrode material for fuel cell and other energy-oriented devices (supercapacitor, solar cell, batteries) [45–47]. CNT envisioned as a seamless cylinder rolled up with one atom thickness sheet known as graphene. Depending upon a number of an array of graphene sheet CNT are mainly classified into two types as shown in Fig. 7(a) (i) single-wall carbon nanotubes (SWCNT) and (ii) multiwall carbon nanotubes [48]. SWCNT is a single sheet of rolled-up cylinder with honeycomb lattice of the graphene having diameter ranging between 0.4 and 2 nm whereas, MWCNT consists of two or more layer of graphene sheets with interplanar spacing of about 0.34 nm, diameter ranging between 5 and 20 nm even exceed 100 nm and both of their lengths range from 100 nm to several centimeters [49]. According to the chirality of SWCNT, they further classified into a zigzag, armchair and chiral [50] as shown in Fig. 7(b). Hence, depending upon their chirality, SWCNT can be metallic or semiconductor in nature. For supporting material, there are specific requirements needed to enhance the catalyst activity and to overcome some bottleneck faced in conventional carbon black support [51–53]. Moreover, the electrochemical performance of energy conversion fuel cell catalyst is largely depending upon the properties of materials. Fortunately, CNT has 6

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Fig. 5. (A) TEM image of the CDs/M-rGO at low magnification, (B) HRTEM image of the CDs/M-rGO at high magnification, (C) Schematic procedure for the preparation of graphene-based materials assisted by microbes, (D) Reactions and operations involved in the process of fabricating CDs/M-rGO and the process of fabricating C-rGO [43].

7

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Fig. 6. (A) Schematic illustration of the preparation procedures of N,S-CD/rGO composites and N,S-rGO controls, (B) TEM of N,S-CD/rGO and (C) SEM images of N,SCD/rGO [44].

Fig. 7. (A) Schematic structures of SWCNT and MWCNT typical dimensions of length, diameter, and separation distance between two graphene layers in MWCNTs are shown, (B) Schematic representation of the relation between nanotubes and graphene. Ch = na1 + ma2 is a graphene 2D lattice vector, where a1 and a2 are unit vectors. Integers n and m uniquely define the tube diameter, chirality, and metal versus semiconducting nature [50].

showed a maximum power generation density of 595 mW cm−2 [51]. Besides, the degradation of electrocatalyst, another major hindering is decreased in the durability of PEMFC. The Presence of surface oxides on carbon support leads to electrochemical oxidation. The electrochemical oxidation occurs mainly on the inability of carbon support as mentioned below: high water content, high temperature (50–90 °C), low pH (0.9 V), high oxygen concentration and H2 starvation [69–71]. Hence, CNT has highly conductive and high-temperature operation stability to

3.1. Application of CNT on ORR of PEMFC 3.1.1. Effect of CNT-precious and alloy metal catalysts towards ORR CNT is widely utilizing carbon material for energy conversion and storage systems. In fuel cells, CNT as support decreases Pt content and increases the power density of electrocatalyst towards ORR. At low Pt loading of 0.04 mg cm−2, Pt/CNT prepared within carbon paper by sputtered deposition of Pt nanodots on the surface of the CNT layer 8

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Table 1 Some extraordinary properties of carbon nanotubes [68]. Properties Performance Mechanical Properties

1. 2. 3. 4.

Young’s modulus of SWCNTs ~ 1 TPa Young’s modulus of MWCNTs ~ 1–1.2 TPa Tensile strength of SWCNTs ropes ~ 60 GPa Tensile strength of MWCNTs ~ 0.15 TPa

Thermal properties at room temperature

1. Thermal conductivity of SWCNT ~ 1750–5800 W/mK 2. Thermal conductivity of MWCNT > 3000 W/mK

Electronic properties

1. SWCNT bandgap When n-m is divisible by 3 (0 eV, metallic) When n-m is not divisible by 3 (0.4–2 eV, semiconducting) 2. MWCNTs band gap ~ 0 eV (non-semiconducting)

Electrical properties

Typical resistivity of SWCNT and MWCNTs = 10-6 Ωm Typical maximum current density = 107 – 109 Acm−2 Typical quantized conductance (measured) = 12.9 kΩ−1

particle size of 3.1 nm, as showed in Fig. 8. Moreover, the synthesized Pt/PDDA-CNT catalyst is fabricated as bucky paper catalyst layer by a simple vacuum filtration method. A gradient structure of bucky paper catalyst layer confirmed high platinum utilization up to 90% and improved catalyst durability. From CV measurements, the electrochemical surface area of bucky paper catalyst layer of Pt/PDDA-CNT showed nearly 92.53% of Pt/PDDA-CNT electrocatalyst. Additionally, a single cell test of bucky paper catalyst layer of Pt/PDDA-CNT exhibited the best fuel cell performance with a maximum power density of 520.7 mW cm−2 [85]. To improve the catalytic activity of ORR and at the same time to reduce the Pt loading, Pt alloy with transition metals plays a significant role. Herein, we discuss CNT support Pt metal alloys reported by various researchers with improved performance. Recently, to increase the performance of the catalytic activity and to improve the corrosion resistance, the octahedral PtNi catalyst has been developed [86]. CNT supported Octahedral PtNi nanoparticles prepared via the surfactantassisted solvothermal method. RDE and accelerated degradation test (ADT) measurements of octahedral PtNi/CNT revealed 5.5 and 8.5 times superior mass activity and specific activity with improved stability than Pt/C. Also, the membrane electrode assembly fabricated with octahedral PtNi/CNT exhibited maximum power density and cell voltage at 600 mA cm−2. Moreover, the precursors utilized for energy conversion electrocatalyst also has a great impact on ORR. CNT supported Pt/Au synthesized through microemulsion method by varying concentration of Pt: Au (10:10, 15:05, 20:00, 7.5:12.5, 0.5:15, 00.20)/ MWCNT ratios. Among the different compositions, Pt: Au (10:10, 15:05)/MWCNT showed an increased electrochemical surface area than others. Thus improved performance of composite Pt: Au (10:10, 15:05)/ MWCNT was due to the presence of high Au concentration, which results in decreased Pt-OH formation. Moreover, the RDE measurement carried out in the presence and absence of methanol. Due to methanol oxidation, Pt: Au(10:10)/MWCNT exhibited excellent electrocatalyst activity in both the measurement at 0.7 vs RHE and has degradation loss of only 10% in accelerated degradation test [87]. Likewise, PtCo supported by carboxylate-functionalized MWCNT fabricated by the one-step ultrasonic method [88]. From the CV curve, it was observed that on O2 saturated PtCO/MWCNT has higher catalytic activity towards ORR. The Eonset of PtCo/MWCNT has positively shifted almost around 40 mV, likewise E1/2 also positively shifted about 0.73 V, which is comparably higher than Pt/C. The attribution to these results was different band configuration between the Pt, and Co alloy leads to activity towards ORR. Moreover, CNT wrapped with conducting polymers supported on Pt metal alloy catalyst also another way to reduce the cost of Pt loading and to improve ORR activity. Recently, PtCo alloy supported by polyaniline (PANI) wrapped CNT catalyst synthesized for PEMFC [89]. CNT

improve the durability of PEMFC. For instance, Shoa et al. reported that the degradation of Pt/CNT is 1.9 times lesser than that of Pt/C [72]. To enhance the performance of PEMFC, Pt nanoparticles with tiny particle size and uniform dispersion on supporting material are highly essential. Hence synthesizing methods are an essential aspect to improve catalytic performance [73]. Generally, Pt support carbon catalyst was prepared via chemical reduction method using NaBH4 or ethylene glycol as a reducing agent to attain uniform dispersion of nanoparticles over support. Recently, Pt nanoparticle on CNT was successfully prepared by a modified chemical reduction method using NaBH4 and ethylene glycol through microwave-assisted as well as conventional heating routes [74]. The homogeneous dispersion and less agglomeration of Pt particles are recorded in Pt/CNT-MW catalyst with an average particle size of 2.7 nm which was comparably smaller than Pt/CNT-CH (3.8 nm) catalyst. The electrochemically active surface obtained by Pt/ CNT-MW and Pt/CNT-CH were 41.4 m2 g−1 and 59.7 m2 g−1. This improved performance was attributed due to fast and uniform heating of reaction mixture and growth of metal nuclei by the microwave-assisted method. Since pristine CNT is chemically inert, therefore CNT is needed to be functionalized for uniform dispersion of metal nanoparticles on the surface of CNTs. By the addition of functional groups and defects, the surface morphology of CNT can effectively functionalize. Generally, CNT are functionalized via two methods: (i) Covalent functionalization and (ii) non-covalent functionalization [75-77]. In covalent method, CNT are functionalized using polyvinylpyrrolidone (PVP) [78], poly (sodium 4-styrenesulphonate) [79], poly (diallyldimethylammoniumchloride) [80], whereas in non-covalent method, 1-pyrenemethylamine [81], 1-aminopyrene [82] and so on were reported by various research groups. Zhang et al. reported a comparative study on poly (allylamine hydrochloride) functionalized CNT (PAH-CNT) and acid-treated (COOH) CNTs (COOH-CNT) as effective electrocatalyst for ORR. The electrochemical studies revealed that Pt/PAH-CNT exhibited high electrochemical surface area (71.0 m2 g−1Pt) and ORR activity (32.4 mA g−1) which is 27% larger and 1.85 times higher than that of Pt/COOH-CNTs. Hence, the Presence of PAH, CNTs and Pt nanoparticles interact with each other to form triple-phase boundary nanostructures which were found to be an added advantage to enhanced electrochemical stability [83]. Likewise, recently in-situ deposited Pt nanoparticles on PDDA functionalized MWCNT by the polyfunctionalized method were reported. The resultant catalyst displayed high density and the corresponding surface functional group on MWCNT without any structural damage [84]. Similarly, Pt/PDDA-CNT catalyst was prepared by the ethylene glycol reduction method with Pt loading of 40%. Pt/PDDACNTs exhibited enhanced electrocatalytic activity and electrochemical stability due to good dispersion of Pt nanoparticles with an average 9

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Fig. 8. CV (50 mV s−1, N2-saturated 0.1 M HClO4) and LSV (5 mV s−1, 1600 rpm, O2-saturated 0.1 M HClO4) curves of Pt/PDDA-CNTs catalysts before and after ADT, BPCL-PDDA before and after ADT and single cell performance of MEA-PDDA [85].

promoted mainly because of crystalline spinel structure of Co3O4, which facilitates electron between Co2+ and Co3+. Also, RDE measurement revealed that Co3O4/CNT exhibited improved definite Eonset and E1/2 along with 3.1 times higher kinetic current density than pristine CNT and almost similar to 20 wt% Pt/C. Moreover, the specific surface area for Co3O4/CNT was 373 m2 g−1 which is 3 times higher than nitrogen-doped supports and its average porous diameter and volume to be 1.32 cm3 g−1 and 6.9 nm, evidently proving that Co3O4/ CNT as outstanding non-precious metal electrocatalyst [93]. In connection with the above report, recently, Shen et al. also prepared Co3O4/CNT as a bifunctional catalyst. Herein, they fabricated electrocatalyst by combining cobalt-based ZIF-67 precursor and 2-methylimidazole ligand together to form 3D Co3O4 polyhedron–CNT hybrid interlinked structure. In O2 saturated 0.1 M KOH solution, the obtained Co3O4/CNT exhibited excellent electrocatalyst activity with low Eonset (0.89 V), high E1/2 (0.076 V) and high current density of −6.28 mA cm−2 [94]. On functionalizing the pristine CNT with metals or metal oxides, the defect sites formed on the surface of the CNT act as anchoring sites for deposition of nanoparticles. Herein, Ni/Co3O4 spinel nanocrystal with an average crystal size of 2 nm has homogeneously deposited on pyridine based polybenzimidazole (PyPBI) coated MWCNT. PyPBI layer coated over MWCNT with a thickness of less than 1 nm. PyPBI layer acts as an anchoring site for uniform deposition of Ni/Co3O4 spinel nanocrystal by opening more possibilities of metal oxide binding sites on the surface of the CNT. Electrochemical studies displayed an improved ORR performance for MWCNT-PyPBI- Ni/Co3O4 in 1 M KOH medium. From the analysis, Ni plays a key aspect during the whole process: (i) The incorporation of Ni in Co3O4 is responsible for homogeneous distribution. On the solvothermal process, Ni metal restricted the movement of spinal oxide nanocrystal on the PyPBI layer, which resulted in uniform, distribution of Ni/Co spinel nanocrystals on MWCNT-PyPBI. (ii) With the same context, incorporation of Ni in Co3O4 on MWCNT-PyPBI catalyst attributed improved electrical conductivity, which was almost near to benchmark Pt/C [95].

wrapped with 10% wt of PANI showed a decrease in crystalline size due to the availability of nitrogen in PANI and reduced the metal ions present in the CNT surface. Additionally, on the increase in PANI content increases the electrical conductivity and induce more hydrophilic property. ORR measurements revealed that PtCo/10PANICNT exhibited higher stability by obtaining the kinetic current density of 36.9 mA cm−2 in O2 saturated 0.5 M H2SO4. Similarly, Pt alloying with different metals (Ni, Co, Cr, Pd) supported on CNT reported. Herein, instead of PANI, CNT wrapped with the 10 wt% of PANI and found PtCr/10PANICNT with enhanced stability and lower surface area loss of 14.8% [90]. 3.1.2. Effect of CNT-nonprecious metal catalyst towards ORR Designing non-precious metal with carbon support has a considerable improvement in energy-oriented aspects. Most recently, Zhao et al. reported carbon-supported bimetallic electrocatalyst for ORR electrocatalyst. They synthesized the hybrid nanostructure of CuFeO2-CNT via a simple one-pot hydrothermal method. During the physical characterization of the obtained catalyst, an interesting phenomenon was observed, in their Surface morphology. It was found that, after the addition of CNT to the CuFeO2 nanoparticles, increased crystallinity and decreased in particle size was observed, which helps to enhance the surface area of CuFeO2-CNT to 372.5 m2 g−1 towards ORR. Furthermore, the LSV test on RDE in 0.1 M KOH alkaline solution revealed good electron transport of 3.70 and followed the four-electron pathway [91]. Dong et al. planned low cost highly efficient cathode catalyst by combining different morphology of 1D materials as MnO2 nanowiresCNT hybrid composite towards ORR. The resultant MnO2/CNT catalyst exhibit an enhanced peak current density of 1.77 mA cm−2 at −0.36 V. Moreover, even after 20,000 s, the current density of MnO2/CNT retains 79%, which demonstrates its state of stability and durability than its individual and commercial Pt/C [92]. Concerning the above report, simple and highly efficient bifunctional electrocatalyst was developed using Co3O4 supported on nondopant CNT by a simple chemical method for ORR and OER. From TEM and HRTEM characterizations it was confirmed the unique morphology of Co3O4 nanocapsules grown over CNTs with the spinel crystal structure. RDE measurement for Co3O4/CNT electrocatalyst exhibited excellent activity towards ORR in 0.1 M KOH solution. It pronounced that the number of electron transfer for Co3O4/CNT was 3.96, which were comparable the same as Pt/C (3.98) and confirmed that Co3O4/CNT followed 4 electron pathways. This superior electron transfer was

3.1.3. Effect of nitrogen-CNT-precious/non-precious metal catalyst towards ORR Nitrogen doping in carbon support materials such as CNT has increased to a greater extent due to its change in electrical conductivity and chemical reactivity. Nitrogen-doped into CNTs generates a defect which frees the chemical inertness of the CNT and retains the electrical conductivity. Nitrogen has high electronegativity than carbon that enables electron transfer from carbon to oxygen results in an increase in 10

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Fig. 9. (A) ORR of CoFe@NCNTs, Fe@NCNTs, Co@NCNTs, Pt/C, and RuO2 by LSV tests in O2/N2-saturated 0.5 M H2SO4 at 1600 rpm [114] and (B) ORR polarization curves of (b) Mn-N-C/CNT-800 for 5000 cycles CV scanning (50 mV s−1) in an O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm [116].

are notable changes occurs in their electronic and structural properties which lead to high performances as supporting material. Therefore, the incorporation of nitrogen provides good electron transfer by sidestepping damage of the CNT structure. Besides, nitrogen-doped CNT plays a crucial part by prohibiting unwanted reactions such as agglomeration and migration of Pt nanoparticles [108]. Before discussing the topic, it is more worthy to note that in the last four years’ nitrogen-doped transition metal supported CNT has studied extensively, among them, Co and Fe are the individual and most alloyed metal catalysts to improve the catalytic activity towards ORR [109–111]. Wang and co-workers innovatively prepared Co-N-CNT electrocatalyst for PEMFC ORR using CNT as building brick and PEGPPG-PEG as templates by pyrolyzing urea, cobalt acetate, and PEG-PPGPEG precursors together. Here, PEG-PPG-PEG plays a vital role in the reaction by inciting the growth of CNT and reduced agglomeration of Co nanoparticles during the pyrolysis process. The ORR performance was evaluated via electrochemical test; the LSV test on RDE showed similar Eonset and E1/2 potential to that of Pt/C of Co-N-CNT in either alkaline or acidic environment. The ORR current density displayed the same current density in 0.1 M KOH alkaline and 0.1 M HClO4 solution acidic solutions. Also, the reduction of oxygen was found to be a direct 4 electron pathway in either medium. ADT test of Co-N-CNT, after 10,000 s, there was no current loss obtained in alkaline media, whereas 95% of initial current was retained in acidic media, indicating that CoN-CNT performed almost more or less equal in both medium and far better than Pt/C [112]. Most recently, a highly efficient FeCo nanoparticle wrapped nitrogen-doped graphitic CNT (FeCo@G-CNT-FD) was successfully fabricated [113]. Herein, the excellent strategy were conducted; Dicyandiamide was utilized as a source of precursor and prepared GCNTFD by freeze-dry treatment. The synthesized GCNT-FD exhibited a nanofibrous structure, which was an advantage for the formation of NGCNT on pyrolysis at 600–800 °C. The obtained FeCo@G-CNT-FD displayed superior catalytic activity because of the presence of more N and C in dicyandiamide, and it also achieved a high surface area 317.9 m2/ g−1. Likewise using CoFe alloy, another group reported, uniform deposition of CoFe nanoparticles over N doped CNT (CoFe-NCNT) by simple pyrolysis of metal precursors. The prepared CoFe-NCNT exhibited enhanced ORR performance in KOH as compared to the individually synthesized metal on support (Co-NCNT and Fe-NCNT) and 20 wt% Pt/C. CoFe-NCNT displayed an average number of electron transfer 3.91 and yield loss was less than 7%, indicating 4 electron pathways and water as a by-product. The ORR curves in Fig. 9(a) showed that after 15,000 cycles, only minimal loss of current density

the electronic property of the carbon materials that leads to high ORR activity [96]. Pyrrole and pyridinic type of nitrogen are the main reasons for the nitrogen-carbon bond in catalytic sites in both acid and alkaline media [97]. When nitrogen doped into honeycomb carbon structure, it changes its electronic structure and producing n-type semiconductor over the limited Fermi level and thus chemically active for the ORR. Terrones et al. absorbed an additional feature in the electronic properties of N-CNT. In order to study the feature observed, they performed ab initio calculations and tunneling microscopy. The result suggests that doping of nitrogen on the graphene surface increases the Fermi energy level of the donor state and decreases the bandgap. Therefore, the presence of an additional feature demonstrates that nitrogen-doped materials may have metallic properties [98]. Nitrogen can be doped into carbon materials by two methods. (i) In-Situ doping and (ii) post doping [99] Nitrogen-doped carbon materials such as carbon nanotubes [100], carbon nanofibers [101], carbon nanocoils [102], and carbon nanosphere [103] are doped via in-situ doping. The method for synthesizing nitrogen-doped carbon materials by chemical vapor deposition (CVD), pyrolysis, carburization, and other modified CVD methods are low-temperature in-situ doping method. Arc discharge and laser ablation methods are high-temperature in-situ methods [104,105]. According to first principle calculation, the nitrogen atom has high electron affinity which enhanced the adsorption of Pt in nitrogen-doped CNT (CNx) than compared to undoped CNT, thus indicates Pt/CNx has high stability. The Pt/CNT and Pt/CNx for different nitrogen contents (x = 1.5, 5.4 and 8.4%) and melamine (200, 800 and 2000 mg), were synthesized by a floating catalyst CVD method. The size of the Pt nanoparticles decreases with an increase in nitrogen content. After 4000 voltammetry cycle, the initial electrochemical surface area of Pt/CNx (x = 1.5, 5.4 and 8.4%) remains 20.2%, 26.6% and 42.5% which was evidently higher than nitrogen undoped Pt/CNT (11.2%). The resultant catalyst exhibited higher electrochemical stability by increased nitrogen doping [106]. The precursors exploited for growing CNTs is also one of the essential parameters for increasing the performance of energy conversion and storage devices. For instance, nitrogen-doped CNT fabricated from ethylenediamine (ED-CNT) precursor showed higher and better catalyst activity towards ORR than pyridine precursor (PY-CNT). This is because the ethylenediamine precursor molecule has one nitrogen atom for each carbon atom, whereas pyridine precursor molecules contain a single nitrogen particle for every five carbons. So that nitrogen to carbon ratio is high during the synthesis of ED-CNT, which attributes the presence of more functional sites [107]. Since as nitrogen content increases, there 11

Average particle size (nm)

1.04

67.5

38.7 37.7 72.46

2–3

CNT based electrocatalyst Pt/ED-CNT 3.73

4.06 4.09 2.9

3.2 2.0

Porous 2–18

NCF-800

Pt/PY-CNT Pt/CNT PFSA-Pt/CNT

Pt/CNT PtAu/MWCNT

Mn/Co-BNCNTs-800

12

22.5

2.33 ± 0.71

3.7

5.49 3.9

4.0

Pt:Au(10:10)/MWCNT

PtCu/CNT

PtCo/10PAN–CNT Pt-Pd/CNT-S

6.58 6.72 6.33 2.3

PtCo/PANI-CNT PtCr/PANI-CNT PtPd/PANI-CNT Pt/MWCNT

MnO2/CNT

34.44

6.76

PtNi/PANI-CNT

44.87 66.87 68.36 50

63.58

24.52

30.2

PtNi-H/MWCNT

PtCu/CNT

63.34 50.7

3.8 12.8

Pt/CNT-CH PtNi/CNT

41.4 34.8

2.7

Pt/CNT-MWH

57.9

182.6

52

5.46

4±2

N-CDs/GO

1.77

378 382 402 0.360

364

4.41

293 452

407 451

After 10,000 s, Pt:Au(10:10)/MWCNT has 11% reduction in current density, which was greater than Pt/C (60%)

270 600

365

5.25

625

18.4

2

N,S-CDs/rGo

After 2000 s, N-CDs retain 90.6% of current than Pt/ C (32.9%) After 10000 s, only 3% decrease in current density whereas 15% for Pt/C 2

Maximum power density/current density (mW cm−2/mA cm−2)

2–6

289.54

Electrochemical surface area (ESA) (m2 g−1)

CDs/M-rGo

CDs based electrocatalyst N-CDs/G 2–6

Catalyst support

Table 2 RDE measurement of CDs and CNT supported precious and non-precious metal catalysts.

[92]

[136]

[90]

[135]

[121]

[89] [122]

[134]

[87]

[86]

[74]

[115]

[133]

[132]

[108]

[41]

[40]

[44]

[43]

[140]

Ref

(continued on next page)

Smaller Pt nanoparticles on CNT rise more active sites and improved electronic conductivity with excellent electron transport From 1D CNT bundle, MnO2 nanowires were produced with more active sites and 3D network of MnO2/CNT hybrid increase stability towards ORR.

The performance of PtCu/CNT catalyst was due to their reaction time, template and Pt concentration The reason for PtNi-H/MWCNT ORR activity was due to alteration of Pt binding energy and reduction on Pt-Pt bond length PANI act as dispersant and stabilizer for Pt nanoparticles to decrease the aggregation and improve uniform dispersion of PtM alloys on CNT and improved the stability of the catalyst

The ORR activity of Pt-Pd/CNT is based on the influence of surfactants, in which SDS separated the MWCNT bundles and uniformly dispersed Pt-Pd nanoparticles on the surface of MWCNT

The ORR activity is based on the polyol synthesis method. On addition of PVP as stabilizer reduces, Pt loading and increase Cu contents on CNT

Graphitic structure of CNT is potential support for octahedral catalysts with high durability. The ORR activity of composite Pt: Au (10:10, 15:05)/MWCNT was due to the presence of high Au concentration, which results in decreased Pt-OH formation.

The multidimensional structure of PtAu/MWCNT easy the transportation of water or gases to catalyst sites. The enhanced performance of Mn/Co-BNCNTs-800 was mainly due to maintaining certain parameters such as t precursors ratio and carbonization temperature The improved performance of Pt/CNT was due to the fast and uniform heating of reaction mixture and growth of metal nuclei by the microwave-assisted method.

PFSA acts as a triple-phase boundary for smooth proton conduction on Pt surface and its dual role as binder and stabilizer further improves the ORR activity

The highlight of this work is based on influence of enriched nitrogen content precursors along with N doping.

The performance of N-CDs was due to amount of N-doping, which changed the electronic structure of CDs. The presence of heteroatoms N, S and P on both CDs and rGo enriched the performance towards ORR. N,S-CDs/rGo have active graphitic N and C-S-C/S-N on the surface of nanoparticle which enhanced the ORR activity. Incorporating N-CDs into GO layer at high temperature alters the electronic structure of CDs, resulting in excellent ORR. Doping of N into CDs at high temperature converts 0D CDs into 3D porous network attribute as efficient catalyst.

Comments/highlights

M.M. Mohideen, et al.

Applied Energy 257 (2020) 114027

Applied Energy 257 (2020) 114027 [139]

[118]

[138]

2.972 0.132 0.273

760

The enhanced ORR activity was recorded due to the modification of CNT with melamine–formaldehyde resin. The improved performance due to 3D-CNT structure combined with N-HCNS without Fe and each Fe atoms attached with HCNS without introduction of CNT During oxidation of CNT, partially unzipping of CNT occurs on their surface. Therefore, the resultant PECNT has a dual structure of 1D CNT in its inner core and unrevealed graphene layer on its outer surface. Herein, the presence of polypyrrole CNT as support produce N functional groups alone and along with Co, they form Co-Nx active sites at the same time during carbonization, results in improved performance 0.62

131

Comments/highlights Maximum power density/current density (mW cm−2/mA cm−2)

[137]

observed for CoFe-NCNT than Pt/C, thus indicating catalyst attains excellent stability. Furthermore, under the same condition on the addition of methanol to the reaction medium, there are no further changes have been observed, thus proved CoFe-NCNT has better methanol tolerant ability. From all these results, it was indicating that the enhanced performance of CoFe-NCNT mainly attributed due to the synergistic effect of Fe, Co and N [114]. In the same background of the above two reports, Mn-Co alloy nanoparticles supported on bamboo structured N doped CNT synthesized via a simple one-step pyrolysis method. Here, by varying Mn: Co ratio (1:1, 2:1 and 1:2) and pyrolysis temperature (700, 800 and 900 °C), ORR performance of the catalyst was investigated concerning individual incorporation of Mn and Co in N-CNT catalyst and noted as Mn-BNCNT and CO-BNCNT respectively. ORR measurement revealed that catalyst (Mn/CO-BNCNT) pyrolyzed at 800C with Mn: Co ratio of 1:1 exhibited best catalytic activity by obtaining the current density of 5.25 mA cm−2 at 1600 rpm in 0.1 M KOH and favor 4 electron pathway. During pyrolysis at 800 °C, mesoporous structure and a large number of defects were formed. Hence bamboo-like structure and presence of significant defects attributed to the improved catalytic performance of Mn/Co-BNCNT-800 [115]. In another study most recently, an attempt made to comparative study in effect of nitrogen doping into different transition metals (M = Co, Mn, Fe, and Ni) supported on CNT and their activity towards ORR was evaluated. All the samples were pyrolyzed at 800 °C and noted as Co-N-C/CNT-800, Mn-N-C/CNT-800, Fe-C/CNT-800, and Ni-N-C/ CNT-800, respectively. The electrocatalyst activity of all the samples possessed different activities depending upon their graphitic degree and content of nitrogen present. Among the different catalysts, Mn-N-C/ CNT-800 exhibited outstanding performance towards ORR as following: (i) ORR performance of Mn-N-C/CNT-800 proved good catalyst activity when compared to commercial Pt/C. This improved performance attributed due to the Mn oxide particles uniformly distributed over the nitrogen-doped CNT. (ii) Mn-N-C/CNT-800 attains a higher electron transfer number of 4.12 among other metal catalysts due to higher graphitization degree and higher limiting current obtained by Mn-N-C/ CNT-800. (iii) ADT measurement of Mn-N-C/CNT shown in Fig. 9(b) revealed that, after continuous 5000 cycles, notably there was no positive half-wave potential were observed along with only 5.57% of loss in current density was recorded and proved excellent stability and moreover, after 2000cycle, there is no change in current was observed on addition of methanol [116].

Electrochemical surface area (ESA) (m2 g−1)

350

297

114.5760 ± 0.2867

153.7 142.8 197.1

Average particle size (nm)

1.8 ± 0.6

130

3.5

10.57 7.67 11.90

Pt/CNT-MF

CNT-Fe/NHCNS

Pt3Sc/PECNT

Co-NCNT800 Co-CNT800 NCNT800

4. Implementation of electrocatalyst from RDE to the real application of PEMFC

Catalyst support

Table 2 (continued)

Ref

M.M. Mohideen, et al.

From the above-discussed sections, we can conclude CDs and CNT are promising supporting carbon material for ORR activity. Table 2 shows CDs and CNT supported-precious and non-precious metal catalysts and their corresponding RDE performance. Besides this performance, electrocatalyst exhibiting excellent activity in RDE measurement fails to extend its performance in the application of PEMFC under realistic conditions. To address these issues, the following section reviewed with electrocatalyst works in the practical application of PEMFC. Firstly, before discussing the performance of the catalyst, below mentioned vital point should be maintained while synthesizing electrocatalyst [117].

• Scale-up electrocatalyst synthesis methods • Activating more active sites • Improving excellent mass transfer to the catalyst layer • Fabrication of MEA with optimized three-phase boundary • Removal of inhomogeneity sites from the surface of the catalyst layer

Recently, Garapat and co-worker fabricated partially exploited CNT 13

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Fig. 10. (A) ECSA and (B) polarisation curve comparison for the Pt/C and Pt/N-CNT GDEs before and after the accelerated stress test. The insert in (A) shows the CVs in the range 0.05–1.2 V vs. RHE [120].

To show the difference from other studies, Rivera-Lugo et al. prepared PtCu/CNT catalyst by different methods (reverse microemulsion, chemical reduction, and sonication assisted method) to study their impact on the template, reaction time and concentration for PEMFC application. Among the synthesized methods, electrocatalyst prepared from galvanic displacement method showed improved performance and their ORR activity was discussed as follows. Firstly, the effect of the template was investigated for PtCu/CNT through CV analysis displayed an oxidation peak with improved catalyst activity towards ORR. Secondly, in reaction time, PtCu/CNT initially maintains with the same ratio, but after 1 h, 2 h, and 4 h, there is a reduction in platinum observed. As a result, a decrease in catalyst activity was observed during the aging time (−147, −70 and −66 mA mg−1 for 4 h, 2 h, and 1 h). Interestingly, the mass activity of PtCu/CNT was high for the catalyst prepared at 4 h. Thirdly, the effect of Pt concentration in the solution was evaluated. From RDE measurement, the PtCu/CNT catalyst prepared from 2.50 mM concentration of Pt shows the higher surface area of 28.66 cm−2 mg−1 which was comparably greater than catalyst prepared from 1.25 mM (18.28 cm−2 mg−1) and 3.75 mM −2 −1 (13.25 cm mg ). Moreover, to study the practical application of fuel cells, MEA was prepared by PtCu/CNT, and their corresponding polarization curve was observed. The MEA exhibited a higher maximum power density of 452 mW cm−2 than that of Pt/C (3 5 8). This result was attributed mainly due to the solvent-free Pt nanoparticles, good gas transport and promising properties of CNT [121]. In respect to the above work, now we are discussing the influence of surfactant in the synthesis and properties of MWCNT. Recently, Bharati et al. designed MWCNT supported Pt-Pd catalyst by using anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) as surfactants [122]. Both surfactants used under the microwave-assisted method to prepare Pt-Pd/CNT-SDS and Pt-Pd/CNTCTAB catalysts. Interestingly, they found the strange behavior of surfactants on MWCNT, which improved the catalyst activity as well as economically viable and accessible synthesis methods attracted to the PEMFC application. The schematic mechanism of both surfactants on Pt-Pd/CNT was shown in Fig. 11. From the physical characterization, it was observed that surfactant SDS separated the MWCNT bundles and uniformly dispersed Pt-Pd nanoparticles on the surface of MWCNT whereas, in CTAB surfactant, agglomeration of nanoparticles was observed. Moreover, in the real operating condition of PEMFC, Pt-Pd/ CNT-SDS and Pt-Pd/CNT-CTAB catalysts polarization and power density curves displayed that Pt-Pd/CNT-SDS has a lower cell voltage of 0.18 V, a higher mass activity which of about 4 times greater than PtPd/CNT-CTAB and obtained maximum power density of 451 mW cm−2.

(PECNT) as cathode support for Pt3Sc alloy as an efficient electrocatalyst for application of PEMFC. The as-prepared Pt3Sc/PECNT showed the increased mass activity of 80.5 mA mgPt−1 towards ORR in 0.5 M H2SO4, which is 1.8 times higher than that of Pt/C. Besides, Pt3Sc/PECNT exhibited enhanced Eonset, E1/2 due to the synergistic effect of Pt and transition Se metal; as a result, there was a shift in the dband center of Pt was observed which removes O]O bond at lower energy. Besides RDE measurement, Pt3Sc/PECNT extends its performance towards a single-cell PEMFC performance test. At low catalyst loading of Pt3Se/CNT, about 0.25 mgPt cm in MEA, exhibited a maximum power density of 760 mW cm−2. The result obtained by the Pt3Se/CNT catalyst almost reached 3/4th of the 2020 target fixed by DOE (0.125 mg cm−2 and 1 kW cm−2). [118]. Concerning the above work, Chandran et al. also used PECNT as support for Pd3Co alloy as promising electrocatalyst. The RDE studies confirmed the enhanced performance of Pd3Co/PECNT as a cathode catalyst. In a single-cell PEMFC performance test, at catalyst loading of 0.5 mg cm−2 in both catalyst layer exhibited the maximum power density of 327, 240 and 96 mW cm−2 at 60 °C [119]. From the result obtained from both groups showing promising performance in the practical application of PEMFC, which was due to the utilization of PECNT. This highlight of PECNT can be achieved, during the oxidation of CNT, partially unzipping of CNT occurs on their surface. Therefore, the resultant PECNT has a dual structure of 1D CNT in its inner core and unrevealed graphene layer on its outer surface. So the combined effect of both dimensional increases its electrical conductivity and surface area to help fast electron transport through carbon materials. From past decades or even presently, in some commercially available metal-based electrocatalyst, carbon black (CB) is used as carbon support. The utilization of CB slows down the reaction kinetics by limiting its catalyst activity, stability and durability during application. Mardle et al. demonstrated that 1D Pt nanorods grew on N-CNT and deposited directly to the gas diffusion layer. The as-prepared Pt/N-CNT worked under real operating conditions, and the RDE measurement towards ORR exhibited a relatively high maximum power density of 1.23 fold at low platinum loading. To further evaluate the mass activity and stability of Pt/N-CNT, the MEA test was conducted, and their corresponding CV and polarization curves were shown in Fig. 10. It was observed that Pt/N-CNT has low surface area of 21.9 m2 g−1 due to 1D nanorod and at 0.9 V, the mass activity of 0.065 A mg−1 is achieved which was higher than that of commercial Pt/C (0.060 A mg−1). Additionally, the ADT was examined to understand the stability of the catalyst. After 3000 accelerated stress test cycles, Pt/N-CNT retains higher surface area with minimum loss of 41% than compared to Pt/C (76%) [120]. 14

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Fig. 11. Possible reaction mechanism for formation of (a) Pt-Pd/CNT-C with CTAB surfactant, and (b) Pt-Pd/CNT-S with SDS surfactant [122].

5. Future growth and target on FCEV

developing non-precious metal catalysts would decrease the cost competitiveness of fuel cells efficiently [128]. Beyond this hindrance, hydrogen storage, production, and distribution of hydrogen refueling stations (HRS) have a paramount role in commercializing FCEV. At present, the average cost of HRS is $2.4 million for 500 kg/day, which may be doubled in upcoming decades. Form Eqs. (3) and (5), we can able to calculate the need for HRS in the future [129].

To subdue the above-discussed barrier and to step forward to projected growth on FCEV, DOE has targeted to decrease the platinum content to reduce the system cost, catalyst loading and to heighten the durability. In 2002, the platinum loading was about 1.0 mg−1 which further reduced into 0.125 mg−1 in 2017 [123,124]. However, still, it was an identified barrier to reach the ultimate cost target and to largescale production of FCEV. Alternatively, the development of platinum group metal (PGM) free catalysts in fuel cell cost comparably 200 times lesser than commercial platinum catalysts [125]. According to the DOE report, from 2006 to present, the fuel cell system cost decreases from US $50 KW−1 for 100,000 to US$45 KW−1 for 500,000 systems per year. Therefore, the cost minimization in the PEMFC system raises the manufacturing of FCEV into a more comprehensive volume. However, DOE fixed a near term target of US$40 KW−1 in 2020 and a long term target of US$30 KW−1 in 2025 for large scale production [126,127]. Hence, to accomplish the proposed goal, reducing platinum loading and

Days to refuel =

No of HRSs =

Autonomous range kmperday

Days to refuel Max No of FCEVs per HRS

(4)

(5)

From the international energy agency (IEA) report, 12,952 FCEV and 376 HRS were found at the end of 2018. Currently, the US, Asian countries and European countries have estimated a near term and long term target of producing FCEV and HRS. Asian countries such as Korea 15

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Fig. 12. Announced targets, visions and projections of FCEV and HRS of different countries [130].

Oxygen reduction reaction occurring in CDs and CNT supported precious and non-precious metal electrocatalysts for energy conversion fuel cell was reviewed on this manuscript. In order to acquire high efficient and low-cost electrocatalyst reducing Pt content or replacing Pt with inexpensive metals are challengable. Based on this, they are two ways: alloying Pt with other transition metals and hybriding carbonaceous materials with non-precious metals. These two ways have been carried out from past decades, but still, there is a lack due to the slow kinetics of oxygen reduction reaction. As the review addressed, Carbon materials such as CDs are recently widespread its application in the field of energy conversion electrocatalyst and CNT are already existing support material from the past decade to the present year for both energy conversion and storage devices. Herein, both materials possess high catalytic activity, but CDs highlights state-of-the-art catalyst in this review. The new strategy of incorporating metal oxide with carbon dots achieve excellent interaction between metal and carbon support, uniform dispersion of metal nanoparticles and N-doping in CDs further tune its physical and chemical properties for better ORR activity. From this review, our point of view for future perspective, more research work has to be carried out in a controlled manner for active electrocatalysts as follow:

estimate producing 81,000 FCEV and 310 HRS in 2020 as near term target and 6,200,000 vehicles and 1200 HRS in 2040 which includes 5,900,000 passenger cars, 120,000 taxis, 60,000 buses, and 120,000 trucks as long term achievement. Similarly, in Japan and China have a target of about 40,000, 200,000, 800,000 and 5,000, 50,000, 1,000,000 FCEVs and 160, 320 and 100, 300 and 1000 HRS in 2020, 2025 and 2030. The announced targets, visions and projections of FCEV and HRS of different countries were shown in Fig. 12 [130]. To know about indepth discussion, we suggest review focusing mainly on FCEV and HRS to readers [131].

6. Conclusions The recent progress of PEMFC and its application on hydrogen FCEV were the main outlines of this review. At present in the modern world, the need for sustainable energy conversion and storage devices is highly required utmost for domestic transportation due to demand in fossil fuel. The hydrogen FCEV is an alternative to future transportation for day to day purposes due to a reduction in emission by combining with clean energy sources. Hydrogen FCEV converts hydrogen and oxygen to electrical energy which is almost more than 90% pollution-free and environmentally friendly. Although having larger advantages than other energy devices, it still suffers from commercial availability in the market. The barriers for its commercialization are high cost due to the utilization of platinum in fuel cell stack and another is an oxygen reduction reaction. These two barriers not only limits its domestic availability also affects the durability and performance of FCEV. Therefore to address these problems US DOE have fixed a target to reduce the amount of platinum loading and to increase the performance of PEMFC.

1. Much attention is the need for the following parameters: pyrolysis temperature, synthesis steps, and carbon and transition metal precursors play a significant role in ORR. On pyrolysis of metal precursors at a suitable temperature typically above 800–1000 °C, electrocatalyst with high activity and durability can be developed. 2. At present, carbon-based monometallic and bimetallic electrocatalysts were mostly fabricated, but most of their performance in 16

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fuel cells is not remarkable. So designing more tri-metallic electrocatalysts may enhance ORR activity by fascinating more active sites, good electron transport to the membrane electrode assembly. 3. Finally, the cost of the fuel cell was increasing competition among renewable energy devices. The carbon-based precious metal-free electrocatalysts are present strategy to accelerate ORR in fuel cells, but still, further improvement in the synthesis method for designing electrocatalysts are highly appreciable. We hope electrocatalysts fabricating at room temperature may be a breakthrough for enhancing ORR activity in real fuel cell tests. 4. Moreover, we suggest that it was the correct period for focusing on carbon dots to apply in an oxygen reduction reaction with promising transition metals. In the future, this kind of new catalysts for fuel cells are welcome with red carpet for the great changeover from high cost to affordable and readily available in the market. Moreover, the ultimate advantage of increasing ORR can model fuel cell as promising alternative energy and increases the power economic growth of the country like China by deploying the largest producer and consumer of energy.

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