Sulfonated graphene oxide/Nafion composite membranes for high temperature and low humidity proton exchange membrane fuel cells

Iron oxide (Fe₃O₄) nanoparticles anchored over sulfonated graphene oxide (SGO) and Nafion/Fe₃O₄–SGO composites were fabricated and applied as potential proton exchange membranes in proton exchange membrane fuel cells (PEMFCs) operated at high temperature and low humidity. Fe₃O₄ nanoparticles bridge SGO and Nafion through electrostatic interaction/hydrogen bonding and increased the intrinsic thermal and mechanical stabilities of Nafion/Fe₃O₄–SGO composite membranes. Nafion/Fe₃O₄–SGO composite membranes increased the compactness of ionic domains and enhanced the water absorption and proton conductivity while restricting hydrogen permeability across the membranes. The proton conductivity of Nafion/Fe₃O₄–SGO (3 wt%) composite membrane at 120 °C under 20% relative humidity (RH) was 11.62 mS cm⁻¹, which is 4.74 fold higher than that of a pristine recast Nafion membrane. PEMFC containing the Nafion/Fe₃O₄–SGO composite membrane delivered a peak power density of 258.82 mW cm⁻² at a load current density of 640.73 mA cm⁻² while operating at 120 °C under 25% RH and ambient pressure. In contrast, under identical operating conditions, a peak power density of only 144.89 mW cm⁻² was achieved with the pristine recast Nafion membrane at a load current density of 431.36 mA cm⁻². Thus, Nafion/Fe₃O₄–SGO composite membranes can be used to address various critical problems associated with commercial Nafion membranes in PEMFC applications.

Preparation process of Nafion/Fe₃O₄–SGO composite membranes.     

Ladder-type sulfonated poly(arylene perfluoroalkylene)s for high performance proton exchange membrane fuel cells

Sulfonated poly(arylene perfluoroalkylene)s containing a sulfone-bonded ladder structure (SPAF-P-Lad) were synthesized by treating the precursor SPAF-P polymers with oleum as a novel proton exchange membrane for fuel cells. SPAF-P-Lad membranes had excellent solubility in polar organic solvents and high molecular weight (M_n = 145.4–162.9 kDa, M_w = 356.9–399.1 kDa) to provide bendable membranes with ion exchange capacity (IEC) ranging from 1.76 to 2.01 meq. g⁻¹. SPAF-P-Lad membranes possessed higher proton conductivity than that of the precursor SPAF-P membranes because of the stronger water affinity. Compared with SPAF-P membranes (T_g: 72–90 °C, Young''s modulus: 0.08–0.42 GPa; yield stress: 5.7–15.1 MPa), SPAF-P-Lad membranes showed better mechanical stability to humidity and temperature and improved tensile properties (Young''s modulus: 0.51–0.59 GPa; yield stress: 23.9–29.6 MPa). The selected membrane, SPAF-mP-Lad, exhibited improved fuel cell performance, in particular, under low humidity with air; the current density at 0.5 V was 0.56 A cm⁻², while that for SPAF-pP was 0.46 A cm⁻². The SPAF-mP-Lad membrane endured an open circuit voltage hold test for 1000 h with average decay of as small as 70 μV h⁻¹. A series of post-analyses including current–voltage characteristics, molecular structure, molecular weight, and IEC suggested very minor degradation of the membrane under the accelerated testing conditions.

Sulfone-bonded ladder-type sulfonated poly(arylene perfluoroalkylene)s (SPAF-P-Lad) were synthesized by an easy method to achieve high thermo-mechanical stability, proton conductivity, fuel cell performance and remarkable in situ durability.     

Possible scenario of forming a catalyst layer for proton exchange membrane fuel cells

Ionomer in the catalyst layer provides an ion transport channel which is essential for many electrochemical devices. As the ionomer and electrochemical catalyst are packed together in the catalyst layer, it is difficult to have a clear image of the ionomer distribution in the catalyst layer and how the ionomer is in contact with Pt or carbon. A highly dispersed catalyst was deposited on the TEM SiN grid directly using the same (ultrasonic spray) or a similar way as the catalyst was deposited on the membrane. By analyzing the distribution of various elements (C, F, S, Pt etc.), we found that the ionomer may coexist in the catalyst layer in three ways: ionomer covered Pt particles due to the relatively strong interaction between Pt and the ionomer; ionomer covered C particles; packed free ionomer in between the aggregated catalyst particles. The results show that the ionomer is prone to covering the surface of Pt particles as further evidenced by the accelerated degradation test (ADT).

Ionomer in the catalyst layer provides an ion transport channel which is essential for many electrochemical devices.     

PtRu/C catalyst slurry preparation for large-scale decal transfer with high performance of proton exchange membrane fuel cells

The large-area membrane-electrode assembly (MEA) has been fabricated using the decal transfer method with a methanol (MeOH)-based PtRu/C catalyst slurry. The stability of slurry dispersion is important when using a large-area decal transfer method to ensure the integrity of the electrode. In order to prepare stable and well dispersed catalyst slurry, a suitable solvent for the PtRu/C catalyst should be selected. We considered the physical properties of various organic solvents, including ionomer solubility, dielectric constant, and catalyst particle surface physical properties. We found that the MeOH-based PtRu/C slurry dispersion showed the best stability and dispersibility of catalyst–ionomer agglomerates. It was also confirmed that the MeOH-based slurry has the most suitable characteristics for coating the slurry on the substrate film. The decal technique-based MEA using this slurry showed excellent performance when compared with the spray method-based MEA. Furthermore, the large-area PtRu/C MEA with an active area of 51.84 cm² was fabricated and excellent performance was realized even when a reforming gas was used.

A large-area membrane-electrode assembly (MEA) has been fabricated using the decal transfer method with a methanol-based PtRu/C catalyst slurry and its excellent performance was realized by using reformed hydrogen gas.     

A graphene oxide polymer brush based cross-linked nanocomposite proton exchange membrane for direct methanol fuel cells

Functional polymer brush modified graphene oxide (FPGO) with functional linear polysiloxane brushes was synthesized via surface precipitation polymerization (sol–gel) and chemical modification. Then, FPGO was covalently cross-linked to the sulfonated polysulfone (SPSU) matrix to obtain novel SPSU/FPGO cross-linked nanocomposite membranes. Meanwhile, SPSU/GO composite membranes and a pristine SPSU membrane were fabricated as control groups. Reduced agglomeration of the inorganic filler and better interfacial interaction, which are benefit to increase diffusion resistance of methanol and to generate continuous channels for fast proton transportation at elevated temperature, were observed in SPSU/FPGO cross-linked membranes. Moreover, the enhanced membrane stability (thermal, oxidative and dimensional stability) and good mechanical performance also guaranteed their proton conducting durability. It is noteworthy that the SPSU/FPGO-1 cross-linked membrane possesses the best comprehensive properties among all the prepared membranes and Nafion®117, it acquires the highest proton conductivity of 0.462 S cm⁻¹ at 90 °C under hydrated conditions together with a low methanol permeability of 1.71 × 10⁻⁶ cm² s⁻¹ at 30 °C. The resulting high membrane selectivity displays the great potential of the SPSU/FPGO cross-linked membrane for DMFCs application.

A novel proton exchange nanocomposite which was cross-linked by functional graphene oxide polymer brushes shows interesting and comprehensive advantages for DMFCs.     

Efficient synthesis of Pt–Co nanowires as cathode catalysts for proton exchange membrane fuel cells

A simple and efficient method was used to prepare highly active and durable carbon-supported ultrathin Pt–Co nanowires (NWs) as oxygen reduction reaction (ORR) catalysts for the cathode in a proton exchange membrane fuel cell (PEMFC). Chromium hexacarbonyl plays a significant role in making Pt and Co form an alloyed NW, which acts as both a reducing agent and a structure directing agent. The nanocrystal exhibits a uniform nanowire morphology with a diameter of 2 nm and a length of 30 nm. In half cell tests, the Pt–Co NWs/C catalyst has a mass activity of 291.4 mA mg_Pt⁻¹, which is significantly better than commercial Pt/C catalysts with 85.5 mA mg_Pt⁻¹. And after the accelerated durability test (ADT), Pt–Co NWs/C shows an electrochemically active surface area (ECSA) loss of 19.1% while the loss in the commercial catalyst is 41.8%. Also, the membrane electrode assembly (MEA) was prepared using Pt–Co NWs/C as the cathode catalyst, resulting in a maximum power density of 952 mW cm⁻², which is higher than that of Pt/C. These results indicate that the one-dimensional structure of the catalyst prepared herein is favorable to improve the activity and durability, and the application of the catalyst in the MEA is also realized.

A simple and efficient method was used to prepare highly active and durable carbon-supported ultrathin Pt–Co nanowires (NWs) as oxygen reduction reaction (ORR) catalysts for the cathode in a proton exchange membrane fuel cell (PEMFC).     

Improvement in physical properties of single-layer gas diffusion layers using graphene for proton exchange membrane fuel cells

Gas diffusion layer (GDL) is an important component related to the efficiency of proton exchange membrane fuel cells (PEMFCs). Nevertheless, the preparation cost of the conventional GDL is high. In our previous studies, a single-layer gas diffusion layer (SL-GDL) prepared by a simple and cost-effective process has been used for PEMFCs, and it achieved 85% efficiency of a commonly used commercial GDL. In this study, improvement in physical properties of a series of single-layer gas diffusion layers, SL-GDL-Gx (x = 1–3), via uniform distribution of graphene in the SL-GDL, and the application of SL-GDL-Gx in PEMFCs are studied. The results indicate that the presence of well-distributed graphene layers in SL-GDL-Gxs causes an increase in the surface roughness and the formation of irregular slender interstices, leading to the enhancement of gas permeability while maintaining the microporous layer (MPL)-like microstructure and retaining good loading and efficient utilization of the catalyst. Moreover, the electrical resistivities significantly decreased and the mechanical properties improved. These improvements in physical properties are significantly beneficial for the performance of PEMFC. The single-cell performance tests show that the best performance measured at 80 °C under 99.9% relative humidity (RH) conditions is obtained from the PEMFC (FC-2) fabricated with SL-GDL-G2 and is 46% higher than that from FC-0 with SL-GDL-G0 without graphene and 15% higher than that from FC-3 with the commercial GDL. Furthermore, the performances of FC-2 measured at 50–80 °C under 15% RH are all much higher than those of FC-3. The results indicate that SL-GDL-G2 prepared via a cost-effective method is a potential GDL for PEMFCs.

Gas diffusion layer (GDL) is an important component related to the efficiency of proton exchange membrane fuel cells (PEMFCs).     

Recovery of expensive Pt/C catalysts from the end-of-life membrane electrode assembly of proton exchange membrane fuel cells

A simple and economical route using an environmental friendly solvent is reported for recovering catalyst from the end-of-life membrane electrode assembly. This precious component is recovered in the form of Pt/C and ionomer powder. The catalyst exhibited an electrochemical surface area of 42 m² g⁻¹ and remarkable stability, showcasing its potential for second life applications.

A sustainable approach for the recovery of EoL MEAs and possibility of pushing the recovered products back into the supply chain.

The drive for the decarbonization of the automotive industry owing to the adverse effects of global warming on both environment and human health has propelled the development of emission free, hydrogen-based fuel cell vehicles (FCVs). However, the complete diffusion of FCVs in fossil fuel-powered internal combustion (IC) engine dominated global vehicle market is limited by their high cost.^1,2 This is majorly due to the usage of Pt-based catalysts in electrochemical reactions generating power, which contributes ∼35–45% to the cost of FCVs.^3,4 The mass production of FCVs to cater to road transport as substitutes to IC-based engines will further increase the requirement of Pt. This is a bottleneck for the wide-scale applications of FCVs; besides being expensive, Pt has limited and geographically restricted natural reserves. Also, the mass employment of FCVs requires readiness with respect to the waste management strategies for end-of-life (EoL) components for the effective utilization of their precious components. This has boosted research on the recycling/recovering Pt from the EoL membrane electrode assemblies (MEAs) of fuel cells. Moreover, the efficient recycling of precious components from EoL MEAs presents sustainable waste management of fuel cell components as well as the development of cost-effective alternative technologies with the recycled components.Recovery strategies for the EoL MEAs of fuel cells are typically categorized into four classes: (i) high-temperature combustion processes, (ii) acid dissolution process, (iii) electrochemical process-based recovery route, and (iv) alcohol treatment method. All these techniques have been proved efficient for the recovery of Pt, but suffer from drawbacks towards effective implementation, hence presenting avenue for better recovery processes. The high-temperature combustion methods are most commonly employed for the Pt recovery from EoL MEAs. However, besides being energy intensive, this process also leads to the emission of toxic HF, arising from high fluorine-containing components, perfluorosulfonic acid (PFSA) membranes and polytetrafluoroethylene (PTFE) binders in MEAs.⁵ The acid dissolution technique for the recovery of Pt utilizes highly corrosive acids, such as HCl, H₂SO₄, HNO₃, aqua regia (HNO₃/HCl) and piranha acid (H₂SO₄/H₂O₂), requires specialized infrastructure and also suffers from hazardous emissions such as HCl vapour, Cl₂, NO_x and SO₂.^6,7 Electrochemical methods of the Pt recovery often employs corrosive or toxic electrolytes, which also lead to harmful emissions.^8,9 These recovery processes also result in the complete loss of the other expensive component, i.e., the Nafion membrane. The alcohol treatment process offers the possibility of membrane recovery; however, utilizes large quantity of alcohols making the process cost-intensive.¹⁰ Besides, this process results in the recovery of larger particle-sized Pt-based catalysts (>50 μm), making them unsuitable for direct catalytic applications.¹¹ Considering the limitations of current recovery technologies, the development of a simple and environmental benign route with the potential for the direct application of recovered products is imperative for the efficient utilization of natural resources and development of sustainable recovery centres. Sustainable EoL recovery technologies are also vital to support future Pt-based technologies and take away burden from limited Pt natural reserves.Herein, a low-temperature hydrothermal treatment is reported for the recovery of EoL MEAs of a proton exchange membrane fuel cell (PEMFC) in 50 : 50 v/v of water and isopropanol (IPA) solution under ambient pressure, as presented in Fig. 1.Open in a separate windowFig. 1Schematic representation of the recovery process of EoL MEA.The presence of the aqueous solution of IPA aids in the disruption of the bond between the fluorocarbon-containing Nafion membrane and the coated Pt/C catalysts, facilitating catalyst detachment.¹² Furthermore, the hydrothermal treatment in the aqueous solution of IPA results in the dissolution of the membrane,¹³ and subsequently Pt/C catalysts were recovered via a simple vacuum filtration technique. The Nafion membrane was recovered in the form of an ionomer-enriched filtrate solution, which can be used to prepare a new Nafion membrane via recasting.¹⁴ The recovered ionomer solution was further heat-treated for 12 h at 80 °C for the evaporation of volatile solvents to yield ionomer powder. This offers the possibility of reuse and desirable modification of the recovered Nafion membrane as powder, which can be dissolved in large number of solvents.¹⁵ Fig. 2 shows the Fourier transform infrared (FTIR) spectrum of the recovered ionomer enriched solution (IES-R) and fresh commercial 5 wt% Nafion dispersion. The spectra display molecular segments similar to commercial Nafion dispersion. The strong band in the region 3700–3000 cm⁻¹ is attributed to the O–H stretching of water. However, the shoulder peak at 2978 cm⁻¹ and sharp peak at 1640 cm⁻¹ correspond to hydrated H₃O⁺.¹⁶ The peak at 1236 cm⁻¹ can be attributed to the stretching vibrations of SO₃⁻ groups.¹⁷ The peaks in the range 1400–1000 cm⁻¹ can be ascribed to the CF₂ stretching vibrations.^16,17 The peak appeared at 963 cm⁻¹ ascribes the to C–O–C stretching modes, whereas the region between 800–500 cm⁻¹ mostly corresponds to C–F groups.^18,19 The FTIR spectroscopy results confirm the dissolution of the Nafion membrane from EoL MEAs, resulting in the formation of an ionomer-enriched solution. This also reveals the efficacy of the employed recovery route to reclaim the precious membrane component of EoL MEAs.Open in a separate windowFig. 2FTIR spectra of IES-R and commercial 5 wt% Nafion dispersion.The recovery of Pt/C catalysts and Nafion membrane from the EoL MEAs of PEMFC via the low temperature non-toxic aqueous alcohol-based route was further confirmed through X-ray diffraction studies. Fig. 3a shows the X-ray diffractogram of recovered ionomer powder (IP-R) obtained after the evaporation of solvents from the IES-R solution and Pt/C catalyst (Pt/C-R). The recovered ionomer powder showed the characteristic peak of Nafion at 16.6° and 39.9°, corresponding to the polyfluorocarbon chains of Nafion.^20,21 The X-ray diffractogram of Pt/C-R displays characteristic diffraction planes, corresponding to the face centered cubic (fcc) Pt lattice. The peaks at 39.8°, 46.3°, 67.5°, 81.4° and 85.9° correspond to the (111), (200), (220), (311) and (222) planes of the fcc Pt lattice. The average crystallite size of the recovered Pt was calculated to be 6.1 nm using the Scherrer equation utilizing the most abundant reflections of Pt (111). It is noteworthy to mention that much smaller crystallite sized Pt nanoparticles (NPs) were recovered in this study employing the non-toxic aqueous IPA solvent-based low temperature recovery route as compared to Pt NPs (30 nm) recovered using a corrosive, concentrated H₂SO₄ solution.²¹ A small hump at around 26° in the diffractogram of Pt/C-R corresponds to carbon support. The morphology and particle size distribution of the recovered catalyst were investigated via transmission electron microscopy (TEM). Fig. 3b shows the TEM image and the corresponding particle size distribution histogram of Pt/C-R. Spherical Pt NPs with an average particle size of 5.5 ± 0.9 nm were found to be uniformly distributed on the carbon support. This value is in close agreement with the crystallite size determined using the XRD data. The attainment of smaller sized Pt NPs substantiates the prospective of the recovered catalyst for direct applications in fuel cells signifying avenue for the efficient utilization of the recovered products from EoL MEAs. Furthermore, the total amount of recovered Pt/C was 0.74 g, which is 98.7% of the catalyst loading on untreated EoL CCM, revealing the efficacy of the recovery process. The Pt wt% in the recovered catalyst was determined via the thermogravimetric analysis under air atmosphere, and the metal content was found to be ∼17 wt% (Fig. S1^†). This value was used for electrochemical calculations.Open in a separate windowFig. 3(a) X-ray diffractogram of IP-R and Pt/C-R, and (b) TEM microgram and particle size distribution histogram of the Pt/C-R catalyst.The catalytic activity of the Pt/C-R catalyst for fuel cell applications was assessed via cyclic voltammetry (CV) in an acidic medium (0.5 M H₂SO₄). The performance was compared with the Pt/C-C catalyst to assess the applicability of the recovered catalyst in real time applications. Fig. 4a shows the cyclic voltammograms of Pt/C-C and Pt/C-R catalysts. Both the catalysts exhibited the characteristic hydrogen adsorption/desorption feature (0–0.3 V) of the active Pt surface. Pt/C-C exhibited a higher current density as compared to Pt/C-R, indicating superior catalytic activity. This is also evident from the electrochemical surface area (ECSA) calculation of the catalysts. The ECSA represents the active Pt sites available for participation in an electrochemical reaction. Hence, ECSA is a measure of catalytic activity and was calculated using the following equation.ECSA = Q_H/(0.21 × [Pt])where Q_H is the coulombic charge for hydrogen desorption in mC cm⁻², 0.21 is the coulombic charge required to oxidize a monolayer of H₂ in mC cm⁻², and [Pt] is the Pt loading on the working electrode in mg cm⁻². Pt/C-C exhibited higher ECSA (83 m² g⁻¹) as compared to Pt/C-R (42 m² g⁻¹), which is anticipated considering the derivation of the recovered catalyst from EoL MEA. However, Pt/C-R exemplified ∼50% ECSA of the fresh catalyst showcasing its potential for subsequent usage as a catalyst in fuel cells. It is also noted that Pt/C-R exhibited similar ECSA to that of the Pt catalyst prepared from the H₂PtCl₆ precursor obtained after the recovery process based on electrochemical dissolution in 1 M HCl (43 m² g⁻¹).^22,23 This highlights the advantage of the employed recovery technique as free from secondary processing such as dissolution of spent Pt and its redeposition/reduction to obtain re-usable catalyst for subsequent applications.Open in a separate windowFig. 4(a) CV plots of Pt/C-C and Pt/C-R catalyst, and (b) CV plots of the Pt/C-R catalyst before and after 5000 cycles of the stability test at a scan rate of 50 mV s⁻¹ in N₂ saturated 0.5 M H₂SO₄.The stability of the catalyst was also evaluated to access its potential for further applications. The stability of Pt/C-R was evaluated through measurement of changes in ECSA after subjecting the catalyst to 5000 potential cycles in the range of 0.6–1.0 V at the scan rate of 100 mV s⁻¹ in the N₂ saturated 0.5 M H₂SO₄ electrolyte. The CVs of Pt/C-R before and after the stability tests are presented in Fig. 4b. No significant suppression of hydrogen desorption peaks in the voltammograms were observed implying the minimal loss of ECSA. Pt/C-R retained ∼93% of its initial ECSA (42 m² g⁻¹vs. 39 m² g⁻¹) with a loss rate of 0.0006 m² g⁻¹ per cycle. The results indicate remarkable stability of the recovered catalyst exemplifying its potential for second life applications. The electrochemical results demonstrated the potential of the recovered catalyst for direct fuel cell applications. The practical viability of the Pt/C-R catalyst for fuel cell applications was assessed through single cell PEMFC performance. Fig. 5 shows the polarization and power density curves of the Pt/C-R catalyst. The cell exhibited an open circuit potential of 0.922 V and maximum power density of 134 mW cm⁻², revealing the functioning of the recovered catalyst in PEMFC. However, studies are in progress on improving the current density of the recovered catalyst for PEMFC applications.Open in a separate windowFig. 5Single cell polarization and power density curves of the Pt/C-R catalyst recorded at the cell temperature of 60 °C with humidified H₂ and air gases.To summarize, the study reports a simple, cost and energy effective, low temperature hydrothermal route utilizing an environmentally friendly aqueous IPA solvent for the recovery of two precious components of EoL MEAs of PEMFC, Pt/C catalyst and Nafion membrane. The molecular composition of the recovered ionomer enriched solution was investigated through FT-IR spectroscopic studies, and it was found to be similar to that of commercial Nafion dispersion, indicating the efficacy of the recovery route as well the recovered product for the utilization of precious components of EoL MEAs for second life applications. The recovered Pt/C exhibited well dispersed Pt NPs on the carbon support with average particle size of 5.5 ± 0.9, nm revealing the effectiveness of the recovery route in the attainment of smaller Pt NPs with prospective for direct applications. In addition, the process demonstrated a high Pt/C catalyst recovery rate of 98.7%. The electrochemical studies supported the potential of the Pt/C-R catalyst for fuel cell applications. Pt/C-R exhibited an ECSA of 42 m² g⁻¹ and remarkable stability with loss of only ∼7% of ECSA after 5000 cycles of stability tests, signifying their viability for fuel cell applications. Pt/C-R also showed ∼50% ECSA of a commercial, fresh Pt/C catalyst disclosing the avenue for the further utilization of the recovered catalyst for niche applications. Furthermore, a single cell PEMFC with the Pt/C-R catalyst demonstrated maximum power density of 134 mW cm⁻², showcasing the applicability of recovered Pt/C as an alternative and sustainable candidate for the development of cost effective PEMFCs. Also, the effective recovery of the Pt/C catalyst in the non-toxic solvent system at a low temperature under mild reaction conditions and utilizing simple equipment is an astounding advantage of this methodology. In addition, the recovery of the active ionomer along with Pt/C is another novel feature of the adopted recovery approach. Added to this two-fold advantage, the whole recovery process exemplifies a sustainable approach for both precious component recovery from EoL MEAs as well as the possibility of pushing the recovered products back into the supply chain.     

Probing of complex carbon nanofiber paper as gas-diffusion electrode for high temperature polymer electrolyte membrane fuel cell

The development of fuel cells is an important part of alternative energy studies. High-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is a very promising and commercialized type of fuel cell since it allows the use of hydrogen contaminated with CO. However, current advances in HT-PEMFC are based on searching for more sustainable materials for the membrane electrode assembly. The key issue is to find new, more stable carbonaceous Pt-electrocatalyst supports instead of the traditional carbon black powder. In the present study, we primarily demonstrate a new electrode design concept. Complex carbon nanofiber paper (CNFP) electrodes, obtained by polyacrylonitrile (PAN) electrospinning with further pyrolysis at 900–1200 °C, are suitable for platinum deposition and were probed as the gas-diffusion electrode for HT-PEMFC. Complex composite electrodes were obtained by introducing zirconium and nickel salts into the electrospinning PAN solution. After pyrolysis, ZrO_x and Ni(0) nanoparticles were distributed in the CNFP throughout the whole nanofiber volume, as it is seen in the high-resolution transmission electron microscopy images. The samples were thoroughly studied by X-ray photoelectron, Raman and impedance spectroscopy, cyclic voltammetry, and elemental analysis. The MEAs designed on platinized composite CNFPs demonstrate higher performance at 180 °C compared to non-composite ones and are comparable with commercial Celtec® P1000.

Complex composite carbon nanofiber fuel cell electrode shows advantages compared to non-composite and less durable commercial carbon black ones.     

Partially fluorinated copolymers containing pendant piperidinium head groups as anion exchange membranes for alkaline fuel cells

A new series of partially fluorinated copolymers with varying alkyl side chain length (C3, C6 and C9) and piperidinium head groups have been synthesized and characterized in detail in an effort to improve membrane properties for alkaline fuel cell applications. The copolymers (QPAF4-Cx-pip) provided thin and bendable membranes by solution casting, and achieved high hydroxide ion conductivity up to 97 mS cm⁻¹ in water at 80 °C. Membrane properties such as water absorbability, conductivity, and mechanical properties were tunable with the side chain length. The copolymer main chain and the piperidinium groups were both alkaline stable and the membranes retained high conductivity in 4 M KOH at 80 °C for as long as 1000 h, however, conductivity was lost in 8 M KOH due to Hofmann degradation of the side chain. QPAF4-C3-pip copolymer with the best-balanced properties as anion exchange membrane functioned well in a hydrogen/oxygen alkaline fuel cell to achieve 226 mW cm⁻² peak power density at 502 mA cm⁻² current density under fully humidified conditions with no back pressure.

Piperidinium functionalized partially fluorinated copolymers with varying alkyl spacer length were synthesized and evaluated as anion exchange membranes to achieve improved performance in alkaline fuel cells.     

N-Enriched carbon nanofibers for high energy density supercapacitors and Li-ion batteries

Nitrogen enriched carbon nanofibers have been obtained by one-step carbonization/activation of PAN-based nanofibers with various concentrations of melamine at 800 °C under a N₂ atmosphere. As synthesised carbon nanofibers were directly used as electrodes for symmetric supercapacitors. The obtained PAN-MEL fibers with 5% melamine stabilised at 280 °C and carbonized at 800 °C under a nitrogen atmosphere showed excellent electrochemical performance with a specific capacitance of up to 166 F g⁻¹ at a current density of 1A g⁻¹ using 6 M KOH electrolyte and a capacity retention of 109.7% after 3000 cycles. It shows a 48% increase as compared to pristine carbon nanofibers. Two electrode systems of the CNFM5 sample showed high energy densities of 23.72 to 12.50 W h kg⁻¹ at power densities from 400 to 30 000 W kg⁻¹. When used as an anode for Li-ion battery application the CNFM5 sample showed a high specific capacity up to 435.47 mA h g⁻¹ at 20 mA g⁻¹, good rate capacity and excellent cycling performance (365 mA h g⁻¹ specific capacity even after 200 cycles at 100 mA g⁻¹). The specific capacity obtained for these nitrogen enriched carbon nanofibers is higher than that for pristine carbon nano-fibers.

Nitrogen enriched carbon nanofibers have been obtained by one-step carbonization/activation of PAN-based nanofibers with various concentrations of melamine at 800 °C under a N₂ atmosphere.     

Coaxial heterojunction carbon nanofibers with charge transport and electrocatalytic reduction phases for high performance dye-sensitized solar cells

Novel coaxial heterojunction carbon nanofibers, fabricated by electro-spinning a mixture of hydro-pitch and polyacrylonitrile, served as the counter electrode for dye-sensitized solar cells. Their high power conversion efficiency, being comparable to that of Pt CE, was achieved due to their good conductivity and high heteroatom content.

Coaxial CNFs featured high conductivity derived from HP, and high N content and defective sites derived from PAN.

The counter electrode (CE) in dye-sensitized solar cells (DSSCs) has multiple functions, including the catalytic reduction of I₃⁻ to I⁻ and the regeneration of dye molecules, and is one of the key dominant components that governs the practical applications of DSSCs to a great degree.¹ Noble metal platinum (Pt) with its low electrical resistance and excellent electrocatalytic activity was the first to be applied as such and is now widely used as CEs in DSSCs. However, there is limited availability of Pt sources, and this has led to the high cost of CEs, which has hindered the practical applications of DSSCs.² To date, various efforts have been made to develop techniques for the production of inexpensive yet high performance CEs to replace the Pt CEs. Of the alternatives now available, carbon materials that are categorized as zero-dimensional (0D) to three-dimensional (3D) structures have attracted much attention as candidates for Pt-free electrodes in view of the advantages of low cost as well as good electrochemical stability; these include carbon black,³ carbon nanoparticles,⁴ carbon nanotubes,^5–9 graphene,^10–14 and graphite¹⁵ and their composites.^16–19 It is believed that a large quantity of defects in the carbon CEs may lead to high catalytic activities, while good electric conductivity can result in fast charge transportation. Nevertheless, how to combine and integrate the high electrical conductivity and abundant defects into one carbon electrode in a balanced way to fabricate high performance CEs with tuned structure remains a major challenge.Recently, one dimensional (1D) core–shell nanofibers have received much attention because a good combination of electrical conductivity and catalytic activity can be realized and achieved in one electrode material.²⁰ However, the core and shell in these fibers are two separate phases, which limits the rapid charge transition and increases the electrical resistance when such nanofibers are used as CEs in DSSCs. Moreover, the tedious and complicated fabrication process greatly limits their large-scale production. As such, it is necessary to explore a new approach for this kind of material, in which electrical conductivity and catalytic activity are combined well.Herein, we present 1D coaxial carbon nanofibers (CNFs) fabricated by the electrospinning method from two kinds of carbon precursors: hydrogenated pitch and polyacrylonitrile (PAN). The adopted hydro-pitch (HP) features planar aromatic hydrocarbon molecules and is more easily transformed into an optically anisotropic, graphitizable carbon structure,²¹ thus leading to high conductivity (Table S1, ESI^†). PAN tends to form a carbonaceous structure with high nitrogen (N) content and abundant defects; it is widely acknowledged that high electrochemical activities could be achieved by the different nitrogen species and defects.^{7,16,17,22–24} Benefiting from different nucleation mechanisms, a novel coaxial core/shell structure has been produced within the matrix, in which hydro-pitch contributes to the core phase, while PAN is responsible for the shell phase. Such CNFs with two phases and heterogeneous characteristics are very unique when they are applied as CEs in DSSCs, with the core playing the role of cable tunnel for charge transporting and the shell providing active sites for catalyzing the reduction of I₃⁻ to I⁻, as shown in Scheme 1.Open in a separate windowScheme 1Illustration of the structure and functions of heterojunction carbon nanofibers as CEs in DSSCs.The process of fabricating our CNFs (denoted as 0.15-HCNF and 1-HCNF, in which 0.15 and 1 are the weight ratios between hydro-pitch and PAN) is illustrated in Fig. S5,^† and the morphologies of different CNFs and their corresponding diameter distributions are shown in Fig. 1a–c and a₁–c₁. The average diameter is 0.33 μm for PAN CNFs (PCNF), 0.36 μm for 0.15-HCNF, and 0.76 μm for 1-HCNF, indicative of a size-increased behavior with the increase in the ratio between the hydro-pitch and PAN. The obtained CNFs in sample 1-HCNF are cross-linked or joined together, as can be seen in Fig. 1c, leading to the inner parts being exposed and the observation of different phases.Open in a separate windowFig. 1Scanning electron microscopy (SEM) images of (a) PCNF, (b) 0.15-HCNF and (c) 1-HCNF.The HCNFs were examined in much detail by transmission electron microscopy (TEM) to yield information about the inner structure, and the typical TEM images are shown in Fig. 2. It can be clearly seen that two phases in the axial direction are obviously observed in the HCNF samples (Fig. 2b and c) in comparison to PCNF (Fig. 2a). Energy-dispersive X-ray spectra (EDX) of the shell region show the high nitrogen content of 9.3 at% (Fig. 2e), which implies that the carbon shell is derived from PAN. To further gain insight into the contributions of these two precursors, sample 1-HCNF was treated with KOH at high temperature to remove the shell, and the corresponding sample was further characterized by TEM and EDX, the detailed results of which are shown in Fig. 2d and f. It can be seen that the shell layer was removed, and the content of nitrogen was only 1 at% in the core phase (Table S2, ESI^†), in comparison to there being only 0.4 wt% (Table S3, ESI^†) of nitrogen in the hydro-pitch.Open in a separate windowFig. 2TEM images of (a) PCNF, (b) 0.15-HCNF, (c) 1-HCNF, and (d) the core of the 1-HCNF fiber after KOH treatment for 1 h at 700 °C in N₂. (e and f) The EDX spectra of the fiber shell and core.With all of this information in mind, it was deduced that the core phase was mainly derived from the hydro-pitch, while the shell was mainly from PAN.It is interesting that this result is different from that reported by Yang.²⁵ In general, the low molecular weight pitch tends to be pushed to the outer surface during the solvent evaporation process.^26,27 Nevertheless, in the present system the hydro-pitch is more easily fixed in the inner part, which was evidenced by density functional theory (DFT) calculations (Fig. 3). As shown in Fig. 3a, the binding energy between the hydro-pitch and two PAN units (E_HP) is 5.6 kcal mol⁻¹, which is 1.3 kcal mol⁻¹ higher than that between pitch and PAN units (E_P, 4.3 kcal mol⁻¹). When three PAN units were applied, an increase of 25% was observed for E_HP, which reached 7.0 kcal mol⁻¹. The newly formed hydrogen bonding between aliphatic hydrogen in hydro-pitch and the cyano group in the PAN unit is responsible for the remarkable increase in binding energy, while the number of E_P only slightly increased from 4.3 to 4.4 kcal mol⁻¹, and no new bonding was observed. As a result, compared to the pitch, the hydro-pitch could be more easily fixed in PAN units due to the strong binding caused by the multi-interaction between the aliphatic hydrogen and cyano groups. In addition, the hydro-pitch is always wrapped by several PAN long chain molecules, as shown in Fig. S5.^† Therefore, a kind of heterojunction core–shell structure was finally produced, in which the core was attributed to the hydro-pitch while the shell was attributed to PAN.Open in a separate windowFig. 3The spatial configurations and binding energies between two kinds of pitch molecules and PAN units. (a) Hydro-pitch with two PAN units, (b) molecular pitch and two PAN units, (c) hydro-pitch and three PAN units, (d) molecular pitch and three PAN units.The HCNF samples were also analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectra to reveal the changes in the surface configuration caused by the addition of hydro-pitch (Fig. S2, ESI^†). Compared to PCNF (7.6 at%, Table S4, ESI^†), sample 0.15-HCNF maintained 4.6 at% nitrogen content, and the same I_D/I_G ratio (1.07) was also obtained for the two samples. With an increase in hydro-pitch in the case of 1-HCNF, a slightly lower nitrogen content (4.2 at%) and I_D/I_G number (1.01) were observed, which could be attributed to the exposure of the hydro-pitch-based core, as mentioned in Fig. 1c. The XPS spectrum of N 1s is shown in Fig. S3 in the ESI^† and the ratios of different nitrogen species are summarized in Table S5.^† It is well known that these nitrogen species in the carbon network could produce highly electrocatalytically active sites,^22,23 while the defects in carbon materials could also play the same role.The HCNFs exhibited unique characteristics in which the shell retained the high nitrogen content and defects, while the core derived from hydro-pitch featured high conductivity. Such an integrated structure in one electrode is so attractive that it could be used as a high-performance CE in DSSCs, as shown in Scheme 1. In this case, it has great potential as an electrocatalyst for Pt replacement in DSSCs. Benefiting from this, the device performance for DSSCs with PCNF, Pt, and two HCNF CEs was determined and the results are shown in

Fabrication of an anion exchange membrane with textured structure for enhanced performance of direct borohydride fuel cells

Developing electrolyte membranes with a simple preparation process and high performance is a top priority for the commercialization of fuel cells. Inspired by solar cell texturing to improve its conversion efficiency, this study prepares a textured membrane by increasing the roughness of a glass plate. The structures of the textured membrane and the flat membrane are characterized and compared. The membranes are assembled in fuel cells for performance testing. The surface area of the textured membrane is 1.27 times that of the flat membrane, which increases the size of the three-phase boundary in fuel cells. The maximum power density of the fuel cell using the textured membrane is 1.17 times of the cell using the flat membrane at 60 °C. The excellent performance of the cell using the textured membrane profit from the enlargement of the three-phase boundary. This work offers a simple way to develop outstanding-performance membranes by changing their surface roughness.

A textured membrane is simply prepared with a rough glass plate, and the improved cell performance benefits from the increased three-phase boundary.     

Anhydrous proton conductivity of electrospun phosphoric acid-doped PVP-PVDF nanofibers and composite membranes containing MOF fillers

A high-temperature proton exchange membrane was fabricated based on polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone (PVP) blend polymer nanofibers. Using electrospinning method, abundant small ionic clusters can be formed and agglomerated on membrane surface, which would facilitate the proton conductivity. To further enhance the conductivity, phosphoric acid (PA) retention as well as mechanical strength, sulfamic acid (SA)-doped metal–organic framework MIL-101 was incorporated into PVP-PVDF blend nanofiber membranes. As a result, the anhydrous proton conductivity of the composite SA/MIL101@PVP-PVDF membrane reached 0.237 S cm⁻¹ at 160 °C at a moderate acid doping level (ADL) of 12.7. The construction of long-range conducting network by electrospinning method combined with hot-pressing and the synergistic effect between PVP-PVDF, SA/MIL-101 and PA all contribute to the proton conducting behaviors of this composite membrane.

A composite SA/MIL101@PVP-PVDF membrane was fabricated via electrospinning and reached a conductivity of 0.237 S cm⁻¹ at 160 °C with a moderate acid doping level (12.7).     

Proton exchange membrane and bio-Fenton micro fuel cells for energy harvesting,gas leakage detection,and dye degradation

The present work focuses on the non-conventional design and operation of micro fuel cells. Two different kinds of fuel cells, Proton Exchange Membrane (PEM) and Biological Fenton (BF) based fuel cells, are fabricated to harvest energy. For the PEM fuel cell, H₂ and O₂ are generated by Mg/HCl reaction and Fenton''s reaction respectively, and are subsequently fed into two terminals of the PEM fuel cell. For the BF fuel cell, the reaction product of hemoglobin (Hb) with hydrogen peroxide (H₂O₂) is used as a source of chemical fuel to generate electrical energy within the fuel cell. An array of PEM microscale fuel cells is fabricated to scale up the reaction which can be used for MEMS/NEMS applications. Furthermore, the application of this adhesive and flexible PEM fuel cell as a hydrogen leakage sensor is demonstrated. In the BF fuel cell, an electronic imbalance across a carbon tape is generated owing to the formation of reactive hydroxyl radicals and concurrent electrons in the system. The generation of a highly oxidizing hydroxyl radical is also utilized to degrade Methylene Blue (MB) dye along with energy harvesting. This multi-purpose fuel cell can be synergistically used in industrial applications of waste treatment as well as energy production.

Flexible proton exchange membrane and bio-Fenton fuel cells for the application of energy harvesting, dye degradation and gas leakage sensor.     

Ceria nanorods as highly stable free radical scavengers for highly durable proton exchange membranes

Chemically durable proton exchange membranes containing free radical scavengers have technically matured in recent years, and commercial products have come into the market. The most general type of free radical scavenger is ceria, which has been proven in many studies. However, the migration of cerium is inevitable in raw ceria particles, and the migrated cerium species can aggregate in catalyst layers, causing performance loss of fuel cells. In this work, the morphology of ceria was changed from conventional nanoparticles to nanorods, and the migration of cerium was mitigated significantly. Both ex situ Fenton''s degradation tests and in situ fuel cell accelerated degradation tests (ADTs) indicated that ceria nanorods have free radical scavenging properties comparable to those of ceria nanoparticles. Moreover, the immobilization of ceria particles and antidissolving properties have been verified by Fenton''s degradation tests, electric field tests and fuel cell ADTs.

Morphology regulation induced high stability of ceria in proton exchange membrane.     

Diffusivity and free anion concentration of ionic liquid composite polybenzimidazole membranes

In this article, PBI composite membranes containing the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf₂) at 1, 5, 10, 20 and 50 wt% (named PBI-IL-x) have been prepared by a casting method. The internal morphology of the membranes was analyzed by scanning electron microscopy (SEM), revealing that the incorporation of IL promotes the formation of porous channels. Thermal and mechanical stability was confirmed by thermogravimetric analysis (TGA) and tensile test measurements. The ionic transport through membranes was analysed by means of electrochemical impedance spectroscopy (EIS), showing a dependence on the IL loading, reaching a highest conductivity value of 1.8 × 10⁻² S cm⁻¹ for the PBI-IL-50 membrane at 160 °C. The experimental results showed a Vogel–Fulcher–Tammann (VFT) type relation for the ionic conductivity with temperature and the calculated activation energies suggest that ionic conduction in the films can occur by both hopping and vehicle-type mechanisms. Eyring''s absolute rate theory was also used to obtain activation enthalpy and entropy from the temperature dependence of the conductivity. Diffusivity and free ion number density were obtained by means of electrode polarization analysis to obtain more insight into the conduction in these composite membranes. Finally, the Debye length was calculated and related to both transport parameters.

PBI composite membranes containing 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf₂) at 1, 5, 10, 20 and 50 wt% have been prepared and the conductivity has been analyzed by electrochemical impedance spectroscopy.     

Hybrid composite of Nafion with surface-modified electrospun polybenzoxazine (PBz) fibers via ozonation as fillers for proton conducting membranes of fuel cells

Nafion was investigated for its compatibility in the preparation of hybrid composites with electrospun Polybenzoxazine (PBz) surface-modified fibers by evaluating the effects on the surface and structure of the composite. A PBz fiber mat was first crosslinked by thermal treatment after electrospinning to enhance the mechanical integrity of the fibers prior to modification. Further surface modification via free radical ozonation was carried out by potentiating oxygen-based functional groups of hydroxyl radicals (–OH) onto fibers'' exposed surfaces. The sequential modifications by crosslinking and ozone treatment were evaluated by analyzing surface properties using XPS, ATR-FTIR and water contact angle which determined the enhanced properties of the fibers that were beneficial to the target functionality. Electron spectroscopy confirmed that fibers'' surfaces were changed with the new surface chemistry without altering the chemical structure of PBz. The presence of higher oxygen-based functional groups on fibers'' surfaces based on the resulting atomic compositions was correlated with the change in surface wettability by becoming hydrophilic with contact angle ranging from 21.27° to 59.83° compared to hydrophobic pristine PBz fibers. This is due to electrophilic aromatic substitution with hydroxyl groups present on the surfaces of the fibers endowed by ozonation. The resulting surface-modified fiber mat was used for the preparation of composites by varying two process parameters, the amount of Nafion dispersion and its homogenization and curing time, which was evaluated for compatibility and interaction as fillers to form hybrid composites. The analyses of SEM images revealed the effects of shorter homogenization and curing time on composites with rougher and wrinkled surfaces shown on the final hybrid composite''s structure while decreasing the amount of Nafion at the same homogenization time but longer curing time showed its influence on improvement of compatibility and surface morphology.

Nafion compatibility in the preparation of hybrid composites with electrospun Polybenzoxazine (PBz) surface-modified fibers via ozonation by evaluating the effects on the surface and structure of the composite.     

Potential carbon nanomaterials as additives for state-of-the-art Nafion electrolyte in proton-exchange membrane fuel cells: a concise review

Proton-exchange membrane fuel cells (PEMFCs) have received great attention as a potential alternative energy device for internal combustion engines due to their high conversion efficiency compared to other fuel cells. The main hindrance for the wide commercial adoption of PEMFCs is the high cost, low proton conductivity, and high fuel permeability of the state-of-the-art Nafion membrane. Typically, to improve the Nafion membrane, a wide range of strategies have been developed, in which efforts on the incorporation of carbon nanomaterial (CN)-based fillers are highly imperative. Even though many research endeavors have been achieved in relation to CN-based fillers applicable for Nafion, still their collective summary has rarely been reported. This review aims to outline the mechanisms involved in proton conduction in proton-exchange membranes (PEMs) and the significant requirements of PEMs for PEMFCs. This review also emphasizes the improvements achieved in the proton conductivity, fuel barrier properties, and PEMFC performance of Nafion membranes by incorporating carbon nanotubes, graphene oxide, and fullerene as additives.

We summarize here recent advances in carbon nanomaterials as additives for the state-of-the-art Nafion electrolytes for proton-exchange membrane fuel cells.