Transition-metal and nitrogen-codoped carbon-based (TM-N/C) catalysts are promising candidates for catalyzing the oxygen reduction reaction (ORR). However, TM-N/C catalysts suffer from insufficient ORR activity, unclear active site structure, and poor durability, particularly in acidic solution. Herein, we report single Co atom and N codoped carbon nanofibers (Co–N/CNFs) catalyst with high durability and desirable ORR activity in both acidic and alkaline solutions. The half-wave potential of the ORR shows a negligible decrease after a 10 000-cycle accelerated durability test. The high ORR durability is originated from the structural stability of the atomically dispersed Co-based active site, as revealed by probing analysis and density functional theory calculations. A passive direct methanol fuel cell with the Co–N/CNFs cathode delivers a maximal power density of 16 mW cm–2 and a remarkable stability during a 200 h test, demonstrating the application potential of Co–N/CNFs. The breakthrough of the highly durable TM-N/C ORR catalyst could open an avenue for affordable and durable fuel cells.Keywords: cobalt single atoms; durability; oxygen reduction reaction; transition-metal/nitrogen-doped carbon nanofibers;

•Controllable synthesis of ultrathin Pd NRs with tunable length and a tunable SPR.•The ORR activity can be improved by an increase of the apparent pre-exponential factor.•The enhancement of the proton tunneling contribution upon SPR excitation is verified.•Such plasmon-enhanced electrocatalysis is universal to the proton-coupled reactions.The electrocatalytic activity of metallic nanostructures is commonly tuned by changing their structure, composition, size and morphology to decrease the activation energy of a rate-determining elementary step. Here, we report that the electrocatalytic activity of the excited plasmonic model Pd nanostructured electrocatalyst for oxygen reduction reaction (ORR) can be enhanced significantly by increasing the kinetic pre-exponential factor. The localized surface plasmon resonance (SPR) effect promotes a two-orders of magnitude increase in the apparent pre-exponential factor in the Arrhenius equation. Combined experimental and theoretical analyses of the kinetic H/D isotope effect indicate that the observed increase in the apparent pre-exponential factor is attributable to an increased quantum–mechanical proton tunneling arising from the local electric field enhancement upon SPR excitation. Intriguingly, the ORR activity enhancement can be observed under simulated solar light illumination, implying potential for fuel cell applications. The methodology developed in this study is applicable to other proton-coupled electron-transfer reactions, thus opening a promising route to manipulating electrocatalytic reactivity by tuning the kinetic pre-exponential factor and suggesting potential for fuel cell applications.Download high-res image (57KB)Download full-size image

•3D carbon aerogel-supported PtNi intermetallic nanoparticle is prepared.•The catalyst exhibits much enhanced activity and durability for the ORR.•The size and intermetallic structure nearly keep unchangeable under durability test.Highly active and cost-effective oxygen reduction reaction (ORR) catalysts that have high metal loading and enhanced durability are desirable for the practical application in direct methanol fuel cells. Here, the preparation of a three dimensional (3D) carbon-based aerogel (CA) composed of graphene and multi-walled carbon nanotubes is reported and used as a support for an ordered PtNi intermetallic catalyst (OPtNi/CA) with a metal loading of 80 wt%. X-ray diffraction and transmission electron microscopic measurements confirm the formation of highly dispersed ordered PtNi intermetallic nanoparticles with a mean particle size of ca. 15.0 ± 1.0 nm. The as-prepared catalyst exhibits enhanced activity and durability for the ORR when compared to the Pt/C catalyst from BASF. The mass and specific activities of the ORR at 0.90 V on OPtNi/CA is ca. 1.4 and 1.8 times higher, respectively, than that using the commercial Pt/C catalyst. After an accelerated stress test, the mean particle size of the OPtNi/CA catalyst nearly kept unchanged. Both the improved activity and durability of the OPtNi/CA catalyst could be ascribed to the formation of an intermetallic compound, the uniform dispersion of PtNi nanoparticles, and the 3 D structure of the support.

Reduction of the Pt loading required in cathodes is crucial for the development of passive direct methanol fuel cells (DMFCs). Herein, a novel membrane electrode assembly (MEA) that utilizes a MWCNT–Pt nanocomposite cathodic catalyst layer (CCL) with a 3D network structure is shown to require significantly less Pt loading. With a CCL Pt loading of 0.5 mg cm−2, the maximum power density of the prepared DMFC is 19.2 ± 0.4 mW cm−2 using 2.0 M methanol solution at 25 ± 1 °C, which is higher than that of the power density by a conventional MEA with twice the Pt loading (1.0 mg cm−2). Electrochemical tests show that the structure of the CCL decreases the charge transfer resistance of the cathode reaction and greatly increases the cathode catalyst utilization in comparison with the conventional MEA. The enhanced MEA performance is attributed to the discontinuous distributions of the Pt MWCNT structures and the formation of a cross-twined network within the CCL. This study could provide a promising way to reduce the cost of future commercialized DMFCs.

Ordered nanostructured cathodes based on controlled vertically aligned Pt nanotubes are fabricated via combining sacrificial template method and in-situ galvanic replacement for ultra-low Pt loading passive direct methanol fuel cells (DMFCs). The Pt nanotubes are composed of highly dispersed Pt nanoparticles shell with an average thickness of ca. 15 nm. Employed as the oxygen reduction reaction (ORR) electrocatalyst, it exhibits similar activity but better durability than that of the commercial Pt black. Serving as the cathode for passive DMFC, a high electrochemical specific area of 77.48 m2g−1(Pt) is obtained by the ordered nanostructured cathode, which is 3 times higher than that of the conventional Pt/C based cathode (19.46 m2g−1(Pt)), indicating a significant improvement in the Pt catalyst utilization. Impressively, the Pt loading at the cathode side can be reduced to approximately 1/7 (0.12 mgcm−2) of the conventional MEA without sacrificing cell performance by using the vertically aligned Pt nanotubes catalytic layer. This method may provide a new way to fabricate ordered nanostructured cathode for ultra-low Pt loadings fuel cells.

•An ionic cross-linked TA-SPEEK membrane was synthesized as a PEM.•Ionic-covalent cross-linked C-SPEEK-x were prepared by TA-SPEEK and cross-linker.•TA-SPEEK and C-SPEEK-x showed reduced methanol permeability and improved stability.•C-SPEEK-25 had a stable DMFC's performance for more than 400 h.A novel ionic cross-linked sulfonated poly(ether ether ketone) containing equal content of sulfonic acid and pendant tertiary amine groups (TA-SPEEK) has been initially synthesized for the application in direct methanol fuel cells (DMFCs). By adjusting the ratio of p-xylene dibromide to tertiary amine groups of TA-SPEEK, a series of ionic-covalent cross-linked membranes (C-SPEEK-x) with tunable degree of cross-linking are prepared. Compared with the pristine membrane, the ionic and ionic-covalent cross-linked proton exchange membranes (PEMs) exhibit reduced methanol permeability and improved mechanical properties, dimensional and oxidative stability. The proton conductivity and methanol selectivity of protonated TA-SPEEK and C-SPEEK-x at 25 °C is up to 0.109 S cm−1 and 3.88 × 105 S s cm−3, respectively, which are higher than that of Nafion 115. The DMFC incorporating C-SPEEK-25 exhibits a maximum power density as high as 35.3 mW cm−2 with 4 M MeOH at 25 °C (31.8 mW cm−2 for Nafion 115). Due to the highly oxidative stability of the membrane, no obvious performance degradation of the DMFC is observed after more than 400 h operation, indicating such cost-effective ionic-covalent cross-linked membranes have substantial potential as alternative PEMs for DMFC applications.Download high-res image (161KB)Download full-size image

•Microporous organic polymer networks were used for modifying Nafion membrane.•This strategy improves the interface compatibility between fillers and polymeric matrix.•It also alleviates cavity problem between fillers and polymeric matrix.•Nafion-MOPN-x shows good dimensional stability and low methanol permeability.A series of new Nafion composite membranes have been prepared by the self-assembly of microporous organic polymer networks (MOPNs) and Nafion. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) reveal that MOPNs and Nafion membranes have good miscibility affording homogeneous composite membranes Nafion-MOPN-x. Compared with the recast Nafion membrane, Nafion-MOPN-x composite membranes show excellent thermal and mechanical properties, good dimensional stability, low methanol permeability, and proper proton conductivity. Also, the passive direct methanol fuel cell (DMFC) of Nafion-MOPN-3 membrane presents a maximum power density of 21.5 mW cm−2 at 25 °C. These results show the introduction of MOPNs can improve the interface compatibility and lessen the cavity problem between fillers and polymeric matrix during chemical blend. Therefore, it casts a new light on developing stable polymer electrolyte membranes (PEMs) for fuel cell application.

Synthesis of highly active and durable Pt based catalysts with a high metal loading for fuel cells’ applications still remains a big challenge. The three-dimensional (3D) graphene aerogel (GA) not only possess the intrinsic property of graphene, but also have abundant pore architecture for anchoring metal nanoparticles, thus would be suitable as metal catalysts’ support. This work reports a simple and mild one-step co-reduction synthesis of Pt nanoparticles highly loaded on 3D GA and the use as durable oxygen reduction catalyst. Both X-ray diffraction and TEM measurements confirm that Pt nanoparticles (ca. 60 wt.% Pt loading) with an average diameter of ca. 3.2 nm are uniformly decorated on the homogeneously interconnected pores of 3D GA even after a heat treatment at 300 °C. Such a Pt/GA catalyst exhibits significantly enhanced electrocatalytic activity and improved durability for the oxygen reduction reaction. The enhancement in both catalytic activity and durability may result from the unique 3-D architecture structure of GA, the uniform dispersion of Pt nanoparticles, and the interaction between the Pt nanoparticles and GA. The GA-supported Pt can serve as a highly active catalyst for fuel cell applications.

•ZnO nanorods are used as efficient sacrificial template to fabricate nano-network structure (NNS) within anode.•The DMFC with 50% reduced anode catalyst loading exhibits an enhanced performance.•The DMFC with NNS exhibits a significant increase in catalyst utilization and mass transfer efficiency.In this study, zinc oxide (ZnO) nanorods were used as efficient sacrificial template to fabricate nano-network structure (NNS) within anode catalyst layer (CL) and micro-porous layer (MPL) of a membrane electrode assembly (MEA). It resulted in a significant reduction of noble metal catalyst loading and enhancement of performance of a passive direct methanol fuel cell (DMFC). At a Pt–Ru loading of 1.0 mg cm−2, the MEA with NNS in anode CL exhibited a maximal power density of 28.4 mW cm−2 using 2 M methanol solution as fuel at 25 °C. However, for the conventional MEA with a Pt–Ru loading of 2.0 mg cm−2, the maximal power density was 29.0 mW cm−2. With the increase in Pt–Ru loading to 2.0 mg cm−2, the maximal power densities of the MEAs with NNS in anode CL and in both anode CL and MPL reached 38.6 and 40.2 mW cm−2, respectively. The improved performance of the MEAs with NNS was attributed to its property of higher catalyst utilization, higher mass transfer efficiency, and lower charge-transfer resistance compared to the conventional MEAs. However, the construction of NNS within both anode CL and MPL led to a relatively serious problem of cathode flooding, which restrained the further improvement in DMFC's performance. This study provided a perspective to fabricate novel pore structure to obtain high performance of passive DMFCs with low noble metal catalyst loading and low concentration methanol solution.

Mn–Ru binary oxides have been synthesized via a hydrothermal approach and their performance as bifunctional catalysts for the air electrode is evaluated in non-aqueous Li–O2 secondary batteries. Characterization of the catalysts by X-ray diffractometry and transmission electron microscopy confirms that the as-prepared oxides contain γ-MnO2 and hydrous RuO2 with the morphology of fusiform nanorods and nanoparticles, respectively. Linear scanning voltammetric measurements reveal that the binary oxides exhibit remarkable electrocatalytic activities towards both the oxygen reduction and oxygen evolution reactions. Li–O2 batteries with Mn–Ru oxides as cathode catalysts show a higher discharge voltage plateau and a higher specific capacity of 6500 mA h gcarbon−1 than those with Ketjen black (KB) alone, and the subsequent charge voltage is ca. 500 mV lower than that with KB. Moreover, much enhanced cyclability of Li–O2 batteries is achieved with a capacity retention ratio of 73.2% after 5 cycles. The cyclability is maintained for 50 cycles without sharp decay under a limited discharge depth of 1100 mA h gcarbon−1, suggesting that such a bifunctional electrocatalyst is a promising candidate for the air electrode in Li–air batteries.

The sluggish oxygen reduction kinetics and insufficient durability of cathode catalysts restrict the practical application of proton exchange membrane fuel cells. This study focuses on the structural transformation of carbon-supported Pt3Cr from a disordered to an ordered phase and on the effect of such structural transformation on oxygen reduction reaction (ORR) activity and durability. X-ray diffraction and transmission electron microscopy results confirm the formation of carbon-supported Pt3Cr intermetallic nanoparticles with a mean particle size of ca. 7.2 nm. Line scanning EDX reveals that the practical Pt–Cr atomic ratio is approximately 3:1. X-ray photoelectron spectroscopy results indicate that the proportion of metallic Pt increases while the binding energy of Pt 4f decreases with such structural transformation. The Pt3Cr/C intermetallic nanoparticles exhibit enhanced mass and specific activities toward the ORR compared with commercial Pt/C but slightly lower mass activity than the disordered Pt3Cr/C alloy nanoparticles. After the accelerated durability test for 5000 cycles, the Pt3Cr intermetallic nanoparticles displayed negligible decay in ORR mass activity; however the ORR mass activity on the isordered Pt3Cr alloy decreases to ca. 50%. Much enhanced durability of the Pt3Cr/C intermetallic nanoparticles toward the ORR is definitely caused by the much higher structural and compositional stabilities of the Pt3Cr/C intermetallic nanoparticles than that of the disordered Pt3Cr/C alloy nanoparticles, suggesting that the Pt3Cr intermetallic nanoparticles may serve as highly active and durable ORR electrocatalysts for practical application.

Three-dimensional ordered mesoporous (3DOM) NiFe2O4 materials with tunable pore size ranging from 5.0 to 25.1 nm have been synthesized via a hard template and used as bifunctional electrocatalysts for rechargeable Li–O2 batteries. Characterization of the catalysts by X-ray diffraction and transmission electron microscopy confirms the formation of a single-phase 3DOM NiFe2O4 structure. Linear scanning voltammetry measurements reveal that Ketjen black (KB) carbon-supported 3DOM NiFe2O4 exhibits a decreased overpotential for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) than commonly used KB. A reduction in both the ORR and OER overpotentials increases with the mean pore size of 3DOM NiFe2O4 materials. Importantly, Li–O2 batteries with 3DOM NiFe2O4 materials as the cathode catalysts exhibit a significant enhancement in the discharge capacity, rate capability, and cyclability, and these performances increases with the mean pore size of 3DOM NiFe2O4 materials. For a Li–O2 battery equipped with a 3DOM NiFe2O4 catalyst with a maximum mean pore size of 25.1 nm, a long cycling life of up to 100 cycles under the limiting capacity of 1000 mAh gC–1 is achieved, strongly indicating that the mesoporous size of the bifunctional catalysts plays a crucial role in enhancing the performance of Li–O2 batteries. The combined use of 3DOM NiFe2O4 with a maximal pore size of 25.1 nm and a poly(vinylidene difluoride hexafluoropropylene) separator with a tuned pore structure further improves the Li–O2 battery performance, highlighting the importance of the pore structure in the development of bifunctional catalysts and separators.Keywords: bifunctional electrocatalysts; lithium−oxygen batteries; nickel ferrite; ordered mesoporous structure; tunable pore size

•MgO nanoparticles are used as a sacrificial template to construct the porous anode.•The DMFC with porous anode exhibits a significant increase in catalyst utilization.•The DMFC with 50% reduced anode catalyst loading exhibits an enhanced performance.Simple addition of magnesium oxide (MgO) nanoparticles as a sacrificial pore-former into the catalytic layer (CL) and micro-porous layer (MPL) in the anode of a membrane electrode assembly (MEA) leads to the formation of porous anodic structure, thus greatly enhancing the performance of a passive direct methanol fuel cell. At the same PtRu(1:1) loading of 2.0 mg cm−2, the MEAs with porous CL and with both porous MPL and CL exhibit the maximal power densities of 37.0 and 43.7 mW cm−2 at a temperature of 25 °C and with 3 M of methanol solution, respectively. When the PtRu loading decreases to 1.0 mg cm−2, the maximum power density of an MEA with both porous MPL and CL is ca. 32.8 mW cm−2, which is even higher than that of a conventional MEA with a PtRu loading of 2.0 mg cm−2. The improved performance of the novel MEA can be ascribed to an increased electrochemical surface area, a decreased charge-transfer resistance as well as an efficient mass transfer of methanol after the formation of porous structure in the anode. The present work provides a very simple but very effective way to reduce the dosage of the noble metal catalysts for fuel cells.

•A nanofiber network catalytic structure (NNCL) is used in the anode of a DMFC.•The use of the NNCL leads to a significant increase in catalyst utilization.•The DMFC with 55% reduced anode catalyst loading exhibits a comparable performance.A novel membrane electrode assembly (MEA) that utilizes a nanofiber network catalytic layer (NNCL) structure in the anode of a passive direct methanol fuel cell (DMFC) leads to a significant decrease in noble metal catalyst loading of a DMFC. When the PtRu (1:1) loading within the NNCL is 1.0 mg cm−2, the maximum power density of a DMFC is ca. 33.0 ± 1.9 mW cm−2, which is even slightly higher than that with a conventional MEA with a PtRu loading of 2.0 mg cm−2. Electrochemical tests show that such a NNCL exhibits a great increase in catalyst utilization and a decrease in charge-transfer resistance of the anode in comparison with the conventional MEA. The improved performance of the novel MEA is definitely due to the formation of the nanofiber network structure in the anode. This study provides a promising way to decrease the utilization of the noble metal catalysts for the proton exchange membrane fuel cells.

•FSPES-x membranes with perfluoroalkyl sulfonic acid groups were synthesized.•The perfluorosulfonated reaction without employing any metal and basic catalysts.•FSPES-3 shows high conductivity, OCV and superior cell performance.A new series of cardo poly(arylene ether sulfone/nitrile)s FSPES-x with perfluoroalkyl sulfonic acid groups have been successfully prepared by the perfluorosulfonic acid lactone ring-opening reaction without using any metal or base catalysts. These materials have been characterized by IR, NMR and TGA. The results indicate that this simple and metal-free method of preparation is highly efficient for controlling both the degree of perfluorosulfonation and the position of the sulfonate group and no side reactions such as crosslinking is observed. The FSPES-x membranes (IEC = 1.17–1.64 m equiv g−1) show the desired characteristics such as good film-forming ability, excellent thermal and mechanical properties, low methanol permeability, high conductivity (up to 0.083 S cm−1 at room temperature), as well as appropriate cell performance compared to Nafion®117. With these properties, such fluorinated sulfonic acid side-chain-type polymers are promising PEM materials for application in fuel cells.

•Pd flower-like nanostructured networks was prepared via a novel CO-assistant method.•The size and morphology of the materials are temperature depended.•The novel materials indicated enhanced activity for formic acid electrooxidation.Novel Pd flower-like nanostructured networks are synthesized via a simple CO-assisted reduction. The morphology and size of the Pd nanostructures are found to strongly depend on the temperature and solvent during the synthesis process. Such Pd flower-like nanostructured networks exhibit a much enhanced activity of about 3 times of that on conventional Pd nanoparticles towards the electrocatalytic oxidation of formic acid. The specific activity of formic acid oxidation on Pd nanostructures is also greatly improved, indicating that the formation of flower-like nanostructured networks is beneficial for the electrooxidation of formic acid. Thus, it could be served as highly active catalyst for formic acid electrooxidation although the stability needs to be greatly improved.Novel Pd flower-like nanostructured networks, synthesized via a simple CO-assisted reduction, exhibit enhanced electrocatalytic activity of about 3 times of that on Pd nanoparticles for formic acid oxidation.

•Magnesia is used as a sacrificial support for the synthesis of PtRu black catalyst.•The PtRu black electrocatalysts show high alloying degree and small particle size.•Catalytic activity for MeOH oxidation is ca. twice that of commercial PtRu black.•In-house PtRu black catalysts have enhanced durability over commercial catalysts.Synthesis of small particle size, highly alloyed PtRu black catalysts for application in direct methanol fuel cells remains a substantial challenge. In this work, PtRu (1:1) black catalysts have been synthesized using Pt carbonyl complex and RuCl3 as initial precursors and magnesia nanoparticles as sacrificial templates. Magnesia is used to prevent the aggregation of synthesized PtRu nanoparticles during heat treatment, and thus promoting the controllable formation of PtRu black of high alloy composition and small particle size. X-ray diffraction shows that the PtRu black nanoparticles have a single-phase face-centered cubic structure and that the degree of alloying increases with treatment temperature. The mean particle size of the PtRu black catalysts, after heat-treatment from 250 to 300 °C, is found to be ca. 3 nm, only slightly larger than that of commercial Johnson-Matthey PtRu black, but more highly alloyed. Electrochemical measurements indicate that the catalytic activity for methanol oxidation on in-house prepared PtRu black catalysts is about twice that of the commercial product, with greater durability, indicating that the degree of alloying between Pt and Ru plays an important role in improving both catalytic activity and durability of the catalysts when used for methanol oxidation.

•Pd-hollow/C was prepared via galvanic replacement reaction with Kirkendall effect.•The electron transfer originally results in the lattice contraction in the hollow Pd.•The formation of hollow structure promotes the activity for formic acid oxidation.•The down-shift of d-band center is also origin to the enhanced activity of hollow Pd.Hollow metal nanocrystals with tuned electronic and geometric structure are highly desirable for the efficient catalytic and/or electrocatalytic reactions. Herein, we report the synthesis of carbon-supported Pd hollow nanocrystal (Pd-hollow/C) catalyst through a galvanic replacement reaction combined with Kirkendall effect without the use of polymeric stabilizer. The Pd-hollow structure is verified by scanning transmission electron microscopy. Noticeable lattice contraction in the Pd-hollow nanocrystal has been observed by high resolution transmission electron microscopy and X-ray diffraction with a decrease in the Pd (111) lattice distance. X-ray photoelectron spectroscopy indicates that the surface Pd atoms donate more electrons to the overlap with the sub-layer atoms, suggesting a strengthened d-hybridization and a down-shift of d-band center relative to the Fermi level on the surface. Electrochemical measurements show that the Pd-hollow/C catalyst exhibits a significantly enhanced electrocatalytic activity toward formic acid oxidation. The collective effects of the hollow structure and down-shift of Pd d-band center could explain well such an enhanced catalytic activity. The present study provides new insights into the relevancy of lattice parameter, electronic structure with catalytic property, and suggests design features for excellent nanostructured catalysts.Both structural effect and variation of electronic structure in the hollow-structured Pd supported on carbon play important roles in enhancing electrocatalytic activity for formic acid oxidation.

Methanol crossover through the proton exchange membrane from the anode to the cathode is a major obstacle for the development of the direct methanol fuel cells (DMFCs). This work focuses on the exploration of a novel methanol-blocking membrane prepared by layer-by-layer assembly of poly(diallyldimethylammonium chloride) (PDDA) and graphene oxide (GO) nanosheets onto the surface of Nafion® membrane. Results demonstrate that the bilayers are methanol-blocking, which formed a dense and uniform thin film structure on the surface of Nafion® film. The methanol permeability measurement across the composite membrane shows a decrease in methanol diffusion coefficient by 67% in comparison to the pristine membrane. The limiting current density on the cathode due to methanol crossover across the membrane with PDDA-GO bilayer structure is about 1/3 of that with pristine membrane, indicative of a reduction of methanol crossover by 63%. The use of Nafion®-PDDA-GO leads to the enhanced power density and improved energy efficiency of the passive DMFC. This study highlights the perspective of the application of GO as methanol-blocking thin film on the surface of Nafion® membrane in the DMFC.

Polypyrrole nanowire networks (PPNNs), as anodic micro-porous layer (MPL) of passive direct methanol fuel cells (DMFCs), are grown in-situ on the surface of carbon paper through an electrochemical polymerization. Passive DMFC with the novel MPL achieves a 28.3% increment in maximum power density from 33.9 mW cm−2 to 43.5 mW cm−2 compared with the conventional layer with similar PtRu(1:1) loading of 2.0 mg cm−2 and operating with 4 M methanol solution at 25 °C. When the PtRu loading is decreased to 1.0 mg cm−2, the maximum power density of the DMFC still reaches 34.3 mWcm−2, which shows a comparative value with the conventional layer. The enhanced performance should be ascribed to the introduction of PPNNs, significantly improves catalyst utilization and mass transfer of methanol on the anode.

•A new cathodal diffusion layer (CDL) is fabricated by using mesoporous carbon (MC).•The adoption of MC improves oxygen transport and the back diffusion of water.•The passive DMFC with the novel CDL yields a specific energy of 946 Whkg−1 at 25 °C.A new cathodal diffusion layer (CDL) that utilizes mesoporous carbons (MCs) is fabricated and evaluated for the passive direct methanol fuel cell (DMFC). Experimental results reveal that the CDL with MCs (MC-CDL) can accelerate oxygen transport and promote the back diffusion of water from cathode to anode, leading to an increasing power density and enhanced discharge stability. The passive DMFC utilizing such an MC-CDL yields a specific energy of ca. 946 Wh kg−1 at 25 °C, which is 2.3 times higher than that of the theoretical specific energy of lithium-ion batteries (∼410 Wh kg−1, LiCoO2/graphite).

Graphene-carbon nanotubes (G-CNTs) composite material was prepared as the anodic micro porous layer (MPL) carbon material to fabricate the anodic diffusion layer for a passive direct methanol fuel cell (DMFC). The crack-free anodic MPL was obtained with the G-CNTs, leading to significant improvement in the cell's performance. The maximum power density of∼41.6mWcm−2 at a temperature of ∼25 °C was achieved using the G-CNTs as the anodic MPL material. H adsorption/desorption and CO stripping indicated that the enhanced performance of the passive DMFC by adding G-CNTs in anodic MPLs could be attributed to the improvement in catalyst utilization. CO stripping measurements showed∼36.1% increase in the electrochemical active surface area of the electrode with G-CNTs within MPL than that of Vulcan XC-72R carbon. AC impedance data demonstrated that the charge transfer resistance of the anode reaction decreased with G-CNTs as the anodic MPL material.

Ordered mesoporous NiCo2O4 (OM NiCo2O4) materials have been synthesized via KIT-6 template and used as bifunctional electrocatalyst for rechargeable Li-O2 batteries. Characterization of the catalyst by X-ray diffractometry and transmission electron microscopy confirms the formation of a single-phase, 3-dimensional, ordered mesoporous NiCo2O4 structure. The as-prepared OM NiCo2O4 exhibits a specific surface area of 95.5 m2 g−1 with mesoporous peaks between 3 and 5 nm. Linear scanning voltammetric measurements reveal that OM NiCo2O4 exhibits slightly higher catalytic activity for the oxygen reduction reaction but much higher activity for the oxygen evolution reaction than Ketjen black (KB) carbon. The Li-O2 battery utilizing OM NiCo2O4 shows a slightly higher discharge voltage plateau and a higher specific capacity of 4120 mAh g−1 than that with pure KB, and the subsequent charge voltage is much lower than that with KB. Moreover, an enhanced cyclability of Li-O2 battery with OM NiCo2O4 cathode is observed with a capacity retention ratio of 65.4% after 5 cycles. When restricting the discharge capacity at 1000 mAh g−1, Li-O2 battery with OM NiCo2O4 cathode shows an improved cyclability and the cut-off voltage is as high as 2.4 V after 20 cycles, suggesting that OM NiCo2O4 could be as a promising catalyst for Li-O2 batteries.

In this paper a simple and rapid fabrication method for a microfluidic direct methanol fuel cell using polydimethylsiloxane (PDMS) as substrate is demonstrated. A gold layer on PDMS substrate as seed layer was obtained by chemical plating instead of conventional metal evaporation or sputtering. The morphology of the gold layer can be controlled by adjusting the ratio of curing agent to the PDMS monomer. The chemical properties of the gold films were examined. Then catalyst nanoparticles were grown on the films either by cyclic voltammetry or electrophoretic deposition. The microfluidic fuel cell was assembled by simple oxygen plasma bonding between two PDMS substrates. The cell operated at room temperature with a maximum power density around 6.28 mW cm−2. Such a fuel cell is low-cost and easy to construct, and is convenient to be integrated with other devices because of the viscosity of the PDMS. This work will facilitate the development of miniature on-chip power sources for portable electronic devices.

This study analyzes the synthesis of carbon-supported core–shell structured Cu@Pd catalysts (Cu@Pd/C) through a galvanic replacement reaction to be utilized in the electrocatalytic oxidation of formic acid. The strategy used in this study explores the relationship among lattice strain, electronic structure, and catalytic performance. X-ray diffraction and X-ray photoelectron spectroscopy indicate that the inclusion of Cu in the nanocatalyst increases lattice strain and results in a downshift of the d-band of palladium. Electrochemical tests show that Cu@Pd/C catalysts exhibit weaker adsorption strength for CO with increased Cu content, which can be attributed to the downshift of the electronic d-band. For the synthesized materials, the Cu@Pd/C catalyst with a Cu:Pd atomic ratio of 27:73 is found to have the highest activity for formic acid oxidation. A peaklike plot between activity and atomic composition is acquired and reveals the relationship among lattice strain, electronic structure, and catalytic performance.

•PVDF-HFP membrane with tuned pore structure is used as a separator for Li–O2 battery.•The Li–O2 battery with PVDF-HFP membrane exhibits an enhanced rate performance.•Discharge capacity of Li–O2 battery increases with the pore size of the PVDF-HFP.•The improved rate performance is due to a faster Li+ transport across the membrane.In this work, the poly(vinylidene difluoride hexafluoropropylene) (PVdF-HFP) membranes with tuned pore structure are prepared and used as a separator for the lithium–oxygen battery. Both oxygen and argon plasma treatments are used to tune the surface pore size and density of the membrane. The discharge capacity of the Li–O2 battery increases with the pore size and density of the membrane, nearly maximal capacity is achieved with a pore size of ca. 1.48 μm. More importantly, the Li–O2 battery using the PVdF-HFP membrane with tuned pore structure exhibits a significantly enhanced rate performance, probably due to a faster Li+ ion transport across the membrane. The highest discharge capacity of 466.1 mAh g−1 is achieved at a current density of 5 mA cm−2 for the Li–O2 battery with the PVdF-HFP thickness of 110 μm and pore size of 1.48 μm. Such a discharge capacity is about 10 times higher than that using commercial PP/PE/PP membrane. To our knowledge, it is the first report that the regulation of the pore structure of separator could significantly improve the rate performance of the Li–O2 battery, which probably provides a new way to solve the low rate characteristics for non-aqueous Li–O2 battery.

Pt and Pd–Pt nanoparticles were anchored on reduced graphene oxide (RGO) with the aid of poly(diallyldimethylammonium chloride) (PDDA), where Pt and Pd ions were first attached to PDDA-functionalized graphene oxide sheets and the encased metal ions and graphene oxide were then reduced simultaneously by ethylene glycol. As supported by transmission electron microscopy, metal nanoparticles, of small particle size even at a high metal loading, were chemically attached to PDDA–RGO. X-ray diffraction indicates that the as-prepared Pd–Pt nanoparticles have a single-phase fcc structure and are principally alloys of Pd and Pt. Among the RGO-supported Pt and Pd–Pt catalysts, Pt nanoparticles anchored on PDDA–RGO exhibit the highest activity for the oxygen reduction reaction (ORR), and the ORR activity of the Pd–Pt alloy electrocatalysts increases with Pt content. All the catalysts demonstrate an enhanced ORR durability when PDDA is present; strongly suggesting that PDDA plays a crucial role in the dispersion and stabilization of the metal nanoparticles on RGO. The ORR activities of the Pd–Pt catalysts remain enhanced even after accelerated durability testing. The formation of a Pt-rich shell, as confirmed by X-ray photoelectron spectroscopy and CO stripping voltammetry, may account for the increased activity.

A novel cathode catalyst layer with discontinuous hydrophobicity gradient distribution is designed and fabricated for passive direct methanol fuel cells (DMFCs). The stepwise hydrophobicity distribution is beneficial to the oxygen diffusion inside the cathodic catalytic layer and to the water removal from the cathode, thus improving the performance and stability of the DMFC. The DMFC with the above-mentioned novel cathodic catalytic layer structure presents a maximum power density of 33.5 mW cm–2 at a temperature of ca. 25 °C, while the DMFC with a conventional structure only gives 27.4 mW cm–2 under the same operating conditions. The enhanced performance of the DMFC with such a novel cathode structure might be attributed to a decreased charge-transfer resistance for the oxygen reduction reaction on the cathode.

The decrease in Nafion ionomer size within the anode catalytic layer for a passive direct methanol fuel cell (DMFC) results in a significant enhancement in fuel cell’s performance. Dynamic light scattering measurement demonstrates that the agglomerate size of Nafion ionomer in the solution decreases and the aggregate particle size distribution becomes narrow until a monodispersed Nafion ionomer was obtained with an increase in heat treatment temperature. The improved performance of the passive DMFC with smaller Nafion ionomer agglomerates within the anode catalytic layer can be ascribed to a decrease in charge-transfer resistance of anodic reaction obtained by electrochemical impedance analysis and to an improvement in catalyst utilization verified by cyclic voltammetric measurement. Furthermore, the small congeries formed between catalyst nanoparticles and Nafion ionomers could lead to a decrease in Nafion loading within the catalytic layer. This study confirms that the decrease in Nafion aggregation within the catalytic ink is beneficial to an improvement in both catalyst and Nafion ionomer utilization, thus enhancing fuel cell’s performance.Highlights► The decreased Nafion ionomer size enhanced the effective contact between catalyst and electrolyte. ► The use with small size of Nafion ionomer leads to an improved performance of a DMFC. ► It can be ascribed to a decrease in charge-transfer resistance of the anodic reaction.

Intermetallic compounds (IMCs) can enhance the electrocatalytic performance of the oxygen reduction reaction (ORR) because of their definite composition and structure and strong electron interaction. Here, we present a one-pot synthesis of structurally ordered Pd2Sn and Pd6Sn3Co nanoparticles supported on carbon material at low temperature without utilizing surfactant. Both XRD and high-resolution TEM results confirm the formation of IMC phase. The incorporation of Co atoms into Pd2Sn leads to a lattice contraction. EDX line scan analysis and ICP-AES demonstrate that the Pd:Sn:Co atomic ratio of ordered Pd6Sn3Co IMC is close to its nominal composition. The X-ray photoelectron spectroscopy indicates that the binding energies of Pd 3d3/2 and Pd 3d5/2 within the Pd6Sn3Co IMC are upshifted as compared with the Pd2Sn IMC, suggesting that a down-shift of d-band center relative to the Fermi level. This shift could explain that the Pd6Sn3Co IMC catalyst exhibits a higher activity and much better durability for the ORR in alkaline medium than the commercial Pt/C and Pd2Sn IMC catalysts.

Mn–Ru binary oxides have been synthesized via a hydrothermal approach and their performance as bifunctional catalysts for the air electrode is evaluated in non-aqueous Li–O2 secondary batteries. Characterization of the catalysts by X-ray diffractometry and transmission electron microscopy confirms that the as-prepared oxides contain γ-MnO2 and hydrous RuO2 with the morphology of fusiform nanorods and nanoparticles, respectively. Linear scanning voltammetric measurements reveal that the binary oxides exhibit remarkable electrocatalytic activities towards both the oxygen reduction and oxygen evolution reactions. Li–O2 batteries with Mn–Ru oxides as cathode catalysts show a higher discharge voltage plateau and a higher specific capacity of 6500 mA h gcarbon−1 than those with Ketjen black (KB) alone, and the subsequent charge voltage is ca. 500 mV lower than that with KB. Moreover, much enhanced cyclability of Li–O2 batteries is achieved with a capacity retention ratio of 73.2% after 5 cycles. The cyclability is maintained for 50 cycles without sharp decay under a limited discharge depth of 1100 mA h gcarbon−1, suggesting that such a bifunctional electrocatalyst is a promising candidate for the air electrode in Li–air batteries.