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The gas diffusion layer (GDL) is an important component of the membrane electrode assembly of fuel cells (FCs). Its roles include supporting the catalyst layer, collecting current, and transferring and redistributing materials. A conventional GDL consists of a backing layer, typically of commercial carbon paper or carbon cloth, but it suffers from its high cost, narrow pore-size distribution, lack of flexibility and poor conductivity, and a micro-porous layer (MPL) is necessary for better gas/liquid management. A novel flexible gas diffusion layer (GDL) was prepared by vacuum filtration of a suspension of carbon fibers (CFs) and highly-dispersed multi-wall carbon nanotubes (MWCNTs) in a polytetrafluoroethylene (PTFE) binder and water repellent. SEM observations, gas permeability and porosity tests indicate that there is a gradient in the concentration of highly-conductive MWCNTs in the CNT-CF GDL network that facilitates electron transport. A multi-level pore structure is formed, which is beneficial to mass transport. The PTFE is distributed uniformly, which is favorable for the discharge of condensed water from the FCs. When the GDL/CNT-CF is used in the cathode, or in both the cathode and anode in direct methanol FCs, the maximum power densities of single cells are increased by 20% and 35%, respectively, compared with those using a commercial GDL consisting of carbon paper with a MPL due to its excellent mass transfer performance.

Fe–N–C endowed with inexpensiveness, high activity, and excellent anti-poisoning power have emerged as promising candidate catalysts for oxygen reduction reaction (ORR). Single-atom Fe–N–C electrocatalysts derived from Fe-doped ZIF-8 represent the top-level ORR performance. However, the current fabrication of Fe-doped ZIF-8 relies on heavy consumption of time, energy, cost and organic solvents. Herein, we develop a rapid and solvent-free method to produce Fe-doped ZIF-8 under microwave irradiation, which can be easily amplified in combination with ball-milling. After rational pyrolysis, Fe–N–C catalysts with atomic FeN4 sites well dispersed on the hierarchically porous carbon matrix are obtained, which exhibit exceptional ORR performance with a half-wave potential of 0.782 V (vs. reversible hydrogen electrode (RHE)) and brilliant methanol tolerance. The assembled direct methanol fuel cells (DMFCs) endow a peak power density of 61 mW cm−2 and extraordinary stability, highlighting the application perspective of this strategy.

The oxygen reduction reaction (ORR) that occurs on the outermost layer of electrocatalysts is significantly affected by the composition and structure of the electrocatalysts. During the preparation of PtM alloy electrocatalysts, high-temperature annealing in an inert or reducing atmosphere could promote the segregation of M toward the core, forming a highly active Pt-skin structure. However, under fuel cell operating conditions, the adsorption of oxygen-containing groups could stimulate the easily dissolved M to segregate to the surface, reducing the activity and stability of the electrocatalysts. In this work, we conducted segregation energy calculation of PtM (M = Cu, Pd, Au) electrocatalysts under specific adsorption (SA), aqueous solution (AS) and an external electric field (EEF) with a density functional theory method. It was found that different factors have different effects on the segregation energy: ΔΔESA ≫ ΔΔEEEF > ΔΔEAS. The coupling effects have also been considered and compared: ΔΔESA+EEF > ΔΔESA+AS > ΔΔEEEF+AS. When including all three factors, the change of segregation energy could reach 1.63 eV. Therefore, operating conditions have a noteworthy influence on the segregation behavior of PtM ORR electrocatalysts, which should be considered in the further design of PtM ORR electrocatalysts.

Charge transport processes involving the proton migration and electron transfer for different parts of membrane electrode assemble (MEA) play an essential role for developing the novel electrode and enhancing the electrochemical performance towards proton exchange membrane fuel cells (PEMFCs). However, the coupled charge transport processes make it difficult for evaluating proton conductivity and electronic conductivity of different parts in MEAs under operation conditions of fuel cells. Here in this work, we propose an experiment approach for separating the electronic conductivity and proton conductivity of different components of MEA at the operating conditions of PEMFCs. This approach involves two different measuring devices, which both consist of electron or proton conducting layers, sealing layers and sample layer, followed by tailoring the thickness of sample layers and via electrochemical impedance spectroscopy (EIS) to quantity the electronic conductivity and proton conductivity of different layers. These experiment results show the great potential in the development of different components of MEA.

The oxygen evolution reaction (OER) plays an important role for multiple energy conversion devices, such as electrochemical water splitting, yet it suffers from high overpotential due to its sluggi...

Fabrication of novel electrode architectures with nanostructured ultrathin catalyst layers is an effective strategy to improve catalyst utilization and enhance mass transport for polymer electrolyte membrane fuel cells (PEMFCs). Herein, we report the design and construction of a nanostructured ultrathin catalyst layer with ordered Pt nanotube arrays, which were obtained by a hard-template strategy based on ZnO, via hydrothermal synthesis and magnetron sputtering for PEMFC application. Because of the crystallographically preferential growth of Pt (111) facets, which was attributed to the structural effects of ZnO nanoarrays on the Pt nanotubes, the catalyst layers exhibit obviously higher electrochemical activity with remarkable enhancement of specific activity and mass transport compared with the state-of-the-art randomly distributed Pt/C catalyst layer. The PEMFC fabricated with the as-prepared catalyst layer composed of optimized Pt nanotubes with an average diameter of 90(±10) nm shows excellent performance with a peak power density of 6.0 W/mgPt at 1 A/cm2, which is 11.6% greater than that of the conventional Pt/C electrode.

Methanol crossover is still a significant barrier to the commercialization of direct methanol fuel cells with wide-used Nafion® membrane. Herein, molecular sieve is introduced into the design of polymer electrolyte membrane to alleviate methanol crossover. The UZM-9 zeolite with an intermediate window size of 0.42 nm can effectively separate hydrated methanol (ca. 1.10 nm) and hydrated proton (ca. 0.23 nm). The methanol diffusion rate through the membrane is effectively suppressed after modified with UZM-9, which is about four times lower than the origin Nafion® membrane. The resulted peak power density reached 80 mW cm−2 with 2 mol L−1 methanol solution feed, which is 2.5-fold higher than that of direct methanol fuel cell with commercial Nafion® membrane. These results open a promising route to alleviate methanol crossover in direct methanol fuel cells.

Comprehension of the structure-activity relationship is of great importance for the rational design of electrocatalysts for the oxygen reduction reaction (ORR). Herein, Fe-N-C catalysts obtained from zeolitic imidazolate framework-8 (ZIF-8) with a tunable size ranging from 30 to 400 nm are precisely synthesized. Structural investigation indicates that the catalyst with smaller size possesses a higher proportion of mesopores originating from particle stacking, which leads to enhanced catalyst utilization and accelerated mass transport. The size effect of the catalyst on ORR activity is systematically investigated by rotation disk electrode (RDE) and direct methanol fuel cell (DMFC) tests. The electrochemical performance of the Fe-N-C catalyst is found to be increased with the reduction of its particle size. The correlation among size, mesoporosity and catalyst performance is discussed, giving new inspiration for the development of rational design strategies of non-precious metal catalysts.

Direct methanol fuel cells (DMFCs) have drawn extensive interest for the past two decades both in scientific research and industrial engineering circles for their advantages of high energy density, environmental friendliness, and easy fuel handling. However, their excessively high costs, especially derived from the massive use of precious metal catalysts in both their anodes and cathodes, hamper the commercialization of this technology to the general public. Though the production of inexpensive catalysts of methanol oxidation remains challenging, non-precious-metal-based catalysts of the oxygen reduction reaction have seen considerable technical progress, yielding remarkable performance levels in hydrogen-fueled polymer electrolyte membrane fuel cells (PEMFCs). Due to the particularities of the electrochemical reactions and mass transport for methanol fuel in DMFC electrodes, highly active non-precious metal catalysts have not yielded sufficiently satisfactory single-cell performances for practical applications. In the current work, rather than exploring the cathodic designs with advanced electrode materials and structures, we estimated the mass transport of methanol and its effects on cathode performance, and then redesigned the anode architecture with ultrathin gas diffusion layers based on carbon nanotube composite materials. By using such an alternative strategy focused on anodic engineering to accelerate methanol transport combined with the use of a methanol-inert cathode, an ultrahigh cell performance, comparable to those of Pt–C-containing cathodes, was achieved even at a low methanol concentration. The peak power density obtained was 141 mW cm−2, a value among the highest obtained from DMFCs with non-precious-metal catalyst cathodes to the best of our knowledge. A wider avenue of DMFC technologies for practical applications might be opened with further development of this work.

Fe-N-C catalysts are promising candidates to replace expensive and scarce Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cell devices. Herein, simultaneous improvement of activity and stability of Fe-N-C is achieved through exposing active sites via a surface modification strategy. Concretely, EDTAFe groups are anchored on the external surface of zeolitic imidazolate framework-8 (ZIF-8) through size limitation, followed by pyrolysis to obtain ZIF@EDTAFe-1%-950, whose surface active site density increases more than 1.7 times as detected by X-ray photoelectron spectroscopy (XPS) and 57Fe Mossbauer spectra. Consequently, 1.7 times improvement of active site utilization efficiency in electrochemical measurements and more than 2 times performance enhancement in direct methanol fuel cells (DMFCs) are achieved due to facilitated mass transport as revealed by oxygen gain voltage and electrochemical impedance spectroscopy (EIS). Furthermore, through engineering robust drainage channels around exposed active sites to alleviate flooding, the assembled DMFC exhibits better stability than that of Pt/C in the first 3 h and remains 83.9% voltage after 24 h at 100 mA cm-2.

Alternative carbon support materials provide great opportunities to fabricate advanced catalysts with enhanced activity and durability, especially for energy conversion and storage technologies bas...

Catalyst poisoning from the impurities in the industrial grade methanol is a major challenge for the large-scale application of direct methanol fuel cells in low cost. In this work, we systematically investigate the impurities influencing on cell performance, and confirm that the adsorption of carbonyl containing intermediate species derived from the partial electro-oxidation of the impurities is a crucial factor leading to performance degradation. Hence, an on-line alleviation strategy by intermittently applying pulse potential (reduction potential on the anode or oxidation potential on the cathode) is proposed. The applied potential will bring a reductive condition on the anode, which releases the active sites via the reduction of carbonyl containing species adsorbed on platinum-ruthenium electro-catalysts. Based on this strategy, the adsorption of carbonyl containing intermediate species are effectively suppressed, and the decay rate declines by nearly two orders of magnitude than that of a single cell under traditional operation, which paves a way for the practical application of direct methanol fuel cells with industrial grade methanol feed.

Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years. In this paper, the ORR activity of defective graphene anchoring single heteroatom (IIIA, IVA, VA, VIA and VIIA) was systematically investigated using a dispersion-corrected density functional theory method. For all of the 34 catalysts, 14 of which were further analyzed, and the Gibbs free energy of each elementary reaction was calculated. According to the scaling relationship between ΔGOOH* and ΔGOH*, we further analyzed the rate-determining step of the remaining 20 catalysts. The results show that when the ORR reaction proceeds in the path O2 → OOH → O → OH → H2O, the reaction energy barriers are lower than 0.8 eV for Te-SV, Sb-DV, Pb-SV, Pb-DV, As-SV, As-DV, B-SV, Sn-SV and N-SV. Our result provides a theoretical basis for further exploration of carbon-based single-atom catalysts for ORR.

The oxygen evolution reaction (OER) plays a critical role in efficient conversion and storage of renewable energy sources, whereas the active sites for the most representative electrocatalysts, Ni–Fe compounds, remain under debate. In this work, we have developed polymeric carbon nitride (PCN) with atomically dispersed N-coordinated Ni–Fe sites to investigate the OER process. The Ni–Fe dual sites consist of adjacent Ni and Fe atoms coordinated with N atoms in the PCN matrix. NiFe-codoped PCN exhibits higher electrocatalytic activity than monometal-doped catalysts, showing a lower overpotential (310 mV at 10 mA cm−2) and smaller Tafel slope (38 mV dec−1) in 1 M KOH, indicating that Ni–Fe dual-metal sites significantly favor the OER process. According to density functional theory calculations based on the oxidized-NiFe@PCN model, it was found that adjacent Ni and Fe atoms co-participate in the OER process for NiFe-codoped PCN, leading to a much lower energy barrier (0.10 or 0.22 eV for U = 1.58 V), while the effect of electronic modification of the single metal active sites by the other component (Ni sites by Fe sites or vice versa) contributes less to activity enhancement, thus leading to a rational explanation on the synergistic effect of the NiFe-based OER catalysts.

Fabricating electrochemical power sources with both high specific energy and power is highly desirable but remains challenging. In this work, a kind of self-charging hybrid electric power device (HEPD) is created with an intrinsic combination of polymer electrolyte membrane fuel cells and supercapacitors on the electrode level. Because of the unique processes of electrocatalytic reactions boosted by the fast pseudocapacitive discharge, remarkably high specific energy (1550 Wh kg–1) and power (4080 W kg–1) can be delivered by the hybrid device. The working mechanism of HEPD is proposed and demonstrated by in situ characterization and extensive electrochemical investigations. The self-charging HEPD with its synergistic electrochemical processes will broaden the applications of electrochemical power sources.