Practical Proton Exchange Membrane Fuel Cell Research Articles

Activity and Durability of Pt/Ir Catalysts for Oxygen Reduction as a Function of Different Platinum Shells

We performed a systematic experimental and theoretical analysis of the oxygen reduction reaction (ORR) activity and durability of Pt/Ir catalysts featuring different platinum shells. Four Pt/Ir catalysts with different platinum monolayers (ML) on an iridium core were synthesized. The nanostructure and composition of the catalysts were studied using scanning transmission electron microscopy with energy–dispersive X–ray spectroscopy, X–ray photoelectron spectroscopy, and scanning electron microscopy with energy–dispersive X–ray spectroscopy. ORR activity and catalyst durability were studied using cyclic voltammetry with rotating disk electrode. Density functional theory calculations were performed to estimate the ORR activity of Pt(111) and nML Pt/Ir(111) surfaces (n = 0, 1, 2). Although the specific ORR activities of the synthesized Pt/Ir catalysts were lower or comparable to those of 50%Pt/C, the mass activities were higher due to the enhanced utilization of platinum. Accelerated stress tests (ASTs) revealed that the durability of 1 ML Pt/Ir surpassed that of the other studied catalysts. The factors influencing the trends in specific and mass activities, durability, and the feasibility of implementing a Pt/Ir system in practical proton exchange membrane fuel cells (PEMFCs) are discussed.

Experimental Study on the Electrochemical Performance of PEMFC under Different Assembly Forces

Proton exchange membrane fuel cell (PEMFC) is of paramount significance to the development of clean energy. The components of PEMFC are assembled using many pairs of nuts and bolts. The assembly champing bolt torque is critical to the electrochemical performance and mechanical stability of PEMFC. In this paper, a PEMFC with the three-channel serpentine flow field was used and studied. The different assembly clamping bolt torques were applied to the PEMFC in three uniform assembly bolt torque and six non-uniform assembly bolt torque conditions, respectively. And then, the electrochemical performance experiments were performed to study the effect of the assembly bolt torque on the electrochemical performance. The test results show that the assembly bolt torque significantly affected the electrochemical performance of the PEMFC. In uniform assembly bolt torque conditions, the maximal power density increased initially as the assembly bolt torque increased, and then decreased on further increasing the assembly torque. It existed the optimum assembly torque which was found to be 3.0 N·m in this work. In non-uniform assembly clamping bolt torque conditions, the optimum electrochemical performance appeared in the condition where the assembly torque of each bolt was closer to be 3.0 N·m. This could be due to the change of the contact resistance between the gas diffusion layer and bipolar plate and mass transport resistance for the hydrogen and oxygen towards the catalyst layers. This work could optimize the assembly force conditions and provide useful information for the practical PEMFC stack assembly.

Improving Oxygen Reduction Reaction Activity through Defect Engineering of Atomically Dispersed Iron Electrocatalysts for Proton Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen-coordinated iron catalysts(Fe-N-Cs) have the potential as an alternative to platinum-group metal catalysts for the oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-N-Cs remain unsatisfactory. To address this issue, a defect engineering strategy has been developed to prepare high-performance PEMFC MEAs using atomically dispersed Fe-N-C catalysts. This strategy involves the use of a zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation to create atomically dispersed iron sites with a controlled number of defects. By adjusting the extent of defect formation in the carbon plane using CO2 activation, it is anticipated that changes in the oxidation state and spin state of the Fe center will modify the electronic structure of the Fe-N4 active sites. The Fe-N-C species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2SO4. Fine-tuning the number of defects can be optimized the ORR activity by adjusting the contribution of the Fe d-orbitals to the reaction intermediate binding energies. The resulting MEA based on the defect-engineered Fe-NC catalyst exhibits remarkable peak power densities in both H2/O2 and H2/air fuel cells, making it one of the most active atomically dispersed catalyst materials at the MEA level.

(Invited) Durable and Highly Active Fe-N-C Catalysts for PEM Fuel Cells

Atomically dispersed and nitrogen-coordinated single iron sites (FeN4) embedded in carbon (Fe-N-C) catalysts are the most promising platinum group metal (PGM)-free catalysts. However, they have yet to match their Pt counterparts for oxygen reduction reaction (ORR) activity and stability in proton exchange membrane fuel cells (PEMFCs). Here, we developed viable Fe-N-C catalysts, which, for the first time, demonstrated competitive activity to that of Pt/C catalysts and dramatically enhanced stability and durability under practical PEMFC operating conditions. The most active Fe-N-C catalyst achieved a record half-wave potential (E1/2 = 0.915 V vs. RHE at 0.6 mgcatcm-2) and an ORR mass activity of 10.5 mA mgcat at 0.9 V in (RDE) tests, exceeding a Pt/C baseline catalyst (60 µgPt cm-2) by 40 mV in acidic electrolytes. This compelling activity of the Fe-N-C catalyst in aqueous acids on rotating disk electrode (RDE) was successfully transferred to fuel cell membrane electrode assembly (MEAs), generating an initial current density of 44.2 mA cm-2 exceeding the U.S. DOE 2025 target (i.e., 44 mA cm-2) at 0.9 VIR-free under O2. Under practical hydrogen-air conditions, record 151 mA cm-2 at 0.8 V and peak power density of 601 mW cm-2 were achieved. Importantly, we discovered that depositing nitrogen-carbon species on the catalyst surface via chemical vapor deposition (CVD) dramatically enhanced catalyst stability, evidenced by performance durability after accelerated stress tests (30 000 square-wave voltage cycles under H2/air) and long-term steady-state life tests (> 300 hours at 0.67 V). Innovative identical location-scanning transmission electron microscopy (IL-STEM) experiments confirmed that the CVD process leads to deposition of nitrogen-doped carbon onto the catalyst surfaces. Along with theoretical modeling, a reconstruction of the carbon structure adjacent to FeN4 sites leads to increased robustness against demetallation and carbon oxidation. This work opens new avenues for developing earth-abundant iron-based catalysts with extraordinary activity and stability, thus competing with Pt and addressing the cost barrier of current PEMFCs.

Sulfur doped iron-nitrogen-hard carbon nanosheets as efficient and robust noble metal-free catalysts for oxygen reduction reaction in PEMFC

Transition metal-nitrogen-carbon (M-N-C) as a promising substitute for the conventional noble metal-based catalyst still suffers from low activity and durability for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). To tackle the issue, herein, a new type of sulfur-doped iron-nitrogen-hard carbon (S-Fe-N-HC) nanosheets with high activity and durability in acid media were developed by using a newly synthesized precursor of amide-based polymer with Fe ions based on copolymerizing two monomers of 2, 5-thiophene dicarboxylic acid (TDA) as S source and 1, 8-diaminonaphthalene (DAN) as N source via an amination reaction. The as-synthesized S-Fe-N-HC features highly dispersed atomic FeNx moieties embedded into rich thiophene-S doped hard carbon nanosheets filled with highly twisted graphite-like microcrystals, which is distinguished from the majority of M-N-C with soft or graphitic carbon structures. These unique characteristics endow S-Fe-N-HC with high ORR activity and outstanding durability in 0.5 M H2SO4. Its initial half-wave potential is 0.80 V and the corresponding loss is only 21 mV after 30,000 cycles. Meanwhile, its practical PEMFC performance is a maximum power output of 628.0 mW cm−2 and a slight power density loss is 83.0 mW cm−2 after 200-cycle practical operation. Additionally, theoretical calculation shows that the activity of FeNx moieties on ORR can be further enhanced by sulfur doping at meta-site near FeN4C. These results evidently demonstrate that the dual effect of hard carbon substrate and S doping derived from the precursor platform of amid-polymers can effectively enhance the activity and durability of Fe-N-C catalysts, providing a new guidance for developing advanced M-N-C catalysts for ORR.

Insight into Defect Engineering of Atomically Dispersed Iron Electrocatalysts for High-Performance Proton Exchange Membrane Fuel Cell.

Atomically dispersed and nitrogen coordinated iron catalysts (Fe-NCs) demonstrate potential as alternatives to platinum-group metal (PGM) catalysts in oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-NCs remain unsatisfactory. Herein, improved MEA performance is achieved by tuning the local environment of the Fe-NC catalysts through defect engineering. Zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation is employed to construct atomically dispersed iron sites with a controlled defect number. The Fe-NC species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83V in 0.5M H2 SO4 . Variation in the number of defects allows for fine-tuning of the reaction intermediate binding energies by changing the contribution of the Fe d-orbitals, thereby optimizing the ORR activity. The MEA based on a defect-engineered Fe-NC catalyst is found to exhibit a remarkable peak power density of 1.1W cm-2 in an H2 /O2 fuel cell, and 0.67W cm-2 in an H2 /air fuel cell, rendering it one of the most active atomically dispersed catalyst materials at the MEA level. This article is protected by copyright. All rights reserved.

Numerical Study on the Influence of Variable Section-Enhanced Mass Transfer Flow Field Design on PEMFC Performance Based on Underrib Convection Flow

The optimal design of variable section is a flow field optimization method that changes the pressure of the reacting gas in the flow channel by setting up a contraction surface in the flow channel, which is effective in improving many aspects of the proton exchange membrane fuel cell (PEMFC) performance. However, there are also problems such as uneven oxygen distribution in some of the flow channels, and the current density and net output power are not significantly improved. To find a balance among gas transfer efficiency, drainage capacity, uniformity of current density distribution, and output performance, this paper investigates the design of a variable cross-section-enhanced mass transfer flow field for PEMFC flow channel contraction by establishing a three-dimensional numerical model, starting from the sidewall width direction and the bottom height direction of the flow channel cross-section dimensions, in order to assess the effects of variable cross-section contraction at the bottom and sidewalls of the flow channel on the cell performance. The polarization curves, mass transfer capacity, current density distribution, and net output power of the variable cross-section contraction flow field are analyzed and compared with the straight flow field. On this basis, the optimum size parameters and operating parameter combinations are determined. Variable cross-sectional contraction flow field improves PEMFC output performance. The contraction of the variable cross-section contraction flow field at the sidewalls effectively improves reactive gas transfer efficiency and drainage capacity, improves current density distribution, and increases the net output power of the PEMFC. In particular, the sidewall contraction flow field has a 9.0% increase in power density, a 6.45% increase in the effective mass transfer coefficient (EMTC), a 6.52% increase in uniformity of current density distribution, and a 6.05% net power increment compared to the straight flow field, respectively. The best output performance, the highest oxygen concentration, and the lowest water concentration are achieved with a combination of parameters: number of sidewall contractions N = 6 , length L f = 6 mm , and width W f = 1.2 mm . And a wide range of air stoichiometry ratios are applied when the contraction of the variable cross-section contraction flow field is located at the sidewalls. Increasing the cathode stoichiometry ratio from 10 to 50 increased the cell power density by 25.5% and from 50 to 90 increased the cell power density by 9.5%. The simulation results can provide theoretical basis for the application of variable section flow field design for practical PEMFC.

Surface Anion Promotes Pt Electrocatalysts with High CO Tolerance in Fuel-Cell Performance.

Platinum reaches considerable activity and stability as an electrocatalyst but is not always capable of maintaining such performance under CO poisoning, particularly in CO residual fuels for practical proton-exchange membrane fuel cells (PEMFCs). In this work, we report that surface anions including a series of nonmetal elements on Pt nanoparticles result in outstanding CO tolerance for electrocatalysts in fuel cells. In particular, phosphorus surface-anion-modified Pt (denoted as P-Pt) possesses more than 10-fold enhancement of CO tolerance (only 8.4% decay) than commercial Pt/C, which can serve as a robust electrocatalyst both in CO poisoning half cells and full cells. Moreover, the general mechanism and principle were proposed, stating that surface anions should be selected preferentially to offer electron feedback to downshift the d-band center for the Pt surface, successfully weakening CO adsorption and leading to high-tolerance capability. We anticipate that surface anions on a Pt surface can bring robust electrocatalysts for practical PEMFCs and offer novel insights for high-performance Pt-based electrocatalysts.

Degradation prediction of proton exchange membrane fuel cell using auto-encoder based health indicator and long short-term memory network

As durability of proton exchange membrane fuel cell (PEMFC) remains as the main obstacle for its larger scale commercialization, predicting PEMFC degradation progress is thus an effective way to extend its lifetime. To realize reliable prediction, a novel health indicator (HI) extraction method based on auto-encoder is proposed in this paper, with which PEMFC future voltage can be predicted by long short-term memory network (LSTM). The effectiveness and robustness of proposed approach is investigated with test data simulating vehicle operation conditions, and accurate prediction performance can be observed, with the maximum root mean square error (RMSE) of 0.003513. Moreover, by comparing with two commonly prognostic methods including attention-based gated recurrent unit network and polarization model-LSTM, the proposed method can provide better predictions under various operating conditions. Furthermore, with the proposed method, the degradation mechanism of PEMFC can also be analyzed. Therefore, the proposed prognostic method can predict reliable PEMFC degradation progress and its corresponding degradation mechanisms, which will be beneficial in practical PEMFC systems for taking appropriate strategies to guarantee PEMFC durability.

Simulation study on magnetic field distribution of PEMFC

As a promising next-generation energy source, proton exchange membrane fuel cell (PEMFC) still suffers from durability and reliability issues, where PEMFC performance will decay during its operation. In this study, a three-dimensional, multi-component and multi-physics PEMFC model is developed to investigate the effect of PEMFC performance degradation on its external magnetic field. By comparing simulation results and experimental data, the capability of the developed model in simulating magnetic field due to PEMFC current is demonstrated. With developed model, different PEMFC degradation mechanisms, including flooding, dehydration, PEMFC aging are simulated, and the amplitudes and distributions of magnetic field under different mechanisms are investigated. Moreover, considering local defects may happen in practical PEMFC systems, its influence on magnetic field distribution is also studied. From the results, the correlation between PEMFC performance degradation and its magnetic field distribution is clarified, which will be beneficial for researches utilizing PEMFC magnetic field for analyzing PEMFC performance variation.

Durable and High-Power Iron-Based Cathodes for Proton-Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen-coordinated single iron sites (FeN4) embedded in carbon (Fe-N-C) catalysts are the most promising platinum group metal (PGM)-free catalysts. However, they have yet to match their Pt counterparts for oxygen reduction reaction (ORR) activity and stability in proton exchange membrane fuel cells (PEMFCs). Here, we developed viable Fe-N-C catalysts, which, for the first time, demonstrated competitive activity to that of Pt/C catalysts and dramatically enhanced stability and durability under practical PEMFC operating conditions. The most active Fe-N-C catalyst achieved a record half-wave potential (E1/2 = 0.915 V vs. RHE at 0.6 mgcatcm-2) and an ORR mass activity of 10.5 mA mgcat at 0.9 V in (RDE) tests, exceeding a Pt/C baseline catalyst (60 µgPt cm-2) by 40 mV in acidic electrolytes. This compelling activity of the Fe-N-C catalyst in aqueous acids on rotating disk electrode (RDE) was successfully transferred to a fuel cell membrane electrode assemblies (MEAs), generating an initial current density of 44.2 mA cm-2 exceeding the U.S. DOE 2025 target (i.e., 44 mA cm-2) at 0.9 VIR-free under O2. Under practical hydrogen-air conditions, record 151 mA cm-2 at 0.8 V and peak power density of 601 mW cm-2 were achieved. Importantly, we discovered that depositing nitrogen-carbon species on the catalyst surface via chemical vapor deposition (CVD) dramatically enhanced catalyst stability, evidenced by performance durability after accelerated stress tests (30 000 square-wave voltage cycles under H2/air) and long-term steady-state life tests (> 300 hours at 0.67 V). Innovative identical location-scanning transmission electron microscopy (IL-STEM) experiments confirmed that the CVD process leads to deposition of nitrogen-doped carbon onto the catalyst surfaces. Along with theoretical modeling, a reconstruction of the carbon structure adjacent to FeN4 sites leads to increased robustness against demetallation and carbon oxidation. This work opens new avenues for developing earth-abundant iron-based catalysts with extraordinary activity and stability, thus competing with Pt and addressing the cost barrier of current PEMFCs.

Evaluation Method for Feature Selection in Proton Exchange Membrane Fuel Cell Fault Diagnosis

Considering the fact that various features can be used in proton exchange membrane fuel cell (PEMFC) fault diagnosis, while the lack of feature evaluation method brings great difficulty in selecting appropriate features at practical PEMFC applications, a generalized feature evaluation and selection method is urgently required in PEMFC fault diagnosis. This article proposes a novel feature evaluation method, where feature discrimination capacity and robustness are evaluated. With the proposed method, features providing accurate and consistent diagnostic performance can be determined. In this study, features widely used in existing PEMFC fault diagnosis are utilized, which are extracted from either PEMFC voltage or multisensor signals, and their effectiveness in identifying faults at different PEMFC systems is investigated. Results demonstrate that with the proposed evaluation method, available features from various PEMFC test data can be ranked based on their diagnostic results. From the findings, appropriate features for PEMFC fault diagnosis can be determined. Moreover, early stage PEMFC faults can also be distinguished with high ranking features. This will be beneficial in practical PEMFC systems, where mitigation strategies can be taken to remove the effect due to early stage faults.

Optimal model evaluation of the proton-exchange membrane fuel cells based on deep learning and modified African Vulture Optimization Algorithm

ABSTRACT A new optimized design of a hybrid AlexNet/Extreme Learning Machine (ELM) network to provide an optimal identification tool for the Proton-exchange membrane fuel cells (PEMFCs) is presented in this study. The major concept is to reduce the error amount between the empirical output voltage and the evaluated output voltage of the PEM fuel cell stack model using the proposed hybrid AlexNet/ELM. For enhancing the model formation of the AlexNet/ELM, a modified version of the African Vulture Optimization (MAVO) Algorithm, which is a new metaheuristic, is suggested. To analyze the efficiency of the suggested method, it is applied to a practical PEMFC benchmark case study for identification purposes. Then, the method is confirmed by comparison of the experimental data and standard AlexNet/ELM. The achievements indicated the better confirmation of the suggested AlexNet/ELM network with the experimental data. The results show that the highest relative error for training and test is 0.03% and 0.05342%, respectively, which shows a promising result for the study.

Efficient and durable gas diffusion electrode for proton exchange membrane fuel cell via in-situ growth of Pt nanowires on dual microporous layer

Catalyst layer (CL) is the key factor of affecting the performance of proton exchange membrane fuel cells (PEMFCs). Herein, a facile and universal strategy is proposed to in situ grow Pt nanowires (Pt-NWs) on a dual microporous layer (MPL) as a novel CL for PEMFCs application. The dual MPL is composed of hydrophobic layer and hydrophilic layer, where the former facilitates mass transport and the latter favors the in-situ growth of Pt-NWs. During the whole preparation, only formic acid is used as reducing agent, no surfactants or templates are involved. The as-prepared Pt-NWs are uniformly grown on the surface of carbon particles to form a unique CL. The effect of dual MPL structure on PEMFCs performance is investigated, by which an optimal carbon powder content ratio (4:1) between hydrophobic layer and hydrophilic layer is determined. The single cell tests reveal that the Pt-specific power of the resultant MEA with 0.108 mg cm−2 Pt-NWs is up to 7.23 W mg−1, about twice higher than that for commercial Pt/C MEA with 0.2 mg cm−2 Pt loading. Furthermore, the resulted GDE shows enhanced durability compared to commercial one, indicating the good potential of in-situ prepared Pt-NWs electrode for practical PEMFC applications.

Accurate parameters extraction of PEMFC model based on metaheuristics algorithms

The electro-chemical proton exchange membrane fuel cell (PEMFC) is an inventing electrical generator from chemical reaction process as a green energy source. An accurate PEMFC model with its precise parameters should be used to carefully fitting of polarization curve to best study and design of its characteristics and performance. This paper introduces an accurate PEMFC model based on recent metaheuristics algorithms to evaluate precisely the unknown parameters of PEMFC. Algorithms of; Whale Optimization Algorithm (WOA) , Weighted Differential Evolution Algorithm (WDE) , Differential evolution algorithm with strategy adaptation (SADE) , Moth- Flame Optimization Algorithm (MFO) , adaptive differential evolution with optional external archive (JADE), Improved mine blast algorithm (IMBA), Gray Wolf Optimizer (GWO), Dragonfly algorithm (DA), Differential EVOLUTION ALGORITHM (DE) , Cumulative Population Distribution Information in Differential Evolution (CPIJDE) , Differential evolution based on covariance matrix learning (COBIDE) , Covariance Matrix Adaptation Evolution Strategy (CMA-ES) , Bernstain-search differential evolution algorithm (BSD), Backtracking Search Optimization Algorithm (BSA) , Bezier Search Differential Evolution Algorithm (BESD ) , DIFFERENTIAL SEARCH ALGORITHM (DSA) and Bijective DSA (B-DSA) , Biogeography-based optimization (BBO); have been applied to estimate model of PEMFC. The verification of the suggested optimizing algorithms is applied on three practical PEMFC stacks of BCS 500-W PEM, 500 W SR-12PEM and 250 W stacks, for different operating conditions. The accuracies of the PEMFC extracted parameters are measured in sum of square errors (SSE) between the results obtained by the optimizing parameters and the test results of the fuel cell stacks in the objective function. Also, the applied methods have been validated as compared results with different research works that were listed in literatures. Moreover, the polarization curves of the applied methods are clear and coinciding with manufacturing polarization curves for all the case study results. So, the suggested PEMFC optimizing model has superiority on the comparative models with respect to the system accuracy and convergence process.

Engineering Pt Nanoparticles onto Resin‐Derived Iron and Nitrogen Co‐Doped Porous Carbon Nanostructure Boosts Oxygen Reduction Catalysis

AbstractThe high cost and deficiency of long‐term durability of carbon supported Pt catalysts have been regarded as obstacle that impeding their practical proton exchange membrane fuel cells (PEMFCs) technologies. Furtherly, iron and nitrogen co‐doped carbons (Fe/NC) are emerging as a steady electrocatalyst. Unfortunately, there are less‐active sites in acidic electrolyte. Herein, combining the advantages of Fe/NC and Pt catalysts, we propose an in‐situ surfactant‐free self‐templating solution reduction strategy to construct Pt nanoparticles (NPs) onto well‐devised Fe/NC porous nanostructures in order to manufacture highly performing Pt−Fe/NC catalyst for oxygen reduction reaction (ORR). The obtained Pt−Fe/NC catalyst displays excellent ORR performance, realizing an account of 2.53 improvement in specific activity and an account of 1.20 strengthening in mass activity toward ORR compared to the commercial Pt/C catalysts. Stability tests confirm that the as‐synthesized Pt−Fe/NC catalyst is more electrochemically steady than the commercial Pt/C catalysts on account of the electronic interaction between Pt and N which result from the difference in electronegativity. Our work provides a promising strategy for exploring the accomplishment of better ORR in acid electrolytes.

N, P, and S tri-doped holey carbon as an efficient electrocatalyst for oxygen reduction in whole pH range for fuel cell and zinc-air batteries

Although, metal-free electrocatalysts have been exhibiting attractive oxygen reduction reaction (ORR) performance comparable to Pt/C in alkaline electrolytes, their activity in acidic and neutral mediums is unsatisfactory, hindering their widespread application in energy devices, such as proton exchange membrane fuel cells (PEMFCs), neutral metal-air batteries, and biofuel cells. Herein, N,P,S tri-doped holey carbon (NPS-HC) nanomaterials have been prepared using a one-step method with large specific surface area (1656.0 m2 g−1), abundant holes and edges, and high heteroatom content (7.57 at.%). The optimized NPS-HC exhibits outstanding ORR performance in universal pH mediums, being closely comparable to the commercial Pt/C. The NPS-HC is also utilized as air cathode in alkaline (peak power density: 286.6 mW cm−2 @ 488.1 mA cm−2) and neutral (85.7 mW cm−2 @ 228.8 mA cm−2) electrolytic Zn-air batteries as well as the ORR cathode in a practical PEMFC (275.1 mW cm−2) with comparable or superior performances than the previous reports. Such excellent electrocatalytic performance is attributed to the N,P,S tri-doping induced synergistic charge transfer and spin redistribution along with the hierarchical holey structure with exposed active sites and facilitated mass transport. Thus, this earth-abundant carbon-based nanomaterial holds great potential for high-performance pH-universal ORR catalysis in various energy-related technologies.

Highly Accessible Atomically Dispersed Fe-N x Sites Electrocatalyst for Proton-Exchange Membrane Fuel Cell.

Atomically dispersed transition metal‐Nx sites have emerged as a frontier for electrocatalysis because of the maximized atom utilization. However, there is still the problem that the reactant is difficult to reach active sites inside the catalytic layer in the practical proton exchange membrane fuel cell (PEMFC) testing, resulting in the ineffective utilization of the deeply hided active sites. In the device manner, the favorite structure of electrocatalysts for good mass transfer is vital for PEMFC. Herein, a facile one‐step approach to synthesize atomically dispersed Fe‐Nx species on hierarchically porous carbon nanostructures as a high‐efficient and stable atomically dispersed catalyst for oxygen reduction in acidic media is reported, which is achieved by a predesigned hierarchical covalent organic polymer (COP) with iron anchored. COP materials with well‐defined building blocks can stabilize the dopants and provide efficient mass transport. The appropriate hierarchical pore structure is proved to facilitate the mass transport of reactants to the active sites, ensuring the utilization of active sites in devices. Particularly, the structurally optimized HSAC/Fe‐3 displays a maximum power density of up to 824 mW cm−2, higher than other samples with fewer mesopores. Accordingly, this work will offer inspirations for designing efficient atomically dispersed electrocatalyst in PEMFC device.

Analysis of a combined proton exchange membrane fuel cell and organic Rankine cycle system for waste heat recovery

ABSTRACT Proton exchange membrane fuel cell (PEMFC) has become a promising alternative power source due to its quiet operation, low emission, and high theoretical energy conversion efficiency. However, a relatively large fraction of the energy (around 40 ~ 50%) is dissipated into the ambient environment by the cooling system, leading to a waste of thermal energy. The present study aims to improve the practical PEMFC performance by harvesting the thermal energy from the cooling system utilizing an organic Rankine cycle (ORC) system. The R134a and R245fa are selected as the working liquids for ORC to harvest energy from the PEMFC. A combined PEMFC and ORC model is established based on the first and second law thermodynamic analysis. It is found that the cycle efficiency (ηorc ) of R134a (about 11.33%) is higher than that of R245fa (about 9.16%). The combined system efficiency (ηsys ) for R134a can be increased by approximately 5.84%, compared with that of the fuel cell stack alone. For R134a, the effect of changes in the operating temperature and current density of the PEMFC stack on its cycle efficiency (ηorc ) is investigated. The results indicate that as the working temperature of the PEMFC stack increases, both the ORC and combined system efficiencies (ηorc and ηsys ) increase, while with the increase of the current density of the PEMFC, the ORC cycle efficiency (ηorc ) remains almost constant, and the combined system efficiency (ηsys ) decreases. The exergy efficiency is found to have the same variation trends with the energy efficiency.

Impact of Solvent on Catalyst Layer for Anion Exchange Membrane Fuel Cell

Anion exchange membrane fuel cell (AEMFC) is attractive energy conversion device that is an alternative to proton exchange membrane fuel cell (PEMFC) which required expensive materials, including platinum-based catalysts and perfluorosulfonic acid (PFSA) ionomers. Operating under the alkaline environment, enables the use of various types of low cost platinum-free-catalysts and the inexpensive metal stack hardware, allowing the system to be configured at a much lower cost. Over the past several years, the development of anion exchange membrane (AEM) and ionomer (AEI) with high OH- conductivity and kinetically fast water diffusivity has resulted in significantly improved power performance of AEMFC.However, the problem of water imbalance during AEMFC operation becomes a hindrance to achieving more higher performance. In the anode, one water per electron is generated by hydrogen oxidation reaction (HOR) and water is consumed by the oxygen reduction reaction (ORR) as a reactant in the cathode. Also, since the OH- migration from the cathode to the anode causes electro-osmotic drag and water molecules are transport to same direction, the difference of water content in both electrodes becomes larger. This water imbalance causes flooding in the anode and making it difficult to diffuse H2 gas to the Pt surface, moreover, water dry-out in the cathode inhibit the OH- conduction of AEIs. These series of processes, finally, reduce the Pt utilization due to the depletion of reactants. Considering the previous studies, despite using AEM and AEI with sufficient OH- conductivity, high-loading of platinum (0.4 mgPt cm-2 or more) was used to achieve power performance. Compared to practical PEMFC (0.2 mgPt cm-2 or less), the amount of platinum is too large, and this low Pt utilization problem is fatal drawback to AEMFC commercialization, which is aimed at lower prices.To improve the Pt utilization of AEMFC, the redox-active site, called the triple-phase-boundary, where the contact region of catalyst, H2 or O2 reactant gas, and OH- conducting ionomer, should be maximized in catalyst layer. In manufacturing a catalyst layer having a plenty of triple-phase-boundary, inducing a homogeneous ionomer distribution is one of the most important components. As a similar perspective, the research on the ionomer distribution has recently been conducted intensively at PEMFC field. The most representative way to control the ionomer distribution is tuning the interaction between the ionomer, solvent, and catalyst. In our previous study, the size of the ionomer aggregates was controlled by changing interaction between the solvent and ionomer using different solvents. As a result, the more homogeneous ionomer distribution in the cathode catalyst layer lead to the better performance due to the facile proton migration, however, nanoscale ionomer aggregates can induce the pore clogging with a severe mass transport resistance. However, AEMFC has not yet reported any papers related to ionomer distribution, even only a few papers studied about pore structure of the catalyst layer. Thus, in-depth study of the catalyst layer needs to be required and constructing optimized catalyst layer that containing homogeneous ionomer distribution with high porosity is absolutely necessary for efficient use of the catalyst.Herein, we presented a rational design of the catalyst layer by controlling the ionomer distribution and pore structure via solvent selection of the catalyst slurry. The analytical methods introduced in this study can be universally used for any AEIs and catalysts, and a strategy for preparing a catalyst layer with high Pt utilization considering the size of AEI and catalyst will greatly contribute to the cost reduction of AEMFC for commercialization. As the solvent of this study, dimethyl sulfoxide (DMSO), methanol (MeOH), and isopropanol (IPA) were selected, which have highly different dielectric constant and commonly used for making catalyst layer. Additionally, water was selected as a cosolvent to improve the dispersity of the catalyst slurry. By changing a solvent into higher dielectric constant, homogeneous ionomer distribution was constructed in catalyst layer, achieving high electrochemical surface area (ECSA) (72.1 m2 gPt-1) in the cathode. The selection of solvent for inducing a low absolute zeta-potential on the catalyst surface enables the formation of highly porous catalyst layer, which can result in improved a power performance in anode. For the first time, we observed nanomorphology of AEI dispersed on the carbon surface by scanning transmission electron microscopy (STEM). The structure and performance of the catalyst layer different by solvents were analyzed using electron microscope and electrochemical methods. Finally, we discuss the optimized catalyst layer design for AEMFC to provide a guideline for future direction.