Single-Crystalline Cathodes for Advanced Li-Ion Batteries: Progress and Challenges.

Single-crystalline cathodes are the most promising candidates for high-energy-density lithium-ion batteries (LIBs). Compared to their polycrystalline counterparts, single-crystalline cathodes have advantages over liquid-electrolyte-based LIBs in terms of cycle life, structural stability, thermal stability, safety, and storage but also have a potential application in solid-state LIBs. In this review, the development history and recent progress of single-crystalline cathodes are reviewed, focusing on properties, synthesis, challenges, solutions, and characterization. Synthesis of single-crystalline cathodes usually involves preparing precursors and subsequent calcination, which are summarized in the details. In the following sections, the development issues of single-crystalline cathodes, including kinetic limitations, interfacial side reactions, safety issues, reversible planar gliding and micro-cracking, and particle size distribution and agglomeration, are systematically analyzed, followed by current solutions and characterization techniques. Finally, this review is concluded with proposed research thrusts for the future development of single-crystalline cathodes.

High-Performance Zinc-Air Batteries Based on Bifunctional Hierarchically Porous Nitrogen-Doped Carbon.

Active and durable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the cathode are required for high-performance rechargeable metal-air batteries. Herein, the synthesis of hierarchically porous nitrogen-doped carbon (HPNC) with bifunctional oxygen electrocatalysis for Zn-air batteries is reported. The HPNC catalyst possesses a large surface area of 1459 m2 g-1 and exhibits superior electrocatalytic activity toward ORR and OER simultaneously with a low OER/ORR overpotential of 0.62 V, taking the difference between the potential at 10 mA cm-2 for OER and half-wave potential for ORR in 0.1 m KOH. Adopting HPNC as the air cathode, primary and rechargeable Zn-air batteries are fabricated. The primary batteries demonstrate a high open-circuit potential of 1.616 V, a specific capacity of 782.7 mAh gZn -1 and a superb peak power density of 201 mW cm-2 . The rechargeable batteries can be cycled stably for over 360 cycles or 120 h at the current density of 5 mA cm-2 . As elucidated by density functional theory, N-doping is preferred on defective sites with pentagon configuration and on the edge in the form of pyridinic-N-type. The high content of these two motifs in HPNC leads to the superior ORR and OER activities, respectively.

Investigation of non-precious metal cathode catalysts for direct borohydride fuel cells.

Borohydride crossover in anion exchange membrane (AEM) based direct borohydride fuel cells (DBFCs) impairs their performance and induces cathode catalyst poisoning. This study evaluates three non-precious metal catalysts, namely LaMn0.5Co0.5O3 (LMCO) perovskite, MnCo2O4 (MCS) spinel, and Fe-N-C, for their application as cathode catalysts in DBFCs. The rotating disk electrode (RDE) testing shows significant borohydride tolerance of MCS. Moreover, MCS has exhibited exceptional stability in accelerated durability tests (ADTs), with a minimal reduction of 10 mV in half-wave potential. DFT calculations further reveal that these catalysts predominantly adsorb over , unlike commercial Pt/C which preferentially adsorbs . In DBFCs, MCS can deliver a peak power density of 1.5 W cm-2, and a 3% voltage loss after a 5 hours durability test. In contrast, LMCO and Fe-N-C have exhibited significantly lower peak power density and stability. The analysis of the TEM, XRD, and XPS results before and after the single-cell stability tests suggests that the diminished stability of LMCO and Fe-N-C catalysts is due to catalyst detachment from carbon supports, resulting from the nanoparticle aggregation during the high-temperature preparation process. Such findings suggest that MCS can effectively mitigate the fuel crossover challenge inherent in DBFCs, thus enhancing its viability for practical application.

Review of Decompression Damage of the Polymer Liner of the Type IV Hydrogen Storage Tank.

The type IV hydrogen storage tank with a polymer liner is a promising storage solution for fuel cell electric vehicles (FCEVs). The polymer liner reduces the weight and improves the storage density of tanks. However, hydrogen commonly permeates through the liner, especially at high pressure. If there is rapid decompression, damage may occur due to the internal hydrogen concentration, as the concentration inside creates the pressure difference. Thus, a comprehensive understanding of the decompression damage is significant for the development of a suitable liner material and the commercialization of the type IV hydrogen storage tank. This study discusses the decompression damage mechanism of the polymer liner, which includes damage characterizations and evaluations, influential factors, and damage prediction. Finally, some future research directions are proposed to further investigate and optimize tanks.

Oxygen-deficient TiO2-x interlayer enabling Li-rich Mn-based layered oxide cathodes with enhanced reversible capacity and cyclability.

The unique anion redox mechanism of Li-rich Mn-based layered oxide (LMLO) cathodes endows them with a higher specific capacity compared with conventional cathodes. However, the irreversible anion redox reactions can cause structural degradation and sluggish electrochemical kinetics in the cathode, resulting in a poor electrochemical performance in the batteries. Thus, to address these issues, a single-sided conductive oxygen-deficient TiO2-x interlayer was applied on a commercial Celgard separator as a coating layer towards the LMLO cathode. After coating TiO2-x, the initial coulombic efficiency (ICE) of the cathode increased from 92.1% to 95.8%, the capacity retention improved from 84.2% to 91.7% after 100 cycles, and the rate performance of the cathode was significantly enhanced from 91.3 mA h g-1 to 203.9 mA h g-1 at 5C. Operando differential electrochemical mass spectroscopy (DEMS) showed that the coating layer could restrain the release of oxygen in the battery, especially from the initial formation process. The X-ray photoelectron spectroscopy (XPS) results demonstrated that the favorable oxygen absorption by the TiO2-x interlayer benefitted the suppression of side reactions and cathode structural evolution and favored the formation of a uniform cathode-electrolyte interphase on the LMLO cathode. This work provides an alternative path to address the issue of oxygen release in LMLO cathodes.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction.

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Advanced and Stable Metal-Free Electrocatalyst for Energy Storage and Conversion: The Structure-Effect Relationship of Heteroatoms in Carbon.

Ever-developing energy device technologies require the exploration of advanced materials with multiple functions. Heteroatom-doped carbon has been attracting attention as an advanced electrocatalyst for zinc-air fuel cell applications. However, the efficient use of heteroatoms and the identification of active sites are still worth investigating. Herein, a tridoped carbon is designed in this work with multiple porosities and high specific surface area (980 m-2 g-1). The synergistic effects of nitrogen (N), phosphorus (P), and oxygen (O) in micromesoporous carbon on oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysis are first investigated comprehensively. Metal-free N-, P-, and O-codoped micromesoporous carbon (NPO-MC) exhibits attractive catalytic activity in zinc-air batteries and outperforms a number of other catalysts. Combined with a detailed study of N, P, and O dopants, four optimized doped carbon structures are employed. Meanwhile, density functional theory (DFT) calculations are made for the codoped species. The lowest free energy barrier for the ORR can be attributed to the pyridine nitrogen and N-P doping structures, which is an important reason for the remarkable performance of NPO-MC catalyst in electrocatalysis.

Effects of Electrostatic Force and Network Structure on the Stability of Proton-Exchange Membrane Fuel Cell Catalyst Ink.

The stability of the catalyst slurry of a proton-exchange membrane fuel cell (PEMFC) is of great significance to its large-scale production and commercialization. In this study, three kinds of slurries with different stabilities were prepared using different probe ultrasonic powers. The influence of electrostatic force and network structure on slurry stability was also studied. In addition, the catalyst layer (CL) and membrane electrode assembly (MEA) were further tested to determine the relationship between slurry stability, CL, and MEA performance. The results showed that the slurry prepared with 600 W dispersion power had the least agglomeration on day 12, which is due to the clusters in the slurry having the smallest average particle size and the largest surface area, thereby allowing them to absorb the most Nafion and have the largest electrostatic force to inhibit agglomeration. However, the slurry with 1200 W dispersion power had the least sedimentation after 9.4 days because the strength of the network structure in the slurry strengthened the most, resulting in a significant increase in viscosity and inhibition of sedimentation. Electrochemical tests showed that the MEA gradually exhibited worse electrical performance and higher impedance due to the agglomeration of catalyst particles caused by the standing process. Altogether, this study provides insights to better understand and regulate the stability of catalyst slurries.

A Super Uniform Hydrophobic Gas Diffusion Layer for a Proton Exchange Membrane Fuel Cell.

The design and optimization of the gas diffusion layer (GDL) play a crucial role in the improvement of proton exchange membrane fuel cell performance. Hydrophobic treatment of a GDL is an important method for facilitating mass transfer, while conventional Teflon treatment is not uniform and leads to an increase in ohmic and heat resistance. Herein, a homogeneous molecular hydrophobic GDL was prepared by liquid phase synthesis, and a two-dimensional non-isothermal model was developed to investigate the transfer mechanism. The peak power density of cells with the GDL described above was improved by 46% compared to that of the conventional GDL. The ohmic and mass transport resistance decreased by 15% and 52%, respectively, under a current density of 1 A cm-2 using the uniform hydrophobic GDL. The simulation results proved that the uniform hydrophobic GDL eliminates the hydrophilic dots, which prevents the formation of water pools and reduces the resistance to gas flow. The water saturation of the conventional GDL reaches 0.19 at a current density of 1 A cm-2, and the saturation of a modified GDL under the same conditions is only 0.13. A dimensionless parameter, Tf, is proposed to characterize the resistance of oxygen diffusion. In conclusion, molecular-level uniform hydrophobic treatment can effectively reduce the ohmic and mass transfer resistance of a GDL and effectively improve the performance of fuel cells.

Effect of Microstructural Damage on the Thermomechanical Properties of Electrodes in Proton Exchange Membrane Fuel Cells.

Advanced functional materials composed of multiple nanoscale phases, including pores and interfaces, have been extensively applied in the fields of new energy, architecture, and aerospace. However, insufficient knowledge of the thermomechanical properties resulting from material failures, such as interfacial delamination and porosity deformations, which limit the durability and lifetime of these materials, has hindered their further application, demanding a deeper understanding of microstructural changes. Based on the fuel cell electrode, we explore a multiscale prediction model that correlates the atomic interactions between interfaces with a microscopic thermomechanical model to illuminate the effects of interface binding characteristics on the materials' mechanical response and heat conduction mechanisms. Compared with experimental measurements and theoretical calculations at the macroscopic scale, our model excels in predicting the initiation and propagation of interfacial debonding and the thermal conductivity of the electrode, with the resistance factors for the interface, pores, and cracks taken into consideration. This work provides guidance for designing robust electrodes resistant to thermomechanical failure and serves as a reference method for predicting damage in heterogeneous porous materials.

Influence of Degassing Treatment on the Ink Properties and Performance of Proton Exchange Membrane Fuel Cells.

Degradation occurs in catalyst inks because of the catalytic oxidation of the solvent. Identification of the generation process of impurities and their effects on the properties of HSC ink and LSC ink is crucial in mitigating them. In this study, gas chromatography-mass spectrometry (GC-MS) and cyclic voltammetry (CV) showed that oxidation of NPA and EA was the primary cause of impurities such as acetic acid, aldehyde, propionic acid, propanal, 1,1-dipropoxypropane, and propyl propionate. After the degassing treatment, the degradation of the HSC ink was suppressed, and the concentrations of acetic acid, propionic acid, and propyl propionate plummeted from 0.0898 wt.%, 0.00224 wt.%, and 0.00046 wt.% to 0.0025 wt.%, 0.0126 wt.%, and 0.0003 wt.%, respectively. The smaller particle size and higher zeta potential in the degassed HSC ink indicated the higher utilization of Pt, thus leading to optimized mass transfer in the catalyst layer (CL) during working conditions. The electrochemical performance test result shows that the MEA fabricated from the degassed HSC ink had a peak power density of 0.84 W cm-2, which was 0.21 W cm-2 higher than that fabricated from the normal HSC ink. However, the introduction of propionic acid in the LSC ink caused the Marangoni flux to inhibit the coffee ring effect and promote the uniform deposition of the catalyst. The RDE tests indicated that the electrode deposited from the LSC ink with propionic acid possessed a mass activity of 84.4 mA∙mgPt-1, which was higher than the 60.5 mA∙mgPt-1 of the electrode deposited from the normal LSC ink.

A Review of the Transition Region of Membrane Electrode Assembly of Proton Exchange Membrane Fuel Cells: Design, Degradation, and Mitigation.

As the core component of a proton exchange fuel cell (PEMFC), a membrane electrode assembly (MEA) consists of function region (active area), structure region, and transition region. Situated between the function and structure regions, the transition region influences the reliability and durability of the MEA. The degradation of the electrolyte membrane in this region can be induced by mechanical stress and chemical aggression. Therefore, prudent design, reliable and robust structure of the transition region can greatly help avoid early failure of MEAs. This review begins with the summarization of current structural concepts of MEAs, focusing on the transition region structures. It can be seen that aiming at better repeatability and robustness, partly or total integration of the materials in the transition region is becoming a development trend. Next the degradation problem at the transition region is introduced, which can be attributed to the hygro-thermal environment, free radical aggression, air pressure shock, and seal material decomposition. Finally, the mitigation approaches for the deterioration at this region are summarized, with a principle of avoiding the exposure of the membrane at the edge of the catalyst-coated membrane (CCM). Besides, durability test methods of the transition region are included in this review, among which temperature and humidity cycling are frequently used.

A High-Durability Graphitic Black Pearl Supported Pt Catalyst for a Proton Exchange Membrane Fuel Cell Stack.

Graphitized black pearl (GBP) 2000 supported Pt nanoparticle catalysts is synthesized by a formic acid reduction method. The results of a half-cell accelerated degradation test (ADT) of two protocols and a single-cell ADT show that, Pt/GBP catalyst has excellent stability and durability compared with commercial Pt/C. Especially, the survival time of Pt/GBP-membrane electrode assembly (MEA) reaches 205 min, indicating that it has better reversal tolerance. After the 1003-hour durability test, the proton exchange membrane fuel cell (PEMFC) stack with Pt/GBP presents a slow voltage degradation rate of 5.19% and 36 µV h-1 at 1000 mA cm-2. The durability of the stack is improved because of the durability and stability of the catalyst. In addition, the post morphology characterizations indicate that the structure and particle size of the Pt/GBP catalyst remain unchanged during the dynamic testing protocol, implying its better stability under dynamic load cycles. Therefore, Pt/GBP is a valuable and promising catalyst for PEMFC, and considered as an alternative to classical Pt/C.

Mitigation of Oxygen Evolution and Phase Transition of Li-Rich Mn-Based Layered Oxide Cathodes by Coating with Oxygen-Deficient Perovskite Compounds.

Li-rich Mn-based layered oxide cathodes with a high discharge capacity hold great promise for high energy density lithium-ion batteries. However, application is hampered by voltage and capacity decay and gas evolution during cycling due to interfacial side reactions. Here, we report coating by oxygen-deficient perovskite La0.9Sr0.1CoO3 using the Pechini process. X-ray photoelectron spectroscopy and scanning transmission electron microscopy both exhibit a uniform coating layer with a high oxygen vacancy concentration. The coating effectively mitigates the first cycle irreversible capacity loss and voltage decay while increasing cyclability. Optimized coating improves capacity retention from 55.6% to 84.8% after 400 cycles at 2 C. Operando differential electrochemical mass spectroscopy shows that such a coating can significantly mitigate the release of oxygen and carbon dioxide. Electrochemical impedance spectroscopy and post-mortem analysis indicate that the coating layer forms a stable interface and restricts structure evolution and cation mixing during cycling, conferring these cathode materials with better cycling and voltage stability. The perovskite can be applied to other cathodes with high voltage and capacity to suppress interfacial side reactions toward developing stable high energy density batteries.

Advanced Reversal Tolerant Anode in Proton Exchange Membrane Fuel Cells: Study on the Attenuation Mechanism during Fuel Starvation.

Fuel starvation at the anode of a proton exchange membrane fuel cell can lead to the increase of anode potential and the reversal of cell voltage followed by water electrolysis and carbon corrosion. A material-based approach (with high active water electrolysis catalysts) does not have much influence on the electrochemical performance, and carbon corrosion can be effectively avoided compared with the complex active control system. However, the membrane electrode assembly shows poor reversal tolerant performance during the hydrogen starvation test in previous studies, and the degradation mechanism is unclear. Therefore, reversal tolerant anode electrodes are designed in this article, and the voltage decrease mechanism is investigated comprehensively. The results exhibit that the increase of anode potential is mainly caused by the increase of mass transport resistance. Additionally, the voltage reversal time can be up to 5020 min, and the degradation rate of cell voltage at 1.2 A cm-2 can be as low as 0.12% h-1 after the first fuel starvation test.

Pre-Lithiation Strategies for Next-Generation Practical Lithium-Ion Batteries.

Next-generation Li-ion batteries (LIBs) with higher energy density adopt some novel anode materials, which generally have the potential to exhibit higher capacity, superior rate performance as well as better cycling durability than conventional graphite anode, while on the other hand always suffer from larger active lithium loss (ALL) in the first several cycles. During the last two decades, various pre-lithiation strategies are developed to mitigate the initial ALL by presetting the extra Li sources to effectively improve the first Coulombic efficiency and thus achieve higher energy density as well as better cyclability. In this progress report, the origin of the huge initial ALL of the anode and its effect on the performance of full cells are first illustrated in theory. Then, various pre-lithiation strategies to resolve these issues are summarized, classified, and compared in detail. Moreover, the research progress of pre-lithiation strategies for the representative electrochemical systems are carefully reviewed. Finally, the current challenges and future perspectives are particularly analyzed and outlooked. This progress report aims to bring up new insights to reassess the significance of pre-lithiation strategies and offer a guideline for the research directions tailored for different applications based on the proposed pre-lithiation strategies summaries and comparisons.

Mechanism and Model for Optimizing Polytetrafluoroethylene Distribution to Improve the Electrical and Thermal Conductivity of Treated Carbon Fiber Paper in Fuel Cells.

Employing polytetrafluoroethylene (PTFE)-treated carbon fiber paper (CFP) as the substrate of the gas diffusion layer (GDL) is a common practice to improve water management in proton exchange membrane fuel cells (PEMFCs), but the resulting increase in electrical and thermal resistance is a critical problem that restricts the performance output of PEMFCs. Hence, studying the mechanism and prediction model for both the electrical and thermal conductivity in CFP is essential. This work established a mathematical graph theory model for CFP electrical and thermal conductivity prediction based on the observation and abstraction of the CFP characteristic structures. For the PTFE-treated CFP, the electrical and thermal conductivity of CFP can be effectively increased by optimizing the PTFE distribution in CFP. A "filter net effect" mechanism was proposed to reasonably explain PTFE distribution's influence on the CFP performance. Finally, the equivalent effect of multiple factors on conductivity was revealed using contour maps, which provides inspiration for further reducing the electrical and thermal resistance in CFP.

Effect of Dispersion Solvents and Ionomers on the Rheology of Catalyst Inks and Catalyst Layer Structure for Proton Exchange Membrane Fuel Cells.

This study investigated the effects of the dielectric constant (ε) of a dispersion solvent and ionomer content on the rheology of graphitized carbon (GC)-supported Pt catalyst ink and the structure of catalyst layers (CLs). The ionomer dispersions and catalyst inks were tested using rheological techniques, zeta (ξ) potential, and dynamic light scattering measurements. Results showed that increases in the solvent ε or ionomer content increased the ξ-potential of catalyst particles in the ink, which reduced the catalyst agglomerate size. Steady-state and oscillation scans showed that the Pt/GC catalyst ink had shear-thinning properties and gel-like behavior. The ink with a solvent ε of 40 tended to be more Newtonian fluid, with low yield stress (σy). The ionomer content altered the rheology of the ink by changing the internal interaction of inks. Solvents with ε of 70 and 55 enhanced the adsorption of ionomers onto catalysts, thereby increasing the adhesion between ink particles and reducing the risk of CL cracking. As the ionomer content increased, the catalyst absorbed more ionomers in inks, increasing the fracture toughness of CLs, which reduced the crack width.

Enhanced PEMFC durability with graphitized carbon black cathode catalyst supports under accelerated stress testing.

The anti-corrosion properties of the carbon substrates of cathode catalysts play a vital role in the commercialization of fuel cell vehicles. Our report reveals the enhanced durability of graphitized carbon black catalyst substrates in polymer electrolyte membrane fuel cells (PEMFCs), tested under simulated start-stop cycling and high potential holding conditions. Graphitized carbon treated at various temperatures is used as the support for Pt catalysts. The catalyst utilizing graphitized carbon treated at 1800 °C demonstrates superior antioxidation properties and the inhibition of Pt particle coarsening. The decay ratio of the potential at 1000 mA cm-2 has been reduced from 34.9% (commercial Pt/C) to 0.5% during high potential holding accelerated stress testing. Correspondingly, the growth of Pt particles is reduced from 0.95 nm (commercial Pt/C) to 0.08 nm; that is, the coalescence of Pt particles is effectively alleviated upon using graphitized carbon black.

Synthesis of Anti-poisoning Spinel Mn-Co-C as Cathode Catalysts for Low-Temperature Anion Exchange Membrane Direct Ammonia Fuel Cells.

Low-temperature anion exchange membrane direct ammonia fuel cells (AEM-DAFCs) have emerged as a potential power source for transportation applications with the recognition that liquid ammonia is a carbon-free hydrogen carrier and facilitates storage, refill, and distribution. However, ammonia crossover from the cell anode to cathode can decrease the fuel efficiency, drop the voltage, and poison the cathode catalysts. In this work, the Mn-Co spinel on three different carbon supports [BP2000, Vulcan XC-72R, and multiwalled carbon nanotubes (MWCNTs)] has been successfully synthesized and demonstrated a high oxygen reduction reaction (ORR) activity with good ammonia tolerance. The structure and composition of the obtained Mn-Co-C catalysts were characterized by high-angle annular dark-field scanning transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All three catalysts exhibit superb ammonia tolerance, and Mn-Co-BP2000 demonstrates the highest ORR activity, even better than the commercial Pt-C in the presence of ammonia. When paired with the commercial PtIr-C anode, the Mn-Co-BP2000 cathode improved the peak power density of single cells from 100.1 mW cm-2 for the Pt-C cathode to 128.2 mW cm-2 under a 2 bar backpressure in both electrodes at 80 °C. All the results have manifested that Mn-Co-BP2000 is a good cathode catalyst for low-temperature AEM-DAFCs.