Anode Design Research Articles

Mechanically Robust Bismuth-Embedded Carbon Microspheres for Ultrafast Charging and Ultrastable Sodium-Ion Batteries.

Advancements in the development of fast-charging and long-lasting microstructured alloying anodes with high volumetric capacities are essential for enhancing the operational efficiency of sodium-ion batteries (SIBs). These anodes, however, face challenges such as declined cyclability and rate capability, primarily due to mechanical degradation reduced by significant volumetric changes (over 252%) and slow kinetics of sodium-ion storage. Herein, we introduce a novel anode design featuring densely packed bismuth (Bi) embedded within highly conductive carbon microspheres to overcome the aforementioned challenges. Remarkably, the high loading Bi anode within carbon microspheres with a high tap density of 2.59 g cm-3 possesses significant mechanical strength exceeding 590 MPa and limits volume swelling of only 10.9% post-sodiation. This anode demonstrates a high volumetric capacity (908.3 mAh cm-3), ultrafast chargeability (200 A g-1, full charge/discharge in just 5.5 s), and outstanding cyclability over 12,000 cycles and maintains exceptional cycling stability even at -30 °C. The full cell paired with a Na3V2(PO4)3 cathode retains over 80% capacity after 600 cycles at 36 C, demonstrating a remarkable rate capability of 126 C (full charge/discharge in 28.6 s). Our comprehensive experimental evaluations and chemo-mechanical simulations shed light on the mechanisms underpinning the anode's superior performance. This development marks a significant advancement in the design of durable and fast-charging anodes for high-performance SIBs.

Structural Design and Challenges of Micron-Scale Silicon-Based Lithium-ion Batteries.

Currently, lithium-ion batteries (LIBs) are at the forefront of energy storage technologies. Silicon-based anodes, with their high capacity and low cost, present a promising alternative to traditional graphite anodes in LIBs, offering the potential for substantial improvements in energy density. However, the significant volumetric changes that silicon-based anodes undergo during charge and discharge cycles can lead to structural degradation. Furthermore, the formation of excessive solid-electrolyte interphases (SEIs) during cycling impedes the efficient migration of ions and electrons. This comprehensive review focuses on the structural design and optimization of micron-scale silicon-based anodes from both materials and systems perspectives. Significant progress is made in the development of advanced electrolytes, binders, and conductive additives that complement micron-scale silicon-based anodes in both half and full-cells. Moreover, advancements in system-level technologies, such as pre-lithiation techniques to mitigate irreversible Li+ loss, have enhanced the energy density and lifespan of micron-scale silicon-based full cells. This review concludes with a detailed classification of the underlying mechanisms, providing a comprehensive summary to guide the development of high-energy-density devices. It also offers strategic insights to address the challenges associated with the large-scale deployment of silicon-based LIBs.

Mutual suppression of Mn3O4 and SiOx in an innovative anode design for enhanced cycling stability.

We designed a SiOx@C/Mn3O4 composite material with ultrafine particle size using a simple sol-gel method and calcination process. SiOx and Mn3O4 components produce a mutual suppression effect during the charge/discharge process to mitigate volume expansion and maintain the long-term stability of composite.

A Universal Design of Lithium Anode via Dynamic Stability Strategy for Practical All-solid-state Batteries.

All-solid-state Li-metal battery (ASSLB) chemistry with thin solid-state electrolyte (SSE) membranes features high energy density and intrinsic safety but suffers from severe dendrite formation and poor interface contact during cycling, which hampers the practical application of rechargeable ASSLB. Here, we propose a universal design of thin Li-metal anode (LMA) via a dynamic stability strategy to address these issues. The ultra-thin LMA (20 μm) is in-situ constructed with uniform highly Li-ion conductive solid-electrolyte interphase and composite-polymer interphase (CPI) via electroplating process. As a result, the passivation layer with poor Li-ion conduction on Li anode can be dissolved and small surface resistance can be achieved due to the good compatibility of CPI to SSEs. The cycling of Li symmetric cell with Li6PS5Cl thin film electrolyte (< 100 μm) shows a high critical current density of > 2.0 mA cm-2 with excellent cycling stability at 1.0 mA cm-2. The ASSLBs paring with Ni-rich LiNi0.6Mn0.2Co0.2O2 cathode demonstrated the feasibility of engineered LMA design by presenting good rate capability from 0.1C to 1.0 C at room temperature, as well as long-term cycling stability (81% retention after 100 cycles). This work represents a general pathway to make thin dendrite-free LMA available for high-energy-density ASSLBs.

A Three‐Tiered Golf Anode towards Ultralong‐Life Zn‐Mn Aqueous Batteries

Zn‐Mn aqueous batteries (ZMABs) are widely recognized as a promising candidate for large‐scale energy storage due to their cost‐effectiveness, high safety and environmental friendliness. However, the practical application of ZMABs is hindered by inherent electrical contact loss, hydrogen evolution and dendrite growth on traditional anodes. Here, a three‐tiered golf anode with high conductivity is developed to simultaneously enhance the reversibility of Zn and Mn metals. The three‐tiered golf anode is achieved by inner zinc powder, intermediate carbon and outer bismuth layer, which can effectively enhance the Gibbs free energy and mitigate the volume change of the anode, thus enabling stable Zn‐Mn cycling. As a proof of concept, Zn‐Mn full cells (coupled to CNT@MnO2) achieve an ultralong cycle life with 94.6% capacity retention at 2 A g−1 after 16,000 cycles. This study presents valuable insights into the structural design of anodes aimed at enhancing the stability and durability of ZMABs.

A Three-Tiered Golf Anode towards Ultralong-Life Zn-Mn Aqueous Batteries.

Zn-Mn aqueous batteries (ZMABs) are widely recognized as a promising candidate for large-scale energy storage due to their cost-effectiveness, high safety and environmental friendliness. However, the practical application of ZMABs is hindered by inherent electrical contact loss, hydrogen evolution and dendrite growth on traditional anodes. Here, a three-tiered golf anode with high conductivity is developed to simultaneously enhance the reversibility of Zn and Mn metals. The three-tiered golf anode is achieved by inner zinc powder, intermediate carbon and outer bismuth layer, which can effectively enhance the Gibbs free energy and mitigate the volume change of the anode, thus enabling stable Zn-Mn cycling. As a proof of concept, Zn-Mn full cells (coupled to CNT@MnO2) achieve an ultralong cycle life with 94.6% capacity retention at 2 A g-1 after 16,000 cycles. This study presents valuable insights into the structural design of anodes aimed at enhancing the stability and durability of ZMABs.

Theoretical investigation of cycling stability in TiS2 anode for zinc-ion rechargeable batteries

Abstract To advance the development of aqueous zinc-ion batteries (ZIBs), which are considered a promising alternative for sustainable energy storage, it is a crucial step to explore zinc-free metal anodes to prevent zinc-dendrite formation. Titanium disulfide (TiS2) has emerged as a potential anode material for ZIBs, due to its sizeable interlayer spacing and favorable electrochemical properties. However, its stability during battery cycles must be thoroughly investigated to assess its feasibility from a theoretical perspective. Herein, the first-principles calculations are performed to theoretically investigate the stability during the cycles of zinc-intercalated TiS2 (Zn x TiS2). Despite its formation energies lying above the thermodynamic convex hull, Zn x TiS2 demonstrates robust mechanical and dynamical stabilities over the entire range of zinc-ion intercalation, suggesting its ability to maintain structural integrity under cycling conditions of ZIBs. Our evaluation of structural, elastic, electrochemical, and electronic properties reveals significant changes during the zinc-ion intercalated process, such as the reduction of the open-circuit voltage and changes in the interlayer spacing. These findings indicate that while TiS2 shows promise as an anode material from a theoretical aspect, addressing the irreversible structural changes observed experimentally is essential. Our insights into the mechanism of zinc-ion intercalation in TiS2 provide valuable guidance for future design and optimization of zinc-free metal anodes.

Challenges and Strategies for Zinc Metal Anode

Aqueous zinc‐ion batteries (AZIBs) are received intensive attention for their inherent safety, economic feasibility, competitive electrochemical performance, and environmental friendliness. However, the rampant side reactions such as dendrite growth, hydrogen evolution, corrosion, and passivation of zinc metal anode seriously hinder the stability of AZIBs. This concept is aim to enhancing the practical viability of AZIBs, with a particular focus on the stability of zinc metal anodes. Here, we summeriazed the strategies for achieving high reversibility zinc metal anode, and points out the shortcomings that still exist in current research. Then the crucial role of regulation electric double layer (EDL) layer, zinc ion transfer number and electrolyte design is summerized in detail. Finally, we consider that the design of zinc anode should focus more on practicality and commercialization, and provide a perspective for the future development direction of zinc anode. We hope to provide an inspiration for the design of stable zinc metal anodes in AZIBs from this concept.

Unveiling Covalent Triazine Frameworks for Lithium Metal Anodes: Recent Developments and Prospective Advances.

Lithium metal batteries (LMBs) are distinguished by their elevated energy densities which represent themselves as the formidable contenders for the forthcoming generation of energy storage technologies. Nonetheless, their cycling efficiency is hindered owing to unregulated growth of lithium dendrites and unstable solid electrolyte interphase (SEI). This raises serious safety concerns while rendering LMBs unfeasible for real-world implementation. Covalent Triazine Frameworks (CTFs) have emerged as a promising class of 2D nanomaterials due to their unique properties such as high surface area, chemical stability, tailorable properties, porosity and high N-containing groups. These groups serve as an efficient acceptor for Li. Consequently, the problem of lithium dendrite formation is significantly reduced. This review offers an extensive examination of CTF based anode materials utilized to address the challenges associated with lithium dendrites in LMBs. It is outline future prospects and provide recommendations for the design and engineering of lithium metal anodes (LMAs) and architectures that can make LMBs viable for practical use. This review also highlights promising strategies for surmounting challenges to ensure the safety and efficiency of LMBs.

Enhancing Lithium-Ion Batteries with a 3D Conductive Network Silicon-Carbon Nanotube Composite Anode.

To meet the rising demand for energy storage, high-capacity Si anode-based lithium-ion batteries (LIBs) with extended cycle life and fast-charging capabilities are essential. However, Si anodes face challenges such as significant volume expansion and low electrical conductivity. This study synthesizes a porous spherical Si/Multi-Walled Carbon Nanotube (MWCNT)@C anode material via spray drying, combining Si nanoparticles, MWCNT dispersion, sucrose, and carboxymethyl cellulose (CMC). The MWCNT incorporation creates a robust 3D conductive network within the porous microspheres, enhancing Li+ diffusion and improving fast-charging/discharging performance. After 300 cycles at 1 A g-1, the material achieved a discharge capacity of 536.6 mA h g-1 with 80.5% capacity retention. Additionally, integrating a 3D network of Single-Walled Carbon Nanotubes (SWCNTs) further enhanced capacity retention in a binder-free, self-supporting electrode created through vacuum filtration. The Si/MWCNT@C//LiFePO4 full cell exhibited an initial Coulombic efficiency (ICE) exceeding 80%, with a specific capacity of 72.4 mA h g-1 and 79.8% capacity retention after 400 cycles at 1 A g-1. This study offers a promising strategy for improving the performance and structural design of Si anodes.

(Invited) Metal-Assisted Chemical Etch of Semiconductors: Fundamentals and Applications

Metal-Assisted Chemical Etch (MacEtch), an unorthodox semiconductor etching method, enables damage-free fabrication of semiconductor nanostructures with unprecedentedly high aspect ratios. By harnessing the intricate interplay of local catalysis and electron transfer effects at the metal-semiconductor-solution interface, MacEtch transcends the definition of conventional chemical etching. In this talk, I will introduce the MacEtch characteristics and present a systematic study of the ensemble electrochemical mechanisms that govern the photo-enhanced MacEtch process of wide and ultrawide bandgap semiconductors including GaN, SiC, and Ga2O3, as well as related heterojunctions. The effects of local cathode and anode design, energy-band alignments, and solution chemistry on MacEtch are correlated with the underlying electronic mechanisms of carrier generation, annihilation, transport, and extraction. I will also provide a few examples of device applications of MacEtch. First, nearly size-independent external quantum efficiency (EQE) for III-N µLED fabricated using the MacEtch approach is achieved, overcoming one of the biggest hurdles in µLED technology -- the efficiency degradation with shrinking pixel size, due to plasma-etching damage. Secondly, fabricated by MacEtch, smooth and high aspect ratio sidewall finFETs of β-Ga2O3, an emerging ultrawide bandgap semiconductor with a bandgap of ~ 4.8 eV, are demonstrated to have a record-low specific on-resistance and DC transfer characteristics with near zero hysteresis. We believe that the simplicity, versatility, and scalability of MacEtch will continue to catalyze a myriad of applications in electronics, photonics, energy, quantum technologies, and bio-sensing.

Design of high-capacity composite anode for next generation lithium-ion batteries

Ga is in primary focus for high-capacity anodes of Li-ion batteries (LIBs). Ga-based anode can form Li2Ga with Li at room temperature. Again, decoupling of Li and Ga is easy compared to other high-capacity materials like Si. Low melting temperature, volume changes, and particle agglomeration of Ga-based electrodes in lithiation and delithiation processes are serious challenges toward its commercialization. To overcome these obstacles, we propose a new composite using Mn-doped GaFeP engineered on N-rGO. Three Mn doping ratios (1, 5, and 10 %) were studied for optimization of electrochemical performance. We observed that the 5 % Mn-doped GaFeP/N-rGO delivered an impressive reversible capacity of 1099.9 mAh/g after 200 cycles at 0.3 A/g. Its retention rate capacity (97.6 %) was higher than those of other electrodes. Furthermore, an excellent rate performance and substantial long-term stability of 671.9 mAh/g was achieved at 1.5 A/g after 850 cycles. The reaction mechanism of the high-performance composite electrode is expounded. We hope our study provides new insights and guidelines to the development of Mn-doped Ga-based electrodes with high capacities, minimal volume change, and long cycle stabilities for next-generation LIBs.

Design of Li-Si Alloy Anodes for Enhancing Mechanical Stability and Electrochemical Performance of Sulfide-Based All-Solid-State Batteries

Li metal all-solid-state batteries, valued for their higher energy density and stability, face issues with lithium metal anodes, such as dendrite structures formation. To address these issues, alloy-based anodes such as Si, Al, and In are replacing lithium metal due to their high-capacity and lithiophilic characteristics. However, there is an issue of large volume change (~300 %) in the alloy anodes during the charge-discharge cycling Pre-lithiation, is considered one of the effective solutions to mitigate volume expansion in the alloy anode. In the present study, we prepared full cells consisting of LixSi (x = 1, 1.7, 2.3, 3.25, 4.4) anodes, argyrodite (Li6PS5Cl) electrolytes, and sulfur composite cathodes, to study the effect of the lithium concentration on the mechanical stability as well as electrochemical performance of sulfide based all-solid-state batteries. During the charge-discharge cycles, we measured the voltage profiles and electrochemical impedance spectra using an embedded Indium reference electrode. Microstructure of the solid electrolyte and distribution of lithium ions were analyzed using secondary electron microscope (SEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). High lithium concentrations (LixSi, x = 3.25, 4.4) exhibited high areal capacity ( ~ 3.5 mAh/cm2), however the non-uniform deposition of lithium on the anode surface during alloying led to the formation of lithium dendrite structures. For a low lithium concentration, LiSi demonstrates limited capacity due to a low lithium-ion diffusion coefficient. The medium concentration, Li1.71Si, showed comparable areal capacity to that of high lithium concentration and maintained high stability even under high cut-off voltage condition without dendrite formation. Thus, this study indicates that the medium lithium concentration (x = 1.71) alloy anode provides superior chemo-mechanical stability as well as comparable areal capacity, to the high lithium concentration (x = 3.25, 4.4).

Lithium, Ether, and Graphite: A Retrospective Trilogy of the Anode-Electrolyte Interface

Ethers represent a family of solvents widely applied in various battery chemistries, except in the most successful battery—Li-ion batteries (LIBs). It is believed that ether electrolytes are incompatible with graphite anodes in LIBs due to detrimental solvent co-intercalation and exfoliation. Here, we demonstrate that ether electrolytes at standard salt concentration can enable outstanding reversibility and fast-charge capability of graphite anodes in LIBs. In short, for ether electrolytes, the anion and the solid-electrolyte interphase (SEI) govern the electrochemical behaviors of graphite anodes. Firstly, we point out that exfoliation and co-intercalation are not always the root causes of the documented cell failures using graphite anodes. Matching the reductive stability between solvents and salts is key to resolving the interphase problem. Reductively unstable lithium bis(fluorosulfonyl)imide (LiFSI) is the optimal salt paired with 1,3-dioxolane (DOL), enabling weakened Li-solvent interaction, preferential reduction of the FSI anion, and a homogeneous SEI enriched with inorganic species. Consequently, electrolyte reduction and Li-DOL co-intercalation are inhibited. Secondly, reductively stable LiBF4 paired with dimethoxyethane (DME) realizes improved cyclability of graphite based on a cointercalation mechanism. We conducted a holistic investigation into the nature, function, and formation of the heterogeneous SEI. Specifically, we revealed a self-terminating, heterogeneous interphase based on grainy LiF located on the catalytically active edge plane of natural graphite. Thirdly, we discovered a novel cointercalation mechanism, namely, in-situ synthesis of ternary graphite-intercalation compounds in a new electrolyte. We characterize the progressive cointercalation via operando synchrotron X-ray analyses, supporting reduced volume expansion and high structural integrity of the cointercalated graphite. The resulting t-GIC chemistry delivers unprecedented fast charging (1 minute) and ultralong lifetimes exceeding 10,000 cycles. The full cells based on cointercalated graphite display remarkable cycling stability upon a 15-minute charging and excellent rate capability even at -40°C. In these three case studies combined as a trilogy with strong correlation, we clarify a long-standing confusing issue in the LIBs community—that is, the compatibility between ether electrolytes and graphite anodes. This trilogy summarizes and reflects on past experiences with the anode/electrolyte interface (mostly failures of ether electrolytes when cycling graphite anodes) dating back to the 1970s. Learning from history, we exemplify different intercalation chemistries of graphite based on DOL, DME, and another common ether solvent, paired with 1M lithium salts, all showing approximately 99.9% Coulombic efficiency, high capacity retention, and rapid interphase kinetics. Our findings not only shed light on the enigmatic interphase formed by ether electrolytes but also offer critical insights into future electrolyte design for graphite anodes operated under extreme conditions.

Long-life and highly stable Zn metal anodes achieved by Ag nano-coating via electron beam evaporation

Aqueous zinc-ion batteries (AZIBs) offer advantages such as high specific capacity, affordability, and minimal assembly environment requirements, positioning them as a promising candidate for next-generation energy storage system. However, the performance of the AZIBs is greatly influenced by dendrite formation and side reactions occurring on the surface of the zinc metal anode. In this study, a high-stability Zn@Ag anode material was prepared using electron beam evaporation. The presence of Ag coating lowers the nucleation overpotential for Zn and encourages its preferential growth on the (002) crystal face, thereby inhibiting dendrite formation. The Zn@Ag symmetric battery exhibits outstanding performance in Zn deposition and stripping, achieving 2070 h at a current density of 1 mA cm−2/1 mAh cm−2. Additionally, it shows stable cycling for 1580 h even at a higher current of 5 mA cm−2. Furthermore, the enhanced corrosion resistance and inhibition of hydrogen evolution reaction (HER) in Zn@Ag compared to bare Zn contribute to its superior stability. The full battery Zn@Ag||MnO2 exhibits higher capacity and superior long-term cycling performance than Zn||MnO2. This approach offers a potential solution for optimizing design of Zn anodes in AZIBs.

(Invited) Mechanisms Governing Lithium Metal and Alloy Anodes in Solid-State Batteries

Solid-state batteries offer the promise of improved energy density and safety compared to lithium-ion batteries, but electro-chemo-mechanical evolution and degradation of materials and interfaces can play an outsized role in limiting their performance. Here, I will present my group’s recent work on understanding structural evolution, interfacial dynamics, and chemo-mechanics in solid-state batteries with both lithium metal and alloy-based anodes. Lithium metal batteries are especially beneficial if used in an “anode-free” configuration in which there is no lithium initially present at the anode current collector. Using X-ray tomography, cryo-FIB, and finite-element modeling, we show that lithium metal anode-free solid-state batteries are intrinsically limited by current concentrations at the end of stripping due to localized lithium depletion. This causes accelerated short circuiting compared to lithium-excess cells. Based on these results, the beneficial influence of metal alloy interfacial layers on controlling lithium evolution and mitigating contact loss from localized lithium depletion will be introduced and discussed. In the second part of the talk, the characteristics of dense, foil-based alloy anodes are introduced. Alloy foils such as aluminum and tin are shown to exhibit improved interfacial stability and better long-term cycling in solid-state batteries compared to liquid-cell batteries due to reduced solid-electrolyte interphase formation. A new design for multiphase aluminum foil alloy anodes is introduced, in which microstructural control over the foil is shown to play a key role in significantly improving reversibility in solid-state batteries. This anode design mitigates chemo-mechanical degradation and offers a paradigm that does away with slurry coating, potentially reducing manufacturing costs. The influence of stack pressure on electrode evolution is investigated. Taken together, these findings show the promise of both lithium metal and alloy anodes for solid-state batteries, with divergent reaction mechanisms giving rise to different operating conditions necessary for each class of materials.

Rigid-Flexible Coupling Realized by Synergistic Engineering of the Graphitic-Amorphous Architecture for Durable and Fast Potassium Storage.

Graphite anodes hold great potential for potassium-ion batteries (PIBs), yet their practical application is hindered by poor cycle performance caused by substantial interlayer expansion. Herein, a partial graphitic carbon (PGC) is elaborately engineered via the catalytic effect of ferric citrate using pitch as a carbon precursor. Systematically varying the catalyst content enables an optimal PGC design integrating a highly graphitized phase providing abundant active sites for K-ion intercalation, balanced with an amorphous carbon region that accommodates volume expansion and facilitates ion diffusion. The optimized PGC12 electrode exhibits a high reversible capacity of 281.9mAhg-1, characterized by a prolonged low-potential plateau region, and excellent cycle stability with a capacity retention of 94.8% after 300 cycles. It also realizes an impressive rate capability with a retained capacity of 222.2mAhg-1 at 1C. Moreover, the assembled K-ion full-cell delivers an exceptional energy density of 148.2Whkg-1. In-situ XRD and DFT simulations further verify the distinct phase transition mechanisms and reaction dynamics across different carbon configurations. This work elucidates the impact of carbon configurations on K-storage performance and proposes a structural model for efficient K-ion storage, which is instrumental in the rational design and advancement of carbon anodes in PIBs.

Hybrid Electrolyte Strategies for High-Energy Sodium-Based Batteries

Sodium-based batteries are emerging as a promising and crucial technology for sustainable energy storage due to the abundance and cost-effectiveness of sodium compared to lithium. While these batteries offer an attractive option for large-scale applications, they face challenges such as lower voltage and energy density. To overcome these limitations and enhance high-energy sodium-based battery performance, researchers are exploring innovative hybrid electrolyte strategies. Hybrid electrolytes, which combine solid and liquid systems, provide improvements in safety, efficiency, and performance.This study explores various hybrid approaches, including organic-solid-aqueous hybrids, quasi-solid anolytes, and over-saturated catholytes. These methods aim to address the limitations of sodium-ion batteries and pave the way for high-performance energy storage solutions. For instance, an organic-solid-aqueous hybrid system lets different non-aqueous and aqueous materials adapt, which leads to wider voltage ranges. Additionally, using non-aqueous sodium metal anodes and aqueous ferrocyanide redox cathodes in these hybrid electrolyte systems demonstrates viable voltage ranges and reversible performance. Research on quasi-solid electrolytes showed potential in developing thicker anodes, while over-saturated catholyte research explored the use of solid cathode materials.The diverse hybrid electrolyte approaches have revealed promising possibilities for both aqueous and non-aqueous sodium-based batteries. These advancements enhance battery stability and durability, leading to higher voltage and energy density across various applications. Recent advancements in sodium anode and aqueous cathode designs have enabled the application of sodium-based batteries in different types, including seawater and sodium-flow batteries. One significant innovation is the use of NASICON ceramic electrolytes, which offer improved ionic conductivity and stability, allowing for further material adaptations and optimization of battery performance.In conclusion, the exploration of hybrid electrolyte strategies for sodium-based batteries represents a novel approach towards realizing the full potential of this technology. As research and development progress, sodium-based batteries with hybrid electrolytes could become foundational for sustainable energy storage, contributing to a cleaner and more efficient energy future.

Catalyst-Integrated Porous Transport Layer for Polymer Electrolyte Membrane Water Electrolysis

Introduction Widespread commercialization of polymer electrolyte membrane (PEM) water electrolyzers requires significant cost reduction. A large amount of platinum metal group (PGM) loading is a primary factor in increasing the electrolyzer cost1. In particular, Ir catalysts and Pt coating on the porous transport layer (PTL) are necessary for the anode. Our research group has developed the Ir catalyst-integrated PTL by increasing the surface area of Ti-based PTL with surface treatment and coating Ir on the PTL, as shown in Fig. 1 2–4. Ir catalysts directly deposited on the PTL also act as a protective coating to prevent Ti oxidation of the PTL. In other words, this alternative anode design has two kinds of oxygen evolution reaction (OER) sites: the catalyst layer and the Ir coating layer on PTL. Suppose the total amount of iridium in this alternative anode is the same as in the conventional structure, and the electrolysis performance is sufficient. In that case, this alternative anode design can reduce the PGM loading for the conventional platinum coating on PTL. Here, we evaluate the impact of this alternative anode design on the initial cell performance and durability. Experimental Titanium microfiber sheets with a nominal porosity of ca. 70% (Nikko Techno, Ltd., Osaka, Japan) were used as the substrates acting as catalyst supports and PTLs. Chemical etching with NaOH solution was performed to increase the surface area of the titanium PTL. First, titanium sheets were etched in an aqueous 1 M NaOH solution at 60 °C for 1 h. After that, the etched titanium sheets were washed under ultra-sonication in 0.01M HNO3 solution for 30 min and then washed in deionized water at room temperature for 10 min. Heat treatment was then performed at 400 °C in 5% H2-N2 gas for 30 min. The Ir catalyst-integrated PTL was prepared by depositing iridium onto the NaOH-etched titanium sheets via arc plasma deposition. Electrocatalyst paste for the PEM electrolysis cathodes was prepared by dispersing the Pt/C (Pt 46.5 wt.%, TEC10E50E, Tanaka Kikinzoku Kogyo Co., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. IrO2 electrocatalyst paste for the PEMWE anodes was prepared by dispersing the IrO2 catalyst (IrO2 (IV) TYPE II, Tokuriki Honten Co. Ltd., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. Using a spray printing system (Nordson, USA), these anode and cathode electrocatalyst pastes were spray-printed onto the electrolyte membrane (Nafion 212, E. I. du Pont de Nemours and Co., Wilmington, USA). The Ir catalyst-integrated or Pt-coated PTLs were used for the alternative or conventional anode. The durability tests were performed at 1.7 V holding for 100 h. Results and discussion Demonstrating the OER activity of Ir coating on the surface-modified PTL is essential. We compared the electrolysis cell performance using the conventional Pt-coated PTL and the Ir catalyst-integrated PTL. The cell with Ir catalyst-integrated PTL has lower activation overvoltage than conventional Pt-coated PTL. This lower activation overvoltage suggests that the Ir catalyst on the PTL can contribute to the oxygen evaluation reaction. After the durability test, the activation overvoltage of the conventional anode increased, while that of the alternative anode hardly changed. Iridium oxide has one order of magnitude higher electronic conductivity than platinum oxide5. Therefore, relatively high electronic conduction between the catalyst layer and the Ir catalyst-integrated PTL could maintain the cell performance. References The International Renewable Energy Agency, Green Hydrogen Cost Reduction, (2020).M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 167, 124523 (2020).M. Yasutake, Z. Noda, J. Matsuda, S. M. Lyth, M. Nishihara, K. Ito, A. Hayashi, and K. Sasaki, Int. J. Hydrogen Energy, 49, 169 (2024).M. Yasutake, Z. Noda, J. Matsuda, S. M. Lyth, M. Nishihara, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 170, 124507 (2023).T. Nobuo, K. Nasu, A. Fujimori, and K. Shiratori, Electronic Conduction in Oxides, 2nd ed., p. 30, (1983). Acknowledgments This study was supported by the Center of Innovation (COI) Program Grant Number JPMJCE1318 by the Japan Science and Technology Agency (JST), and the Fukuoka Strategy Conference for Hydrogen Energy. Figure 1

Innovative Anode Design Incorporating Sub-Catalyst Layer for Proton Exchange Membrane Water Electrolyzers with Ultra-Low Iridium Loading

Proton exchange membrane water electrolyzers (PEMWE) have garnered increasing attention in recent years due to their capability to produce clean, high-purity hydrogen. The optimization of the catalyst layer structure within these systems is critical, particularly in reducing the loading of platinum-group metal (PGM) anode catalysts. Under conditions of ultralow iridium loading, ion and mass transport are severely impeded, which hampers the kinetic performance of oxygen evolution reaction (OER) catalysts.In this study, we propose an innovative membrane electrode assembly (MEA) design for PEMWE, wherein an ultra-thin sub-catalyst layer is constructed atop the conventional catalyst layer. This sub-catalyst layer, fabricated via spray coating with an ultra-low iridium loading of less than 0.05 mg/cm², possesses a high surface area and is engineered to enhance the interfacial contact area between the catalyst layer and the porous transport layer. The integration of this sub-catalyst layer markedly reduces both kinetic and ohmic resistances, thereby lowering the overall overpotential.Our findings demonstrate that this novel approach results in a significant increase in the current density of the electrolyzer by 400 mA/cm² and a reduction in high-frequency resistance (HFR) by 35 mΩ cm², when compared to the conventional MEA structure. This study highlights the potential of the proposed MEA design to improve the efficiency and performance of PEMWE systems, contributing to the advancement of clean hydrogen production technologies.