Metal-Free Materials for Electrocatalytic Oxygen Evolution Reaction: Status and Prospects

Abstract

The oxygen evolution reaction (OER) is essential to the operation of various renewable energy technologies, particularly in water electrolysis and fuel cells. Nevertheless, the broad application of benchmark materials derived from iridium (Ir) and ruthenium (Ru) is seriously hindered by their scarcity. Owing to their abundance, sustainability, and tunable properties, metal-free materials have been regarded as potential alternative options to traditional metal-based electrocatalysts for OER. This review comprehensively overviews recent advancements in metal-free materials for electrocatalytic OER, focusing on their design strategies, catalytic mechanisms, and performance evaluation. Different types of metal-free OER catalysts, including carbon-based materials, carbon nitrides, and organic materials are discussed in detail. Additionally, recent progress in understanding the structure-activity relationships and approaches to enhancing their catalytic performance is highlighted. Finally, the challenges and future prospects in the realm of metal-free OER catalysts are critically presented, providing valuable insights into potential directions for future research endeavors.

Keywords

defect engineering / doping / metal-free OER catalysts / surface modification

References

[1] Y. Zhao, D. P. Adiyeri Saseendran, C. Huang, et al., “Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design,” Chemical Reviews 123 (2023): 6257–6358.

[2] C. Feng, M. B. Faheem, J. Fu, Y. Xiao, C. Li, and Y. Li, “Fe-Based Electrocatalysts for Oxygen Evolution Reaction: Progress and Perspectives,” ACS Catalysis 10 (2020): 4019–4047.

[3] L. An, C. Wei, M. Lu, et al., “Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment,” Advanced Materials 33 (2021): 2006328.

[4] S. Gbadamasi, S. Loomba, M. Haris, et al., “Breaking the Inactivity of MXenes to Drive Ampere-Level Selective Oxygen Evolution Reaction in Seawater,” Materials Science and Engineering: R: Reports 160 (2024): 100835.

[5] H. Yang, P. Guo, R. Wang, et al., “Sequential Phase Conversion-Induced Phosphides Heteronanorod Arrays for Superior Hydrogen Evolution Performance to Pt in Wide pH Media,” Advanced Materials 34 (2022): 2107548.

[6] Q. Wang, S. Kaushik, X. Xiao, and Q. Xu, “Sustainable Zinc–Air Battery Chemistry: Advances, Challenges and Prospects,” Chemical Society Reviews 52 (2023): 6139–6190.

[7] J. Chen, Y. Ha, R. Wang, et al., “Inner Co Synergizing Outer Ru Supported on Carbon Nanotubes for Efficient pH-Universal Hydrogen Evolution Catalysis,” Nano-Micro Letters 14 (2022): 186.

[8] Y. J. Kim, A. Lim, J. M. Kim, et al., “Highly Efficient Oxygen Evolution Reaction via Facile Bubble Transport Realized by Three-Dimensionally Stack-Printed Catalysts,” Nature Communications 11 (2020): 4921.

[9] M. Görlin, J. Halldin Stenlid, S. Koroidov, et al., “Key Activity Descriptors Of Nickel-Iron Oxygen Evolution Electrocatalysts in The Presence of Alkali Metal Cations,” Nature Communications 11 (2020): 6181.

[10] C. Wang, L. Jin, H. Shang, H. Xu, Y. Shiraishi, and Y. Du, “Advances in Engineering RuO₂ Electrocatalysts Towards Oxygen Evolution Reaction,” Chinese Chemical Letters 32 (2021): 2108–2116.

[11] S. Czioska, K. Ehelebe, J. Geppert, et al., “Heating up the OER: Investigation of IrO₂ OER Catalysts as Function of Potential and Temperature,” ChemElectroChem 9 (2022): e202200514.

[12] W. Li, J. Liu, P. Guo, et al., “Co/CoP Heterojunction on Hierarchically Ordered Porous Carbon as a Highly Efficient Electrocatalyst for Hydrogen and Oxygen Evolution,” Advanced Energy Materials 11 (2021): 2102134.

[13] S. S. Siwal, W. Yang, and Q. Zhang, “Recent Progress of Precious-Metal-Free Electrocatalysts for Efficient Water Oxidation in Acidic Media,” Journal of Energy Chemistry 51 (2020): 113–133.

[14] X. Lei, Q. Tang, Y. Zheng, et al., “High-Entropy Single-Atom Activated Carbon Catalysts for Sustainable Oxygen Electrocatalysis,” Nature Sustainability 6 (2023): 816–826.

[15] F. Shen, Y. Wang, G. Qian, et al., “Bimetallic Iron-Iridium Alloy Nanoparticles Supported on Nickel Foam as Highly Efficient and Stable Catalyst for Overall Water Splitting At Large Current Density,” Applied Catalysis, B: Environmental 278 (2020): 119327.

[16] Y. Zhang, C. Gan, Q. Jiang, P. Lang, W. Wang, and J. Tang, “Precious Metal-Free CoFe Layered Double Hydroxide as an Efficient Catalyst for Oxygen Evolution Reaction,” Materials Science and Engineering: B 282 (2022): 115800.

[17] Y. Zhang, X. Fan, J. Jian, D. Yu, Z. Zhang, and L. Dai, “A General Polymer-Assisted Strategy Enables Unexpected Efficient Metal-Free Oxygen-Evolution Catalysis on Pure Carbon Nanotubes,” Energy & Environmental Science 10 (2017): 2312–2317.

[18] V. S. Sapner, P. P. Chavan, and B. R. Sathe, “L-Lysine-Functionalized Reduced Graphene Oxide as a Highly Efficient Electrocatalyst for Enhanced Oxygen Evolution Reaction,” ACS Sustainable Chemistry & Engineering 8 (2020): 5524–5533.

[19] B. Fei, Z. Chen, J. Liu, et al., “Ultrathinning Nickel Sulfide With Modulated Electron Density for Efficient Water Splitting,” Advanced Energy Materials 10 (2020): 2001963.

[20] Z. Liu, Z. Zhao, Y. Wang, et al., “In Situ Exfoliated, Edge-Rich, Oxygen-Functionalized Graphene From Carbon Fibers for Oxygen Electrocatalysis,” Advanced Materials 29 (2017): 1606207.

[21] S. Chen, J. Duan, M. Jaroniec, and S.-Z. Qiao, “Nitrogen and Oxygen Dual-Doped Carbon Hydrogel Film as a Substrate-Free Electrode for Highly Efficient Oxygen Evolution Reaction,” Advanced Materials 26 (2014): 2925–2930.

[22] Z. Chen, M. Chen, X. Yan, et al., “Vacancy Occupation-Driven Polymorphic Transformation in Cobalt Ditelluride for Boosted Oxygen Evolution Reaction,” ACS Nano 14 (2020): 6968–6979.

[23] C. Xing, Y. Xue, B. Huang, et al., “Fluorographdiyne: A Metal-Free Catalyst for Applications in Water Reduction and Oxidation,” Angewandte Chemie International Edition 58 (2019): 13897–13903.

[24] D. Liu, P. Guo, Q. Wang, et al., “Local Proton-Mediated Synthesis of a High-Entropy Borate Library,” Advanced Materials 37 (2025): 2414067.

[25] D. Liu, P. Guo, H. Pan, and R. Wu, “Emerging High-Entropy Compounds for Electrochemical Energy Storage and Conversion,” Progress in Materials Science 145 (2024): 101300.

[26] H. B. Tao, L. Fang, J. Chen, et al., “Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction,” Journal of the American Chemical Society 138 (2016): 9978–9985.

[27] J. Liu, D. Liu, X. Yan, et al., “Nifeco-OH/Nite Nanoarrays With Amorphous/Crystalline Interfaces for Highly Efficient Oxygen Evolution Reaction,” Journal of Materials Chemistry A 12 (2024): 13160–13167.

[28] Q. Bai, D. Liu, X. Yan, et al., “Oxygen Vacancy-Enriched Amorphous Transition Metal Ternary Oxides Toward Highly Efficient Oxygen Evolution Reaction,” ACS Materials Letters 6 (2024): 2948–2956.

[29] Z.-J. Chen, T. Zhang, X.-Y. Gao, et al., “Engineering Microdomains of Oxides in High-Entropy Alloy Electrodes Toward Efficient Oxygen Evolution,” Advanced Materials 33 (2021): 2101845.

[30] R. He, L. Yang, Y. Zhang, et al., “A 3d-4d-5d High Entropy Alloy as a Bifunctional Oxygen Catalyst for Robust Aqueous Zinc-Air Batteries,” Advanced Materials 35 (2023): 2303719.

[31] W. Li, B. Liu, D. Liu, et al., “Alloying Co Species Into Ordered and Interconnected Macroporous Carbon Polyhedra for Efficient Oxygen Reduction Reaction in Rechargeable Zn-Air Battery,” Advanced Materials 34 (2022): 2109605.

[32] J. Lee, H. Jung, Y. S. Park, et al., “Chemical Transformation Approach for High-Performance Ternary NiFeCo Metal Compound-Based Water Splitting Electrodes,” Applied Catalysis, B: Environmental 294 (2021): 120246.

[33] P. Guo, D. Liu, H. Yang, et al., “Dynamic Restructuring of Carbon Nanotube-Supported High-Entropy Alloys Enabling Efficient Oxygen Electrocatalysis,” ACS Nano 20 (2026): 4152–4165.

[34] Y. Sun, R. Li, X. Chen, et al., “A-Site Management Prompts the Dynamic Reconstructed Active Phase of Perovskite Oxide OER Catalysts,” Advanced Energy Materials 11 (2021): 2003755.

[35] J. A. Dias, M. A. S. Andrade, H. L. S. Santos, M. R. Morelli, and L. H. Mascaro, “Lanthanum-Based Perovskites for Catalytic Oxygen Evolution Reaction,” ChemElectroChem 7 (2020): 3173–3192.

[36] H. J. Song, H. Yoon, B. Ju, and D.-W. Kim, “Highly Efficient Perovskite-Based Electrocatalysts for Water Oxidation in Acidic Environments: A Mini Review,” Advanced Energy Materials 11 (2021): 2002428.

[37] Y. Li, G. Chen, H.-C. Chen, et al., “Tailoring The Surface Cation Configuration of Ruddlesden–Popper Perovskites For Controllable Water Oxidation Performance,” Energy & Environmental Science 16 (2023): 3331–3338.

[38] J. Li, F. Yang, Y. Du, et al., “The Critical Role of A, B-Site Cations and Oxygen Vacancies on the OER Electrocatalytic Performances of Bi_0.15Sr_0.85Co_1−xFe O_3−δ (0.2 ≤ x ≤ 1) Perovskites in Alkaline Media,” Chemical Engineering Journal 451 (2023): 138646.

[39] D. Liu, P. Guo, X. Yan, Y. He, and R. Wu, “Manipulating the Configuration Entropy of Layered Hydroxides Toward Efficient Oxygen Evolution Reaction for Anion Exchange Membrane Electrolyzer,” Materials Today 80 (2024): 101–112.

[40] D. Liu, X. Yan, P. Guo, et al., “Inert Mg Incorporation to Break the Activity/Stability Relationship in High-Entropy Layered Hydroxides for the Electrocatalytic Oxygen Evolution Reaction,” ACS Catalysis 13 (2023): 7698–7706.

[41] Z. Wang, W. A. Goddard, and H. Xiao, “Potential-Dependent Transition of Reaction Mechanisms For Oxygen Evolution on Layered Double Hydroxides,” Nature Communications 14 (2023): 4228.

[42] M. Görlin, P. Chernev, J. Ferreira de Araújo, et al., “Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni–Fe Oxide Water Splitting Electrocatalysts,” Journal of the American Chemical Society 138 (2016): 5603–5614.

[43] J. Kim, W. Ko, J. M. Yoo, et al., “Structural Insights Into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis,” Advanced Materials 34 (2022): 2107868.

[44] P. Guo, L. Shi, D. Liu, et al., “Fe-Doping-Induced Cation Substitution and Anion Vacancies Promoting Co₃O₄ Hexagonal Nanosheets for Efficient Overall Water Splitting,” Materials Today Catalysis 1 (2023): 100002.

[45] P. Guo, D. Liu, Q. Wang, et al., “Balancing the Surface Reconstruction of Spinel Oxides for Efficient Oxygen Evolution Reaction,” Nano Letters 25 (2025): 10209–10217.

[46] L. Li, H. Yang, J. Miao, et al., “Unraveling Oxygen Evolution Reaction on Carbon-Based Electrocatalysts: Effect of Oxygen Doping on Adsorption of Oxygenated Intermediates,” ACS Energy Letters 2 (2017): 294–300.

[47] X. Xie, L. Du, L. Yan, et al., “Oxygen Evolution Reaction in Alkaline Environment: Material Challenges and Solutions,” Advanced Functional Materials 32 (2022): 2110036.

[48] X. Zhang, M. Jin, F. Jia, et al., “Noble-Metal-Free Oxygen Evolution Reaction Electrocatalysts Working at High Current Densities Over 1000 mA cm⁻²: From Fundamental Understanding to Design Principles,” Energy & Environmental Materials 6 (2023): e12457.

[49] S. Zhao, D.-W. Wang, R. Amal, and L. Dai, “Carbon-Based Metal-Free Catalysts for Key Reactions Involved in Energy Conversion and Storage,” Advanced Materials 31 (2019): 1801526.

[50] T. Noor, L. Yaqoob, and N. Iqbal, “Recent Advances in Electrocatalysis of Oxygen Evolution Reaction Using Noble-Metal, Transition-Metal, and Carbon-Based Materials,” ChemElectroChem 8 (2021): 447–483.

[51] X. Liu and L. Dai, “Carbon-Based Metal-Free Catalysts,” Nature Reviews Materials 1 (2016): 16064.

[52] F. M. Toma, A. Sartorel, M. Iurlo, et al., “Efficient Water Oxidation at Carbon Nanotube–Polyoxometalate Electrocatalytic Interfaces,” Nature Chemistry 2 (2010): 826–831.

[53] Y.-N. Sun, J. Yang, X. Ding, et al., “Synergetic Contribution Of Nitrogen and Fluorine Species in Porous Carbons as Metal-Free and Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries,” Applied Catalysis, B: Environmental 297 (2021): 120448.

[54] X. Xiao, X. Li, Z. Wang, et al., “Robust Template-Activator Cooperated Pyrolysis Enabling Hierarchically Porous Honeycombed Defective Carbon as Highly-Efficient Metal-Free Bifunctional Electrocatalyst for Zn-Air Batteries,” Applied Catalysis, B: Environmental 265 (2020): 118603.

[55] Q. Liu, Y. Wang, L. Dai, and J. Yao, “Scalable Fabrication of Nanoporous Carbon Fiber Films as Bifunctional Catalytic Electrodes for Flexible Zn-Air Batteries,” Advanced Materials 28 (2016): 3000–3006.

[56] C. Tang, H.-F. Wang, X. Chen, et al., “Topological Defects in Metal-Free Nanocarbon for Oxygen Electrocatalysis,” Advanced Materials 28 (2016): 6845–6851.

[57] C. Hu and L. Dai, “Multifunctional Carbon-Based Metal-Free Electrocatalysts for Simultaneous Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution,” Advanced Materials 29 (2017): 1604942.

[58] S. Mondal, B. Mohanty, M. Nurhuda, et al., “A Thiadiazole-Based Covalent Organic Framework: A Metal-Free Electrocatalyst Toward Oxygen Evolution Reaction,” ACS Catalysis 10 (2020): 5623–5630.

[59] Z. Yuan, J. Li, M. Yang, et al., “Ultrathin Black Phosphorus-On-Nitrogen Doped Graphene for Efficient Overall Water Splitting: Dual Modulation Roles of Directional Interfacial Charge Transfer,” Journal of the American Chemical Society 141 (2019): 4972–4979.

[60] H.-W. Zhang, Y.-X. Lu, B. Li, et al., “Acid-Induced Topological Morphology Modulation of Graphitic Carbon Nitride Homojunctions as Advanced Metal-Free Catalysts for OER and Pollutant Degradation,” Journal of Materials Science & Technology 86 (2021): 210–218.

[61] H. Choi, S. Surendran, Y. Sim, et al., “Enhanced Electrocatalytic Full Water-Splitting Reaction by Interfacial Electric Field in 2D/2D Heterojunction,” Chemical Engineering Journal 450 (2022): 137789.

[62] Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi, and K. Hashimoto, “Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation,” Nature Communications 4 (2013): 2390.

[63] Y.-Y. Li, B.-X. Zhou, H.-W. Zhang, et al., “Doping-Induced Enhancement of Crystallinity in Polymeric Carbon Nitride Nanosheets to Improve Their Visible-Light Photocatalytic Activity,” Nanoscale 11 (2019): 6876–6885.

[64] Z. Li, S. Ji, H. Liu, et al., “Constructing Asymmetrical Coordination Microenvironment With Phosphorus-Incorporated Nitrogen-Doped Carbon to Boost Bifunctional Oxygen Electrocatalytic Activity,” Advanced Functional Materials 34 (2024): 2314444.

[65] D. Li, B. Ren, Q. Jin, H. Cui, and C. Wang, “Nitrogen-Doped, Oxygen-Functionalized, Edge- and Defect-Rich Vertically Aligned Graphene for Highly Enhanced Oxygen Evolution Reaction,” Journal of Materials Chemistry A 6 (2018): 2176–2183.

[66] F. Gui, Q. Jin, D. Xiao, et al., “High-Performance Zinc-Air Batteries Based on Bifunctional Hierarchically Porous Nitrogen-Doped Carbon,” Small 18 (2022): 2105928.

[67] K. Kordek, L. Jiang, K. Fan, et al., “Two-Step Activated Carbon Cloth With Oxygen-Rich Functional Groups as a High-Performance Additive-Free Air Electrode for Flexible Zinc-Air Batteries,” Advanced Energy Materials 9 (2019): 1802936.

[68] X. Lu, W.-L. Yim, B. H. R. Suryanto, and C. Zhao, “Electrocatalytic Oxygen Evolution at Surface-Oxidized Multiwall Carbon Nanotubes,” Journal of the American Chemical Society 137 (2015): 2901–2907.

[69] Z. Pei, H. Tan, J. Gu, et al., “A Polymeric Hydrogel Electrocatalyst for Direct Water Oxidation,” Nature Communications 14 (2023): 818.

[70] E. Mirzakulova, R. Khatmullin, J. Walpita, et al., “Electrode-Assisted Catalytic Water Oxidation by a Flavin Derivative,” Nature Chemistry 4 (2012): 794–801.

[71] C. Yang, Z.-D. Yang, H. Dong, et al., “Theory-Driven Design and Targeting Synthesis of a Highly-Conjugated Basal-Plane 2D Covalent Organic Framework for Metal-Free Electrocatalytic OER,” ACS Energy Letters 4 (2019): 2251–2258.

[72] C.-T. He, L.-H. Yu, H. Liu, et al., “Post-Oxidation of All-Organic Electrocatalysts to Promote O−O Coupling in Water Oxidation,” Nature Communications 16 (2025): 4389.

[73] T. Y. Ma, S. Dai, M. Jaroniec, and S. Z. Qiao, “Graphitic Carbon Nitride Nanosheet–Carbon Nanotube Three-Dimensional Porous Composites as High-Performance Oxygen Evolution Electrocatalysts,” Angewandte Chemie International Edition 53 (2014): 7281–7285.

[74] Q. Jiang, L. Xu, N. Chen, H. Zhang, L. Dai, and S. Wang, “Facile Synthesis of Black Phosphorus: An Efficient Electrocatalyst for the Oxygen Evolving Reaction,” Angewandte Chemie International Edition 55 (2016): 13849–13853.

[75] A. Sadhukhan, A. Karmakar, K. Koner, et al., “Functionality Modulation Toward Thianthrene-Based Metal-Free Electrocatalysts for Water Splitting,” Advanced Materials 36 (2024): 2310938.

[76] R. Prasannachandran, T. V. Vineesh, A. Anil, B. M. Krishna, and M. M. Shaijumon, “Functionalized Phosphorene Quantum Dots as Efficient Electrocatalyst for Oxygen Evolution Reaction,” ACS Nano 12 (2018): 11511–11519.

[77] Y. Wang, N. Xu, R. He, L. Peng, D. Cai, and J. Qiao, “Large-Scale Defect-Engineering Tailored Tri-Doped Graphene as a Metal-Free Bifunctional Catalyst for Superior Electrocatalytic Oxygen Reaction in Rechargeable Zn-Air Battery,” Applied Catalysis, B: Environmental 285 (2021): 119811.

[78] L. Li, S. Hagiwara, C. Jiang, et al., “Boron Monosulfide as an Electrocatalyst for the Oxygen Evolution Reaction,” Chemical Engineering Journal 471 (2023): 144489.

[79] U. P. Kadel, “Metal-Free Electrocatalysts for Oxygen Evolution Reaction and Photocatalysts for Carbon Dioxide Reduction Reaction,” Photochemical Sciences Ph.D. Dissertations (2018).

[80] K. Barman, G. Askarova, R. Jia, G. Hu, and M. V. Mirkin, “Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode,” Journal of the American Chemical Society 145 (2023): 5786–5794.

[81] H. Li, F. Xie, and M.-T. Zhang, “Metal-Free Electrocatalyst for Water Oxidation Initiated by Hydrogen Atom Transfer,” ACS Catalysis 11 (2021): 68–73.

[82] M. Zhao, J. Zhang, H. Xiao, T. Hu, J. Jia, and H. Wu, “Facile In Situ Synthesis of a Carbon Quantum Dot/Graphene Heterostructure as an Efficient Metal-Free Electrocatalyst for Overall Water Splitting,” Chemical Communications 55 (2019): 1635–1638.

[83] K. Qu, Y. Zheng, Y. Jiao, X. Zhang, S. Dai, and S.-Z. Qiao, “Polydopamine-Inspired, Dual Heteroatom-Doped Carbon Nanotubes for Highly Efficient Overall Water Splitting,” Advanced Energy Materials 7 (2017): 1602068.

[84] B. Z. Desalegn, H. S. Jadhav, and J. G. Seo, “Highly Efficient g-C₃N₄ Nanorods With Dual Active Sites as an Electrocatalyst for the Oxygen Evolution Reaction,” ChemCatChem 11 (2019): 2870–2878.

[85] X. Wang, A. Guo, Y. Fu, M. Tan, W. Luo, and W. Yang, “Porphyrin-Conjugated Microporous Polymer Nanospheres as Electrocatalysts for Nitrobenzene Detection and Oxygen Evolution Reaction,” ACS Applied Nano Materials 6 (2023): 3226–3235.

[86] Q. Hu, G. Li, X. Liu, et al., “Superhydrophilic Phytic-Acid-Doped Conductive Hydrogels as Metal-Free and Binder-Free Electrocatalysts for Efficient Water Oxidation,” Angewandte Chemie International Edition 58 (2019): 4318–4322.

[87] Y.-P. Zhu, J. Ran, and S.-Z. Qiao, “Scalable Self-Supported Graphene Foam for High-Performance Electrocatalytic Oxygen Evolution,” ACS Applied Materials & Interfaces 9 (2017): 41980–41987.

[88] F. Shiravani, J. Tashkhourian, and B. Haghighi, “One-Step Synthesis Of Graphitic Carbon-Nitride Doped With Black-Red Phosphorus as a Novel, Efficient and Free-Metal Bifunctional Catalyst and Its Application for Electrochemical Overall Water Splitting,” Sustainable Energy & Fuels 5 (2021): 3229–3239.

[89] S. S. Shinde, J.-Y. Yu, J.-W. Song, Y.-H. Nam, D.-H. Kim, and J.-H. Lee, “Highly Active and Durable Carbon Nitride Fibers as Metal-Free Bifunctional Oxygen Electrodes for Flexible Zn-Air Batteries,” Nanoscale Horizons 2 (2017): 333–341.

[90] X. Yu, M. Zhang, J. Chen, Y. Li, and G. Shi, “Nitrogen and Sulfur Codoped Graphite Foam as a Self-Supported Metal-Free Electrocatalytic Electrode for Water Oxidation,” Advanced Energy Materials 6 (2016): 1501492.

[91] M. A. Ahsan, T. He, K. Eid, et al., “Tuning the Intermolecular Electron Transfer of Low-Dimensional and Metal-Free BCN/C₆₀ Electrocatalysts via Interfacial Defects for Efficient Hydrogen and Oxygen Electrochemistry,” Journal of the American Chemical Society 143 (2021): 1203–1215.

[92] J. Sun, S. E. Lowe, L. Zhang, et al., “Ultrathin Nitrogen-Doped Holey Carbon@Graphene Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions in Alkaline and Acidic Media,” Angewandte Chemie International Edition 57 (2018): 16511–16515.

[93] T. Li, Y. Wang, T. Chen, G. Wang, C. Qiu, and W. Hu, “Ionic Liquid In-Situ Functionalized Carbon Nanotube Film as Self-Supported Metal-Free Electrocatalysts for Oxygen Evolution,” Chemical Engineering Journal 484 (2024): 149767.

[94] B. Li, Q. Fang, Y. Si, et al., “Ultra-Thin Tubular Graphitic Carbon Nitride-Carbon Dot Lateral Heterostructures: One-Step Synthesis and Highly Efficient Catalytic Hydrogen Generation,” Chemical Engineering Journal 397 (2020): 125470.

[95] R. Gao, Q. Dai, F. Du, D. Yan, and L. Dai, “C₆₀-Adsorbed Single-Walled Carbon Nanotubes as Metal-Free, pH-Universal, and Multifunctional Catalysts for Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution,” Journal of the American Chemical Society 141 (2019): 11658–11666.

[96] A. García-Miranda Ferrari, D. A. C. Brownson, and C. E. Banks, “Investigating the Integrity of Graphene Towards the Electrochemical Oxygen Evolution Reaction,” ChemElectroChem 6 (2019): 5446–5453.

[97] B. H. R. Suryanto, T. Fang, S. Cheong, R. D. Tilley, and C. Zhao, “From the Inside-Out: Leached Metal Impurities in Multiwall Carbon Nanotubes for Purification or Electrocatalysis,” Journal of Materials Chemistry A 6 (2018): 4686–4694.

[98] Q. Liu, K. Liu, J. Huang, C. Hui, X. Li, and L. Feng, “A Review of Modulation Strategies for Improving the Catalytic Performance of Transition Metal Sulfide Self-Supported Electrodes for the Hydrogen Evolution Reaction,” Dalton Transactions 53 (2024): 3959–3969.

[99] Z. Chen, R. Wu, Y. Liu, et al., “Ultrafine Co Nanoparticles Encapsulated in Carbon-Nanotubes-Grafted Graphene Sheets as Advanced Electrocatalysts for the Hydrogen Evolution Reaction,” Advanced Materials 30 (2018): 1802011.

[100] Y. Cheng, C. Xu, L. Jia, et al., “Pristine Carbon Nanotubes as Non-Metal Electrocatalysts for Oxygen Evolution Reaction of Water Splitting,” Applied Catalysis, B: Environmental 163 (2015): 96–104.

[101] X. Yang, H. He, T. Lv, and J. Qiu, “Fabrication of Biomass-Based Functional Carbon Materials for Energy Conversion and Storage,” Materials Science and Engineering: R: Reports 154 (2023): 100736.

[102] J. Zhang, Z. Zhao, Z. Xia, and L. Dai, “A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions,” Nature Nanotechnology 10 (2015): 444–452.

[103] H. B. Yang, J. Miao, S.-F. Hung, et al., “Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of Highly Efficient Metal-Free Bifunctional Electrocatalyst,” Science Advances 2 (2016): e1501122.

[104] J. Zhang and L. Dai, “Nitrogen, Phosphorus, and Fluorine Tri-Doped Graphene as a Multifunctional Catalyst for Self-Powered Electrochemical Water Splitting,” Angewandte Chemie International Edition 55 (2016): 13296–13300.

[105] Y. Zhu, T. Zhang, and J. Y. Lee, “Nitrogenated-Graphite-Encapsulated Carbon Black as a Metal-Free Electrocatalyst for the Oxygen Evolution Reaction in Acid,” ChemElectroChem 5 (2018): 583–588.

[106] T. Sun, J. Wang, C. Qiu, et al., “B, N Codoped and Defect-Rich Nanocarbon Material as a Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions,” Advanced Science 5 (2018): 1800036.

[107] Y. Zhao, N. Yang, H. Yao, et al., “Stereodefined Codoping of sp-N and S Atoms in Few-Layer Graphdiyne for Oxygen Evolution Reaction,” Journal of the American Chemical Society 141 (2019): 7240–7244.

[108] X. Zheng, X. Cao, K. Zeng, et al., “Cotton Pad-Derived Large-Area 3D N-Doped Graphene-Like Full Carbon Cathode With an O-Rich Functional Group for Flexible All Solid Zn-Air Batteries,” Journal of Materials Chemistry A 8 (2020): 11202–11209.

[109] F. Qiang, J. Feng, H. Wang, et al., “Oxygen Engineering Enables N-Doped Porous Carbon Nanofibers as Oxygen Reduction/Evolution Reaction Electrocatalysts for Flexible Zinc-Air Batteries,” ACS Catalysis 12 (2022): 4002–4015.

[110] Y. P. Zhu, Y. Jing, A. Vasileff, T. Heine, and S.-Z. Qiao, “3D Synergistically Active Carbon Nanofibers for Improved Oxygen Evolution,” Advanced Energy Materials 7 (2017): 1602928.

[111] K. Kordek-Khalil, A. Moyseowicz, P. Rutkowski, and G. Gryglewicz, “Excellent Performance of Nitrogen-Doped Carbon Nanofibers as Electrocatalysts for Water Splitting Reactions,” International Journal of Hydrogen Energy 52 (2024): 494–506.

[112] J. Liu, S. Zhao, C. Li, et al., “Carbon Nanodot Surface Modifications Initiate Highly Efficient, Stable Catalysts for Both Oxygen Evolution and Reduction Reactions,” Advanced Energy Materials 6 (2016): 1502039.

[113] V. Mazánek, J. Luxa, S. Matějková, et al., “Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the Key Role of Metallic Impurities in Graphene-Based Electrocatalysis,” ACS Nano 13 (2019): acsnano.8b07534.

[114] F. Zoller, S. Häringer, D. Böhm, J. Luxa, Z. Sofer, and D. Fattakhova-Rohlfing, “Carbonaceous Oxygen Evolution Reaction Catalysts: From Defect and Doping-Induced Activity Over Hybrid Compounds to Ordered Framework Structures,” Small 17 (2021): 2007484.

[115] G.-L. Chai, Z. Hou, D.-J. Shu, T. Ikeda, and K. Terakura, “Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect,” Journal of the American Chemical Society 136 (2014): 13629–13640.

[116] J. Wang, H. Kong, J. Zhang, Y. Hao, Z. Shao, and F. Ciucci, “Carbon-Based Electrocatalysts for Sustainable Energy Applications,” Progress in Materials Science 116 (2021): 100717.

[117] X. Wang, A. Vasileff, Y. Jiao, Y. Zheng, and S.-Z. Qiao, “Electronic and Structural Engineering of Carbon-Based Metal-Free Electrocatalysts for Water Splitting,” Advanced Materials 31 (2019): 1803625.

[118] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, and J. Nakamura, “Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts,” Science 351 (2016): 361–365.

[119] J. Zhang and L. Dai, “Heteroatom-Doped Graphitic Carbon Catalysts for Efficient Electrocatalysis of Oxygen Reduction Reaction,” ACS Catalysis 5 (2015): 7244–7253.

[120] S. Wang, E. Iyyamperumal, A. Roy, Y. Xue, D. Yu, and L. Dai, “Vertically Aligned BCN Nanotubes as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-Doping With Boron and Nitrogen,” Angewandte Chemie International Edition 50 (2011): 11756–11760.

[121] Y. Li, W. Li, D. Liu, et al., “Carboxyl Functionalized Double-Walled Carbon Nanotubes for Oxygen Evolution Reaction,” Electrochimica Acta 419 (2022): 140395.

[122] W. Wang, J. Luo, and S. Chen, “Carbon Oxidation Reactions Could Misguide the Evaluation of Carbon Black-Based Oxygen-Evolution Electrocatalysts,” Chemical Communications 53 (2017): 11556–11559.

[123] W.-J. Ong, L.-L. Tan, Y. H. Ng, S.-T. Yong, and S.-P. Chai, “Graphitic Carbon Nitride (g-C₃N₄)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer to Achieving Sustainability?,” Chemical Reviews 116 (2016): 7159–7329.

[124] B.-B. Xu, X.-B. Fu, X.-M. You, et al., “Synergistic Promotion of Single-Atom Co Surrounding a PtCo Alloy Based on a g-C₃N₄ Nanosheet for Overall Water Splitting,” ACS Catalysis 12 (2022): 6958–6967.

[125] B. K. Satpathy, C. R. Raj, and D. Pradhan, “Facile Room Temperature Synthesis of CoSn(OH)₆/g-C₃N₄ Nanocomposite for Oxygen Evolution Reaction,” Electrochimica Acta 433 (2022): 141250.

[126] H. Niu, X. Wan, X. Wang, et al., “Single-Atom Rhodium on Defective g-C₃N₄: A Promising Bifunctional Oxygen Electrocatalyst,” ACS Sustainable Chemistry & Engineering 9 (2021): 3590–3599.

[127] J. S. Shaikh, M. Rittiruam, T. Saelee, et al., “First-Principles and Experimental Insight of High-Entropy Materials as Electrocatalysts for Energy-Related Applications: Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions,” Materials Science and Engineering: R: Reports 160 (2024): 100813.

[128] F. Besharat, F. Ahmadpoor, Z. Nezafat, et al., “Advances in Carbon Nitride-Based Materials and Their Electrocatalytic Applications,” ACS Catalysis 12 (2022): 5605–5660.

[129] M. A. Wahab, J. Joseph, L. Atanda, et al., “Nanoconfined Synthesis of Nitrogen-Rich Metal-Free Mesoporous Carbon Nitride Electrocatalyst for the Oxygen Evolution Reaction,” ACS Applied Energy Materials 3 (2020): 1439–1447.

[130] S. Chen, J. Duan, J. Ran, and S.-Z. Qiao, “Paper-Based N-Doped Carbon Films for Enhanced Oxygen Evolution Electrocatalysis,” Advanced Science 2 (2015): 1400015.

[131] A. Torres-Pinto, A. M. Díez, C. G. Silva, et al., “Tuning Graphitic Carbon Nitride (g-C₃N₄) Electrocatalysts for Efficient Oxygen Evolution Reaction (OER),” Fuel 360 (2024): 130575.

[132] K. Ding, Y. Ye, J. Hu, et al., “Aerophilic Triphase Interface Tuned by Carbon Dots Driving Durable and Flexible Rechargeable Zn-Air Batteries,” Nano-Micro Letters 15 (2023): 28.

[133] S. S. Zance and S. Ravichandran, “Electrochemical Surface Modification of Carbon for Enhanced Water Electrolysis,” Applied Physics A 125 (2019): 456.

[134] W. Zhang, C.-H. Xu, H. Zheng, R. Li, and K. Zhou, “Oxygen-Rich Cobalt–Nitrogen–Carbon Porous Nanosheets for Bifunctional Oxygen Electrocatalysis,” Advanced Functional Materials 32 (2022): 2200763.

[135] H. Jin, H. Huang, Y. He, et al., “Graphene Quantum Dots Supported by Graphene Nanoribbons With Ultrahigh Electrocatalytic Performance for Oxygen Reduction,” Journal of the American Chemical Society 137 (2015): 7588–7591.

[136] Y. Li, W. Zhou, H. Wang, et al., “An Oxygen Reduction Electrocatalyst Based on Carbon Nanotube–Graphene Complexes,” Nature Nanotechnology 7 (2012): 394–400.

[137] Q. Liu and J. Zhang, “Graphene Supported Co-g-C₃N₄ as a Novel Metal–Macrocyclic Electrocatalyst for the Oxygen Reduction Reaction in Fuel Cells,” Langmuir 29 (2013): 3821–3828.

[138] M. Tahir, C. Cao, N. Mahmood, et al., “Multifunctional g-C₃N₄ Nanofibers: A Template-Free Fabrication and Enhanced Optical, Electrochemical, and Photocatalyst Properties,” ACS Applied Materials & Interfaces 6 (2014): 1258–1265.

[139] T. Y. Ma, J. L. Cao, M. Jaroniec, and S. Z. Qiao, “Interacting Carbon Nitride and Titanium Carbide Nanosheets for High-Performance Oxygen Evolution,” Angewandte Chemie International Edition 55 (2016): 1138–1142.

[140] Y. Zhao, F. Zhao, X. Wang, et al., “Graphitic Carbon Nitride Nanoribbons: Graphene-Assisted Formation and Synergic Function for Highly Efficient Hydrogen Evolution,” Angewandte Chemie International Edition 53 (2014): 13934–13939.

[141] Z. Peng, S. Yang, D. Jia, et al., “Homologous Metal-Free Electrocatalysts Grown on Three-Dimensional Carbon Networks for Overall Water Splitting in Acidic and Alkaline Media,” Journal of Materials Chemistry A 4 (2016): 12878–12883.

[142] T. Y. Ma, J. Ran, S. Dai, M. Jaroniec, and S. Z. Qiao, “Phosphorus-Doped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes,” Angewandte Chemie International Edition 54 (2015): 4646–4650.

[143] G. Y. Jang, S. Y. Kang, H. J. Choi, et al., “Graphite Carbon Nitride-Based Metal-Free Bifunctional Electrocatalysts for Anion Exchange Membrane Water Electrolyzer,” Journal of Electroanalytical Chemistry 987 (2025): 119111.

[144] Q. Yan, G.-F. Huang, D.-F. Li, M. Zhang, A.-L. Pan, and W.-Q. Huang, “Facile Synthesis and Superior Photocatalytic and Electrocatalytic Performances of Porous B-Doped g-C₃N₄ Nanosheets,” Journal of Materials Science & Technology 34 (2018): 2515–2520.

[145] E.-X. Han, Y.-Y. Li, Q.-H. Wang, et al., “Chlorine Doped Graphitic Carbon Nitride Nanorings as an Efficient Photoresponsive Catalyst for Water Oxidation and Organic Decomposition,” Journal of Materials Science & Technology 35 (2019): 2288–2296.

[146] J. Kim, A. P. Tiwari, M. Choi, et al., “Boosting Bifunctional Oxygen Electrocatalysis of Graphitic C₃N₄ Using Non-Covalently Functionalized Non-Oxidized Graphene Aerogels as Catalyst Supports,” Journal of Materials Chemistry A 10 (2022): 15689–15697.

[147] Y.-X. Lin, W.-J. Feng, J.-J. Zhang, et al., “A Polyimide Nanolayer as a Metal-Free and Durable Organic Electrode Toward Highly Efficient Oxygen Evolution,” Angewandte Chemie International Edition 57 (2018): 12563–12566.

[148] C. Mo, J. Jian, J. Li, et al., “Boosting Water Oxidation on Metal-Free Carbon Nanotubes via Directional Interfacial Charge-Transfer Induced by an Adsorbed Polyelectrolyte,” Energy & Environmental Science 11 (2018): 3334–3341.

[149] C. Liu, F. Liu, H. Li, et al., “One-Dimensional van der Waals Heterostructures as Efficient Metal-Free Oxygen Electrocatalysts,” ACS Nano 15 (2021): 3309–3319.

[150] S. Lu, W. Zhou, Y. Shi, C. Liu, Y. Yu, and B. Zhang, “Phenanthrenequinone-Like Moiety Functionalized Carbon for Electrocatalytic Acidic Oxygen Evolution,” Chem 8 (2022): 1415–1426.

[151] R. Wang, Z. Zhang, J. Suo, et al., “Exploring Metal-Free Ionic Covalent Organic Framework Nanosheets as Efficient OER Electrocatalysts via Cationic-π Interactions,” Chemical Engineering Journal 478 (2023): 147403.

[152] K. Prakash, I. Ahmed, R. Deka, and S. M. Mobin, “Metal Free Donor–Acceptor Covalent Organic Frameworks as Efficient Electrocatalysts for Oxygen Evolution Reaction,” Journal of Materials Chemistry A 13 (2025): 30214–30223.

[153] A. Basak, A. Karmakar, S. Dutta, et al., “Metal-Free Electrocatalytic Alkaline Water Splitting by Porous Macrocyclic Proton Sponges,” Angewandte Chemie International Edition 64 (2025): e202419377.

[154] L.-H. Yu, X.-F. Zhang, Z.-M. Ye, et al., “Engineering p-Orbital States via Molecular Modules in All-Organic Electrocatalysts Toward Direct Water Oxidation,” Advanced Science 12 (2025): 2410507.

[155] D. Yang, L. Zhang, X. Yan, and X. Yao, “Recent Progress in Oxygen Electrocatalysts for Zinc–Air Batteries,” Small Methods 1 (2017): 1700209.

[156] X. Feng, X. Ding, and D. Jiang, “Covalent Organic Frameworks,” Chemical Society Reviews 41 (2012): 6010–6022.

[157] X. Cui, S. Lei, A. C. Wang, et al., “Emerging Covalent Organic Frameworks Tailored Materials for Electrocatalysis,” Nano Energy 70 (2020): 104525.

[158] A. Zitolo, V. Goellner, V. Armel, et al., “Identification of Catalytic Sites for Oxygen Reduction in Iron- and Nitrogen-Doped Graphene Materials,” Nature Materials 14 (2015): 937–942.

[159] J. Li, C. A. Triana, W. Wan, et al., “Molecular and Heterogeneous Water Oxidation Catalysts: Recent Progress and Joint Perspectives,” Chemical Society Reviews 50 (2021): 2444–2485.

[160] L. Wu, A. Nayak, J. Shao, and T. J. Meyer, “Crossing the Bridge From Molecular Catalysis to a Heterogenous Electrode in Electrocatalytic Water Oxidation,” Proceedings of the National Academy of Sciences 116 (2019): 11153–11158.

[161] H. N. Nong, L. J. Falling, A. Bergmann, et al., “Key Role of Chemistry Versus Bias in Electrocatalytic Oxygen Evolution,” Nature 587 (2020): 408–413.

[162] S. W. Boettcher and Y. Surendranath, “Heterogeneous Electrocatalysis Goes Chemical,” Nature Catalysis 4 (2021): 4–5.

[163] F. Geneste and C. Moinet, “Electrochemically Linking TEmpo to Carbon via Amine Bridges,” New Journal of Chemistry 29 (2005): 269–271.

[164] N. Ullah, R. Ullah, S. Khan, and Y. Xu, “Boron Nitride-Based Electrocatalysts for HER, OER, and ORR: A Mini-Review,” Frontiers of Materials Science 15 (2021): 543–552.

[165] W. Zhan, H. Wang, J. Gao, et al., “Hexagonal Boron Nitride/Reduced Graphene Oxide Heterostructures as Promising Metal-Free Electrocatalysts for Oxygen Evolution Reaction Driven by Boron Radicals,” Small Structures 4 (2023): 2300167.

[166] Y. Lu, B. Li, N. Xu, et al., “One-Atom-Thick Hexagonal Boron Nitride Co-Catalyst for Enhanced Oxygen Evolution Reactions,” Nature Communications 14 (2023): 6965.

[167] M. Kumar, B. Meena, P. Subramanyam, D. Suryakala, and C. Subrahmanyam, “Recent Trends in Photoelectrochemical Water Splitting: The Role of Cocatalysts,” NPG Asia Materials 14 (2022): 88.

[168] X. Li, J. Wang, Y. Fang, H. Zhang, X. Fu, and X. Wang, “Roles of Metal-Free Materials in Photoelectrodes for Water Splitting,” Accounts of Materials Research 2 (2021): 933–943.

[169] G. Zhang, Z.-A. Lan, and X. Wang, “Surface Engineering of Graphitic Carbon Nitride Polymers With Cocatalysts for Photocatalytic Overall Water Splitting,” Chemical Science 8 (2017): 5261–5274.

[170] J. Zhang, M. Grzelczak, Y. Hou, et al., “Photocatalytic Oxidation of Water by Polymeric Carbon Nitride Nanohybrids Made of Sustainable Elements,” Chemical Science 3 (2012): 443–446.

[171] C. Feng, Z. Wang, Y. Ma, Y. Zhang, L. Wang, and Y. Bi, “Ultrathin Graphitic C₃N₄ Nanosheets as Highly Efficient Metal-Free Cocatalyst for Water Oxidation,” Applied Catalysis, B: Environmental 205 (2017): 19–23.

[172] T. Zhou, S. Chen, J. Wang, et al., “Dramatically Enhanced Solar-Driven Water Splitting of BiVO₄ Photoanode via Strengthening Hole Transfer and Light Harvesting by Co-Modification of CQDs and Ultrathin β-FeOOH Layers,” Chemical Engineering Journal 403 (2021): 126350.

[173] G. Nazir, A. Rehman, J. H. Lee, et al., “A Review of Rechargeable Zinc–Air Batteries: Recent Progress and Future Perspectives,” Nano-Micro Letters 16 (2024): 138.

[174] K. Zhu, X. Zhu, and W. Yang, “Application of In Situ Techniques for the Characterization of NiFe-Based Oxygen Evolution Reaction (OER) Electrocatalysts,” Angewandte Chemie International Edition 58 (2019): 1252–1265.

[175] G. Zhao, Y. Yao, W. Lu, et al., “Direct Observation of Oxygen Evolution and Surface Restructuring on Mn₂O₃ Nanocatalysts Using In Situ and Ex Situ Transmission Electron Microscopy,” Nano Letters 21 (2021): 7012–7020.

[176] T. Tarnev, H. B. Aiyappa, A. Botz, et al., “Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media,” Angewandte Chemie International Edition 58 (2019): 14265–14269.

[177] K. J. Jenewein, L. Torresi, N. Haghmoradi, A. Kormányos, P. Friederich, and S. Cherevko, “Navigating the Unknown With AI: Multiobjective Bayesian Optimization of Non-Noble Acidic OER Catalysts,” Journal of Materials Chemistry A 12 (2024): 3072–3083.

[178] R. Ding, X. Wang, A. Tan, J. Li, and J. Liu, “Unlocking New Insights for Electrocatalyst Design: A Unique Data Science Workflow Leveraging Internet-Sourced Big Data,” ACS Catalysis 13 (2023): 13267–13281.

[179] Z. Fang, S. Li, Y. Zhang, et al., “The DFT and Machine Learning Method Accelerated the Discovery of DMSCs With High ORR and OER Catalytic Activities,” Journal of Physical Chemistry Letters 15 (2024): 281–289.

[180] Y. Ying, K. Fan, X. Luo, J. Qiao, and H. Huang, “Unravelling the Origin of Bifunctional OER/ORR Activity for Single-Atom Catalysts Supported on C₂N by DFT and Machine Learning,” Journal of Materials Chemistry A 9 (2021): 16860–16867.

[181] Q. Zhu, Y. Huang, D. Zhou, et al., “Automated Synthesis of Oxygen-Producing Catalysts From Martian Meteorites by a Robotic AI Chemist,” Nature Synthesis 3, no. 2024 (2024): 319–328.

RIGHTS & PERMISSIONS

2026 The Author(s). Interdisciplinary Materials published by Wuhan University of Technology and John Wiley & Sons Australia, Ltd.