{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T15:40:59Z","timestamp":1785426059051,"version":"3.56.0"},"reference-count":58,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2026,4,22]],"date-time":"2026-04-22T00:00:00Z","timestamp":1776816000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"National Science Foundation","award":["CBET # 1747826"],"award-info":[{"award-number":["CBET # 1747826"]}]},{"award":["CBET # 1747826"],"award-info":[{"award-number":["CBET # 1747826"]}],"id":[{"id":"https:\/\/ror.org\/021nxhr62","id-type":"ROR","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Nanomaterials"],"abstract":"<jats:p>Electrolysis of water is a promising emission-free approach of hydrogen production, making water electrolyzers important for many renewable energy systems. Electrochemical electrodes enriched with nanocatalysts can significantly advance such technologies, but the use of nanomaterials, deployed as packed powders or painted films, is generally limited by durability and reusability challenges. To overcome these deficiencies, we have fabricated hierarchical hybrid electrode (HHE) monoliths comprising carpet-like arrays of multiwalled carbon nanotubes covalently bonded to porous reticulated carbon foams that are further functionalized with strongly attached nanocatalysts. This paper presents our investigation of HHE materials with CNT carpets and palladium nanoparticle (PdNP) catalysts in two key electrolysis reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Their performances in different electrolytes have been evaluated using cyclic voltammetry, linear sweep voltammetry and Tafel analysis. This architecture provided multi-faceted advantages, and the contribution of each nanocomponent in the monolith has been analyzed. The presence of Pd-NP in the HHE also improved the electrode\u2019s tolerance to Cl\u2212 ions, which is very promising for saline water electrolysis. These studies indicate that the HHE architecture of electrochemical electrodes can be a versatile and tunable option for future electrochemical systems relevant to renewable energy applications.<\/jats:p>","DOI":"10.3390\/nano16090500","type":"journal-article","created":{"date-parts":[[2026,4,22]],"date-time":"2026-04-22T10:38:14Z","timestamp":1776854294000},"page":"500","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Hierarchical Hybrid Electrodes (HHE) for Enhancing the Performance of Water Electrolysis Systems"],"prefix":"10.3390","volume":"16","author":[{"given":"Sanskar","family":"Shrestha","sequence":"first","affiliation":[{"name":"Department of Mechanical Engineering, Maine College of Engineering and Computing, the University of Maine, Orono, ME 04469, USA"},{"name":"Frontier Institute for Research in Sensor Technologies, the University of Maine, Orono, ME 04469, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-5231-5728","authenticated-orcid":false,"given":"Sathvik","family":"Peddamalla","sequence":"additional","affiliation":[{"name":"Frontier Institute for Research in Sensor Technologies, the University of Maine, Orono, ME 04469, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1389-2920","authenticated-orcid":false,"given":"Wenhu","family":"Wang","sequence":"additional","affiliation":[{"name":"Frontier Institute for Research in Sensor Technologies, the University of Maine, Orono, ME 04469, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sharmila M.","family":"Mukhopadhyay","sequence":"additional","affiliation":[{"name":"Department of Mechanical Engineering, Maine College of Engineering and Computing, the University of Maine, Orono, ME 04469, USA"},{"name":"Frontier Institute for Research in Sensor Technologies, the University of Maine, Orono, ME 04469, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,4,22]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"521","DOI":"10.1016\/j.ijhydene.2024.08.245","article-title":"An overview of pure hydrogen production via electrolysis and hydrolysis","volume":"84","author":"Chang","year":"2024","journal-title":"Int. J. Hydrogen Energy"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"13793","DOI":"10.1016\/j.egyr.2022.10.127","article-title":"An overview of water electrolysis technologies for green hydrogen production","volume":"8","author":"Kumar","year":"2022","journal-title":"Energy Rep."},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Takach, M., Sarajli\u0107, M., Peters, D., Kroener, M., Schuldt, F., and Von Maydell, K. (2022). Review of Hydrogen Production Techniques from Water Using Renewable Energy Sources and Its Storage in Salt Caverns. Energies, 15.","DOI":"10.3390\/en15041415"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"101426","DOI":"10.1016\/j.rineng.2023.101426","article-title":"Hydrogen production by water electrolysis technologies: A review","volume":"20","year":"2023","journal-title":"Results Eng."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"121001","DOI":"10.1088\/1748-9326\/ab57b3","article-title":"Persistent fossil fuel growth threatens the Paris Agreement and planetary health","volume":"14","author":"Jackson","year":"2019","journal-title":"Environ. Res. Lett."},{"key":"ref_6","unstructured":"Organisation for Economic Co-Operation and Development (2023). World Energy Outlook 2023, Organisation for Economic Co-Operation and Development."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"86994","DOI":"10.1007\/s11356-022-23323-y","article-title":"A review of water electrolysis\u2013based systems for hydrogen production using hybrid\/solar\/wind energy systems","volume":"29","author":"Nasser","year":"2022","journal-title":"Environ. Sci. Pollut. Res."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"5315","DOI":"10.1038\/s41467-023-41107-x","article-title":"Global land and water limits to electrolytic hydrogen production using wind and solar resources","volume":"14","author":"Tonelli","year":"2023","journal-title":"Nat. Commun."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"304","DOI":"10.1038\/s41586-021-03907-3","article-title":"Photocatalytic solar hydrogen production from water on a 100-m2 scale","volume":"598","author":"Nishiyama","year":"2021","journal-title":"Nature"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"234856","DOI":"10.1016\/j.jpowsour.2024.234856","article-title":"A review on fundamentals for designing hydrogen evolution electrocatalyst","volume":"613","author":"Qadeer","year":"2024","journal-title":"J. Power Sources"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"7568","DOI":"10.1002\/anie.201710556","article-title":"The hydrogen evolution reaction in alkaline solution: From theory, single crystal models, to practical electrocatalysts","volume":"57","author":"Zheng","year":"2018","journal-title":"Angew. Chem. Int. Ed."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"2725","DOI":"10.1021\/acsmaterialslett.4c00745","article-title":"Advanced device architecture strategies for decoupled water splitting: A review","volume":"6","author":"Mathur","year":"2024","journal-title":"ACS Mater. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"159391","DOI":"10.1016\/j.apsusc.2024.159391","article-title":"Precisely tuning the Ni3N\/Pt interface to boost the catalytic activity of alkaline hydrogen evolution reaction","volume":"653","author":"Zhou","year":"2024","journal-title":"Appl. Surf. Sci."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"12329","DOI":"10.1021\/ja405351s","article-title":"An investigation of thin-film Ni\u2013Fe oxide catalysts for the electrochemical evolution of oxygen","volume":"135","author":"Louie","year":"2013","journal-title":"J. Am. Chem. Soc."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"13925","DOI":"10.1021\/ja5082553","article-title":"Metal\u2013organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes","volume":"136","author":"Ma","year":"2014","journal-title":"J. Am. Chem. Soc."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"105270","DOI":"10.1016\/j.nanoen.2020.105270","article-title":"Iridium-based nanomaterials for electrochemical water splitting","volume":"78","author":"Chen","year":"2020","journal-title":"Nano Energy"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"4499","DOI":"10.1021\/acssuschemeng.7b04732","article-title":"P-doped Ag nanoparticles embedded in N-doped carbon nanoflake: An efficient electrocatalyst for the hydrogen evolution reaction","volume":"6","author":"Ji","year":"2018","journal-title":"ACS Sustain. Chem. Eng."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"43704","DOI":"10.1039\/D0RA09438B","article-title":"Carbon nanotubes: Functionalisation and their application in chemical sensors","volume":"10","author":"Norizan","year":"2020","journal-title":"RSC Adv."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"179106","DOI":"10.1016\/j.jallcom.2025.179106","article-title":"A review of multi-walled carbon nanotubes-based electrochemical sensors to detect heavy metals for food packaging applications","volume":"1017","author":"Lu","year":"2025","journal-title":"J. Alloys Compd."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"942","DOI":"10.1039\/D0NA00822B","article-title":"Critical challenges and advances in the carbon nanotube\u2013metal interface for next-generation electronics","volume":"3","author":"Daneshvar","year":"2021","journal-title":"Nanoscale Adv."},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Choi, C., Yun, T.G., and Hwang, B. (2023). Dispersion stability of carbon nanotubes and their impact on energy storage devices. Inorganics, 11.","DOI":"10.3390\/inorganics11100383"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"129406","DOI":"10.1016\/j.snb.2020.129406","article-title":"Understanding and improving carbon nanotube-electrode contact in bottom-contacted nanotube gas sensors","volume":"331","author":"Jung","year":"2020","journal-title":"Sens. Actuators B Chem."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Pallilavalappil, S., Jabeen, S., Rosely, C.V.S., Animasree, V.P., Manoj, N., John, H., and Pillai, S.C. (2024). Metal and metal oxide-based electrocatalysts for the hydrogen evolution reaction. Advances in Photocatalysis, Electrocatalysis and Photoelectrocatalysis for Hydrogen Production, Royal Society of Chemistry.","DOI":"10.1039\/9781837674664-00336"},{"key":"ref_24","first-page":"100359","article-title":"Asymmetric ruthenium-iron dipole enabling fast alkaline water splitting on ruthenium-doped iron-nickel layered double hydroxides","volume":"5","author":"Li","year":"2025","journal-title":"Mater. Rep. Energy"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"2985","DOI":"10.1021\/acsmaterialslett.4c00789","article-title":"New Nanocomposites Derived from Cation-Nonstoichiometric Bax(Co, Fe, Zr, Y)O3\u2212\u03b4 as Efficient Electrocatalysts for Water Oxidation in Alkaline Solution","volume":"6","author":"Abdelghafar","year":"2024","journal-title":"ACS Mater. Lett."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"2493","DOI":"10.1002\/cssc.201900393","article-title":"Ruthenium nanoparticles for catalytic water splitting","volume":"12","author":"Creus","year":"2019","journal-title":"ChemSusChem"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"12186","DOI":"10.1039\/C9DT02201E","article-title":"A hierarchically-assembled Fe\u2013MoS2\/Ni3S2\/nickel foam electrocatalyst for efficient water splitting","volume":"48","author":"Xue","year":"2019","journal-title":"Dalton Trans."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"1900699","DOI":"10.1002\/adma.201900699","article-title":"Ultrafine dual-phased carbide nanocrystals confined in porous nitrogen-doped carbon dodecahedrons for efficient hydrogen evolution reaction","volume":"31","author":"Lu","year":"2019","journal-title":"Adv. Mater."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"108517","DOI":"10.1016\/j.mtcomm.2024.108517","article-title":"Prospects of functionalized carbon nanotubes for supercapacitors applications","volume":"38","author":"Meskher","year":"2024","journal-title":"Mater. Today Commun."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"159742","DOI":"10.1016\/j.jallcom.2021.159742","article-title":"Carbon nanotube-based nanomaterials for high-performance sodium-ion batteries: Recent advances and perspectives","volume":"873","author":"Tao","year":"2021","journal-title":"J. Alloys Compd."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"267","DOI":"10.1016\/j.carbon.2015.11.078","article-title":"Electrochemical charge storage in hierarchical carbon manifolds","volume":"99","author":"Narayanan","year":"2015","journal-title":"Carbon"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"197","DOI":"10.1016\/j.cej.2015.01.027","article-title":"Hierarchical hybrid carbon nano-structures as robust and reusable adsorbents: Kinetic studies with model dye compound","volume":"268","author":"Vijwani","year":"2015","journal-title":"Chem. Eng. J."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"100096","DOI":"10.1016\/j.cartre.2021.100096","article-title":"Hierarchical nanostructured surface design for robust and flexible multifunctional devices","volume":"5","author":"Wang","year":"2021","journal-title":"Carbon Trends"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"478381","DOI":"10.1155\/2012\/478381","article-title":"Dechlorination of environmental contaminants using a hybrid nanocatalyst: Palladium nanoparticles supported on hierarchical carbon nanostructures","volume":"2012","author":"Vijwani","year":"2012","journal-title":"J. Nanotechnol."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"712","DOI":"10.1016\/j.apsusc.2012.09.146","article-title":"Palladium nanoparticles on hierarchical carbon surfaces: A new architecture for robust nano-catalysts","volume":"263","author":"Vijwani","year":"2012","journal-title":"Appl. Surf. Sci."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"15","DOI":"10.1016\/j.jwpe.2018.05.016","article-title":"Robust nanocatalyst membranes for degradation of atrazine in water","volume":"25","author":"Vijwani","year":"2018","journal-title":"J. Water Process Eng."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"232258","DOI":"10.1016\/j.jpowsour.2022.232258","article-title":"A comparative study of high density vertically aligned carbon nanotubes grown onto different grades of aluminum\u2014Application to supercapacitors","volume":"553","author":"Querne","year":"2023","journal-title":"J. Power Sources"},{"key":"ref_38","first-page":"023103","article-title":"Computational study on entanglement length and pore size of carbon nanotube buckypaper","volume":"100","author":"Li","year":"2012","journal-title":"Appl. Phys. Lett."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"1871","DOI":"10.1021\/acsenergylett.4c00266","article-title":"Tafel slope plot as a tool to analyze electrocatalytic reactions","volume":"9","author":"Park","year":"2024","journal-title":"ACS Energy Lett."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"180","DOI":"10.1002\/smll.200400118","article-title":"Chemically functionalized carbon nanotubes","volume":"1","author":"Balasubramanian","year":"2004","journal-title":"Small"},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"96","DOI":"10.1016\/j.apcatb.2014.07.049","article-title":"Pristine carbon nanotubes as non-metal electrocatalysts for oxygen evolution reaction of water splitting","volume":"163","author":"Cheng","year":"2014","journal-title":"Appl. Catal. B Environ."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"11878","DOI":"10.1021\/acs.jpcc.5c02416","article-title":"Electrochemical and electrocatalytic behavior of platinum\u2013palladium bulk alloy single crystal electrodes in alkaline media","volume":"129","author":"Melle","year":"2025","journal-title":"J. Phys. Chem. C"},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"138388","DOI":"10.1016\/j.jcis.2025.138388","article-title":"Palladium-chloride ion coordination stabilizes NiFe layered double hydroxides for alkaline seawater oxidation at industrial current densities","volume":"700","author":"Ren","year":"2025","journal-title":"J. Colloid Interface Sci."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"28209","DOI":"10.1039\/C8RA05639K","article-title":"Three-dimensional flower-like NiCo2O4\/CNT for efficient catalysis of the oxygen evolution reaction","volume":"8","author":"Ma","year":"2018","journal-title":"RSC Adv."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"37927","DOI":"10.1021\/acsomega.3c03314","article-title":"Cobalt ferrite surface-modified carbon nanotube fibers as an efficient and flexible electrode for overall electrochemical water splitting reactions","volume":"8","author":"Fatima","year":"2023","journal-title":"ACS Omega"},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"396","DOI":"10.1016\/j.jcis.2020.08.051","article-title":"Carbon nanotube boosting electrocatalytic oxygen evolution of NiFe-polyphenol coordination catalyst through donor-acceptor modulation","volume":"582","author":"Huang","year":"2020","journal-title":"J. Colloid Interface Sci."},{"key":"ref_47","doi-asserted-by":"crossref","unstructured":"Sun, J., Wang, S., Wang, Y., Li, H., Zhou, H., Chen, B., Qu, K., and Zhao, J. (2019). One simple strategy towards nitrogen and oxygen codoped carbon nanotube for efficient electrocatalytic oxygen reduction and evolution. Catalysts, 9.","DOI":"10.3390\/catal9020159"},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"1573","DOI":"10.1007\/s10800-021-01600-x","article-title":"Fluorine doped CNTs for efficient OER activity outperforming iridium supported carbon electrocatalyst","volume":"51","author":"Ali","year":"2021","journal-title":"J. Appl. Electrochem."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"14500","DOI":"10.1021\/acs.jpcc.2c04567","article-title":"Adsorption and absorption energies of hydrogen with palladium","volume":"126","author":"Schwarzer","year":"2022","journal-title":"J. Phys. Chem. C"},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"361","DOI":"10.1016\/j.jiec.2020.06.003","article-title":"Relationship between hydrogen binding energy and activity for hydrogen evolution reaction by palladium supported on sulfur-doped ordered mesoporous carbon","volume":"89","author":"Kim","year":"2020","journal-title":"J. Ind. Eng. Chem."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"8973","DOI":"10.1021\/jp991519o","article-title":"Theoretical density functional analysis of maleic anhydride chemisorption on Pd(111), Re(0001), and bimetallic PdML\/Re(0001) and PdML\/Mo(110) pseudomorphic overlayers","volume":"103","author":"Pallassana","year":"1999","journal-title":"J. Phys. Chem. B"},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"4876","DOI":"10.1039\/C8NR10281C","article-title":"W2C nanodot-decorated CNT networks as a highly efficient and stable electrocatalyst for hydrogen evolution in acidic and alkaline media","volume":"11","author":"Hu","year":"2019","journal-title":"Nanoscale"},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"13216","DOI":"10.1038\/ncomms13216","article-title":"Ultrasmall and phase-pure W2C nanoparticles for efficient electrocatalytic and photoelectrochemical hydrogen evolution","volume":"7","author":"Gong","year":"2016","journal-title":"Nat. Commun."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"160","DOI":"10.1007\/s40820-021-00679-3","article-title":"Engineering ruthenium-based electrocatalysts for effective hydrogen evolution reaction","volume":"13","author":"Yang","year":"2021","journal-title":"Nano-Micro Lett."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"26978","DOI":"10.1016\/j.ijhydene.2022.06.020","article-title":"Ru\/MoO2 decorated on CNT networks as an efficient electrocatalyst for boosting hydrogen evolution reaction","volume":"47","author":"Liu","year":"2022","journal-title":"Int. J. Hydrogen Energy"},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"3399","DOI":"10.1039\/C6SC00077K","article-title":"Heteronanowires of MoC\u2013Mo2C as efficient electrocatalysts for hydrogen evolution reaction","volume":"7","author":"Lin","year":"2016","journal-title":"Chem. Sci."},{"key":"ref_57","doi-asserted-by":"crossref","first-page":"1272","DOI":"10.1038\/s41467-020-15069-3","article-title":"Ruthenium anchored on carbon nanotube electrocatalyst for hydrogen production with enhanced Faradaic efficiency","volume":"11","author":"Kweon","year":"2020","journal-title":"Nat. Commun."},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"7855","DOI":"10.1021\/acscatal.7b02718","article-title":"Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution Reactions","volume":"7","author":"Zhang","year":"2017","journal-title":"ACS Catal."}],"container-title":["Nanomaterials"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2079-4991\/16\/9\/500\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,4,24]],"date-time":"2026-04-24T04:39:15Z","timestamp":1777005555000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2079-4991\/16\/9\/500"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,4,22]]},"references-count":58,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2026,5]]}},"alternative-id":["nano16090500"],"URL":"https:\/\/doi.org\/10.3390\/nano16090500","relation":{},"ISSN":["2079-4991"],"issn-type":[{"value":"2079-4991","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,4,22]]}}}