Research progress on polymer electrochemical hydrogen evolution

1 Introduction

The widespread use of traditional fossil fuels has triggered global energy crises and environmental pollution, making the development of renewable clean energy an urgent priority (Turner, 2004; Zhang et al., 2019). Currently, the key to advancing the widespread application of electrochemical energy conversion technologies lies in developing highly efficient and cost-effective electrochemical reactions, including the hydrogen evolution reaction (HER) (Lin et al., 2022; Adil et al., 2023a; Hou et al., 2019; Saleem et al., 2022), oxygen evolution reaction (OER) (Adil et al., 2022a; Adil et al., 2020), oxygen reduction reaction (ORR) (Hou et al., 2019), carbon dioxide reduction reaction (CO₂ RR) (Saleem et al., 2022), and nitrate reduction reaction (NO₃ RR). Water electrolysers decompose water via the cathodic HER and anodic OER to produce hydrogen and oxygen. Hydrogen serves as a carrier for clean energy, converting chemical energy into electricity through the HOR and ORR in fuel cells (Wu et al., 2021; Neil, 2020; Tan and Tan, 2017; Dong et al., 2022). CO₂ RR combined with OER can transform carbon dioxide into hydrocarbon fuels and high-value-added chemicals. The charging and discharging processes of rechargeable metal-air batteries involve the reversible reaction pair of OER and ORR (Jia et al., 2015; Ding and Wang, 2013; Coelho et al., 2012). Consequently, researchers have developed various catalysts, among which precious metals and their oxides represent the most efficient catalysts, exhibiting outstanding activity and stability in hydrogen-oxygen related reactions (Tian and Zhu, 2020; Sun et al., 2021). However, issues such as high cost, scarcity, and insufficient durability limit their large-scale application. Consequently, developing highly efficient, durable, and low-cost catalysts has become a core direction for advancing clean electrochemical energy technologies. Amid dual challenges of energy crises and environmental pollution, hydrogen energy has garnered global attention as a clean, high-energy-density carrier. As a primary hydrogen production method, the efficiency and cost of the electrochemical hydrogen evolution reaction (HER) largely depend on the performance of electrocatalysts (Yang et al., 2019; Adil et al., 2023b; Adil et al., 2024). Although Pt-based noble metal electrocatalysts exhibit high reactivity, their high cost limits widespread application in catalysis, making the design of novel, highly active, and stable hydrogen evolution electrocatalysts an urgent priority (Friedman et al., 2018). In recent years, two-dimensional (2D) materials have demonstrated significant application potential and research value in electrocatalytic hydrogen evolution due to their unique physicochemical properties, including tunable interlayer spacing, edge active sites, and superior conductivity (Guo et al., 2018; Venugopal et al., 2022; Adil et al., 2022b; Bansal et al., 2025). Composed of only a single or few atomic layers, these materials exhibit immense application potential across electronic, optical, mechanical, and other fields (Adil et al., 2025; Song et al., 2004). Furthermore, two-dimensional nanostructures effectively mitigate stress induced by volume changes and reduce damage to material structures caused by reaction-generated bubbles. This optimizes the adsorption/desorption strength of intermediates, resulting in superior electrocatalytic activity and stability.

Over the past few decades, researchers have developed various catalysts for hydrogen production. Early research focused on inorganic semiconductor catalysts such as metal oxides, sulfides, and nitrides. However, their high cost, significant metal toxicity, and limited stability restrict their long-term application. To overcome these issues, researchers have turned their attention to organic polymer semiconductors with unique electronic structures and excellent optoelectronic properties, particularly polymers featuring continuous delocalized π-conjugated systems. Their π-conjugated systems facilitate rapid charge transfer and separation. Furthermore, conjugated polymers allow for molecular-level structural design, enabling easy adjustment of their electronic properties and catalytic activity. Consequently, polymers offer novel approaches and extensive research opportunities for developing high-performance electrocatalysts for hydrogen evolution.

Polymers encompass intrinsically microporous polymers (PIMs), hypercrosslinked polymers (HCPs), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), covalent organic polymers (COPs), covalent triazine frameworks (CTFs), and porous aromatic frameworks (PAFs). Compared to traditional electrocatalysts (metals or their oxides), polymers offer tunable pore structures, high specific surface areas, and diverse chemical compositions. Functionalization can be achieved through pre-design or post-modification of organic linkers. Furthermore, polymers are composed of light elements linked by strong covalent bonds, exhibiting excellent properties such as low density and high chemical stability. These characteristics endow polymers with broad application potential in the field of electrocatalysis. This review summarizes the practical applications of polymers in various electrocatalytic reactions and analyzes their current application prospects in relevant research fields. The principle of electrocatalysis involves interactions between the catalyst and reactants that lower the reaction energy barrier and optimize the reaction pathway, thereby achieving efficient energy conversion. When reactants interact with the active sites and functional units of polymers, corresponding changes occur in the polymer’s electronic structure, surface chemistry, and conductivity, thereby influencing its electrocatalytic performance. By controlling the pore structure, functional groups, and distribution of active sites within polymers, highly efficient catalysis for specific electrocatalytic reactions can be achieved. Polymers can be applied in reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂ RR), and nitrate reduction reaction (NO₃ RR). This review summarizes the research progress of polymers in these electrocatalytic fields.

2 Synthesis strategies of polymer-based electrocatalysts for HER

Unlike inorganic two-dimensional materials (e.g., MXenes, transition metal dichalcogenides (TMDs), carbides), which have been extensively reviewed in existing literature, this section focuses exclusively on the synthesis strategies of polymer-based electrocatalysts for HER. Specifically, the following subsections (2.1–2.6) will systematically elaborate on the synthetic methodologies, key characteristics, and HER-related performance of five major polymer systems: covalent organic frameworks (COFs), conjugated microporous polymers (CMPs), linear conjugated polymers (e.g., polyaniline, polypyrrole, polythiophene), donor–acceptor (D–A) conjugated polymers, and metal–organic polymers (including metal nanoparticle-functionalized polymers). These polymer systems have emerged as promising HER electrocatalyst candidates, attributed to their tunable chemical structures, adjustable electronic properties, and functional versatility—features that enable precise regulation of catalytic active sites, electron transfer efficiency, and mass transport capacity through tailored synthesis strategies. A concise summary of the core synthesis methods, advantages, and limitations of these polymer-based electrocatalysts will also be provided to establish a comprehensive overview of their synthetic landscape.

2.1 Covalent organic frameworks (COFs)

COFs are crystalline porous polymers with periodic ordered structures formed by covalent bonds (Cote et al., 2005). Their well-defined pore channels and predictable active site distribution make them attractive for HER electrocatalysis. Solvothermal synthesis is the most widely used method for COF preparation. Typically, boronic acid and catechol derivatives are condensed in mixed solvents (e.g., dioxane/mesitylene) at 120 °C for 3–7 days. For HER-oriented COFs, metalloporphyrin or bipyridine units are often incorporated as active centers. Patra et al. reported a porphyrin-based metal-free covalent organic polymer, TpPAM, synthesized via Schiff base condensation between trimethylenecarbonylresorcinol and 5,10,15,20-tetra(4-aminophenyl)-21H,23H-porphine (Patra et al., 2017). This material exhibited outstanding HER catalytic performance with a Faradaic efficiency as high as 98% and maintained 91% of its initial current density after 1000 cycles.

Ionothermal synthesis employs molten salts (e.g., ZnCl₂) as both solvent and catalyst at 400 °C–600 °C (Kuhn et al., 2008). This method produces highly stable triazine-based COFs (CTFs) with excellent HER activity. Mechanochemical synthesis (solvent-free ball milling at room temperature) offers a green and rapid route (<1 h), though the resulting polymers show lower crystallinity (Biswal et al., 2013).

2.2 Conjugated microporous polymers (CMPs)

CMPs are amorphous but permanently porous networks combining π-conjugated backbones with micropores (<2 nm). Their synthetic flexibility allows diverse heteroatom (N, S, P) doping (Zhou, 2024).

Sonogashira–Hagihara cross-coupling (Pd-catalyzed coupling between aryl halides and terminal alkynes) is the most common route for CMPs, yielding materials with tunable pore sizes (0.5–2 nm) and high surface areas (up to 1000 m²/g) (Cooper, 2009). Suzuki polycondensation (coupling between aryl boronic acids and aryl halides) offers better tolerance to functional groups and is suitable for incorporating HER-active moieties (e.g., thiophene, pyrazine). Yamamoto coupling (Ni(0)-catalyzed homo-coupling of aryl dihalides) produces highly conjugated networks with excellent charge transport properties (Weber and Thomas, 2008).

2.3 Linear conjugated polymers (polyaniline, polypyrrole, polythiophene)

These classic conducting polymers have been revisited for HER due to their metal-free nature and ease of processing. Electrochemical polymerization enables direct growth on electrode surfaces (e.g., glassy carbon, carbon cloth) via cyclic voltammetry or chronoamperometry in monomer-containing electrolyte (e.g., 0.1 M aniline in 0.5 M H₂SO₄) (Malinauskas, 1999). This method allows precise control over film thickness and morphology. Chemical oxidative polymerization uses oxidants such as (NH₄)₂S₂O₈ or FeCl₃ in acidic aqueous media. The resulting powders can be drop-cast or spray-coated onto electrodes. Doping with heteroatoms (e.g., N, S) during synthesis significantly enhances HER activity.

2.4 Donor–acceptor (D–A) conjugated polymers

D–A polymers, consisting of electron-rich donor and electron-deficient acceptor units along the backbone, exhibit tunable electronic structures and efficient intramolecular charge transfer—beneficial for electrocatalytic HER (Cheng et al., 2009). Stille cross-coupling (Pd-catalyzed coupling between organotin donors and organohalide acceptors) provides high molecular weight polymers with alternating D–A structures. Common HER-relevant acceptors include benzothiadiazole, diketopyrrolopyrrole, and naphthalene diimide (Gu et al., 2024). Suzuki–Miyaura coupling (a tin-free alternative using boronic esters) offers better environmental compatibility. Post-polymerization metalation (e.g., with Co, Ni, or Fe) is often applied to introduce catalytic metal centers (Bai et al., 2024).

2.5 Metal–organic polymers and metal nanoparticle-functionalized polymers

Metal-based polymers exhibit highly efficient electrocatalytic activity. The metallization strategy—introducing metallic active sites into porous organic polymers—has become a recognized approach.

Ranjeet (2022) successfully synthesized a bimetallic (Co and Ni) porphyrin two-dimensional polymer (2DP3) through the condensation reaction of metallated octahydroxyporphyrin and fumaric acid. At 10 mA·cm^-2, this polymer exhibited an overpotential of 105 mV, a Tafel slope of 74.1 mV·dec⁻¹, and a Faradaic efficiency of 97%. The enhanced HER performance was attributed to its polar porous structure and Ni ion incorporation.

Metal nanoparticle-functionalized polymers further enhance catalyst activity and stability. Das (2013) synthesized an imine-linked 2D crystalline COF as a carrier for Ru ions, forming Ru@COF with an onset overpotential of 159 mV and Tafel slope of 79 mV·dec⁻¹. Fang et al. reported an imidazole-based POPs material loaded with Pt nanoparticles (∼2.28 nm), achieving an overpotential of 56 mV at 10 mA·cm^-2 and stability over 4000 cycles.

Table 1 summarizes the key synthetic methods for polymer-based HER electrocatalysts. Solvothermal and cross-coupling methods yield high-performance catalysts but require inert conditions and extended reaction times (Arshad et al., 2026; Cui et al., 2024). Electrochemical and mechanochemical approaches offer rapid, scalable, and greener alternatives.

TABLE 1. Comparison of synthesis methods and performance of polymer-based HER electrocatalysts.

GCE, glassy carbon electrode; FE, faradaic efficiency; η₁₀ = overpotential at 10 mA·cm-2.*.

2.6 Applications of polymers in the hydrogen evolution reaction (HER)

HER is the process of producing hydrogen through water electrolysis, primarily occurring at the cathode. With the growing demand for clean energy, researchers have developed polymer materials capable of efficiently catalyzing hydrogen evolution reactions. By rationally designing the chemical structure and electronic properties of these materials, their catalytic activity and stability can be enhanced.

Metal-free polymers containing functional groups with paired electrons, such as benzene, thiophene, and pyrrole, can effectively reduce the material’s bandgap through conjugation effects and molecular orbital overlap. Patra et al. reported a porphyrin-based metal-free covalent organic polymer, TpPAM, and its application in electrocatalytic HER (Patra et al., 2017). TpPAM was synthesized via a Schiff base condensation reaction between trimethylenecarbonylresorcinol and 5,10,15,20-tetra(4-aminophenyl)-21H,23H-porphine, exhibiting outstanding HER catalytic performance with a Faradaic efficiency as high as 98%. The material maintained 91% of its initial current density after 1000 cycles, demonstrating excellent stability.

Metal-based polymers exhibit highly efficient electrocatalytic activity and are widely regarded as ideal catalysts for the hydrogen evolution reaction (HER). The metallization strategy, which involves introducing metallic active sites into porous organic polymers to achieve high catalytic activity, has become a recognized approach with significant research value. Ranjeet (2022) successfully synthesized a bimetallic (Co and Ni) porphyrin two-dimensional sheet polymer (2DP3) through the condensation reaction of metallated octahydroxyporphyrin and fumaric acid, which served as an electrocatalyst for HER. At a current density of 10 mA·cm^-2, this polymer exhibited an overpotential of 105 mV, a Tafel slope of 74.1 mV·dec⁻¹, and a Faraday efficiency of 97%. The enhanced HER performance of the polymer was primarily attributed to its polar porous structure and the incorporation of Ni ions into the Co framework. The polar porous structure enhances the accessibility of active catalytic sites to proton sources, while Ni ion doping improves material conductivity and reduces the adsorption energy barrier for active species, further optimizing catalytic performance.

Metal nanoparticle-functionalized polymers can enhance the activity and stability of catalysts, thereby yielding highly efficient catalytic materials. Das (2013) successfully synthesized an imine-linked two-dimensional crystalline COF through the condensation reaction of benzene-1,3,5-triformaldehyde with 3,4-diaminobenzohydrazide. This two-dimensional COF served as an ideal carrier for encapsulating ruthenium (Ru) ions, forming the Ru@COF composite material. Experimental results demonstrate that Ru@COF exhibits outstanding HER activity in acidic media, with an onset overpotential of only 159 mV and a Tafel slope of 79 mV·dec⁻¹. Fang et al. (et al.) reported an imidazole-based POPs material. Through the silane chemistry and alkylation reaction of tetrakis [4-(1H-imidazol-1-yl)phenyl] methane (IM) with 1,4-bis(bromomethyl)benzene (2BrB) via silane chemistry and alkylation, successfully achieving surface modification on a glassy carbon electrode (GCE). Subsequently, Pt nanoparticles with an average size of approximately 2.28 nm were loaded onto the electrode surface via reduction of PtCl₆ ²⁺. The imidazole-based polymer not only effectively suppressed Pt nanoparticle agglomeration and loss but also enhanced their dispersion and stability. The imidazole moiety facilitated electron transfer through π-π interactions. The modified electrode loaded with Pt nanoparticles exhibited a low overpotential of 56 mV at a current density of 10 mA cm^-2 and demonstrated stability over 4000 cycles.

3 Enhancing HER using polymers as catalysts

By modifying the functional catalysts on the electrodes, the energy barrier in the water electrolysis process for producing green hydrogen can be overcome. Precious metals, non-precious metals, their alloys and other derivatives, carbon compounds, and polymers have been widely and effectively used as catalysts and/or supports to enhance the electrocatalytic hydrogen evolution reaction (HER). Although the performance of conductive polymers as catalysts and conductive supports has been extensively and rigorously evaluated, these materials and their electrocatalytic properties have not yet received the attention and in-depth research they deserve in the field of electrocatalysis, particularly in the context of HER.

An efficient HER catalyst should possess the following characteristics: large specific surface area, minimal overpotential, high exchange current density, excellent stability, durability, and reproducibility. Polymers are receiving widespread attention in the field of HER catalysis and catalyst design (Venugopal et al., 2022). In this field, there are numerous opportunities for the integrated application of polymers, including direct use as catalysts, catalyst supports, fixing agents, adsorption sites for reaction intermediates, proton trapping agents, hydrophilic surfaces, porous materials, and even as templates for catalyst synthesis, thereby facilitating the development of catalysts with ideal structures to enhance HER efficiency.

4 Mechanism of thermal decomposition of polymeric compound

Overall, previous studies have confirmed that random chain scission primarily governs the thermal cracking (non-catalytic cracking) of polymers through a radical mechanism. This process involves complex chain reactions, including chain initiation, chain propagation, and chain termination (Zhang et al., 2019; Jia et al., 2015; Dong et al., 2022; Jia et al., 2015; Ding and Wang, 2013; Coelho et al., 2012). As shown in Figure 1a, the heat-induced chain initiation stage begins with the random homolytic cleavage of C–C bonds in long polymer chains, thereby forming primary radicals (PRs).

In contrast to the radical mechanism of thermal cracking (non-catalytic), catalytic cracking is a cracking process primarily based on the carbocation mechanism (Coelho et al., 2012; Friedman et al., 2018; Guo et al., 2018; Venugopal et al., 2022; Adil et al., 2022b; Bansal et al., 2025). Before discussing the mechanism of catalytic cracking, we will first examine the zeolite catalysts commonly used in this reaction and their acid sites (see Figure 1b). As is well known, zeolites are microporous materials with a regular tetrahedral network structure composed of alumina and silica.

As mentioned earlier, the primary C–C bond-breaking reactions in the catalytic cracking process occur at Brønsted acid sites. Therefore, the strength and density of Brønsted acids have a significant impact on catalytic performance. On the other hand, the role of Lewis acids in the catalytic cracking of polyolefins should not be overlooked. As another characteristic of zeolite catalysts, the pore structure plays a crucial role in the accessibility of these acid sites, the diffusion of reactants, and shape selectivity, which in turn may further influence the catalyst’s activity, stability, and selectivity. Before conducting a comprehensive discussion of zeolite-catalyzed polyolefin conversion influenced by acid sites and pore structure, this paper presents some fundamental principles to clarify this issue. First, it is difficult to separate the effects of acidity and pore structure, which makes comparisons between different zeolite catalysts challenging. Second, polyolefin upgrading reactions have been conducted in various reactors and under a wide range of conditions. The methods used for product collection and quantification vary across the literature, complicating the comparison of specific numerical values and potentially leading to misleading results. Therefore, we prefer to discuss the influence of acidity in specific types of zeolites and compare trends across different studies, rather than focusing on specific numerical values.

Acidity is not the sole criterion for comparing the catalytic performance of different zeolite catalysts in polyolefin cracking reactions. The pore structure of zeolite catalysts affects the accessibility of their acid sites to polyolefin reactants or any intermediates, thereby ultimately influencing catalytic activity. More importantly, different zeolites possess inherent topological and microporous structures that directly affect their catalytic selectivity and stability. Furthermore, pore size dynamics govern the diffusion and adsorption of substrate and intermediate molecules, while the presence of mesopores in zeolites enhances mass transfer of polyolefin reactants and product molecules (Jia et al., 2015; Ding and Wang, 2013; Coelho et al., 2012; Tian and Zhu, 2020; Sun et al., 2021; Yang et al., 2019). Therefore, studies on the structure–activity relationship in polyolefin cracking must consider both acidity and pore structure.

In summarizing the findings from the existing literature, we have outlined the key structure-activity relationships in polyolefin catalytic conversion reactions catalyzed by various zeolite catalysts (Figure 2). Although higher catalytic activity, stability, and product selectivity can be achieved by adjusting acidity, pore structure, and morphology, the application of pure zeolite catalysts in polyolefin catalytic conversion is limited due to their unique catalytic sites (i.e., a single catalytic active component). Furthermore, there are some drawbacks, such as low selectivity for a single product or similar products, which increases the additional costs of product separation and purification. Furthermore, catalyst deactivation is difficult to avoid during long-term reactions in industrial plants. Therefore, the metal/zeolite catalysts described below represent a solution worth considering.

5 Summary and outlook

In this review, we summarize a series of studies on the application of conjugated polymers in electrocatalytic hydrogen evolution and analyze the unique advantages they exhibit in this field: (1) The structural designability of conjugated polymers at the molecular level enables precise tuning of their electronic structures and the introduction of highly efficient catalytic active sites (e.g., heteroatoms, metal coordination centers), providing more active centers for electrocatalytic reactions (Otep et al., 2020). (2) The highly delocalized π-electron system of conjugated polymers effectively facilitates charge transport under applied potential. Through interfacial engineering, conjugated polymers can form heterojunctions with conductive substrates or construct graded porous architectures, accelerating the hydrogen evolution reaction kinetics (Emin et al., 2026). (3) The high chemical stability of conjugated polymers allows them to protect active metal species from aggregation or dissolution during long-term electrolysis, while simultaneously promoting electron transfer from the electrode to the active sites (Wang et al., 2023).

Despite significant progress in polymer-based electrocatalytic hydrogen evolution, several challenges remain to be addressed: (1) Most polymer electrocatalysts still rely on precious metals (e.g., Pt, Ru) as active centers or co-catalysts, resulting in high application costs. Therefore, developing inexpensive, earth-abundant, metal-free polymer electrocatalysts with high activity represents a major challenge; (2) The hydrogen evolution reaction occurs at specific active sites (e.g., N, S, P heteroatoms, metal–N coordination, or edge defects). The number, intrinsic activity, and accessibility of these sites are critical to performance. While molecular design can introduce active sites in some polymers, systematic studies on the relationship between polymer structure and active site density remain limited; (3) Although some understanding of the reaction mechanisms for electrocatalytic hydrogen evolution exists, the correlation between polymer structure (porosity, conjugation length, heteroatom doping), active site environment, and electrocatalytic performance remains incompletely elucidated; (4) While many developed polymers exhibit promising HER performance in laboratory-scale three-electrode systems, significant challenges remain before practical commercialization. Key issues include long-term stability under high current densities, electrode fabrication scalability, and electrochemical stability in harsh acidic or alkaline electrolytes.

To address these challenges, it is imperative to leverage the inherent advantages of polymers to accelerate their development and achieve high-performance electrocatalysts. First, D–A conjugated polymers can be designed to enhance intramolecular charge transfer (ICT) effects, thereby reducing charge transport resistance and improving HER kinetics (Xia et al., 2023). Second, structural modifications such as altering conjugation length, introducing electron-withdrawing/donating substituents, or controlling pore architecture can effectively regulate charge transfer and active site exposure, thereby tuning electrocatalytic properties. Third, composite formulations with conductive materials (e.g., carbon nanotubes, graphene, carbon cloth) can achieve synergistic optimization of conductivity and catalytic activity, improving HER efficiency. Furthermore, for polymers with limited electrochemical stability, incorporating crosslinking groups or conductive protective layers (e.g., carbon coatings) can enhance their durability during prolonged electrolysis. Finally, leveraging in situ spectroscopic techniques (e.g., Raman, XPS, XAFS) and theoretical computational methods (e.g., DFT) provides deeper insights into the adsorption energetics of hydrogen intermediates (*H), charge transfer pathways, and active site evolution under working conditions, offering theoretical guidance for catalyst design and performance optimization.

Beyond the currently understood HER pathways (Volmer–Heyrovský and Volmer–Tafel), exploring alternative reaction mechanisms or synergistic effects in bimetallic polymer systems may enable highly efficient and selective electrocatalytic hydrogen production. Transitioning polymer-based electrocatalysts from laboratory-scale testing (e.g., rotating disk electrode, glassy carbon electrode) to practical membrane electrode assembly (MEA) electrolyzers requires extended development. Advancing interdisciplinary integration—such as leveraging computational materials science to predict active sites, combined with in situ characterization techniques (e.g., electrochemical atomic force microscopy, in situ Raman spectroscopy) to monitor catalyst evolution under operation—will enable future breakthroughs in performance limits. This will advance the large-scale application of low-cost, high-efficiency, durable polymer-based cathodes for water electrolysis.

In summary, we hope this review provides actionable design strategies for future research on conjugated polymer electrocatalysts for the hydrogen evolution reaction, thereby addressing issues such as poor intrinsic conductivity, limited active site density, and insufficient long-term stability in polymer electrocatalysts. Furthermore, we believe that as global carbon neutrality goals advance, research outcomes in this field will not only inject new momentum into the hydrogen energy industry but also potentially extend to other electrocatalytic systems (e.g., CO₂ reduction, oxygen reduction, nitrate reduction), spearheading the innovation of next-generation clean energy technologies.

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Footnotes

Edited by: Ajay Kumar, Los Alamos National Laboratory (DOE), United States

Reviewed by: Panagiotis Bexis, Los Alamos National Laboratory (DOE), United States