Bimodal Heterogeneous Functionality in Redox-Active Conjugated Microporous Polymer toward Electrocatalytic Oxygen Reduction and Photocatalytic Hydrogen Evolution

Microporous Organic Ladder Polymer with Vertically Aligned Quinones for Sodium-Ion Battery

Sodium-ion battery is emerging as a promising technology in the post-lithium-ion battery era to meet the high demand for portable energy storage devices. Custom-designed organic materials have been pursued as sustainable alternative electrodes for sodium-ion batteries, offering a solution that bypasses the need for traditional high-temperature synthesis. However, the challenge lies in achieving the desired electrochemical properties through precise structural modulation and the incorporation of redox-active functional groups. In this study, a triptycene-based microporous organic ladder polymer is developed featuring redox-active quinone moieties, designed as an anode material for high-performance sodium-ion batteries. The vertically aligned quinone moieties in the porous ladder polymer prevent the eclipsed stacking of layers, thereby enhancing the exposure of electroactive sites to electrolyte ions. Additionally, the ladder polymer exhibits almost unimodal pores due to its structural rigidity, facilitating fast Na+ ion diffusion. The redox-active quinone moieties host Na+ ions, and the microporosity supports capacitive ion storage. Consequently, a high reversible specific capacity of 316 mAh g-1 has been achieved. This study introduces a novel design strategy to develop redox-active microporous ladder polymers by carefully selecting organic building units for efficient Na+ ion storage.

One-Component Catalytic Electrodes from Metal-Organic Frameworks Covalently Linked to an Anion Exchange Ionomer

Anion-conducting organic-inorganic polymers (OIPs), constructed using metal-organic framework (MOF)-like structures with non-toxic, non-rare catalytic metals (Fe3+, Zr4+), have been developed. The incorporation of MOF-like structures imparts porosity to the polymers, classifying them as porous organic polymers (POPs). The combination between catalytic activity, ion conduction, and porosity allows the material to act as one-component catalytic electrodes. A high catalytic activity is expected since the entire surface area contributes to electrocatalysis, rather than being restricted to triple-phase boundaries. The synthesis involved anchoring a synthon onto a commercial polymer, assembling organo-metallic moieties, and functionalizing with quaternary ammonium (QA) groups. Two hybrid materials, Zr-POP-QA and Fe-POP-QA, were thoroughly characterized by NMR, FTIR, XPS, BET surface area (≈200 m2/g), and TGA. The resulting electrodes demonstrated a high electrochemically active surface area and a high efficiency for the oxygen reduction reaction (ORR), a critical process for energy storage and conversion technologies. The performance was characterized by a 4-electron reduction pathway, a high onset potential (≈0.9 V vs. RHE), and a low Tafel slope (≈0.06 V). We attribute this efficiency to the high active surface area, which results from the simultaneous presence of catalytic transition metal ions (Zr or Fe) and ion conducting groups.

Ultrathin, large area β-Ni(OH)2 crystalline nanosheet as bifunctional electrode material for charge storage and oxygen evolution reaction

Bifunctional electrode materials are highly desirable for meeting increasing global energy demands and mitigating environmental impact. However, improving the atom-efficiency, scalability, and cost-effectiveness of storage systems, as well as optimizing conversion processes to enhance overall energy utilization and sustainability, remains a significant challenge for their application. Herein, we devised an optimized, facile, economic, and scalable synthesis of large area (cm2), ultrathin (∼2.9 ± 0.3 nm) electroactive nanosheet of β-Ni(OH)2, which acted as bifunctional electrode material for charge storage and oxygen evolution reaction (OER). The β-Ni(OH)2 nanosheet electrode shows the volumetric capacity of 2.82 Ah.cm-3(0.82 µAh.cm-2) at the current density of 0.2 mA.cm-2. The device shows a high capacity of 820 mAh.cm-3 with an ultrahigh volumetric energy density of 0.33 Wh.cm-3 at 275.86 W.cm-3 along with promising stability (30,000 cycles). Furthermore, the OER activity of ultrathin β-Ni(OH)2 exhibits an overpotential (η10) of 308 mV and a Tafel value of 42 mV dec-1 suggesting fast reaction kinetics. The mechanistic studies are enlightened through density functional theory (DFT), which reveals that additional electronic states near the Fermi level enhance activity for both capacitance and OER.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction

The widespread application of fuel cells is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR), which traditionally necessitates the use of high-cost platinum group metal catalysts. The indispensability of these metal catalysts stems from their ability to overcome kinetic barriers, but their high cost and scarcity necessitate alternative strategies. In this context, porous organic polymers (POPs), which are built up from the molecular level, are emerging as promising precursors to produce carbonaceous catalysts owning to their cost-effectiveness, high electrical conductivity, abundant active sites and extensive surface area accessibility. To enhance the intrinsic ORR activity and optimize the performance of these electrocatalysts, recognizing, designing, and increasing the density of active sites are identified as three crucial steps. These steps, which form the core of our review, serve to elucidate the link between the material structure design and ORR performance evaluation, thereby providing valuable insights for ongoing research in the field. Leveraging the precision of polymer skeletons based on molecular units, POP-derived carbonaceous catalysts provide an excellent platform for in-depth exploration of the role and working mechanism for the specific active site during the ORR process. In this review, the recent advances pertaining to the synthesis techniques and electrochemical functions of various types of active sites, pinpointed from POPs, are systematically summarized, including heteroatoms, surficial substituents and edge/defects. Notably, the structure-property relationship, between these active sites and ORR performance, are discussed and emphasized, which creates guidelines to shed light on the design of high-performance ORR electrocatalysts.

Heterocyclic Modulated Electronic States of Alkynyl-Containing Conjugated Microporous Polymers for Efficient Oxygen Reduction

The oxygen reduction reaction (ORR) is a key process in green energy conversion technology. Heteroatom doping has been proven to be a prospective strategy to prepare metal-free carbon-based electrocatalysts, but such methods often suffer from uncontrollable catalyst frameworks and imprecise active sites. Herein, an organic heterocyclic strategy is adopted to modulate the charge redistribution of alkynyl-containing conjugated microporous polymers (CMPs) by introducing varied five-membered heterocyclic structures. Among these CMPs, the S, 2N-containing thiadiazole heterocyclic molecule (CMP-Tdz) with carbonized alginate materials (C_CA ) displays a remarkable quasi-four-electron-transfer ORR pathway, exhibiting an excellent half-wave potential (E_1/2 ) of 0.77 V, coupled with superior methanol tolerance and electrochemical stability, which are among the highest performance in the metal-free organic catalytic material systems. Density functional theory calculations prove that the high catalytic performance of these catalysts originates from the sp-hybridized C atom (site-2) which is activated by their adjacent heterocyclic structures. Importantly, the five-membered heterocyclic structures can also modulate the local charge distribution, and increase dipole moment, with significantly improved catalytic kinetics. This incorporation of chemically designed heterocyclic-containing alkynyl-CMPs provides a new approach to developing efficient metal-free carbon-based electrocatalysts for fuel cells.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.

Metal-Free Catalysis: A Redox-Active Donor-Acceptor Conjugated Microporous Polymer for Selective Visible-Light-Driven CO2 Reduction to CH4

Achieving more than a two-electron photochemical CO₂ reduction process using a metal-free system is quite exciting and challenging, as it needs proper channeling of electrons. In the present study, we report the rational design and synthesis of a redox-active conjugated microporous polymer (CMP), TPA-PQ, by assimilating an electron donor, tris(4-ethynylphenyl)amine (TPA), with an acceptor, phenanthraquinone (PQ). The TPA-PQ shows intramolecular charge-transfer (ICT)-assisted catalytic activity for visible-light-driven photoreduction of CO₂ to CH₄ (yield = 32.2 mmol g^-1) with an impressive rate (2.15 mmol h^-1 g^-1) and high selectivity (>97%). Mechanistic analysis based on experimental results, in situ DRIFTS, and computational studies reveals that the potential of TPA-PQ for catalyzing photoreduction of CO₂ to CH₄ was energetically driven by photoactivated ICT upon surface adsorption of CO₂, wherein adjacent keto groups of PQ unit play a pivotal role. The critical role of ICT for stimulating photocatalysis is further illustrated by synthesizing another redox-active CMP (TEB-PQ), bearing triethynylbenzene (TEB) and PQ, that shows 8-fold lesser activity for photoreduction toward CO₂ to CH₄ (yield = 4.4 mmol g^-1) as compared to TPA-PQ. The results demonstrate a novel concept for CO₂ photoreduction to CH₄ using an efficient, sustainable, and recyclable metal-free robust organic photocatalyst.

A red luminescent Eu3+ doped conjugated microporous polymer for highly sensitive and selective detection of aluminum ions

A Eu³⁺ doped-CMP composite can be used as a chemosensor for highly sensitive and selective detection of Al³⁺.