Arnab Dutta

The large-scale deployment of electrochemical CO ₂ conversion requires the development of catalysts that are both highly selective and stable under operating conditions.

From molecular catalysts to hybrid materials and photoreactors, 3d transition metals enable increasingly efficient and robust photocatalytic CO ₂ conversion, defining a pathway from mechanistic insight to scalable solar-fuel devices.

Developing molecular catalysts capable of bidirectional CO2/CO conversion is crucial for advancing carbon‐neutral energy technologies, yet achieving such bidirectionality remains a formidable challenge. In recent years, several complexes, including [Co(II)(bpy)]2+, have been shown to catalyze the two-electron reduction of CO2 to CO selectively but not the reverse reaction of CO oxidation. One of the key features of metal complexes demonstrating bidirectionality (see e.g. ACS Catal. 2021, 11, 5934), is the access to a range of oxidation states. Since accessing high oxidation states for a Co centre is usually quite energy demanding, one alternative solution is to design ligands that can stabilize low oxidation states. Redox non-innocent ligands can stabilize formally very low oxidation states by storing the excess electrons on the ligands. Here, we present a cobalt complex bearing two redox‐non‐innocent bis(2‐(phenylazo)pyridine) (PAP) ligands that enables energy‐efficient, bidirectional CO2/CO electrocatalysis under mild conditions. Cooperative electron redistribution between the cobalt centre and the PAP ligand manifold facilitates multielectron storage and proton‐coupled electron transfer, allowing seamless switching between CO2 reduction and CO oxidation with minimal structural reorganisation. The catalyst displays a pronounced bias toward CO2 reduction, operating at an overpotential as low as 0.27 V with a turnover frequency of ~30 s-1 and Faradaic efficiency of 85%, while selectively oxidising CO back to CO2 with comparable activity. Spectroelectrochemical characterisation and multireference computational analysis reveal a mechanistic landscape governed by ligand‐centred redox flexibility and shared catalytic intermediates. This work introduces a rare example of a homogeneous, bidirectional cobalt catalyst, establishing a general design principle for next‐generation molecular systems that operate near thermodynamic potentials and close the carbon cycle through bidirectional electrocatalysis.

The efficient and reversible interconversion of protons and molecular hydrogen is essential for commercially viable hydrogen energy systems. Natural hydrogenases catalyze this process with unmatched efficiency but lack operational stability outside biological conditions. Inspired by their catalytic activity, a water-soluble nonheme iron complex, Fe(pap)3(ClO4)2 (pap = 2-(phenylazo)pyridine), has been developed as a molecular catalyst exhibiting bifunctional homogeneous hydrogen evolution (HER) and hydrogen oxidation (HOR) activity in aqueous media. The complex achieves a turnover number of ∼800 for HOR and delivers HER with ∼90% Faradaic efficiency at overpotentials near 150 mV, maintaining activity under mild conditions. Ligand redox activity facilitates electron and proton transfer, enabling efficient HER and HOR. Beyond mechanistic relevance, the catalyst is integrated into a prototype fuel cell and electrolyzer. In HOR fuel cell mode, an open-circuit voltage of 0.98 V and a peak power density of 0.87 mW·cm–2 is observed, while in a separate PEM electrolyzer operated in HER mode, sustained hydrogen evolution is observed under acidic conditions. This study highlights the potential of nonheme molecular iron catalysts to enable translation of molecular electrocatalysis toward device-relevant hydrogen conversion.

Rechargeable metal‐air batteries offer exceptional theoretical energy densities but require durable bifunctional electrocatalysts that efficiently mediate oxygen reduction (ORR) and oxygen evolution (OER). Here, a triazole‐based cobalt complex (CoTzN) is developed to bridge molecular‐level mechanistic understanding with practical device integration. In homogeneous media, CoTzN catalyzes bidirectional ORR/OER, while electrochemical and spectroelectrochemical studies identify key Co(III)–OOH and Co(III)–O• intermediates. These molecular insights guide catalyst heterogenization on carbon nanotubes through π–π interactions (CoTzN‐P) and covalent amide anchoring (CoTzN‐C). CoTzN‐C exhibits enhanced charge transfer, stability across pH 7–14, and a highly selective four‐electron ORR pathway (n ≈ 3.95) with 450 rapid charge‐discharge cycles over ≈30 h, while a non‐aqueous Li‐air battery operates for 80 prolonged cycles (≈160 h). This molecular‐to‐device framework demonstrates how coordination‐level catalyst design can be translated into robust hybrid electrodes for cross‐platform rechargeable metal–air batteries.

MXenes have emerged as a versatile two-dimensional platform in advanced catalytic systems because of their tunable surface chemistry, metallic conductivity, and structural diversity. Nevertheless, severe agglomeration and limited colloidal stability may undermine their catalytic performance. Integrating MXenes with low-dimensional materials through rational nanoscale construction mitigates these limitations while enhancing mass and charge transport, interfacial performance, dispersion, and catalytic activity. This review focuses on the design, functionality, and catalytic applications of MXene-based composites. It begins with a brief introduction to the chemical and crystal structures of MAX phases and MXenes, followed by an overview of representative MXene-based composites, including 0D–2D, 2D–2D, and 3D–2D systems. The article then discusses their applications in heterogeneous catalysis, photocatalysis, biocatalysis, and electrocatalysis. Finally, we summarize the perspectives, highlighting the current challenges and future opportunities in the rational design and industrial transformation of MXene composite nanoscale construction.

Developing efficient, low-cost bifunctional electrocatalysts for overall water splitting is crucial for sustainable hydrogen production via water electrolysis. Herein, a NiS2/CoTe heterostructure with tunable compositional ratios was successfully synthesized via a facile hydrothermal method and systematically investigated for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) applications. Structural characterization revealed that crystalline NiS2 nanoparticles were uniformly anchored on conductive CoTe nanosheets, forming a well-defined nanoscale heterointerface. Among the synthesized catalysts, the optimized NiS2/CoTe-90:10 heterostructure exhibited the highest catalytic activity, requiring overpotentials of only 168 and 363 mV to achieve a current density of 10 mA/cm2 for HER and OER, respectively. NiS2/CoTe-90:10 further exhibited superior long-term durability and high faradaic efficiencies of 98.3 ± 0.5% and 97.8 ± 0.3% for HER and OER, respectively. The catalyst achieved H2 and O2 production rates of 414.3 and 194.4 μmol/h, respectively, while achieving turnover frequency (TOF) of 6.16 × 10−2 s−1 for HER and 12.21 × 10−3 s−1 for OER, representing a twofold and 1.4-fold enhancement over pristine NiS2, respectively. This improved performance originates from the synergistic coupling between NiS2 and CoTe, accelerated interfacial charge transfer, and enlarged electrochemically active surface area. Moreover, a single-stack anion-exchange membrane alkaline electrolyzer using the NiS2/CoTe-90:10 achieved a cell voltage of 1.78 V at 10 mA/cm2. This work provides valuable insights into the nanoscale heterointerface engineering of transition-metal chalcogenides to rationally design next-generation bifunctional electrocatalysts for efficient water splitting.

Vitamin B12 (cobalamin) is an evolutionarily optimized Co-corrin redox cofactor capable of accessing low-valent cobalt states to mediate challenging biological transformations. Here, we reengineer this biological redox platform into an electrode-addressable catalyst for aqueous nitrate-to-ammonium electrocatalysis and demonstrate that catalytic selectivity is governed by the cooperative interplay between axial-ligand electronic tuning and local proton management. An isostructural series of lower-axial variants (C1–C4) was prepared by systematically modifying the native dimethylbenzimidazole environment, culminating in an l-histidine-ligated derivative (C4) bearing proximal −COOH/–NH2 functionality. In mildly acidic MOPS buffer (pH 5.81), C4 exhibited the highest performance, achieving ∼90% NH4+ Faradaic efficiency at −1.1 V vs SCE while effectively suppressing H2 and hydroxylamine formation compared with the other derivatives. Electrochemical studies reveal that axial-ligand engineering modulates the Co(III)/Co(I) redox manifold, facilitating access to the catalytically active Co(I) state and promoting nitrate activation. Mechanistic investigations support a pathway in which electrochemical reduction induces Co–CN bond dissociation and solvent coordination to generate a Co(I)-solvent intermediate, followed by nitrate binding and proton-coupled electron transfer through nitrite and hydroxylamine intermediates to ammonium. 15N isotopic labeling and in situ spectroelectrochemical measurements confirm the formation of nitrate-derived NH4+ and validate the proposed catalytic intermediates. To evaluate the transferability of this molecular design, the catalysts were immobilized on graphene oxide. The heterogeneous systems preserved the activity trend observed in solution, with C4@GO exhibiting the highest NH4+ selectivity, demonstrating that the electronic effects imparted by lower axial-ligand engineering remain operative after immobilization. Together, these results establish vitamin B12 as a versatile Co-corrin platform and identify axial-ligand-controlled electronic tuning coupled with spatially programmed proton delivery as a general strategy for designing efficient molecular and hybrid electrocatalysts for selective multiproton, multielectron nitrate reduction in water.

Inspired by the electron‐transfer function of biological ferredoxins, we report a biomimetic iron–sulfur cluster‐functionalized porous organic polymer ( FeS‐POP ) for efficient oxygen evolution reaction (OER). FeS‐POP is constructed through polymerization‐induced restructuring of a diamagnetic diiron carbonyl precursor ( FeS‐Com ), wherein Fe─Fe bond cleavage and decarbonylation trigger a diamagnetic‐to‐paramagnetic transformation, generating atomically dispersed cubane [Fe ₄ S ₄ ] active centers within a conjugated porous framework. FeS‐POP exhibits excellent OER activity in alkaline media, delivering 10 mA cm ⁻² at an overpotential of 390 mV with a Tafel slope of 68 mV dec ⁻¹ , ∼89% Faradaic efficiency, and remarkable durability, positioning it among the leading polymer‐based OER electrocatalysts reported thus far. Importantly, synchrotron Fe K‐edge XAFS establishes a cubane [Fe ₄ S ₄ (OH) ₄ ] motif as the catalytically active center and reveals its structural resilience throughout prolonged electrolysis, thereby elucidating the atomic‐level structure–activity relationship governing OER. Complementary DFT calculations reveal the thermodynamic driving force for cluster reconstruction and show that cooperative multi‐metallic interactions within the [Fe ₄ S ₄ ] core facilitate O–O bond formation through a low‐barrier redox‐isomerization pathway. Our findings principally shed light on the molecular mechanism underlying this remarkable diamagnetic‐to‐paramagnetic transformation, establishing a synthetic blueprint for engineering biomimetic iron–sulfur active sites in porous electrocatalysts.

The clinical utility of platinum-based chemotherapeutics is limited by severe toxicity and acquired resistance, motivating the development of alternative metallo drugs. Here, we report a series of rhenium complexes, Re[HL1–4]3, synthesized via reduction of Re(VII) precursors followed by coordination with aromatic thiohydrazide ligands. The lead complex, Re[HL2]3, adopts an unusual trigonal-prismatic geometry and a favorable redox profile, correlating with enhanced cytotoxic potency. It demonstrates selective antiproliferative activity against cancer cells with minimal toxicity toward nonmalignant cells in both 2D and 3D culture systems. Mechanistic studies reveal DNA binding, induction of DNA damage, and reactive oxygen species (ROS) generation, triggering apoptosis via PARP and caspase 3 cleavage. Pharmacokinetic analysis indicates moderate stability and plasma retention, supporting sustained therapeutic levels while limiting accumulation-related toxicity. In a syngeneic 4T1 murine breast cancer model, Re[HL2]3 significantly inhibited tumor growth without observable systemic toxicity, supporting its further preclinical evaluation.