Hai-Hua Huang

The trade‐off of the Volmer reaction for *H formation and the Heyrovsky/Tafel Reaction for *H desorption is a crucial challenge in alkaline hydrogen evolution reaction (HER). Thus, aside from the conventional construction of bifunctional active sites to accelerate the water dissociation step, the electronic retrimming of each metal site is also non‐negligible. Herein, an efficient HER electrocatalyst combining Pt single‐atoms and Pt nanoparticles on the etched boron nitride (eBN) coated carbon nanotube (Pt‐eBN@CNT) is synthesized, where both the Volmer and Heyrovsky steps are promoted by the synergistic adsorption of the composite active sites and the electron‐withdrawing effect of eBN carrier, respectively. The proposed catalyst shows impressive performance in alkaline media with an overpotential of 25.1 ± 1.7 mV at 10 mA cm⁻², and a TOF of 17.1 ± 1.3 s⁻¹ at 0.15 V vs RHE under extremely low metal loading of 6 µg_Pt cm⁻² (catalyst loading of 0.77 mg cm⁻²), significantly better than those for the commercial benchmark Pt/C (44.2 ± 4.1 mV, 0.9 ± 0.1 s⁻¹ at catalyst loading of 0.77 mg cm⁻²; 156.1 ± 0.6 mV, 1.6 ± 0.1 s⁻¹ at metal loading of 6 µg_Pt cm⁻²) under the same conditions. Moreover, a membrane‐electrode‐assembly (MEA) electrolyzer with Pt‐eBN@CNT as the cathodic catalyst exhibits a notable charge transfer amount (stands for a long service life) of 2.0 × 10⁶ C·cm⁻² within high‐temperature electrocatalysis at 1.0 A·cm⁻², significantly higher than those of state‐of‐the‐art Pt‐based electrocatalysts. This work demonstrates the important role of active‐site combination coupled with carrier effect in optimizing the kinetics of multi‐step electrocatalytic reaction.

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO ₂ reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene‐appended Co ^II macrocyclic catalyst in comparison with its pyrene‐free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene‐tethered catalyst in noble‐metal‐free CO ₂ photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO‐bound intermediates and revealed their different CO‐binding behavior, demonstrating that the pyrene group endows the corresponding Co ^II catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance.

A new binuclear cobalt complex with electron-donating group (–OCH₃) accelerates the visible-light-driven conversion of CO₂ to CO in a water-containing system.

A carbazolide- bis (NHC) Ni ^II catalyst (1; NHC, N -heterocyclic carbene) for selective CO ₂ photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO ₂ to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol- bis (NHC) Ni ^II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

The fulfillment of a high quantum efficiency for photocatalytic CO₂ reduction presents a key challenge, which can be overcome by developing strategies for dynamic attachment between photosensitizer and catalyst. In this context, we exploit the use of coordinate bond to connect a pyridine-appended iridium photosensitizer and molecular catalysts for CO₂ reduction, which is systematically demonstrated by ¹H nuclear magnetic resonance titration, theoretical calculations, and spectroscopic measurements. The mechanistic investigations reveal that the coordinative interaction between the photosensitizer and an unmodified cobalt phthalocyanine significantly accelerates the electron transfer and thus realizes a remarkable quantum efficiency of 10.2% ± 0.5% at 450 nm for photocatalytic CO₂-to-CO conversion with a turn-over number of 391 ± 7 and nearly complete selectivity, over 4 times higher than a comparative system with no additional interaction (2.4%±0.2%). Moreover, the decoration of electron-donating amino groups on cobalt phthalocyanine can optimize the quantum efficiency up to 27.9% ± 0.8% at 425 nm, which is more attributable to the enhanced coordinative interaction rather than the intrinsic activity. The control experiments demonstrate that the dynamic feature of coordinative interaction is important to prevent the coordination occupancy of labile sites, also enabling the wide applicability on diverse non-noble-metal catalysts.

The fulfilment of a high quantum efficiency for photocatalytic CO₂ reduction presents a key challenge, which can be overcome by developing new strategies for dynamic attachment between photosensitizer and catalyst. In this context, we have for the first time exploited the use of coordinate bond to connect a pyridine-appended iridium photosensitizer (IrQPY) and molecular catalysts for CO₂ reduction, which has been systematically demonstrated by ¹H NMR titration, theoretical calculations, electrochemistry, and spectroscopic measurements. The mechanistic investigations reveal that the coordinative interaction between IrQPY and an unmodified cobalt phthalocyanine significantly accelerates the electron transfer and thus realizes a remarkable quantum efficiency of 10.2%±0.5% at 450 nm for photocatalytic CO₂-to-CO conversion with nearly complete selectivity, over 4 times higher than a comparative system with no additional interaction (2.4%±0.2%). Moreover, the decoration of electron-donating amino groups on cobalt phthalocyanine has optimized the quantum efficiency up to 27.9%±0.8% at 425 nm, which is more attributable to the enhanced coordinative interaction rather than the intrinsic activity. The control experiments have indicated that the dynamic feature of coordinative interaction is important to prevent the coordination occupancy of labile sites and back electron transfer, also enabling the wide applicability on diverse non-noble-metal catalysts.

Metal-organic layers with ordered structure and molecular tunability are of great potential as heterogeneous catalysts due to their readily accessible active sites. Herein, we demonstrate a facile template strategy to prepare metal-organic layers with a uniform thickness of three metal coordination layers (ca. 1.5 nm) with graphene oxide as both template and electron mediator. The resulting hybrid catalyst exhibits an excellent performance for CO₂ photoreduction with a total CO yield of 3133 mmol g^–1_MOL (CO selectivity of 95%), ca. 34 times higher than that of bulky Co-based metal-organic framework. Systematic studies reveal that well-exposed active sites in metal-organic layers, and facile electron transfer between heterogeneous and homogeneous components mediated by graphene oxide, greatly contribute to its high activity. This work highlights a facile way for constructing ultrathin metal-organic layers and demonstrates charge transfer pathway between conductive template and catalyst for boosting photocatalysis.

The electronic effect is crucial for the electrocatalytic reduction of CO₂. In contrast to the previous understanding of the monotonic influence of the electronic effect on the selectivity of CO₂ reduction, the dual roles of the electronic effect on the selectivity are revealed in the present study, i.e., (1) the electronic effect on redox originating from σ-donation and (2) the electronic effect on π-back-donation, via comprehensive DFT studies on four representative classes of pincer NHC Ni^II catalysts. On the one hand, the electron-rich C, B-coordinating (CCC and CBC) ligands guarantee that the catalysts possess the driving force to reduce CO₂ in a lower-electron reduction state (Ni^I), leading to lower free energy barriers for the formation of HCOOH, which results from the lower ligand-field deformation energies for the hydride transfer and the stronger p–σ* interactions in the metal–hydride intermediates. In contrast, the less electron-rich N-coordinating (CNC and C(B)NC) ligands require an Ni⁰ electron reduction state to reduce CO₂, preferring kinetic-controlled CO formation due to the higher free energy barriers for the generation of HCOOH. This redox effect well explains the unprecedented experimentally observed selectivity of HCOOH in the stronger electron donor CCC-Ni system, which is different from the traditional understanding of the electronic effect. On the other hand, at an identical reduction state, the electronic effect plays an important role in tuning the back-donation ability of the metal center, benefiting the π-back-donation in the metal–carbonyl intermediates, and thus favors the formation of CO. This back-donation effect is consistent with the traditional understanding of selectivity. This work provides comprehensive insights into the dual role of the electronic effect on the selectivity for CO₂ reduction, which can be instructive for the future design and development of catalysts.

Metal-organic layers (MOLs) with ordered structure and molecular tunability are of great potential as heterogeneous catalysts due to their readily accessible active sites. Herein, we demonstrate a facile template strategy to prepare MOLs with a uniform thickness of three metal coordination layers (ca. 1.5 nm) with graphene oxide (GO) as both template and electron mediator. The resulting MOL@GO exhibits an outstanding performance for CO2 photoreduction with a record-high total CO yield of 3133 mmol/gMOL among all the metal-organic framework and MOL catalysts (CO selectivity of 95%). This performance is ca. 34 times higher than that of bulky Co-MOF, [CoL(H2O)2]•0.5H2O (H2L = 5-(1H-1,2,4-triazol-1-yl)isophthalic acid). Systematic studies reveal that well exposed active sites in MOLs, and facile electron transfer between heterogeneous and homogeneous components mediated by GO, greatly contribute to its high activity. This work highlights a facile way for constructing ultrathin MOLs and demonstrate charge transfer pathway between conductive template and catalyst for boosting photocatalysis.