Adopting Back Reduction Current as an Additional Output Signal for Achieving Photoelectrochemical Differentiated Detection.

Photoelectrochemical (PEC) sensors are usually based on a single output signal, that is, the photocurrent change caused by the (photoelectro)chemical reaction between target analytes and photoelectrodes. However, the photocurrent may be influenced by redox species other than the target analyte; therefore, modifying the surface of photoelectrodes with probes that selectively bind to the analyte is essential. Moreover, even though various surface modification methods have been developed, distinguishing molecularly similar chemicals using PEC sensing systems remains a significant challenge. To address these selectivity issues, we proposed a photoanode-based PEC sensor that utilizes a cathodic transient current as a second output signal in addition to the photocurrent, which arises from the back reduction of photo-oxidized species. Factors influencing the back reduction were investigated by observing the transient photocurrent of hematite photoanodes in the presence of model redox probes. The chemical environment around the electrode-electrolyte interface was manipulated by altering the electrolyte composition or modifying the electrode surface. The favorable interaction between the electrode surface and redox species led to an increase in the extent of back reduction and the cathodic transient current. In addition, the extent of back reduction also depends on the chemical identity of the redox species, such as the kinetics of subsequent chemical reactions. Therefore, the synergistic combination of the photocurrent and the cathodic transient current enabled the differentiated detection of various catecholamine neurotransmitters with a single pristine photoelectrode, which has never been achieved using traditional PEC methods. Revisiting the transient photocurrent can complement conventional PEC applications and offers possibilities for more effective semiconductor-based applications.

Universal Suzuki-Miyaura Catalyst-Transfer Polymerization for Precision Synthesis of Strong Donor/Acceptor-Based Conjugated Polymers and Their Sequence Engineering.

Catalyst-transfer polymerization has revolutionized the field of polymer synthesis due to its living character, but for a given catalyst system, the polymer scope is rather narrow. Herein we report a highly efficient Suzuki-Miyaura catalyst-transfer polymerization (SCTP) that covers a wide range of monomers from electron-rich (donor, D) to electron-deficient (acceptor, A) (hetero)arenes by rationally designing boronate monomers and using commercially available Buchwald RuPhos and SPhos Pd G3 precatalysts. Initially, we optimized the controlled polymerization of 3,4-propylenedioxythiophene (ProDOT), benzotriazole (BTz), quinoxaline (QX), and 2,3-diphenylquinoxaline (QXPh) by introducing new boronates, such as 4,4,8,8-tetramethyl-1,3,6,2-dioxazaborocane and its N-benzylated derivative, to modulate the reactivity and stability of the monomers. As a result, PProDOT, PBTz, PQX, and PQXPh were prepared with controlled molecular weight and narrow dispersity (D < 1.29) in excellent yield (>85%). A detailed investigation of the polymer structures using 1H NMR and MALDI-TOF spectrometry supported the chain-growth mechanism and the high initiation efficiency of the SCTP method. In addition, the use of RuPhos-Pd showing excellent catalyst-transfer ability on both D/A monomers led to unprecedented controlled D-A statistical copolymerization, thereby modulating the HOMO energy level (from -5.11 to -4.80 eV) and band gap energy (from 1.68 to 1.91 eV) of the resulting copolymers. Moreover, to demonstrate the living nature of SCTP, various combinations of D-A and A-A block copolymers (PBTz-b-PProDOT, PQX-b-PProDOT, and PQX-b-PBTz) were successfully prepared by the sequential addition method. Finally, simple but powerful one-shot D-A block copolymerization was achieved by maximizing the rate difference between a fast-propagating pinacol boronate donor and a slow-propagating acceptor to afford well-defined poly(3-hexylthiophene)-b-poly(benzotriazole).

Zeolitic Imidazolate Framework-Derived Pt-Co in Nanofibrous Networks as Stable Oxygen Reduction Electrocatalysts with Low Pt Loading.

Proton-exchange membrane fuel cell technology is a key component in the future zero-carbon energy system, generating power from carbon-free fuels, such as green hydrogen. However, the high Pt loading in conventional fuel cell electrodes to maintain electrocatalytic activity and durability, especially on the cathode for oxygen reduction, is the Achilles heel for the worldwide deployment of fuel cell technologies. To minimize Pt consumption for oxygen reduction, we synthesized Pt-Co-based electrocatalysts with meticulous structuring from micrometer to the atomic scale based on reaction pathways. The resulting Pt-Co-based electrocatalysts contain only 1.9 wt% Pt, which is 20 times lower than the conventional Pt-C catalysts for fuel cells. By utilizing electrospinning and in situ synthesis, we anchored three-dimensionally structured zeolitic imidazolate frameworks on continuously connected nanofibrous electrospun mats. The Pt-Co@Pt-free nanowire (PC@PFN) electrocatalysts contain Pt-Co nanoparticles (NPs) and non-Pt elements, Co-containing sites comprising NPs, nanoclusters, and N-coordinated Co single atoms. Despite the ultralow Pt loading in PC@PFN, the mass activity exceeds the U.S. Department of Energy 2025 target by 2.8 times and retains 85.5% of the initial activity after 80,000 durability test cycles, possibly owing to synergistic reaction pathways between Pt and non-Pt sites.

Functional Integration of Catalysts with Si Nanowire Photocathodes for Efficient Utilization of Photogenerated Charge Carriers.

Low-cost catalysts with high activity and durability are necessary to achieve efficient large-scale energy conversion in photoelectrochemical cell (PEC) systems. An additional factor that governs the construction of photoelectrodes for PECs is the spatial control of the catalysts for efficient utilization of photogenerated charge carriers. Here, we demonstrate spatial decoupling of the light-absorbing and catalytic components in hierarchically structured Si-based photocathodes for the hydrogen evolution reaction (HER). By simply modifying a well-known metal-assisted chemical etching procedure, we fabricated a Si nanowire (NW) array-based photocathode with Ag-Pt catalysts at the base and small amounts of the Pt catalyst at the NW tips. This approach simultaneously mitigates the parasitic light absorption by the catalytic layers and recombination of charge carriers owing to the long transport distance, resulting in improved photoelectrochemical HER performance under simulated AM 1.5G illumination.

Enhanced H2 Evolution at Patterned MoSx-Modified Si-Based Photocathodes by Incorporating the Interfacial 3D Nanostructure of Ag.

Photoelectrochemical cells represent one of the promising ways to renewably produce hydrogen (H2) as a future chemical fuel. The design of a catalyst/semiconductor junction for the hydrogen evolution reaction (HER) requires various factors for high performance. In catalytic materials, an intrinsic activity with fast charge-transfer kinetics is important. Additionally, their thermodynamic property and physical adhesion should be compatible with the underlying semiconductor for favorable band alignment and stability during vigorous H2 bubble formation. Moreover, catalysts, especially non-noble materials that demand a large amount of loading, should be adequately dispersed on the semiconductor surface to allow sufficient light absorption to generate excitons. One of the methods to simultaneously satisfy these conditions is to adopt an interfacial layer between the semiconductor and active materials in HER. The interfacial layer efficiently extracts the electrons from the semiconductor and conveys those to the catalytically active surface. We demonstrate Ag as a 3D interfacial nanostructure of patterned MoSx catalysts for photoelectrochemical HER. The nanostructured porous Ag layer was introduced by a simple chemical process, followed by photoelectrochemical deposition of MoSx to form MoSx/Ag nanostructures in cross-shaped catalyst pattern arrays. Ag modulated the surface electronic property of MoSx to improve the reaction kinetics as well as helped a charge transport at the Ag|p-Si(100) junction. The physically stable adhesion of catalysts was also achieved despite the ∼40 nm thick catalysts owing to the interfacial Ag nanostructure. This work contributes to further understand the complex multistep HER from light absorption to charge transfer to protons, helping to develop cost-effective and efficient photocathodes.

Robust and High Spatial Resolution Light Addressable Electrochemistry Using Hematite (α-Fe2O3) Photoanodes.

Light addressable/activated electrochemistry (LAE) has recently attracted attention as it can provide spatially resolved electrochemical information without using pre-patterned electrodes whose sizes and positions are unchangeable. Here, we propose hematite (α-Fe2O3) as the photoanode for LAE, which does not require any sort of surface modification for protection or facilitating charge transfer. As experimentally confirmed with various redox species, hematite is stable enough to be used for repetitive electroanalytical measurements. More importantly, it offers exceptionally high spatial resolution so that the "virtual electrode" is exactly as large as the light spot owing to the short diffusion length of the minority carriers. Quantitative analysis of dopamine in this study shows that the hematite-based photoanode is a promising platform for many potential LAE applications including spatially selective detection of oxidizable biomolecules.