Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M-B (M = Ni, Co) Catalysts on Silicon Nanowire Arrays

Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

NiO/Poly(4-alkylthiazole) Hybrid Interface for Promoting Spatial Charge Separation in Photoelectrochemical Water Reduction

Conjugated polymers are emerging as alternatives to inorganic semiconductors for the photoelectrochemical water splitting. Herein, semi-transparent poly(4-alkylthiazole) layers with different trialkylsilyloxymethyl (R₃SiOCH₂-) side chains (PTzTNB, R = n-butyl; PTzTHX, R = n-hexyl) are applied to functionalize NiO thin films to build hybrid photocathodes. The hybrid interface allows for the effective spatial separation of the photoexcited carriers. Specifically, the PTzTHX-deposited composite photocathode increases the photocurrent density 6- and 2-fold at 0 V versus the reversible hydrogen electrode in comparison to the pristine NiO and PTzTHX photocathodes, respectively. This is also reflected in the substantial anodic shift of onset potential under simulated Air Mass 1.5 Global illumination, owing to the prolonged lifetime, augmented density, and alleviated recombination of photogenerated electrons. Additionally, coupling the inorganic and organic components also enhances the photoabsorption and amends the stability of the photocathode-driven system. This work demonstrates the feasibility of poly(4-alkylthiazole)s as an effective alternative to known inorganic semiconductor materials. We highlight the interface alignment for polymer-based photoelectrodes.

Designing transition-metal-boride-based electrocatalysts for applications in electrochemical water splitting

Investigating renewable and clean energy materials as alternatives to fossil fuels can be foreseen as a potential solution to the global problems of energy shortages and environmental pollution. Recently, transition metal boride (TMB)-based materials have emerged as the rising star as efficient electrocatalysts for hydrogen evolution reaction (HER) and/or oxygen evolution reaction (OER). In this review, an overview of the most recent developments in the use of TMB-based materials as electrocatalysts for HER/OER or overall water splitting has been presented. Initially, we provide a comprehensive introduction of the fundamentals of electrochemical water splitting. Then, the synthesis approaches of TMB materials are summarized and compared. Emphasis is put on the various strategies for further improving the electrocatalytic performance of TMBs. Finally, challenges and future perspectives for TMBs in water-splitting applications are proposed.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Hydrogen (H2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO2

Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under conditions compatible with those for photoanodes are desired. Herein, Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO₂ protective layer, followed by decorating the electrode surface with Ni and Ni-Mo catalysts. The carbon nanolayer, denoted as C_PDA, was formed directly on the Si surface by in situ self-polymerization of dopamine, followed by carbonization of the polydopamine (PDA) coating. The performance of the as-fabricated Si photocathodes with and without the C_PDA layer was comparatively studied for PEC hydrogen evolution reaction (HER) in alkaline electrolytes to evaluate the effect of the sandwich C_PDA layer in between the Si substrate and the TiO₂ layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the C_PDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the C_PDA layer. Accordingly, the short-circuit photocurrents of the Ni and Ni-Mo-decorated photocathodes with the C_PDA layer were enhanced by a factor of 2.8-3.3, and their open-circuit photovoltages were enlarged by 0.14-0.22 V, compared to those of the corresponding electrodes without the C_PDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the C_PDA layer also exhibited an improved stability for PEC HER in alkaline solutions, with a faradaic efficiency of 90-97% in the initial hour.

Silicon based photoelectrodes for photoelectrochemical water splitting

Solar water splitting using Si photoelectrodes in photoelectrochemical (PEC) cells offers a promising approach to convert sunlight into sustainable hydrogen energy, which has recently received intense research. This review summarizes the recent advances in the development of efficient and stable Si photoelectrodes for solar water splitting. The definition and representation of efficiency and stability for Si photoelectrodes are firstly introduced. We then present several basic strategies for designing highly efficient and stable Si photoelectrodes, including surface textures, protective layer, catalyst loading and the integration of the system. Finally, we highlight the progress that has been made in Si photocathodes and Si photoanodes, respectively, with emphasis on how to integrate Si with protective layer and catalyst.

Co2B and Co Nanoparticles Immobilized on the N-B-Doped Carbon Derived from Nano-B4C for Efficient Catalysis of Oxygen Evolution, Hydrogen Evolution, and Oxygen Reduction Reactions

A novel hybrid electrocatalyst of Co₂B and Co nanoparticles immobilized on N-B-doped carbon derived from nano-B₄C (Co₂B/Co/N-B-C/B₄C) is in situ synthesized by pyrolysis of nano-B₄C supporting Co(OH)₂ nanoparticles with melamine. The Co₂B and Co nanoparticles are formed and anchored on the generated N and B codoped carbon and undecomposed B₄C. The hybrid exhibits remarkable catalytic performances toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)-a very small potential of 1.53 V at 10 mA cm^-2 for the OER and a high catalytic kinetics and superior durability for the ORR-which are superior to the RuO₂ and Pt/C catalyst, respectively. Most impressively, the hybrid delivers a very small potential gap of 710 mV, which is lower than those of most bifunctional electrocatalysts reported. In addition, the hybrid also shows a satisfying hydrogen evolution reaction performance offering a small overpotential of 220 mV at 10 mA cm^-2 and wonderful stability. The excellent trifunctional catalytic performances issue from synergetic effects of Co₂B, metal Co, Co/N-doped carbon, and B self-doped carbon coexisting in the hybrid with good interaction mutually. This work provides a new-type efficient multifunctional catalyst for regenerative fuel cell and overall water-splitting technologies.

Boosting PEC performance of Si photoelectrodes by coupling bifunctional CuCo hybrid oxide cocatalysts

Silicon (Si) is an attractive candidate for photoelectrochemical (PEC) water splitting because of its small band gap, fast carrier mobility and abundant reserves. However, the PEC performance has been severely limited by the sluggish kinetics of oxygen/hydrogen evolution reaction at the Si photoelectrode/electrolyte interface and poor stability in the aqueous environment. Herein, the bifunctional CuCo hybrid oxides (CuCo-HO) cocatalysts have been integrated with ultrathin TiO₂ decorated n-type and p-type Si nanowires (NWs) to simultaneously improve the photoactivity and stability of Si photoelectrodes. The thickness of TiO₂ layer, the concentration of CuCo-precursor and the hydrothermal reaction time have been investigated to optimize the PEC performance. The enhancement mechanism is studied and mainly ascribed to the increased light harvesting, small charge transfer resistance and high carrier density of Si NWs/TiO₂/CuCo-HO photoelectrodes.

Carbon Nanotube-Supported Amorphous Co–B for Hydrogenation of M-chloronitrobenzene

A series of carbon nanotube (CNT)-supported amorphous Co–B alloy catalysts were prepared by selectively depositing Co–B particles inside and/or outside of CNTs. The effects of the nanotubular structure on the physiochemical properties of the amorphous Co–B alloys were studied. It was found that the internal loading enhanced the thermal stability of the amorphous Co–B alloys and inhibited the loss of Co compared with the external loading. The internal loading also increased the proportion of elemental Co in the Co–B alloys, while the loading method did not change the valence states of either Co or B. The internally loaded Co–B particles exhibited higher hydrogenation activity for m-chloronitrobenzene ( m-CNB) than the externally loaded analogue. The kinetics of m-CNB hydrogenation were also studied.

Electrochemical Growth of Copper Hydroxy Double Salt Films and Their Conversion to Nanostructured p-Type CuO Photocathodes

New electrochemical synthesis methods were developed to produce copper hydroxy double salt(Cu-HDS) films with four different intercalated anions (NO₃^-, SO₄^2-, Cl^-, and dodecyl sulfate (DS)) as pure crystalline films as deposited (Cu₂NO₃(OH)₃, Cu₄SO₄(OH)₆, Cu₂Cl(OH)₃, and Cu₂DS(OH)₃). These methods are based on p-benzoquinone reduction, which increases the local pH at the working electrode and triggers the precipitation of Cu²⁺ and appropriate anions as Cu-HDS films on the working electrode. The resulting Cu-HDS films could be converted to crystalline Cu(OH)₂ and CuO films by immersing them in basic solutions. Because Cu-HDS films were composed of 2D crystals as a result of the atomic-level layered structure of HDS, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures, creating high surface areas that cannot be obtained by direct deposition of CuO, which has a 3D atomic-level crystal structure. The resulting nanostructures allowed the CuO films to facilitate electron-hole separation and demonstrate great promise for photocurrent generation when investigated as a photocathode for a water-splitting photoelectrochemical cell. Electrochemical synthesis of Cu-HDS films and their facile conversion to CuO films will provide new routes to tune the morphologies and properties of the CuO electrodes that may not be possible by other synthesis means.