Selective Catalytic Electroreduction of CO2 at Silicon Nanowires (SiNWs) Photocathodes Using Non-Noble Metal-Based Manganese Carbonyl Bipyridyl Molecular Catalysts in Solution and Grafted onto SiNWs

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of -0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s^-1 . To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.

Using Light and Electrons to Bend Carbon Dioxide: Developing and Understanding Catalysts for CO2 Conversion to Fuels and Feedstocks

Our global society generates an unwieldy amount of CO₂ per unit time. Therefore, the capture of this greenhouse gas must involve a diverse set of strategies. One solution to this problem is the conversion of CO₂ into a more useful chemical species. Again, a multiplicity of syntheses and products will be necessary. No matter how elegant the chemistry is, commercial markets often have little use for a small set of compounds made in tremendous yield. Following this reasoning, the Bocarsly Research Group seeks to develop new electrochemical and photochemical processes that may be of utility in the conversion of CO₂ to organic compounds. We focus on investigating proton-coupled charge transfer mechanisms that produce both C1 and carbon-carbon bonded products (C2+).In early work, we considered the reduction of CO₂ to formate at electrocatalytic indium and tin electrodes. These studies demonstrated the key role of surface oxides in catalyzing the reduction of CO₂. This work generated efficient systems for the formation of formate and paved the way to studies using non-copper, intermetallic electrocatalysts for the generation of C2+ species. Most notable is the efficient formation of oxalate at an oxidized Cr₃Ga electrode. Oxalate has recently been suggested as a potential nonfossil, alternate organic feedstock.Separately, we have focused on the electrocatalytic effects of pyridine on the reduction of CO₂ in aqueous electrolyte. These studies demonstrated that electrodes that normally yield a low hydrogen overpotential (Pd and Pt) show suppressed H₂ evolution and strongly enhanced activity for CO₂ reduction in the presence of pyridinium. Methanol was observed to form in high Faradaic yield at low overpotential using this system. The 6-electron, 6-proton reduction of CO₂ in the presence of pyridinium was intriguing, and significant effort was placed on understanding the mechanism of this reaction both on metal electrodes and on semiconducting photocathodes. P-GaP electrodes were found to provide exceptional behavior for the formation of methanol using only light as the energy source.The pyridinium studies highlighted the role of protons in the overall reduction of CO₂, stimulating our interest in the chemistry of MnBr(bpy)(CO)₃ and related compounds. This complex was reported to electrochemically reduce CO₂ to CO. We saw these reports as an opportunity to study the detailed nature of the proton-coupled electron transfer (PCET) mechanism associated with CO₂ reduction. Our investigation of this system revealed the role of hydrogen-bonding in CO₂ reduction and pointed the way for the construction of a photochemical process for CO generation using a [(bpy)(CO)₃Mn(CN)Mn(bpy)(CO)₃]⁺ photocatalyst.Based on our studies to date, it appears likely that heterogeneous systems can be assembled to convert CO₂ into products that are "beyond C2 products." This may open up new practical chemistry in the area of fossil-based replacements for both synthesis and fuels. Systems with pragmatic efficiencies are close to reality. Electrochemical reactors using heterogeneous electrocatalysts show the stability and product selectivity needed to generate industrial opportunities. Continued growth of mechanistic understanding is expected to facilitate the chemical design of cogent systems for the taming of CO₂.

Reticular-Chemistry-Inspired Supramolecule Design as a Tool to Achieve Efficient Photocatalysts for CO2 Reduction

Photocatalytic CO₂ reduction into C1 products is one of the most trending research subjects of current times as sustainable energy generation is the utmost need of the hour. In this review, we have tried to comprehensively summarize the potential of supramolecule-based photocatalysts for CO₂ reduction into C1 compounds. At the outset, we have thrown light on the inert nature of gaseous CO₂ and the various challenges researchers are facing in its reduction. The evolution of photocatalysts used for CO₂ reduction, from heterogeneous catalysis to supramolecule-based molecular catalysis, and subsequent semiconductor-supramolecule hybrid catalysis has been thoroughly discussed. Since CO₂ is thermodynamically a very stable molecule, a huge reduction potential is required to undergo its one- or multielectron reduction. For this reason, various supramolecule photocatalysts were designed involving a photosensitizer unit and a catalyst unit connected by a linker. Later on, solid semiconductor support was also introduced in this supramolecule system to achieve enhanced durability, structural compactness, enhanced charge mobility, and extra overpotential for CO₂ reduction. Reticular chemistry is seen to play a pivotal role as it allows bringing all of the positive features together from various components of this hybrid semiconductor-supramolecule photocatalyst system. Thus, here in this review, we have discussed the selection and role of various components, viz. the photosensitizer component, the catalyst component, the linker, the semiconductor support, the anchoring ligands, and the peripheral ligands for the design of highly performing CO₂ reduction photocatalysts. The selection and role of various sacrificial electron donors have also been highlighted. This review is aimed to help researchers reach an understanding that may translate into the development of excellent CO₂ reduction photocatalysts that are operational under visible light and possess superior activity, efficiency, and selectivity.

Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Photoelectrochemical CO2 reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO2 reduction reaction can give rise to high product selectivity. In this context, a RuII complex [Ru(HO-tpy)(6-mbpy)(NCCH3)]2+ (HO-tpy = 4'-hydroxy-2,2':6',2''-terpyridine; 6-mbpy = 6-methyl-2,2'-bipyridine) was immobilised on a thin SiOx layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO2 reduction and the p-Si/SiOx junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an SN2-type reaction could be established.

Study of Excited States and Electron Transfer of Semiconductor-Metal-Complex Hybrid Photocatalysts for CO2 Reduction by Using Picosecond Time-Resolved Spectroscopies

A semiconductor-metal-complex hybrid photocatalyst was previously reported for CO₂ reduction; this photocatalyst is composed of nitrogen-doped Ta₂ O₅ as a semiconductor photosensitizer and a Ru complex as a CO₂ reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO₂ reduction with hybrid photocatalysts. Herein, the excited-state dynamics of several hybrid photocatalysts are described by using time-resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst. The results show that electron transfer from the semiconductor to the metal-complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge-transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.

Effects of Chemically-Modified Polypyridyl Ligands on the Structural and Redox Properties of Tricarbonylmanganese(I) Complexes

Carbonyl complexes with manganese(I) as the central metal are very attractive catalysts. The introduction of redox-active ligands, such as quinones and methyl viologen analogs into these catalysts, would be expected to lead to superior catalyst performances, since they can function as excellent electron carriers. In this study, we synthesized four tricarbonylmanganese(I) complexes containing typical bidentate polypyridyl ligands, including 1,10-phenanthroline (phen) and 2,2′-bipyridine (bpy) frameworks bound to redox-active ortho-quinone/catechol or methyl viologen-like units. The molecular structures of the resulting complexes were determined by X-ray crystallography to clarify their steric features. As expected from the infrared (IR) data, three CO ligands for each complex were coordinated in the facial configuration around the central manganese(I) atom. Additionally, the structural parameters were found to differ significantly between the quinone/catechol units. Electrochemical analysis revealed some differences between them and their reference complexes, namely [MnBr(CO)₃(phen)] and [MnBr(CO)₃(bpy)]. Notably, interconversions induced by two-electron/two-proton transfers between the quinone and catechol units were observed in the phenanthroline-based complexes. This work indicated that the structural and redox properties in tricarbonylmanganese(I) complexes were significantly affected by chemically modified polypyridyl ligands. A better understanding of structures and redox behaviors of the present compounds would facilitate the design of new manganese complexes with enhanced properties.

Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO2 reduction remains a highly creative approach.

Photocathode functionalized with a molecular cobalt catalyst for selective carbon dioxide reduction in water

Artificial photosynthesis is a vibrant field of research aiming at converting abundant, low energy molecules such as water, nitrogen or carbon dioxide into fuels or useful chemicals by means of solar energy input. Photo-electrochemical reduction of carbon dioxide is an appealing strategy, aiming at reducing the greenhouse gas into valuable products such as carbon monoxide at low or without bias voltage. Yet, in such configuration, there is no catalytic system able to produce carbon monoxide selectively in aqueous media with high activity, and using earth-abundant molecular catalyst. Upon associating a p-type Cu(In,Ga)Se2 semi-conductor with cobalt quaterpyridine complex, we herein report a photocathode complying with the aforementioned requirements. Pure carbon dioxide dissolved in aqueous solution (pH 6.8) is converted to carbon monoxide under visible light illumination with partial current density above 3 mA cm-2 and 97% selectivity, showing good stability over time.

Manganese Telluride Carbonyl Complexes: Facile Syntheses and Exotic Properties-Reversible Transformations, Hydrogen Generation, Paramagnetic, and Semiconducting Properties

A novel family of five Mn-Te-CO complexes was prepared via facile syntheses: mono spirocyclic [Mn₄Te(CO)₁₆]^2- (1), four-membered Mn₂Te₂ ring-type [Mn₂Te₂(CO)₈]^2- (2), hydride-containing square pyramidal [HMn₃Te₂(CO)₉]^2- (3), and dumbbell-shaped [Mn₆Te₆(CO)₁₈]^4- (4) and [Mn₆Te₁₀(CO)₁₈]^4- (5). Electron-precise complexes 4 and 5 exhibit unusual paramagnetism arising from two types of Mn atoms in different oxidation states, as determined by X-ray photoelectron spectroscopy, electron paramagnetic resonance, and density functional theory (DFT) calculations. The structural transformations from small-sized Mn₄Te 1 and Mn₂Te₂ 2 to the largest Mn₆Te₁₀ 5 were controllable, the off/on magnetic-switched transformation between HMn₃Te₂ 3 and 5 was reversible, and the magnetic transformation between Mn₆Te₆ 4 and 5 was observed. Interestingly, the reversible dehydridation and hydridation between the HMn₃Te₂-based cluster 3 and [Mn₃Te₂(CO)₉]^- were successfully accomplished, in which the release of a high yield of H₂ was detected by gas chromatography. In addition, upon the addition of CO, cluster 3 first forms a carbonyl-inserted intermediate [HMn₃Te₂(CO)₁₀]^2- (3'), detected by the high resolution ESI-MS, which is readily transformed to a dimeric dihydrido cluster [{HMn₃Te₂(CO)₁₀}₂]^2- (6) with the introduction of O₂. These low- to high-nuclearity complexes exhibit rich redox properties with semiconducting behavior in solids, possessing low but tunable energy gaps (1.06-1.62 eV) due to efficient electron transport via nonclassical C-H···O(carbonyl) interactions. The structural nature, reversible structural transformations, controllable on/off magnetic switches, electron communication networks, and associated chemical properties for hydrogen generation are discussed in detail and supported by DFT calculations, density of states, band structures, and noncovalent interaction analyses.

Synergetic catalysis of a cobalt-based coordination polymer for selective visible-light driven CO2-to-CO conversion

Herein, based on the strategy of synergetic catalysis, we report a cobalt-based coordination polymer PEI₆-Co. As a heterogeneous catalyst, PEI₆-Co shows a selectivity of 95% and a yield of 1170 mmol g⁻¹ for visible-light-driven CO₂-to-CO conversion in a water containing system, which is almost 2.8 times that of the mononuclear cobalt catalyst CoL¹ and is comparable to that of the dinuclear cobalt catalyst Co₂L.

Herein, based on the strategy of synergetic catalysis, we report a cobalt-based coordination polymer PEI₆-Co for the visible-light-driven CO₂-to-CO conversion in water containing system.

Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light

CO₂ reduction to carbon feedstocks using heterogeneous photocatalysts is an attractive means of addressing both climate change and the depletion of fossil fuels. Of particular importance is the development of a photosystem capable of functioning in response to visible light, which accounts for the majority of the solar spectrum, representing a kind of artificial photosynthesis. Hybrid systems comprising a metal complex and a semiconductor are promising because of the excellent electrochemical (and/or photocatalytic) activity of metal complexes during CO₂ reduction and the ability of semiconductors to efficiently oxidize water to molecular O₂ . Here, the development of hybrid photocatalysts and photoelectrodes for CO₂ reduction in combination with water oxidation is described.

An Unexpected Iron (II)-Based Homogeneous Catalytic System for Highly Efficient CO2-to-CO Conversion under Visible-Light Irradiation

We present two as-synthesized Fe(II)-based molecular catalysts with 1,10-phenanthroline (phen) ligands; Fe(phen)₃Cl₂ (1) and [Fe(phen)₂(CH₃CH₂OH)Cl]Cl (2), and their robust catalytic properties for the conversion of CO₂ to CO in DMF/TEOA (DMF = N,N’-dimethylformamide; TEOA = triethanolamine) solution containing Ru(bpy)₃²⁺ and BIH (1,3-dimethyl-2-phenyl-2,3- dihydro-1H-benzo-[d]-imidazole). High turnover numbers (TONs) of 19,376 were achieved with turnover frequencies (TOFs) of 3.07 s⁻¹ for complex 1 (1.5 × 10⁻⁷ M). A quantum efficiency of 0.38% was observed after 5 h irradiated by 450 nm monochromatic light. The generation rate of CO₂ and H₂ were tuned by optimizing the experimental conditions, resulting in a high CO selectivity of 90%. The remarkable contribution of the photosensitizer to the total TON_CO was found being 19.2% (as shown by tests under similar conditions without catalysts) when BIH was employed as a sacrificial electron donor. The product selectivity in complex 2 reached 95%, and the corresponding TON_CO and TOF_CO were 33,167 and 4.61 s⁻¹ in the same concentration with complex 1 used as catalyst; respectively. This work provides guidance for future designs of simple, highly efficient and selective molecular catalytic systems that facilitate carbon-neutral solar-to-fuel conversion processes

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Phosphine oxide-based tricarbonylrhenium(i) complexes from phosphine/phosphine oxide and dihydroxybenzoquinones

Neutral phosphine oxide (P[double bond, length as m-dash]O) donor-based organometallic complexes [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (1), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (2), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (3), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (4), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PCy3}] (5), and [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PPh3}] (6) were assembled from phosphine/phosphine oxide, a dihydroxybenzoquinone donor and Re2(CO)10via a one-pot solvothermal approach. The soft phosphine donor was transformed into a hard phosphine oxide donor during the formation of 1, 3, 4, 5, and 6. The complexes 1-6 were air and moisture stable and were soluble in polar organic solvents. The complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopic methods. The molecular structures of 1, 2, 4, and 6 were analyzed by single-crystal X-ray diffraction analysis. The UV-Visible absorption studies indicated that 1-6 in THF display strong visible light absorption in the range of ∼350-700 nm.

Photoelectrochemical Characterisation on Surface-Inverted Black Silicon Photocathodes by Using Platinum/Palladium Co-catalysts for Solar-to-Hydrogen Conversion

Black silicon (bSi) has recently captured research attention in photoelectrochemical (PEC) solar-to-hydrogen (STH) conversion devices. Because nanostructuring of silicon retains the photovoltaic attributes of the material, it also provides a range of excellent physicochemical properties, such as a vast active-site-rich electrochemical interface, owing to a high aspect ratio, and important light-scattering attributes, which significantly improve photoconversion. One method to gain control over p-type bSi interface energetics is surface inversion of the p-type interface by phosphorus doping to introduce a shallow n⁺ -emitter layer, which provides a thin p-n junction at the interface of the nanostructures. Although this concept has been suggested in the literature, it has not been demonstrated experimentally for a platinum/palladium co-catalysed bSi photocathode device for STH conversion. Herein, preliminary investigations and proof-of-concept studies are reported for the fabrication and PEC characterisation of surface-inverted p-type bSi photocathodes prepared by wet chemical etching. The PEC tests on p-bSi|n⁺ photocathodes show that, for both metal nanoparticles (Pt and Pd), the catalytic activity for proton conversion is increased; this is evident from an anodic shift in the onset potentials shifts to 0.24 and 0.29 V and an increase in photocurrent by 9 and 13.8 mA cm^-2 , respectively, at 0 V versus a reversible hydrogen electrode, as a result of introducing the emitter layer.

Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts

The utilization of CO₂ via electrochemical reduction constitutes a promising approach toward production of value-added chemicals or fuels using intermittent renewable energy sources. For this purpose, molecular electrocatalysts are frequently studied and the recent progress both in tuning of the catalytic properties and in mechanistic understanding is truly remarkable. While in earlier years research efforts were focused on complexes with rare metal centers such as Re, Ru, and Pd, the focus has recently shifted toward earth-abundant transition metals such as Mn, Fe, Co, and Ni. By application of appropriate ligands, these metals have been rendered more than competitive for CO₂ reduction compared to the heavier homologues. In addition, the important roles of the second and outer coordination spheres in the catalytic processes have become apparent, and metal-ligand cooperativity has recently become a well-established tool for further tuning of the catalytic behavior. Surprising advances have also been made with very simple organocatalysts, although the mechanisms behind their reactivity are not yet entirely understood. Herein, the developments of the last three decades in electrocatalytic CO₂ reduction with homogeneous catalysts are reviewed. A discussion of the underlying mechanistic principles is included along with a treatment of the experimental and computational techniques for mechanistic studies and catalyst benchmarking. Important catalyst families are discussed in detail with regard to mechanistic aspects, and recent advances in the field are highlighted.

Semiconductor-Based Photoelectrochemical Conversion of Carbon Dioxide: Stepping Towards Artificial Photosynthesis

The photoelectrochemical (PEC) carbon dioxide reduction process stands out as a promising avenue for the conversion of solar energy into chemical feedstocks, among various methods available for carbon dioxide mitigation. Semiconductors derived from cheap and abundant elements are interesting candidates for catalysis. Whether employed as intrinsic semiconductors or hybridized with metallic cocatalysts, biocatalysts, and metal molecular complexes, semiconductor photocathodes exhibit good performance and low overpotential during carbon dioxide reduction. Apart from focusing on carbon dioxide reduction materials and chemistry, PEC cells towards standalone devices that use photohybrid electrodes or solar cells have also been a hot topic in recent research. An overview of the state-of-the-art progress in PEC carbon dioxide reduction is presented and a deep understanding of the catalysts of carbon dioxide reduction is also given.

Photocatalytic Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex

The invention of efficient systems for the photocatalytic reduction of CO₂ comprising earth-abundant metal catalysts is a promising approach for the production of solar fuels. One bottleneck is to design highly selective and robust molecular complexes that are able to transform the CO₂ gas. The Cu^II quaterpyridine complex [Cu(qpy)]²⁺ (1) is found to be a highly efficient and selective catalyst for visible-light driven CO₂ reduction in CH₃ CN using [Ru(bpy)₃ ]²⁺ (bpy: bipyridine) as photosensitizer and BIH/TEOA (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole/triethanolamine) as sacrificial reductant. The photocatalytic reaction is greatly enhanced by the presence of H₂ O (1-4 % v/v), and a turnover number of >12 400 for CO production can be achieved with 97 % selectivity, which is among the highest of molecular 3d CO₂ reduction catalysts. Results from Hg poisoning and dynamic light scattering experiments suggest that this photocatalyst is homogenous. To the best of our knowledge, 1 is the first example of molecular Cu-based catalyst for the photoreduction of CO₂ .

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode

The development of high-performance electrocatalytic systems for the controlled reduction of CO₂ to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)₃] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO₂ under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO₂ reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn⁰ species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

Silicon nanowire heterostructures for advanced energy and environmental applications: a review

Semiconductor nanowires (NWs), in particular Si NWs, have attracted much attention in the last decade for their unique electronic properties and potential applications in several emerging areas. With the introduction of heterostructures (HSs) on NWs, new functionalities are obtained and the device performance is improved significantly in many cases. Due to the easy fabrication techniques, excellent optoelectronic properties and compatibility of forming HSs with different inorganic/organic materials, Si NW HSs have been utilized in various configurations and device architectures. Herein, we review the recent developments in Si NW HS-based devices including the fabrication techniques, properties (e.g., light emitting, antireflective, photocatalytic, electrical, photovoltaic, sensing etc) and related emerging applications in energy generation, conversion, storage, and environmental cleaning and monitoring. In particular, recent advances in Si NW HS-based solar photovoltaics, light-emitting devices, thermoelectrics, Li-ion batteries, supercapacitors, hydrogen generation, artificial photosynthesis, photocatalytic degradation of organic dyes in water treatment, chemical and gas sensors, biomolecular sensors for microbial monitoring etc have been addressed in detail. The problems and challenges in utilizing Si NW HSs in device applications and the key parameters to improve the device performance are pointed out. The recent trends in the commercial applications of Si NW HS-based devices and future outlook of the field are presented at the end.

Boron-doped diamond semiconductor electrodes: Efficient photoelectrochemical CO2 reduction through surface modification

Competitive hydrogen evolution and multiple proton-coupled electron transfer reactions limit photoelectrochemical CO2 reduction in aqueous electrolyte. Here, oxygen-terminated lightly boron-doped diamond (BDDL) thin films were synthesized as a semiconductor electron source to accelerate CO2 reduction. However, BDDL alone could not stabilize the intermediates of CO2 reduction, yielding a negligible amount of reduction products. Silver nanoparticles were then deposited on BDDL because of their selective electrochemical CO2 reduction ability. Excellent selectivity (estimated CO:H2 mass ratio of 318:1) and recyclability (stable for five cycles of 3 h each) for photoelectrochemical CO2 reduction were obtained for the optimum silver nanoparticle-modified BDDL electrode at -1.1 V vs. RHE under 222-nm irradiation. The high efficiency and stability of this catalyst are ascribed to the in situ photoactivation of the BDDL surface during the photoelectrochemical reaction. The present work reveals the potential of BDDL as a high-energy electron source for use with co-catalysts in photochemical conversion.

Directed Assembly of Nanoparticle Catalysts on Nanowire Photoelectrodes for Photoelectrochemical CO2 Reduction

Reducing carbon dioxide with a multicomponent artificial photosynthetic system, closely mimicking nature, represents a promising approach for energy storage. Previous works have focused on exploiting light-harvesting semiconductor nanowires (NW) for photoelectrochemical water splitting. With the newly developed CO2 reduction nanoparticle (NP) catalysts, direct interfacing of these nanocatalysts with NW light absorbers for photoelectrochemical reduction of CO2 becomes feasible. Here, we demonstrate a directed assembly of NP catalysts on vertical NW substrates for CO2-to-CO conversion under illumination. Guided by the one-dimensional geometry, well-dispersed assembly of Au3Cu NPs on the surface of Si NW arrays was achieved with facile coverage tunability. Such Au3Cu NP decorated Si NW arrays can readily serve as effective CO2 reduction photoelectrodes, exhibiting high CO2-to-CO selectivity close to 80% at -0.20 V vs RHE with suppressed hydrogen evolution. A reduction of 120 mV overpotential compared to the planar (PL) counterpart was observed resulting from the optimized spatial arrangement of NP catalysts on the high surface area NW arrays. In addition, this system showed consistent photoelectrochemical CO2 reduction capability up to 18 h. This simple photoelectrode assembly process will lead to further progress in artificial photosynthesis, by allowing the combination of developments in each subfield to create an efficient light-driven system generating carbon-based fuels.