Silicon Photoelectrodes Prepared by Low-Cost Wet Methods for Solar Photoelectrocatalysis

ConspectusThe electrochemical conversion of sunlight by photoelectrochemical cells (PECs) is based on semiconductor electrodes that are interfaced with a liquid electrolyte. This approach is highly promising, first, because it can be employed for the generation of a chemical fuel (e.g., H2) to store solar energy that can be used on-demand to generate electricity when the sun is not available. Second, it can be seen as a concept reminiscent of photosynthesis, where CO2 is converted into a valuable feedstock by solar energy. Thus, photoelectrochemical cells are sometimes referred to as “artificial leaves”. Silicon, being the main semiconductor in the electronics and photovoltaic sector, is a prime candidate to be used as the light absorber and the substrate for building photoelectrochemical cells. However, Si alone has “poor-to-no photoelectrochemical performance”. This is caused by its weak electrocatalytic activity for cathodic reactions (namely, the hydrogen evolution reaction (HER), the CO2 reduction reaction (CDRR), and the N2 reduction reaction) and by its deactivation in the anodic regime, prohibiting its use for the oxygen evolution reaction (OER). This latter reaction is essential for supplying electrons to generate a solar fuel. Due to these problems, layers that both protect and are catalytically active are typically employed on Si photoelectrodes but require rather sophisticated manufacturing processes (e.g., atomic layer or electron beam deposition), which hinders research and innovation in this field. Nevertheless, our group and others have demonstrated that these layers are not always required and that highly active and stable Si-based photoelectrodes can be manufactured using simple wet processes, such as drop casting, electroless deposition, or aqueous electrodeposition. In this Account, we first introduce the topic and the possible structures that can be easily obtained starting from commercial Si wafers. Then, we discuss strategies that have been employed to manufacture photocathodes based on p-type Si. Among these, we describe Si photocathodes coated with metal, inorganic compounds such as metal sulfides, and more original constructs, such as those based on macromolecules composed of a catalytic Mo3S4 core and a polyoxometallate macrocycle. Also, we discuss the elaboration and the advantages of Si photocathodes obtained by grafting organometallic catalysts which are promising candidates for reaching excellent selectivity for the CDRR. Then, the manufacturing of photoanodes based on n-Si is reviewed with an emphasis on those prepared by electrodeposition of a transition metal such as Ni and Fe. The effect of the catalyst morphology, density, and Si structuration is discussed, and future developments are proposed

Single-Component and Mixed Ferrocene-Terminated Alkyl Monolayers Covalently Bound to Si(111) Surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 Å from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 × 10-11 to ∼3.5 × 10-10 mol cm-2 by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25−0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E°‘ = 0.50 ± 0.01 V vs SCE, and the rate constant of electron transfer kapp was about 50 s-1

Electrochemically Directed Micropatterning of a Conducting Polymer Covalently Bound to Silicon

Electrochemically Directed Micropatterning of a Conducting Polymer Covalently Bound to Silico

Electron Transfer-Induced Conformational Changes of Highly Hindered Aromatic Compounds. The Case of Hexakis(alkylsulfonyl)benzenes

The monoelectronic reduction of hexakis(alkylsulfonyl)benzenes (alkyl = methyl 1a, ethyl 1b, butyl 1c, iso-propyl 1d, and iso-butyl 1e) to the corresponding radical anion in dimethylformamide involves two widely separated redox systems, except in the case of 1e that shows a single perfectly reversible system. Electrochemical data supported by calculations of molecular modeling (DFT and PM3 methods) show the existence of a four-member square scheme for which the neutral and radical anion species can both exist under chair and boat conformations. The relative stability of the conformers was found to be strongly dependent on the nature of the alkyl substituents. Generally, the most stable neutral forms adopt a chairlike geometry, and the radical anions adopt a boatlike geometry. For the most hindered compound 1e, the steric contribution of the (iso-butylsulfonyl) substituents becomes so strong that the conformational changes are considerably slowed, resulting in a frozen chair conformation

Multiredox Tetrathiafulvalene-Modified Oxide-Free Hydrogen-Terminated Si(100) Surfaces

Tetrathiafulvalene (TTF) monolayers covalently bound to oxide-free hydrogen-terminated Si(100) surfaces have been prepared from the hydrosilylation reaction involving a TTF-terminated ethyne derivative. FTIR spectroscopy characterization using similarly modified porous Si(100) substrates revealed the presence of vibration bands assigned to the immobilized TTF rings and the Si–CC– interfacial bonds. Cyclic voltammetry measurements showed the presence of two reversible one-electron systems ascribed to TTF/TTF^.+ and TTF^.+/TTF²⁺ redox couples at ca. 0.40 and 0.75 V vs SCE, respectively, which compare well with the values determined for the electroactive molecule in solution. The amount of immobilized TTF units could be varied in the range from 1.7 × 10^–10 to 5.2 × 10^–10 mol cm^–2 by diluting the TTF-terminated chains with inert <i>n</i>-decenyl chains. The highest coverage obtained for the single-component monolayer is consistent with a densely packed TTF monolayer

Conducting Ferrocene Monolayers on Nonconducting Surfaces

The redox activity of a ferrocenyl monolayer grafted on an n-type Si(111) substrate was investigated by scanning electrochemical microscopy (SECM) in conditions where the substrate plays the role of an insulator. This approach permits the differentiation between the different possible electron-transfer and mass-transport pathways occurring at the interface. As an exciting result, the thin ferrocenyl monolayer behaves like a purely conducting material, highlighting very fast electron communication between immobilized ferrocenyl headgroups in a 2D-like charge-transport mechanism

Optimization of Three-Dimensional-Printed Catalytic Electrodes for Alkaline Water Electrolysis Guided by the Experimental Design Methodology

Dihydrogen (H2) is considered nowadays as a sustainable energy carrier and exhibits a high energy density in its highly compressed state. Different countries in the world are developing industrial production and are scaling up water electrolysis processes with clear roadmaps. Alkaline water electrolysis is an affordable technology that avoids the use of platinum group metals with a good compromise of efficiency because of the development of new cost-effective and Earth-abundant materials of electrodes and new anion-exchange membranes. Here, we report on the use of emerging and selective laser melting (SLM) technology for the three-dimensional (3D) printing of binary alloyed NiFe, NiMo, NiCr, and NiCo electrode materials at an applied scale. These materials are preliminarily tested for hydrogen and oxygen evolution half-reactions in alkaline conditions to select the most promising combinations of cathode/anode combinations for overall water electrolysis. On the basis of three key performance metrics (KPMs, namely, overpotential values at 10 and 50 mA cm–2 and the stability in operation over 6 days at 50 mA cm–2), NiCr and NiCo are preferred for the cathode, whereas the anode is NiMo or NiFe. Moreover, to facilitate the release of the electrogenerated gas bubbles, the patterning of 3D-printed electrodes with either roughly conical holes or ramps is considered as a third experimental factor. The water electrolysis process is then fully optimized using the formalism of design of experiments (DOE), which allows to reduce the number of experiments without loss in the quality of conclusions. This study reveals that NiCo and NiMo will be preferentially used as the cathode and anode, respectively, to decrease the cell overvoltage, whereas NiFe as the anode yields the best stability in operation. Besides, the impact of the patterns printed by SLM on the KPMs is somewhat limited, even though the presence of holes is rather beneficial for decreasing cell overvoltage. Interestingly, an approximate calculation of the consumed energy for the production of 1 kg of H2 from the combination of NiCo/NiMo/holes as the cathode/anode/pattern yields 50.6 kWh/kg of H2, which is quite promising if one refers to the 48 kWh industrial target set by European Union for 2030

Scanning Electrochemical Microscopy Investigations of Monolayers Bound to p-Type Silicon Substrates

p-Si type electrodes modified with different organic monolayers were investigated by reaction with radical anion and cation electrogenerated at a microelectrode operating in the configuration of a scanning electrochemical microscope. The method proves to be a convenient tool for investigating both the quality and the redox properties of the layer as previously demonstrated on metallic electrodes especially when the sample cannot be electrically connected. Approach curves recorded with the different mediators were used to investigate the electron-transfer rates across alkyl monolayers bound to p-type silicon substrates. Preliminary results indicate that the interfacial electron transfer occurs via electron tunneling through the organic layer as generally described for SAMs grafted on gold electrodes

Synthesis and Electrochemistry of Carboranylpyrroles. Toward the Preparation of Electrochemically and Thermally Resistant Conjugated Polymers

Pyrrole derivatives covalently linked to a neutral ortho- or anionic nido-carborane cage via a methylene or ethylene spacer arm have been synthesized. Their electrochemical study showed that only the neutral compounds yielded anodically conducting 3-substituted polypyrrole films. In contrast, the anodic oxidation of the anionic derivatives underwent the passivation of the electrode surface. The pyrrole with a two-carbon linkage was the most efficiently electropolymerized, and the relevant polymer films exhibited the best electroactive and conducting properties. These were resistant to highly positive potentials owing to the hydrophobic nature and the electron-withdrawing character of the attached carborane cage. Moreover, this material was more thermally stable than unsubstituted polypyrrole

Efficient and Highly Stable 3D-Printed NiFe and NiCo Bifunctional Electrodes for Practical HER and OER

Considered like a fully renewable and clean energy carrier with the highest energy density, dihydrogen (H2) constitutes the best alternative to fossil fuels for ensuring the sustainability of energy. This technologically and industrially relevant energy vector can be produced by water electrolysis that is now widely recognized as an eco-friendly, scalable, and carbon-free route. In that context, alkaline water electrolysis is an affordable technology because it allows the use of transition metals as electrocatalysts for both hydrogen (HER) and oxygen (OER) evolution reactions. Here, combining catalytically active transition metal alloys (NiFe and NiCo) with a 3D printing technique, namely, selective laser melting (SLM), enables access to ca. 25 cm2 microstructured cylindrical electrodes, efficiently promoting alkaline HER and OER. NiCo is found to be the most efficient electrode for HER, with an overpotential of 210 mV at 10 mA cm–2. It is also the most stable electrode when studied in operation during prolonged electrolysis, with a potential change of only 40 mV after 140 h of electrolysis at 50 mA cm–2 (1.25 A). Regarding the OER, NiFe shows the highest catalytic activity with an overpotential of 300 mV at 10 mA cm–2 and the greatest stability in operation with an electrolysis-induced potential change of only 30 mV. Further surface characterization techniques (scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), contact angle measurements, and analysis of electrogenerated gas bubbles) are used to monitor the electrolysis-induced chemical changes and to get valuable information on the bubble dynamics. Interestingly, electrolysis is found to be beneficial for enhancing the hydrophilicity and/or the aerophobicity of the electrodes, thus facilitating the detachment of the H2 or O2 bubbles