Fadl H. Saadi

Electrocatalysis of the hydrogen-evolution reaction by electrodeposited amorphous cobalt selenide films†

Using an electrochemical method under ambient conditions, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports. These films have been evaluated as electrocatalysts for the hydrogen-evolution reaction. In 0.500 M H2SO4, the cobalt selenide films required an overpotential of ∼135 mV to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm−2. Galvanostatic measurements indicated stability of the electrocatalytic films for >16 h of continuous operation at −10 mA cm−2. The facile preparation method, and the activity of the cobalt selenide films, suggest that electrodeposited metal chalcogenides are potentially attractive earth-abundant electrocatalysts for the hydrogen-evolution reaction.

Interface engineering of the photoelectrochemical performance of Ni-oxide-coated n-Si photoanodes by atomic-layer deposition of ultrathin films of cobalt oxide

Introduction of an ultrathin (2 nm) film of cobalt oxide (CoO_x) onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO_x coating yields stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the equilibrium potential for water oxidation when in contact with 1.0 M KOH(aq) under 1 sun of simulated solar illumination. The photoelectrochemical performance of these electrodes was very close to the Shockley diode limit for moderately doped n-Si(100) photoelectrodes, and was comparable to that of typical protected Si photoanodes that contained np+ buried homojunctions.

Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films.

Significance The development of efficient artificial photosynthetic systems, designed to store solar energy in chemical bonds, requires the pairing of stable light-absorbing electrodes for both the oxidative and reductive half-reactions. The development of such systems has been hindered in part by the lack of semiconducting photoanodes that are stable under the conditions required for the production of O2(g) from water. We demonstrate herein that a reactively sputtered NiOx layer provides a transparent, antireflective, conductive, chemically stable, inherently catalytic coating that stabilizes many efficient and technologically important semiconducting photoanodes under viable system operating conditions, thereby allowing the use of these materials in an integrated system for the sustainable, direct production of fuels from sunlight. Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O2(g). These NiOx coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aqueous potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiOx films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O2(g).

Operando Spectroscopic Analysis of CoP Films Electrocatalyzing the Hydrogen-Evolution Reaction

Transition metal phosphides exhibit high catalytic activity toward the electrochemical hydrogen-evolution reaction (HER) and resist chemical corrosion in acidic solutions. For example, an electrodeposited CoP catalyst exhibited an overpotential, η, of -η < 100 mV at a current density of -10 mA cm-2 in 0.500 M H2SO4(aq). To obtain a chemical description of the material as-prepared and also while effecting the HER in acidic media, such electrocatalyst films were investigated using Raman spectroscopy and X-ray absorption spectroscopy both ex situ as well as under in situ and operando conditions in 0.500 M H2SO4(aq). Ex situ analysis using the tandem spectroscopies indicated the presence of multiple ordered and disordered phases that contained both near-zerovalent and oxidized Co species, in addition to reduced and oxygenated P species. Operando analysis indicated that the active electrocatalyst was primarily amorphous and predominantly consisted of near-zerovalent Co as well as reduced P.

A comparison of the chemical, optical and electrocatalytic properties of water-oxidation catalysts for use in integrated solar-fuel generators

The in situ optical properties and electrocatalytic performance of representative catalysts for the oxygen-evolution reaction (OER) have been considered together to evaluate system-level effects that accompany the integration of OER catalysts into a solar-fuel device driven by a tandem-junction light absorber with a photoanode top cell, i.e., a design that requires incident light to be transmitted through the OER catalyst before reaching a semiconducting light absorber. The relationship between the overpotential and optical transmission of the catalysts determined the attainable efficiencies for integrated solar-fuel devices as well as the optimal band gaps for the photoanode in such devices. The systems investigated generally showed: (1) the OER catalysts dissolved in acid, and were less stable in buffered near-neutral pH electrolytes than in strongly alkaline electrolytes; (2) higher overpotentials were required to drive the OER at a specified current density when the catalysts were operated in contact with near-neutral pH electrolytes than strong alkaline electrolytes; (3) for some of the OER catalysts, the electrocatalytic activity and in situ absorption spectra depended strongly on the preparation method; (4) increasing the loading of the electrocatalyst reduced the overpotential and the optical transmission; (5) for the catalysts studied, the optical transmission and overpotential were generally correlated, and the trend lines did not cross, indicating that based on these factors alone, the optimal approach is to use lower loadings of highly active catalysts, rather than to use a less active but more transparent catalysts; (6) for a solar-fuel device driven by semiconductors operating at the Shockley–Queisser limit and using a continuous film of a given OER catalyst in the path of incident light, the efficiency decrease due to the reduced optical transmittance that accompanies increased OER catalyst loading can be substantially greater than any efficiency increase that might be gained through the reduction in catalytic overpotential by increasing the catalyst loading; and (7) HER catalysts possessed the same performance trade-off when the light is incident through the HER catalysts as is observed for OER catalysts when the light is incident from the OER side.

Pathways to electrochemical solar-hydrogen technologies

Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

Heterogenization of a Water-Insoluble Molecular Complex for Catalysis of the Proton-Reduction Reaction in Highly Acidic Aqueous Solutions

Our long-held interest in the resiliency of electrochemical functionalities upon surface immobilization has herded us from directly chemisorbed electroactive moieties, to anchor group-leashed redox-active couples and to surface-tethered enzyme-inspired molecular catalysts. The latter represent the most intricate because the electrocatalytic activities involve mixed-valence states and may require certain entatic (fractionally rotated) configurations. In this regard, we recently investigated the proton-reduction electrocatalysis by hydrogenase-inspired di-iron complexes at polycrystalline and (111)-faceted Au electrodes.

Correction: Relative costs of transporting electrical and chemical energy

Correction for ‘Relative costs of transporting electrical and chemical energy’ by Fadl H. Saadi et al., Energy Environ. Sci., 2018, DOI: 10.1039/c7ee01987d.