Angel T. Garcia-Esparza

Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion.

Microkinetic analyses of aqueous electrochemistry involving gaseous H2 or O2, i.e., hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are revisited. The Tafel slopes used to evaluate the rate determining steps generally assume extreme coverage of the adsorbed species (θ ≈ 0 or ≈1), although, in practice, the slopes are coverage-dependent. We conducted detailed kinetic analyses describing the coverage-dependent Tafel slopes for the aforementioned reactions. Our careful analyses provide a general benchmark for experimentally observed Tafel slopes that can be assigned to specific rate determining steps. The Tafel analysis is a powerful tool for discussing the rate determining steps involved in electrocatalysis, but our study also demonstrated that overly simplified assumptions led to an inaccurate description of the surface electrocatalysis. Additionally, in many studies, Tafel analyses have been performed in conjunction with the Butler-Volmer equation, where its applicability regarding only electron transfer kinetics is often overlooked. Based on the derived kinetic description of the HER/HOR as an example, the limitation of Butler-Volmer expression in electrocatalysis is also discussed in this report.

Photoelectrochemical and electrocatalytic properties of thermally oxidized copper oxide for efficient solar fuel production

We report the use of a facile and highly scalable synthesis process to control growth products of earth-abundant Cu-based oxides and their application in relevant photoelectrochemical and electrochemical solar fuel generation systems. Characterization of the synthesized Cu(I)/Cu(II) oxides indicates that their surface morphology and chemical composition can be simply tuned by varying two synthesis parameters (time and temperature). UV-Vis spectroscopy and impedance spectroscopy studies are performed to estimate the band structures and electronic properties of these p-type semiconductor materials. Photoelectrodes made of Cu oxides possess favorable energy band structures for production of hydrogen from water; the position of their conduction band is ≈1 V more negative than the water-reduction potential. High acceptor concentrations on the order of 1018–1019 cm−3 are obtained, producing large electric fields at the semiconductor–electrolyte interface and thereby enhancing charge separation. The highly crystalline pristine samples used as photocathodes in photoelectrochemical cells exhibit high photocurrents under AM 1.5G simulated illumination. When the samples are electrochemically reduced under galvanostatic conditions, the co-existence of the oxide with metallic Cu on the surface seems to function as an effective catalyst for the selective electrochemical reduction of CO2.

A simplified theoretical guideline for overall water splitting using photocatalyst particles

Particulate photocatalytic water splitting is the most disruptive and competitive solution for the direct production of solar fuels. Despite more than four decades of work in the field of photocatalysis using powdered semiconductors decorated with catalyst particles, there is no clear consensus on the factors limiting the solar-to-hydrogen efficiency (STH). To understand the intrinsic limitations of the system, we numerically simulated simplified two-dimensional photocatalytic models using classical semiconductor device equations. This work presents the sensitivity of quantum efficiency (QE) to the various semiconductor properties, such as absorption properties and carrier mobilities, and to the dispersion of catalyst particles, which create heterojunctions, the driving force for charge separation. As a result, a pinch-off effect was prevalent underneath the hydrogen evolution site, suggesting an undesired energetic barrier for electron diffusion to the catalyst. The simulation using the values reported in the literature revealed that the QE was exclusively governed by recombination in the bulk of the photocatalyst particles, hindering the charge separation efficiency before reaching the catalysts on the surface. Using some of the reported parameters, our simulation shows that a typical defective n-type semiconductor particle (∼100 nm) ideally exhibits a QE of <5% in the visible light range per particle, which reaches only approximately 10% in a slurry after 4 consecutive absorbing units (1.4% STH, from simulated solar irradiation). Although the present model contains rigid limitations, we use these trends as an initial guideline to pursue photocatalysis by a design strategy, which may result in possible alternatives to achieve higher efficiencies.

An Oxygen-Insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-Based Layer for Overall Water Splitting

For overall water-splitting systems, it is essential to establish O2 -insensitive cathodes that allow cogeneration of H2 and O2 . An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H2 evolution in the presence of O2 . In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O2 gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoOx /Pt/SrTiO3 with inhibited water formation from H2 and O2 , which is the prevailing back reaction on the bare Pt/SrTiO3 photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis.

Tethering Metal Ions to Photocatalyst Particulate Surfaces by Bifunctional Molecular Linkers for Efficient Hydrogen Evolution

A simple and versatile method for the preparation of photocatalyst particulates modified with effective cocatalysts is presented; the method involves the sequential soaking of photocatalyst particulates in solutions containing bifunctional organic linkers and metal ions. The modification of the particulate surfaces is a universal and reproducible method because the molecular linkers utilize strong covalent bonds, which in turn result in modified monolayer with a small but controlled quantity of metals. The photocatalysis results indicated that the CdS with likely photochemically reduced Pd and Ni, which were initially immobilized via ethanedithiol (EDT) as a linker, were highly efficient for photocatalytic hydrogen evolution from Na2S-Na2SO3-containing aqueous solutions. The method developed in this study opens a new synthesis route for the preparation of effective photocatalysts with various combinations of bifunctional linkers, metals, and photocatalyst particulate materials.