Recent Advancement of p‐ and d‐Block Elements, Single Atoms, and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Abstract

is produced via steam reforming process, coal gasification, and water splitting methods. [4] Steam reforming and coal gasification generate hydrogen from the reaction of fossil fuels with water which requires high temperatures (≈1000 °C) and pressures. [4a] The steam reforming and coal gasification utilize the conventional fossil fuels for hydrogen fuel generation, therefore, emitting the flue gas (CO and CO 2 ) to the environment that is severely affecting the climate. In contrast, hydrogen fuel produced from water splitting reaction is a highly favored and clean alternative because of the following advantages: i) water splitting reactions proceed at room temperature and pressure which requires less infrastructure, ii) hydrogen and oxygen gases are produced separately, which thus eliminates the gas separation steps, and iii) both reactant (H 2 O) and products (H 2 and O 2 ) are environmentally friendly, which fulfills the concept of green technology. [5] Fundamentally, water splitting is a two half-cell reaction in which hydrogen is generated at cathode through hydrogen evolution reaction (HER) [6] while oxygen is generated at anode through oxygen evolution reaction (OER). [7] Splitting water for hydrogen generation can be mainly achieved via electrocatalytic and photocatalytic/photoelectrocatalytic systems. In electrochemical water splitting, the hydrogen and oxygen are produced through high thermodynamic potential and platinum-/iridium-based catalysts. [8] In electrochemical water splitting, the consumption of high electric power and the uneconomical catalysts hinder the industrial-scale hydrogen production. Therefore, most of the recent efforts for hydrogen generation through water splitting are directed toward sunlightto-hydrogen conversion, since the light from the sun can be regarded as a free energy resource. [1,2,9] In photoelectrochemical (PEC) system, the most prominent solar-to-hydrogen (STH) conversion process is a photoelectrocatalytic hydrogen generation by water splitting. [2,9] The main components of a typical PEC cell are i) a working electrode composed of a semiconducting material responsible for the current generation, ii) a counter electrode designed to complement the main catalytic process, and iii) an electrolyte system which provides efficient mass transport to the surface of each electrode. [9] Figure 1 summarizes the general photoelectrocatalytic

In situ charge carrier dynamics of semiconductor nanostructures for advanced photoelectrochemical and photocatalytic applications

“…Such an operando spectroscopic measurement is of essential significance and can offer more reliable information on the charge dynamics mechanism for working photoelectrodes and photocatalysts. Future utilization of this technique will benefit from the ongoing development of more sophisticated materials systems, such as single atom-based photoelectrodes [108] and photocatalysts [109], for advanced PEC and photocatalytic applications.…”

Tuning metal single atoms embedded in NxCy moieties toward high-performance electrocatalysis

“…19 In this regard, minimal amounts of metal (M) SAs [20][21][22][23][24][25][26] are exploited to maximize the active surface area and to tune the catalytic activity by coordinating the SAs to glue atoms in defect sites of graphene (Gr) or other 2D materials. 17,[27][28][29][30][31][32][33][34][35][36] However, while most metal NPs behave like conducting metals, metal SAs embedded in supports often lose some of the electric conductivity (s), because once embedded metal SAs are ligated by C/N atoms in vacant sites, the p-conjugation through C/N atoms in the 2D sheet can be lost. 17,27,28,31 This would cause a reduction in the kinetic reaction rate of SA electrocatalysts.…”

Late Transition Metal Doped MXenes Showing Superb Bifunctional Electrocatalytic Activities for Water Splitting via Distinctive Mechanistic Pathways

“…The half‐reactions of HER [ 5,11 ] and OER [ 12–17 ] which take place at cathodic and anodic sites, respectively, can be seen simultaneously in electrocatalytic water splitting. [ 18–25 ] Presently, the promising catalysts for OER and HER are Ru and Ir‐based metal oxides and Pt‐based electrocatalysts, respectively. [ 26,27 ] Despite being highly efficient with low overpotential (η), the large‐scale industrial usage is hampered by its high cost and low availability.…”