Recent Advances in Electrocatalysts toward Alcohol-Assisted, Energy-Saving Hydrogen Production

Abstract

As a pure and sustainable source of power, hydrogen (H2) is the desired chemical candidate for the future energy mix. Water electrolysis has been regarded as an effective method for producing clean and ultrapure hydrogen gas. However, its large-scale applications are hampered by its slow kinetics, particularly due to its slow anodic half-reaction i.e., the oxygen evolution reaction (OER). Another strategy based on chemical-assisted electrocatalytic energy-saving hydrogen production has recently been developed with great potential to address barriers associated with OER. In this case, OER is replaced by organic oxidation reactions that are thermodynamically more favorable, which substantially reduces the voltage required for H2 evolution and also facilitates the co-production of organic value-added products. Oxidation of biomass derivatives, such as alcohols, is the most suitable strategy for producing value-added chemicals with energy-saving hydrogen production. This Review focuses on the characteristics of making electrolytic hydrogen production more cost-efficient by using different alcohols. We have reviewed the fundamentals and key parameters for alcohol-assisted electrochemical hydrogen production and discussed several anodic alcohol oxidation reactions with value-added products. The choice of electrocatalysts, strategies to increase the reaction selectivity, and the possible cell architectures are elaborated in detail.

Electrooxidation of Alcohols on Mixed Copper–Cobalt Hydroxycarbonates in Alkaline Solution

“…[3,4] Therefore, substituting water oxidation by other reactions requiring lower overpotentials while producing H 2 at the cathode, described as a "chemical-assisted hydrogen evolution reaction", could make electrocatalytic hydrogen production economically feasible. [5,6] Additionally, by replacing O 2 formation, possibilities open up also at the anode for the generation of value-added chemicals, which can be paired with H 2 formation at the cathode via water splitting. [7,8] As such, the electrochemical oxidation of organic compounds, for example, alcohols, [9,10] amines, [11] urea, [12] and hydrazine [13] has gained increasing interest in the last years.…”

“…While in HER, two protons coupled to two electrons are transferred, OER requires the transfer of four electrons and protons, thus being a sluggish reaction showing slow kinetics and higher overpotentials which decrease the overall energy efficiency of water electrolysis [3,4] . Therefore, substituting water oxidation by other reactions requiring lower overpotentials while producing H 2 at the cathode, described as a “chemical‐assisted hydrogen evolution reaction”, could make electrocatalytic hydrogen production economically feasible [5,6] . Additionally, by replacing O 2 formation, possibilities open up also at the anode for the generation of value‐added chemicals, which can be paired with H 2 formation at the cathode via water splitting [7,8] …”

Replacing Anodic Oxygen Evolution Reaction with Organic ­Oxidation: The Importance of Metal (Oxy)Hydroxide Formation as the Active Oxidation Catalyst

“…also affect the activity. 51 Although transitionand noble-metal-based catalysts were reported for the anodic oxidation of the organic compounds, 5,7,32,33 in this account, we have considered only the transition-metal-based catalysts. The improvement of the cell voltage, faradaic efficiency (FE), and selectivity for the value-added products are particularly described when anodic OER is replaced by AOR.…”

“…As described in section 3, transition-metal catalysts produced excellent catalytic activity and selectivity for the value-added product during the anodic oxidation of organic compounds. 5,7,32,33 The transition-metal-based catalysts undergo anodic electrochemical activation to form M(O) x (OH) y as the active catalyst in the alkaline medium (Figure 2). 2,36,37,41 The activation process depends on the structure, particle size of the precatalyst, applied anodic potential, and the pH of the electrolyte solution.…”

Boosting Hydrogen Production by Selective Anodic Electrooxidation of Ethanol over Trimetallic PdSbBi Nanoparticles: Composition Matters

“…Hydrogen has emerged as one of the most attractive energy carriers due to its virtue of clean and high energy density. Electrocatalytic water splitting is extensively studied as a promising approach to produce ultrapure hydrogen fuel under mild conditions. − However, the performance of water electrolysis is limited by the sluggish anodic oxygen evolution reaction (OER), which requires much higher overpotentials than the cathodic hydrogen evolution reaction (HER). , Moreover, the hydrogen and oxygen mixing in the electrolyzer increases the explosion risk even though a proton exchange membrane is used in the system. , To overcome these drawbacks, attempts have been made to replace the OER in water splitting by the oxidation of organic substrates. − Compared with the OER, the electrooxidation of organic species is a thermodynamically favorable route and possesses high energy conversion efficiency. , Among the organic substrate candidates, ethanol is particularly attractive relying on its low toxicity, easy storage, and potential for bio-sourcing. , The electrochemical ethanol oxidation reaction (EOR) is a complicated process with multielectron transportation. In particular, the incomplete oxidation of ethanol to acetic acid by delivering four electrons is an appealing energy-efficient and value-added pathway. − Therefore, the electrocatalytic system integrated the hydrogen fuel generation from water reduction at the cathode and acetic acid production from ethanol oxidation at the anode is of great significance from economic and energetic points of view.…”