Electrocatalytic Hydrogen Oxidation in Alkaline Media: From Mechanistic Insights to Catalyst Design

Abstract

With the potential to circumvent the need for scarce and cost-prohibitive platinum-based catalysts in proton-exchange membrane fuel cells, anion-exchange membrane fuel cells (AEMFCs) are emerging as alternative technologies with zero carbon emission. Numerous noble metal-free catalysts have been developed with excellent catalytic performance for cathodic oxygen reduction reaction in AEMFCs. However, the anodic catalysts for hydrogen oxidation reaction (HOR) still rely on noble metal materials. Since the kinetics of HOR in alkaline media is 2–3 orders of magnitude lower than that in acidic media, it is a major challenge to either improve the performance of noble metal catalysts or to develop high-performance noble metal-free catalysts. Additionally, the mechanisms of alkaline HOR are not yet clear and still under debate, further hampering the design of electrocatalysts. Against this backdrop, this review starts with the prevailing theories for alkaline HOR on the basis of diverse activity descriptors, i.e., hydrogen binding energy theory and bifunctional theory. The design principles and recent advances of HOR catalysts employing the aforementioned theories are then summarized. Next, the strategies and recent progress in improving the antioxidation capability of HOR catalysts, a thorny issue which has not received sufficient attention, are discussed. Moreover, the significance of correlating computational models with real catalyst structure and the electrode/electrolyte interface is further emphasized. Lastly, the remaining controversies about the alkaline HOR mechanisms as well as the challenges and possible research directions in this field are presented.

Phase Engineering of a Ruthenium Nanostructure toward High-Performance Bifunctional Hydrogen Catalysis

“…The Koutecky–Levich plots at 0.1 V vs RHE were obtained to verify the electron transport process (Figure f). The slopes of Koutecky–Levich plots were 12.89, 12.21, 13.05, and 13.32 cm 2 mA –1 rpm 1/2 for MoO x -Ru fcc, MoO x -Ru hcp, Ru/C, and Pt/C, respectively, which verified the H 2 mass-transport controlled process. − Figure g and Table S2 summarize the mass activities of some noble metal-based catalysts for HOR in the literature, and MoO x -Ru fcc presented the highest mass activity. Finally, the catalytic stability of these catalysts was evaluated by chronoamperometry measurement at 0.1 V vs RHE in H 2 -saturated 0.1 M KOH at a rotating speed of 1600 rpm (Figure h).…”

NiFe Alloy Electrocatalysts toward Efficient Alkaline Hydrogen Oxidation

“…So far, PGM-free alkaline HOR electrocatalysts are mainly Nibased ones. [9,10] However, Ni has inappropriate hydrogen binding energy (HBE) and hydroxyl binding energy (OHBE), which have hindered the alkaline HOR activity. [11,12] In order to tune HBE and OHBE, many efforts have been contributed to alloying with transition metal, doping with non-metal elements and modifying surface with metal oxides.…”

“…So far, PGM‐free alkaline HOR electrocatalysts are mainly Ni‐based ones [9,10] . However, Ni has inappropriate hydrogen binding energy (HBE) and hydroxyl binding energy (OHBE), which have hindered the alkaline HOR activity [11,12] .…”

Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis

“…Building an interface between a metal and an oxide is an important example of the multifunctionality. The synergistic interactions between the support (typically, an oxide) and active phase (e.g., metal nanoparticles) have shown the superior activity in CO oxidation and the water–gas shift reaction. − The critical role of metal/oxide interfaces is also elucidated in several electrocatalytic reactions, including hydrogen oxidation and evolution reactions in alkaline and neutral media. − Thus, metal/oxide junctions are potentially a key factor to overcome the challenges of electrocatalysis on metal surfaces. − In the metal/oxide junction system, the electrocatalytic activity enhancement will be determined by interface formation between the metal and the oxide. Nevertheless, clear design principles for building metal/oxide systems with desired catalytic characteristics are still lacking.…”