Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Abstract

Surface science research fixated on phenomena and processes that transpire at the electrodeelectrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical-analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined singlecrystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; i.e., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surfacescience protocols remain unchanged but the experimental capabilities have been escalated by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes results selected from ongoing studies of earth-abundant electrocatalysts for the reactions that underpin artificial photosynthesis: nickel-molybdenum alloys for the hydrogen evolution reaction, calcium birnessite as a heterogeneous analogue for the oxygen-evolving complex in natural photosynthesis, and single-crystalline copper in relation to the carbon dioxide reduction reaction.

Oxygen-tolerant proton reduction catalysis: much O2 about nothing?

“…15,103 The wide scope for structural and geometric modification through methods such as doping, nanostructuring or controlled deposition of multifunctional layers has allowed rational surface design to maximise catalytic turnover and stability. 12,104,105 Their use includes a few disadvantages however, as they have generally low 'per atom activity' and ascertaining the exact nature of the catalytically active site and mechanism can be difficult. Heterogeneous surfaces are considerably less sensitive to O 2 than molecular complexes and hydrogenases (presumably due to the absence of fragile organic ligand frameworks) and many proton reducing surfaces are active O 2 reduction catalysts.…”

Spectroscopic identification of active sites for the oxygen evolution reaction on iron-cobalt oxides

“…This approach has revealed that descriptors, such as metal-oxygen bond strength 5 or the population level of antibonding orbitals 6 can successfully describe catalytic performance, and has ultimately led to the modern understanding of scaling relations in electrocatalysis 7 – 10 . The accuracy of design principles and predictive models, however, requires accurate structural models for the electrocatalyst material based on experimental identification of reaction sites and intermediates 11 – 15 . Reports of amorphization under electrocatalytic OER conditions introduces the possibility that the catalytically active species assumes a fundamentally different structure than the original material 16 – 18 .…”

Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS2 and MoSe2

“…16,20,21 However, the well-defined surfaces of single crystals provide unique opportunities for the rigorous study of electron−solution interfaces, active sites, and mechanisms of electrocatalysis. 22 We compare herein the electrocatalytic activity of single crystals of MoS 2 and MoSe 2 for the HER in aqueous acidic and alkaline solutions and compare the dependence of their electrocatalytic activity on the density of step edges created by electrochemical etching. Furthermore, we compare the electrocatalytic activity of thin films of MoS 2 and MoSe 2 in acid and base.…”

“…Bulk single crystals are among the worst MX 2 -based hydrogen-evolution catalysts with respect to the overpotential required to drive the HER at a particular current density. For example, bulk single crystals of MoS 2 possess negligible catalytic activity for the HER when operated in contact with acidic or neutral aqueous solutions, typically requiring the application of potentials more negative than −1.0 V vs the reversible hydrogen electrode (RHE) to drive the HER at a current density of −10 mA cm –2 , whereas MoS 2 nanoparticles require an overpotential of −150 mV to drive the HER at the same current density. ,, However, the well-defined surfaces of single crystals provide unique opportunities for the rigorous study of electron–solution interfaces, active sites, and mechanisms of electrocatalysis . We compare herein the electrocatalytic activity of single crystals of MoS 2 and MoSe 2 for the HER in aqueous acidic and alkaline solutions and compare the dependence of their electrocatalytic activity on the density of step edges created by electrochemical etching.…”