The Influence of Nanoconfinement on Electrocatalysis

Abstract

The use of nanoparticles and nanostructured electrodes are abundant in electrocatalysis. These nanometric systems contain elements of nanoconfinement in different degrees, depending on the geometry, which can have a much greater effect on the activity and selectivity than often considered. In this Review, we firstly identify the systems containing different degrees of nanoconfinement and how they can affect the activity and selectivity of electrocatalytic reactions. Then we follow with a fundamental understanding of how electrochemistry and electrocatalysis are affected by nanoconfinement, which is beginning to be uncovered, thanks to the development of new, atomically precise manufacturing and fabrication techniques as well as advances in theoretical modeling. The aim of this Review is to help us look beyond using nanostructuring as just a way to increase surface area, but also as a way to break the scaling relations imposed on electrocatalysis by thermodynamics.

Keywords: Electrocatalysis; Electrochemistry; Ion Migration; Nanotechnology; Scaling Relationships.

Figure 1

Types of nanoconfined systems. a) Schematic representation of nanoconfined systems: i) nanowires and pillars, ii) parallel plates, iii) interconnecting meso‐ and nanopores, and iv) an isolated channel system. b) Computational model of the confined microenvironment for the CO₂RR with two adjacent nanoparticles, where the configuration of *COOH is optimised within the confined structure, where the gap d is infinite, 10 Å, 8 Å, and 6 Å. c) Top and side views of the H adsorption configurations on graphene‐covered metal surfaces for Ni and Cu (left) as well as Pd, Rh, Pt, Au, and Ag (right) substrates. d) Kinetic mass and specific activities at 0.9 V (RHE) for porous NiPt films with and without ionic liquid as a function of the dealloyed depth. Representation of the ORR on porous electrodes at both high (top) and low (bottom) overpotential. e) Schematic representation of nanozyme nanoparticles with passivated exterior surfaces and catalytic active sites inside substrate channels, together with a voltammogram displaying the specific activity for the ORR on spherical nanoparticles with isolated channels (nanozymes), interconnected pores, and no confinement, at low overpotentials. Adapted, with permission, from: b) Ref. [2a], Copyright 2020 American Chemical Society; c) Ref. [2c], Copyright 2016 American Chemical Society; d) Ref. , Copyright 2010 Nature; e) Ref. , Copyright 2018 American Chemical Society.

Figure 2

Increased collisions as a result of nanoconfined space. Schematic depiction of a) a planar electrode, where reactants often only interact with the surface once before diffusing back into the bulk electrolyte; a mesoporous sheet of b) interconnected pores, and c) non‐interconnected pores, both acting as “partition membranes”; and d) an isolated channel only open at one end.

Figure 3

Conditions where nanoconfinement are most prominent. a) Schematic illustration of the volume of the porous electrode surface that participates in the reaction when the reaction is fast (mass transport limited, top) and slow (kinetically limited, bottom). b) Linear sweep voltammograms of a flat and porous Pt electrode in oxygen‐saturated 0.1 m phosphate buffer containing chloride. Currents are normalised to the electrochemical surface area. The effect of the degree of adsorption on collision frequency as a result of nanoconfinement for c) adsorption reactions (e.g. oxidation of 1‐butanol) and d) non‐adsorption reactions (e.g. oxidation of 2‐butanol). Kinetic current densities calculated from Koutecký–Levich plots at 0.8–1.0 V (RHE) for nanozymes with e) small (black), medium (blue), large (red) channels, and mesoporous particles (white); f) small channel particles without surfactant (black), medium channel particles without surfactant (blue), large channel particles without surfactant (red), and mesoporous particles without surfactant (white). Error bars represent the standard deviation from triplicate experiments. Adapted, with permission, from: a) Ref. [12b], Copyright 2010 Elsevier; b) Ref. [20a], Copyright 2010 American Chemical Society; c), d) Ref. [20b], Copyright 2013 Royal Society of Chemistry; e), f) Ref. [4a], Copyright 2020 Royal Society of Chemistry.

Figure 4

Concentration profiles in nanoconfined space. Counterion and co‐ion concentration profiles in a 20 Å diameter nanopore for a single 1 : 1 electrolyte with a surface charge density of −0.05 C m⁻² and an ionic strength of a) 0.5 mol L⁻¹ and b) 1.0 mol L⁻¹. Adapted, with permission, from Ref. , Copyright 2008 AIP Publishing.

Figure 5

Electrical double layer overlapping in nanoconfined space. a) Simulated H⁺ concentration distribution in 1 nm, 1.5 nm, and 3 nm diameter channels during the diffusion‐limited reduction of oxygen, showing higher H⁺ concentrations in narrower channels. The depletion of H⁺ ions is insignificant regardless of the selected applied potential. Kinetic current densities for the ORR at 0.95–1.0 V (RHE) for nanozymes with small (<2 nm; red) and large (>2 nm; black) channels at different HClO₄ concentrations: b) 1.0 mol L⁻¹, c) 0.1 mol L⁻¹, and d) 0.01 mol L⁻¹. Each voltammogram is accompanied with a scheme, where the line represents the equipotential profile separating the EDL from the bulk solution, and the arrows represent migration into the channels when EDL overlapping occurs. Adapted, with permission, from Ref. [4a], Copyright 2020 Royal Society of Chemistry.

Figure 6

Breaking scaling relationships with nanoconfinement. a) The potential needed to drive each oxygen‐evolution reaction step on RuO₂ (top) and IrO₂ (bottom) as a function of channel width. Discrete symbols represent the highest limiting potential in the plots on the left, which have been translated to overpotentials in the volcano plot on the right. Arrows indicate trends in activity as d _mm decreases. b) Schematic representation of processes occurring at the specific potential windows with active (red) or inactive (black) nanozyme cascade mechanisms, and c) product formation rates of C₃H₈O in moles per second and gram of nanozyme particles. The vertical lines separate three distinct potential windows. Adapted, with permission, from: a) Ref. [2f], Copyright 2015 European Chemical Societies Publishing, and b), c) Ref. [4b], Copyright 2019 American Chemical Society.