Metal-free carbonaceous electrocatalysts and photocatalysts for water splitting

Abstract

Water splitting driven by sunlight or renewable resource-derived electricity has attracted great attention for sustainable production of hydrogen from water. Current research interest in this field is focused on the development of earth-abundant photo- or electrocatalytic materials with high activity and long-term stability for hydrogen and/or oxygen evolution reactions. Due to their unique properties and characteristics, carbon and related carbon-based materials show great potential to replace some of the existing precious metal catalysts in water splitting technology. This tutorial review summarizes the recent significant progress in the fabrication and application of metal-free carbonaceous materials as photo- or electrocatalysts for water splitting. Synthetic strategies and applications of various carbonaceous materials, including graphitic carbon nitride (g-C3N4), graphene, carbon nanotubes (CNTs) as well as other forms of carbon-containing materials, for electrochemical or photochemical water splitting are presented, accompanied by a discussion of the key scientific issues and prospects for the future development of metal-free photo- and electrocatalysts.

One‐Dimensional Earth‐Abundant Nanomaterials for Water‐Splitting Electrocatalysts

“…In this work, we will focus our discussion on those materials with 1D morphologies. Other comprehensive reviews on HER electrocatalysts can be found elsewhere 6, 8, 11, 12, 15, 36…”

“…Among many nanostructured electrode candidates, carbon nanotubes (CNTs) are the most recognized one, due to their convenient and large‐scale synthesis, high intrinsic conductivity and environmental benignity 55. Nitrogen doping has also been interrogated to further increase the conductivity and electrocatalytic activity of CNTs, due to the comparatively higher electron negativity of nitrogen atoms than carbon 12. In 2014, Dai and coworkers reported the formation of nanoscale NiO/Ni heterostructures on the CNT sidewalls as an active and stable HER catalyst 56.…”

“…Hydrogen has been well recognized as one of the most abundant and promising candidates for clean energy supply to replace fossil fuels since the beginning of 1970's, as the only combustion product is water 6, 7, 8, 9, 10, 11, 12, 13. Although hydrogen is one of the most abundant elements in the world, free hydrogen molecules do not exist naturally.…”

“…The electrical water splitting represents ≈ 4% contribution of global hydrogen fuel production to date. Compared to other approaches, as the chemical reaction of water splitting is indeed the reverse reaction of hydrogen combustion, this process enables the complete and close cycle of hydrogen with zero carbon emission, and is thereby regarded as a substantially important and promising method 8, 9, 10, 11, 12, 13, 14…”

Multidimensional Evolution of Carbon Structures Underpinned by Temperature‐Induced Intermediate of Chloride for Sodium‐Ion Batteries

“…[[qv: 9a,b]] With this evolution, the carbon materials would have large changes in their properties. For 1D carbon structures, it can be summarized to be of two main kinds: (1) carbon nanotubes (CNTs) mainly with sp‐hybridized carbon atoms, containing single‐walled and multiwalled carbon nanotubes, are deemed with wrapping graphite layers and perfect tubes in structure, showing the high modulus, strength, and conductivity;10 (2) carbon fibers (CNFs) mostly with sp 2 ‐hybridized carbon structure, displaying the nonsequence monofilaments with the length/diameter ratios larger than 100 times 11. As well known, graphene is the hot topic among 2D carbon structures, including graphene oxides (GOs) and reduced graphene oxides, which are composed of the linking of carbon's six‐atom rings to form a 2D honeycomb network 12.…”