Carbon-based electrocatalysts for water splitting at high-current-densities: A review

Abstract

Electrocatalytic water splitting is a promising strategy to generate hydrogen using renewable energy under mild conditions. Carbon-based materials have attracted attention in electrocatalytic water splitting because of their distinctive features such as high specific area, high electron mobility and abundant natural resources. Hydrogen produced by industrial electrocatalytic water splitting in a large quantity requires electrocatalysis at a low overpotential at a large current density. Substantial efforts focused on fundamental research have been made, while much less attention has been paid to the high-current-density test. There are many distinct differences in electrocatalysis to split water using low and high current densities such as the bubble phenomenon, local environment around active sites, and stability. Recent research progress on carbon-based electrocatalysts for water splitting at low and high current densities is summarized, significant challenges and prospects for carbon-based electrocatalysts are discussed, and promising strategies are proposed.

1. Introduction

Hydrogen emerges global wide applications in petroleum refining, industrial production of ammonia and methanol, fuel cells, and metallurgical industry[1–2]. Compared with steam reforming of methanol and coal gasification that produce hydrogen, electrochemical water splitting has various advantages[3], such as high purity carbon-free hydrogen, use of renewable sources, nontoxic reagent and scale flexibility[4–6]. Typically, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the two steps in electrocatalytic water splitting[7]. OER requires 4 electrons during the process, while 2 electrons are needed in HER[8]. Besides, a larger than the thermodynamic potential should be required in electrolysis due to the intrinsic barriers[9]. Reducing the overpotential and enhancing the stability is of enormous economic benefit to realize industrial applications[10–11].

In fundamental laboratory research at low current densities[12], the charge state of carbon-based catalysts can be easily regulated through engineering structure defects, heteroatom engineering, heterojunction engineering, and structure regulation[13]. For example, defective carbon materials and carbon-based noble/non-noble metal materials are potential substitutes for noble metal catalysts due to their multiple superiorities[14], such as excellent electronic conductivity, high surface area, excellent resistance to corrosion, low cost and structural flexibility. Breaking the sp2 carbon lattice with electroneutrality can lead to uneven charge distribution in the carbon matrix that serve as new electrocatalytic active sites. A series of defective carbon electrocatalysts exhibiting superior HER or OER performance have been synthesized by plasma etching, oxidation, irradiation, heteroatom removal[10], etc. The carbon-based catalysts are able to achieve more excellent electrocatalytic performance by the modification with metal species. Besides, the charge transfer between the carbon host and metal or metal compounds can regulate the electronic structure of each component[15–16]. Owing to the modification by metal species, carbon-based catalysts can achieve increased electrocatalytic performance.

The interconnection of the charge transfer path on carbon-based materials and the electrocatalytic water splitting behavior have been reviewed previously[12]. Although great progress in advanced electrocatalysts under laboratory conditions has been achieved[17–19], the laboratory research is usually carried out in dilute solution at a low current density[20]. Actually, the optimized electrocatalysts, exhibiting an excellent HER and OER performance at a low current density usually, do not display an expected performance at a high current density[21]. Because current density greatly impacts the electron transfer at the interface.