Hydrogen energy has become one of the most attractive candidates to replace traditional fossil fuels because of its lack of pollution and its high energy density. Electrocatalytic water splitting is a “green” and sustainable way to produce hydrogen but is still not sufficiently efficient at this stage. In recent years, Ce-based materials have become very popular as the electrocatalysts for water splitting primarily because of the multivalence state of Ce and easily formed oxygen vacancies readily formed in CeO2. However, until now, this interesting subject has seldom been reviewed, especially for electrocatalysts for alkaline water splitting. Herein, we outline and discuss recent progress on the active sites of Ce-based electrocatalysts for hydrogen evolution and oxygen evolution. Oxygen vacancies and interfaces between CeO2 and mixed metal components could provide optimized binding of hydrogen evolution reaction (HER) intermediates, thus promoting HER performance. For the oxygen evolution reaction (OER), Ce3+/Ce4+ redox, oxygen vacancies, and exogenous transition metals could optimize the binding of OER intermediates toward top catalytic activities. The aim of this review is to seek an overall understanding about the reaction sites in Ce-based electrocatalysts for water splitting, which may provide a guide for the future development of HER and OER Ce-based electrocatalysts toward industrial applications.

Cu is commonly modified with sulfur to obtain high selectivity for formate since S can promote the formation of the key *OCHO intermediate along the formate pathway. In the present work, we demonstrate that Cu-vacancies on copper sulfide can surprisingly switch the formate pathway to the CO pathway, and the concentration of Cu vacancies can deterministically regulate the CO faradaic efficiency and partial current density. The J CO of SNC@Cu1.96S (Cu1.96S coated sulfur, nitrogen-co-doped carbon) can reach 37.2 mA cm–2 in an H cell, which is the highest among the Cu-based catalysts and comparable to other top CO production catalysts. According to DFT calculations, the Cu vacancies formed in copper sulfide change the electronic structures of the S sites in such a way that the H* takes a large Gibbs free energy, which in turn suppresses the formation of formate. However, the resulting fewer surface Cu cations and more surface S anions weakens the adsorbate–metal interaction, synergizing the adsorption structural transition of the surface intermediates from *OCHO (two O–Cu bonds) to *COOH (one C–Cu bond) in favor of CO production.

For electrochemical CO2 reduction to HCOOH, an ongoing challenge is to design energy efficient electrocatalysts that can deliver a high HCOOH current density (JHCOOH) at a low overpotential. Indium oxide is good HCOOH production catalyst but with low conductivity. In this work, we report a unique corn design of In2O3-x@C nanocatalyst, wherein In2O3-x nanocube as the fine grains dispersed uniformly on the carbon nanorod cob, resulting in the enhanced conductivity. Excellent performance is achieved with 84% Faradaic efficiency (FE) and 11 mA cm−2JHCOOH at a low potential of − 0.4 V versus RHE. At the current density of 100 mA cm−2, the applied potential remained stable for more than 120 h with the FE above 90%. Density functional theory calculations reveal that the abundant oxygen vacancy in In2O3-x has exposed more In3+ sites with activated electroactivity, which facilitates the formation of HCOO* intermediate. Operando X-ray absorption spectroscopy also confirms In3+ as the active site and the key intermediate of HCOO* during the process of CO2 reduction to HCOOH.