Review of Anodic Catalysts for SO2 Depolarized Electrolysis for “Green Hydrogen” Production

Abstract

In the near future, primary energy from fossil fuels should be gradually replaced with renewable and clean energy sources. To succeed in this goal, hydrogen has proven to be a very suitable energy carrier, because it can be easily produced by water electrolysis using renewable energy sources. After storage, it can be fed to a fuel cell, again producing electricity. There are many ways to improve the efficiency of this process, some of them based on the combination of the electrolytic process with other non-electrochemical processes. One of the most promising is the thermochemical hybrid sulphur cycle (also known as Westinghouse cycle). This cycle combines a thermochemical step (H2SO4 decomposition) with an electrochemical one, where the hydrogen is produced from the oxidation of SO2 and H2O (SO2 depolarization electrolysis, carried out at a considerably lower cell voltage compared to conventional electrolysis). This review summarizes the different catalysts that have been tested for the oxidation of SO2 in the anode of the electrolysis cell. Their advantages and disadvantages, the effect of platinum (Pt) loading, and new tendencies in their use are presented. This is expected to shed light on future development of new catalysts for this interesting process.

Behavior of S, SO, and SO3 on Pt (001), (011), and (111) surfaces: A DFT study

“…It has been shown that atmospheric SO 2 has a detrimental effect, not only on the environment but also human life [5,6]. With more countries and governments enforcing limitations on industry to reduce SO 2 emissions [5][6][7], new technologies are emerging for either the capture [8] or reutilisation of SO 2 [9,10].…”

“…While various other metals have been investigated [21], including Cu [22][23][24][25], Ni [26][27][28], Ag [29,30], Rh [31,32], Pd [24,[32][33][34][35][36], in addition to Pt [24,31,[37][38][39][40], which is still the best performing catalyst in terms of activity and stability [41][42][43]. However, major difficulties are still experienced in experiments, in part due to the occurrence of various co-adsorbed surface sulphur species, including elemental sulphur (S), sulphur oxide (SO) and sulphur trioxide (SO 3 ), amongst others [9].…”

Influence of Anion Chaotropicity on the SO2 Oxidation Reaction: When Spectator Species Determine the Reaction Pathway

“…The development of human society is largely based on energy consumption. Fossil fuels are the most used and oldest energy source, which has caused a number of detrimental impacts, such as increased CO 2 emissions, leading to environmental changes such as global warming. This fact by itself is already a “red alert” and is changing perceptions about energy use, and environmentally friendly energy sources, such as solar, wind power and photovoltaics, are emerging as alternatives of high interest .…”

“…Nevertheless, the use of these products as fuels allows a closed carbon cycle, reducing new CO 2 emissions, but it does not address the high levels of this pollutant already in the air. The HER, on the other hand, could be a more interesting way to obtain solar fuels, since it is possible to generate “green hydrogen” and the HER presents the highest combustion power. The counterpart of this process is the O 2 evolution reaction (OER) .…”

“…One interesting process for this assisted water electrolysis is the SO 2 oxidation reaction (SO 2 OR) . This pathway is not only an alternative pathway with a lower overpotential than the OER; it also presents an opportunity to consume SO 2 , a pollutant emitted by fossil fuel combustion, by generating H 2 SO 4 , a chemical of great industrial interest.…”