H₂ production through electro-oxidation of SO₂: identifying the fundamental limitations

Abstract

Sulphur dioxide (SO 2 ), a known industrial pollutant and pulmonary irritant, is emitted to the atmosphere in excess of 120 Mt per annum. Great strides have been taken to reduce SO 2 emissions, but with the growth of specifically China, and to a lesser extent India, it is on the rise again. The electrolysis of aqueous solutions of dissolved SO 2 holds huge environmental potential in that SO 2 is converted to sulphuric acid (H 2 SO 4 ) and at the same time hydrogen gas is produced. A further benefit or incentive is that a sulphur depolarised electrolyser (SDE) operates at an applied potential that is about one volt lower than that of a regular water electrolyser. In taking this technology forward the greatest improvement to be made is in developing a suitable electrocatalyst, which is also the 'lowest hanging fruit' in that very limited research and development has been conducted on the electrocatalyst for this process. In this work, density functional theory is employed to model the electro-oxidation of SO 2 on single crystal planes of the 4d and 5d transition metals. Two reaction mechanisms are considered, a HSO 3 intermediate pathway and a SO 3 intermediate pathway. The binding energies of all intermediates are found to scale with the surface reactivity (measured as the adsorption of OH). Irrespective of the pathway water needs to be activated and reduction of SO 2 to elemental sulphur must be avoided. This requirement alone calls for an electrode potential of at least 0.7-0.8 V for all the investigated transition metals and thus challenges the proclaimed goal to operate the SDE at 0.6 V. A high chemical barrier is further found to severely limit the oxidation reaction on reactive metals. A much higher catalytic activity can be obtained on precious metals but at the cost of running the reaction at high overpotentials.

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

“…As mentioned before, the electrolysis step is one of the parts that has to be optimized. One of the proposed goals for this step is to operate at 0.6 V and 0.5 A/cm 2 [19]; however, this has not been yet reached.…”

Influence of the Electrode and Chaotropicity of the Electrolyte on the Oscillatory Behavior of the Electrocatalytic Oxidation of SO2

“…However, the need of a suitable catalyst for this reaction that provides a low kinetic barrier is challenging and the main motivation for studies in this field [1][2][3][4][5][6][7][8][9][10][11][12][13] . Recent reports 9 indicate that Au and Pt should be the most electroactive catalysts (with the order in activity Pt > Au). This mechanism is in disagreement with proposed by previous experimental reports [11][12][13] .…”

Mixing thermodynamics and electronic structure of the Pt1−xNix(0 ≤x≤ 1) bimetallic alloy

“…However, the future development of regenerative fuel cells is hindered by the large over-potential exceeding the 1.23 eV required by the oxygen producing anode and slow reaction kinetics, which reduces the efficiency of the process. [4][5][6] Pt-based electrocatalysts exhibit excellent activity for the ORR reaction, [7][8][9][10][11][12][13][14] but poor performance for the OER reaction, while state of the art IrO 2 and RuO 2 OER electrocatalysts are only moderately active towards the ORR reaction. 2 Electrocatalysts which combine two of these precious metals, Pt, Ir and Ru, show reasonable activity towards both the ORR and the OER reactions, but their use is limited due to low abundance and high cost.…”