Enabling direct H₂O₂ electrosynthesis of 100 % selectivity at 100 mA cm⁻² using a continuous flow sulfite/air fuel cell

Highlights

• A novel continuous flow sulfite/air fuel cell to electrosynthesis hydrogen peroxide from the air.

• 100 % selectivity at a current density of up to 100 mA cm^−2.

• A remarkable productivity of 1.86 mmol cm^−2 h−1.

• Low energy consumption of 1.96 kWh kg⁻¹ HO₂^–

Abstract

Direct electrosynthesis of hydrogen peroxide from the air is a promising alternative to the complex and energy-intensive anthraquinone process but generally suffers a low productivity and high energy consumption now. Here we provide a direct synthesis strategy using a continuous flow sulfite/air fuel cell, where the traditional oxygen evolution anode is replaced by a sulfite depolarized anode. By optimizing the carbon catalyst and cell parameters for hydrogen peroxide electrosynthesis, the prototype cell can achieve 100 % selectivity at a current density of up to 100 mA cm⁻² and retain the selectivity for > 50 h. Up to 23.12 g/L HO₂^–, the cell can be operated continuously at remarkable productivity of 1.86 mmol cm^−2 h−1, where the energy consumption is only 1.96 kWh kg⁻¹ HO₂^–. The maximum electrosynthesis HO₂^– concentration was 35.02 g L⁻¹. As a win–win strategy, we apply the desulfurization solution from the ammonia flue gas desulfurization process as the anolyte, which not only enhances the direct electrosynthesis of hydrogen peroxide from the air but also recovers sulfur from flue gas to (NH₄)₂SO₄ fertilizer.

Introduction

Hydrogen peroxide (H₂O₂) is widely used as a versatile and eco-friendly oxidant in a variety of industries, e.g. chemical and medical industries, and environmental governance [1], [2], [3]. Today, almost 99 % of H₂O₂ is manufactured via the energy-intensive multi-step anthraquinone method, which requires large-scale infrastructure and expensive palladium catalysts, as well as by-produces vast waste [4], [5], [6]. In response, an alternative route for direct production of H₂O₂ through the electrochemical 2e⁻ oxygen reduction reaction (ORR) route has largely developed [7], [8], [9], [10].

$$\mathrm{A}\mathrm{t}\mathrm{p}\mathrm{H}<7,{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2e^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2{E}^0=+0.695\mathrm{V}vs.\mathrm{S}\mathrm{H}\mathrm{E}$$

$$\mathrm{A}\mathrm{t}7<\mathrm{p}\mathrm{H}<11.6,{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+2e^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{O}\mathrm{H}}^{-}{E}^0=-0.133\mathrm{V}vs.\mathrm{S}\mathrm{H}\mathrm{E}$$

$$\mathrm{A}\mathrm{t}\mathrm{p}\mathrm{H}>11.6,{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2e^{-}\to {\mathrm{H}\mathrm{O}}_2^{-}+{\mathrm{O}\mathrm{H}}^{-}{E}^0=-0.065\mathrm{V}vs.\mathrm{S}\mathrm{H}\mathrm{E}$$

$${\mathrm{H}}_2{\mathrm{O}}_2\leftrightarrow {\mathrm{H}\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}p{K}_a=11.6$$

The electrochemical approach can be driven by renewable power for on-site H₂O₂ production, which is one-step, easy operation, under moderate conditions, suitable for various scale applications, and require simple devices [4], [11].

Recently, research on H₂O₂ electrosynthesis includes catalyst development, reaction engineering, reactor development, and parameter optimization [5], [7], [12]. Extensive researchers have focused on catalyst synthesis [4], [5], [7], e.g. Pd-based catalysts, carbon-based materials, and MNC single-atom catalysts. Carbon-based materials are promising catalysts for H₂O₂ electrosynthesis, as they are cost-effective, earth-abundant, and electrochemically stable [5], [6]. They allow the incorporation of a variety of functional groups through various well-developed methods to further enhance activity and selectivity [1], [4]. For example, Zhu et al. [13] reported a KOH-treated reduce method to induce the etheric group (COC) on the surface of graphene oxide, which can achieve 100 % selectivity for H₂O₂. Except for the catalyst, anode reaction and reactor configuration greatly affect H₂O₂ production and energy consumption [11], [14]. H-cells are widely used in the evaluation of catalysts. But it cannot represent the accurate performances of catalysts or electrodes for industrial applications, because it is non-continuous and scalable [7]. The flow filter-press cell is a promising structure for the H₂O₂ electrosynthesis reactor [11], [14]. The filter-press cell is one of the fully developed electrochemical reactors in electrochemical industries, e.g. fuel cell, water electrolysis, and electrosynthesis [15]. Xia et.al. [2] developed a self-driven H₂/O₂ fuel cell using a solid electrolyte filter-press cell to direct electrosynthesis up to 20 wt% pure H₂O₂ solution. Besides, several other reactor configurations have been developed for pollution degradation, such as rotating disc reactor [16], and jet-cell [17]. Nevertheless, the published energy consumption of H₂O₂ electrosynthesis was in the range of 6.0–22.1 kWh kg⁻¹, which was much higher than the anthraquinone route [11], [17], [18]. Moreover, the electrosynthesis of H₂O₂ generally suffered low productivity, 0.05–0.12 mmol cm⁻²h⁻¹ today [2], [14].

Our goal is to develop a promising method to enhance H₂O₂ productivity and reduce energy consumption. Herein, we propose a strategy to reduce the cell voltage by replacing the traditional oxygen evolution anode with a sulfite depolarized anode, which can operate as a sulfite/air fuel cell [19], [20]. As presented in Fig. 1, the theoretical cell voltage drop is −0.509 V in an alkaline medium. For win–win goals, we choose the desulfurization solution from the ammonia flue gas desulfurization process as an anolyte, which not only reduces the cost of the reagent but also achieves resource recovery from waste. Moreover, we also test and compare the influence of some usual active methods for the 2e⁻-ORR activity of Vulcan XC-72R carbon material. 100 % selectivity of H₂O₂ electrosynthesis at 100 mA cm⁻² is achieved using a homemade prototype cell.