Photovoltaic powered solar hydrogen production coupled with waste SO2 valorization enabled by MoP electrocatalysts

https://doi.org/10.1016/j.apcatb.2021.121045Get rights and content

Highlights

  • Solar H2 production employing SOR as an alternative anodic reaction is presented.

  • SOR enables the reduction in thermodynamic barrier for H2 production.

  • Bifunctional MoP-based catalysts for SOR–HER is developed.

  • SOR-based PV-EC exhibits photocurrent of 17.23 mA cm–2 with durability of 50 h.

Abstract

In this study, we demonstrated high-rate H2 generation by coupling with the sulfite oxidation reaction (SOR) as an alternative to the oxygen evolution reaction for solar H2 production. The emerging and cost-effective molybdenum phosphide electrocatalyst was appropriately optimized and used as a bifunctional catalyst in an alkaline electrolyte for both SOR and HER. Powered by state-of-the-art perovskite–Si tandem photovoltaics, a remarkable photocurrent density of over 17 mA cm−2 was achieved in the HER coupled with the SOR. In addition to the significantly enhanced photocurrent, the SOR can further reduce the overall cost of solar H2 production owing to the elimination of the expensive membranes required for H2 and O2 gas separation. Considering the high global demand for desulfurization via the SOR, the strategy proposed here will enable practical H2 production from renewable sources while effectively converting the toxic SO2 gas into a value-added product for the chemical industry.

Graphical Abstract

Solar hydrogen production system employing SOR as alternative anodic reaction is presented. Photoassisted electrochemical device consists of MoP-based catalysts and perovskite–Si tandem PV. Our system based on SOR demonstrates remarkable hydrogen generation photocurrent density of over 17 mA cm–2 under 1-sun illumination with prolonged durability of 50 h.

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Introduction

Despite the growth in renewable energy sources over the past few decades, fossil fuels continue to be the dominant energy sources worldwide. Their use for electricity generation, e.g., continued to rise to 68% of the worldwide electricity in 2017 [1]. Although fossil fuel power plants can generate reliable electricity at low prices, carbon-based fuels inevitably produce large amounts of carbon dioxide (CO2) and sulfur dioxide (SO2) gases, which induce irrecoverable climate change and atmospheric pollution [2]. In comparison, hydrogen (H2) is a storable clean chemical fuel that does not emit CO2 and SO2 during combustion. However, because 95% of the H2 produced currently originates from fossil fuels (i.e., gray H2), the H2 production process still produces CO2 as a by-product [3]. In this regard, H2 generation by solar water splitting (2H2O 2H2+O2) is recognized as one of the simplest solutions for producing clean H2 without generating harmful by-products [4].

Despite its promising concept, the typical solar water splitting system faces a fundamental challenge for practical H2 generation owing to the high potential energy required to drive the H2 evolution reaction (HER) with the oxygen evolution reaction (OER). Specifically, a potential of 1.23 V is required to provide the thermodynamic driving force along with a potential of approximately 0.5 V for the overpotential associated with the reaction kinetics [5]. The OER significantly limits the overall reaction owing to its sluggish kinetics and four-electron transfer step [6]. Moreover, not only is generating oxygen unnecessary particularly because of its abundance but also becomes of the explosive nature of its mixture with H2. Thus, an additional gas separation infrastructure is necessary to prevent gas accumulation, thereby raising the initial capital cost [7]. Thus, many studies have explored alternative reactions to the OER [8], [9], [10]. However, there are few reactions that satisfy all requirements for an ideal alternative, including a low thermodynamic energy requirement when coupled with the HER as well as a global demand commensurate with the scale of global H2 production, which is mainly used in the chemical industry.

The sulfite oxidation reaction (SOR) can potentially meet these requirements. The standard electrode potential of the SOR under alkaline conditions (Eq. (1)) does not require any thermodynamic energy input when coupled with the HER (Eq. (2)) and must only overcome the kinetic barrier (Eq. (3)) [11].SO42− + H2O + 2e→SO32− + 2OH Ered0 = – 0.93 V vs. NHE2 H2O + 2e→ H2 + 2OH Ered0 = – 0.83 V vs. NHESO32− + H2O → SO42−+ H2 Ecell = +0·10 V

The SOR can be used for the desulfurization of sulfur dioxide (SO2) in the form of flue gas, which is typically produced during the combustion and extraction of sulfur-containing fossil fuels. SO2 emissions cause a broad range of environmental issues related to agriculture and climate (e.g., acid rain) and even threaten human health [12]. Most fossil fuel-based power plants and oil refineries have been controlled at ultra-low emission levels of SO2 (i.e., 35 mg Nm−3) to meet the strict environmental protection standard for utilizing flue gas desulfurization (FGD) technology [13]. Accordingly, the market size of the global FGD reached USD 25.1 billion in 2019 and is expected to grow owing to the massive SO2 emissions [14]. Among the conventional FDG technologies, including magnesium-enhanced lime, dual alkali, and ammonia-based methods, limestone–gypsum wet desulfurization is the most commonly used because of its low removal cost [15]. However, the limestone used in the process leads to not only release CO2, but also leave behind gypsum (CaSO4), which is now considered to pose a new environmental issue [16].

Another typical treatment involves using aqueous sodium hydroxide (NaOH) solution to absorb SO2 [17]. In the absence of CO2 emission and gypsum formation, an aqueous solution of Na2SO3 is produced as a by-product. After SO2 absorption, the solution is purged with air to oxidize into Na2SO4 because not only it is a chemically inert and nontoxic compound but can also be used in the paper industry, glass production, and various other applications [17]. However, this additional aeration process costs some energy and simultaneously causes loss of the potential energy in SO32−. If the SOR can be integrated with solar H2 generation, substantial economic benefits can be expected. The integration of solar-powered H2 production and sulfite oxidation with natural gas refinery plants requiring massive sulfur recovery can become an excellent solution. Although H2 production by the photoelectrochemical oxidation of SO32− using BiVO4 photoanodes has been demonstrated, [18], [19] H2 could not be generated under unbiased conditions. Moreover, the resulting H2 production current density is orders of magnitude lower than the practical solar H2 conversion benchmark of 10 mA cm–2 owing to the lack of an optimal SOR catalyst. HER and SOR experiments should be performed in an alkaline electrolyte because, in an acid electrolyte, disproportionation of HSO3 to elemental sulfur and SO42− occurs instead of H2 production [11]. Thus, developing a highly active bifunctional electrocatalyst for the HER and the SOR in alkaline electrolytes is essential for efficient H2 production.

Molybdenum phosphide (MoP) is the best choice as a highly active bifunctional electrocatalyst in a basic electrolyte. MoP has been extensively investigated as a promising candidate for substituting commercial Pt-based electrocatalysts owing to its low cost, suitable electronic structure (Pt-like) for the HER, good electrical conductivity, and even high chemical stability over a wide pH range [20], [21], [22]. Even in alkaline media, most MoP catalysts exhibit long-term stability without severe deterioration, unlike other metal phosphides [23], [24]. To enhance the electrocatalytic performance of MoP, various synthetic strategies were explored. For example, nanostructuring can enlarge the exposed active surface area, [25], [26] and designing binder-free electrodes can be further beneficial for reducing the "dead volume" and ensuring rapid charge transfer between the catalytic material and the conductive substrate [27], [28]. Moreover, fabrication of bimetallic or multimetallic hybrid catalysts by doping or alloying can optimize their electronic structure toward a specific electrochemical reaction [29], [30]. Hence, developing MoP-based materials with outstanding electrocatalytic properties for the HER as well as presenting excellent performance for the SOR will significantly contribute to achieving cost-effective and efficient H2 production.

Herein, we present an unassisted H2 generation system employing the SOR as an alternative anodic reaction. MoP-based electrodes were carefully optimized to reveal the high catalytic activities for both HER and SOR in alkaline electrolytes. It was shown that depositing a small amount of Pt on the MoP-based electrodes causes the activity of MoP for the HER to exceed that of commercial Pt/C catalyst in a basic electrolyte. To supply the required energy to drive the HER and the SOR, we employed state-of-the art tandem photovoltaics (PVs), which utilizes a wide-bandgap perovskite (~ 1.7 eV)–Si tandem configuration with a maximum power conversion efficiency (PCE) of 26.6%. Combined with the highly efficient bifunctional electrocatalyst and PV, the PV-powered H2 evolution system coupled with the SOR demonstrates a remarkable H2 generation photocurrent density of over 17 mA cm–2 under 1-sun illumination. It corresponds to 21% solar-to-hydrogen conversion efficiency in a traditional water splitting cell (HER+OER), with a prolonged durability of up to 80 h. Our approach not only enables simultaneous SO2 removal and enhanced H2 generation but also eliminates the use of expensive gas-separation membranes because the OER is replaced by the SOR without evolved gas.

Section snippets

Materials

MoO2 powder was synthesized using the electrical explosion method with a Mo wire (Nano Technology Inc.) using an electrical pulse equipment (NTi-10 C, Nano Technology Inc.). H2PtCl6·6H2O, poly(triaryl amine) (PTAA), toluene, cesium iodide (CsI), lead bromide (PbBr2), lead thiocyanate (Pb(SCN)2), N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), polyethyleneimine (PEIE), 5 wt% Nafion solution, and methanol were purchased from Sigma Aldrich (St. Louis, MO, USA). NaH2PO2.2 H2O,

Catalyst preparation and characterization

The MoP electrode was prepared by dip-coating into an-electrically exploded MoO2 precursor solution, followed by thermal phosphidation. Porous carbon cloth (PCC) was chosen as a substrate because of its high electrical conductivity and porous microstructure for maximizing the contact area between the catalytic material and the conductive substrate (Fig. S1) [27]. The resulting MoP electrode (denoted as MoP/PCC) was further decorated with Pt by the ethanol oxidation method (denoted as Ptx

Conclusions

Our unbiased H2-generating PV–EC system uses solar energy to produce H2 along with waste treatment. By coupling with SOR at the anode, the photoassisted device exhibits an exceptionally high H2 generation photocurrent density of over 17 mA cm–2 under 1-sun illumination as well as sufficient operational stability for 50 h. Replacement of the anodic OER with the SOR allowed reduction in the required potential for H2 generation, which was supplied by perovskite–Si tandem PV. The development of

CRediT authorship contribution statement

Jaemin Park: Conceptualization, Methodology, Validation, Writing – original draft. Hyunseok Yoon: Validation, Writing – original draft. Dong-Yeop Lee: Methodology, Resources. Su Geun Ji: Methodology, Resources. Wooseok Yang: Writing – review & editing. S. David Tilley: Writing – review & editing. Myeong-Chang Sung: Resources. Ik Jae Park: Resources. Jeiwan Tan: Investigation. Hyungsoo Lee: Investigation. Jin Young Kim: Writing – review & editing. Dong-Wan Kim: Writing – review & editing. Jooho

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT, Republic of Korea (2021R1A3B1068920, 2021M3H4A1A03049662, and 2021M3D1A2051636). This research was also supported by the Yonsei Signature Research Cluster Program of 2021, Republic of Korea (2021-22-0002).

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