Reduced titania nanorods and Ni–Mo–S catalysts for photoelectrocatalytic water treatment and hydrogen production coupled with desalination

https://doi.org/10.1016/j.apcatb.2020.119745Get rights and content

Highlights

  • A novel ternary hybrid photoelectrochemical process is presented.

  • Thermochemically reduced TiO2 nanorod arrays are developed for treatment of urea.

  • Ni-Mo-S composite electrocatalysts are developed for H2 production.

  • The photoanodic and cathodic processes are coupled with desalination.

  • The high efficiency ternary processes are achieved.

Abstract

This study presents a ternary hybrid solar desalination process coupled with photoelectrocatalytic water treatment and H2 production in a single device. The desalination of brackish water in the desalination cell is initiated via photoinduced charge generation with a thermochemically reduced TiO2 nanorod array photoanode. The chlorides transferred to the neighboring anolyte at ion-transport efficiency of ∼100% are photoelectrochemically transformed into reactive chlorine species responsible for the decomposition of urea into nitrate in the anolyte. Simultaneously, the H2 production with a Ni–Mo–S (Ni2S3/MoS2) composite catalyst grown onto porous Ni substrate is achieved at Faradaic efficiency of ∼90% in the catholyte concentrated with desalted Na+. Regardless of the operation condition, the H2 energy contributes to the reduction in the energy consumption for desalination by 25%–30%. The overall ternary hybrid process is understood systematically, and the physiochemical properties and electrochemical behavior of the Ni–Mo–S catalysts are examined.

Introduction

Photoelectrocatalytic (PEC) water splitting has received long attention as a viable process for hydrogen production (R1: 2H2O + 2e→ H2 + 2OH) while evolving oxygen from water (R2: 2H2O → O2 + 4H+ + 4e, E° = 1.229 V) in a single device [[1], [2], [3], [4], [5], [6]]. Significant efforts have been made to address the challenge, particularly with materials that are abundant in nature and are stable for long-term periods [[7], [8], [9], [10]]. The solar-to-hydrogen efficiency grows gradually, recently reaching approximately 6% for oxide photoanodes [11,12]. Many non-noble metal-based HER electrocatalysts (e.g., Ni-based composites) also have been developed as alternatives to platinum group metals (Table S1) [[13], [14], [15]]. One of the limiting factors for attaining higher efficiencies is the kinetically sluggish oxygen evolution reaction (OER), which requires a large overpotential (η > 0.2 V) [15,16]. To address the challenge, chlorine evolution reaction (CER) has often been employed. The CER with two-electron transfer (R3: 2Cl→ Cl2 + 2e, E° = 1.358 V; R4: Cl + H2O → HClO + H+ + 2e, E° = 1.482 V) occurs kinetically faster than the OER with four-electron transfer [17]. In addition, the CER requires less overpotentials than those for the OER although the standard redox potential of the former is greater by ∼0.13 V than that of the latter [18]. This suggests that even with an OER-favoring catalyst, the CER must occur competitively decreasing the OER efficiency and become predominant [[19], [20], [21], [22], [23], [24]]. However, if the role of an anodic reaction is to simply produce proton/electron pairs, then the CER would be a choice in terms of overpotential [21,25].

Considering this, saline water (e.g., brackish water with 10 g L−1 NaCl; seawater with the salinity of 36 g L−1) is suitable for CER and HER [26,27]. Unfortunately, using CER with direct saline water splitting can cause several technical problems. For example, toxic Cl2 gas can be evolved and mixed with H2 in an undivided cell. In addition, dissolved chlorine species are readily reduced to Cl and interfere with HER [22,28,29]. Employing membranes can effectively separate both gases and partially inhibit dissolved chlorine species crossover. However, the organics inherently present in the saline water can clog membranes, reducing the proton transfer to the cathode.

With this in mind, we have long attempted to couple the CER and HER in a single PEC hybrid system with contaminated saline water [[19], [20], [21],24,28,[30], [31], [32]]. A primary difference of the hybrid from conventional saline water splitting is the concomitant decomposition of (in)organics in saline water by reactive chlorine species (RCS, represented by Cl2∙− and HClO) via CER. Recently, we have designed an indirect saline water splitting system that drives the desalination of brackish water and seawater while treating contaminated water with photoanode (e.g., TiO2 nanorod arrays) and simultaneously producing value-added chemicals with porous metal catalysts [20,21]. The configuration of the ternary hybrid process is similar to the electrodialysis and microbial desalination (Scheme 1). However, the hybrid process accompanying the CER and HER inevitably requires an energy input (G° > 0) in addition to sunlight. Accordingly, the specific energy consumption (SEC) for desalination was approximately 4.3 kW h m−3 [21]. This value is comparable with those of conventional electrodialysis (1–15 kW h m−3) yet greater than that of the state-of-the-art reverse osmosis (∼2.8 kW h m−3). The formation of a pH difference over 10 between anolyte (∼ pH 2) and catholyte (∼ pH 12) due to the absence of proton transfer is another challenge. This suggests that the photoanodes and cathodes must be durable and exhibit their activities under the given electrolyte conditions.

To improve the viability of the ternary hybrid system, we adopted three strategies in this study. The first strategy employed was synthesis of thermochemically reduced TiO2 nanorod arrays for the PEC decomposition of urea. Although TiO2 absorbs only UV light, it is highly stable in a wide pH range and suitable to drive CER at an acidic pH. In addition, urea is the largest constituent in animal urine, with 240 million tons discharged daily [31,33]. Second, we specifically designed and optimized photoelectrochemical multi-cell stack device with r-TNA comprised five desalination cell arrays (Scheme 1). The number of desalination cell arrays did not alter the CER and HER kinetics while enhancing the desalination rate. As a result, the SEC values were lowered inversely proportional to the number of desalination cells, reaching approximately 1.1 kW h m−3. Finally, Ni–Mo–S composites, as a non-noble metal-based HER electrocatalyst, were hydrothermally synthesized onto the as-fabricated porous Ni substrates. We chose these composites because the metal sulfides (e.g., Ni2S3 and MoS2) typically exhibit high electrocatalytic activities in alkaline solutions. When assembled into the optimized multi-cell stack device, the Ni–Mo–S electrocatalyst showed the HER efficiencies similar to those of Pt. When the produced H2 was converted to electricity at approximately 50% efficiency, then the SEC could be decreased by 25–30%.

Section snippets

Synthesis of materials and characterization

TiO2 nanorod array (TNA) electrodes were synthesized onto fluorine-doped SnO2 glass substrates (FTO, Pilkington, ∼500 nm-thick FTO layer) via a hydrothermal reaction followed by heat treatment with H2 gas [21]. In brief, the as-cleaned FTO substrates were placed in a Teflon-lined stainless steel autoclave with titanium butoxide (0.6 mL, 97%, Aldrich), HCl (20 mL, 35%, Junsei), and deionized water (20 mL, 18 MΩ cm). After the autoclave was treated at 150 °C for 4 h, the as-obtained TNA grown

Photoelectrocatalytic activity of TNA and r-TNA

The as-synthesized r-TNA shows well-arrayed orthorhombic rods with dimensions of ∼0.2 μm × 0.2 μm × 1.4 μm vertically free-standing onto the FTO substrates (Fig. 1a). The TNA exhibited the same configuration (Fig. S2), indicating that the thermochemical H2 treatment did not affect the TNA morphology. Both samples showed the same XRD pattern, with the rutile structure (2θ = 36.1°, 41.3°, 62.8°, 69°, and 70° for (101), (111), (002), (301), and (112), respectively; ICDD no. 01-077-0441) (Fig. 1b).

Conclusions

This study demonstrated that the desalination process could be coupled to photoelectrocatalytic processes with r-TNA photoanode for water treatment and Ni–Mo–S cathode for H2 evolution in an arrayed desalination device. The photoinduced charge transfer initiated the desalination, and the desalted ions transferred from the saline water into electrolytes drove the water treatment and H2 evolutions. Then, the photoelectrocatalytic reactions accelerated the desalination and vice versa until the

CRediT authorship contribution statement

Seonghun Kim: Methodology, Validation, Formal analysis, Investigation. Dong Suk Han: Formal analysis, Writing - review & editing. Hyunwoong Park: Conceptualization, Methodology, Validation, Formal analysis, Resources, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This research was supported by the National Research Foundation of Korea (2018R1A6A1A03024962, 2019R1A2C2002602, and 2019M1A2A2065616) and by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Materials/Components Technology Development Program (No. 20011360, Development low cost electrodes manufacturing technology to improve both wastewater treatment and H2 generation efficiencies). This publication was made possible by a grant from the Qatar National Research Fund under its

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