Michael
Walsh
a,
Jeannie Z. Y.
Tan
a,
Sanjay
Nagarajan
b,
Kenneth
Macgregor
c,
John M.
Andresen
a,
M. Mercedes
Maroto-Valer
a and
Sudhagar
Pitchaimuthu
*a
aResearch Centre for Carbon Solutions (RCCS), Institute of Mechanical, Processing and Energy Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, UK. E-mail: S.Pitchaimuthu@hw.ac.uk
bDepartment of Chemical Engineering, University of Bath, Bath BA27AY, UK
cThe Scotch Whisky Research Institute, The Robertson Trust Building, Research Avenue North, Riccarton, Edinburgh, EH14 4AP, UK
First published on 24th November 2023
This study reports a promising and innovative approach for electrochemical green H2 generation using distillery industry wastewater. We employed solvothermally derived Ni2Se3 nanoparticles with a particle size of ∼50 nm as the anode catalyst material to effectively oxidise the acetic acid present in the distillery wastewater. The utilisation of a Ni2Se3 nanoparticle-coated stainless steel electrode significantly enhanced the current density (282 mA cm−2) in the electrochemical cell compared to the pristine SS (stainless steel) electrode (146 mA cm−2) at 2 V RHE. Also, the distillery wastewater electrolyte based cell exhibits higher current density compared to conventional freshwater (i.e., NaOH-based) electrolyte. The distillery wastewater electrolyte demonstrated remarkable H2 gas evolution (∼15 mL h−1 cm−2), showcasing its potential for sustainable H2 generation. However, it was observed that the aggressive bubbling effect at the cathode led to a lower H2 evolution reaction activity when compared to the freshwater-based electrolyte, which displayed a H2 production rate of ∼22 mL h−1 cm−2. These findings underscore the potential of employing Ni2Se3 as an effective oxidation catalyst in the production of H2 gas from pre-treated brewery wastewater H2 gas. The utilisation of Ni2Se3 nanoscale water oxidation catalysts in this context opens up new possibilities for both wastewater treatment and H2 production, paving the way for a more sustainable and resource-efficient future.
In the electrochemical method, 9 kg of water is required to produce 1 kg of H2.6 With regards to the global target of net-zero emissions, the availability of sustainable H2 is key to achieving these aims. This is evident in the goal of ∼15 Mt green H2 generation by 2030. In this context, it becomes clear that a significant amount of freshwater will be required.7 Currently, ∼20.5 billion litres of freshwater are consumed annually in the electrolysis process for H2 production. However, it is important to note that this accounts for only 1.5 parts per million (ppm) of the Earth's available freshwater. When compared to sectors like irrigated agriculture, which utilise over 2700 billion cubic meters of water each year, the water requirement for H2 production remains relatively minimal.8 Despite its low impact on the overall water supply chain, concerns about freshwater scarcity necessitate a reduction in water extractions from all available sources. Therefore, it becomes crucial to explore solutions that enable H2 production to utilise the abundant wastewater resources available on Earth. By tapping into wastewater resources, the water footprint of H2 can be further reduced, alleviating pressure on the freshwater supply chain and promoting sustainability.
Recent studies had examined the potential of using industrial wastewater as a raw material for photo- and electrocatalytic H2 generation.9–13 This promising development suggested that industrial wastewater could be a viable alternative to freshwater resources. One approach involved identifying regional wastewater sources, such as the wastewater produced from Scotch whisky production, which is currently discharged into local rivers after physical or chemical treatment processes. However, resource recovery from this wastewater stream is crucial. By implementing systems for recovery, treatment, and reuse, a significant portion of the wastewater can be repurposed for activities, such as boiler feed water and cooling processes, leading to reduced disposal costs.14,15 Depending on the composition of the brewery wastewater, the presence of sugars, alcohol, and soluble starch allow the wastewater to be utilised for biogas production. However, there is still considerable potential for utilising this wastewater in the electrolysis process due to its high biomass content.16,17
In recent years, a wide range of electrocatalytic materials have been developed for the H2 evolution reaction (HER), but less research has been carried out on the development of materials for the water oxidation process.18 To achieve successful green H2 generation with minimal energy input, designing an efficient, low-cost, sustainable water oxidation catalyst is indispensable. Currently, existing water oxidation catalysts, such as RuO2 and IrO2, are known to be highly efficient, but their high costs and corrosive behaviour limit their industrial deployment.19,20 Metal chalcogenides, particularly nickel chalcogenides, are receiving profound attention due to their catalytic activity for HER or oxygen evolution reactions (OER) in alkaline solutions.21 However, enhancing the number of active sites remains a challenge for producing highly efficient HER and OER catalysts via a facile synthesis process.22–24 Nickel selenides (NiSe) are the second largest studied nickel chalcogenides for electrocatalytic water splitting.25,26 Four different types of nickel selenides are reported in the literature Ni1−xSe (0 < x < 0.15), NiSe2, Ni3Se4 and Ni2Se3.27 In general, nickel selenides are black in colour and do not dissolve in water. The insolubility of nickel selenides in water is a crucial parameter that makes them perfect for electrocatalytic water splitting applications. Interestingly, there is no other solvent that is capable of dissolving nickel selenides, but they tend to dissolve readily in highly oxidising acids, such as HNO3.27 Owing to its stability in neutral water and neutral water electrolyte solutions, NiSe is an appropriate catalyst for electrochemical water splitting in neutral and near-neutral media.
This study aims to explore the potential advantages of employing NiSe as a water oxidation electrocatalyst for the simultaneous generation of O2 and H2 gas from distillery industry wastewater. Several important factors were investigated in this research, including the stability of the NiSe coating, the performance of the catalyst over an extended period of time, and the overall stability of the materials before and after the electrolysis process. To assess the effectiveness of NiSe in the distillery wastewater electrolyte, all the obtained results are compared with those obtained in a freshwater (i.e., NaOH-based) electrolyte. This comparison provided a comprehensive evaluation of the performance and suitability of NiSe as an oxidative electrocatalyst to enhance H2 generation from distillery wastewater.
Evs. RHE = Evs. Hg/HgO + 0.098 + 0.059pH | (1) |
The electrochemically active surface area (ECSA) of the Ni2Se3 coating on SS was determined based on cyclic voltammetry (CV) measurements. Various scan rates ranging from 0.001 V s−1 to 0.1 V s−1 at non-faradaic region potential were applied. The peak current was recorded for each scan rate and plotted against the current density.
The chemical environment of the as-synthesised Ni2Se3 powder was analysed with XPS. The full-scan spectrum of NiSe2 (Fig. 2a) confirmed the presence of Ni, Se, C, and O. The high-resolution of Ni 2p (Fig. 2b) shows that the characteristic peaks at 852.3 and 869.3 eV were attributed to Ni2+, and the peaks located at 854.4 and 872.3 eV were originated from Ni3+.31–33 The Ni3+ most likely originated from the surface oxide phase. In addition, the broad peaks centred at 859.5 and 878.9 eV could be assigned to the satellite peaks (labelled as sat. in Fig. 2b). The Se 2d spectra displayed the characteristic peaks at 54.5 eV and 55.1 eV which correspond to Se 3d5/2 and Se 3d3/2, respectively, indicated the existence of Se2− (Fig. 2b).
Fig. 2 (a) High resolution of Ni 2p, (b) Se 3d, and (c) O 1s spectra of XPS using the as-synthesized Ni2Se3 powder sample. |
Meanwhile, the broad peak located at 57.8 eV confirmed the oxidation state of Se.34,35 The peak at 531.8 eV (Fig. 2c) was attributed to the oxygen of metal oxides, which may have resulted from the surface oxidising after exposure to air.36
The electrocatalytic activity of the Ni2Se3 anode was analysed in two different electrolytes, namely fresh water (NaOH solution only) and distillery wastewater (distillery wastewater and NaOH). To compare the performance, J–V plots obtained from linear sweep voltammetry of SS and Ni2Se3/SS electrodes in the presence of different anolyte were studied. During the process of water splitting for O2 generation, an applied potential is employed in the cell. This potential initiates the oxidation of water molecules on the anode, resulting in the production of O2.
Simultaneously, the by-product protons (H+) are transported to the cathode surface through the membrane. At the cathode, the protons undergo reduction and combine with electrons supplied by the circuit, generating H2. The water splitting reactions occurring at the anode and cathode (alkaline environment) can be explained by the following reactions (eqn (2)–(4)):37,38
Cathode: 2H2O + 2e− → H2 + 2OH− (E0 = −0.83 V vs. SHE) | (2) |
Anode: 2OH− → H2O + 2e− + ½O2 (E0 = 0.4 V vs. SHE) | (3) |
Overall: H2O → H2 + ½O2 | (4) |
These reactions elucidate the fundamental processes taking place during water electrolysis, in which the electrocatalytic activity of Ni2Se3 was crucial in enhancing the efficiency of these reactions. The current density generated by electrochemical cells indicate the electrocatalytic efficiency of the catalysts in O2 and H2 evolutions through OER and HER reactions (eqn (1) and (2)), respectively. Notably, the Ni2Se3/SS electrode (Fig. 3) exhibits a significantly higher current density of 223 mA cm−2 compared to the pristine SS electrode (146 mA cm−2) in freshwater-based electrolyte, evidencing Ni2Se3 possessed superior OER activity. In particular, the Ni2Se3 catalyst reduced the overpotential of the pristine SS electrode from approximately 410 mV (@10 mA cm−2) to 240 mV (@10 mA cm−2), effectively demonstrating its efficacy in facilitating water oxidation reactions. Furthermore, the current density of Ni2Se3 experiences a notable improvement of ∼21%, reaching 282 mA cm−2 at 2.1 V RHE in distillery industry wastewater, compared to the freshwater-based electrolyte (223 mA cm−2 at 2.1 V RHE). The increase in current density was attributed to the presence of organic moieties in the distillery wastewater that was evidenced using nuclear magnetic resonance (NMR) analysis (ESI Fig. S2a†). The current enhancement in distillery wastewater electrolyte was further examined by conducting cyclic voltammetry (CV) in the potential region between 0.9–1.9 V vs. RHE (Fig. 3b). Subsequently, we compare the CV results of Ni2Se3/SS anode using both freshwater and distillery wastewater electrolytes (Fig. 4).
A strong oxidation peak around 1.5 V RHE in the distillery wastewater-based electrolyte, which was absent in the freshwater electrolyte, was observed. This peak was attributed to the presence of acetate species in the distillery wastewater, which underwent an anodic oxidation reaction. It was proposed that when in contact with the electrocatalyst, acetate undergoes oxidation, generating oxidative electrons that significantly contribute to H2 production by enhancing the rate of proton reduction at the cathode:39
2CH3COO− → 2CO2 + C2H6 + 2e− | (5) |
At the cathode: 2H+ + 2e− → H2 | (6) |
An alternative route for acetic acid oxidation, where acetic acid oxidation yields 8e− (eqn (7))40 and these oxidative electrons transfer to the cathode via the external circuit, which effectively reduces the protons under alkaline conditions, contributing to H2 generation (eqn (6)).
CH3COO− + 6OH− → 2CH4 + 4O2 + H+ + 8e− | (7) |
The proton diffusion coefficient (Dn) of distillery wastewater was estimated using CV plots at different scan rates (Fig. S4†) and determined using the following relation (eqn (8)).
Ip = 2.69 × 105 × n3/2 × A × D1/2 × C × ν1/2 | (8) |
To compare the water oxidation performance of Ni2Se3 in with the commercial RuO2, we conducted LSV tests (as illustrated in Fig. S4†) under identical alkaline conditions, using a freshwater-based electrolyte. The current density of Ni2Se3 (Fig. S4†) reveals that it was comparable, particularly at higher applied potentials around 2 V RHE, when compared to the commercial RuO2 catalyst. It's worth mentioning that RuO2 is not the most ideal water oxidation candidate for demonstrating in alkaline electrolysis, as indicated by the Pourbaix diagram.41
The concentration of organic species (acetic acid) within the water significantly influences the overall performance of hydrogen generation during water splitting.42,43 To investigate this effect, LSV tests were conducted using a Ni2Se3/SS anode in three distinct distillery water electrolytes: DW1, DW2, and DW3. These samples were collected at different stages of the water treatment process at the distillery industry site. The LSV results for DW1, DW2, and DW3 are presented in Fig. S5.† The data clearly indicates variations in current density across the distillery wastewater samples. This disparity in the current density was attributed to differences in the concentration of organic matter and the presence of heavy metals, such as copper, in the wastewater. Hence, the critical factor determining H2 generation performance was the specific stage at which the samples were collected at the distillery water treatment site.
Tafel analysis is an invaluable tool that provides insights into both the activity and kinetics of electrochemical reactions simultaneously.44,45 By plotting logJ vs. V overpotential (RHE) (Fig. 4), the Tafel slope offers valuable information about the catalyst's self-oxidation and double-layer charging effects.46 Additionally, Tafel slope data allows one to determine the required increase in overpotential to enhance the reaction rate by a factor of ten. In the case of Ni2Se3, the Tafel slope (Fig. 4) was estimated through chronoamperometry experiments. Notably, in distillery wastewater electrolyte, the Tafel slope of Ni2Se3 is significantly reduced to 93.1 mV dec−1, compared to the freshwater-based electrolyte (106.7 mV dec−1). This reduction suggests an improved performance of Ni2Se3 as an oxygen evolution reaction (OER) catalyst when operating in distillery wastewater. Furthermore, when compared to the conventional OER catalyst PtC (120 mV dec−1) in literature, the lower Tafel slope value of synthesised Ni2Se3 nanoparticles in the current work implies the potential of catalyst quality.47 Nonetheless, when comparing the Tafel slope values of Ni2Se3 with those reported in previous literature, it becomes evident that there is room for further enhancement in the quality of Ni2Se3. This improvement can be achieved through various approaches, including careful substrate selection, innovative doping techniques, and the meticulous design of nanostructures. These strategies, as discussed in references, offer avenues for optimizing the performance and efficiency of Ni2Se3 catalysts.48–53 A recent review conducted by Anantharaj et al.54 has presented a comprehensive comparison of Tafel slope values of nickel selenide in electrochemical water oxidation reactions. Their findings revealed that Tafel values fall within the range of 24 to 128 mV dec−1 across various nanostructures, substrates, and electrolyte conditions. These results demonstrated the influence of different electrolyte compositions on the Tafel slope values of Ni2Se3 catalyst, emphasising the importance of considering the electrolyte conditions for optimizing the catalyst's performance in specific applications.
Electrochemical Impedance Spectroscopy (EIS) is a valuable tool for gaining insights into charge transfer resistance at electrode/electrolyte interfaces involved in electrochemical reactions.55,56 It offers a means to understand the various components contributing to resistance, such as the catalyst,57 electrolyte,58 and membrane,59 and how they collectively impact overall water splitting performance. In our current study, we employ EIS to investigate the reasons behind the superior current density achieved with distillery wastewater-based electrolytes in comparison to freshwater-based electrolytes. The EIS measurement was presented in Nyquist plots (Fig. 5) from the electrochemical cells featuring Ni2Se3/SS anodes and Ni foam cathode with different electrolytes. The impedance measurements were conducted at the open circuit potential (OCP), a state in which the catalyst's effect does not significantly impact the overall resistance of the cells. The diameter of the semicircles depicted in Fig. 5 corresponds to the charge transfer resistance values of the electrode/electrolyte interfaces. A wider semicircle signifies higher resistance, whereas a narrower one indicates lower resistance. In this context, the distillery wastewater-based electrolysis exhibited notably lower charge transfer resistance than its freshwater counterpart. This phenomenon was attributed to the presence of organic substances and potentially copper particles in the distillery wastewater, both of which contribute to lowering the overall electrolyte resistance. The lower electrolyte resistance is one of the reasons for the enhanced electrochemical performance in distillery water-based electrolytes.
The recovery of H2 generation from wastewater pollutants treatment is an innovative approach in recent times. Notably, a wastewater electrolysis cell opens pathways for decentralised H2 production with simultaneous on-site wastewater treatment.60,61 The current density and corresponding H2 production exhibited from the electrochemical cell with a Ni2Se3 anode is presented in Fig. 6. Though Ni2Se3 has shown higher current density in distillery wastewater-based electrolyte, an aggressive bubbling effect was observed at the cathode over time (see the Video in ESI†), This had the effect of reducing the catalytic activity of the Ni cathode in the production of H2. As a result, the cell using freshwater electrolyte produced 0.37 mL min−1; whereas distillery wastewater-based cell resulted in 0.25 mL min−1. Overall, Ni2Se3 coated SS electrode was capable to produce ∼13 mL h−1 cm−2 of H2 from distillery wastewater. In the case of O2 evolution at anode compartment, we observed distillery waste-based electrolyte produces higher oxygen evolution than fresh water based electrolyte (Fig. S6†). This is most likely due to the reduction of acetate (CH3COO−), which was formed after mixing NaOH into the distillery waste solution that contains notable amounts acetic acid, forming CH4 and O2 as illustrated in eqn (3). As a result, distillery wastewater produced three times the amount of CH4 than that of freshwater, and a continuous evolution of O2 throughout the electrocatalytic measurement, which could be observed in the NMR (0.23 ppm in 1H NMR) and GC (Fig. S2B†), respectively.
Fig. 6 H2 generation of Ni2Se3/SS electrodes using fresh water (black) and distillery wastewater based electrolyte (red). Note that the concentration of electrolytes was 1.0 M NaOH. |
To assess the stability of the Ni2Se3/SS electrode in water oxidation reactions, a chronopotentiometry experiment was conducted at a current density of 10 mA cm−2. Fig. S7† illustrates that the cell potential remained notably stable around 1.6 V RHE for a duration of 12 hours. This observation suggests that the Ni2Se3/SS electrode exhibited high stability when it exposed in distillery wastewater-based electrolyte. Following the completion of the 12 h chronoamperometry studies, we conducted a surface examination of the Ni2Se3/SS anode under SEM (Fig. S8a and b†). The SEM images reveal a slight increase in the size of the Ni2Se3 nanoparticles after the electrochemical reaction. This phenomenon was attributed to two factors: (a) the formation of a thin oxidation layer, SeO2, during the reactions. (b) The agglomeration effect on the particles. Notably, despite these changes, there was no significant accumulation of particulate matter on the surface of the Ni2Se3 catalyst before or after the reaction. This observation underscored the significant surface stability of Ni2Se3 against electrochemical corrosion reactions, particularly in its ability to oxidise the organic substances present in the distillery wastewater (Fig. 3).
For a comprehensive assessment of catalytic performance and scalability potential, it is more informative to focus on a key metric: the turnover frequency (TOF). This metric quantifies the rate at which molecules (such as H2 and O2 in the context of water splitting) are produced per second per active site while maintaining a specific overpotential. In the present work, this approach is crucial for comparing the intrinsic catalytic activity of Ni2Se3 in freshwater and distillery water. As a pivotal measure in catalysis, the TOF per active Ni2Se3 site under freshwater and distillery water was computed for the OER, assuming 100% faradaic efficiency, and is presented in the accompanying Fig. 7. The TOF of oxygen generation using Ni2Se3/SS anode can be estimated using the relation:62,63
TOF = [total oxygen turnovers (cm−2)/active sites density] | (9) |
Active sites density = (number of active sites × ECSA) | (10) |
Fig. 7 TOF of Ni2Se3/SS anode in electrolytic oxygen generation performance in fresh water and distillery wastewater. |
A detailed estimation of total oxygen turnovers can be found in ESI.† ECSA calculations were performed using the following equation:
ECSA = (CDL)/Cs | (11) |
Ni2Se3/SS anode exhibited a significantly higher TOF in freshwater when compared to distillery wastewater (Fig. 7). This observation suggested that Ni2Se3 has a notable capacity to catalyse the electrochemical oxidation of water molecules for oxygen generation at its active sites. In contrast, the lower TOF observed in distillery wastewater suggested a reduced capacity for oxygen generation. This could be attributed to the presence of competitive catalytic activities that favoured the oxidation of organic molecules, hampering the electrochemical water oxidation process.
We have comparatively analysed the electrocatalytic performance of Ni2Se3 synthesized in this study with recent reports on nickel selenide-based anodes, which were synthesized through different techniques, used in water splitting reactions (Table 1). Notably, the Ni2Se3, which was synthesized via the solvothermal method, exhibited a significantly lower overpotential of 240 mV compared to other counterparts.
Catalyst | Synthesis technique | Coating surface | Reference electrode | Electrolyte | Overpotential (mV) measured @ 10 mA cm−2 | References |
---|---|---|---|---|---|---|
NiSe2 | Chemical vapour deposition | Glassy carbon | Ag/AgCl | 1 M KOH | 230 | 65 |
NiSe/NiOx | Solvothermal | Ni foam | Hg/HgO | 1 M KOH | 243 | 66 |
NiSe | Electrodeposition | Au coated glass | Hg/HgO | 1 M KOH | 290 | 67 |
NiSe2 | Solvothermal | Glassy carbon | Ag/AgCl | 1 M KOH | 378 | 68 |
NiSe2/MoSe2 | Hydrothermal/selenization | Glassy carbon | Hg/HgO | 1 M KOH | 295 | 69 |
NiSe2/NiO nanosheet | Selenization | Glassy carbon | Hg/HgO | 1 M KOH | 300 | 70 |
NiSe-2 | Electrodeposition | Ni foam | SCE | 1 M KOH | 252 | 71 |
NiSe@Ni | Electrodeposition/selenization | Stainless steel | Ag/AgCl | 1 M KOH | 290 | 72 |
Ni2Se3 | Solvothermal | Stainless steel | Hg/HgO | 1 M NaOH | 240 | Present work |
It is worth to note that when handling distillery wastewater, several considerations should be considered. Firstly, it is important to note that freshly collected distillery water samples tend to yield higher H2 production due to the potential presence of alcohol molecules, which may evaporate into the atmosphere. To mitigate this, one of the recommended practices is to store the distillery samples at freezing temperatures to preserve the alcohol content. Additionally, conducting experiments involving varied temperatures can provide valuable insights into the impact of temperature on current density enhancement. Exploring the effects of temperature variations in electrochemical experiments could help to uncover the potential strategies for optimising H2 production. Additionally, it is crucial to further test this electrolyte in a flow cell type setup. Such testing would help address critical issues related to electrolyte evaporation and any other associated challenges that may arise in practical applications. Flow cell configurations allow for continuous operation and can provide insights into the long-term stability and performance of the electrolyte, helping to assess its feasibility for scalable sustainable H2 production from distillery wastewater. Furthermore, analysing the distillery wastewater after electrolysis can provide valuable information regarding the recovery of value-added products. Although this aspect is beyond the scope of the present study, investigating the potential recovery of valuable substances from the electrolyzed wastewater could be a worthwhile avenue for future research. Moreover, it is worth mentioning that the electrochemical activity of the NiSe nanoscale catalyst can be further improved through various approaches. These approaches can include modifying the coating method on steel substrates and exploring the use of highly conducting substrates as alternatives to steel.
To mitigate the bubbling effect at the cathode, effective approaches involve the incorporation of ultrasonic treatment73 or cavitation treatment74 at the cathode. For instance, a study conducted by Ehrnst and colleagues73 showcased an enhancement in the HER performance. This improvement stems from the intense local electromechanical coupling induced by acoustic forcing. This acoustic influence disrupts the tetrahedrally-coordinated hydrogen bond network of water molecules at the electrode–electrolyte interface. Consequently, it leads to the generation of a higher concentration of “free” water molecules, making them more accessible to catalytic sites on the unmodified polycrystalline electrode. By optimizing these factors, the performance and efficiency of the NiSe catalyst can be enhanced for future applications.
Footnote |
† Electronic supplementary information (ESI) available: Details of SEM, CV, NMR, results. See DOI: https://doi.org/10.1039/d3se01445b |
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