Designing Electrocatalysts for High-Current-Density Freshwater/Seawater Splitting

Open AccessRenewablesREVIEWS20 Feb 2024Designing Electrocatalysts for High-Current-Density Freshwater/Seawater Splitting Madiha Rafiq, Zanling Huang, Chaoran Pi, Liangsheng Hu, Fushen Lu, Kaifu Huo and Paul K. Chu Madiha Rafiq Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063 , Zanling Huang Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063 , Chaoran Pi Wuhan National Laboratory for Optoelectronics (WNLO), School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074 , Liangsheng Hu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063 Chemistry and Chemical Engineering Guangdong Laboratory, Shantou 515061 , Fushen Lu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063 Chemistry and Chemical Engineering Guangdong Laboratory, Shantou 515061 , Kaifu Huo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Wuhan National Laboratory for Optoelectronics (WNLO), School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074 Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518063 and Paul K. Chu Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong https://doi.org/10.31635/renewables.023.202300043 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Electrocatalytic water splitting is crucial to renewable and clean hydrogen generation. Achieving high efficiency and stability in hydrogen generation by freshwater/seawater electrolysis at a high current density (HCD) using low-cost electrode materials is of utmost importance for the future hydrogen economy. However, conventional freshwater/seawater electrolysis suffers from low current density due to inefficient electrocatalysts and competitive reactions of the chlorine evolution reaction (ClER), consequently hampering its industrial adoption. Advanced surface and interface engineering techniques are essential for the development of efficient and long-lasting electrodes for freshwater and seawater electrolysis at HCD. In the review, we begin by discussing the fundamental aspects of freshwater/seawater splitting, focusing on recent advancements and strategies to increase the efficiency at HCD. We then comprehensively discuss the rational design strategies for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at HCD together with the associated fundamental electrode reactions by considering the thermodynamic and kinetic aspects of the catalytic efficiency, selectivity, and corrosion resistance. It is followed by a discussion of some existing issues and limitations of HCD freshwater/seawater splitting and viable solutions. Finally, the issues facing the field and possible future research directions for efficient large-scale industrial water splitting are discussed. Download figure Download PowerPoint Introduction In 2018, worldwide energy consumption increased to 9938 million tons, as reported by the International Energy Agency. Around 70% of the energy was generated by fossil fuels, which has caused environmental concerns such as the release of carbon dioxide exceeding 33 gigatons.1 Researchers are thus actively looking for renewable energy alternatives such as wind, solar, tidal, and hydroelectric energies. Electricity generation is the principal method to utilize these sources.2 Presently, electricity contributes to approximately 20% of the world's energy consumption, and by 2040, it is projected to surpass oil and coal to reach >30%.1 Unfortunately, because of technical and geographic constraints, electricity produced by these renewable methods may not be easily accessible and integrated into the electric grid.3 One solution is to use electricity from renewable sources to produce stable chemical products that can be transported, such as hydrogen gas.4 Hydrogen energy has many advantages including high energy density, abundant natural availability, and the release of only water.5–9 Unfortunately, 96% of industrial hydrogen is currently generated by methanol reforming, coal gasification, and steam methane reforming, which are untenable due to the use of nonrenewable sources and CO2 emission.10,11 In this respect, water splitting (2H2O + energy → 2H2 + O2) powered by green and renewable methods such as sun, wind, and tidal waves is a clean approach to generate hydrogen.12–14 In general, analytical-grade hydrogen and oxygen gasses are generated by water splitting, which involves two separate half-cell reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).15 Molecular hydrogen (H2) is formed by releasing electrons on the cathode, whereas oxygen (O2) is generated by accepting electrons on the anode. Theoretically, the ideal potential required for electrolysis of water is 1.23 V corresponding to 237.1 kJ mol−1.16,17 However, a substantially greater operational voltage of 1.8–2.0 V is usually required to overcome the overpotentials in HER/OER and ohmic resistance generated by transportation of ions in the electrolytic cell.18,19 Therefore, electrocatalysts that can promote the reaction kinetics despite the kinetic barrier, especially for OER with the four-electron reaction are necessary. Although significant progress has been made in recent years, their efficacy is still insufficient to meet the demand for industrial-scale implementation. Furthermore, most of the reported catalysts have only been tested at low current densities of 10–100 mA cm−2.20–23 On the contrary, the industrial standard for H2/O2 production exceeds 1000 mA cm−2 while maintaining stability, but only a small number of catalysts has been documented for high current density (HCD) water splitting (Figure 1). Therefore, to accomplish industrial-scale HCD water splitting, researchers have focused on developing efficient electrocatalysts with low overpotentials. Figure 1 | Electrochemical approaches for HCD fresh/seawater splitting at HCD based on Web of Science search results. Download figure Download PowerPoint Currently, electrolysis-based technologies for splitting water include alkaline electrolyzers and proton exchange membrane (PEM) electrolyzers using fresh/DI water as the source.7 However, the high global demand can lead to fresh water shortage,24 and seawater electrolysis is appealing as seawater constitutes 96.5% of the surface water on earth.25 Nonetheless, the shipping cost of seawater to inland regions and other logistic issues must be addressed. If H2 can be made locally using seawater by green energy and long-term storage can be realized, it is a desirable technique. However, Hausmann and colleagues26 have hypothesized that even if all the electricity in the world comes from hydrogen, only 0.4% of the freshwater resource will be used. Therefore, there is a school of thought that freshwater water splitting is still practical and inexpensive. Owing to the controversy, more data and research are needed. From the research point of view, according to the published data on electrocatalytic water splitting, only 6% of the papers are related to HCD electrocatalytic seawater splitting (Figure 1). Many reviews have discussed the efficacy of various electrocatalysts for HCD water splitting by focusing on noble metals, transition metals, and non-noble or metal-free catalysts. In comparison, there are fewer reviews addressing the specifics of catalyst design and morphology, even though these factors are key to the efficiency of catalysts in HCD water splitting. In this review, we focus on the development of electrocatalysts for industrial applications (Figure 2). We begin by providing an overview on the basic principles of HCD water splitting and discussing the various reactions and side reactions in freshwater and seawater electrolysis. We then summarize the current strategies to optimize the structural features that impact the performance of catalysts. It is followed by a description of electrocatalysts with ultrahigh HER, OER, and overall water-splitting activity and the corresponding detrimental effects of HCD on the electrocatalysts. Last, a brief assessment of the obstacles and future prospects is presented. Figure 2 | Typical strategies for the design and engineering of electrocatalysts for HCD freshwater/seawater splitting. Download figure Download PowerPoint Fundamental Principles of Electrocatalytic Freshwater/Seawater Splitting Electrocatalytic water splitting is known to involve two distinct reactions, HER and OER, which take place along various reaction routes based on the electrochemical characteristics of the electrode surface. The HER process includes the interaction of two electrons and a proton, while OER, on the other hand, has a multielectron transfer mechanism that sequentially forms intermediate species and removes four protons for each oxygen molecule that is evolved. The general equation for overall water splitting is 2H2O → H2 + O2. Mechanism of HER To explore the more complex electrochemical reactions involving several electron–proton transfers, one can start with the simplest electrochemical process known as HER. Even though it seems simple, HER is actually a complex, multistep process on the electrode surface. It involves the adsorption, reduction, and desorption of the reactants, intermediates species, and products through different reactions according to the catalyst properties. The following are the most commonly accepted reactions in alkaline and acidic environments. In an alkaline environment: Volmer step: H 2 O + * + e − → H * + OH − (1) Heyrovsky reaction: H 2 O + e − + H * → H 2 + OH − + * (2) Tafel reaction: 2 H * → H 2 + 2 * (3) In an acidic environment: Volmer step: H 3 O + + * + e − → H * + H 2 O (4) Heyrovsky reaction: H + + e − + H * → H 2 + * (5) Tafel reaction: 2 H * → H 2 + 2 * (6) The initial step is hydrogen adsorption on the catalyst site, known as the Volmer step (eqs 1 and 4). The reaction proceeds either by the Heyrovsky step (eqs 2 and 5) or the Tafel step (eqs 3 and 6), depending upon the extent to which the catalyst surfaces interact with protons. If the surface adsorbed H* couples with another H+ + e− from the electrolyte and forms a H2 molecule, it follows the Heyrovsky step. However, combining the two adsorbed H٭ generates a H2 molecule followed by the Tafel route. These steps show some standard theoretical Tafel slopes such as 120 mV dec−1 (Heyrovsky step), 40 mV dec−1 (Volmer step), and 30 mV dec−1 (Tafel step), from which the HER reaction mechanism can be determined.27,28 Mechanism of OER OER is considered a fundamental step in water splitting. However, OER is more sluggish and often regarded as the more kinetically and thermodynamically challenging reaction.29 Specifically, the slower kinetics of OER can be attributed to the four electron–proton transfer mechanism.30 Similar to HER, the morphology of the catalyst and pH of the electrolyte has a major effect on the OER mechanism.31 The following are the four-step reactions under basic and acidic conditions:32 In an alkaline environment: * + OH − → OH * + e − (7) OH * + OH − → O * + H 2 O * + e − (8) O * + OH − → OOH * + e − (9) OOH * + OH − → * + O 2 + e − + H 2 O (10) In an acidic environment: * + H 2 O → OH * + H + + e − (11) OOH * → O * + H + + e − (12) O * + H 2 O → OOH * + H + + e − (13) OOH * → O 2 * + H + + e − (14) According to the above-mentioned reaction step, OER starts with the adsorption of multiple intermediate species (i.e., O*, OH*, and OOH*). However, only adsorption of OH* is considered for the first stage in OER. Oxygen is then produced either by the diffusion and interactions of two O* species or by the decomposition of the adsorbed OOH* species in both acidic and basic environments. Challenges for HER/OER in seawater electrolysis Natural seawater electrolysis is similar to the electrolysis of fresh water. Nevertheless, catalysts used in seawater electrolysis encounter more complications than those in freshwater splitting due to corrosion caused by local changes in seawater pH, the detrimental impacts of dissolved impurity ions including Na+, Ca2+, Br−, Mg2+, and I−, the existence of microbes/bacteria, and the competitive nature of the chloride oxidation reaction (ClER).33 During seawater splitting, in the case of HER, the active center on the surface of the electrode will be covered by some precipitates or calcium/magnesium hydroxide generated due to the variation in pH during electrolysis to lower the efficiency of the catalyst. Moreover, bacteria/microorganisms and tiny particles severely affect the robustness of the electrode materials by degrading and poisoning the catalyst surface.2 These issues can be addressed by creating a catalyst that is resistant to corrosion using an appropriate membrane and electrolyzer to isolate the catalyst from the interfering ions or contaminants. In the meantime, seawater pH can be adjusted with the use of a buffer solution to prevent the production of precipitates.34 The most problematic aspect of OER is the influence of Cl− (chloride ions) in seawater, which invariably produces complex side reactions such as the chlorine evolution reaction (ClER). The ClER in basic and acidic conditions can be expressed as follows.35 In a basic condition: Cl − + 2 OH − → ClO − + H 2 O + 2 e − (15) In an acidic condition: 2 Cl − → Cl 2 + 2 e − (16) While ClER faces thermodynamic challenges in an acidic medium, its faster kinetics allow it to compete with OER.36 Nevertheless, the pH variation does not affect the equilibrium potential of the chlorine evolution process but in an alkaline environment, the difference in the voltage between oxidation of chloride and production of oxygen increases to 480 mV. Even under these conditions, oxygen evolution still competes with hypochlorite formation. Therefore, a pH greater than 7.5 is commonly utilized when developing noble metal-free setups in seawater electrolysis.37,38 Consequently, a higher potential difference between OER and ClER in a basic medium is advantageous to selective seawater splitting in comparison to acidic electrolytes. Strategies for Designing HCD Systems It has been reported that the promising catalysts for HER/OER show activity only at low current densities (≤200 mA cm−2). Nevertheless, when subjected to current densities required by industrial applications (≥200 mA cm−2), these materials typically deliver poor performance due to the slow reaction kinetics, inadequate exposure of active sites, and leaching of the catalyst into the solution due to the rapid evolution of H2/O2 bubbles.39 Therefore, to foster commercial applications of catalytic energy conversion, some important issues such as the catalyst dimensionality/morphology, surface chemistry, and catalyst electrolyte interplay, need to be considered for the development of highly efficient, durable, and economical electrode materials. Advanced electrode design and structures Past studies have concentrated on the enhancement of HER/OER electrocatalysts through heterostructure formation, coupling, defect engineering, and doping,16 but it is still challenging to build catalysts that achieve high efficiency for a long time and under HCD conditions. To tackle these challenges, electrocatalysts need to be designed with careful consideration given to their morphology and architecture in order to support high current densities and robust gas evolution while allowing rapid ion and charge transfer. Low-dimensional electrocatalysts have several advantages over their bulk counterparts.40 These advantages include a vastly enlarged active surface area and improved surface activity due to the modified interface between the catalyst and electrolyte, which allows effective transportation of electrons/ions and products. Additionally, electrocatalysts with low dimensionality possess various chemical and physical properties and are capable of assembling into hierarchical 3D structures. In recent years, high electrocatalytic activity has been reported for nano electrocatalysts with a variety of structures, including zero-dimensional (0D) nanoparticles (NPs), quantum dots, nanospheres, one-dimensional (1D) nanorods, nanowires, and two-dimensional (2D) nanosheets (Figure 3a).41 By taking advantage of the low dimensionality, many active sites are made available to reactants leading to high reaction rates per electrode area under HCD conditions at a low overpotential. Li et al.42 demonstrated a Co2P/Ni2P nanohybrid comprising interconnected Co2P and Ni2P NPs on a CoNi support that acts as a highly effective pH-universal catalyst with good HER performance. Experimental investigations show that Co2P/Ni2P requires only 51, 65.7, and 46 mV overpotentials to yield 10 mA cm−2 in 1 M KOH, 1 M PBS, and 20 mA cm−2 in 0.5 M H2SO4 with good durability in all these media. At a cell voltage of 0.2 V, this catalyst can attain high current densities of 177, 1700, and 1000 mA cm−2, respectively, in neutral, alkaline, and acidic media. We attribute the notable activity of the catalyst primarily to the collaborative effects of three key factors: the large surface area, rigorous electrical connections enabling rapid transfer of electrons between the support and catalyst surface, and synergistic effects produced by the Co2P and Ni2P NPs. Generally, with regard to minimization of the surface energy, metal atoms often prefer to form 0D NPs, but 1D nanorods, nanowires, and 2D nanosheets offer superior electron mobility, more surface-active sites, and more surface areas. Qian et al.43 developed robust Co9S8/Ni3S2 1D nanowire arrays under different applied potentials for up to 1440 h (Figure 3b). The highly efficient electrocatalyst only needs a 1.88 V potential to achieve 200 mA cm−2.43 Wu et al.44 grown 2D layer double hydroxide nanosheets of boron-doped NiCo on nickel foam (NF) for HER. A-NiCo LDH/NF shows small overpotentials of 286 and 381 mV at HCD of 500 and 1000 mA cm–2 and high sustainability at 1000 mA cm–2 for 72 h.44 Zou et al.45 created Ni-Fe-OH nanoflakes on Ni3S2/NF for 500 and 1000 mA cm−2 at 369 and 480 mV overpotentials, respectively (Figure 3c–e). Figure 3 | (a) Summary of the different configurations. Reproduced with permission from ref 41. Copyright 2021 John Wiley and Sons. (b) Schematic depiction of uniform FeOOH/Co9S8/Ni3S2 synthesis. Reproduced with permission from ref 43. Copyright 2023 American Chemical Society. (c) Schematic presentation of Ni-Fe-OH@Ni3S2/NF synthesis. (d) OER performance of Ni-Fe-OH@Ni3S2/NF and control samples. (e) Stability test at 1000 mA cm−2. Reproduced with permission from ref 45. Copyright 2017 John Wiley and Sons. Download figure Download PowerPoint Combining different dimensional morphologies on one catalyst can boost the efficacy and robustness of the electrode materials.46 For instance, Wang et al.47 combined the advantages of 3D spherical and 2D film nanostructures to generate the efficient 3D porous nanoflower Co6Ni4P/NF catalyst. Incorporating Ni gives the catalyst a 3D nanosphere shape, while Co leads to the growth of 2D nanofilms. By tuning the ratio of Ni to Co, the catalyst achieves a 1000 mA cm−2 current density at small overpotentials of 336 mV and 373 mV for HER and OER, respectively. Yoo et al.48 used a hydrothermal strategy followed by calcination to load IrO2 NPs on a hierarchical MnO2 NS on reduced graphene oxide (IrO2@MnO2/rGO) (Figure 4a). The hierarchical MnO2/rGO structure in the IrO2@MnO2/rGO catalyst possesses a large surface area along with an overabundance of IrO2 active sites and synergistic effects rendered by IrO2 and MnO2. IrO2@MnO2/rGO requires overpotentials of 250 and 270 mV for OER and HER at 50 mA cm−2, respectively, in KOH, whereas only 2.27 V@100 mA cm−2 is required for natural seawater electrolysis.48 Liu et al.49 reported highly active electrocatalysts with good HER properties requiring a 98 mV overpotential to reach a 1000 mA cm−2 current density (Figure 4b–d). The h-NiMoFe electrocatalyst shows a strong synergistic relationship between Ni and Fe/Mo, which stabilizes the hydroxide structure even at high current densities.49 Kou et al.50 used a 3D printing technique to prepare a 3DPNi (3D printed nickel) electrode decorated with carbon-doped NiO to catalyze HER and OER (Figure 4e), and the catalyst requires an overpotential of 245 mV for HER and 425 mV for OER to generate a current density of 1000 mA cm−2. Figure 4 | (a) Schematic illustration of the synthesis of IrO2 NPs decorated on MnO2 on rGO. Reproduced with permission from ref 48. Copyright 2023 American Chemical Society. (b) linear sweep voltammetry (LSV) curves of h-NiMoFe, Ni foam, and Pt. (c) Ratios of Δη/Δlog|j| at different current densities. (d) Comparative graphs of j at 100 mV of h-NiMoFe and recently reported catalysts. Reproduced with permission from ref 49. Copyright 2021 Royal Society of Chemistry. (e) 3D-printed 3DPNi and deposition of C-Ni1−xO. Reproduced with permission from ref 50. Copyright 2020 John Wiley and Sons. Download figure Download PowerPoint Surface chemistry The electrochemical reaction encompasses three fundamental stages, namely adsorption, activation, and desorption, which include the interplay of reactant molecules and the surface of the catalyst. According to the classical Sabatier principle, a highly reactive catalyst must have the appropriate strength for molecules/intermediates adsorption or reaction products desorption, which is neither too weak nor too strong.51 In accordance with this theory, several methods have been used to tune the adsorption and desorption energies by changing the surface characteristics and features of the catalyst. Charge state regulation is one of the most effective.52 Zhao et al.53 regulated the charge state of cobalt (Co) and observed that a low charge state of cobalt is advantageous to HER, while a high charge state of cobalt benefits OER. This manipulation allows the as-obtained CoxSey electrocatalyst to serve as the bifunctional electrocatalyst for seawater splitting with good performance. Fine-tuning the mass ratio of Co and Se is a simple way to alter the charge state of Co species to improve the activity of the CoxSey catalyst. Lu et al.54 shown that the self-regulatory process of manganese (Mn) charge states from Mn4+ to Mn3+ increases oxygen vacancies (Ov) in the MnO2 matrix. Moreover, Fe3+ ions are intercalated into MnO2 to improve the electronic structure and exploit the synergistic effect for improved electron mobility and OH adsorption on the cation-inserted MnO2/NF electrodes during OER. Interface engineering also leads to surface charge tuning due to the transfer of electrons from the strong interactions between different catalytically active sites.55,56 Apart from surface charge regulation, strain engineering is a successful strategy to modulate the surface chemistry of materials, which in turn improves the catalytic activity.41 Generally, strain engineering is primarily achieved by the substitution of heteroatoms or a lattice mismatch. However, in high-current water splitting, attention typically shifts to the role played by lattice mismatch. For instance, Wang et al.57 devised and created lattice-strained homogeneous NiSxSe1-x hybrids that exhibit impressive properties in both HER and OER in KOH media. To investigate the impact of lattice strain, ultraviolet photoelectron spectroscopy (UPS) reveals that the NiS0.5Se0.5 catalyst with 2.7% strain shows the highest Fermi energy level and lowest work function, which verifies the highest electrical conductivity (Figure 5a–c). Furthermore, the width of the d-band shrinks along the positive shift of the d-band center as the lattice strain increases. This upshift elevates the antibonding d states, which in turn modify the adsorption capability of the intermediates and enhance catalyst activity.57 Engineering the metallic phases or edges may also optimize the conductivity of catalysts. For example, Yang et al.58 prepared metallic 2H and 3R phases of niobium disulfide by chemical vapor deposition (CVD) method. The van der Waals forces from layers of the metallic 2H phase of NbS2 and optimum free energy for hydrogen result in an ultra-HCD of 5000 mA cm−2 at the lowest overpotential of 420 mV (Figure 5d–f). Yao et al.59 reported interfacial sp C-O-Mo hybridization to improve the charge kinetics and facilitate H2O dissociation producing good HER performance with a 1850 mV overpotential at the 1200 mA cm−2 current density (Figure 5g,h). Figure 5 | (a) Relationship between lattice strain and calculated bulk moduli of the NiSxSe1-x samples. (b) UPS data. (c) Electron exchange on the surface with lattice strain. Reproduced with permission from ref 57. Copyright 2020 John Wiley and Sons. (d) LSV curves of the catalysts with different phases and benchmark Pt in 0.5 M H2SO4. (e) Hydrogen adsorption on different phases of NbS2 is depicted by the free energy diagram. (f) Charge density between H adsorbed on Nb-terminated 2H and 3R Nb1.35S2. Reproduced with permission from ref 58. Copyright 2019 Springer Nature. (g) Model for relationship between C–O–Mo bonds and the active centers. (h) HER performance of the GDY/MoO3 and the control samples at ampere level current density. Reproduced with permission from ref 59. Copyright 2021 American Chemical Society. Download figure Download PowerPoint Optimization of the catalyst-electrolyte chemistry The relationship between the surface of the catalyst and electrolyte has drawn a lot of interest in order to gain fundamental knowledge about the catalytically active sites. HCD electrode materials can be optimized by tailoring the catalyst-electrolyte chemistry by altering the bond strengths between the intermediates and active sites, modulating the mass transfer efficacy, and maintaining a suitable pH (Figure 6a). The reaction mechanism and activity of a catalyst can be modified by the interactions with water at the interface. For example, Jin et al.61 shown that Ni-SN@C promotes water adsorption while inhibiting adsorption of hydrogen molecules, consequently forming hydronium ions on the surface of the catalyst in an alkaline solution. However, on the Ni@C and Ni3N catalysts, adsorbed H is transformed into H2 molecules rather than generating H3O+ as the intermediate under HCD. Furthermore, in the mass transfer process, robust desorption of gas bubbles might provide a swift mass transfer process at HCD. Nevertheless, adherence and accumulation of gas bubbles considerably limits mass transfer and inhibits the active sites. Figure 6 | (a) Catalyst-electrolyte interplay. Reproduced with permission from ref 60. Copyright 2022 John Wiley and Sons. (b) Microstructure of a Ni-Mo-B with the corresponding air bubble contact angles. (c) Cassie model and (d) smooth surface model. Reproduce