Over the years, significant advances have been made to boost the efficiency of water splitting by carefully designing economic electrocatalysts with augmented conductivity, more accessible active ...sites, and high intrinsic activity in laboratory test conditions. However, it remains a challenge to develop earth‐abundant catalysts that can satisfy the demands of practical water electrolysis, that is, outstanding all‐pH electrolyte capacity, direct seawater splitting ability, exceptional performance for overall water splitting, superior large‐current‐density activity, and robust long‐term durability. In this context, considering the features of increased active species loading, rapid charge, and mass transfer, a strong affinity between catalytic components and substrates, easily‐controlled wettability, as well as, enhanced bifunctional performance, the self‐supported electrocatalysts are presently projected to be the most suitable contenders for practical massive scale hydrogen generation. In this review, a comprehensive introduction to the design and fabrication of self‐supported electrocatalysts with an emphasis on the design of deposited nanostructured catalysts, the selection of self‐supported substrates, and various fabrication methods are provided. Thereafter, the recent development of promising self‐supported electrocatalysts for practical applications is reviewed from the aforementioned aspects. Finally, a brief conclusion is delivered and the challenges and perspectives relating to promotion of self‐supported electrocatalysts for sustainable large‐scale production of hydrogen are discussed.
This review presents deep insights into the design strategies for self‐supported electrocatalysts with a special emphasis on the critical parameters that are mandatory for their successful application in industrial water electrolysis.
Electrocatalytic water splitting is the key process for the formation of green fuels for energy transport and storage in a sustainable energy economy. Besides electricity, it requires water, an ...aspect that seldomly has been considered until recently. As freshwater is a limited resource (<1% of earth's water), lately, plentiful reports were published on direct seawater (around 96.5% of earth's water) splitting without or with additives (buffers or bases). Alternatively, the seawater can be split in two steps, where it is first purified by reverse osmosis and then split in a conventional water electrolyser. This quantitative analysis discusses the challenges of the direct usage of non-purified seawater. Further, herein, we compare the energy requirements and costs of seawater purification with those of conventional water splitting. We find that direct seawater splitting has substantial drawbacks compared to conventional water splitting and bears almost no advantage. In short, it is less promising than the two-step scenario, as the capital and operating costs of water purification are insignificant compared to those of electrolysis of pure water.
In this analysis, we show that direct seawater splitting with or without additives faces significant challenges and bears almost no advantage with respect to the costs and energy demands to purify water prior to water electrolysis.
Transition metal hydroxides (M‐OH) and their heterostructures (X|M‐OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active ...electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy‐efficient alkaline HER.
This review brings out the key advancements made in the field of alkaline HER with metal hydroxides and their heterostructures and also provides a detailed and critical analysis of strategies and perspectives used with highlights on future prospects at the end.
Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class ...of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.
This perspective provides an overview of the versatility of intermetallic compounds for electrochemical water splitting along with their synthetic strategies, catalytic efficiencies as well as their active structures under operating conditions.
Over the years, cobalt phosphates (amorphous or crystalline) have been projected as one of the most significant and promising classes of nonprecious catalysts and studied exclusively for the ...electrocatalytic and photocatalytic oxygen evolution reaction (OER). However, their successful utilization of hydrogen evolution reaction (HER) and the reaction of overall water‐splitting is rather unexplored. Herein, presented is a crystalline cobalt phosphate, Co3(OH)2(HPO4)2, structurally related to the mineral lazulite, as an efficient precatalyst for OER, HER, and water electrolysis in alkaline media. During both electrochemical OER and HER, the Co3(OH)2(HPO4)2 nanostructure was completely transformed in situ into porous amorphous CoOx
(OH) films that mediate efficient OER and HER with extremely low overpotentials of only 182 and 87 mV, respectively, at a current density of 10 mA cm−2. When assemble as anode and cathode in a two‐electrode alkaline electrolyzer, unceasing durability over 10 days is achieved with a final cell voltage of 1.54 V, which is superior to the recently reported effective bifunctional electrocatalysts. The strategy to achieve more active sites for oxygen and hydrogen generation via in situ oxidation or reduction from a well‐defined inorganic material provides an opportunity to develop cost‐effective and efficient electrocatalysts for renewable energy technologies.
A crystalline lazulite cobalt phosphate is identified as a low‐cost preelectrocatalyst for generating remarkably active and durable electrocatalysts for unifying the hydrogen evolution reaction, oxygen evolution reaction, and overall water‐splitting in alkaline media. Under oxidizing and reducing electrochemical environments, the restructuring (corrosion) of highly crystalline particles results in two different in situ‐generated amorphously active phases, yielding low overpotentials and cell potential.
As the kinetically demanding oxygen evolution reaction (OER) is crucial for the decarbonization of our society, a wide range of (pre)catalysts with various non‐active‐site elements (e.g., Mo, S, Se, ...N, P, C, Si…) have been investigated. Thermodynamics dictate that these elements oxidize during industrial operation. The formed oxyanions are water soluble and thus predominantly leach in a reconstruction process. Nevertheless, recently, it was unveiled that these thermodynamically stable (oxy)anions can adsorb on the surface or intercalate in the interlayer space of the active catalyst. There, they tune the electronic properties of the active sites and can interact with the reaction intermediates, changing the OER kinetics and potentially breaking the persisting OER *OH/*OOH scaling relations. Thus, the addition of (oxy)anions to the electrolyte opens a new design dimension for OER catalysis and the herein discussed observations deepen the understanding of the role of anions in the OER.
(Oxy)anions can surface‐adsorb on and intercalate into oxygen evolution reaction (OER) electrocatalysts. There, they can affect the catalyst's electronic properties and interlayer spacing, stabilizing OER intermediates and potentially breaking the *OH/*OOH scaling relations, as described in this Minireview. Thus, adding (oxy)anions to the electrolyte opens a new OER design dimension.
Transition metals, in particular noble metals, are the most common species in metal‐mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of ...nontransition metals, that is, s‐, p‐, and f‐block metals with high natural abundance in the earth‐crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline‐earth metals, rare‐earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge‐based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure–catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s‐, p‐, and f‐block metals species toward the discovery of advanced polymetal‐containing electrocatalysts for practical water splitting.
This review pioneeringly presents the unique and essential role of some s‐, p‐, and f‐block metal species enabling transition metal‐based electrocatalysts to efficiently drive hydrogen evolution and oxygen evolution reaction.
The single source precursor (SSP) approach was used to prepare highly active CoP bifunctional electro(pre)catalysts for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and ...overall water splitting (OWS) reaction starting from a molecular β-diketiminato Co(
i
) cyclo-P
4
complex. Crystalline or amorphous CoP particles were attained depending on the preparation route. Notably, the amorphous CoP displayed higher activity compared to the crystalline CoP on nickel foam (NF) and fluorinated tin oxide (FTO) substrates due to its unique electronic properties and surface characteristics. During the OER, severe oxidation to Co-oxy(hydroxides)/oxides by the loss of P was found to be crucial to increase the concentration of CoO
x
active sites. Interestingly, complete leaching of surface P from CoP and surface Co enrichment occurred during the HER. Finally, an OWS device was fabricated where the amorphous CoP outperformed the crystalline CoP with respect to low OWS cell voltage (with a difference of 130 mV) and enhanced stability for 5 days.
Amorphous CoP outperforms the crystalline phase in the OER, HER and overall water splitting with low overpotential and remarkable long-term stability.
The strikingly high catalytic performance and stability of manganese substituted cobalt oxide spinel (Mn
Co
O
) over pristine cobalt oxide spinel (Co
O
) for the alkaline electrochemical water ...oxidation is reported. The different role of cations could be uncovered along with the detection of drastic surface-structural changes during the catalysis using spectroscopic and microscopic methods.
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