ConspectusHydrogen is an ideal energy carrier and plays a critical role in the future energy transition. Distinct from steam reforming, electrochemical water splitting, especially powered by ...renewables, has been considered as a promising technique for scalable production of high-purity hydrogen with no carbon emission. Its commercialization relies on the reduction of electricity consumption and thus hydrogen cost, calling for highly efficient and cost-effective electrocatalysts with the capability of steadily working at high hydrogen output. This requires the electrocatalysts to feature (1) highly active intrinsic sites, (2) abundant accessible active sites, (3) effective electron and mass transfer, (4) high chemical and structural durability, and (5) low-cost and scalable synthesis. It should be noted that all these requirements should be fulfilled together for a practicable electrocatalyst. Much effort has been devoted to addressing one or a few aspects, especially improving the electrocatalytic activity by electronic modulation of active sites, while few reviews have focused on the synergistic modulation of these aspects together although it is essential for advanced electrochemical water splitting.In this Account, we will present recent innovative strategies with an emphasis on our solutions for synergistically modulating intrinsic active sites, electron transportation, mass transfer, and gas evolution, as well as mechanical and chemical durability, of non-precious-metal electrocatalysts, aiming for cost-effective and highly efficient water splitting. The following approaches for coupling these aspects are summarized for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). (1)
. The electronic structure of a catalytic site determines the adsorption/desorption of reactive intermediates and thus intrinsic activity. It can be tuned by heterogeneous doping, strain effect, spin polarization, etc. Coupling these effects to optimize the reaction pathways or target simultaneously the activity and stability would advance electrocatalytic performance. (2)
. The crystallinity, crystalline phase, crystalline facets, crystalline defects, etc. affect both activity and stability. Coupling these effects with electronic modulation would enhance the activity together with stability. (3)
. It will focus on concurrently modulating electronic structure for improving the intrinsic activity and morphology for increasing accessible active sites, especially through single action or processing. The mass transfer and gas evolution properties can also be enhanced by morphological modulation to enable water splitting at large output. (4)
. Electrocatalytic reaction generally consists of a couple of elementary reactions. Each one may need a specific active site. Designing and combining various components targeting every elementary step on a space-limited catalyst surface will balance the intermediates and these steps for accelerating the overall reaction. (5)
Taking all these strategies together into account is necessary to integrate all above essential features into one electrocatalyst for enabling high-output water electrolysis. Beyond the progress made to date, the remaining challenges and opportunities is also discussed. With these insights, hopefully, this Account will shed light on the rational design of practical water-splitting electrocatalysts for the cost-effective and scalable production of hydrogen.
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Practical electrochemical water splitting requires cost‐effective electrodes capable of steadily working at high output, leading to the challenges for efficient and stable electrodes for the oxygen ...evolution reaction (OER). Herein, by simply using conductive FeS microsheet arrays vertically pre‐grown on iron foam (FeS/IF) as both substrate and source to in situ form vertically aligned NiFe(OH)x nanosheets arrays, a hierarchical electrode with a nano/micro sheet‐on‐sheet structure (NiFe(OH)x/FeS/IF) can be readily achieved to meet the requirements. Such hierarchical electrode architecture with a superhydrophilic surface also allows for prompt gas release even at high output. As a result, NiFe(OH)x/FeS/IF exhibits superior OER activity with an overpotential of 245 mV at 50 mA cm−2 and can steadily output 1000 mA cm−2 at a low overpotential of 332 mV. The water‐alkali electrolyzer using NiFe(OH)x/FeS/IF as the anode can deliver 10 mA cm−2 at 1.50 V and steadily operate at 300 mA cm−2 with a small cell voltage for 70 h. Furthermore, a solar‐driven electrolyzer using the developed electrode demonstrates an exceptionally high solar‐to‐hydrogen efficiency of 18.6%. Such performance together with low‐cost Fe‐based materials and facile mass production suggest the present strategy may open up opportunities for rationally designing hierarchical electrocatalysts for practical water splitting or diverse applications.
A 3D hierarchical NiFe(OH)x/FeS/IF electrode with a nano/micro sheet‐on‐sheet structure exhibits superior oxygen evolution reaction activity and durability with a low overpotential of 261 mV at 100 mA cm−2 and 332 mV at 1000 mA cm−2. The water‐alkali electrolyzer, using it as an anode, achieves stable overall water splitting at 300 mA cm−2 with a small cell voltage.
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Single‐atom catalysts (SACs) have been at the frontier of research field in catalysis owing to the maximized atomic utilization, unique structures and properties. The atomically dispersed and ...catalytically active metal atoms are necessarily anchored by surrounding atoms. As such, the structure and composition of anchoring sites significantly influence the catalytic performance of SACs even with the same metal element. Significant progress has been made to understand structure–activity relationships at an atomic level, but in‐depth understanding in precisely designing highly efficient SACs for the targeted reactions is still required. In this review, various anchoring sites in SACs are summarized and classified into five different types (doped heteroatoms, defect sites, surface atoms, metal sites, and cavity sites). Then, their impacts on catalytic performance are elucidated for electrochemical reactions based on their distance from the metal center (first coordination shell and beyond). Further, SACs anchored on two typical types of hosts, carbon‐ and metal‐based materials, are highlighted, and the effects of anchoring points on achieving the desirable atomic structure, catalytic performance, and reaction pathways are elaborated. At last, insights and outlook to the SAC field based on current achievements and challenges are presented.
Single‐atom catalysts (SACs) with isolated metal atoms stabilized on various supports by surrounding atoms have emerged as a new frontier. The atomic structure and composition of anchoring sites significantly influence the catalytic performance of SACs. An overview of the correlation between the property of anchoring points and catalytic performance to guide the design of SACs is provided.
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Developing low‐cost, highly efficient, and durable electrocatalysts for oxygen evolution reaction (OER) is essential for the practical application of electrochemical water splitting. Herein, it is ...discovered that organic small molecule (hexabromobenzene, HBB) can activate commercial transition metal (Ni, Fe, and NiFe) foam by directly evolving metal nanomeshes embedded in graphene‐like films (M‐NM@G) through a facile Br‐induced solid‐phase migration process. Systematic investigations indicate that HBB can conformally generate graphene‐like network on bulk metal foam substrate via the cleavage of CBr bonds and the formation of CC linkage. Simultaneously, the cleaved CBr fragments can efficiently extract metal atoms from bulk substrate, in situ producing transition metal nanomeshes embedded in the graphene‐like films. As a result, such functional nanostructure can serve as an efficient OER electrocatalyst with a low overpotential and excellent long‐term stability. Specifically, the overpotential at 100 mA cm−2 is only 208 mV for NiFe‐NM@G, ranking the top‐tier OER electrocatalysts. This work demonstrates an intriguing general strategy for directly transforming bulk transition metals into nanostructured functional electrocatalysts via the interaction with organic small molecules, opening up opportunities for bridging the application of organic small molecules in energy technologies.
Transition metal nanomeshes embedded in graphene‐like film (M‐NM@G) are achieved via a newly developed Br‐induced solid‐phase migration method enabled by directly treating commercial transition metal foam with organic small molecules. NiFe‐NM@G exhibits excellent oxygen evolution reaction performance with a low overpotential of 208 mV at a current density of 100 mA cm−2, as well as excellent long‐term stability.
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Highly efficient and stable bifunctional electrocatalysts for oxygen reduction and evolution are essential for aqueous rechargeable Zn–air batteries, which require highly active sites as well as ...delicate structural design for increasing effective active sites and facilitating mass/electron transfer. Herein, a scalable and facile self‐catalyzed growth strategy is developed to integrate highly active Co–N–C sites with 3D brush‐like nanostructure, achieving Co–N–C nanobrushes with Co,N‐codoped carbon nanotube branches grown on Co,N‐codoped nanoparticle assembled nanowire backbones. Systematic investigations suggest that nanobrushes deliver significantly improved electrocatalytic activity compared with nanowire or nanotube counterparts and the longer nanotube branches give the better performance. Benefiting from the increase of accessible highly active sites and enhanced mass transfer and electron transportation, the present Co–N–C nanobrush exhibits superior electrocatalytic activity and durability when used as a bifunctional oxygen catalyst. It enables a rechargeable Zn–air battery with a high peak power density of 246 mW cm−2 and excellent cycling stability. These results suggest that the reported synthetic strategy may open up possibilities for exploring efficient electrocatalysts for diverse applications.
3D hierarchical Co–N–C brush‐like nanostructure is fabricated through a self‐catalyzed chemical vapor deposition, which enables a rechargeable Zn–air batteries a high peak power density of 246 mW cm−2 and excellent cycling stability. The strategy can also be easily extended to prepare Fe‐based M–N–C nanobrush analogs.
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Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient ...activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such a Se-doped FeOOH electrode via an FeSe precatalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm–2 is secured at a low overpotential of 348 mV. The application of such an Fe-rich OER electrode in a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.
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The exploration of new efficient OER electrocatalysts based on nonprecious metals and the understanding of the relationship between activity and structure of electrocatalysts are important to advance ...electrochemical water oxidation. Herein, we developed an efficient OER electrocatalyst with nickel boride (Ni3B) nanoparticles as cores and nickel(II) borate (Ni‐Bi) as shells (Ni‐Bi@NB) via a very simple and facile aqueous reaction. This electrocatalyst exhibited a small overpotential of 302 mV at 10 mA cm−2 and Tafel slope of 52 mV dec−1. More interestingly, it was found that the OER activity of Ni‐Bi@NB was closely dependent on the crystallinity of the Ni‐Bi shells. The partially crystalline Ni‐Bi catalyst exhibited much higher activity than the amorphous or crystalline analogues; this higher activity originated from the enhanced intrinsic activity of the catalytic sites. These findings open up opportunities to explore nickel(II) borates as a new class of efficient nonprecious metal OER electrocatalysts, and to improve the electrocatalyst performance by modulating their crystallinity.
A new winner! Crystallinity‐dependent activity was demonstrated on a new efficient electrocatalyst for the oxygen evolution reaction (OER) which consists of a nickel(II) borate thin layer on nickel boride (NB) nanoparticles (Ni‐Bi@NB) (see figure). The partially crystalline Ni‐Bi catalyst exhibits excellent OER performance.
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Developing bifunctional efficient and durable non-noble electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable and challenging for overall ...water splitting. Herein, Co–Mn carbonate hydroxide (CoMnCH) nanosheet arrays with controllable morphology and composition were developed on nickel foam (NF) as such a bifunctional electrocatalyst. It is discovered that Mn doping in CoCH can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and tune the electronic structure of Co center to effectively boost its intrinsic activity. As a result, the optimized Co1Mn1CH/NF electrode exhibits unprecedented OER activity with an ultralow overpotential of 294 mV at 30 mA cm–2, compared with all reported metal carbonate hydroxides. Benefited from 3D open nanosheet array topographic structure with tight contact between nanosheets and NF, it is able to deliver a high and stable current density of 1000 mA cm–2 at only an overpotential of 462 mV with no interference from high-flux oxygen evolution. Despite no reports about effective HER on metal carbonate hydroxides yet, the small overpotential of 180 mV at 10 mA cm–2 for HER can be also achieved on Co1Mn1CH/NF by the dual modulation of Mn doping. This offers a two-electrode electrolyzer using bifunctional Co1Mn1CH/NF as both anode and cathode to perform stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm–2. These findings may open up opportunities to explore other multimetal carbonate hydroxides as practical bifunctional electrocatalysts for scale-up water electrolysis.
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Multifractality is ubiquitously observed in complex natural and socioeconomic systems. Multifractal analysis provides powerful tools to understand the complex nonlinear nature of time series in ...diverse fields. Inspired by its striking analogy with hydrodynamic turbulence, from which the idea of multifractality originated, multifractal analysis of financial markets has bloomed, forming one of the main directions of econophysics. We review the multifractal analysis methods and multifractal models adopted in or invented for financial time series and their subtle properties, which are applicable to time series in other disciplines. We survey the cumulating evidence for the presence of multifractality in financial time series in different markets and at different time periods and discuss the sources of multifractality. The usefulness of multifractal analysis in quantifying market inefficiency, in supporting risk management and in developing other applications is presented. We finally discuss open problems and further directions of multifractal analysis.
Photonic analogue of topological insulator was recently predicted by arranging ε/μ (permittivity/permeability)-matched bianisotropic metamaterials into two-dimensional superlattices. However, the ...experimental observation of such photonic topological insulator is challenging as bianisotropic metamaterial is usually highly dispersive, so that the ε/μ-matching condition can only be satisfied in a narrow frequency range. Here we experimentally realize a photonic topological insulator by embedding non-bianisotropic and non-resonant metacrystal into a waveguide. The cross coupling between transverse electric and transverse magnetic modes exists in metacrystal waveguide. Using this approach, the ε/μ-matching condition is satisfied in a broad frequency range which facilitates experimental observation. The topologically non-trivial bandgap is confirmed by experimentally measured transmission spectra and calculated non-zero spin Chern numbers. Gapless spin-filtered edge states are demonstrated experimentally by measuring the magnitude and phase of the fields. The transport robustness of the edge states is also observed when an obstacle was introduced near the edge.
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