Hierarchically scaffolded CoP/CoP2 nanoparticles: controllable synthesis and their application as a well-matched bifunctional electrocatalyst for overall water splitting
TL;DR: The results demonstrate that the preparation route based on the LDH precursor may provide an alternative for investigating diverse TMPs as bifunctional electrocatalysts for water splitting.
Abstract: Transition metal phosphide (TMP) nanostructures have stimulated increasing interest for use in water splitting owing to their abundant natural sources and high activity for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Typically, the preparation of hierarchical TMPs involves the utilization of expensive or dangerous phosphorus sources, and, in particular, the understanding of topotactic transformations of the precursors to crystalline phases—which could be utilized to enhance electrocatalytic performance—remains very limited. We, herein, report a controllable preparation of CoP/CoP2 nanoparticles well dispersed in flower-like Al2O3 scaffolds (f-CoP/CoP2/Al2O3) as a bifunctional electrocatalyst for the HER and OER via the phosphorization of a flower-like CoAl layered double hydroxide precursor. Characterization by in situ X-ray diffraction (XRD) monitored the topotactic transformation underlying the controllable formation of CoP/CoP2via tuning the phosphorization time. Electrocatalytic tests showed that an f-CoP/CoP2/Al2O3 electrode exhibited a lower onset potential and higher electrocatalytic activity for the HER and OER in the same alkaline electrolyte than electrodes of flower-like and powdered CoP/Al2O3. The enhanced electrochemical performance was experimentally supported by measuring the electrochemically active surface area. The f-CoP/CoP2/Al2O3 composite further generated a current density of 10 mA cm−2 at 1.65 V when used as a bifunctional catalyst for overall water splitting. Our results demonstrate that the preparation route based on the LDH precursor may provide an alternative for investigating diverse TMPs as bifunctional electrocatalysts for water splitting.
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TL;DR: The electrochemical stability of TMPs and TMCs predicted using Pourbaix diagrams and their morphological, structural and compositional evolution under OER conditions are discussed.
219 citations
TL;DR: In this article, the authors summarized the recent progress in the development of bifunctional electrocatalysts for several important electrochemical redox reactions which are critical to implement conversion between electrical energy and chemical energy.
Abstract: There remains a grand challenge to develop sustainable and pollution-free energy to replace the current dominant but gradually “depleting” fossil fuels. Electrochemical energy conversion presents a promising “bridge” to mitigate energy shortage issues and minimize the ecological implications by synergy with the ever-increasing sustainable energies (e.g., wind and solar). It thus calls for exploring high-activity, low-cost, and long-durability electrocatalysts to facilitate the electrochemical reactions involved in electrochemical energy conversion systems. This review focuses on summarizing the recent progress in the development of bifunctional electrocatalysts for several important electrochemical redox reactions which are critical to implement conversion between electrical energy and chemical energy, especially for bifunctional electrocatalysts applied in a single electrochemical conversion system, including water electrolyzers with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), zinc–air batteries with the OER and oxygen reduction reaction (ORR), and fuel cells with small molecule (e.g., glucose, urea, and hydrazine hydrate) oxidation reactions (MOR) and the ORR. Special emphasis has been placed on investigating the progress of electrocatalyst synthesis and strategies for improving the electrocatalytic performance and the associated devices' performance in terms of activity, stability, power density etc. We also put forward the major challenges and prospects in the development of bifunctional electrocatalysts for potential applications in a variety of energy devices.
196 citations
TL;DR: In this article, an efficient and stable overall water splitting system; cobalt nitride-vanadium oxynitride nanohybrid, prepared from the polyaniline (PANI) mediated synthetic protocol was reported.
Abstract: Excellent Electrochemical water splitting with remarkable durability can provide a solution for the increasing global energy demand. Herein, we report an efficient and stable overall water splitting system; cobalt nitride-vanadium oxynitride nanohybrid, prepared from the polyaniline (PANI) mediated synthetic protocol. This nanohybrid produces significantly higher water oxidation, proton reduction as well as overall water splitting performances compared to the cobalt nitride, vanadium oxynitride or even noble metal catalyst systems. Only 263 mV overpotential is required to reach 10 mA cm−2 current density for oxygen evolution reaction (OER) and 118 mV for the same in case of hydrogen evolution reaction (HER). Finally, the bifunctional nanohybrid has been explored for the alkaline overall water splitting at cell voltage of 1.64 V to attain 10 mA cm−2 current density with long term stability for 100 h. Post-catalytic analyses have revealed the formation of defect rich amorphous CoOx sites leading to high OER activity, whereas crystalline Co(OH)2-Coδ+-N (0
178 citations
TL;DR: In this paper, the composition engineering of pristine metal-organic frameworks (MOFs) and their derivatives, such as heteroatom-doped carbon, metal oxides/phosphides/sulfides/selenides and their hybrids, is summarized.
Abstract: Engineering highly efficient and cost-effective catalysts for the electrochemical oxygen evolution reaction (OER) is crucial for accelerating the development of renewable energy techniques due to the pivotal role of OER in rechargeable metal–air batteries and water electrolyzers. Recently, porous nonprecious metal catalysts (PNMCs) have received tremendous interest for application in the OER. Metal–organic frameworks (MOFs) built from metal ions and organic linkers have been demonstrated to be promising precursors for preparing PNMCs for the OER owing to their tunable structures/compositions, high surface area and large pore volume. Benefitting from the versatility of compositional/structural modification, MOFs and MOFs-derived PNMCs are robust in generating various high density active sites, which greatly contribute to their electrochemical OER performances. In this review, we first summarize the composition engineering of pristine MOFs and their derivatives, such as heteroatom-doped carbon, metal oxides/phosphides/sulfides/selenides and their hybrids, followed by structural/morphology engineering, including nano-coating and nano-perforating, and the configuration of high-dimensional architecture and single atom sites. Simultaneously, insights toward an in-depth understanding of the actual active sites and the intrinsic mechanism for OER enhancement are highlighted combined with experimental and theoretical studies. Finally, the challenges and perspectives for engineering MOF-based electrocatalysts are addressed.
137 citations
References
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TL;DR: A unified theoretical framework highlights the need for catalyst design strategies that selectively stabilize distinct reaction intermediates relative to each other, and opens up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions.
Abstract: BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.
7,062 citations
TL;DR: The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER.
Abstract: Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.
4,898 citations
TL;DR: Delamination of LDHs is an interesting route for producing positively charged thin platelets with a thickness of a few atomic layers, which can be used as nanocomposites for polymers or as building units for making new designed organic- inorganic or inorganic-inorganic nanomaterials.
Abstract: Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. Delamination of LDHs is an interesting route for producing positively charged thin platelets with a thickness of a few atomic layers, which can be used as nanocomposites for polymers or as building units for making new designed organic-inorganic or inorganic-inorganic nanomaterials. The synthesis of nanosized LDH platelets can be generally classified into two approaches, bottom-up and top-down. It requires modification of the LDH interlamellar environment and then selection of an appropriate solvent system. In DDS intercalated LDHs, the aliphatic tails of the DDS- anions exhibit a high degree of interdigitation in order to maximize guest-guest dispersive interactions. Bellezza reported that the LDH colloids can also been obtained by employing a reverse microemulsion approach.
2,616 citations
11 Jan 2017
TL;DR: In this article, the authors investigate progress towards photo-electrocatalytic water-splitting systems, with special emphasis on how they might be incorporated into photoelectrocaralyst systems.
Abstract: Sunlight is by far the most plentiful renewable energy resource, providing Earth with enough power to meet all of humanity's needs several hundred times over. However, it is both diffuse and intermittent, which presents problems regarding how best to harvest this energy and store it for times when the sun is not shining. Devices that use sunlight to split water into hydrogen and oxygen could be one solution to these problems, because hydrogen is an excellent fuel. However, if such devices are to become widely adopted, they must be cheap to produce and operate. Therefore, the development of electrocatalysts for water splitting that comprise only inexpensive, earth-abundant elements is critical. In this Review, we investigate progress towards such electrocatalysts, with special emphasis on how they might be incorporated into photoelectrocatalytic water-splitting systems and the challenges that remain in developing these devices. Splitting water is an attractive means by which energy — either electrical and/or light — is stored and consumed on demand. Active and efficient catalysts for anodic and cathodic reactions often require precious metals. This Review covers base-metal catalysts that can afford high performance in a more sustainable and available manner.
2,369 citations
TL;DR: An overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented, and specific strategies to further improve the catalytic efficiency and stability of T MPs by structural engineering are demonstrated.
Abstract: The urgent need of clean and renewable energy drives the exploration of effective strategies to produce molecular hydrogen. With the assistance of highly active non-noble metal electrocatalysts, electrolysis of water is becoming a promising candidate to generate pure hydrogen with low cost and high efficiency. Very recently, transition metal phosphides (TMPs) have been proven to be high performance catalysts with high activity, high stability, and nearly ∼100% Faradic efficiency in not only strong acidic solutions, but also in strong alkaline and neutral media for electrochemical hydrogen evolution. In this tutorial review, an overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented. The effects of phosphorus (P) on HER activity, and their synthetic methods of TMPs are briefly discussed. Then we will demonstrate the specific strategies to further improve the catalytic efficiency and stability of TMPs by structural engineering. Making use of TMPs as cocatalysts and catalysts in photochemical and photoelectrochemical water splitting is also discussed. Finally, some key challenges and issues which should not be ignored during the rapid development of TMPs are pointed out. These strategies and challenges of TMPs are instructive for designing other high-performance non-noble metal catalysts.
2,104 citations