Bifunctional Catalysts For Water Splitting Research Articles

Co-doped Pyrrhotite Fe7S8 nanosheets as bifunctional electrocatalysts for water splitting

The development of low-price and highly efficient catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has always been challenging with regard to water splitting. Pyrrhotite Fe7S8 has become a research hotspot due to its abundant reserves, high electrical conductivity and excellent catalytic activity. In general, introducing metal heteroatoms into transition metal sulfides (TMSs) can effectively enhance the activity and stability of TMSs. Herein, Co-doped pyrrhotite Fe7S8 nanosheets were obtained by a straightforward two-step method consisting of hydrothermal treatment followed by subsequent heat treatment. Results from XPS analysis and contact angle measurements showed that doping optimized the chemical environment around iron atoms and enhanced the adsorption of water molecules on the surface of Fe7S8, thereby greatly improving the electrocatalytic efficiency of water electrolysis. Compared with other counterparts, Co0·95Fe6·05S8 exhibited superior OER activity with an overpotential of 311 mV at a current density of 10 mA cm−2, and higher performance towards HER with an overpotential of 284 mV at 10 mA cm−2 in alkaline solution. In sum, Co0·95Fe6·05S8 is promising as a bifunctional catalyst for water splitting under alkaline conditions.

Two-dimensional metallic MoS2-amorphous CoNi(OH)2 nanocomposite for enhanced electrochemical water splitting in alkaline solutions

We report the preparation of nanocomposite consisting of metallic MoS2 and electrodeposited amorphous CoNi(OH)2 nanosheets on carbon paper for efficient electrochemical water splitting in the alkaline solutions. The synergistic effect between metallic MoS2 and amorphous CoNi(OH)2 nanosheets enables the excellent catalytic activity of nanocompsite towards hydrogen evolution reaction (HER) in 0.1 M KOH with a low overpotential of 178 mV at a current density of 10 mA/cm2 and Tafel slope of 60.9 mV/decade. The current density of MoS2-CoNi(OH)2 nanocomposite for HER at the overpotential of more than 0.190 V is even higher than that of the commercial Pt/C catalyst. Moreover, the MoS2-CoNi(OH)2 nanocomposite is also active towards electrochemical oxygen evolution and can act as a bifunctional catalyst for water splitting in 0.1 M KOH. Our work povides a simple and effective way to engineer phases and prepare two-dimensional nanocomposites for effecient water splitting in alkaline aqueous solutions.

Advantageous metal-atom-escape towards super-hydrophilic interfaces assembly for efficient overall water splitting

Controllably constructing the functional microscopic surfaces, especially via the escape of metal atoms from extremely stable texture to achieve super-hydrophilic interfaces, as efficient bifunctional catalysts for water splitting is a daunting challenge but critical for sustainable economic development. Based on hydrothermal coordination strategy, we take advantage of strong oxidizing property of nitrate ions to realize temperature-controllable atom-escape of metal Ni for self-assembling super-hydrophilic hydroxide FeNi–OH on nickel base as electrocatalysts of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Such distinctive Ni source, resulting in improved ohmic contact and mechanical robustness of catalysts, in conjunction with super-hydrophilicity-accelerated bubble separation and electrolyte contact, endows the catalyst with excellent electrocatalytical activity for both HER and OER in alkaline solution. Super-hydrophilic FeNi–OH delivers the current density of 50 mA cm−2 at overpotentials of 236 mV for OER and 196 mV for HER. Water electrolysis with FeNi–OH as anode and cathode demands a cell voltage of 1.664 V to drive 50 mA cm−2 in 1.0 M KOH, continuously operating for 25 h. This study opens up an avenue in establishing super-hydrophilic structure via self-assembly of the escaped atoms, broadening the horizon on low-cost iron-containing catalyst preparation for energy conversions.

Constructing Ultrathin W-Doped NiFe Nanosheets via Facile Electrosynthesis as Bifunctional Electrocatalysts for Efficient Water Splitting.

Exploring cost-effective and efficient bifunctional electrocatalysts via simple fabrication strategies is strongly desired for practical water splitting. Herein, an easy and fast one-step electrodeposition process is developed to fabricate W-doped NiFe (NiFeW)-layered double hydroxides with ultrathin nanosheet features at room temperature and ambient pressure as bifunctional catalysts for water splitting. Notably, the NiFeW nanosheets require overpotentials of only 239 and 115 mV for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, to reach a current density of 10 mA/cm2 in alkaline media. Their exceptional performance is further demonstrated in a full electrolyzer configuration with the NiFeW as both anode and cathode catalysts, which achieves a low cell voltage of 1.59 V at 10 mA/cm2, 110 mV lower than that of the commercial IrO2 (anode) and Pt (cathode) catalysts. Moreover, the NiFeW nanosheets are superior to various recently reported bifunctional electrocatalysts. Such remarkable performances mainly ascribe to W doping, which not only effectively modulates the electrocatalyst morphology but also engineers the electronic structure of NiFe hydroxides to boost charge-transfer kinetics for both the OER and HER. Hence, the ultrathin NiFeW nanosheets with an efficient fabrication strategy are promising as bifunctional electrodes for alkaline water electrolyzers.

Facile fabrication of flower-like CuS/MnCO3 microspheres clusters on nickel foam as an efficient bifunctional catalyst for overall water splitting

Utilizing the abundant elements on earth to product inexpensive, high-active and stable catalysts for water splitting is very significant but still remains serious challenge to produce hydrogen. Herein, heterostructures of CuS/MnCO3 on nickel foam substrate are firstly successfully synthesized via a facile one-step hydrothermal strategy. The as-prepared electrocatalyst displays an enhanced oxygen evolution reaction (OER) performance in alkaline conditions with a minimum overpotential of 70 mV and a small Tafel slope of 42.5 mV/dec to achieve 10 mA cm−2. The catalyst also exhibits an excellent HER activity with a low overpotential of 143 mV and the Tafel slope of 51.4 mV/dec to acquire 10 mA cm−2 in 1.0 M KOH. Moreover, when the CuS/MnCO3//CuS/MnCO3 electrode is applied for the overall water splitting, the electrolyzer cell device affords 10 mA cm−2 at a relative low voltage of 1.43 V, which is one of the best catalysts ever reported. In stability test, its activity first decreases and then remains stable in 1 M KOH solution for about 10 h, indicating that the electrode has good electrochemical stability. Density functional theory calculations (DFT) show that MnCO3 has a stronger adsorption energy for water than CuS does, indicating that MnCO3 is a real active center and CuS plays a certain synergistic effect. This work not only provides a low-cost and efficient bifunctional catalyst for water splitting technology, but also extends the application of bifunctional catalyst based on transition metal sulfide and carbonate compound.

Recycling valuable cobalt from spent lithium ion batteries for controllably designing a novel sea-urchin-like cobalt nitride-graphene hybrid catalyst: Towards efficient overall water splitting

Along with the continuous consumption in lithium-ion batteries (LIBs), the price of cobalt is inevitably going up in recent years. Therefore, recycling valuable Co element from spent devices, and boosting its service efficiency are becoming two indispensable approaches to promote the utilization of Co in various energy conversion/storage devices. Herein, we realize the recovery of Co from spent LIBs and synthesize a three–dimensional (3D) sea-urchin-like cobalt nitride composite material (labeled as CoN-Gr-2), which is used as a bi-functional catalyst for water splitting. Benefiting from the intrinsic high conductivity, larger surface area and unique 3D sea–urchin–like architecture, CoN-Gr-2 shows an excellent electron transfer efficiency, highly exposed active sites as well as the superior mass transport capacity. The CoN-Gr-2 catalyst exhibits low overpotentials of 128.9 mV and 280 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which are comparable to the commercial 20 wt% Pt/C and RuO2 catalysts. Moreover, when adopting CoN-Gr-2 as both anode and cathode materials for overall water splitting (in 1.0 M KOH electrolyte), the assembled cell achieves a current density of 10 mA cm−2 at 1.61 V, which almost close to that of Pt/C||RuO2 benchmark (1.60 V), demonstrating its superior water-splitting efficiency. Meanwhile, the CoN catalysts exhibit strong chemical interaction with the Gr support, suppressing the aggregation of CoN catalysts and maintains their high activity during HER and OER reactions. So, the cell exhibits a high current retention of 97.3% after 40 h. This work successfully develops an industrial chain from recycling Co wastes in spent energy devices to controllably designing 3D sea-urchin-like CoN-Gr with high water splitting efficiency. Therefore, it could further promote the efficient utilization of valuable Co element in various energy devices.

In-situ growth of Fe–Co Prussian-blue-analog nanocages on Ni(OH)2/NF and the derivative electrocatalysts with hierarchical cage-on-sheet architectures for efficient water splitting

To design an efficient and cost-effective electrocatalyst based on Prussian blue and its analogs are a promising choice to realize energy transformation and storage via water-splitting. Herein, a facile and practical method is developed to in-situ grow Fe–Co Prussian-blue-analog (PBA) nanocages with an open hole in each face center on Ni(OH)2/NF substrate to form the hierarchical cage-on-plate structure. Furthermore, the Fe–Co PBA nanocages attached to Ni(OH)2/NF plates are hydrogenated and nitrogenized into FeCoNi/NF and FeCoNiN/NF electrodes, respectively. As-prepared electrodes successfully retain the 3D hierarchical micro-nano structures of Fe–Co PBA@Ni(OH)2/NF precursor and can be used as a bifunctional water-splitting catalyst for overall water splitting. Compared to FeCoNi/NF, FeCoNiN/NF shows more efficiency and durability in the electrolytic water splitting tests in alkaline media. For the FeCoNiN/NF electrocatalyst, ultralow overpotentials for hydrogen evolution reaction (HER) are only 56 and 290 mV at current densities of 10 and 500 mA cm−2. Meanwhile, overpotentials for oxygen evolution reaction (OER) are 267 and 374 mV at current densities of 50 and 500 mA cm−2. The FeCoNiN/NF electrode can act both the cathode and the anode for overall water splitting, this electrolyzer only requires a cell voltage of 1.492 V to afford a current density of 10 mA cm−2. This electrolyzer can stably deliver a viable high current density of 625 mA cm−2 for 40 h to meet the condition of industrial application.

Trace amounts of Ru-doped Ni–Fe oxide bone-like structures via single-step anodization: a flexible and bifunctional electrode for efficient overall water splitting

A trace amount of Ru-doped Ni–Fe oxide film with an in situ formed 3D bone-like structure is an outstanding, efficient, durable, and flexible bifunctional catalyst for water splitting.

Pulse electrodeposited CoFeNiP as a highly active and stable electrocatalyst for alkaline water electrolysis

A high-performance CoFeNiP/NF bifunctional catalyst for water splitting was prepared via a pulse-electrodeposition method. Doping with iron and nickel significantly modulates the electronic state of CoP, resulting in enhanced activity and stability.

Theoretical investigation on hbox {BeN}_{{2}} monolayer for an efficient bifunctional water splitting catalyst

The search for an active, stable, and abundant semiconductor-based bifunctional catalysts for solar hydrogen production will make a substantial impact on the sustainable development of the society that does not rely on fossil reserves. The photocatalytic water splitting mechanism on a hbox {BeN}_{{2}} monolayer has here been investigated by using state-of-the-art density functional theory calculations. For all possible reaction intermediates, the calculated changes in Gibbs free energy showed that the oxygen evolution reaction will occur at, and above, the potential of 2.06 V (against the NHE) as all elementary steps are exergonic. In the case of the hydrogen evolution reaction, a potential of 0.52 V, or above, was required to make the reaction take place spontaneously. Interestingly, the calculated valence band edge and conduction band edge positions for a hbox {BeN}_{{2}} monolayer are located at the potential of 2.60 V and 0.56 V, respectively. This indicates that the photo-generated holes in the valence band can oxidize water to oxygen, and the photo-generated electrons in the conduction band can spontaneously reduce water to hydrogen. Hence, the results from the present theoretical investigation show that the hbox {BeN}_{{2}} monolayer is an efficient bifunctional water-splitting catalyst, without the need for any co-catalyst.

Self-supported electrode of NiCo-LDH/NiCo2S4/CC with enhanced performance for oxygen evolution reaction and hydrogen evolution reaction

The exploitation of bifunctional electrocatalysts for stable and efficient water splitting is of great significant for the development of clean energy industry. Herein, a hierarchical NiCo-LDH/NiCo2S4/CC electrode is prepared on carbon cloth by coupling NiCo-LDH nanosheets with NiCo2S4 nanoneedles through a simple process. This NiCo-LDH/NiCo2S4/CC electrode serves as an efficient bifunctional electrocatalyst for overall water splitting with excellent activity and stability in alkaline media. The enhanced electrocatalytic performance is ascribed to the hierarchical structure, the synergistic effect of NiCo-LDH and NiCo2S4 as well as the phase transformation of NiCo2S4 in alkaline condition. The NiCo-LDH/NiCo2S4/CC electrode requires overpotentials of 254 and 219 mV to deliver the current density of 50 mA cm-2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1 M KOH aqueous solution, respectively. The Tafel results further reveal favorable reaction kinetics during electrolysis. When used as the bifunctional electrode for overall water splitting, the NiCo-LDH/NiCo2S4/CC performs superior activity with a voltage of 1.64 V at the current density of 50 mA cm-2. This work offers a new approach for the exploitation of high-performance and cost-efficient bifunctional catalysts for water splitting.

A co-coordination strategy to realize janus-type bimetallic phosphide as highly efficient and durable bifunctional catalyst for water splitting

In this work, a Janus-type dual-ligand metal-organic frameworks derived bimetallic (Fe, Co)P nanoparticles embedded carbon nanotube (CNT) skeleton (DLD-FeCoP@CNT) is presented and synthesized via a facile “co-coordination synthesis” strategy. The DLD-FeCoP@CNT hybrid shows much better performances for OER and HER with much lower Tafel slope of 39.6 (57.1) mV dec-1, an overpotential of 286 (166) mV @ 10 mA cm-2 and better stability for OER (HER) in 1 M KOH. Being both cathode and anode for water splitting, it requires only a low voltage of 1.67 V to obtain 10 mA cm-2 with nearly 100% faradaic efficiency, which is close to Pt/C//RuO2 cell. Density functional theory calculations based on the bimetallic phosphide (Fe0.3Co0.7P) model reveal that, compared with the monometallic FeP or CoP, the enhanced catalytic activities of Fe0.3Co0.7P is mainly manifested in its free energy of H adsorption (ΔGH*) closer to zero, larger binding strength for H2O and higher electrical conductivity.

Single Ni Atoms and Clusters Embedded in N-Doped Carbon "Tubes on Fibers" Matrix with Bifunctional Activity for Water Splitting at High Current Densities.

Among the bifunctional catalysts for water splitting, recently emerged transition-metal single-atom catalysts are theoretically considered to possess high potential, while the experimental activity is not satisfactory yet. Herein, an exceptionally efficient trifunctional metal-nitrogen-carbon (M-N-C) catalyst electrode, composed of a hierarchical carbon matrix embedding isolated nickel atoms with nickel-iron (NiFe) clusters, is presented. 1D microfibers and nanotubes grow sequentially from 2D nanosheets as sacrificial templates via two stages of solution- and solid-phase reactions to form a 1D hierarchy. Exceptionally efficient bifunctional activity with an overpotential of only 13 mV at 10 mA cm-2 toward hydrogen evolution reaction (HER) and an overpotential of 210 mV at 30 mA cm-2 toward oxygen evolution reaction (OER) is obtained, surpassing each monofunctional activity ever reported. More importantly, an overpotential of only 126 and 326 mV is required to drive 500 mA cm-2 toward the HER and OER, respectively. For the first time, industrial-scale water splitting with two bifunctional catalyst electrodes with a current density of 500 mA cm-2 at a potential of 1.71 V is demonstrated. Lastly, trifunctional catalytic activity including oxygen reduction reaction is also proven with a half-wave potential at 0.848 V.

Nanoporous NiFeMoP alloy as a bifunctional catalyst for overall water splitting

High performance bifunctional catalysts for water splitting are very promising. Transition metal phosphides as catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted considerable attention due to their high performance. However, the catalyst with excellent properties still remains a significant challenge. Herein, the nanoporous NiFeMoP(np-NiFeMoP) ribbon was prepared by quenching and dealloying method. It was found that np-NiFeMoP showed excellent HER and OER performance in 1 M KOH. The overpotential of OER is as low as 197 mV at a current density of 20 mA·cm−2. When the current density is 10 mA·cm−2, the overpotential of HER is 223 mV. Moreover, np-NiFeMoP only needs a cell voltage of 1.41 V when current density is 10 mA·cm−2 for water splitting. Our current work may provide some new insights on rationally constructing nanoporous structure with rich active sites to boost the catalytic performances for overall water splitting.

Electrodeposited cobalt phosphides with hierarchical nanostructure on biomass carbon for bifunctional water splitting in alkaline solution

Low cost and high-performance bifunctional catalysts for water splitting under alkaline conditions still face many challenges. CoP/C were synthesized by electrodeposition in the presence of different sodium hypophosphite concentrations with porous carbon membranes as working electrodes. The morphology of CoP/C is strongly dependent on the concentration of sodium hypophosphite. We prepared the CoP/C with varying contents of phosphorus to investigate influence of phosphorus content on the morphology of CoP/C and the corresponding catalytic capability of water splitting in alkaline conditions. When the sodium hypophosphite in the electrodeposition electrolyte reaches to 0.6 M, the malformed octahedral CoP particles appear on nanofibers of carbon membrane coated with CoP films. The CoP/C with such hierarchical structure exhibits superior electrocatalytic activities for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with the overpotential of 140 mV and 250 mV at a current density of 10 mA cm−2, respectively in the alkaline solution. Furthermore, the CoP/C with hierarchical structure as both anode and cathode for water splitting achieves 10 mA cm−2 only need 1.56 V. The CoP/C with hierarchical structure keeps stable catalytic performance even after 24 h. This work provides a facile strategy for designing high-efficient phosphides based catalyst for overall water splitting.

The self-supported Zn-doped CoNiP microsphere/thorn hierarchical structures as efficient bifunctional catalysts for water splitting

The hierarchical electrocatalysts with the superaerophobic property are in favor of removal of gas bubbles and supply of numerous active sites to improve the electrochemical catalytic activity for water splitting. In this work, the self-supported Zn-doped CoNiP (Zn–CoNiP/NF) hierarchical structure that consisted of nanothorns and radial microspheres were prepared with via a facile precursor phosphorization method. Zn-doping could tailor the initial electronic state of CoNiP to improve the hydrogen or oxygen atom binding, which effectively promoted the catalytic kinetics of water splitting. The hierarchical structure not only offered more catalytic active sites for water splitting, but also facilitated gas releasing due to the superaerophobic interface. The hierarchical Zn–CoNiP structure showed a high hydrogen evolution reaction (HER) efficiency in electrolytes with pH 0–14, which yielded low overpotentials of 73 mV in acidic solution and 34 mV in alkaline media at current density of 10 mA cm−2. Furthermore, it also had evidenced by an overpotential of 310 mV for oxygen evolution reaction (OER) at the current density of 50 mA cm−2. Note that the required cell initial voltage was 1.51 V for overall water splitting. These results were comparable or even better than many non-noble catalysts.

Boosting electrocatalytic water splitting via metal-metalloid combined modulation in quaternary Ni-Fe-P-B amorphous compound

Design and synthesis of highly efficient and cost-effective bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remain a big challenge. Herein, a quaternary amorphous nanocompound Ni-Fe-P-B has been synthesized by a facile, scalable co-reduction method. The Ni-Fe-P-B exhibits high electrocatalytic activity and outstanding durability for both HER and OER, delivering a current density of 10 mAcm−2 at overpotentials of 220 and 269 mV, respectively. When loaded on carbon fiber paper (CFP) as a bifunctional catalyst, the Ni-Fe-P-B@CFP electrode requires a low cell voltage of 1.58 V to obtain 10 mAcm−2 for overall water splitting with negligible recession over 60 h. The excellent catalytic performances of Ni-Fe-P-B mainly benefit from the metal-metalloid combined composition modulation and the unique amorphous structure. This work provides new insights into the design of robust bifunctional catalysts for water splitting, and may promote the development of multicomponent amorphous catalysts.

Single-crystalline CoFe nanoparticles encapsulated in N-doped carbon nanotubes as a bifunctional catalyst for water splitting

We utilize a simple heat-treatment process to synthesize single-crystalline CoFe alloy nanoparticles encapsulated in N-doped carbon nanotubes, which exhibit excellent catalytic activity and superior stability towards overall water splitting.

Core-shell cobalt particles Co@CoO loaded on nitrogen-doped graphene for photocatalytic water-splitting

It is of great significance to explore a bifunctional catalyst that can produce both hydrogen and oxygen to accelerate the development of water-splitting technology. In this work, Co@CoO/NG was obtained via calcinating ZIF-67 and in-situ preparation process, which exhibited excellent performance (water oxidation AQE 10.22% at λ = 450 nm and oxygen production rate 543198 μmol g−1 h−1 and hydrogen production rate 330 μmol−1 g−1 h−1). A comprehensive analysis of SEM, XRD, TEM, UV–vis, EIS, and PL showed that Co@CoO/NG-7 prepared has a perfect skeleton and more crystal defects, which can provide more reactive sites. The core-shell structure Co@CoO has a synergistic effect with graphene, which is beneficial to the light absorption, separation of photo-generated charges. Meanwhile, cyclic experiments of water oxidation and water reduction showed that the catalyst exhibited high stability during the reaction process. This study has provided a universal strategy to design efficient bifunctional catalyst for water-splitting.

Integrating Rh Species with NiFe-Layered Double Hydroxide for Overall Water Splitting.

NiFe-layered double hydroxide (LDH) is thought of as a promising bifunctional water-splitting catalyst, owing to its excellent performances for alkaline oxygen evolution reactions (OERs). However, it shows extremely poor activity toward hydrogen evolution reactions (HERs) due to the weak hydrogen adsorption. We demonstrated that the integration of Rh species and NiFe-LDH can dramatically improve its HER kinetics without sacrificing the OER performance. The Rh species were initially integrated into NiFe-LDH as oxidized dopants and metallic clusters (< 1 nm). In 1 M KOH electrolyte, an overpotential of 58 mV is needed to catalyze 10 mA cm-2 HER current density. Furthermore, this catalyst only requires 1.46 V to drive an electrolyzer at 10 mA cm-2. A strong interaction between metallic Rh clusters and NiFe hydroxide during the HER process is revealed. The theoretical calculation shows the Rh ions replace Fe ions as the major active sites that are responsible for OERs.