Self‐Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation

Abstract

7337wileyonlinelibrary.com energy and the development of regenerative fuel cells and rechargeable metal-air batteries. [1][2][3][4] Note that overpotentials originating from the polarization phenomenon occurring at the electrodes induce a larger voltage window than the theoretical minimum one (1.23 V) to afford thermodynamic driving force. [ 5,6 ] As such, the sluggish apparent reaction kinetics necessitates the utilization of noble-metal-based electrocatalysts to achieve respectable performance, i.e., Pt for HER and IrO 2 / RuO 2 for OER, [ 7,8 ] though the scarcity and the consequent unfavorable cost prohibit the scale-up deployment in energy devices. Accordingly, great efforts have been made toward effi cient earth-enriched materials, such as OER catalysts working under strongly alkaline conditions and HER catalysts operating in strongly acidic mediums, due to the thermodynamic convenience and application prospect in alkaline electrolyzers or proton-exchange membrane. [9][10][11][12][13][14][15] On the one hand, pairing the OER and HER catalysts together in the same electrolyte is of practical values to accomplish overall water splitting, which remains diffi cult to achieve owing to the incompatibility of the stability and activity for the same catalyst systems in the operating pH regions, thereby leading to inferior effi ciency; on the other hand, the different catalysts intended to separate HER and OER may need distinct synthetic strategies and instruments with lowthroughput preparation processes. [16][17][18][19][20] Therefore, it is still a grand challenge to exploit bifunctional electrocatalysts in terms of not only featuring high effi ciency toward both hydrogen and oxygen evolution reactions, but also simplifying the system and reducing the costs.Transition Co-based catalysts have been regarded as promising alternatives to noble metals to drive the half reactions, for instance, metal Co, [ 21 ] CoS, [ 22 ] and CoSe, [ 23 ] for HER and Co oxides/(oxy)hydroxides designed for OER. [ 24,25 ] Indeed, CoO xcarbon composites have been implemented to be bifunctional and effective electrocatalysts for overall water splitting in base, wherein the involvement of conductive carbonaceous hosts is to overcome the disadvantages of self-accumulation and insuffi cient electric conductivity as to oxides. [ 26,27 ] Nonetheless, the sophisticated preparation and instability of active phases in acid propose an obstacle to further optimize the technology and fi nd Water splitting for the production of hydrogen and oxygen is an appealing solution to advance many sustainable and renewable energy conversion and storage systems, while the key fact depends on the innovative exploration regarding the design of effi cient electrocatalysts. Reported herein is the growth of CoP mesoporous nanorod arrays on conductive Ni foam through an electrodeposition strategy. The resulting material of well-defi ned mesoporosity and a high specifi c surface area (148 m 2 g −1 ) can be directly employed as a bifunctional and fl exible working elect...

Straightforward synthesis of nitrogen-doped carbon nanotubes as highly active bifunctional electrocatalysts for full water splitting

“…The OER activity of 20-NMWNT is significantly improved in 1 M NaOH and shows an onset potential of only $1.5 V. The overpotentials required for OER current density of 10 mA cm À2 (denoted as N OER10 ) are measured to be 360 mV and 320 mV in 0.1 and 1 M NaOH, respectively. These values are comparable to those of the recently reported highly active transition metals [17,25,26,[44][45][46] and superior to the most active metal free catalysts [5,24,33,37,3 8,42,47-50] for OER (for more comparison see Table S2). It is noteworthy that the performance of 20-NMWNT is comparable to that of the most active metal-free catalysts reported very recently including N,S-CNT [47] (N OER10 of 360 mV in 1 M KOH), the most active N-doped metal-free catalyst, N-doped graphene nanoribbons (N-GRW) which has been recently reported by Liu et al [37], showing N OER10 of 360 mV in 1 M KOH, and also nitrogen, phosphorus and oxygen tri-doped porous graphite carbon@oxi-dized carbon cloth (ONPPGC/OCC) [38] developed as a metal-free bifunctional electrocatalyst but requiring significantly higher N OER10 of 410 mV in 1 M KOH.…”

Ternary NiCoP nanosheet arrays: An excellent bifunctional catalyst for alkaline overall water splitting

“…The high-resolution P 2p spectrum shows two main peaks at 134.1 and 129.3 eV ( Fig. 2(d)), with the lower binding energy (BE) peak corresponding to metal phosphides, and the other reflecting the oxidized metal phosphate species on the surface, arising from exposure of superficial NiCoP to air [23,29]. After peak deconvolution of the Co 2p 3/2 core level region, two main peaks appear at 778.8 and 781.7 eV, with the peak located at 794.2 eV assigned to oxidized metal phosphate ( Fig.…”

“…Recently, MPCs have been reported to show good catalytic OER performance, attributable to the formation of metal oxo/hydroxo species on the catalyst surface, which create a heterostructure with high OER activity [12,22]. While significant progress has been made in the study of binary transition metal phosphides as bifunctional catalysts [12,22,23], the application of ternary metal phosphides in hydrogen or oxygen evolution reactions has been scarcely reported, to the best of our knowledge. In addition, previous spectroscopic and theoretical studies demonstrated that the incorporation of additional metal atoms into HER and OER catalysts is crucial for enhancing their electrocatalytic activity by tuning of the electronic structure [24][25][26][27].…”

Acid promoted Ni/NiO monolithic electrode for overall water splitting in alkaline medium

“…Although a variety of non-noble metal-based nanostructured materials such as Mo 2 C/MoO 2 [11], Co 9 S 8 /graphene [12], CoPS [13], CoMoS [14], NiMoN [15] have been reported, the facts are still far from satisfactory since these materials are either for oxygen evolution reaction (OER) catalysts working in strongly alkaline conditions or for hydrogen evolution reaction (HER) catalysts operating in strongly acidic mediums due to the thermodynamic convenience [5,[16][17][18]. The inferior efficiency caused by the disparity of the stability and activity for the same catalyst system in the operating pH ranges [19,20] limits the practical implementations of these materials as a catalytic electrode in an integrated electrolyser. Therefore, a formidable challenge for electrocatalytic water splitting is to explore and design bifunctional catalysts that can work well for both HER and OER in the same alkaline medium to accomplish overall water splitting.…”