Phosphorus‐Based Electrocatalysts: Black
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REVIEW www.advmatinterfaces.de Phosphorus-Based Electrocatalysts: Black Phosphorus, Metal Phosphides, and Phosphates Xin Li and John Wang* their efficiencies, the catalysts employed Enormous progresses have been made in developing advanced energy con- are basically required to overcome the version and storage technologies, which inevitably require high-performance high energy barriers and sluggish kinetics electrocatalysts. Recently, phosphorus (P)-based materials have drawn of the electrochemical HER, OER, and ORR.[3] The benchmarking catalysts for tremendous attention as a class of promising electrocatalysts and presented ORR and HER are platinum (Pt)-based intrinsic electrochemical activity and widely tunable property. In a timely noble metal catalysts, whereas iridium response to the ongoing interests, issues faced in P-based inorganic mate- (Ir) and ruthenium (Ru) oxides are highly rials, and the approaches to address them in relation to energy conversion active toward the OER.[4] Nevertheless, reactions, including hydrogen evolution reaction, oxygen evolution reaction, the high cost and scarcity of these pre- and oxygen reduction reaction are investigated. The emphasis is focused cious metal-based catalysts have notably hindered their wide applications and on the key strategies to modulate black phosphorus, metal phosphides, and commercialization.[5] phosphates-based electrocatalysts, although it is impossible to include all Therefore, developing highly active, possible P-compounds. The strategies are analyzed and discussed in four low cost, and sustained catalysts for these aspects, namely, morphological engineering, tuning in crystallinity, composi- key electrocatalytic processes is para- tional tailoring, and material hybridization. They are aimed to modulate the mount and apparently rewarding for the sustainable and large-scale implementa- key parameters involved in electrocatalysis, such as the adsorption energy, tion of clean energy devices.[6] To reduce, and density of the active sites, electrical conductivity, and durability of and hopefully to eliminate the usage of these P-based inorganic electrocatalysts. With the thorough understandings these precious metal-based catalysts, there established for the rational design of the efficient electrocatalysts, the chal- have been intensive researches, in order lenges and perspectives in the future development of these high-performance to develop the practically and economi- [7] P-based systems are briefly looked into. cally viable alternative electrocatalysts. In this connection, phosphorus (P) is one of the most earth-abundant elements, thereby P-based materials have been con- 1. Introduction sidered being employed for electrocatalysis.[8] Indeed, P-based inorganic materials, especially the black phosphorus, metal The ongoing quick rise in energy consumption, concerns in phosphides, and metal phosphates, have attracted immense the sustainability of traditional fuels, such as oil and coals, attention recently as a class of promising electrocatalysts, and and their detrimental environmental effects, have stimulated presented intrinsic electrochemical activity and widely tunable worldwide efforts to develop highly efficient, sustainable, clean, properties.[9] and low-cost energy technologies.[1] Wherein metal–air bat- Black phosphorus (BP) is a layer-structured semiconductor, teries, fuel cells, and water splitting, etc., as highly efficient in which individual atomic layers are stacked and held together and renewable energy conversion and storage systems, have by van der Waals interactions.[10,11] Until recently, BP stands been among the best-known promising energy technologies.[2] out as a group of promising catalysts that have been investi- In these systems, hydrogen evolution reaction (HER), oxygen gated and shown to exhibit catalytic activities, owing to their evolution reaction (OER), and oxygen reduction reaction (ORR) unique puckered layer-structure, tunable band gap, and high are of great importance. To promote these reactions and raise carrier mobility.[12,13] As suggested by recent researches, 2D catalysts with reduced layer numbers or the thickness can present increased surface area and hence expose additional [14] Dr. X. Li, Prof. J. Wang active sites, in comparison with their bulk counterparts. Department of Materials Science and Engineering To this end, phosphorene, as the building block for BP, pos- National University of Singapore sesses a monolayer (or a few layer) sheet structure, and has Singapore 117574, Singapore been explored for electrocatalysis and expected to promote the E-mail: [email protected] catalytic efficiency.[15,16] Nevertheless, because of the densely The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202000676. packed lone-pair p-electrons of phosphorus atoms, it would be hard for phosphorene to adsorb hydrogen, leading to the large DOI: 10.1002/admi.202000676 hydrogen adsorption energy, and thus poor HER electrocatalytic Adv. Mater. Interfaces 2020, 2000676 2000676 (1 of 32) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de activity.[17] On the other hand, the adsorption of O* on pristine strategies in attempts to achieve better HER, OER, or ORR phosphorene is too strong, which leads to poor OER/ORR per- performance. First, we discuss the main requirements that a formances. Thereby, their catalytic activities are rather mod- desired electrocatalyst should fulfil. After that, various modu- erate/marginal, and the research is relatively scarce, with some lating strategies to develop efficient BP, metal phosphides, studies remaining at the theoretical stage.[18] and phosphates catalysts will be discussed in four aspects: Metal phosphides, especially transition metal phosphides i) morphological engineering, including the efforts in estab- (TMPs), have been researched as a group of the most prom- lishing various freestanding, nanostructured, and mesoporous/ ising HER catalysts, due to their hydrogenase-like catalytic hollow textures; ii) tuning in crystallinity with crystalline and mechanisms, good conductivity, low cost, and abundant amorphous materials involved; iii) compositional tailoring, reserves.[19] Generally, the positive charged metal-site and including tuning in the chemical ratio, elemental doping, and negative charged P-site could function synergistically as the surface modification; iv) material hybridization, including syn- hydride- and proton-acceptor to complete HER, respectively.[20] ergies with other functional phases, such as carbon-based, and Additionally, P anions are able to protect the MPs from dis- the control in their interfaces. In connection with these four solving and thus enable MPs a good durability during the HER aspects, we will also visit selected methods to synthesize the process. With respect to OER that under oxidation potentials, designed catalysts. In the end, we look into the challenges and MPs can be partially or completely oxidized into metal oxides/ perspectives in the further development of high-performance hydroxides, which become the active sites for OER, as has been P-based electrocatalysts, for HER, OER, and ORR. reported in several previous studies.[21,22] As such, phosphides are capable of working as bifunctional electrocatalysts for water splitting. However, the binding of hydrogen for phosphides 2. Key Requirements for a High-Performing is generally stronger or weaker than the optimum during the Electrocatalyst HER process.[23] Also, the OER performance of MPs remains inferior compared to that of the noble metal based catalysts, A high-performance electrocatalyst can be identified by the test pointing toward the space for further improvements.[24] Various results showing the “ideal values” for key catalytic parameters, stoichiometric ratios and constituent elements provide metal including overpotential, exchange current density, Tafel slope, phosphides with various compositions and structures, which and so forth. To this end, there are several primary requirements allow fine tuning of their (electronic) structures and thus open that a high-performance electrocatalyst should fulfil, which can up a new platform for designing a large range of active HER be briefly listed as a moderate/appropriate adsorption energy, and OER catalysts.[25] high surface area, good conductivity, and long stability, which Metal phosphates are also deemed as one group of the attrac- will be briefly discussed in this section. tive materials for electrocatalysis. The phosphate (Pi) groups can function as proton acceptor which facilitate the oxidation of metal atoms. Simultaneously, the local metal geometry can be 2.1. Moderate Adsorption Energy adjusted and induce distorted local metal geometry, beneficial to the water adsorption and oxidation. Thus, metal phosphates The first requirement is a “moderate adsorption energy” to the are ready to be among the most promising OER electrocata- involved intermediates. Generally, an electrocatalyst functions lysts.[26] Also, the diverse structures together with the versatile to adsorb reactant(s) on the surface, then forms the adsorbed compositions of metal phosphates allow fine tuning of their intermediate(s) and thereby promotes the electrochemical reac- electronic configurations and thus make some of them the ideal tions involving charge transfer. Therefore, it is of apparent platforms for designing and engineering high-performance importance to estimate the differential adsorption