Recent Progress of Vacancy Engineering for Electrochemical Energy Conversion Related Applications

Abstract

Efficient electrocatalysts are key requirements for the development of ecofriendly electrochemical energy‐related technologies and devices. It is widely recognized that the introduction of vacancies is becoming an important and valid strategy to promote the electrocatalytic performances of the designed nanomaterials. In this review, the significance of vacancies (i.e., cationic vacancies, anionic vacancies, and mixed vacancies) on the improvement of electrocatalytic performances via three main functionalities, including tuning the electronic structure, regulating the active sites, and improving electrical conductivity, is systematically discussed. Recent achievements in vacancy engineering on various hotspot electrocatalytic processes are comprehensively summarized, with focus on the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), CO2 reduction reaction (CO2RR), and their further applications in overall water‐splitting and zinc–air battery devices. The recent development of vacancy engineering for other energy‐related applications is also summarized. Finally, the challenges and prospects of vacancy engineering to regulate the electrocatalytic performances of different electrochemical reactions are discussed.

Rational Construction of a N, F Co‐doped Mesoporous Cobalt Phosphate with Rich‐Oxygen Vacancies for Oxygen Evolution Reaction and Supercapacitors

“…On the other hand, in the F 1s high-resolution spectrum, the binding energy of the NCoHPOF catalyst splits into two peaks at 684.3 and 686.8 eV, corresponding to the CoÀ FÀ Co and FÀ Co bonds. [31][32][33] While, after calcination, for NCoHPOF-450, it has only one weaker peak at 686.3 eV, which can be ascribed to the release of HF in the pyrolysis process, in line with TGA result; meanwhile, three peaks at 531.3, 532.4 and 533.9 eV can be responded to the lattice oxygen, vacancy oxygen and surface adsorbed hydroxy species, [5,[45][46][47] respectively, it should be noted that the peak of oxygen vacancy is different from many reported results, which should be due to the chemical shift and binding energy changes of the oxygen vacancies in metal phosphate system with different environment. [45][46][47] In addition, the P 2p spectrum of NCoHPOF and NCoHPOF-450 can be divided into two peaks at P 2p3/2 (133.4 eV) and P 2p1/2 (134.2 eV), attributed to PÀ O bond, especially, the P 2p spectrum of NCoHPOF-450 shifted 0.2 eV toward higher binding energy compared with that of NCoHPOF, which might be ascribed to the formation of oxygen vacancies and pyrophosphate.…”

“…Moreover, Tafel slope could evaluate the reaction kinetics of the electrocatalyst during OER process, as shown in Figure 5d, the Tafel slope of the NCoHPOF-450 catalyst is 57.11 mV dec À 1 in 1.0 M KOH electrolyte, which is superior to that of IrO 2 (76.25 mV dec À 1 ) and NCoHPOF (102.42 mV dec À 1 ), NCoHPOF-400 (100.27 mV dec À 1 ), NCoHPOF-500 (75.23 mV dec À 1 ), NCoHPOF-550 (70.22 mV dec À 1 ) and NCoHPOF-600 (83.94 mV dec À 1 ), thus demonstrating that the NCoHPOF-450 catalyst possesses the fastest reaction kinetic. [5,[45][46][47][48][49][50][51][52][53][54] According to the above characterization results, the superb OER activity of NCoHPOF-450 electrocatalysts should be related to the N, F co-doping reconstructs its electronic structure and composition. Especially, during calcination process, the formation of disordered structure creates rich oxygen vacancies and mesoporous structure, which can increase its active site and facilitate mass transfer and the release of O 2 during OER process, consequently, improving its OER activity.…”

“…[31][32][33][34][35][36] Notably, compared with NCoHPOF, the ratio of Co 2 + /Co 3 + in NCoHPOF-450 increases from 1.68 to 1.76, which should be ascribed to the appearance of oxygen vacancies. [5,[45][46][47] Besides, from N 1s spectrum, a characteristic peak at 401.7 eV is corresponding to the NÀ H bond of NH 4 + cations in NCoHPOF, after calcination, while, there are two fitted peaks at 401.9 and 399.9 eV, attributed to the formation of the CoÀ N bond. On the other hand, in the F 1s high-resolution spectrum, the binding energy of the NCoHPOF catalyst splits into two peaks at 684.3 and 686.8 eV, corresponding to the CoÀ FÀ Co and FÀ Co bonds.…”

“…As shown in Figure S7, NCoHPOF-450 has the strongest symmetrical EPR signal at g = 2.003 than that of other as-prepared materials in this study, further confirming the rich oxygen vacancies in these N, F co-doped monometallic cobalt phosphates. [5,[45][46][47][48][49][50][51][52] Therefore, its reconstruction of electronic structure and rich oxygen vacancies of NCoHPOF-450 should contribute to improving its electrochemical performance. Hereby, we investigated the electrocatalytic performance of the asprepared materials at different calcination temperatures and IrO 2 as reference standard.…”

“…Inorganic porous materials display interesting physical properties, such as magnetic, heterogeneous catalysis, adsorption/ separation, and ion-exchange due to their diverse composition of metal ions and unique porous structure. [1][2][3][4][5][6][7][8][9] Among them, porous transition-metal phosphates have been attracting wide attention in renewable energy conversion and storage technology, [2,3] such as supercapacitors, [10,11] metal-air batteries, especially, hydrogen energy from water splitting. [2,[12][13][14][15][16] However, the overpotential at 10 mA/cm 2 for oxygen evolution reaction (OER) in the process of water splitting, corresponding to 10 % solar-to-fuel conversion efficiency, is still significantly higher than the standard potential of~1.23 V because it involves multiple proton-coupled electron transfers (PCET) process, leading to its sluggish kinetics.…”