Metal-Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges

Hydrogen is in limelight as an environmental benign alternative to fossil fuels from few decades. To bring the concept of hydrogen economy from academic labs to real world certain challenges need to be addressed in the areas of hydrogen production, storage, and its use in fuel cells. Crystalline metal-organic frameworks (MOFs) with unprecedented surface areas are considered as potential materials for addressing the challenges in each of these three areas. MOFs combine the diverse chemistry of molecular linkers with their ability to coordinate to metal ions and clusters. The unabated flurry of research using MOFs in the context of hydrogen energy related activities in the past decade demonstrates the versatility of this class of materials. In the present review, we discuss major strategical advances that have taken place in the field of “hydrogen economy and MOFs” and point out issues requiring further attention.     

Thermodynamics study of hydrogen storage materials

The growing use of conventional energy such as fossil fuels results in problems degrading our environment. Hydrogen is frequently discussed as a clean energy in the future without pollution. However, efficient and safe storage of hydrogen constitute a key challenge and unresolved problem. One of the main options is solid-state storage technology. A successful solid-state reversible storage material should meet the requirements of high storage capacity, suitable thermodynamic properties, reversibility and fast adsorption and desorption kinetics. This feature article focuses mainly on the development of thermodynamic improvement of hydrogen storage materials in the past few years including the complex hydride, ammonia borane, and metal-organic frameworks.     

储氢材料第一性原理计算的研究进展

综述了第一性原理计算在储氢材料研究中取得的成果和最新的进展。 第一性原理计算在储氢材料研究中的应用主要有以下4个方面： 1）研究纳米结构的储氢性能; 2） 研究储氢材料中掺杂和缺陷的作用及对储氢性能的影响; 3）研究储氢机理; 4）确定氢化物的几何结构以及预测新型储氢材料。 同时展望了第一性原理计算在储氢领域中的应用前景。     

Active MgH2?Mg Systems for Reversible Chemical Energy Storage

An overview of the importance of and methods available for heat storage in the form of sensible and latent heat is followed by a discussion of the advantages and disadvantages of reversible thermochemical energy storage compared to conventional energy sources such as fuels, i.e. irreversible chemical energy carriers. Of the reversible metal-hydride–metal systems, the MgH₂? Mg system is particularly attractive as a hydrogen and a high-temperature heat storage material because of its high hydrogen content and the high energy content of the Mg–H bond. The advances made in this area over the past few years, namely in catalytic hydrogenation and the doping of magnesium powders, have led to the development of “active MgH₂? Mg systems” for energy storage. The first experimental results on high-temperature heat storage (also with cooling) by coupling a MgH₂? Mg storage system with a low-temperature metal hydride storage system are presented.     

Photoresponsive Carbon-Azobenzene Hybrids: A Promising Material for Energy Devices

Advancements in renewable energy technology have been a hot topic in the field of photoresponsive materials for a sustainable community. Organic compounds that function as photoswitches is being researched and developed for use in a variety of energy storage systems. Azobenzene photoswitches can be used to store and release solar energy in solar thermal fuels. This review draws out the significance of azobenzene as photoswitches and its recent advances in solar thermal fuels. The recent developments of nano carbon templated azobenzene, their interactions and the effect of substituents are highlighted. The review also introduces their applications in solar thermal fuels and concludes with the challenges and future scope of the material. The advancements of solar thermal fuels with cost effective and desired optimal properties can be explored by scientists and engineers from different technological backgrounds.     

储氢研究进展

氢能是21世纪主要的新能源之一。作为一种新型的清洁能源,氢的廉价制取、安全高效储存与输送及规模应用是当今研究的重点课题,而氢的储存是氢能应用的关键。储氢材料能可逆地大量吸放氢,在氢的储存与输送过程中是一种重要载体。本文综述了目前所采用或正在研究的主要储氢材料与技术,如高压气态储氢、低温液态储氢、金属氢化物储氢、化学氢化物储氢、吸附储氢、金属有机骨架储氢等,比较了各种储氢的优缺点,并指出其相关发展趋势。     

Research progress on transition metal oxide based electrode materials for asymmetric hybrid capacitors

In the past few years, the increasing energy consumption of traditional fossil fuels has posed a huge threat to human health. It is very imperious to develop the sustainable and renewable energy storage and conversion devices with low cost and environment friendly features. Hybrid supercapacitors are emerging as one of the promising energy devices with high power density, fast charge-discharge process and excellent cycle stability. However, morphology and structure of the electrode materials exert serious effect on their electrochemical performances. In this review, we summarized recent progresses in transition metal oxide based electrode materials for supercapacitors. Different synthesis routes and electrochemical performances of electrode materials and storage mechanisms of supercapacitor devices have been presented in details. The future developing trends of supercapacitor based on metal oxide electrode materials are also proposed.     

金属-有机框架材料在液相催化化学制氢中的研究进展

金属-有机框架(MOFs)材料是由金属簇节点或金属离子与有机配体连接而成的典型的无机-有机杂合物, 作为一类新兴的无机多孔晶态材料, MOFs因具有高度有序的多孔性、 结构可裁剪性、 高比表面积及灵活多变的骨架类型等优点而在工业合成、 能源开发、 环境治理和生物制药等领域展现出广阔的应用前景. 本文从氢能源的开发利用出发, 总结了近年来MOFs基纳米复合材料在催化化学制氢方面的研究进展. 讨论了常见的含氢量高的化学储氢材料, 包括氨硼烷、 甲酸和水合肼等; 催化材料主要有单一MOFs、 MOF基贵金属和非贵金属复合材料及MOF基衍生材料等. 最后, 对MOF基复合材料在液相催化化学储氢中的应用前景进行了展望.     

Electrochemical Oxidation of Carbon‐Containing Fuels and Their Dynamics in Low‐Temperature Fuel Cells

Fuel cells can convert the energy that is chemically stored in a compound into electrical energy with high efficiency. Hydrogen could be the first choice for chemical energy storage, but its utilization is limited due to storage and transport difficulties. Carbon‐containing fuels store chemical energy with significantly higher energy density, which makes them excellent energy carriers. The electro‐oxidation of carbon‐containing fuels without prior reforming is a more challenging and complex process than anodic hydrogen oxidation. The current understanding of the direct electro‐oxidation of carbon‐containing fuels in low‐temperature fuel cells is reviewed. Furthermore, this review covers various aspects of electro‐oxidation for carbon‐containing fuels in non‐steady‐state reaction conditions. Such dynamic investigations open possibilities to elucidate detailed reaction kinetics, to sense fuel concentration, or to diagnose the fuel‐cell state during operation. Motivated by the challenge to decrease the consumption of fossil fuel, the production routes of the fuels from renewable resources also are reviewed.     

金属-碳基储氢材料计算与实验研究*

氢能以其资源丰富和环境友好性成为未来最具发展潜力的能源。储氢技术是氢能应用中的关键问题。随着计算材料学的发展,利用密度泛函和量子机制第一性原理研究已知材料储氢性能和寻找潜在的新型优良储氢载体已成为当前研究储氢材料的有效方法。本文综述了近年来金属-碳基储氢材料中的金属修饰碳纳米管、C₆₀材料和过渡金属-乙烯复合物的理论计算与实验研究进展,并对该领域未来的研究工作进行了展望。     

Electrochemical hydrogen storage in porous carbons with acidic electrolytes: Uncovering the potential

Electrochemical hydrogen storage in porous carbon materials is emerging as a cost-effective hydrogen storage and transport technology with competitive power and energy densities. The merits of electrochemical hydrogen storage using porous conductive carbon-based electrodes are reviewed. The employment of acidic electrolytes in such storage systems is compared with alkaline electrolytes. The recent innovations of a proton battery for smaller-scale electricity storage, and a proton flow reactor system for larger (grid)-scale storage and bulk export of hydrogen produced from renewable energy, are briefly described. It is argued that such systems, along with variants proposed by others, all of which rely on electrochemical hydrogen storage in porous carbons, can contribute to the search for energy storage technologies essential for the transition to a zero-emission global economy.     

储氢材料的研究进展

日益严峻的能源危机和环境污染,使得发展清洁的可再生能源成为世界各国的重要课题。氢能源以其可再生性和良好的环保效应成为未来最具发展潜力的能源载体,氢能被公认为人类未来的理想能源,而氢的储存是发展氢能技术的难点之一。介绍了各类材料的储氢功能特点和近年来几类主要储氢材料的研究进展,并指出了储氢材料的发展方向。     

Lowering reaction temperature: Electrochemical ammonia synthesis by coupling various electrolytes and catalysts

Ammonia is a vital emerging energy carrier and storage medium in the future hydrogen economy, even presenting relevant advantages compared with methanol due to the higher hydrogen content(17.6 wt% for ammonia versus 12.5 wt% for methanol). The rapidly growing demand for ammonia is still dependent on the conventional high-temperature and high-pressure Haber–Bosch process, which can deliver a conversion rate of about 10%–15%. However, the overall process requires a large amount of fossil fuels,resulting in serious environmental problems. Alternatively, electrochemical routes show the potential to greatly reduce the energy consumption, including sustainable energy sources and simplify the reactor design. Electrolytes perform as indispensable reaction medium during electrochemical processes, which can be further classified into solid oxide electrolytes, molten salt electrolytes, polymer electrolytes, and liquid electrolytes. In this review, recent developments and advances of the electrocatalytic ammonia synthesis catalyzed by a series of functional materials on the basis of aforementioned electrolytes have been summarized and discussed, along with the presentation and evaluation of catalyst preparation, reaction parameters and equipment.     

新型储氢材料及其热力学与动力学问题

氢能作为一种理想的二次能源受到了国内外科研工作者的广泛关注,研制可以在室温和较低压力下方便、安全、高效地储存氢能的材料是氢能发展的瓶颈.到目前为止,固态储氢材料以能量密度高及安全性好等优势被认为极具应用前景,其中以轻质元素构成的氢化物(包括硼氢化物/铝氢化物(可用通式A(MH4)n表示,其中A是碱金属(Li,Na,K)或碱土金属(Be,Mg,Ca);M是硼或铝;n=1～4)、氨基氢化物(如LiNH2等))、氨硼烷(NH3BH3)、金属有机骨架材料(MOFs)是新型储氢材料研究领域的热点,本文将着重就目前这几类储氢材料的研究当中所涉及到的一些热力学及动力学问题进行总结探讨.     

Photocatalytic hydrogen production

The efficient storage of solar energy in chemical fuels, such as hydrogen, is essential for the large-scale utilisation of solar energy systems. Recent advances in the photocatalytic production of H(2) are highlighted. Two general approaches for the photocatalytic hydrogen generation by homogeneous catalysts are considered: HX (X = Cl, Br) splitting involving both proton reduction and halide oxidation via an inner-sphere mechanism with a single-component catalyst; and sensitized H(2) production, employing sacrificial electron donors to regenerate the active catalyst. Future directions and challenges in photocatalytic H(2) generation are enumerated.     

Hydrogen storage in metal-organic frameworks by bridged hydrogen spillover

The possible utilization of hydrogen as the energy source for fuel-cell vehicles is limited by the lack of a viable hydrogen storage system. Metal-organic frameworks (MOFs) belong to a new class of microporous materials that have recently been shown to be potential candidates for hydrogen storage; however, no significant hydrogen storage capacity has been achieved in MOFs at ambient temperature. Here we report substantially increased hydrogen storage capacities of modified MOFs by using a simple technique that causes and facilitates hydrogen spillover. Thus, the storage of 4 wt % is achieved at room temperature and 100 atm for the modified IRMOF-8. The adsorption is reversible, and the rates are fast. That has made MOFs truly promising for hydrogen storage application.     

Chemoenzymatic Hydrogen Production from Methanol through the Interplay of Metal Complexes and Biocatalysts

Microbial methylotrophic organisms can serve as great inspiration in the development of biomimetic strategies for the dehydrogenative conversion of C₁ molecules under ambient conditions. In this Concept article, a concise personal perspective on the recent advancements in the field of biomimetic catalytic models for methanol and formaldehyde conversion, in the presence and absence of enzymes and co-factors, towards the formation of hydrogen under ambient conditions is given. In particular, formaldehyde dehydrogenase mimics have been introduced in stand-alone C₁-interconversion networks. Recently, coupled systems with alcohol oxidase and dehydrogenase enzymes have been also developed for in situ formation and decomposition of formaldehyde and/or reduced/oxidized nicotinamide adenine dinucleotide (NADH/ NAD⁺). Although C₁ molecules are already used in many industries for hydrogen production, these conceptual bioinspired low-temperature energy conversion processes may lead one day to more efficient energy storage systems enabling renewable and sustainable hydrogen generation for hydrogen fuel cells under ambient conditions using C₁ molecules as fuels for mobile and miniaturized energy storage solutions in which harsh conditions like those in industrial plants are not applicable.     

Hydrophilic Conjugated Materials for Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution is viewed as a promising green strategy to utilize the inexhaustible solar energy and provide clean hydrogen fuels with zero‐emission characteristic. The nature of semiconductor‐based photocatalysts is the key point to achieve efficient photocatalytic hydrogen evolution. Conjugated materials have been recently emerging as a novel class of photocatalysts for hydrogen evolution and photocatalytic reactions due to their electronic properties can be well controlled via tailor‐made chemical structures. Hydrophilic conjugated materials, a subgroup of conjugated materials, possess multiple advantages for photocatalytic applications, thus spurring remarkable progress on both material realm and photocatalytic applications. This minireview aims to provide a brief review of the recent developments of hydrophilic conjugated polymers/small molecules for photocatalytic applications, and special concern on the rational molecular design and their impact on photocatalytic performance will be reviewed. Perspectives on the hydrophilic conjugated materials and challenges to their applications in the photocatalytic field are also presented.     

电化学氢压缩和纯化与竞争技术的对比:Ⅰ.优缺点

毫无疑问,氢将在我们未来的能量组合中发挥重要作用,因为它可以储存可再生电(电-氢)并在燃料电池中可逆地转化为电能,更不用说它在(石油)化学工业中的广泛应用了.然而,在这些应用中需要纯氢,而如今的制氢仍主要基于化石燃料,因而不能被视为纯氢.因此,大规模的氢气净化是必须的.此外,氢是最轻的气体,它的体积能量含量远远低于它的竞争燃料,除非它在高压下被压缩(通常70MPa),使压缩不可避免.本文将详细说明目前可用于氢气净化和压缩的方法.这将表明在现有的技术中,也可以实现氢气净化的电化学氢压缩机(EHC)与目前工业规模上使用的经典技术相比具有许多优势.EHC有其热力学和操作上的优点,但也易于使用.然而,只有达到足够的性能, EHCs的部署才是可行的,这意味着他们的基础材料应遵守一些规范.本文将详述这些规范.     

A review on non-noble metal based electrocatalysis for the oxygen evolution reaction

In order to find a clean, efficient and sustainable new energy source that can replace fossil fuels, hydrogen energy is considered to be the most ideal choice. Electrocatalytic oxygen evolution plays a vital role in the development of hydrogen energy, promotes the research of new electrocatalysts, and is dedicated to find materials with high electrocatalytic efficiency. This article discusses in detail the major developments in OER electrocatalysts, including recently reported metal and non-metal based materials. Metal-based catalysts, although having the advantages of high catalytic activity, have disadvantages such as poor stability and low selectivity, which hinder the further application of such materials. Non-metallic based materials avoid such disadvantages and exhibit very substantial performance in overall water decomposition. This review provides useful knowledge of a well-designed OER electrocatalyst and a possible strategy for OER/HER dual-function catalytic performance for future development.