ReviewAdvances in alkaline water electrolyzers: A review
Introduction
The world economy is constantly expanding. There are two influencing factors related to that expansion: the population growth and progress in personal comfort. Both factors affect the current fossil economy by increasing consumption and generating greater amount of greenhouse gases (GHG). The International Energy Agency (IEA) indicates a world consumption in 2015 of 9.383 Mtoe (1.1 × 105 TWh). This amount represents an increase of 18.23% and 99.64% over the past ten (2005) and fourty (1975) years, respectively. Besides, CO2 emissions in 2015 were 32.294 MTon, compared to 15.484 MTon in 1975 (109% increment) [1]. This situation is widely accepted as critical, hence worldwide environmental impact studies and environmental protection policies are generated. Moreover, the fact that fossil fuels are neither renewable nor evenly distributed across the globe leads to geopolitical conflicts and unequal situations.
Around the world, proposed solutions focus on the production of renewable energy. However, the share of renewable energies has not grown significantly (from 12.7% in 1975 to 13.5% in 2015). Besides costs issues, the global experience indicates that advances are needed to solve technical problems related to energy fluctuations produced in renewable sources. To achieve high integration of renewable energy, it is necessary to have the ability to accumulate the excess of energy to be consumed at a time when consumption exceeds production. Fig. 1 shows the variety of available technologies for energy storage. While some technologies such as supercapacitors or flywheels are used to store a reduced amount of power (up to 10MW) for a short time (up to an hour) and redeliver it quickly, for the case raised, it is necessary to use other technologies such as Compressed Air Energy Storage (CAES), Pumped Hydro Energy Storage (PHES) or hydrogen.
So far, the most common way to store large amounts of energy is PHES. The biggest disadvantage of this technology is related to its requirements on specific geographical features for installation and political conditions. It is here that among the methods of energy storage, hydrogen production currently takes relevance for its energy density, high energy capacity and transportability [2], [3].
Moreover, in the same direction, there is the concern about pollution in the transportation sector. Along with the development of electric vehicles, the hydrogen appears as an interesting energy vector. Both technologies, electric and H2-based vehicles, share the benefit of eliminating urban pollution and, depending on the original source, reducing or eliminating pollution in the whole process [4]. The union of these two sectors, electricity and transport, generates what is disclosed as hydrogen economy. The hydrogen economy is stated as an integral solution for the problem of producing, storing and supplying energy including all final uses while succeeding in GHG mitigation.
The industrial use of hydrogen dates from almost a century ago with a wide consumption in the chemical and oil industries (89% of consumption share) [5]. However, progress must be achieved in various issues in order to accomplish competitiveness of these technologies and develop this economic concept. Issues such as the efficiency and cost of production, storage and transport, are concepts that several companies, research centers and governments are developing.
Several reviews can be found that present the different technologies related to the use of hydrogen. Abdalla et al. [6] published a review of hydrogen technologies making a detailed explanation and comparison of current storage methods. Zhang et al. [7] present a brief and well-organized compendium of production, storage and electricity generation technologies. Dutta [8] summarizes development models for the hydrogen economy in various countries along with an explanation of hydrogen production, storage and utilization. Mazloomi and Gomes [9] discuss the economic aspects of centralized and distributed production. In addition, they present the risks inherent in the production, storage and distribution stages, proposing possible risk-reduction techniques.
At the same time, there are studies such as [10] that detail the steps to be followed in order to reach a mature hydrogen economy. Among those steps there are the Power-to-Gas [11], [12], the use of fossil hydrogen to power vehicles [13], [14], [15], [16] and the integration of electrolyzers with renewable energies in microgrids [17], [18]. All these developments bring hydrogen technologies taking into account the necessary economic issues in order for it to be sustainable over time. To do this, it will be necessary that companies, governments and research centers cooperate together in this direction [13].
This paper provides an overview of the hydrogen production technologies, specifically emphasizing production from alkaline electrolysis. Mueller-Langer et al. [19] in their techno-economic assessment assure that natural gas steam reforming, coal and biomass gasification and water electrolysis will play a significant role in the short and medium term. Besides, electrolysis occupies until today a dominant position as it is the only technology that can use directly the power surplus from renewable and fluctuating energies like wind mills or solar panels [7] so it has a concrete perspective on the use of this type of energy as the axis of the hydrogen economy. Among CO2-neutral H2 production, electrolysis highlight because it produces high purity hydrogen and it has an infrastructure already developed being a well-established technology [20], [21]. In the same direction, alkaline electrolysis is a mature and reliable technology which stands out from other types of electrolysis based on cost and simplicity [22].
The remainder of this paper is organized as follows. In Section 2, hydrogen production technologies are compared according to efficiency, costs and environmental consequences. After that, in Section 3, water electrolysis, as the most certain solution for ecofriendly hydrogen production, is described. Then, in Section 4, necessary developments in alkaline electrolyzers in the short and long term are displayed. Finally, conclusions in Section 6 reinforce the necessity to advance research to achieve the reduction of pollution through the hydrogen economy.
Fig. 2 shows the different methods of hydrogen production presented in Section 2. It highlights the approach outlined in this paper, explaining its organization.
Section snippets
Hydrogen production methods
There are several methods of hydrogen production with different stages of development. Currently, its production is mainly based on the reforming of fossil fuels (78%) and coal gasification (18%). From the pending 4% of alternate resources, the main technology is the electrolysis of water as a byproduct from chlor-alkali process [23], [24]. Despite the current use of hydrogen produced by the last process, this technology will not be considered in the analysis because in the long term and taking
Water electrolysis
Electrolysis is the method through which the water molecule is separated into hydrogen and oxygen by applying an electric current [42]. Although there are different methods, which are introduced below, they share the same global reaction
Developments in alkaline electrolysis
In recent decades, advances have been made in this type of electrolyzers called as advanced alkaline electrolyzers. The most important points of development are [21]:
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Zero-gap configuration. It consists of minimizing the distance between electrodes to reduce the ohmic losses.
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New materials for the diaphragm. Previously made of asbestos, the use of inorganic membranes is investigated. Some are based on antimony polyacid impregnated with polymers [78], on porous composite composed of a polysulfone
Discussions
As presented in the current review, there are interesting alternative methods for the production of hydrogen with virtually zero emissions, among them highlighting the production from biomass and electrolysis. Its biggest disadvantage is the economic cost superior to industrial processes such as the SMR in both construction and operation. It can be seen that these three technologies will coexist in the medium term, waiting for the proportion of SMR to gradually decrease, generating two
Conclusions
The search for alternative methods of power generation and transport has developed the concept of hydrogen economy. While today hydrogen is obtained mainly from hydrocarbons, new technologies to achieve lower GHG emissions are being developed and consolidated. This paper summarizes the different methods of hydrogen production with emphasis on the current status of alkaline electrolysis. Among hydrogen methods, electrolysis stands out for ease of connection to renewable energies, obtainable
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