Carbon dioxide capture and mitigation forms a key part of the technological response to combat climate change and reduce CO 2 emissions. Solid materials capable of reversibly absorbing CO 2 have been the focus of intense research for the past two decades, promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this Review, we explore the fundamental aspects underpinning solid CO 2 sorbents based on alkali and alkaline earth metal oxides operating at mid - to high temperature : how their structure, chemical composition and morphology impact their performance and long -term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines including materials discovery, synthesis, and in situ characterization to present a coherent understanding of the mechanisms of CO 2 absorption both at sur faces and within solid materials. 1 Introduction and objectives - CO 2 capture by metal oxides Humankind faces an urgent challenge to rapidly reduce greenhouse gas emissions in order to reach a carbon neutral society by 2050, the point at which environmental changes caused by climate change may become irreversible [1], [2] . Amongst the many technologies being developed towards this goal, it has been argued that carbon dioxide capture , utilization and storage (CCUS) is a necessary component , being both a mature and relatively inexpensive measure that can support other initiatives in electrochemical storage, renewable energy conversion and improving industrial efficiency [3]–[6]. Not only is CC US an environmental necessity, but it also represents a significant economic opportunity [7], [8] . Recent governmental and industrial white papers highlight its vital role in “Power to X ” (P2X) schemes that seek to harness electricity to synthesiz e various energy -dense fuel sources by utilizing H 2 from electrolysis combined with CO 2 from capture [9]–[13]. P2X represents just one of several 2 proposed industrial processes aiming to use CO 2 as a feedstock [14]–[20], underlining the need for efficient CC US implementation. Within the area of carbon dioxide capture, liquid amines have seen the most use at scale in pilot plants designed to reversibly capture CO 2 and yielding a purified stream of CO 2 for subsequent sequestration [18], [21] –[23]. However , amines and especiall y their regeneration incur a relatively high energy cost , and there exist concerns regarding their toxicity [24], motivating research over the past two decades to develop alternative materials. CO 2 capture by metal oxides has been of industrial relevance for more than 150 years, ever since Tessié du Motay and Marechal [25] used CaO to absorb the CO 2 generated in a gasification process to increase the purity of H 2. The use of oxides for CO 2 capture has emerged only recently when Silaban and Harrison suggested using CaO to capture CO 2 from emissions in 1995 [26]. In 1999, Shimizu et al. [27] proposed a post -combustion CO 2 capture process comp osed of two interconnected fluidized bed reactors, in which CO 2 is continuously absorbed from a flue gas stream by CaO, forming CaCO 3, followed by its t hermal decompos ition to CaO to obtain a stream of sufficiently pure CO 2 (> 96 %) for sequestration ( R1 in Table 1) [28]. This process, termed calcium (or carbonate) looping (CaL), formed the basis for research on CO 2 capture by metal oxides at mid- to high temperatures (> 200 °C) to the present day. The higher operating temperatures of metal oxides compared to liquid amines gives the promise of a lower energy penalty incurred if implemented into power plants, driving interest in their further development [29], [30] . There has been a great deal of res earch aimed at understanding various aspects of the CO 2 absorption reaction at elevated temperature , including the materials that are of potential interest , and various modifications and optimization s that can be performed to achieve benchmarks set by gove rnments and industry. This review aims to focus on the chemistry, engineering and material science aspects that underpin mid- to high temperature CO 2 capture using solid oxide materials (e.g. CaO, MgO and Li4SiO 4) in order to consolidate the current state of understanding of the parameters that drive capture performance ; for general overviews of solid sorbents suitable for CO 2 capture at low temperature (< 200 °C), e.g. carbon -based materials, zeolites, metal -organic -frameworks (MOF) or hydrotalcite -like co mpounds, which all do not necessarily form thermodynamically stable carbonates, we refer to relevant review papers [31]–[39]. The different disciplines concerned in the development of metal oxide sorbents can offer complementary insights into CO 2 absorption, but their communities are often disparate; this review aims to bring together the contributions of each field, and to encourage more interdisciplinary studies and experimental approaches . 3 CaO-based sorbents are by far the most widespread in ter ms of research literature, much of which is focused on the optimization of the archetypal CaO on an industrial process scale; for summaries of this work, we refer to a number of review papers [40]–[50] and also to papers cove ring specifically Mg - [51], [52] and Li -based [53], [54] sorbe nts. In contrast, this review will investigate questions at a much more fundamental level, including the influence of material structure, ionic conduction and particle morphology on CO 2 absorption, and how the structural and operando analysis of a wide ran ge of Ca, Mg and Li containing materials reveals trends tying performance to these chemo -physical properties. This includes theoretical approaches utilizing screening methodologies based on first principle analysis using density functional theory (DFT), as well as large -scale experimental screening which can guide the rational design of novel materials. This review also covers work applying new analytical techniques to study CO 2 capture materials including in situ X -ray diffraction (XRD), Raman spectroscopy, nuclear magnetic resonance (NMR) and synchrotron -based techniques, attempting to uncover causal links between performance and the various chemical and physical features to a material . The hope is to offer researchers in this field a broad survey of the underlying problems inherent in solid oxide CO 2 capture, and to highlight the most advanced research efforts in material synthesis, structural manipulation and material characterization s to present common findings across different fields providing advances in fundamental understanding. 4 2 Fundamentals of the carbonation and calcination reactions The following sections discuss fundamental aspects of the carbonation and calcination reaction s for the most widely used sorbents for CO 2 at elevated temperature, i.e. CaO and MgO, including their structural transformations and kinetics. Further, the role of the gas environment and reasons for sorbent deactivation are assessed. In order to simplify the analysis, we ch ose to focus solely on unmodified CaO -CaCO 3 and MgO -MgCO 3 as model systems, given that the majority of previous research has involve d these materials or modifications thereof, and that they represent the basis of the most advanced syste ms in terms of practical implementation. 2.1 Thermodynamic properties of alkali metal oxide sorbents The applicability of any material for CO 2 capture at elevated temperature s (> 200 °C) is driven by the requirements of the process in which the material is to be used. For post -combustion CO 2 capture, CO 2 concentrations in the off -gas from industrial process es typically range from ~ 3 – 15 vol.% CO 2 depending on the type of fuel used , but can reach as high as 35 – 40 vol.% in cement plants or in the iron and steel industry [55]–[58]. Ideally, CO 2 is removed completely from the respective gas stream before it is released to the atmosphere. In pre -combustion CO 2 capture, e.g. for integrated gasification combined cycle (IGCC) power plants, partial pressures of CO 2 from coal gas can range from 0.5 to 10 bar [59]–[61]. Figure 1 plots the equilib rium partial pressure of CO 2, pCO2,eq , as a function of temperature T for various single metal oxide sorbents. Combinations of T and pCO2 above the respective pCO2,eq -curves i mply the material exists as carbonate, whereas below the curve the material’s thermodynamically stable state is its oxide form. 5 Figure 1. Equilibrium partial pressure of CO 2, pCO2,eq , as a function of temperature for alkali (green ) and alkaline earth (blue) metal oxide -carbonate systems. Horizontal grey lines indicate p CO2 of 0.05, 0.15 and 1 bar, respectively. Note that the pCO2,eq for the system s K2O-K2CO 3, Rb 2O-Rb2CO 3 and Cs 2O-Cs2CO 3 are < 10-3 bar for temperatures 100 – 1500 °C and are thus not seen in the figure . The curves were computed using thermodynamic data provided by the NASA Glenn Research Center [62]. The blue dashed line indicates the p CO2,eq for the system MgO -MgCO 3 reported in [63], [64] . The inset plots the maximum CO 2 capture efficiency as a function of temperature for the system CaO -CaCO 3 for d ifferent p CO2; for a CO 2 capture efficiency of zero the temperature for the transition between CaO and CaCO 3 is obtained for the given p CO2. Taking the system CaO -CaCO 3 and a gas stream containing 15 vol.% CO 2 (equivalent to pCO2 ≈ 0.15 bar at atmospheric pressure), the maximum CO 2 capture efficiency at 600 °C is (pCO2 – pCO2,eq )/pCO2 = (0.15 – 0.0032)/0.15 = 97.9 %, but decreases to (0.15 – 0.0358)/0.15 = 76.1 % at 700 °C. The r emoval of almost all (99.9 %) CO 2 from th e gas stream is thermodyna mically feasible up to temperatures of 570 °C. We note that the thermodynamic properties of the CaO -CaCO 3 system enable also the capture of CO 2 from ambient air ( pCO2 = 4·10-4 bar) [65]–[67]. Turni ng to the release of CO 2 from the sorbent (i.e. the decomposition of CaCO 3), temperatures greater than ~ 900 °C are required to obtain a pure stream of CO 2 (pCO2 ≈ 1 bar). Surprisingly, the thermodynamic data for the system MgO -MgCO 3 is inconsistent ; for temperatures up to 520 °C, the pCO2,eq differ by at least one order of magnitude from