Accessibility assessment and optimization of public charging stations from a categorical perspective: A case study in Nanjing based on charging travel chains

• Dual-stage travel chain model integrates driving and walking accessibility analysis. • "Spatial Efficiency–Supply-Demand Diagnosis–Site Selection Optimisation" framework. • G2SFCA and cumulative methods assess access to fast, slow, and ultra-fast PCSs. • XGBoost model predicts demand and optimizes siting based on supply-demand gaps. • Category-specific optimization strategies for fast, slow, and ultra-fast PCSs. This study examines supply–demand imbalances and service inefficiencies in the layout of public charging stations (PCSs) for new energy vehicles in central Nanjing. A framework of "Spatial Efficiency–Supply and Demand Diagnosis–Site Selection Optimisation" is established. Theoretically, a dual-stage travel chain accessibility model is introduced, evaluating both driving and walking accessibility while integrating road hierarchy, vehicle ownership, and public facility data. Methodologically, an improved Gaussian two-step floating catchment area method (G2SFCA) and a cumulative opportunity model (COM) quantify accessibility; spatial autocorrelation tests supply–demand matching; and multi-scale geographically weighted regression with grid search-optimised extreme gradient boosting provides grid-level siting recommendations. Strategically, a category-based response is proposed to align PCSs with urban functions. Results show that despite high overall density, fast PCSs exhibit service gaps in old urban areas; slow PCSs are evenly distributed but weakly integrated with communities; and ultra-fast PCSs must balance industrial coverage with premium service demand. Supply–demand clustering highlights key mismatch zones. At the same time, the optimised siting map suggests enhancing utilisation of fast PCSs in high-demand grids, strengthening the community service network of slow PCSs, and coupling industrial and high-end functions of ultra-fast PCSs. Overall, the framework enhances coordinated dual-stage accessibility and offers a transferable approach to reconcile infrastructure provision with urban constraints in dense cities, providing a robust basis for precise transport infrastructure planning under carbon-neutrality goals.